Shorewall FAQs Shorewall Community Tom Eastep 2006-05-18 2001-2006 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 empty!!! Once you have installed the .deb package and before you attempt to configure Shorewall, please heed the advice of Lorenzo Martignoni, the Shorewall Debian Maintainer: For more information about Shorewall usage on Debian system please look at /usr/share/doc/shorewall/README.Debian provided by [the] shorewall Debian package. If you install using the .deb, you will find that your /etc/shorewall directory is empty. This is intentional. The released configuration file skeletons may be found on your system in the directory /usr/share/doc/shorewall/default-config. Simply copy the files you need from that directory to /etc/shorewall and modify the copies. Note that you must copy /usr/share/doc/shorewall/default-config/shorewall.conf and /usr/share/doc/shorewall/default-config/modules to /etc/shorewall even if you do not modify those files.
(FAQ 44) I can't install/upgrade the RPM — I keep getting the message "error: failed dependencies:iproute is needed..." Answer: Read the Installation Instructions!
(FAQ 50) When I install/upgrade I get multiple instance of the message "warning: user teastep does not exist - using root" Answer: You may safely ignore this warning message. It was caused by a minor packaging error that has since been corrected. It makes no difference to Shorewall's operation.
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 first example in the rules file documentation shows how to do port forwarding under Shorewall. 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 #> 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 #> - <external IP> 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) Ok -- 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 should be set to the IP address of your firewall's internal interface). Your ISP is blocking that particular port inbound. 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 iptables -t nat -Z . This clears the NetFilter counters in the nat table. Try to connect to the redirected port from an external host. As root type shorewall show nat 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 promt, type "shorewall show zones" then be sure that you have specified in the DEST column the first zone in the list that matches the OUT=<dev> and the DEST= <ip>from the REJECT/DROP log message.
(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? 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: Let's assume the following: External IP address is 206.124.146.176 on eth0. 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: ETH0_IP=`find_interface_address eth0` For users of Shorewall 2.1.0 and later: 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
(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 30) I'm confused about when to use DNAT rules and when to use ACCEPT rules. 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. 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? Ian Allen has written a Paper about DNAT and Linux.
(FAQ 48) How do I Set up Transparent 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 Bind Version 9 views (or using a separate DNS server for local clients) 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. 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: All traffic redirected through use of this hack will look to the server as if it came from the firewall (192.168.1.254) rather than from the original client! In /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS loc eth1 detect routeback In /etc/shorewall/masq: #INTERFACE SUBNET 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: 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 client to automatically restart Shorewall each time that you get a new IP address.
(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 hosts'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 Bind Version 9 views. 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 to stupidly 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 SUBNETS ADDRESS eth2 eth2 192.168.2.254 Like the idiotic 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: 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 save and shorewall restore.
(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?
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 > contrack 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 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 zone 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 DROP action (i.e., Auth/DROP). 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 firerwall will block that attempt.
(FAQ 4c) How do I use Shorewall with PortSentry? Here's a writeup describing a nice integration of Shorewall and PortSentry.
(FAQ 51) How do I "Open a Port" with Shorewall Answer: It depends… If the application serving the port is running on the same system as Shorewall then add this rule: #ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT net $FW <protocol> <port number> Where <protocol> is either tcp or udp and <port number> is the port that you wish to "open". If the application serving the port is running on one of the systems in your local network then please see FAQ 1.
Connection Problems
(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 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 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 /etc/shorewall/interfaces; 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.
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://www.fireparse.com http://cert.uni-stuttgart.de/projects/fwlogwatch http://www.logwatch.org http://gege.org/iptables http://home.regit.org/ulogd-php.html 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.
(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? Temporarily add the following rule: DROP net fw 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. 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: Find where klogd is being started (it will be from one of the files in /etc/init.d -- sysklogd, klogd, ...). Modify that file or the appropriate configuration file so that klogd is started with -c <n> where <n> is a log level of 5 or less; or See the dmesg man page (man dmesg). You must add a suitable dmesg command to your startup scripts or place it in /etc/shorewall/start. Under RedHat and Mandriva, the max log level that is sent to the console is specified in /etc/sysconfig/init in the LOGLEVEL variable. Set LOGLEVEL=5 to suppress info (log level 6) messages on the console. Under Debian, you can set KLOGD=-c 5 in /etc/init.d/klogd to suppress info (log level 6) messages on the console. Under SUSE, add -c 5 to KLOGD_PARAMS in /etc/sysconfig/syslog to suppress info (log level 6) messages on the console.
(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. all2<zone>, <zone>2all 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. <zone1>2<zone2> 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. @<source>2<dest> 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 The packet is being logged under the maclist interface option. logpkt The packet is being logged under the logunclean interface option. badpkt The packet is being logged under the dropunclean interface option as specified in the LOGUNCLEAN setting in /etc/shorewall/shorewall.conf . blacklst 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 check 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 or you need the routeback option in the relevant entry in /etc/shorewall/hosts . OUTPUT The packet has a destination IP address that isn't in any of your defined zones("shorewall check" and look at the printed zone definitions). 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. 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) For additional information about the log message, see http://logi.cc/linux/netfilter-log-format.php3. 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: 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 a lot of 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 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.
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 Routing.
(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 stop</quote>, I can't connect to anything. Why doesn't that command work? 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 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 Also, be sure to check the errata for problems concerning the version of iptables (v1.2.3) shipped with RH7.2.
(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. If you are running Shorewall 1.4.10 or later, you can consider setting the detectnets interface option on your local interface (eth1 in the above example). That will cause Shorewall to restrict the local zone to only those networks routed through that interface.
(FAQ 22) I have some iptables commands that I want to run when Shorewall starts. Which file do I put them in? 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 to be sure that the commands will do what they are 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 start (restart)? Using a light-weight shell such as ash can dramatically decrease the time required to start or restart Shorewall. See the SHOREWALL_SHELL variable in shorewall.conf . Use a fast terminal emulator -- in particular the KDE konsole scrolls much faster than the Gnome terminal. Also use the '-q' option if you are restarting remotely or from a slow terminal (or redirect the output to a file as in shorewall restart > /dev/null). Upgrade your hardware. Many people find that even a modest increase in CPU and memory speed (e.g. from P3 with SDRAM to P4 with DDR) helps dramatically. EM64T-capable CPUs (from either AMD or Intel) exhibit quite acceptable restart speeds, even with a fairly complex ruleset. Shorewall also supports a fast start capability. To use this capability: With Shorewall in the started state, run shorewall save. This creates the script /var/lib/shorewall/restore. Use the -f option to the start command (e.g., shorewall -f start). This causes Shorewall to look for the /var/lib/shorewall/restore script and if that script exists, it is run. Running /var/lib/shorewall/restore takes much less time than a full shorewall start. The /etc/init.d/shorewall script that is run at boot time uses the -f option. The /var/lib/shorewall/restore script can be run any time to restore the firewall. The script may be run directly or it may be run indirectly using the shorewall restore command. If you change your Shorewall configuration, you must execute a shorewall start (without -f) or shorewall restart prior to doing another shorewall save. The shorewall save command saves the currently running configuration and not the one reflected in your updated configuration files. Likewise, if you change your Shorewall configuration then once you are satisfied that it is working properly, you must do another shorewall save. Otherwise at the next reboot, you will revert to the old configuration stored in /var/lib/shorewall/restore. Finally, the time that new connections are blocked during shorewall restart can be dramatically reduced by upgrading to Shorewall 3.2 or later. In 3.2 and later releases, shorewall [re]start proceeds in two phases: The current configuration is compiled to produce a shell program taylored for your configuration. If compilation is error-free, the compiled program is run to [re]start your firewall.
(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 run-levels that you run your firewall system at.
(FAQ 45) Why does "shorewall 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".
About Shorewall
(FAQ 10) What Distributions does it work with? Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.
(FAQ 11) What Features does it have? Answer: See the Shorewall Feature List.
(FAQ 12) Is there a GUI? Answer: Yes. Shorewall support is included in Webmin 1.060 and later versions. 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? 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 to I tell which version of Shorewall I am running? At the shell prompt, type: /sbin/shorewall version
(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 36) Does Shorewall Work with the 2.6 Linux Kernel? Shorewall works with the 2.6 Kernels with a couple of caveats: Netfilter/iptables doesn't fully support IPSEC in the 2.6 Kernels prior to 2.6.16 -- kernel and iptables patches are available and the details may be found at the Shorewall IPSEC-2.6 page. The 2.6 Kernels do not provide support for the logunclean and dropunclean options in /etc/shorewall/interfaces. Note that support for those options was also removed from Shorewall in version 2.0.0.
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: 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. The solution is the same as above. Simply substitute the IP address of your ISPs DHCP server.
(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 SUBNET 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 rulesets for different IPs? Answer: Yes. See Shorewall and Aliased Interfaces.
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? 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 conections to let's say the ssh port only from specific IP Addresses on the internet? 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?" 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? 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? 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 examiniation 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 Patch-O-Matic. 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 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 can also delete any ACCEPT rules from $FW->loc and loc->$FW since those rules are redundant with the above policies.