<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"> <article> <!--$Id$--> <articleinfo> <title>Shorewall FAQs</title> <authorgroup> <corpauthor>Shorewall Community</corpauthor> <author> <firstname>Tom</firstname> <surname>Eastep</surname> </author> </authorgroup> <pubdate><?dbtimestamp format="Y/m/d"?></pubdate> <copyright> <year>2001-2011</year> <holder>Thomas M. Eastep</holder> </copyright> <legalnotice> <para>Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled <quote> <ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink> </quote>.</para> </legalnotice> </articleinfo> <caution> <para><emphasis role="bold">This article applies to Shorewall 4.4 and later. If you are running a version of Shorewall earlier than Shorewall 4.4.0 then please see the documentation for that release.</emphasis></para> </caution> <section id="Install"> <title>Installing Shorewall</title> <section id="Howto"> <title>Where do I find Step by Step Installation and Configuration Instructions?</title> <para><emphasis role="bold">Answer:</emphasis> Check out the <ulink url="shorewall_quickstart_guide.htm">QuickStart Guides</ulink>.</para> </section> <section id="faq92"> <title>(FAQ 92) There are lots of Shorewall packages; which one(s) do I install?</title> <para><emphasis role="bold">Answer</emphasis>: When first installing Shorewall 4.4.0 or later, you must install the <emphasis role="bold">shorewall</emphasis> package. If you want to configure an IPv6 firewall, you must also install <emphasis role="bold">shorewall6</emphasis>.</para> <section id="faq92a"> <title>(FAQ 92a) Someone once told me to install shorewall-perl; anything to that?</title> <para><emphasis role="bold">Answer</emphasis>: That was good advice in Shorewall 4.2 and earlier. In those releases, there were two packages that provided the basic firewalling functionality: <emphasis role="bold">shorewall-shell</emphasis> and <emphasis role="bold">shorewall-perl</emphasis>. Beginning with Shorewall 4.4.0, <emphasis role="bold">shorewall-shell</emphasis> is discontinued and <emphasis role="bold">shorewall-perl</emphasis> is renamed <emphasis role="bold">shorewall</emphasis>.</para> </section> </section> <section id="faq37"> <title>(FAQ 37) I just installed Shorewall on Debian and the /etc/shorewall directory is almost empty!!!</title> <para><emphasis role="bold">Answer:</emphasis></para> <important> <para>Once you have installed the .deb package and before you attempt to configure Shorewall, please heed the advice of Lorenzo Martignoni, former Shorewall Debian Maintainer:</para> <para><quote>For more information about Shorewall usage on Debian system please look at /usr/share/doc/shorewall-common/README.Debian provided by [the] shorewall-common Debian package.</quote></para> </important> <para>If you install using the .deb, you will find that your <filename class="directory">/etc/shorewall</filename> directory is almost empty. This is intentional. The released configuration file skeletons may be found on your system in the directory <filename class="directory">/usr/share/doc/shorewall-common/default-config</filename>. Simply copy the files you need from that directory to <filename class="directory">/etc/shorewall</filename> and modify the copies.</para> <section id="faq37a"> <title>(FAQ 37a) I just installed Shorewall on Debian and I can't find the sample configurations.</title> <para><emphasis role="bold">Answer:</emphasis> Beginning with Shorewall 4.4, the samples are in the shorewall package and are installed in <filename class="directory">/usr/share/doc/shorewall/examples/</filename>.</para> </section> </section> <section id="faq14"> <title>(FAQ 14) I can't find the Shorewall 4.4 shorewall-common, shorewall-shell and shorewall-perl packages? Where are they?</title> <para><emphasis role="bold">Answer</emphasis>:In Shorewall 4.4, the <firstterm>shorewall-shell</firstterm> package was discontinued. The <firstterm>shorewall-common</firstterm> and <firstterm>shorewall-perl</firstterm> packages were combined to form a single <firstterm>shorewall</firstterm> package.</para> </section> </section> <section id="Upgrading"> <title>Upgrading Shorewall</title> <section id="faq66"> <title>(FAQ 66) I'm trying to upgrade to Shorewall 4.x; which of these packages do I need to install?</title> <para><emphasis role="bold">Answer:</emphasis> Please see the <ulink url="upgrade_issues.htm">upgrade issues.</ulink></para> </section> <section id="faq34"> <title>(FAQ 34) I am trying to upgrade to Shorewall 4.4 and I can't find the shorewall-common, shorewall-shell and shorewall-perl packages? Where are they?</title> <para><emphasis role="bold">Answer</emphasis>:In Shorewall 4.4, the <firstterm>shorewall-shell</firstterm> package was discontinued. The <firstterm>shorewall-common</firstterm> and <firstterm>shorewall-perl</firstterm> packages were combined to form a single <firstterm>shorewall</firstterm> package. For further information, please see the <ulink url="upgrade_issues.htm">upgrade issues.</ulink>.</para> </section> <section id="faq34a"> <title>(FAQ 34a) I am trying to upgrade to Shorewall 4.4 and I'm getting errors when I try to start Shorewall. Where can I find information about these issues?</title> <para><emphasis role="bold">Answer</emphasis>: Please see the <ulink url="upgrade_issues.htm">upgrade issues</ulink>.</para> </section> <section id="faq34b"> <title>(FAQ 34b) I am trying to upgrade to Shorewall 4.4 and I'm seeing warning messages when I try to start Shorewall. Where can I find information about these issues?</title> <para><emphasis role="bold">Answer</emphasis>: Please see the <ulink url="upgrade_issues.htm">upgrade issues.</ulink></para> </section> <section id="faq76"> <title>(FAQ 76) I just upgraded my Debian (Ubuntu, Kubuntu, ...) system and now masquerading doesn't work? What happened?</title> <para><emphasis role="bold">Answer:</emphasis> This happens to people who ignore <ulink url="Install.htm#Upgrade_Deb">our advice</ulink> and allow the installer to replace their working <filename>/etc/shorewall/shorewall.conf</filename> with one that has default settings. Failure to forward traffic (such as during masqueraded net access from a local network) usually means that <filename><ulink url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename> contains the Debian default setting IP_FORWARDING=Keep; it should be IP_FORWARDING=On.</para> <para><emphasis role="bold">Update</emphasis>: Beginning with Shorewall 4.4.21, there is a <emphasis role="bold">shorewall update</emphasis> command that does a smart merge of your existing shorewall.conf and the new one.</para> </section> </section> <section id="PortForwarding"> <title>Port Forwarding (Port Redirection)</title> <section id="faq1"> <title>(FAQ 1) I want to forward UDP port 7777 to my personal PC with IP address 192.168.1.5. I've looked everywhere and can't find how to do it.</title> <para><emphasis role="bold">Answer:</emphasis> The format of a port-forwarding rule <emphasis>from the net</emphasis> to a local system is as follows:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:<emphasis>local-IP-address</emphasis>[:<emphasis>local-port</emphasis>] <emphasis>protocol</emphasis> <emphasis>port-number</emphasis></programlisting> <para>So to forward UDP port 7777 to internal system 192.168.1.5, the rule is:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.5 udp 7777</programlisting> <para>If you want to forward requests directed to a particular address ( <emphasis>external-IP</emphasis> ) on your firewall to an internal system:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:<emphasis>local-IP-address</emphasis>>[:<emphasis>local-port</emphasis>] <emphasis>protocol</emphasis> <emphasis>port-number</emphasis> - <emphasis>external-IP</emphasis></programlisting> <para>If you want to forward requests from a particular Internet address ( <emphasis>address</emphasis> ):</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net:<emphasis>address</emphasis> loc:<emphasis>local-IP-address</emphasis>[:<emphasis>local-port</emphasis>] <emphasis> protocol</emphasis> <emphasis>port-number</emphasis> -</programlisting> <para>Finally, if you need to forward a range of ports, in the DEST PORT column specify the range as <emphasis>low-port:high-port</emphasis>.</para> <important> <para><emphasis role="bold">The above does not work for forwarding from the local network. If you want to do that, see <link linkend="faq2">FAQ 2</link>.</emphasis></para> </important> <section id="faq1a"> <title>(FAQ 1a) Okay -- I followed those instructions but it doesn't work</title> <para><emphasis role="bold">Answer:</emphasis> That is usually the result of one of four things:</para> <itemizedlist> <listitem> <para>You are trying to test from inside your firewall (no, that won't work -- see <xref linkend="faq2"/>).</para> </listitem> <listitem> <para>You have a more basic problem with your local system (the one that you are trying to forward to) such as an incorrect default gateway (it must be set to the IP address of your firewall's internal interface; if that isn't possible for some reason, see <link linkend="faq1f">FAQ 1f</link>).</para> </listitem> <listitem> <para>Your ISP is blocking that particular port inbound or, for TCP, your ISP is dropping the outbound SYN,ACK response.</para> </listitem> <listitem> <para>You are running Mandriva Linux prior to 10.0 final and have configured Internet Connection Sharing. In that case, the name of your local zone is 'masq' rather than 'loc' (change all instances of 'loc' to 'masq' in your rules). You may want to consider re-installing Shorewall in a configuration which matches the Shorewall documentation. See the <ulink url="two-interface.htm">two-interface QuickStart Guide</ulink> for details.</para> </listitem> </itemizedlist> </section> <section id="faq1b"> <title>(FAQ 1b) I'm still having problems with port forwarding</title> <para><emphasis role="bold">Answer:</emphasis> To further diagnose this problem:</para> <itemizedlist> <listitem> <para>As root, type <quote> <command>shorewall reset</command> </quote> ("<command>shorewall-lite reset</command>", if you are running Shorewall Lite). This clears all Netfilter counters.</para> </listitem> <listitem> <para>Try to connect to the redirected port from an external host.</para> </listitem> <listitem> <para>As root type <quote> <command>shorewall show nat</command> </quote> ("<command>shorewall-lite show nat</command>", if you are running Shorewall Lite).</para> </listitem> <listitem> <para>Locate the appropriate DNAT rule. It will be in a chain called <emphasis><source zone></emphasis>_dnat (<quote>net_dnat</quote> in the above examples).</para> </listitem> <listitem> <para>Is the packet count in the first column non-zero? If so, the connection request is reaching the firewall and is being redirected to the server. In this case, the problem is usually a missing or incorrect default gateway setting on the local system (the system you are trying to forward to -- its default gateway must be the IP address of the firewall's interface to that system unless you use the hack described in <link linkend="faq1f">FAQ 1f</link>).</para> </listitem> <listitem> <para>If the packet count is zero:</para> <itemizedlist> <listitem> <para>the connection request is not reaching your server (possibly it is being blocked by your ISP); or</para> </listitem> <listitem> <para>you are trying to connect to a secondary IP address on your firewall and your rule is only redirecting the primary IP address (You need to specify the secondary IP address in the <quote>ORIG. DEST.</quote> column in your DNAT rule); or</para> </listitem> <listitem> <para>your DNAT rule doesn't match the connection request in some other way. In that case, you may have to use a packet sniffer such as tcpdump or Wireshark to further diagnose the problem.</para> </listitem> <listitem> <para>The traffic is entering your firewall on a different interface (interfaces reversed in <filename>/etc/shorewall/interfaces</filename>?).</para> </listitem> </itemizedlist> </listitem> <listitem> <para>If the packet count is non-zero, check your log to see if the connection is being dropped or rejected. If it is, then you may have a zone definition problem such that the server is in a different zone than what is specified in the DEST column. At a root prompt, type "<command>shorewall show zones</command>" ("<command>shorewall-lite show zones</command>") then be sure that in the DEST column you have specified the <emphasis role="bold">first</emphasis> zone in the list that matches OUT=<dev> and DEST= <ip>from the REJECT/DROP log message.</para> </listitem> <listitem> <para>If everything seems to be correct according to these tests but the connection doesn't work, it may be that your ISP is blocking SYN,ACK responses. This technique allows your ISP to detect when you are running a server (usually in violation of your service agreement) and to stop connections to that server from being established.</para> </listitem> </itemizedlist> </section> <section id="faq1c"> <title>(FAQ 1c) From the Internet, I want to connect to port 1022 on my firewall and have the firewall forward the connection to port 22 on local system 192.168.1.3. How do I do that?</title> <para><emphasis role="bold">Answer:</emphasis>In /<filename>etc/shorewall/rules</filename>:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.3:22 tcp 1022</programlisting> </section> <section id="faq1d"> <title>(FAQ 1d) I have a web server in my DMZ and I use port forwarding to make that server accessible from the Internet. That works fine but when my local users try to connect to the server using the Firewall's external IP address, it doesn't work.</title> <para><emphasis role="bold">Answer:</emphasis> See <link linkend="faq2b">FAQ 2b</link>.</para> </section> <section id="faq1e"> <title>(FAQ 1e) In order to discourage brute force attacks I would like to redirect all connections on a non-standard port (4104) to port 22 on the router/firewall. I notice that setting up a REDIRECT rule causes the firewall to open both ports 4104 and 22 to connections from the net. Is it possible to only redirect 4104 to the localhost port 22 and have connection attempts to port 22 from the net dropped?</title> <para><emphasis role="bold">Answer </emphasis>courtesy of Ryan: Assume that the IP address of your local firewall interface is 192.168.1.1. If you configure SSHD to only listen on that address and add the following rule, then you will have access on port 4104 from the net and on port 22 from your LAN.</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) DNAT net fw:192.168.1.1:22 tcp 4104</programlisting> </section> <section id="faq1f"> <title>(FAQ 1f) Why must the server that I port forward to have it's default gateway set to my Shorewall system's IP address?</title> <para><emphasis role="bold">Answer:</emphasis> Let's take an example. Suppose that</para> <itemizedlist> <listitem> <para>Your Shorewall firewall's external IP address is 206.124.146.176 (eth0) and its internal IP address is 192.168.1.1 (eth1).</para> </listitem> <listitem> <para>You have another gateway router with external IP address 130.252.100.109 and internal IP address 192.168.1.254.</para> </listitem> <listitem> <para>You have an FTP server behind both routers with IP address 192.168.1.4</para> </listitem> <listitem> <para>The FTP server's default gateway is through the second router (192.168.1.254).</para> </listitem> <listitem> <para>You have this rule on the Shorewall system:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:192.168.1.4 tcp 21 - 206.124.146.176</programlisting></para> </listitem> <listitem> <para>Internet host 16.105.221.4 issues the command <command>ftp 206.124.146.176</command></para> </listitem> </itemizedlist> <para>This results in the following sequence of events:</para> <orderedlist> <listitem> <para>16.105.221.4 sends a TCP SYN packet to 206.124.146.176 specifying destination port 21.</para> </listitem> <listitem> <para>The Shorewall box rewrites the destination IP address to 192.168.1.4 and forwards the packet.</para> </listitem> <listitem> <para>The FTP server receives the packet and accepts the connection, generating a SYN,ACK packet back to 16.105.221.4. Because the server's default gateway is through the second router, it sends the packet to that router.</para> </listitem> </orderedlist> <para>At this point, one of two things can happen. Either the second router discards or rejects the packet; or, it rewrites the source IP address to 130.252.100.109 and forwards the packet back to 16.105.221.4. Regardless of which happens, the connection is doomed. Clearly if the packet is rejected or dropped, the connection will not be successful. But even if the packet reaches 16.105.221.4, that host will reject it since it's SOURCE IP address (130.252.100.109) doesn't match the DESTINATION IP ADDRESS (206.124.146.176) of the original SYN packet.</para> <para>The best way to work around this problem is to change the default gateway on the FTP server to the Shorewall system's internal IP address (192.168.1.1). But if that isn't possible, you can work around the problem with the following ugly hack in <filename>/etc/shorewall/masq</filename>:<programlisting>#INTERFACE SOURCE ADDRESS PROTO PORT eth1:192.168.1.4 0.0.0.0/0 192.168.1.1 tcp 21</programlisting></para> <para>This rule has the undesirable side effect of making all FTP connections from the net appear to the FTP server as if they originated on the Shorewall system. But it will force the FTP server to reply back through the Shorewall system who can then rewrite the SOURCE IP address in the responses properly.</para> </section> <section id="faq1g"> <title>(FAQ 1g) I would like to redirect port 80 on my public IP address (206.124.146.176) to port 993 on Internet host 66.249.93.111</title> <para><emphasis role="bold">Answer:</emphasis> This requires a vile hack similar to the one in <link linkend="faq2">FAQ 2</link>. Assuming that your Internet zone is named <emphasis>net</emphasis> and connects on interface <filename class="devicefile">eth0</filename>:</para> <para>In <filename>/etc/shorewall/rules</filename>:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net net:66.249.93.111:993 tcp 80 - 206.124.146.176</programlisting></para> <para>In <filename>/etc/shorewall/interfaces</filename>, specify the <emphasis role="bold">routeback</emphasis> option on eth0:<programlisting>#ZONE INTERFACE BROADCAST OPTIONS net eth0 detect <emphasis role="bold">routeback</emphasis></programlisting></para> <para><filename>/etc/shorewall/masq</filename>;<programlisting>#INTERFACE SOURCE ADDRESS PROTO PORT eth0:66.249.93.111 0.0.0.0/0 206.124.146.176 tcp 993</programlisting></para> <para>and in <filename>/etc/shorewall/shorewall.conf</filename>:</para> <programlisting>IP_FORWARDING=On</programlisting> <para>Like the hack in FAQ 2, this one results in all forwarded connections looking to the server (66.249.93.11) as if they originated on your firewall (206.124.146.176).</para> </section> <section id="faq1h"> <title>(FAQ 1h) How do I set shorewall to allow ssh on port 9022 from net? SSHD is listening on port 22.</title> <para><emphasis role="bold">Answer</emphasis>: Use this rule.</para> <programlisting>#ACTION SOURCE DEST PROTO DEST # PORT(S) REDIRECT net 22 tcp 9022</programlisting> <para>Note that the above rule will also allow connections from the net on TCP port 22. If you don't want that, see <link linkend="faq1e">FAQ 1e</link>.</para> </section> <section id="faq1j"> <title>(FAQ 1j) Why doesn't this DNAT rule work?</title> <para>I added this rule but I'm still seeing the log message below</para> <programlisting>RULE: DNAT scnet:172.19.41.2 dmz0:10.199.198.145 udp 2055 LOG: Sep 21 12:55:37 fw001 kernel: [10357687.114928] Shorewall:scnet2fw:DROP:IN=eth2 OUT= MAC=00:26:33:dd:aa:05:00:24:f7:19:ce:44:08:00 SRC=172.19.41.2 DST=172.19.1.1 LEN=1492 TOS=0x00 PREC=0x00 TTL=63 ID=23035 PROTO=UDP SPT=6376 DPT=2055 LEN=1472</programlisting> <para><emphasis role="bold">Answer</emphasis>: There was already a conntrack entry for the failing connection before you added the rule. Install the <emphasis role="bold">conntrack</emphasis> utility program and use it to delete the entry.</para> <programlisting><command>conntrack -D -s 172.19.41.2 -d 172.19.1.1 -p udp -sport 6367 -dport 2055 </command></programlisting> </section> </section> <section id="faq30"> <title>(FAQ 30) I'm confused about when to use DNAT rules and when to use ACCEPT rules.</title> <para><emphasis role="bold">Answer:</emphasis> It would be a good idea to review the <ulink url="shorewall_quickstart_guide.htm">QuickStart Guide</ulink> appropriate for your setup; the guides cover this topic in a tutorial fashion. DNAT rules should be used for connections that need to go the opposite direction from SNAT/MASQUERADE. So if you masquerade or use SNAT from your local network to the Internet then you will need to use DNAT rules to allow connections from the Internet to your local network.<note> <para>If you use both 1:1 NAT and SNAT/MASQUERADE, those connections that are subject to 1:1 NAT should use ACCEPT rather than DNAT. Note, however, that DNAT can be used to override 1:1 NAT so as to redirect a connection to a different internal system or port than would be the case using 1:1 NAT.</para> </note> You also want to use DNAT rules when you intentionally want to rewrite the destination IP address or port number. In all other cases, you use ACCEPT unless you need to hijack connections as they go through your firewall and handle them on the firewall box itself; in that case, you use a REDIRECT rule.</para> <note> <para>The preceding answer should <emphasis>not</emphasis> be interpreted to mean that DNAT can only be used in conjunction with SNAT. But in common configurations using private local addresses, that is the most common usage.</para> </note> </section> <section id="faq8"> <title>(FAQ 8) I have several external IP addresses and use /etc/shorewall/nat to associate them with systems in my DMZ. When I add a DNAT rule, say for ports 80 and 443, Shorewall redirects connections on those ports for all of my addresses. How can I restrict DNAT to only a single address?</title> <para><emphasis role="bold">Answer</emphasis>: Specify the external address that you want to redirect in the ORIGINAL DEST column.</para> <para>Example:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net net:192.168.4.22 tcp 80,443 - <emphasis role="bold">206.124.146.178</emphasis></programlisting> </section> <section id="faq38"> <title>(FAQ 38) Where can I find more information about DNAT?</title> <para><emphasis role="bold">Answer:</emphasis> Ian Allen has written a <ulink url="http://idallen.com/dnat.txt">Paper about DNAT and Linux</ulink>.</para> </section> <section id="faq48"> <title>(FAQ 48) How do I Set up a Transparent HTTP Proxy with Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> See <ulink url="Shorewall_Squid_Usage.html">Shorewall_Squid_Usage.html</ulink>.</para> </section> </section> <section id="DNS-DNAT"> <title id="DNS">DNS and Port Forwarding/NAT</title> <section id="faq2"> <title>(FAQ 2) I port forward www requests to www.mydomain.com (IP 130.151.100.69) to system 192.168.1.5 in my local network. External clients can browse http://www.mydomain.com but internal clients can't.</title> <para><emphasis role="bold">Answer:</emphasis> I have two objections to this setup.</para> <itemizedlist> <listitem> <para>Having an Internet-accessible server in your local network is like raising foxes in the corner of your hen house. If the server is compromised, there's nothing between that server and your other internal systems. For the cost of another NIC and a cross-over cable, you can put your server in a DMZ such that it is isolated from your local systems - assuming that the Server can be located near the Firewall, of course :-)</para> </listitem> <listitem> <para>The accessibility problem is best solved using <firstterm>Split DNS</firstterm> (either <ulink url="SplitDNS.html">use a separate DNS server</ulink> for local clients or use <ulink url="shorewall_setup_guide.htm#DNS">Bind Version 9 <quote>views</quote></ulink> on your main name server) such that www.mydomain.com resolves to 130.141.100.69 externally and 192.168.1.5 internally. I use a separate DNS server (dnsmasq) here at shorewall.net.</para> </listitem> </itemizedlist> <para>So the best and most secure way to solve this problem is to move your Internet-accessible server(s) to a separate LAN segment with it's own interface to your firewall and follow <link linkend="faq2b">FAQ 2b</link>. That way, your local systems are still safe if your server gets hacked and you don't have to run a split DNS configuration (separate server or Bind 9 views).</para> <para>If physical limitations make it impractical to segregate your servers on a separate LAN, the next best solution it to use Split DNS. Before you complain "It's too hard to set up split DNS!", <ulink url="SplitDNS.html"><emphasis role="bold">check here</emphasis></ulink>.</para> <para>If you really want to route traffic between two internal systems through your firewall, then proceed as described below.<warning> <para>All traffic redirected through use of this technique will look to the server as if it originated on the firewall rather than on the original client! So the server's access logs will be useless for determining which local hosts are accessing the server.</para> </warning></para> <para>Assuming that your external interface is eth0 and your internal interface is eth1 and that eth1 has IP address 192.168.1.254 with subnet 192.168.1.0/24, then:</para> <itemizedlist> <listitem> <para>In <filename>/etc/shorewall/interfaces</filename>:</para> <programlisting>#ZONE INTERFACE BROADCAST OPTIONS loc eth1 detect <emphasis role="bold">routeback</emphasis> </programlisting> </listitem> <listitem> <para>In <filename>/etc/shorewall/masq</filename>:</para> <programlisting>#INTERFACE SOURCE ADDRESS PROTO PORT(S) <emphasis role="bold">eth1:192.168.1.5 eth1 192.168.1.254 tcp www</emphasis></programlisting> <para>Note: The technique described here is known as <firstterm>hairpinning NAT</firstterm> and is described in section 6 of <ulink url="http://www.faqs.org/rfcs/rfc4787.html">RFC 4787</ulink>. In that RFC, it is required that the <emphasis>external IP address</emphasis> be used as the source:</para> <programlisting>#INTERFACE SOURCE ADDRESS PROTO PORT(S) eth1:192.168.1.5 eth1 <emphasis role="bold">130.151.100.69</emphasis> tcp www</programlisting> </listitem> <listitem> <para>In <filename>/etc/shorewall/rules</filename>:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. <emphasis role="bold">DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69</emphasis></programlisting> <para>That rule (and the second one in the previous bullet) only works of course if you have a static external IP address. If you have a dynamic IP address then include this in <filename>/etc/shorewall/params</filename>.</para> <programlisting><command>ETH0_IP=$(find_first_interface_address eth0)</command> </programlisting> <para>and make your DNAT rule:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - <emphasis role="bold">$ETH0_IP</emphasis></programlisting> <para>Using this technique, you will want to configure your DHCP/PPPoE/PPTP/… client to automatically restart Shorewall each time that you get a new IP address.</para> <note> <para>For optional interfaces, use the function <emphasis role="bold">find_first_interface_address_if_any()</emphasis> rather than <emphasis role="bold">find_first_interface_address()</emphasis>. The former will return 0.0.0.0 if the interface has no configured IP address; the latter terminates the calling program.</para> </note> <note id="Call"> <para>If you run Shorewall-lite on your firewall, you must use the following in the firewall's configuration directory <filename>params</filename> file:</para> <programlisting><command>ETH0_IP=$(ssh root@firewall "/sbin/shorewall-lite call find_first_interface_address eth0")</command></programlisting> </note> </listitem> </itemizedlist> <section id="faq2a"> <title>(FAQ 2a) I have a zone <quote>Z</quote> with an RFC1918 subnet and I use one-to-one NAT to assign non-RFC1918 addresses to hosts in Z. Hosts in Z cannot communicate with each other using their external (non-RFC1918 addresses) so they can't access each other using their DNS names.</title> <note> <para>If the ALL INTERFACES column in /etc/shorewall/nat is empty or contains <quote>Yes</quote>, you will also see log messages like the following when trying to access a host in Z from another host in Z using the destination host's public address:</para> <programlisting>Oct 4 10:26:40 netgw kernel: Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1 SRC=192.168.118.200 DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127 ID=1342 DF PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0</programlisting> </note> <para><emphasis role="bold">Answer:</emphasis> This is another problem that is best solved using split DNS. It allows both external and internal clients to access a NATed host using the host's DNS name.</para> <para>Another good way to approach this problem is to switch from one-to-one NAT to Proxy ARP. That way, the hosts in Z have non-RFC1918 addresses and can be accessed externally and internally using the same address.</para> <para>If you don't like those solutions and prefer to route all Z->Z traffic through your firewall then:</para> <orderedlist> <listitem> <para>Set the routeback option on the interface to Z.</para> </listitem> <listitem> <para>Set the ALL INTERFACES column in the nat file to <quote>Yes</quote>.</para> </listitem> </orderedlist> <example id="Example1"> <title>Example:</title> <literallayout>Zone: dmz, Interface: eth2, Subnet: 192.168.2.0/24, Address: 192.168.2.254</literallayout> <para>In <filename>/etc/shorewall/interfaces</filename>:</para> <programlisting>#ZONE INTERFACE BROADCAST OPTIONS dmz eth2 192.168.2.255 <emphasis role="bold">routeback</emphasis> </programlisting> <para>In <filename>/etc/shorewall/nat</filename>, be sure that you have <quote>Yes</quote> in the ALL INTERFACES column.</para> </example> </section> <section id="faq2b"> <title>(FAQ 2b) I have a web server in my DMZ and I use port forwarding to make that server accessible from the Internet as www.mydomain.com. That works fine but when my local users try to connect to www.mydomain.com, it doesn't work.</title> <para><emphasis role="bold">Answer:</emphasis> Let's assume the following:</para> <itemizedlist> <listitem> <para>External IP address is 206.124.146.176 on <filename class="devicefile">eth0</filename> (www.mydomain.com).</para> </listitem> <listitem> <para>Server's IP address is 192.168.2.4</para> </listitem> </itemizedlist> <para>You can enable access to the server from your local network using the firewall's external IP address by adding this rule:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL # PORT DEST <emphasis role="bold">DNAT loc dmz:192.168.2.4 tcp 80 - 206.124.146.176</emphasis></programlisting> <para>If your external IP address is dynamic, then you must do the following:</para> <para>In <filename>/etc/shorewall/params</filename>:</para> <programlisting><command>ETH0_IP=`find_first_interface_address eth0`</command> </programlisting> <para>and make your DNAT rule:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc dmz:192.168.2.4 tcp 80 - <emphasis role="bold">$ETH0_IP</emphasis></programlisting> <warning> <para>With dynamic IP addresses, you probably don't want to use <ulink url="starting_and_stopping_shorewall.htm"><command>shorewall[-lite] save</command> and <command>shorewall[-lite] restore</command></ulink>.</para> </warning> <note> <para>For optional interfaces, use the function <emphasis role="bold">find_first_interface_address_if_any()</emphasis> rather than <emphasis role="bold">find_first_interface_address()</emphasis>. The former will return 0.0.0.0 if the interface has no configured IP address; the latter terminates the calling program.</para> </note> <note> <para>If you use Shorewall-lite, then you need to configure the params file in the firewall's configuration directory as described <link linkend="Call">above</link>.</para> </note> </section> <section id="faq2c"> <title>(FAQ 2c) I tried to apply the answer to FAQ 2 to my external interface and the net zone and it didn't work. Why?</title> <para><emphasis role="bold">Answer:</emphasis> Did you set <emphasis role="bold">IP_FORWARDING=On</emphasis> in <filename>shorewall.conf</filename>?</para> </section> <section> <title>(FAQ 2d) Does Shorewall support hairpinning NAT?</title> <para><emphasis role="bold">Answer:</emphasis> Yes.</para> <para>In the case of simple masquerade/SNAT, see <link linkend="faq2">FAQ 2</link>.</para> <para>For one-to-one (static), NAT, simply place 'Yes' in the ALL INTERFACES column of each entry in <ulink url="manpages/shorewall-nat.html">/etc/shorewall/nat</ulink>.</para> </section> </section> </section> <section id="Blacklisting"> <title>Blacklisting</title> <section id="faq63"> <title>(FAQ 63) I just blacklisted IP address 206.124.146.176 and I can still ping it. What did I do wrong?</title> <para><emphasis role="bold">Answer:</emphasis> Nothing.</para> <para>Blacklisting an IP address blocks incoming traffic from that IP address. And if you set BLACKLISTNEWONLY=Yes in <filename>shorewall.conf</filename>, then only new connections <emphasis role="bold">from</emphasis> that address are disallowed; traffic from that address that is part of an established connection (such as ping replies) is allowed.</para> <note> <para>Beginning with Shorewall 4.4.13, you can use the <option>blacklist</option> option in <ulink url="manpages/shorewall-interfaces.html"><filename>/etc/shorewall/interfaces</filename></ulink> to implement blacklisting by destination IP address.</para> </note> </section> <section id="faq84"> <title>(FAQ 84) I put some IPs in the blacklist file in /etc/shorewall to block the ips but i'm still getting reports from PSAD from those ips saying they're port scanning. Shouldn't being on the blacklist drop all packets from those ips?</title> <para><emphasis role="bold">Answer</emphasis>: You probably forgot to specify the <emphasis role="bold">blacklist</emphasis> option for your external interface(s) in <filename><ulink url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink></filename>.</para> </section> </section> <section id="MSN"> <title>Netmeeting/MSN</title> <section id="faq3"> <title>(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with Shorewall. What do I do?</title> <para><emphasis role="bold">Answer:</emphasis> There is an <ulink url="http://www.kfki.hu/~kadlec/sw/netfilter/newnat-suite/">H.323 connection tracking/NAT module</ulink> that helps with Netmeeting.</para> <para>Look <ulink url="UPnP.html">here</ulink> for a solution for MSN IM but be aware that there are significant security risks involved with this solution. Also check the Netfilter mailing list archives at <ulink url="http://www.netfilter.org">http://www.netfilter.org</ulink>.</para> </section> </section> <section id="Openports"> <title>Open Ports</title> <section id="faq100"> <title>(FAQ 100) With Shorewall started, the output of 'iptables -L' looks like my firewall is wide open!</title> <para><emphasis role="bold">Answer:</emphasis> The problem here is that a bare <command>iptables -L</command> command produces totally useless output. Use <command>shorewall show</command> instead.</para> <note> <para>The <command>shorewall show</command> command is a wrapper around <command>iptables -L -n -v</command>.</para> </note> </section> <section id="faq51"> <title>(FAQ 51) How do I Open Ports in Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> No one who has installed Shorewall using one of the <ulink url="shorewall_quickstart_guide.htm">Quick Start Guides</ulink> should have to ask this question.</para> <para>Regardless of which guide you used, all outbound communication is open by default. So you do not need to 'open ports' for output.</para> <para>For input:</para> <itemizedlist> <listitem> <para>If you installed using the Standalone Guide, then please <ulink url="standalone.htm#Open">re-read this section</ulink>.</para> </listitem> <listitem> <para>If you installed using the Two-interface Guide, then please re-read these sections: <ulink url="two-interface.htm#DNAT">Port Forwarding (DNAT)</ulink>, and <ulink url="two-interface.htm#Open">Other Connections</ulink></para> </listitem> <listitem> <para>If you installed using the Three-interface Guide, then please re-read these sections: <ulink url="three-interface.htm#DNAT">Port Forwarding (DNAT)</ulink> and <ulink url="three-interface.htm#Open">Other Connections</ulink></para> </listitem> <listitem> <para>If you installed using the <ulink url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink> then you had better read the guide again -- you clearly missed a lot.</para> </listitem> </itemizedlist> <para>Also please see the <link linkend="PortForwarding">Port Forwarding section of this FAQ</link>.</para> </section> <section id="faq4"> <title>(FAQ 4) I just used an online port scanner to check my firewall and it shows some ports as <quote>closed</quote> rather than <quote>blocked</quote>. Why?</title> <para><emphasis role="bold">Answer:</emphasis> The default Shorewall setup invokes the <emphasis role="bold">Drop</emphasis> action prior to enforcing a DROP policy and the default policy to all zones from the Internet is DROP. The Drop action is defined in <filename>/usr/share/shorewall/action.Drop</filename> which in turn invokes the <emphasis role="bold">Auth</emphasis> macro (defined in <filename>/usr/share/shorewall/macro.Auth</filename>) specifying the <emphasis role="bold">REJECT</emphasis> action (i.e., <emphasis role="bold">Auth(REJECT)</emphasis>). This is necessary to prevent outgoing connection problems to services that use the <quote>Auth</quote> mechanism for identifying requesting users. That is the only service which the default setup rejects.</para> <para>If you are seeing closed TCP ports other than 113 (auth) then either you have added rules to REJECT those ports or a router outside of your firewall is responding to connection requests on those ports.</para> <para>If you would prefer to 'stealth' port 113, then:</para> <itemizedlist> <listitem> <para>If you are running Shorewall 4.4.20 or earlier, copy /<filename>usr/share/shorewall/action.Drop</filename> to <filename>/etc/shorewall/</filename> and modify the invocation of Auth to <emphasis role="bold">Auth(DROP)</emphasis>.</para> </listitem> <listitem> <para>If you are running Shorewall 4.4.21 or later, in shorewall.conf, set DROP_DEFAULT="Drop(-,DROP)". See the <ulink url="Actions.html">Action HOWTO</ulink> to learn how that magic works.</para> </listitem> </itemizedlist> <section id="faq4a"> <title>(FAQ 4a) I just ran an nmap UDP scan of my firewall and it showed 100s of ports as open!!!!</title> <para><emphasis role="bold">Answer:</emphasis> Take a deep breath and read the nmap manpage section about UDP scans. If nmap gets <emphasis role="bold">nothing</emphasis> back from your firewall then it reports the port as open. If you want to see which UDP ports are really open, temporarily change your net->all policy to REJECT, restart Shorewall and run the nmap UDP scan again.</para> </section> <section id="faq4b"> <title>(FAQ 4b) I have a port that I can't close no matter how I change my rules.</title> <para>I had a rule that allowed telnet from my local network to my firewall; I removed that rule and restarted Shorewall but my telnet session still works!!!</para> <para><emphasis role="bold">Answer:</emphasis> Rules only govern the establishment of new connections. Once a connection is established through the firewall it will be usable until disconnected (tcp) or until it times out (other protocols). If you stop telnet and try to establish a new session your firewall will block that attempt.</para> </section> <section id="faq4c"> <title>(FAQ 4c) How do I use Shorewall with PortSentry?</title> <para><ulink url="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt"><emphasis role="bold">Answer:</emphasis> Here's a writeup</ulink> describing a nice integration of Shorewall and PortSentry.</para> </section> </section> </section> <section id="Connections"> <title>Connection Problems</title> <section id="pseudofaq17"> <title>Why are these packets being Dropped/Rejected? How do I decode Shorewall log messages?</title> <para>Please see <link linkend="faq17">FAQ 17</link>.</para> </section> <section id="faq5"> <title>(FAQ 5) I've installed Shorewall and now I can't ping through the firewall</title> <para><emphasis role="bold">Answer:</emphasis> For a complete description of Shorewall <quote>ping</quote> management, see <ulink url="ping.html">this page</ulink>.</para> </section> <section id="faq15"> <title>(FAQ 15) My local systems can't see out to the net</title> <para><emphasis role="bold">Answer:</emphasis> Every time I read <quote>systems can't see out to the net</quote>, I wonder where the poster bought computers with eyes and what those computers will <quote>see</quote> when things are working properly :-). That aside, the most common causes of this problem are:</para> <orderedlist> <listitem> <para>The default gateway on each local system isn't set to the IP address of the local firewall interface. You can test this by:</para> <orderedlist numeration="loweralpha"> <listitem> <para>At a root shell prompt, type 'shorewall clear'.</para> </listitem> <listitem> <para>From a local system, attempt to ping the IP address of the Shorewall system's internet (external) interface. If that doesn't work, then the default gateway on the system from which you pinged is not set correctly.</para> </listitem> <listitem> <para>Be sure to 'shorewall start' after the test.</para> </listitem> </orderedlist> </listitem> <listitem> <para>The entry for the local network in the <filename>/etc/shorewall/masq</filename> file is wrong or missing.</para> </listitem> <listitem> <para>The DNS settings on the local systems are wrong or the user is running a DNS server on the firewall and hasn't enabled UDP and TCP port 53 from the local net to the firewall or from the firewall to the Internet.</para> </listitem> <listitem> <para>Forwarding is not enabled (This is often the problem for Debian users). Enter this command:</para> <programlisting>cat /proc/sys/net/ipv4/ip_forward</programlisting> <para>If the value displayed is 0 (zero) then set <emphasis role="bold">IP_FORWARDING=On</emphasis> in <filename>/etc/shorewall/shorewall.conf</filename> and restart Shorewall.</para> </listitem> </orderedlist> </section> <section id="faq29"> <title>(FAQ 29) FTP Doesn't Work</title> <para><emphasis role="bold">Answer:</emphasis> See the <ulink url="FTP.html">Shorewall and FTP page</ulink>.</para> </section> <section id="faq33"> <title>(FAQ 33) From clients behind the firewall, connections to some sites fail. Connections to the same sites from the firewall itself work fine. What's wrong?</title> <para><emphasis role="bold">Answer:</emphasis> Most likely, you need to set CLAMPMSS=Yes in <filename><ulink url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>.</para> </section> <section id="faq35"> <title>(FAQ 35) I have two Ethernet interfaces to my local network which I have bridged. When Shorewall is started, I'm unable to pass traffic through the bridge. I have defined the bridge interface (br0) as the local interface in <filename>/etc/shorewall/interfaces</filename>; the bridged Ethernet interfaces are not defined to Shorewall. How do I tell Shorewall to allow traffic through the bridge?</title> <para><emphasis role="bold">Answer:</emphasis> Add the <option>routeback</option> option to <filename class="devicefile">br0</filename> in <filename><ulink url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink></filename>.</para> <para>For more information on this type of configuration, see the <ulink url="SimpleBridge.html">Shorewall Simple Bridge documentation</ulink>.</para> </section> <section id="faq64"> <title>(FAQ 64) I just upgraded my kernel to 2.6.20 (or later) and my bridge/firewall stopped working. What is wrong?</title> <para><emphasis role="bold">Answer:</emphasis> In kernel 2.6.20, the Netfilter <firstterm>physdev match</firstterm> feature was changed such that it is no longer capable of matching the output device of non-bridged traffic. You will see messages such as the following in your log:</para> <programlisting>Apr 20 15:03:50 wookie kernel: [14736.560947] physdev match: using --physdev-out in the OUTPUT, FORWARD and POSTROUTING chains for non-bridged traffic is not supported anymore.</programlisting> <para>This kernel change, while necessary, means that Shorewall zones may no longer be defined in terms of bridge ports. See the<ulink url="bridge-Shorewall-perl.html"> Shorewall-perl bridging documentation</ulink> for information about how to configure bridge/firewalls.<note> <para>Following the instructions in the new bridging documentation will not prevent the above message from being issued.</para> </note></para> </section> <section id="faq85"> <title>(FAQ 85) Shorewall is rejecting connections from my local lan because it thinks they are coming from the 'net' zone.</title> <para>I'm seeing this in my log:</para> <programlisting>Aug 31 16:51:24 fw22 kernel: Shorewall:net2fw:DROP:IN=eth5 OUT= MAC=00:0c:29:74:9c:0c:08:00:20:b2:5f:db:08:00 SRC=10.1.50.14 DST=10.1.50.7 LEN=57 TOS=0x00 PREC=0x00 TTL=255 ID=32302 DF PROTO=UDP SPT=53289 DPT=53 LEN=37</programlisting> <para><emphasis role="bold">Answer</emphasis>: This occurs when the external interface and an internal interface are connected to the same switch or hub. See <ulink url="FoolsFirewall.html">this article</ulink> for details. The solution is to never connect more than one firewall interface to the same hub or switch (an obvious exception is that when you have a switch that supports VLAN tagging and the interfaces are associated with different VLANs).</para> </section> </section> <section id="Logging"> <title>Logging</title> <section id="faq91"> <title>(FAQ 91) I changed the shorewall.conf file in /etc/shorewall/ to spit out logs to /var/log/shorewall.log and it's not happening after I restart shorewall. LOGFILE=/var/log/shorewall.log <-- that should be the correct line, right?</title> <para><emphasis role="bold">Answer</emphasis>: No, that is not correct. The LOGFILE setting tells Shorewall where to find the log; it does not determine where messages are written. See <link linkend="faq6">the next FAQ</link>.</para> </section> <section id="faq6"> <title>(FAQ 6) Where are the log messages written and how do I change the destination?</title> <para><emphasis role="bold">Answer:</emphasis> NetFilter uses the kernel's equivalent of syslog (see <quote>man syslog</quote>) to log messages. It always uses the LOG_KERN (kern) facility (see <quote>man openlog</quote>) and you get to choose the log level (again, see <quote>man syslog</quote>) in your <filename><ulink url="manpages/shorewall-policy.html">policies</ulink></filename> and <filename><ulink url="manpages/shorewall-rules.html">rules</ulink></filename>. The destination for messages logged by syslog is controlled by <filename>/etc/syslog.conf</filename> (see <quote>man syslog.conf</quote>). When you have changed <filename>/etc/syslog.conf</filename>, be sure to restart syslogd (on a RedHat system, <quote>service syslog restart</quote>).</para> <para>It is also possible to <ulink url="shorewall_logging.html">set up Shorewall to log all of Netfilter's messages to a separate file</ulink>.</para> <section id="faq6a"> <title>(FAQ 6a) Are there any log parsers that work with Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> Here are several links that may be helpful:</para> <literallayout> <ulink url="http://www.shorewall.net/pub/shorewall/parsefw/">http://www.shorewall.net/pub/shorewall/parsefw/</ulink> <ulink url="http://aaron.marasco.com/linux.html">http://aaron.marasco.com/linux.html</ulink> <ulink url="http://cert.uni-stuttgart.de/projects/fwlogwatch">http://cert.uni-stuttgart.de/projects/fwlogwatch</ulink> <ulink url="http://www.logwatch.org">http://www.logwatch.org</ulink> </literallayout> <para>I personally use <ulink url="http://www.cert.uni-stuttgart.de.projects/fwlogwatch">fwlogwatch</ulink>. It emails me a report each day from my various systems with each report summarizing the logged activity on the corresponding system; here's a sample:</para> <blockquote> <programlisting>fwlogwatch summary Generated Tuesday March 02 08:14:37 PST 2010 by root. 362 (and 455 older than 86400 seconds) of 817 entries in the file "/var/log/ulog/syslogemu.log" are packet logs, 138 have unique characteristics. First packet log entry: Mar 01 08:16:06, last: Mar 02 08:06:21. All entries were logged by the same host: "gateway". All entries have the same target: "-". Only entries with a count of at least 5 are shown. net-dmz DROP eth2 36 packets from 61.158.162.9 to 206.124.146.177 net-fw DROP eth0 21 packets from 89.163.162.13 to 76.104.233.98 net-fw DROP eth0 19 packets from 61.184.101.46 to 76.104.233.98 net-fw DROP eth0 12 packets from 81.157.214.103 to 76.104.233.98 net-fw DROP eth0 11 packets from 174.37.159.222 to 76.104.233.98 net-fw DROP eth0 10 packets from 221.195.73.86 to 76.104.233.98 net-dmz DROP eth2 9 packets from 202.199.158.6 to 206.124.146.177 net-fw DROP eth2 9 packets from 202.199.158.6 to 206.124.146.176 net-dmz DROP eth2 9 packets from 202.199.158.6 to 206.124.146.178 net-fw DROP eth0 6 packets from 221.192.199.35 to 76.104.233.98 net-fw DROP eth2 5 packets from 61.158.162.9 to 206.124.146.177</programlisting> </blockquote> <para>Fwlogwatch contains a built-in web server that allows monitoring recent activity in summary fashion.</para> </section> <section id="faq6b"> <title>(FAQ 6b) DROP messages on port 10619 are flooding the logs with their connect requests. Can I exclude these error messages for this port temporarily from logging in Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> Temporarily add the following rule:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) DROP net fw udp 10619</programlisting> <para>Alternatively, if you do not set BLACKLIST_LOGLEVEL and you have specifed the 'blacklist' option on your external interface in <filename>/etc/shorewall/interfaces</filename>, then you can blacklist the port. In <filename>/etc/shorewall/blacklist</filename>:</para> <programlisting>#ADDRESS/SUBNET PROTOCOL PORT - udp 10619</programlisting> </section> <section id="faq6d"> <title>(FAQ 6d) Why is the MAC address in Shorewall log messages so long? I thought MAC addresses were only 6 bytes in length.</title> <para><emphasis role="bold">Answer:</emphasis> What is labeled as the MAC address in a Netfilter (Shorewall) log message is actually the Ethernet frame header. It contains:</para> <itemizedlist> <listitem> <para>the destination MAC address (6 bytes)</para> </listitem> <listitem> <para>the source MAC address (6 bytes)</para> </listitem> <listitem> <para>the Ethernet frame type (2 bytes)</para> </listitem> </itemizedlist> <para><example id="Example5"> <title id="Example2">Example</title> <para><programlisting>MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00</programlisting> <itemizedlist> <listitem> <para>Destination MAC address = 00:04:4c:dc:e2:28</para> </listitem> <listitem> <para>Source MAC address = 00:b0:8e:cf:3c:4c</para> </listitem> <listitem> <para>Ethernet Frame Type = 08:00 (IP Version 4)</para> </listitem> </itemizedlist></para> </example></para> </section> </section> <section id="faq16"> <title>(FAQ 16) Shorewall is writing log messages all over my console making it unusable!</title> <para><emphasis role="bold">Answer:</emphasis></para> <para>Just to be clear, it is not Shorewall that is writing all over your console. Shorewall issues a single log message during each <command>start</command>, <command>restart</command>, <command>stop</command>, etc. It is rather your logging daemon that is writing messages to your console. Shorewall itself has no control over where a particular class of messages are written. See the <ulink url="shorewall_logging.html">Shorewall logging documentation</ulink>.</para> <para>The max log level to be sent to the console is available in /proc/sys/kernel/printk:<programlisting>teastep@ursa:~$ <emphasis role="bold">cat /proc/sys/kernel/printk</emphasis> 6 6 1 7 teastep@ursa:~$ </programlisting>The first number determines the maximum log level (syslog priority) sent to the console. Messages with priority <emphasis role="bold">less than</emphasis> this number are sent to the console. On the system shown in the example above, priorities 0-5 are sent to the console. Since Shorewall defaults to using 'info' (6), the Shorewall-generated Netfilter rule set will generate log messages that <emphasis role="bold">will not appear on the console.</emphasis></para> <para>The second number is the default log level for kernel printk() calls that do not specify a log level.</para> <para>The third number specifies the minimum console log level while the fourth gives the default console log level.</para> <para>If, on your system, the first number is 7 or greater, then the default Shorewall configurations will cause messages to be written to your console. The simplest solution is to add this to your <filename>/etc/sysctl.conf</filename> file:<programlisting>kernel.printk = 4 4 1 7</programlisting></para> <para>then<programlisting><command>sysctl -p /etc/sysctl.conf</command></programlisting></para> <section id="faq16a"> <title>(FAQ 16a) cat /proc/sys/kernel/prink returns '4 4 1 7' and still I get dmesg filled up</title> <para><emphasis role="bold">Answer</emphasis>: While we would argue that 'dmesg filled up' is not necessarily a problem, the only way to eliminate that is to <ulink url="shorewall_logging.html">set up Shorewall to log all of Netfilter's messages to a separate file</ulink>.</para> </section> <section id="faq16b"> <title>(FAQ 16b) Why can't I see any Shorewall messages in /var/log/messages?</title> <para>Some people who ask this question report that the only Shorewall messages that they see in <filename>/var/log/messages</filename> are 'started', 'restarted' and 'stopped' messages.</para> <para><emphasis role="bold">Answer:</emphasis> First of all, it is important to understand that Shorewall itself does not control where Netfilter log messages are written. The LOGFILE setting in <filename>shorewall.conf</filename> simply tells the <filename>/sbin/shorewall[-lite]</filename> program where to look for the log. Also, it is important to understand that a log level of "debug" will generally cause Netfilter messages to be written to fewer files in <filename class="directory">/var/log</filename> than a log level of "info". The log level does not control the number of log messages or the content of the messages.</para> <para>The actual log file where Netfilter messages are written is not standardized and will vary by distribution and distribution version. But anytime you see no logging, it's time to look outside the Shorewall configuration for the cause. As an example, recent <trademark>SUSE</trademark> releases use syslog-ng by default and write Shorewall messages to <filename>/var/log/firewall</filename>.</para> <para>Please see the <ulink url="shorewall_logging.html">Shorewall logging documentation</ulink> for further information.</para> </section> <section id="faq16c"> <title>(FAQ 16c) Shorewall messages are flooding the output of 'dmesg'; how to I stop that?</title> <para><emphasis role="bold">Answer</emphasis>: Switch to using <ulink url="???">ulogd</ulink>.</para> </section> <section id="faq16d"> <title>(FAQ 16d) I set LOGFILE=/var/log/shorewall but log messages are still going to /var/log/messages.</title> <para><emphasis role="bold">Answer</emphasis>: See the answer to <link linkend="faq16b">FAQ 16b</link> above.</para> </section> </section> <section id="faq17"> <title>(FAQ 17) Why are these packets being Dropped/Rejected? How do I decode Shorewall log messages?</title> <para><emphasis role="bold">Answer:</emphasis> Logging of dropped/rejected packets occurs out of a number of chains (as indicated in the log message) in Shorewall:</para> <variablelist> <varlistentry id="all2all"> <term><emphasis role="bold"><replaceable>zone</replaceable>2all, <replaceable>zone</replaceable>-all, all2<replaceable>zone</replaceable>, all-<replaceable>zone</replaceable>, all2all or all-all</emphasis></term> <listitem> <para>You have a <filename><ulink url="manpages/shorewall-policy.html">policy</ulink></filename> that specifies a log level and this packet is being logged under that policy. If you intend to ACCEPT this traffic then you need a <ulink url="manpages/shorewall-rules.html">rule</ulink> to that effect.</para> <para>Packets logged out of these chains may have a source and/or destination that is not in any defined zone (see the output of <command>shorewall[-lite] show zones</command>). Remember that zone membership involves both a firewall interface and an ip address.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold"><replaceable>zone1</replaceable>2<replaceable>zone2</replaceable> or <replaceable>zone1-zone2</replaceable></emphasis></term> <listitem> <para>Either you have a <ulink url="manpages/shorewall-policy.html">policy</ulink> for <emphasis>zone1</emphasis> to <emphasis>zone2</emphasis> that specifies a log level and this packet is being logged under that policy or this packet matches a <ulink url="manpages/shorewall-rules.html">rule</ulink> that includes a log level.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">@<replaceable>zone1</replaceable>2<replaceable>zone2</replaceable> or @<replaceable>zone1</replaceable>-<replaceable>zone2</replaceable></emphasis></term> <listitem> <para>You have a policy for traffic from <replaceable>zone1</replaceable> to <replaceable>zone2</replaceable> that specifies TCP connection rate limiting (value in the LIMIT:BURST column). The logged packet exceeds that limit and was dropped. Note that these log messages themselves are severely rate-limited so that a syn-flood won't generate a secondary DOS because of excessive log message. These log messages were added in Shorewall 2.2.0 Beta 7.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold"><replaceable>zone1</replaceable>2<replaceable>zone2</replaceable>~, <replaceable>zone1</replaceable>-<replaceable>zone2</replaceable>~ or ~blacklist<replaceable>nn</replaceable></emphasis></term> <listitem> <para>These are the result of entries in the <ulink url="manpages/shorewall-blrules.html">/etc/shorewall/blrules</ulink> file.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold"><emphasis>interface</emphasis>_mac or <emphasis>interface</emphasis>_rec</emphasis></term> <listitem> <para>The packet is being logged under the <emphasis role="bold">maclist</emphasis> <ulink url="manpages/shorewall-interfaces.html">interface option</ulink>.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">blacklist</emphasis></term> <listitem> <para>The packet is being logged because the source IP is blacklisted in the <filename> <ulink url="manpages/shorewall-blacklist.html">/etc/shorewall/blacklist</ulink> </filename> file.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">INPUT or FORWARD</emphasis></term> <listitem> <para>The packet has a source IP address that isn't in any of your defined zones (<quote><command>shorewall[-lite] show zones</command></quote> and look at the printed zone definitions) or the chain is FORWARD and the destination IP isn't in any of your defined zones. If the chain is FORWARD and the IN and OUT interfaces are the same or they match the same wildcard entry in <ulink url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>, then you probably need the <emphasis role="bold">routeback</emphasis> option on that interface in<filename> <ulink url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink> </filename>, you need the <emphasis role="bold">routeback</emphasis> option in the relevant entry in <filename> <ulink url="manpages/shorewall-hosts.html">/etc/shorewall/hosts</ulink> or you've done something silly like define a default route out of an internal interface.</filename></para> <para>With OPTIMIZE=1 in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>, such packets may also be logged out of a <zone>2all chain or the all2all chain.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">OUTPUT</emphasis></term> <listitem> <para>The packet has a destination IP address that isn't in any of your defined zones(<command>shorewall[-lite] show zones</command> and look at the printed zone definitions).</para> <para>With OPTIMIZE=1 in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>, such packets may also be logged out of the fw2all chain or the all2all chain.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">logflags</emphasis></term> <listitem> <para>The packet is being logged because it failed the checks implemented by the <emphasis role="bold">tcpflags</emphasis> <ulink url="manpages/shorewall-interfaces.html">interface option</ulink>.</para> </listitem> </varlistentry> <varlistentry> <term><emphasis role="bold">sfilter</emphasis></term> <listitem> <para>On systems running Shorewall 4.4.20 or later, either the packet matched the <option>filter</option> <ulink url="manpages/shorewall-interfaces.html">interface option</ulink> or it is being routed out of the same interface on which it arrived and the interface does not have the <option>routeback</option> or <option>routefilter</option> <ulink url="manpages/shorewall-interfaces.html">interface option</ulink>.</para> </listitem> </varlistentry> </variablelist> <example id="Example3"> <title>Here is an example:</title> <programlisting>Jun 27 15:37:56 gateway kernel: Shorewall:<emphasis role="bold">all2all:REJECT</emphasis>:<emphasis role="bold">IN=eth2</emphasis> <emphasis role="bold">OUT=eth1</emphasis> <emphasis role="bold">SRC=192.168.2.2</emphasis> <emphasis role="bold">DST=192.168.1.3 </emphasis>LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF <emphasis role="bold">PROTO=UDP</emphasis> SPT=1803 <emphasis role="bold">DPT=53</emphasis> LEN=47</programlisting> <para>Let's look at the important parts of this message:</para> <variablelist> <varlistentry> <term>all2all:REJECT</term> <listitem> <para>This packet was REJECTed out of the <emphasis role="bold">all2all</emphasis> chain -- the packet was rejected under the <quote>all</quote>-><quote>all</quote> REJECT policy (<link linkend="all2all">all2all</link> above).</para> </listitem> </varlistentry> <varlistentry> <term>IN=eth2</term> <listitem> <para>the packet entered the firewall via eth2. If you see <quote>IN=</quote> with no interface name, the packet originated on the firewall itself.</para> </listitem> </varlistentry> <varlistentry> <term>OUT=eth1</term> <listitem> <para>if accepted, the packet would be sent on eth1. If you see <quote>OUT=</quote> with no interface name, the packet would be processed by the firewall itself.</para> <note> <para>When a DNAT rule is logged, there will never be an OUT= shown because the packet is being logged before it is routed. Also, DNAT logging will show the <emphasis>original</emphasis> destination IP address and destination port number. When a REDIRECT rule is logged, the message will also show the original destination IP address and port number.</para> </note> </listitem> </varlistentry> <varlistentry> <term>SRC=192.168.2.2</term> <listitem> <para>the packet was sent by 192.168.2.2</para> </listitem> </varlistentry> <varlistentry> <term>DST=192.168.1.3</term> <listitem> <para>the packet is destined for 192.168.1.3</para> </listitem> </varlistentry> <varlistentry> <term>PROTO=UDP</term> <listitem> <para>UDP Protocol</para> </listitem> </varlistentry> <varlistentry> <term>DPT=53</term> <listitem> <para>The destination port is 53 (DNS)</para> </listitem> </varlistentry> </variablelist> <para>In this case, 192.168.2.2 was in the <quote>dmz</quote> zone and 192.168.1.3 is in the <quote>loc</quote> zone. I was missing the rule:</para> <programlisting>ACCEPT dmz loc udp 53</programlisting> </example> </section> <section id="faq21"> <title>(FAQ 21) I see these strange log entries occasionally; what are they?</title> <programlisting>Nov 25 18:58:52 linux kernel: Shorewall:net2all:DROP:IN=eth1 OUT= MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00 SRC=206.124.146.179 DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 <emphasis role="bold">PROTO=ICMP</emphasis> <emphasis role="bold">TYPE=3 CODE=3</emphasis> [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00 TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]</programlisting> <para>192.0.2.3 is external on my firewall... 172.16.0.0/24 is my internal LAN</para> <para><emphasis role="bold">Answer:</emphasis> First of all, please note that the above is a very specific type of log message dealing with ICMP port unreachable packets (PROTO=ICMP TYPE=3 CODE=3). Do not read this answer and assume that all Shorewall log messages have something to do with ICMP (hint -- see <link linkend="faq17">FAQ 17</link>).</para> <para>While most people associate the Internet Control Message Protocol (ICMP) with <quote>ping</quote>, ICMP is a key piece of IP. ICMP is used to report problems back to the sender of a packet; this is what is happening here. Unfortunately, where NAT is involved (including SNAT, DNAT and Masquerade), there are many broken implementations. That is what you are seeing with these messages. When Netfilter displays these messages, the part before the "[" describes the ICMP packet and the part between the "[" and "]" describes the packet for which the ICMP is a response.</para> <para>Here is my interpretation of what is happening -- to confirm this analysis, one would have to have packet sniffers placed a both ends of the connection.</para> <para>Host 172.16.1.10 behind NAT gateway 206.124.146.179 sent a UDP DNS query to 192.0.2.3 and your DNS server tried to send a response (the response information is in the brackets -- note source port 53 which marks this as a DNS reply). When the response was returned to to 206.124.146.179, it rewrote the destination IP TO 172.16.1.10 and forwarded the packet to 172.16.1.10 who no longer had a connection on UDP port 2857. This causes a port unreachable (type 3, code 3) to be generated back to 192.0.2.3. As this packet is sent back through 206.124.146.179, that box correctly changes the source address in the packet to 206.124.146.179 but doesn't reset the DST IP in the original DNS response similarly. When the ICMP reaches your firewall (192.0.2.3), your firewall has no record of having sent a DNS reply to 172.16.1.10 so this ICMP doesn't appear to be related to anything that was sent. The final result is that the packet gets logged and dropped in the all2all chain.</para> </section> <section id="faq52"> <title>(FAQ 52) When I blacklist an IP address with "shorewall[-lite] drop www.xxx.yyy.zzz", why does my log still show REDIRECT and DNAT entries from that address?</title> <para>I blacklisted the address 130.252.100.59 using <command>shorewall drop 130.252.100.59</command> but I am still seeing these log messages:</para> <programlisting>Jan 30 15:38:34 server Shorewall:net_dnat:REDIRECT:IN=eth1 OUT= MAC=00:4f:4e:14:97:8e:00:01:5c:23:24:cc:08:00 SRC=130.252.100.59 DST=206.124.146.176 LEN=64 TOS=0x00 PREC=0x00 TTL=43 ID=42444 DF PROTO=TCP SPT=2215 DPT=139 WINDOW=53760 RES=0x00 SYN URGP=0</programlisting> <para><emphasis role="bold">Answer:</emphasis> Please refer to the <ulink url="NetfilterOverview.html">Shorewall Netfilter Documentation</ulink>. Logging of REDIRECT and DNAT rules occurs in the nat table's PREROUTING chain where the original destination IP address is still available. Blacklisting occurs out of the filter table's INPUT and FORWARD chains which aren't traversed until later.</para> </section> <section id="faq81"> <title>(FAQ 81) logdrop and logreject don't log.</title> <para>I love the ability to type 'shorewall logdrop ww.xx.yy.zz' and completely block a particular IP address. However, the log part doesn't happen. When I look in the logdrop chain, there is no LOG prefix.</para> <para><emphasis role="bold">Answer</emphasis>: You haven't set a value for BLACKLIST_LOGLEVEL in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5).</para> </section> <section id="faq36"> <title>(FAQ 36) My log is filling up with these BANDWIDTH messages!</title> <programlisting>Dec 15 16:47:30 heath-desktop kernel: [17182740.184000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00 SRC=10.119.248.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64 ID=62081 PROTO=UDP SPT=67 DPT=68 LEN=308 Dec 15 16:47:30 heath-desktop last message repeated 2 times Dec 15 16:47:30 heath-desktop kernel: [17182740.188000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00 SRC=10.112.70.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64 ID=62082 PROTO=UDP SPT=67 DPT=68 LEN=308 Dec 15 16:47:30 heath-desktop last message repeated 2 times</programlisting> <para><emphasis role="bold">Answer</emphasis>: The Webmin 'bandwidth' module adds commands to <filename>/etc/shorewall/start</filename> that creates rules to log every packet to/from/through the firewall. <emphasis role="bold">DON'T START THE BANDWIDTH SERVICE IN WEBMIN!</emphasis></para> <para>To correct this situation once it occurs, edit <filename>/etc/shorewall/start</filename> and insert 'return 0' prior to the BANDWIDTH rules.</para> </section> </section> <section id="Routing"> <title>Routing</title> <section id="faq32"> <title>(FAQ 32) My firewall has two connections to the Internet from two different ISPs. How do I set this up in Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> See <ulink url="MultiISP.html">this article about Shorewall and Multiple ISPs</ulink>.</para> </section> <section id="faq49"> <title>(FAQ 49) When I start Shorewall, my routing table gets blown away. Why does Shorewall do that?</title> <para><emphasis role="bold">Answer:</emphasis> This is usually the consequence of a one-to-one nat configuration blunder:</para> <orderedlist> <listitem> <para>Specifying the primary IP address for an interface in the EXTERNAL column of <filename>/etc/shorewall/nat</filename> even though the documentation (and the comments in the file) warn you not to do that.</para> </listitem> <listitem> <para>Specifying ADD_IP_ALIASES=Yes and RETAIN_ALIASES=No in /etc/shorewall/shorewall.conf.</para> </listitem> </orderedlist> <para>This combination causes Shorewall to delete the primary IP address from the network interface specified in the INTERFACE column which usually causes all routes out of that interface to be deleted. The solution is to <emphasis role="bold">not specify the primary IP address of an interface in the EXTERNAL column</emphasis>.</para> </section> </section> <section id="Start-Stop"> <title>Starting and Stopping</title> <section id="faq94"> <title>(FAQ 94) After I start Shorewall, ps doesn't show any shorewall process running. What is the Shorewall daemon called?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall is not a daemon. It is a configuration tool that configures your kernel based on the contents of <filename>/etc/shorewall/</filename>. Once the <command>start</command> command completes, Shorewall has done its job and there are no Shorewall processes remaining in the system.</para> </section> <section id="faq7"> <title>(FAQ 7) When I stop Shorewall using <quote>shorewall[-lite] stop</quote>, I can't connect to anything. Why doesn't that command work?</title> <para><emphasis role="bold">Answer:</emphasis> The <command>stop</command> command places the firewall in a safe state; connections that are allowed are governed by the setting of ADMINISABSENTMINDED in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5) and the contents of <ulink url="manpages/shorewall-routestopped.html">shorewall-routestopped</ulink> (5). To totally open the firewall, use the <command>clear</command> command.</para> </section> <section id="faq9"> <title>(FAQ 9) Why can't Shorewall detect my interfaces properly at startup?</title> <para>I just installed Shorewall and when I issue the <command>start</command> command, I see the following:</para> <programlisting>Processing /etc/shorewall/params ... Processing /etc/shorewall/shorewall.conf ... Starting Shorewall... Loading Modules... Initializing... Determining Zones... Zones: net loc Validating interfaces file... Validating hosts file... Determining Hosts in Zones... <emphasis role="bold">Net Zone: eth0:0.0.0.0/0 </emphasis><emphasis role="bold">Local Zone: eth1:0.0.0.0/0</emphasis> Deleting user chains... Creating input Chains... ...</programlisting> <para>Why can't Shorewall detect my interfaces properly?</para> <para><emphasis role="bold">Answer:</emphasis> The above output is perfectly normal. The Net zone is defined as all hosts that are connected through <filename class="devicefile">eth0</filename> and the local zone is defined as all hosts connected through <filename class="devicefile">eth1</filename>. You can set the <emphasis role="bold">routefilter</emphasis> option on an internal interface if you wish to guard against '<firstterm>Martians</firstterm>' (a Martian is a packet with a source IP address that is not routed out of the interface on which the packet was received). If you do that, it is a good idea to also set the <emphasis role="bold">logmartians</emphasis> option.</para> </section> <section id="faq22"> <title>(FAQ 22) I have some iptables commands that I want to run when Shorewall starts. Which file do I put them in?</title> <para><emphasis role="bold">Answer:</emphasis>You can place these commands in one of the <ulink url="shorewall_extension_scripts.htm">Shorewall Extension Scripts</ulink>. Be sure that you look at the contents of the chain(s) that you will be modifying with your commands so that the commands will do what is intended. Many iptables commands published in HOWTOs and other instructional material use the -A command which adds the rules to the end of the chain. Most chains that Shorewall constructs end with an unconditional DROP, ACCEPT or REJECT rule and any rules that you add after that will be ignored. Check <quote>man iptables</quote> and look at the -I (--insert) command.</para> </section> <section id="faq43"> <title>(FAQ 43) I just installed the Shorewall RPM and Shorewall doesn't start at boot time.</title> <para><emphasis role="bold">Answer:</emphasis> When you install using the "rpm -U" command, Shorewall doesn't run your distribution's tool for configuring Shorewall startup. You will need to run that tool (insserv, chkconfig, run-level editor, …) to configure Shorewall to start in the the default run-levels of your firewall system.</para> </section> <section id="faq59"> <title>(FAQ 59) After I start Shorewall, there are lots of unused Netfilter modules loaded. How do I avoid that?</title> <para><emphasis role="bold">Answer:</emphasis> Copy <filename>/usr/share/shorewall[-lite]/modules</filename> to <filename>/etc/shorewall/modules </filename>and modify the copy to include only the modules that you need. An alternative is to set LOAD_HELPERS_ONLY=Yes in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5).</para> </section> <section id="faq68"> <title>(FAQ 68) I have a VM under an OpenVZ system. I can't get rid of the following message:</title> <para>ERROR: Command "/sbin/iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT" failed.</para> <para><emphasis role="bold">Answer:</emphasis> See the <ulink url="OpenVZ.html">Shorewall OpenVZ article</ulink>.</para> </section> <section id="faq73"> <title>(FAQ 73) When I stop Shorewall, the firewall is wide open. Isn't that a security risk?</title> <para>It is important to understand that the scripts in <filename class="directory">/etc/init.d</filename> are generally provided by your distribution and not by the Shorewall developers. These scripts must meet the requirements of the distribution's packaging system which may conflict with the requirements of a tight firewall. So when you say "…when I stop Shorewall…" it is necessary to distinguish between the commands <command>/sbin/shorewall stop</command> and <command>/etc/init.d/shorewall stop</command>.</para> <para><command>/sbin/shorewall stop</command> places the firewall in a <firstterm>safe state</firstterm>, the details of which depend on your <filename>/etc/shorewall/routestopped</filename> file (<ulink url="manpages/shorewall-routestopped.html">shorewall-routestopped</ulink>(5)) and on the setting of ADMINISABSENTMINDED in <filename>/etc/shorewall/shorewall.conf</filename> (<ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>(5)).</para> <para><command>/etc/init.d/shorewall stop</command> may or may not do the same thing. In the case of <trademark>Debian</trademark> systems for example, that command actually executes <command>/sbin/shorewall clear</command> which opens the firewall completely. In other words, in the init script, <command>stop</command> reverses the effect of <command>start</command>.</para> <para>Beginning with Shorewall 4.4, when the Shorewall tarballs are installed on a Debian (or derivative) system, the <filename>/etc/init.d/shorewall</filename> file is the same as would be installed by the .deb. The behavior of <command>/etc/init.d/shorewall stop</command> is controlled by the setting of SAFESTOP in <filename>/etc/default/shorewall</filename>. When set to 0 (the default), the firewall is cleared; when set to 1, the firewall is placed in a safe state.</para> </section> <section id="faq78"> <title>(FAQ 78) After restart and bootup of my Debian firewall, all traffic is blocked for hosts behind the firewall trying to connect out onto the net or through the vpn (although i can reach the internal firewall interface and obtain dumps etc). Once I issue 'shorewall clear' followed by 'shorewall start' it then works, despite the config not changing</title> <para><emphasis role="bold">Answer:</emphasis> Set IP_FORWARDING=On in <filename><ulink url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink></filename>.</para> </section> <section id="faq86"> <title>(FAQ 86) My distribution (Ubuntu) uses NetworkManager to manage my interfaces. I want to specify the upnpclient option for my interfaces which requires them to be up and configured when Shorewall starts but Shorewall is being started before NetworkManager.</title> <para>Answer: I faced a similar problem which I solved as follows:</para> <itemizedlist> <listitem> <para>Don't start Shorewall at boot time (Debian and Ubuntu users may simply set startup=0 in <filename>/etc/default/shorewall</filename>).</para> </listitem> <listitem> <para>In <filename>/etc/network/ip-up.d</filename>, I added a <filename>shorewall</filename> script as follows:</para> <programlisting>#!/bin/sh shorewall status > /dev/null 2>&1 || shorewall start # Start Shorewall if it isn't already running</programlisting> <para>Be sure to secure the script for execute access.</para> </listitem> </itemizedlist> <variablelist> <varlistentry> <term>Update:</term> <listitem> <para>Beginning with Shorewall 4.4.10, there is a new <ulink url="Manpages/shorewall-init.html">Shorewall Init Package</ulink> that is designed to handle this case.</para> </listitem> </varlistentry> </variablelist> </section> <section id="faq90"> <title>(FAQ 90) Shorewall starts fine but after several minutes, it stops. Why is it doing that?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall uses the presence of a chain named <emphasis>shorewall</emphasis> to indicate whether is started or stopped. That chain is created during execution of a successful <emphasis role="bold">start</emphasis>, <emphasis role="bold">restart</emphasis> or <emphasis role="bold">restore</emphasis> command and is removed during <emphasis role="bold">stop</emphasis> and <emphasis role="bold">clear</emphasis>. If <emphasis role="bold">shorewall status</emphasis> indicates that Shorewall is stopped, then something has deleted that chain. Look at the output of <emphasis role="bold">shorewall status</emphasis>; if it looks like this:</para> <blockquote> <programlisting>gateway:~# shorewall status Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:21:41 PDT 2010 Shorewall is <emphasis role="bold">stopped</emphasis> State:<emphasis role="bold">Started</emphasis> (Tue Jul 20 16:01:49 PDT 2010) gateway:~#</programlisting> </blockquote> <para>then it means that something outside of Shorewall has deleted the chain. This usually means that you were running another firewall package before you installed Shorewall and that other package has replaced Shorewall's Netfilter configuration with its own. You must remove (or at least disable) the other firewall package and restart Shorewall.</para> <blockquote> <programlisting>gateway:~# shorewall status Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:26:29 PDT 2010 Shorewall is <emphasis role="bold">stopped</emphasis> State:<emphasis role="bold">Stopped</emphasis> (Wed Jul 21 13:26:26 PDT 2010) gateway:~# </programlisting> </blockquote> <para>then a <emphasis role="bold">shorewall stop</emphasis> command has been executed (if the State shown in the output is <emphasis role="bold">Cleared</emphasis>, then a <emphasis role="bold">shorewall clear</emphasis> command was executed). Most likely, you have installed and configured the <emphasis>shorewall-init</emphasis> package and a required interface has gone down.</para> </section> <section id="faq99"> <title>(FAQ 99) My /var/lib/shorewall-init.log shows that Shorewall is running at boot but after boot 'iptables -L' shows an empty configuration</title> <para><emphasis role="bold">Answer</emphasis>: This is caused by your failure to disable your distributions default iptables configuration tool when you installed Shorewall. Look for a service called 'iptables' that is being started after Shorewall and disable it.</para> </section> </section> <section id="MultiISP"> <title>Multiple ISPs</title> <section id="faq57"> <title>(FAQ 57) I configured two ISPs in Shorewall but when I try to use the second one, it doesn't work.</title> <para><emphasis role="bold">Answer:</emphasis> The Multi-ISP Documentation strongly recommends that you use the <emphasis role="bold">balance</emphasis> option on all providers even if you want to manually specify which ISP to use. If you don't do that so that your main routing table only has one default route, then you must disable route filtering. Do not specify the <emphasis role="bold">routefilter</emphasis> option on the other interface(s) in <filename>/etc/shorewall/interfaces</filename> and disable any <emphasis>IP Address Spoofing</emphasis> protection that your distribution supplies.</para> </section> <section id="faq58"> <title>(FAQ 58) But if I specify 'balance' then won't Shorewall balance the traffic between the interfaces? I don't want that!</title> <para><emphasis role="bold">Answer:</emphasis> Suppose that you want all traffic to go out through ISP1 (mark 1) unless you specify otherwise. Then simply add these two rules as the first marking rules in your <filename>/etc/shorewall/tcrules</filename> file:</para> <programlisting>#MARK SOURCE DEST 1:P 0.0.0.0/0 1 $FW <emphasis>other MARK rules</emphasis></programlisting> <para>Now any traffic that isn't marked by one of your other MARK rules will have mark = 1 and will be sent via ISP1. That will work whether <emphasis role="bold">balance</emphasis> is specified or not!</para> </section> </section> <section> <title>Using DNS Names</title> <section id="faq79"> <title>(FAQ 79) Can I use DNS names in Shorewall configuration file entries in place of IP addresses?</title> <para><emphasis role="bold">Answer</emphasis>: <ulink url="configuration_file_basics.htm#dnsnames">Yes</ulink>, but we advise strongly against it.</para> </section> </section> <section id="TC"> <title>Traffic Shaping</title> <section id="faq67"> <title>(FAQ 67) I just configured Shorewall's builtin traffic shaping and now Shorewall fails to Start.</title> <para>The error I receive is as follows:<programlisting>RTNETLINK answers: No such file or directory We have an error talking to the kernel ERROR: Command "tc filter add dev eth2 parent ffff: protocol ip prio 50 u32 match ip src 0.0.0.0/0 police rate 500kbit burst 10k drop flowid :1" Failed</programlisting><emphasis role="bold">Answer:</emphasis> This message indicates that your kernel doesn't have 'traffic policing' support. If your kernel is modularized, you may be able to resolve the problem by loading the <emphasis role="bold">act_police</emphasis> kernel module. Other kernel modules that you will need include:<simplelist> <member>cls_basic</member> <member>cls_fw</member> <member>cls_u32</member> <member>sch_htb</member> <member>sch_ingress</member> <member>sch_sfq</member> </simplelist></para> </section> <section id="faq97"> <title>(FAQ 97) I enable Shorewall traffic shaping and now my upload rate is way below what I specified</title> <para><emphasis role="bold">Answer</emphasis>: This is likely due to TCP Segmentation Offload (TSO) and/or Generic Segmentation Offload (GSO) being enabled in the network adapter. To verify, install the <firstterm>ethtool</firstterm> package and use the -k command:</para> <programlisting>root@gateway:~# ethtool -k eth1 Offload parameters for eth1: rx-checksumming: on tx-checksumming: on scatter-gather: on tcp-segmentation-offload: <emphasis role="bold">on</emphasis> udp-fragmentation-offload: off generic-segmentation-offload: <emphasis role="bold">on</emphasis> generic-receive-offload: off large-receive-offload: off ntuple-filters: off receive-hashing: off root@gateway:~#</programlisting> <para>If that is the case, you can correct the problem by adjusting the <replaceable>minburst</replaceable> setting in /etc/shorewall/tcinterfaces (complex traffic shaping) or /etc/shorewall/tcdevices (simple traffic shaping). We suggest starting at 10-12kb and adjust as necessary. Example (simple traffic shaping):</para> <programlisting>#INTERFACE TYPE IN-BANDWIDTH OUT-BANDWIDTH eth0 External 50mbit:200kb 5.0mbit:100kb:200ms:100mbit:<emphasis role="bold">10kb</emphasis> </programlisting> <para>Alternatively, you can turn off TSO and GSO using this command in <filename>/etc/shorewall/init</filename>:</para> <programlisting><emphasis role="bold">ethtool -K eth<emphasis>N</emphasis> tso off gso off</emphasis></programlisting> </section> <section> <title>(FAQ 97a) I enable Shorewall traffic shaping and now my download rate is way below what I specified</title> <para><emphasis role="bold">Answer</emphasis>: This is likely due to Generic Receive Offload (GRO) being enabled in the network adapter. To verify, install the <firstterm>ethtool</firstterm> package and use the -k command:</para> <programlisting>root@gateway:/etc/shorewall# ethtool -k eth1 Offload parameters for eth1: rx-checksumming: on tx-checksumming: on scatter-gather: on tcp-segmentation-offload: on udp-fragmentation-offload: off generic-segmentation-offload: on generic-receive-offload: <emphasis role="bold">on</emphasis> large-receive-offload: off ntuple-filters: off receive-hashing: off root@gateway:/etc/shorewall# </programlisting> <para>To work around the issue, use this command:</para> <programlisting><emphasis role="bold">ethtool -K eth</emphasis>N <emphasis role="bold">gro off</emphasis></programlisting> </section> </section> <section id="About"> <title>About Shorewall</title> <section id="faq10"> <title>(FAQ 10) What Distributions does Shorewall work with?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall works with any GNU/Linux distribution that includes the <ulink url="shorewall_prerequisites.htm">proper prerequisites</ulink>.</para> </section> <section id="faq11"> <title>(FAQ 11) What Features does Shorewall have?</title> <para><emphasis role="bold">Answer:</emphasis> See the <ulink url="shorewall_features.htm">Shorewall Feature List</ulink>.</para> </section> <section id="faq12"> <title>(FAQ 12) Is there a GUI?</title> <para><emphasis role="bold">Answer:</emphasis> Yes! Shorewall support is available in Webmin. See <ulink url="http://www.webmin.com">http://www.webmin.com</ulink>. But beware of the issue described in <link linkend="faq36">FAQ 36</link>.</para> </section> <section id="faq13"> <title>(FAQ 13) Why do you call it <quote>Shorewall</quote>?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall is a concatenation of <quote> <emphasis>Shore</emphasis>line</quote> (<ulink url="http://www.cityofshoreline.com">the city where I live</ulink>) and <quote>Fire<emphasis>wall</emphasis> </quote>. The full name of the product is actually <quote>Shoreline Firewall</quote> but <quote>Shorewall</quote> is much more commonly used.</para> </section> <section id="faq23"> <title>(FAQ 23) Why do you use such ugly fonts on your web site?</title> <para><emphasis role="bold">Answer:</emphasis> The Shorewall web site is almost font neutral (it doesn't explicitly specify fonts except on a few pages) so the fonts you see are largely the default fonts configured in your browser. If you don't like them then reconfigure your browser.</para> </section> <section id="faq25"> <title>(FAQ 25) How do I tell which version of Shorewall or Shorewall Lite I am running?</title> <para><emphasis role="bold">Answer:</emphasis> At the shell prompt, type:</para> <programlisting><command>/sbin/shorewall[-lite] version -a</command> </programlisting> <section id="faq25a"> <title>(FAQ 25a) It says 4.4.7.5; how do I know if it is Shorewall-shell or Shorewall-perl?</title> <para><emphasis role="bold">Answer</emphasis>: It is Shorewall-perl. Shorewall-shell is discontinued in Shorewall 4.4.</para> </section> </section> <section id="faq31"> <title>(FAQ 31) Does Shorewall provide protection against....</title> <variablelist> <varlistentry> <term>IP Spoofing: Sending packets over the WAN interface using an internal LAP IP address as the source address?</term> <listitem> <para><emphasis role="bold">Answer:</emphasis> Yes.</para> </listitem> </varlistentry> <varlistentry> <term>Tear Drop: Sending packets that contain overlapping fragments?</term> <listitem> <para><emphasis role="bold">Answer:</emphasis> This is the responsibility of the IP stack, not the Netfilter-based firewall since fragment reassembly occurs before the stateful packet filter ever touches each packet.</para> </listitem> </varlistentry> <varlistentry> <term>Smurf and Fraggle: Sending packets that use the WAN or LAN broadcast address as the source address?</term> <listitem> <para><emphasis role="bold">Answer:</emphasis> Shorwall filters these packets under the <firstterm>nosmurfs</firstterm> interface option in <ulink url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>.</para> </listitem> </varlistentry> <varlistentry> <term>Land Attack: Sending packets that use the same address as the source and destination address?</term> <listitem> <para><emphasis role="bold">Answer:</emphasis> Yes, if the <ulink url="manpages/shorewall-interfaces.html">routefilter interface option</ulink> is selected.</para> </listitem> </varlistentry> <varlistentry> <term>DOS: - SYN Dos - ICMP Dos - Per-host Dos protection</term> <listitem> <para><emphasis role="bold">Answer:</emphasis> Yes.</para> </listitem> </varlistentry> </variablelist> </section> <section id="faq65"> <title>(FAQ 65) How do I accomplish failover with Shorewall?</title> <para><emphasis role="bold">Answer:</emphasis> <ulink url="http://linuxman.wikispaces.com/Clustering+Shorewall">This article by Paul Gear</ulink> should help you get started.</para> </section> </section> <section id="ALIASES"> <title>Alias IP Addresses/Virtual Interfaces</title> <section id="faq18"> <title>(FAQ 18) Is there any way to use aliased ip addresses with Shorewall, and maintain separate rule sets for different IPs?</title> <para><emphasis role="bold">Answer:</emphasis> Yes. See <ulink url="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased Interfaces</ulink>.</para> </section> <section id="faq83"> <title>(FAQ 83) Is there no way to nest the firewall zone or create subzones? I've got a system with Linux-VServers, it's one interface (eth0) with multiple IPs</title> <para><emphasis role="bold">Answer</emphasis>: Beginning with Shorewall 4.4.11 Beta 2, you can <ulink url="Vserver.html">create vserver zones</ulink> that are nested within the firewall zone.</para> <para>Prior to 4.4.11 Beta 2, there is no way to create sub-zones of the firewall zone. But you can use shell variables to make vservers easier to deal with.</para> <para><filename>/etc/shorewall/params</filename>:</para> <programlisting>VS1=fw:192.168.2.12 VS2=fw:192.168.2.13 VS3=fw:192.168.2.14</programlisting> <para><filename>/etc/shorewall/rules</filename>:</para> <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT $VS1 net tcp 25 DNAT net $VS1 tcp 25 etc...</programlisting> </section> </section> <section id="Lite"> <title>Shorewall Lite</title> <section id="faq53"> <title>(FAQ 53) What is Shorewall Lite?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall Lite is a companion product to Shorewall and is designed to allow you to maintain all Shorewall configuration information on a single system within your network. See the <ulink url="Shorewall-Lite.html">Compiled Firewall script documentation</ulink> for details.</para> </section> <section id="faq54"> <title>(FAQ 54) If I want to use Shorewall Lite, do I also need to install Shorewall on the same system?</title> <para><emphasis role="bold">Answer:</emphasis> No. In fact, we recommend that you do <emphasis role="bold">NOT</emphasis> install Shorewall on systems where you wish to use Shorewall Lite. You must have Shorewall installed on at least one system within your network in order to use Shorewall Lite.</para> </section> <section id="faq55"> <title>(FAQ 55) How do I decide which product to use - Shorewall or Shorewall Lite?</title> <para><emphasis role="bold">Answer:</emphasis> If you plan to have only a single firewall system, then Shorewall is the logical choice. I also think that Shorewall is the appropriate choice for laptop systems that may need to have their firewall configuration changed while on the road. In the remaining cases, Shorewall Lite will work very well. At shorewall.net, the two laptop systems have the full Shorewall product installed as does my personal Linux desktop system. All other Linux systems that run a firewall use Shorewall Lite and have their configuration directories on my desktop system.</para> </section> <section id="faq60"> <title>(FAQ 60) What are the compatibility restrictions between Shorewall and Shorewall Lite</title> <para><emphasis role="bold">Answer:</emphasis> There are no compatibility constraints between Shorewall and Shorewall-lite.</para> </section> </section> <section id="VOIP"> <title>VOIP</title> <section id="faq77"> <title>(FAQ 77) Shorewall is eating my Asterisk egress traffic!</title> <para>Somehow, my firewall config is causing a one-way audio problem in Asterisk. If a person calls into the PBX, they cannot hear me speaking, but I can hear them. If I plug the Asterisk server directly into the router, bypassing the firewall, the problem goes away.</para> <para><emphasis role="bold">Answer:</emphasis> There are two things to try when VOIP problems are encountered. Both begin with executing two <command>rmmod</command> commands.</para> <para>If your kernel version is 2.6.20 or earlier:<programlisting>rmmod ip_nat_sip rmmod ip_conntrack_sip</programlisting>If your kernel version is 2.6.21 or later:<programlisting>rmmod nf_nat_sip rmmod nf_conntrack_sip</programlisting></para> <para>The first alternative seems to work for those running recent kernels (2.6.26 or later):</para> <orderedlist> <listitem> <para>Copy <filename>/usr/share/shorewall/module</filename>s to <filename class="directory">/etc/shorewall</filename> (<filename>/usr/share/shorewall/helpers</filename> if you have LOAD_HELPERS_ONLY in shorewall.conf).</para> </listitem> <listitem> <para>Edit the copy and change this line:</para> <blockquote> <para>loadmodule nf_conntrack_sip</para> </blockquote> <para>to</para> <blockquote> <para>loadmodule nf_conntrack_sip sip_direct_media=0</para> </blockquote> </listitem> <listitem> <para><command>shorewall restart</command></para> </listitem> </orderedlist> <para>The second alternative is to not load the sip helpers:</para> <itemizedlist> <listitem> <para>If you are running kernel 2.6.20 or earlier, then change the DONT_LOAD specification in your shorewall.conf to:<programlisting>DONT_LOAD=ip_nat_sip,ip_conntrack_sip</programlisting></para> </listitem> <listitem> <para>If you are running kernel 2.6.21 or later, then change Then change the DONT_LOAD specification in your shorewall.conf to:<programlisting>DONT_LOAD=nf_nat_sip,nf_conntrack_sip</programlisting></para> </listitem> </itemizedlist> </section> </section> <section id="faq40"> <title>IPv6</title> <section id="faq80"> <title>(FAQ 80) Does Shorewall support IPV6?</title> <para>Answer: <ulink url="IPv6Support.html">Shorewall IPv6 support</ulink> is currently available in Shorewall 4.2.4 and later.</para> <section id="faq80a"> <title>(FAQ 80a) Why does Shorewall lPv6 Support Require Kernel 2.6.24 or later?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall implements a stateful firewall which requires connection tracking be present in ip6tables and in the kernel. Linux kernels before 2.6.20 didn't support connection tracking for IPv6. So we could not even start to develop Shorewall IPv6 support until 2.6.20 and there were significant problems with the facility until at least kernel 2.6.23. When distributions began offering IPv6 connection tracking support, it was with kernel 2.6.25. So that is what we developed IPv6 support on and that's all that we initially tested on. Subsequently, we have tested Shorewall6 on Ubuntu Hardy with kernel 2.6.24. If you are running 2.6.20 or later, you can <emphasis role="bold">try</emphasis> to run Shorewall6 by hacking<filename> /usr/share/shorewall/prog.footer6</filename> and changing the kernel version test to check for your kernel version rather than 2.6.24 (20624). But after that, you are on your own.</para> <programlisting>kernel=$(printf "%2d%02d%02d\n" $(echo $(uname -r) 2> /dev/null | sed 's/-.*//' | tr '.' ' ' ) | head -n1) if [ $kernel -lt <emphasis role="bold">20624</emphasis> ]; then error_message "ERROR: $PRODUCT requires Linux kernel <emphasis role="bold">2.6.24</emphasis> or later" status=2 else </programlisting> <para>Update: The above logic is found in <filename>/usr/share/shorewall/prog.footer</filename> in later Shorewall releases.</para> </section> </section> <section> <title>(FAQ 40) I have an interface that gets its IPv6 configuration from radvd. When I start Shorewall6, I immediately loose my default route. Why?</title> <para><emphasis role="bold">Answer</emphasis>: You have configured forwarding on the interface which disables autoconfiguration of the interface. To retain autoconfiguration on the interface when Shorewall6 starts, specify <emphasis role="bold">forwarding=0</emphasis> in the OPTIONS column on the interface's entry in <ulink url="manpages6/shorewall6-interfaces.html">shorewall6-interfaces</ulink> (5).</para> </section> <section> <title id="faq96">(FAQ 96) I am starting to use ipv6, but on my ipv4 FW, when restarting Shorewall . it puts in ip6tables rules. How do i dissable that ?</title> <para>Answer: This is a two-step process.</para> <orderedlist> <listitem> <para>Set DISABLE_IPV6=No in <ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink> (5) and restart Shorewall.</para> </listitem> <listitem> <para>Execute these commands at a root shell prompt:</para> <itemizedlist> <listitem> <para>ip6tables -P INPUT ACCEPT</para> </listitem> <listitem> <para>ip6tables -P OUTPUT ACCEPT</para> </listitem> <listitem> <para>ip6tables -P FORWARD ACCEPT</para> </listitem> </itemizedlist> </listitem> </orderedlist> <para>You will probably want to soon install <ulink url="IPv6Support.html">Shorewall6</ulink> so that you have an IPv6 firewall as well as one for IPv4.</para> </section> </section> <section id="Misc"> <title>Miscellaneous</title> <section id="faq20"> <title>(FAQ 20) I have just set up a server. Do I have to change Shorewall to allow access to my server from the Internet?</title> <para><emphasis role="bold">Answer:</emphasis> Yes. Consult the <ulink url="shorewall_quickstart_guide.htm">QuickStart guide</ulink> that you used during your initial setup for information about how to set up rules for your server.</para> </section> <section id="faq24"> <title>(FAQ 24) How can I allow connections to, let's say, the ssh port only from specific IP Addresses on the Internet?</title> <para><emphasis role="bold">Answer:</emphasis> In the SOURCE column of the rule, follow <quote>net</quote> by a colon and a list of the host/subnet addresses as a comma-separated list.</para> <programlisting>net:<ip1>,<ip2>,...</programlisting> <example id="Example4"> <title>Example:</title> <programlisting>ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22</programlisting> </example> </section> <section id="faq26"> <title>(FAQ 26) When I try to use any of the SYN options in nmap on or behind the firewall, I get <quote>operation not permitted</quote>. How can I use nmap with Shorewall?"</title> <para><emphasis role="bold">Answer:</emphasis> Temporarily remove any <emphasis role="bold">rejNotSyn</emphasis>, <emphasis role="bold">dropNotSyn</emphasis>, <emphasis role="bold">dropInvalid</emphasis>, <emphasis role="bold">NotSyn(...)</emphasis> and <emphasis role="bold">Invalid(...)</emphasis> rules from <filename>/etc/shorewall/rules</filename> and restart Shorewall.</para> </section> <section id="faq27"> <title>(FAQ 27) I'm compiling a new kernel for my firewall. What should I look out for?</title> <para><emphasis role="bold">Answer:</emphasis> First take a look at the <ulink url="kernel.htm">Shorewall kernel configuration page</ulink>. You probably also want to be sure that you have selected the <quote> <emphasis role="bold">NAT of local connections (READ HELP)</emphasis> </quote> on the Netfilter Configuration menu. Otherwise, DNAT rules with your firewall as the source zone won't work with your new kernel.</para> </section> <section id="faq28"> <title>(FAQ 28) How do I use Shorewall as a Bridging Firewall?</title> <para><emphasis role="bold">Answer:</emphasis> Shorewall Bridging Firewall support is available — <ulink url="bridge-Shorewall-perl.html">check here for details</ulink>.</para> </section> <section id="faq39"> <title>(FAQ 39) How do I block connections to a particular domain name?</title> <para>I tried this rule to block Google's Adsense that you'll find on everyone's site. Adsense is a Javascript that people add to their Web pages. So I entered the rule:</para> <programlisting>#ACTION SOURCE DEST PROTO REJECT fw net:pagead2.googlesyndication.com all</programlisting> <para>However, this also sometimes restricts access to "google.com". Why is that? Using dig, I found these IPs for domain googlesyndication.com:<programlisting>216.239.37.99 216.239.39.99</programlisting>And this for google.com:<programlisting>216.239.37.99 216.239.39.99 216.239.57.99</programlisting>So my guess is that you are not actually blocking the domain, but rather the IP being called. So how in the world do you block an actual domain name?</para> <para><emphasis role="bold">Answer:</emphasis> Packet filters like Netfilter base their decisions on the contents of the various protocol headers at the front of each packet. Stateful packet filters (of which Netfilter is an example) use a combination of header contents and state created when the packet filter processed earlier packets. Netfilter (and Shorewall's use of Netfilter) also consider the network interface(s) where each packet entered and/or where the packet will leave the firewall/router.</para> <para>When you specify <ulink url="configuration_file_basics.htm#dnsnames">a domain name in a Shorewall rule</ulink>, the iptables program resolves that name to one or more IP addresses and the actual Netfilter rules that are created are expressed in terms of those IP addresses. So the rule that you entered was equivalent to:</para> <para><programlisting>#ACTION SOURCE DEST PROTO REJECT fw net:216.239.37.99 all REJECT fw net:216.239.39.99 all</programlisting>Given that name-based multiple hosting is a common practice (another example: lists.shorewall.net and www1.shorewall.net are both hosted on the same system with a single IP address), it is not possible to filter connections to a particular name by examination of protocol headers alone. While some protocols such as <ulink url="FTP.html">FTP</ulink> require the firewall to examine and possibly modify packet payload, parsing the payload of individual packets doesn't always work because the application-level data stream can be split across packets in arbitrary ways. This is one of the weaknesses of the 'string match' Netfilter extension available in later Linux kernel releases. The only sure way to filter on packet content is to proxy the connections in question -- in the case of HTTP, this means running something like <ulink url="Shorewall_Squid_Usage.html">Squid</ulink>. Proxying allows the proxy process to assemble complete application-level messages which can then be accurately parsed and decisions can be made based on the result.</para> </section> <section id="faq42"> <title>(FAQ 42) How can I tell which features my kernel and iptables support?</title> <para><emphasis role="bold">Answer:</emphasis> Use the <command>shorewall[-lite] show capabilities</command> command at a root prompt.</para> <programlisting>gateway:~# <command>shorewall show capabilities</command> Shorewall has detected the following iptables/netfilter capabilities: NAT: Available Packet Mangling: Available Multi-port Match: Available Extended Multi-port Match: Available Connection Tracking Match: Available Extended Connection Tracking Match Support: Available Old Connection Tracking Match Syntax: Not available Packet Type Match: Available Policy Match: Available Physdev Match: Available Physdev-is-bridged Support: Available Packet length Match: Available IP range Match: Available Recent Match: Available Owner Match: Available Ipset Match: Available CONNMARK Target: Available Extended CONNMARK Target: Available Connmark Match: Available Extended Connmark Match: Available Raw Table: Available IPP2P Match: Available Old IPP2P Match Syntax: Not available CLASSIFY Target: Available Extended REJECT: Available Repeat match: Available MARK Target: Available Extended MARK Target: Available Mangle FORWARD Chain: Available Comments: Available Address Type Match: Available TCPMSS Match: Available Hashlimit Match: Available Old Hashlimit Match: Not available NFQUEUE Target: Available Realm Match: Available Helper Match: Available Connlimit Match: Available Time Match: Available Goto Support: Available LOGMARK Target: Available IPMARK Target: Available LOG Target: Available Persistent SNAT: Available gateway:~# </programlisting> <para/> </section> <section id="faq19"> <title>(FAQ 19) How do I open the firewall for all traffic to/from the LAN?</title> <para><emphasis role="bold">Answer:</emphasis> Add these two policies:</para> <programlisting>#SOURCE DESTINATION POLICY LOG LIMIT:BURST # LEVEL $FW loc ACCEPT loc $FW ACCEPT </programlisting> <para>You should also delete any ACCEPT rules from $FW->loc and loc->$FW since those rules are redundant with the above policies.</para> </section> <section id="faq88"> <title>(FAQ 88) Can I run Snort with Shorewall?</title> <para><emphasis role="bold">Answer</emphasis>: Yes. In <emphasis>Network Intrusion Detection System (NIDS) mode</emphasis>, Snort is libpcap based (like tcpdump) so it doesn't interfere with Shorewall. We have had reports that users have also been successful in using Snort in <emphasis>inline</emphasis> more with Shorewall, but no HOWTO exists at this time.</para> </section> <section id="faq89"> <title>(FAQ 89) How do I connect to the web server in my aDSL modem from my local LAN?</title> <para>Answer: Here's what I did:</para> <itemizedlist> <listitem> <para>My local network is 172.20.1.0/24, so I set the IP address in the modem to 172.20.1.2.</para> </listitem> <listitem> <para>The IP address of my firewall's interface to the LAN is 172.20.1.254. The logical name of the DSL interface is EXT_IF and my LAN interface is INT_IF.</para> <para>I added the following two configuration entries:</para> <para><filename>/etc/shorewall/masq:</filename></para> <programlisting>#INTERFACE SOURCE ADDRESS COMMENT DSL Modem EXT_IF:172.20.1.2 0.0.0.0/0 172.20.1.254 </programlisting> <para><filename>/etc/shorewall/proxyarp</filename>:</para> <programlisting>#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT 172.20.1.2 EXT_IF INT_IF no yes </programlisting> </listitem> </itemizedlist> <para>If you can't change the IP address of your modem and its current address isn't in your local network, then you need to change this slightly; assuming that the modem IP address is 192.168.1.1:</para> <itemizedlist> <listitem> <para>Do not include an entry in <filename>/etc/shorewall/proxyarp</filename>.</para> </listitem> <listitem> <para>Add an IP address in 192.168.1.0/24 to your external interface using your configuration's network management tools. For Debian-based systems, that means adding this to the interface's stanza in <filename>/etc/network/interfaces</filename>:</para> <programlisting> post-up /sbin/ip addr add 192.168.1.254/24 dev <replaceable>external-interface</replaceable></programlisting> </listitem> <listitem> <para>Your entry in <filename>/etc/shorewall/masq</filename> would then be:</para> <programlisting>#INTERFACE SOURCE ADDRESS COMMENT DSL Modemhttp://ipv6.shorewall.net/SimpleBridge.html EXT_IF:192.168.1.1 0.0.0.0/0 192.168.1.254 </programlisting> </listitem> </itemizedlist> </section> <section> <title id="faq93">(FAQ 93) I'm not able to use Shorewall to manage a bridge. I get the following error: ERROR: BRIDGING=Yes is not supported by Shorewall 4.4.13.3.</title> <para><emphasis role="bold">Answer:</emphasis> If you want to apply firewall rules to the traffic passing between bridge ports, see <ulink url="bridge-Shorewall-perl.html">http://www.shorewall.net/bridge-Shorewall-perl.html</ulink>. If you simply want to allow all traffic between ports, then see <ulink url="SimpleBridge.html">http://www.shorewall.net/SimpleBridge.html</ulink>.</para> </section> <section id="faq95"> <title>(FAQ 95) What is this $FW that I see in the configuration files and documentation?</title> <para><emphasis role="bold">Answer: FW</emphasis> is a <ulink url="configuration_file_basics.htm#Variables">shell variable</ulink> that expands to the name that you gave to the firewall zone in <ulink url="manpages/shorewall-zones.html">shorewall-zones</ulink>(5). The default name for the firewall zone is <emphasis role="bold">fw</emphasis>:</para> <programlisting>#ZONE TYPE OPTIONS <emphasis role="bold">fw</emphasis> firewall</programlisting> <para>So, using the default or sample configurations, writing <emphasis role="bold">$FW</emphasis> is the same as writing <emphasis role="bold">fw</emphasis>. If you give the firewall zone a different name, <emphasis role="bold">gate</emphasis> for example, then writing <emphasis role="bold">$FW</emphasis> would be the same as writing <emphasis role="bold">gate</emphasis>.</para> <programlisting>#ZONE TYPE OPTIONS <emphasis role="bold">gate</emphasis> firewall</programlisting> <section id="faq95a"> <title>Why was that done?</title> <para><emphasis role="bold">Answer:</emphasis> The firewall zone has special semantics, so having a way to refer to it in a configuration-independent way makes writing the documentation, examples, macros, etc. easier.</para> </section> </section> <section id="faq98"> <title>(FAQ 98) How do I Unsubscribe from the Mailing List</title> <para><emphasis role="bold">Answer</emphasis>: There are two ways:</para> <orderedlist> <listitem> <para>On the web</para> <para>Go to <ulink url="https://lists.sourceforge.net/lists/listinfo/shorewall-users">https://lists.sourceforge.net/lists/listinfo/shorewall-users</ulink>. At the bottom of the form is a section entitled "<emphasis role="bold">Shorewall-users Subscribers</emphasis>". At the bottom of that section find:</para> <blockquote> <para>"To <emphasis role="bold">unsubscribe</emphasis> from Shorewall-users, get a password reminder, or change your subscription options <emphasis role="bold">enter your subscription email address</emphasis>:".</para> </blockquote> <para>Enter your email address in the box provided and click on the "<emphasis role="bold"><ulink url="???">Unsubscribe or edit options</ulink></emphasis>" button. That will take you to a second form.</para> <para>At the top of the second form is a box to <emphasis role="bold">enter your password</emphasis> -- enter it there then click the <emphasis role="bold">Unsubscribe</emphasis> button in the center of the form. You will be unsubscribed.</para> <para>If you <emphasis role="bold">don't remember your password</emphasis>, click on the <emphasis role="bold">Remind</emphasis> button at the bottom of the form and your password will be emailed to you.</para> </listitem> <listitem> <para>Via email using this link: <ulink url="mailto:shorewall-users-request@lists.sourceforge.net?subject=unsubscribe">mailto:shorewall-users-request@lists.sourceforge.net?subject=unsubscribe</ulink>. You will receive a confirmation email shortly; follow the instructions in that email.</para> </listitem> </orderedlist> </section> </section> </article>