Shorewall FAQs Shorewall Community Tom Eastep 2001-2008 Thomas M. Eastep 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 GNU Free Documentation License . This article applies to Shorewall 3.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release.
Installing Shorewall
Where do I find Step by Step Installation and Configuration Instructions? Answer: Check out the QuickStart Guides.
(FAQ 37) I just installed Shorewall on Debian and the /etc/shorewall directory is almost empty!!! Answer: 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: 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. If you install using the .deb, you will find that your /etc/shorewall directory is almost empty. This is intentional. The released configuration file skeletons may be found on your system in the directory /usr/share/doc/shorewall-common/default-config. Simply copy the files you need from that directory to /etc/shorewall and modify the copies.
(FAQ 37a) I just installed Shorewall on Debian and I can't find the sample configurations. Answer: With Shorewall 3.x, the samples are included in the shorewall documentation package and are installed in /usr/share/doc/shorewall/examples/. Beginning with Shorewall 4.0, the samples are in the shorewall-common package and are installed in /usr/share/doc/shorewall-common/examples/.
(FAQ 75) I can't find the Shorewall 4.x shorewall-common RPM. Where is it? Answer: If you use Simon Matter's Redhat/Fedora/CentOS rpms, be aware that Simon calls the shorewall-common RPM shorewall. So you should download and install the appropriate shorewall-4.x.y RPM from his site.
Upgrading Shorewall
(FAQ 66) I'm trying to upgrade to Shorewall 4.x; where is the 'shorewall' package? Answer: Please see the upgrade issues.
(FAQ 66a) I'm trying to upgrade to Shorewall 4.x; do I have to uninstall the 'shorewall' package? Answer: Please see the upgrade issues.
(FAQ 66b) I'm trying to upgrade to Shorewall 4.x: which of these packages do I need to install? Answer: Please see the upgrade issues.
(FAQ 76) I just upgraded my Debian (Ubuntu, Kubuntu, ...) system and now masquerading doesn't work? What happened? Answer: This happens to people who ignore our advice and allow the installer to replace their working /etc/shorewall/shorewall.conf with one that has default settings. Failure to forward traffic (such as during masqueraded net access from a local network) usually means that /etc/shorewall/shorewall.conf contains the Debian default setting IP_FORWARDING=Keep; it should be IP_FORWARDING=On.
Port Forwarding (Port Redirection)
(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. Answer: The format of a port-forwarding rule to a local system is as follows: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:local-IP-address[:local-port] protocol port-number So to forward UDP port 7777 to internal system 192.168.1.5, the rule is: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.5 udp 7777 If you want to forward requests directed to a particular address ( external-IP ) on your firewall to an internal system: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:local-IP-address>[:local-port] protocol port-number - external-IP If you want to forward requests from a particular Internet address ( address ): #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net:address loc:local-IP-address[:local-port] protocol port-number - Finally, if you need to forward a range of ports, in the DEST PORT column specify the range as low-port:high-port.
(FAQ 1a) Okay -- I followed those instructions but it doesn't work Answer: That is usually the result of one of four things: You are trying to test from inside your firewall (no, that won't work -- see ). 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 FAQ 1f). Your ISP is blocking that particular port inbound or, for TCP, your ISP is dropping the outbound SYN,ACK response. 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 two-interface QuickStart Guide for details.
(FAQ 1b) I'm still having problems with port forwarding Answer: To further diagnose this problem: As root, type shorewall reset ("shorewall-lite reset", if you are running Shorewall Lite). This clears all Netfilter counters. Try to connect to the redirected port from an external host. As root type shorewall show nat ("shorewall-lite show nat", if you are running Shorewall Lite). Locate the appropriate DNAT rule. It will be in a chain called <source zone>_dnat (net_dnat in the above examples). 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 should be the IP address of the firewall's interface to that system). If the packet count is zero: the connection request is not reaching your server (possibly it is being blocked by your ISP); or 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 ORIG. DEST. column in your DNAT rule); or 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 ethereal to further diagnose the problem. 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 "shorewall show zones" ("shorewall-lite show zones") then be sure that in the DEST column you have specified the first zone in the list that matches OUT=<dev> and DEST= <ip>from the REJECT/DROP log message. 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.
(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? Answer:In /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.3:22 tcp 1022
(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. Answer: See FAQ 2b.
(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? Answer 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 interface and add the following rule then from the net, you will have 4104 listening, from your LAN, port 22. #ACTION SOURCE DEST PROTO DEST PORT(S) DNAT net fw:192.168.1.1:22 tcp 4104
(FAQ 1f) Why must the server that I port forward to have it's default gateway set to my Shorewall system's IP address? Answer: Let's take an example. Suppose that Your Shorewall firewall's external IP address is 206.124.146.176 (eth0) and its internal IP address is 192.168.1.1 (eth1). You have another gateway router with external IP address 130.252.100.109 and internal IP address 192.168.1.254. You have an FTP server behind both routers with IP address 192.168.1.4 The FTP server's default gateway is through the second router (192.168.1.254). You have this rule on the Shorewall system:#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:192.168.1.4 tcp 21 - 206.124.146.176 Internet host 16.105.221.4 issues the command ftp 206.124.146.176 This results in the following set of events: 16.105.221.4 sends a TCP SYN packet to 206.124.146.176 specifying destination port 21. The Shorewall box rewrites the destination IP address to 192.168.1.4 and forwards the packet. 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. 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. 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 /etc/shorewall/masq:#INTERFACE SOURCE ADDRESS PROTO PORT eth1:192.168.1.4 0.0.0.0/0 192.168.1.1 tcp 21 This rule has the undesirable side effect that it makes 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.
(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 Answer: This requires a vile hack similar to the one in FAQ 2. Assuming that your Internet zone is named net and connects on interface eth0: In /etc/shorewall/rules:#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net net:66.249.93.111:993 tcp 80 - 206.124.146.176 In /etc/shorewall/interfaces, specify the routeback option on eth0:#ZONE INTERFACE BROADCAST OPTIONS net eth0 detect routeback And in /etc/shorewall/masq;#INTERFACE SOURCE ADDRESS PROTO PORT eth0:66.249.93.111 0.0.0.0/0 206.124.146.176 tcp 993 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).
(FAQ 30) I'm confused about when to use DNAT rules and when to use ACCEPT rules. Answer: It would be a good idea to review the QuickStart Guide 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. 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.
(FAQ 38) Where can I find more information about DNAT? Answer: Ian Allen has written a Paper about DNAT and Linux.
(FAQ 48) How do I Set up Transparent HTTP Proxy with Shorewall? Answer: See Shorewall_Squid_Usage.html.
DNS and Port Forwarding/NAT
(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. Answer: I have two objections to this setup. 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 :-) The accessibility problem is best solved using Split DNS (either use a separate DNS server for local clients or use Bind Version 9 views on your main name server) such that www.mydomain.com resolves to 130.141.100.69 externally and 192.168.1.5 internally. That's what I do here at shorewall.net for my local systems that use one-to-one NAT. 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 FAQ 2b. 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). 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!", check here. But if you are the type of person who prefers quick and dirty hacks to "doing it right", then proceed as described below. All traffic redirected through use of this hack 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. 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: In /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS loc eth1 detect routeback In /etc/shorewall/masq: #INTERFACE SOURCE ADDRESS PROTO PORT(S) eth1:192.168.1.5 eth1 192.168.1.254 tcp www In /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69 That rule only works of course if you have a static external IP address. If you have a dynamic IP address then include this in /etc/shorewall/params (or your <export directory>/init file if you are using Shorewall Lite on the firewall system): ETH0_IP=`find_first_interface_address eth0` and make your DNAT rule: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - $ETH0_IP 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. If you are running Shorewall 3.2.6 on a Debian-based system, the call to find_first_interface_address in /etc/shorewall/params must be preceded with a load of the Shorewall function library:. /usr/share/shorewall/functions ETH0_IP=`find_first_interface_address eth0`
(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. If the ALL INTERFACES column in /etc/shorewall/nat is empty or contains Yes, 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: 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 Answer: 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. 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. If you don't like those solutions and prefer, incredibly, to route all Z->Z traffic through your firewall then: Set the routeback option on the interface to Z. Set the ALL INTERFACES column in the nat file to Yes. Example: Zone: dmz, Interface: eth2, Subnet: 192.168.2.0/24, Address: 192.168.2.254 In /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS dmz eth2 192.168.2.255 routeback In /etc/shorewall/nat, be sure that you have Yes in the ALL INTERFACES column. In /etc/shorewall/masq: #INTERFACE SOURCE ADDRESS eth2 eth2 192.168.2.254 Like the silly hack in FAQ 2 above, this will make all dmz->dmz traffic appear to originate on the firewall.
(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. Answer: Let's assume the following: External IP address is 206.124.146.176 on eth0 (www.mydomain.com). Server's IP address is 192.168.2.4 You can enable access to the server from your local network using the firewall's external IP address by adding this rule: #ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL # PORT DEST DNAT loc dmz:192.168.2.4 tcp 80 - 206.124.146.176 If your external IP address is dynamic, then you must do the following: In /etc/shorewall/params (or in your <export directory>/init file if you are using Shorewall Lite on the firewall system): ETH0_IP=`find_first_interface_address eth0` and make your DNAT rule: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc dmz:192.168.2.4 tcp 80 - $ETH0_IP With dynamic IP addresses, you probably don't want to use shorewall[-lite] save and shorewall[-lite] restore. If you are running Shorewall 3.2.6 on a Debian-based system, the call to find_first_interface_address in /etc/shorewall/params must be preceded with a load of the Shorewall function library:. /usr/share/shorewall/functions ETH0_IP=`find_first_interface_address eth0`
(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? Answer: Did you set IP_FORWARDING=On in shorewall.conf?
Blacklisting
(FAQ 63) I just blacklisted IP address 206.124.146.176 and I can still ping it. What did I do wrong? Answer: Nothing. Blacklisting an IP address blocks incoming traffic from that IP address. And if you set BLACKLISTNEWONLY=Yes in shorewall.conf, then only new connections from that address are disallowed; traffic from that address that is part of an established connection (such as ping replies) is allowed.
Netmeeting/MSN
(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with Shorewall. What do I do? Answer: There is an H.323 connection tracking/NAT module that helps with Netmeeting. Note however that one of the Netfilter developers recently posted the following:
> I know PoM -ng is going to address this issue, but till it is ready, and > all the extras are ported to it, is there any way to use the h.323 > conntrack module kernel patch with a 2.6 kernel? > Running 2.6.1 - no 2.4 kernel stuff on the system, so downgrade is not > an option... The module is not ported yet to 2.6, sorry. > Do I have any options besides a gatekeeper app (does not work in my > network) or a proxy (would prefer to avoid them)? I suggest everyone to setup a proxy (gatekeeper) instead: the module is really dumb and does not deserve to exist at all. It was an excellent tool to debug/develop the newnat interface.
Look here 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 http://www.netfilter.org.
Open Ports
(FAQ 51) How do I Open Ports in Shorewall? Answer: No one who has installed Shorewall using one of the Quick Start Guides should have to ask this question. Regardless of which guide you used, all outbound communication is open by default. So you do not need to 'open ports' for output. For input: If you installed using the Standalone Guide, then please re-read this section. If you installed using the Two-interface Guide, then please re-read these sections: Port Forwarding (DNAT), and Other Connections If you installed using the Three-interface Guide, then please re-read these sections: Port Forwarding (DNAT) and Other Connections If you installed using the Shorewall Setup Guide then you had better read the guide again -- you clearly missed a lot. Also please see the Port Forwarding section of this FAQ.
(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? Answer: The default Shorewall setup invokes the Drop 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 /usr/share/shorewall/action.Drop which in turn invokes the Auth macro (defined in /usr/share/shorewall/macro.Auth) specifying the REJECT action (i.e., Auth/REJECT). This is necessary to prevent outgoing connection problems to services that use the Auth mechanism for identifying requesting users. That is the only service which the default setup rejects. 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.
(FAQ 4a) I just ran an nmap UDP scan of my firewall and it showed 100s of ports as open!!!! Answer: Take a deep breath and read the nmap man page section about UDP scans. If nmap gets nothing 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 do the nmap UDP scan again.
(FAQ 4b) I have a port that I can't close no matter how I change my rules. 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!!! Answer: 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.
(FAQ 4c) How do I use Shorewall with PortSentry? Answer: Here's a writeup describing a nice integration of Shorewall and PortSentry.
Connection Problems
Why are these packets being Dropped/Rejected? How do I decode Shorewall log messages? Please see FAQ 17.
(FAQ 5) I've installed Shorewall and now I can't ping through the firewall Answer: For a complete description of Shorewall ping management, see this page.
(FAQ 15) My local systems can't see out to the net Answer: Every time I read systems can't see out to the net, I wonder where the poster bought computers with eyes and what those computers will see when things are working properly. That aside, the most common causes of this problem are: The default gateway on each local system isn't set to the IP address of the local firewall interface. The entry for the local network in the /etc/shorewall/masq file is wrong or missing. 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. Forwarding is not enabled (This is often the problem for Debian users). Enter this command: cat /proc/sys/net/ipv4/ip_forward If the value displayed is 0 (zero) then set IP_FORWARDING=On in /etc/shorewall/shorewall.conf and restart Shorewall.
(FAQ 29) FTP Doesn't Work Answer: See the Shorewall and FTP page.
(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. Answer: Most likely, you need to set CLAMPMSS=Yes in /etc/shorewall/shorewall.conf.
(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? Answer: Add the routeback option to br0 in /etc/shorewall/interfaces. For more information on this type of configuration, see the Shorewall Simple Bridge documentation.
(FAQ 64) I just upgraded my kernel to 2.6.20 and my bridge/firewall stopped working. What is wrong? Answer: In kernel 2.6.20, the Netfilter physdev match 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: 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. This kernel change, while necessary, means that Shorewall zones may no longer be defined in terms of bridge ports. See the new Shorewall-shell bridging documentation for information about configuring a bridge/firewall under kernel 2.6.20 and later with Shorewall shell or the Shorewall-perl bridging documentation if you use Shorewall-perl (highly-recommended). Following the instructions in the new bridging documentation will not prevent the above message from being issued.
Logging
(FAQ 6) Where are the log messages written and how do I change the destination? Answer: NetFilter uses the kernel's equivalent of syslog (see man syslog) to log messages. It always uses the LOG_KERN (kern) facility (see man openlog) and you get to choose the log level (again, see man syslog) in your policies and rules. The destination for messages logged by syslog is controlled by /etc/syslog.conf (see man syslog.conf). When you have changed /etc/syslog.conf, be sure to restart syslogd (on a RedHat system, service syslog restart). By default, older versions of Shorewall rate-limited log messages through settings in /etc/shorewall/shorewall.conf -- If you want to log all messages, set: LOGLIMIT="" LOGBURST="" It is also possible to set up Shorewall to log all of its messages to a separate file.
(FAQ 6a) Are there any log parsers that work with Shorewall? Answer: Here are several links that may be helpful: http://www.shorewall.net/pub/shorewall/parsefw/ http://aaron.marasco.com/linux.html http://cert.uni-stuttgart.de/projects/fwlogwatch http://www.logwatch.org I personally use Logwatch. It emails me a report each day from my various systems with each report summarizing the logged activity on the corresponding system. I use the brief report format; here's a sample:
--------------------- iptables firewall Begin ------------------------ Dropped 111 packets on interface eth0 From 58.20.162.142 - 5 packets to tcp(1080) From 62.163.19.50 - 1 packet to udp(6348) From 66.111.45.60 - 9 packets to tcp(192) From 69.31.82.50 - 18 packets to tcp(3128) From 72.232.183.102 - 2 packets to tcp(3128) From 82.96.96.3 - 6 packets to tcp(808,1080,1978,7600,65506) From 128.48.51.209 - 5 packets to tcp(143) From 164.77.223.150 - 12 packets to tcp(873) From 165.233.109.23 - 8 packets to tcp(22) From 202.99.172.175 - 4 packets to udp(2,4081) From 206.59.41.101 - 2 packets to tcp(5900) From 217.91.30.224 - 24 packets to tcp(873) From 218.87.47.114 - 6 packets to tcp(3128) From 220.110.219.234 - 4 packets to tcp(22) From 220.133.116.173 - 5 packets to tcp(3128) ---------------------- iptables firewall End -------------------------
(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? Answer: Temporarily add the following rule: #ACTION SOURCE DEST PROTO DEST PORT(S) DROP net fw udp 10619 Alternatively, if you do not set BLACKLIST_LOGLEVEL and you have specifed the 'blacklist' option on your external interface in /etc/shorewall/interfaces, then you can blacklist the port. In /etc/shorewall/blacklist: #ADDRESS/SUBNET PROTOCOL PORT - udp 10619
(FAQ 6d) Why is the MAC address in Shorewall log messages so long? I thought MAC addresses were only 6 bytes in length. Answer: What is labeled as the MAC address in a Netfilter (Shorewall) log message is actually the Ethernet frame header. It contains: the destination MAC address (6 bytes) the source MAC address (6 bytes) the Ethernet frame type (2 bytes) Example MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00 Destination MAC address = 00:04:4c:dc:e2:28 Source MAC address = 00:b0:8e:cf:3c:4c Ethernet Frame Type = 08:00 (IP Version 4)
(FAQ 16) Shorewall is writing log messages all over my console making it unusable! Answer: Just to be clear, it is not Shorewall that is writing all over your console. Shorewall issues a single log message during each start, restart, stop, etc. It is rather the klogd daemon that is writing messages to your console. Shorewall itself has no control over where a particular class of messages are written. See the Shorewall logging documentation. The max log level to be sent to the console is available in /proc/sys/kernel/printk:teastep@ursa:~$ cat /proc/sys/kernel/printk 6 6 1 7 teastep@ursa:~$ The first number determines the maximum log level (syslog priority) sent to the console. Messages with priority less than 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 will not appear on the console. The second number is the default log level for kernel printk() calls that do not specify a log level. The third number specifies the minimum console log level while the fourth gives the default console log level. 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 /etc/sysctl.conf file:kernel.printk = 4 4 1 7 thensysctl -p /etc/sysctl.conf
(FAQ 16a) Why can't I see any Shorewall messages in /var/log/messages? Some people who ask this question report that the only Shorewall messages that they see in /var/log/messages are 'started', 'restarted' and 'stopped' messages. Answer: First of all, it is important to understand that Shorewall itself does not control where Netfilter log messages are written. The LOGFILE setting in shorewall.conf simply tells the /sbin/shorewall[-lite] 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 /var/log than a log level of "info". The log level does not control the number of log messages or the content of the messages. 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 SUSE releases use syslog-ng by default and write Shorewall messages to /var/log/firewall. Please see the Shorewall logging documentation for further information.
(FAQ 17) Why are these packets being Dropped/Rejected? How do I decode Shorewall log messages? Answer: Logging of dropped/rejected packets occurs out of a number of chains (as indicated in the log message) in Shorewall: man1918 or logdrop The destination address is listed in /usr/share/shorewall/rfc1918 with a logdrop target -- see /usr/share/shorewall/rfc1918 . rfc1918 or logdrop The source or destination address is listed in /usr/share/shorewall/rfc1918 with a logdrop target -- see /usr/share/shorewall/rfc1918 . If you see packets being dropped in the rfc1918 chain and neither the source nor the destination IP address is reserved by RFC 1918, that usually means that you have a old rfc1918 file in /etc/shorewall (this problem most frequently occurs if you are running Debian or one if its derivatives). The rfc1918 file used to include bogons as well as the three ranges reserved by RFC 1918 and it resided in /etc/shorewall. The file now only includes the three RFC 1918 ranges and it resides in /usr/share/shorewall. Remove the stale rfc1918 file in /etc/shorewall. all2zone, zone2all or all2all You have a policy 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 rule to that effect. Beginning with Shorewall 3.3.3, packets logged out of these chains may have a source and/or destination that is not in any defined zone (see the output of shorewall[-lite] show zones). Remember that zone membership involves both a firewall interface and an ip address. zone12zone2 Either you have a policy for zone1 to zone2 that specifies a log level and this packet is being logged under that policy or this packet matches a rule that includes a log level. @source2dest You have a policy for traffic from source to dest 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. interface_mac or interface_rec The packet is being logged under the maclist interface option. blacklist The packet is being logged because the source IP is blacklisted in the /etc/shorewall/blacklist file. INPUT or FORWARD The packet has a source IP address that isn't in any of your defined zones (shorewall[-lite] show zones 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, then you probably need the routeback option on that interface in /etc/shorewall/interfaces , you need the routeback option in the relevant entry in /etc/shorewall/hosts or you've done something silly like define a default route out of an internal interface. In Shorewall 3.3.3 and later versions with OPTIMIZE=1 in shorewall.conf, such packets may also be logged out of a <zone>2all chain or the all2all chain. OUTPUT The packet has a destination IP address that isn't in any of your defined zones(shorewall[-lite] show zones and look at the printed zone definitions). In Shorewall 3.3.3 and later versions with OPTIMIZE=1 in shorewall.conf, such packets may also be logged out of the fw2all chain or the all2all chain. logflags The packet is being logged because it failed the checks implemented by the tcpflags interface option. Here is an example: Jun 27 15:37:56 gateway kernel: Shorewall:all2all:REJECT:IN=eth2 OUT=eth1 SRC=192.168.2.2 DST=192.168.1.3 LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF PROTO=UDP SPT=1803 DPT=53 LEN=47 Let's look at the important parts of this message: all2all:REJECT This packet was REJECTed out of the all2all chain -- the packet was rejected under the all->all REJECT policy (all2all above). IN=eth2 the packet entered the firewall via eth2. If you see IN= with no interface name, the packet originated on the firewall itself. OUT=eth1 if accepted, the packet would be sent on eth1. If you see OUT= with no interface name, the packet would be processed by the firewall itself. 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 original 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. SRC=192.168.2.2 the packet was sent by 192.168.2.2 DST=192.168.1.3 the packet is destined for 192.168.1.3 PROTO=UDP UDP Protocol DPT=53 The destination port is 53 (DNS) In this case, 192.168.2.2 was in the dmz zone and 192.168.1.3 is in the loc zone. I was missing the rule: ACCEPT dmz loc udp 53
(FAQ 21) I see these strange log entries occasionally; what are they? 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 PROTO=ICMP TYPE=3 CODE=3 [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 ] 192.0.2.3 is external on my firewall... 172.16.0.0/24 is my internal LAN Answer: 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 FAQ 17). While most people associate the Internet Control Message Protocol (ICMP) with ping, 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. 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. 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. I have also seen cases where the source IP in the ICMP itself isn't set back to the external IP of the remote NAT gateway; that causes your firewall to log and drop the packet out of the rfc1918 chain because the source IP is reserved by RFC 1918.
(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? I blacklisted the address 130.252.100.59 using shorewall drop 130.252.100.59 but I am still seeing these log messages: 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 Answer: Please refer to the Shorewall Netfilter Documentation. 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.
(FAQ 56) When I start or restart Shorewall, I see these messages in my log. Are they harmful?
modprobe: Can't locate module ipt_physdev modprobe: Can't locate module iptable_raw
Answer: No. These occur when Shorewall probes your system to determine the features that it support. They are completely harmless.
(FAQ 81) logdrop and logreject don't log. 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. Answer: You haven't set a value for BLACKLIST_LOGLEVEL in shorewall.conf (5).
Routing
(FAQ 32) My firewall has two connections to the Internet from two different ISPs. How do I set this up in Shorewall? Answer: See this article on Shorewall and Multiple ISPs.
(FAQ 49) When I start Shorewall, my routing table gets blown away. Why does Shorewall do that? Answer: This is usually the consequence of a one-to-one nat configuration blunder: Specifying the primary IP address for an interface in the EXTERNAL column of /etc/shorewall/nat even though the documentation (and the comments in the file) warn you not to do that. Specifying ADD_IP_ALIASES=Yes and RETAIN_ALIASES=No in /etc/shorewall/shorewall.conf. 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 not specify the primary IP address of an interface in the EXTERNAL column.
Starting and Stopping
(FAQ 7) When I stop Shorewall using <quote>shorewall[-lite] stop</quote>, I can't connect to anything. Why doesn't that command work? Answer: The stop command is intended to place your firewall into a safe state whereby only those hosts listed in /etc/shorewall/routestopped are activated. If you want to totally open up your firewall, you must use the shorewall[-lite] clear command.
(FAQ 8) When I try to start Shorewall on RedHat, I get messages about insmod failing -- what's wrong? Answer: The output you will see looks something like this: /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?) Perhaps iptables or your kernel needs to be upgraded. This problem is usually corrected through the following sequence of commands service ipchains stop chkconfig --delete ipchains rmmod ipchains
(FAQ 8a) When I try to start Shorewall on RedHat I get a message referring me to FAQ #8 Answer: This is usually cured by the sequence of commands shown above in .
(FAQ 9) Why can't Shorewall detect my interfaces properly at startup? I just installed Shorewall and when I issue the start command, I see the following: 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... Net Zone: eth0:0.0.0.0/0 Local Zone: eth1:0.0.0.0/0 Deleting user chains... Creating input Chains... ... Why can't Shorewall detect my interfaces properly? Answer: The above output is perfectly normal. The Net zone is defined as all hosts that are connected through eth0 and the local zone is defined as all hosts connected through eth1. You can set the routefilter option on an internal interface if you wish to guard against 'Martians' (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 logmartians option.
(FAQ 22) I have some iptables commands that I want to run when Shorewall starts. Which file do I put them in? Answer:You can place these commands in one of the Shorewall Extension Scripts. 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 man iptables and look at the -I (--insert) command.
(FAQ 34) How can I speed up Shorewall start (restart)? Answer: Switch to using Shorewall-perl.
(FAQ 69) When I restart Shorewall, new connections are blocked for a long time. Is there a way to avoid that? Answer: Switch to using Shorewall-perl.
(FAQ 43) I just installed the Shorewall RPM and Shorewall doesn't start at boot time. Answer: 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.
(FAQ 45) Why does "shorewall[-lite] start" fail when trying to set up SNAT/Masquerading? shorewall start produces the following output: … Processing /etc/shorewall/policy... Policy ACCEPT for fw to net using chain fw2net Policy ACCEPT for loc0 to net using chain loc02net Policy ACCEPT for loc1 to net using chain loc12net Policy ACCEPT for wlan to net using chain wlan2net Masqueraded Networks and Hosts: iptables: Invalid argument ERROR: Command "/sbin/iptables -t nat -A …" Failed Answer: 99.999% of the time, this error is caused by a mismatch between your iptables and kernel. Your iptables must be compiled against a kernel source tree that is Netfilter-compatible with the kernel that you are running. If you rebuild iptables using the defaults and install it, it will be installed in /usr/local/sbin/iptables. As shown above, you have the IPTABLES variable in shorewall.conf set to "/sbin/iptables".
(FAQ 59) After I start Shorewall, there are lots of unused Netfilter modules loaded. How do I avoid that? Answer: Copy /usr/share/shorewall[-lite]/modules to /etc/shorewall/modules and modify the copy to include only the modules that you need.
(FAQ 61) I just installed the latest Debian kernel and now "shorewall start" fails with the message "ipt_policy: matchsize 116 != 308". What's wrong? Answer: Your iptables is incompatible with your kernel. Either rebuild iptables using the kernel headers that match your new kernel; or if you don't need policy match support (you are not using the IPSEC implementation builtinto the 2.6 kernel) then you can rename /lib/iptables/libipt_policy.so. Beginning with Shorewall 3.4.0, Shorewall no longer attempts to use policy match if you have no IPSEC zones and you have not specified the option on any entry in /etc/shorewall/hosts. The subject message will still appear in your kernel log each time that Shorewall determines the capabilities of your kernel/iptables.
(FAQ 62) I have unexplained 30-second pauses during "shorewall [re]start". What causes that? Answer: This usually happens when the firewall uses LDAP Authentication. The solution is to list your LDAP server(s) as critical in /etc/shorewall/routestopped.
(FAQ 68) I have a VM under an OpenVZ system. I can't get rid of the following message: ERROR: Command "/sbin/iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT" failed. Answer: At a root shell prompt, type the iptables command shown in the error message. If the command fails, you OpenVZ Netfilter/iptables configuration is incorrect. Until that command can run without error, no stateful iptables firewall will be able to run in your VM.
(FAQ 73) When I stop Shorewall, the firewall is wide open. Isn't that a security risk? It is important to understand that the scripts in /etc/init.d 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 /sbin/shorewall stop and /etc/init.d/shorewall stop. /sbin/shorewall stop places the firewall in a safe state, the details of which depend on your /etc/shorewall/routestopped file (shorewall-routestopped(8)) and on the setting of ADMINISABSENTMINDED in /etc/shorewall/shorewall.conf (shorewall.conf(8)). /etc/init.d/shorewall stop may or may not do the same thing. In the case of Debian systems for example, that command actually executes /sbin/shorewall clear which opens the firewall completely. In other words, in the init script's stop reverses the effect of start. One way to avoid these differences is to install Shorewall from the tarballs available from shorewall.net. This places Shorewall outside of the control of the packaging system and provides consistent behavior between the init scripts and /sbin/shorewall (and /sbin/shorewall-lite). For more information on the factors involved when deciding whether to use the Debian package, see this article.
(FAQ 74) When I "<command>shorewall start</command>" or "<command>shorewall check</command>" on my SuSE 10.0 system, I get FATAL ERROR messages and/or the system crashes" Answer: These failures result from trying to load a particular combination of kernel modules. To work around the problem: Copy /usr/share/shorewall/modules to /etc/shorewall/modules Edit /etc/shorewall/modules and remove all entries except for those for the helper modules that you need.
(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 restart' it then works, despite the config not changing Answer: Set IP_FORWARDING=On in /etc/shorewall/shorewall.conf.
Multiple ISPs
(FAQ 57) I configured two ISPs in Shorewall but when I try to use the second one, it doesn't work. Answer: The Multi-ISP Documentation strongly recommends that you use the balance 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 routefilter option on the other interface(s) in /etc/shorewall/interfaces and disable any IP Address Spoofing protection that your distribution supplies.
(FAQ 58) But if I specify 'balance' then won't Shorewall balance the traffic between the interfaces? I don't want that! Answer: 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 /etc/shorewall/tcrules file: #MARK SOURCE DEST 1:P 0.0.0.0/0 1 $FW other MARK rules 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 balance is specified or not!
Using DNS Names
(FAQ 79) Can I use DNS names in Shorewall configuration file entries in place of IP addresses? Answer: Yes, but we advise strongly against it.
Traffic Shaping
(FAQ 67) I just configured Shorewall's builtin traffic shaping and now Shorewall fails to Start. The error I receive is as follows: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" FailedAnswer: 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 act_police kernel module. Other kernel modules that you will need include: cls_u32 sch_htb sch_ingress sch_sfq
About Shorewall
(FAQ 10) What Distributions does Shorewall work with? Answer: Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.
(FAQ 11) What Features does Shorewall have? Answer: See the Shorewall Feature List.
(FAQ 12) Is there a GUI? Answer: Yes! Shorewall 3.x support is available in Webmin 1.300. See http://www.webmin.com
(FAQ 13) Why do you call it <quote>Shorewall</quote>? Answer: Shorewall is a concatenation of Shoreline (the city where I live) and Firewall . The full name of the product is actually Shoreline Firewall but Shorewall is much more commonly used.
(FAQ 23) Why do you use such ugly fonts on your web site? Answer: 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.
(FAQ 25) How do I tell which version of Shorewall or Shorewall Lite I am running? Answer: At the shell prompt, type: /sbin/shorewall[-lite] version
(FAQ 25a) How do I tell which version of Shorewall-perl and Shorewall-shell that I have installed? Answer: At the shell prompt, type: /sbin/shorewall version -a
(FAQ 31) Does Shorewall provide protection against.... IP Spoofing: Sending packets over the WAN interface using an internal LAP IP address as the source address? Answer: Yes. Tear Drop: Sending packets that contain overlapping fragments? Answer: 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. Smurf and Fraggle: Sending packets that use the WAN or LAN broadcast address as the source address? Answer: Shorewall can be configured to do that using the blacklisting facility. Shorewall versions 2.0.0 and later filter these packets under the nosmurfs interface option in /etc/shorewall/interfaces. Land Attack: Sending packets that use the same address as the source and destination address? Answer: Yes, if the routefilter interface option is selected. DOS: - SYN Dos - ICMP Dos - Per-host Dos protection Answer: Shorewall has facilities for limiting SYN and ICMP packets. Netfilter as included in standard Linux kernels doesn't support per-remote-host limiting except by explicit rule that specifies the host IP address; that form of limiting is supported by Shorewall.
(FAQ 65) How do I accomplish failover with Shorewall? Answer: This article by Paul Gear should help you get started.
(FAQ 80) Does Shorewall support IPV6? Answer: Shorewall IPv6 support is currently available in Shorewall 4.2.4 and later.
(FAQ 80a) Why does Shorewall lPv6 Support Require Kernel 2.6.25 or later? Answer: Shorewall implements a stateful firewall which requires connection tracking be present in ip6tables and in the kernel. Linux kernel's before 2.6.20 didn't have connection tracking for IPv6. So we could not even start to develop IPv6 support until 2.6.20. We understand that 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 it has been tested on. If you are running 2.6.20 or later, you can try to run Shorewall6 by hacking /usr/share/shorewall-perl/prog.footer6 and changing the kernel version test to check for your kernel version rather than 2.6.25 (20625). But after that, you are on your own. kernel=$(printf "%2d%02d%02d\n" $(echo $(uname -r) 2> /dev/null | sed 's/-.*//' | tr '.' ' ' ) | head -n1) if [ $kernel -lt 20625 ]; then error_message "ERROR: $PRODUCT requires Linux kernel 2.6.25 or later" status=2 else
RFC 1918
(FAQ 14) I'm connected via a cable modem and it has an internal web server that allows me to configure/monitor it but as expected if I enable rfc1918 blocking for my eth0 interface (the Internet one), it also blocks the cable modems web server. Is there any way it can add a rule before the rfc1918 blocking that will let all traffic to and from the 192.168.100.1 address of the modem in/out but still block all other rfc1918 addresses? Answer: Use of the norfc1918 interface is currently deprecated and support for the option will be removed entirely in a future version. So deleting the option from shorewall-interfaces (5) is the preferred solution. Otherwise, add the following to /etc/shorewall/rfc1918 (Note: If you are running Shorewall 2.0.0 or later, you may need to first copy /usr/share/shorewall/rfc1918 to /etc/shorewall/rfc1918): Be sure that you add the entry ABOVE the entry for 192.168.0.0/16. #SUBNET TARGET 192.168.100.1 RETURN If you add a second IP address to your external firewall interface to correspond to the modem address, you must also make an entry in /etc/shorewall/rfc1918 for that address. For example, if you configure the address 192.168.100.2 on your firewall, then you would add two entries to /etc/shorewall/rfc1918: #SUBNET TARGET 192.168.100.1 RETURN 192.168.100.2 RETURN
(FAQ 14a) Even though it assigns public IP addresses, my ISP's DHCP server has an RFC 1918 address. If I enable RFC 1918 filtering on my external interface, my DHCP client cannot renew its lease. Answer: The solution is the same as above.
(FAQ 14b) I connect to the Internet with PPPoE. When I try to access the built-in web server in my DSL Modem, I get connection Refused. I see the following in my log: Mar 1 18:20:07 Mail kernel: Shorewall:OUTPUT:REJECT:IN= OUT=eth0 SRC=192.168.1.2 DST=192.168.1.1 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=26774 DF PROTO=TCP SPT=32797 DPT=80 WINDOW=5840 RES=0x00 SYN URGP=0 Answer: The fact that the message is being logged from the OUTPUT chain means that the destination IP address is not in any defined zone (see FAQ 17). You need to: Add a zone for the modem in /etc/shorewall/zones: #ZONE TYPE OPTIONS modem ipv4 Define the zone to be associated with eth0 (or whatever interface connects to your modem) in /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS modem eth0 detect Allow web traffic to the modem in /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT fw modem tcp 80 ACCEPT loc modem tcp 80 Note that many of these ADSL/Cable Modems have no default gateway or their default gateway is at a fixed IP address that is different from the IP address you have assigned to your external interface. In either case, you may have problems browsing the modem from your local network even if you have the correct routes established on your firewall. This is usually solved by masquerading traffic from your local network to the modem. /etc/shorewall/masq: #INTERFACE SOURCE ADDRESS eth0 eth1 # eth1 = interface to local network For an example of this when the ADSL/Cable modem is bridged, see my configuration. In that case, I masquerade using the IP address of my local interface!
Alias IP Addresses/Virtual Interfaces
(FAQ 18) Is there any way to use aliased ip addresses with Shorewall, and maintain separate rule sets for different IPs? Answer: Yes. See Shorewall and Aliased Interfaces.
Shorewall Lite
(FAQ 53) What is Shorewall Lite? Answer: 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 Compiled Firewall script documentation for details.
(FAQ 54) If I want to use Shorewall Lite, do I also need to install Shorewall on the same system? Answer: No. In fact, we recommend that you do NOT 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.
(FAQ 55) How do I decide which product to use - Shorewall or Shorewall Lite? Answer: 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.
(FAQ 60) What are the compatibility restrictions between Shorewall and Shorewall Lite Answer: Beginning with version 3.2.3, there are no compatibility constraints between Shorewall and Shorewall-lite.
Shorewall-Perl
(FAQ 70) What is Shorewall-Perl? Answer: Shorewall-perl is a re-implementation of the Shorewall configuration compiler written in Perl.
(FAQ 71) What are the advantages of using Shorewall-perl? Answer: The Shorewall-perl compiler is much faster than the Shorewall-shell compiler. The script generated by the Shorewall-perl compiler uses iptables-restore to instantiate the Netfilter configuration. So it runs much faster than the script generated by the Shorewall-shell compiler and doesn't disable new connections during rule set installation. The Shorewall-perl compiler does more thorough checking of the configuration than the Shorewall-shell compiler does. The error messages produced by the Shorewall-perl compiler are better, more consistent and always include the file name and line number where the error was detected. Going forward, the Shorewall-perl compiler will get all enhancements; the Shorewall-shell compiler will only get those enhancements that are easy to retrofit.
(FAQ 72) Can I switch to using Shorewall-perl without changing my Shorewall configuration? Answer: Maybe yes, maybe no. See the Shorewall Perl article for a list of the incompatibilities between Shorewall-shell and Shorewall-perl.
VOIP
(FAQ 77) Shorewall is eating my Asterisk egress traffic! 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. Answer (requires Shorewall 4.0.6 or later): If your kernel version is 2.6.20 or earlier:rmmod ip_nat_sip rmmod ip_conntrack_sipThen change the DONT_LOAD specification in your shorewall.conf to:DONT_LOAD=ip_nat_sip,ip_conntrack_sipIf your kernel version is 2.6.21 or later:rmmod nf_nat_sip rmmod nf_conntrack_sipThen change the DONT_LOAD specification in your shorewall.conf to:DONT_LOAD=nf_nat_sip,nf_conntrack_sipIf you are running a version of Shorewall earlier than 4.0.6, you can avoid loading the sip helper modules by following the suggestions in FAQ 59.
Miscellaneous
(FAQ 20) I have just set up a server. Do I have to change Shorewall to allow access to my server from the Internet? Answer: Yes. Consult the QuickStart guide that you used during your initial setup for information about how to set up rules for your server.
(FAQ 24) How can I allow connections to, let's say, the ssh port only from specific IP Addresses on the Internet? Answer: In the SOURCE column of the rule, follow net by a colon and a list of the host/subnet addresses as a comma-separated list. net:<ip1>,<ip2>,... Example: ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22
(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?" Answer: Temporarily remove and rejNotSyn, dropNotSyn and dropInvalid rules from /etc/shorewall/rules and restart Shorewall.
(FAQ 27) I'm compiling a new kernel for my firewall. What should I look out for? Answer: First take a look at the Shorewall kernel configuration page. You probably also want to be sure that you have selected the NAT of local connections (READ HELP) on the Netfilter Configuration menu. Otherwise, DNAT rules with your firewall as the source zone won't work with your new kernel.
(FAQ 27a) I just built (or downloaded or otherwise acquired) and installed a new kernel and now Shorewall won't start. I know that my kernel options are correct. The last few lines of a startup trace are these: + run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE + '[' 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE' = 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0. 0/0 -j MASQUERADE' ']' + run_iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE + iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE iptables: Invalid argument + '[' -z '' ']' + stop_firewall + set +x Answer: Your new kernel contains headers that are incompatible with the ones used to compile your iptables utility. You need to rebuild iptables using your new kernel source.
(FAQ 28) How do I use Shorewall as a Bridging Firewall? Answer: Shorewall Bridging Firewall support is available — check here for details.
(FAQ 39) How do I block connections to a particular domain name? 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: #ACTION SOURCE DEST PROTO REJECT fw net:pagead2.googlesyndication.com all However, this also sometimes restricts access to "google.com". Why is that? Using dig, I found these IPs for domain googlesyndication.com:216.239.37.99 216.239.39.99And this for google.com:216.239.37.99 216.239.39.99 216.239.57.99So 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? Answer: 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. When you specify a domain name in a Shorewall rule, 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: #ACTION SOURCE DEST PROTO REJECT fw net:216.239.37.99 all REJECT fw net:216.239.39.99 allGiven 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 FTP 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 Squid. 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.
(FAQ 42) How can I tell which features my kernel and iptables support? Answer: Use the shorewall[-lite] show capabilities command at a root prompt. gateway:~# shorewall show capabilities Loading /usr/share/shorewall/functions... Processing /etc/shorewall/params ... Processing /etc/shorewall/shorewall.conf... Loading Modules... 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 Packet Type Match: Available Policy Match: Available Physdev Match: Available IP range Match: Available Recent Match: Available Owner Match: Available Ipset Match: Available ROUTE Target: Available Extended MARK Target: Available CONNMARK Target: Available Connmark Match: Available Raw Table: Available gateway:~#
(FAQ 19) How do I open the firewall for all traffic to/from the LAN? Answer: Add these two policies: #SOURCE DESTINATION POLICY LOG LIMIT:BURST # LEVEL $FW loc ACCEPT loc $FW ACCEPT You should also delete any ACCEPT rules from $FW->loc and loc->$FW since those rules are redundant with the above policies.