Copyright © 2001-2009 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 ”.
Table of Contents
/etc/shorewall/interfaces; the bridged Ethernet interfaces are not defined to Shorewall. How do I tell Shorewall to allow traffic through the bridge?
This article applies to Shorewall 4.3 and later. If you are running a version of Shorewall earlier than Shorewall 4.3.0 then please see the documentation for that release.
Answer: Check out the QuickStart Guides.
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
Simply copy the files you need from that directory to
/etc/shorewall and modify the
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.
Answer: Please see the upgrade issues.
Answer: Please see the upgrade issues.
Answer: Please see the upgrade issues.
Answer:In Shorewall 4.4, the shorewall-shell package was discontinued. The shorewall-common and shorewall-perl packages were combined to form a single shorewall package. For further information, please see the upgrade issues..
Answer: Please see the upgrade issues.
Answer: Please see the upgrade issues.
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
contains the Debian default setting IP_FORWARDING=Keep; it should be
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.
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 the section called “(FAQ 2) I port forward www requests to www.mydomain.com (IP 18.104.22.168) to system 192.168.1.5 in my local network. External clients can browse http://www.mydomain.com but internal clients can't.”).
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.
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.
#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.3:22 tcp 1022
Answer: See FAQ 2b.
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
Answer: Let's take an example. Suppose that
Your Shorewall firewall's external IP address is 22.214.171.124 (eth0) and its internal IP address is 192.168.1.1 (eth1).
You have another gateway router with external IP address 126.96.36.199 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 - 188.8.131.52
Internet host 184.108.40.206 issues the command ftp 220.127.116.11
This results in the following set of events:
18.104.22.168 sends a TCP SYN packet to 22.214.171.124 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 126.96.36.199. 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 188.8.131.52 and forwards the packet back to 184.108.40.206. 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 220.127.116.11, that host will reject it since it's SOURCE IP address (18.104.22.168) doesn't match the DESTINATION IP ADDRESS (22.214.171.124) 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
#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.
Answer: This requires a vile
hack similar to the one in FAQ 2. Assuming
that your Internet zone is named net and connects
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net net:126.96.36.199:993 tcp 80 - 188.8.131.52
/etc/shorewall/interfaces, specify the
routeback option on
#ZONE INTERFACE BROADCAST OPTIONS net eth0 detect routeback
#INTERFACE SOURCE ADDRESS PROTO PORT eth0:184.108.40.206 0.0.0.0/0 220.127.116.11 tcp 993
Like the hack in FAQ 2, this one results in all forwarded connections looking to the server (18.104.22.168) as if they originated on your firewall (22.214.171.124).
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.
Answer: Specify the external address that you want to redirect in the ORIGINAL DEST column.
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net net:192.168.4.22 tcp 80,443 - 126.96.36.199
Answer: Ian Allen has written a Paper about DNAT and Linux.
Answer: See Shorewall_Squid_Usage.html.
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 188.8.131.52 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:
#ZONE INTERFACE BROADCAST OPTIONS loc eth1 detect routeback
#INTERFACE SOURCE ADDRESS PROTO PORT(S) eth1:192.168.1.5 eth1 192.168.1.254 tcp www
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - 184.108.40.206
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):
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 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 1. Example:
Zone: dmz, Interface: eth2, Subnet: 192.168.2.0/24, Address: 192.168.2.254
#ZONE INTERFACE BROADCAST OPTIONS dmz eth2 192.168.2.255 routeback
/etc/shorewall/nat, be sure that you
have “Yes” in the ALL INTERFACES column.
#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.
Answer: Let's assume the following:
External IP address is 220.127.116.11 on
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 - 18.104.22.168
If your external IP address is dynamic, then you must do the following:
/etc/shorewall/params (or in your
<export directory>/init file if you are
using Shorewall Lite on the firewall system):
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.
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.
Answer: You probably forgot to
specify the blacklist option for your
external interface(s) in
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.
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.
If you installed using the Standalone Guide, then please re-read this section.
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.
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.
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.
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.
Answer: Here's a writeup describing a nice integration of Shorewall and PortSentry.
Please see FAQ 17.
Answer: For a complete description of Shorewall “ping” management, see this page.
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
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:
If the value displayed is 0 (zero) then set IP_FORWARDING=On in
/etc/shorewall/shorewall.conf and restart
Answer: See the Shorewall and FTP page.
Answer: Most likely, you need to
set CLAMPMSS=Yes 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
For more information on this type of configuration, see the Shorewall Simple Bridge documentation.
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 Shorewall-perl bridging documentation for information about how to configure bridge/firewalls.
Following the instructions in the new bridging documentation will not prevent the above message from being issued.
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
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:
It is also possible to set up Shorewall to log all of its messages to a separate file.
Answer: Here are several links that may be helpful:
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 22.214.171.124 - 5 packets to tcp(1080) From 126.96.36.199 - 1 packet to udp(6348) From 188.8.131.52 - 9 packets to tcp(192) From 184.108.40.206 - 18 packets to tcp(3128) From 220.127.116.11 - 2 packets to tcp(3128) From 18.104.22.168 - 6 packets to tcp(808,1080,1978,7600,65506) From 22.214.171.124 - 5 packets to tcp(143) From 126.96.36.199 - 12 packets to tcp(873) From 188.8.131.52 - 8 packets to tcp(22) From 184.108.40.206 - 4 packets to udp(2,4081) From 220.127.116.11 - 2 packets to tcp(5900) From 18.104.22.168 - 24 packets to tcp(873) From 22.214.171.124 - 6 packets to tcp(3128) From 126.96.36.199 - 4 packets to tcp(22) From 188.8.131.52 - 5 packets to tcp(3128) ---------------------- iptables firewall End -------------------------
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
#ADDRESS/SUBNET PROTOCOL PORT - udp 10619
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 2. Example
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)
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
kernel.printk = 4 4 1 7
sysctl -p /etc/sysctl.conf
Some people who ask this question report that the only Shorewall
messages that they see in
'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
/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
Please see the Shorewall logging documentation for further information.
Answer: Logging of dropped/rejected packets occurs out of a number of chains (as indicated in the log message) in Shorewall:
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.
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.
The packet is being logged under the maclist interface option.
The packet is being logged because the source IP is
blacklisted in the
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
, you need the routeback option in the relevant entry in
or you've done something silly like define a default route out of
an internal interface.
With OPTIMIZE=1 in shorewall.conf, such packets may also be logged out of a <zone>2all chain or the all2all chain.
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).
With OPTIMIZE=1 in shorewall.conf, such packets may also be logged out of the fw2all chain or the all2all chain.
The packet is being logged because it failed the checks implemented by the tcpflags interface option.
Example 3. 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:
This packet was REJECTed out of the all2all chain -- the packet was rejected under the “all”->“all” REJECT policy (all2all above).
the packet entered the firewall via eth2. If you see “IN=” with no interface name, the packet originated on the firewall itself.
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.
the packet was sent by 192.168.2.2
the packet is destined for 192.168.1.3
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
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=184.108.40.206 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 220.127.116.11 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 18.104.22.168, 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 22.214.171.124, that box correctly changes the source address in the packet to 126.96.36.199 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 blacklisted the address 188.8.131.52 using shorewall drop 184.108.40.206 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=220.127.116.11 DST=18.104.22.168 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.
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).
Dec 15 16:47:30 heath-desktop kernel: [17182740.184000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00 SRC=10.119.248.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64 ID=62081 PROTO=UDP SPT=67 DPT=68 LEN=308 Dec 15 16:47:30 heath-desktop last message repeated 2 times Dec 15 16:47:30 heath-desktop kernel: [17182740.188000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00 SRC=10.112.70.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64 ID=62082 PROTO=UDP SPT=67 DPT=68 LEN=308 Dec 15 16:47:30 heath-desktop last message repeated 2 times
Answer: The Webmin 'bandwidth'
module adds commands to
creates rules to log every packet to/from/through the firewall. DON'T
START THE BANDWIDTH SERVICE IN WEBMIN!!!!!
To correct this situation once it occurs, edit
/etc/shorewall/start and insert 'return 0' prior to
the BANDWIDTH rules.
Answer: See this article about Shorewall and Multiple ISPs.
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
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.
I get the following errors:
RTNETLINK answers: Numerical result out of range ERROR: Command "ip -4 rule add from all table 254 pref 999" Failed
Answer: This is a known kernel issue -- see http://lkml.org/lkml/2007/3/30/253.
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.
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
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
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.
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.
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".
/etc/shorewall/modules and modify the copy to
include only the modules that you need.
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
Shorewall does not attempt to use policy match if you have no
IPSEC zones and you have not specified the
option on any entry in
subject message will still appear in your kernel log each time that
Shorewall determines the capabilities of your kernel/iptables.
ERROR: Command "/sbin/iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT" failed.
Answer: See the Shorewall OpenVZ article.
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
/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/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-lite). For more information on the
factors involved when deciding whether to use the Debian package, see
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.
Answer: Set IP_FORWARDING=On in
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
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
#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!
Answer: Yes, but we advise strongly against it.
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" Failed
Answer: 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:
Answer: Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.
Answer: See the Shorewall Feature List.
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.
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.
Answer: At the shell prompt, type:
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.
Answer: Shorwall filters these packets under the nosmurfs interface option in /etc/shorewall/interfaces.
Answer: Yes, if the routefilter interface option is selected.
Answer: This article by Paul Gear should help you get started.
Answer: Shorewall IPv6 support is currently available in Shorewall 4.2.4 and 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
support connection tracking for IPv6. So we could not even start to
develop Shorewall IPv6 support until 2.6.20 and there were significant
problems with the facility until at least kernel 2.6.23. When
distributions began offering IPv6 connection tracking support, it was
with kernel 2.6.25. So that is what we developed IPv6 support on and
that's all that we initially tested on. Subsequently, we have tested
Shorewall6 on Ubuntu Hardy with kernel 2.6.24. If you are running
2.6.20 or later, you can try to run
Shorewall6 by hacking
/usr/share/shorewall/prog.footer6 and changing the kernel
version test to check for your kernel version rather than 2.6.24
(20624). But after that, you are on your own.
kernel=$(printf "%2d%02d%02d\n" $(echo $(uname -r) 2> /dev/null | sed 's/-.*//' | tr '.' ' ' ) | head -n1) if [ $kernel -lt 20624 ]; then error_message "ERROR: $PRODUCT requires Linux kernel 2.6.24 or later" status=2 else
Answer: Yes. See Shorewall and Aliased Interfaces.
Answer: There is no way to create sub-zones of the firewall zone. But you can use shell variables to make vservers easier to deal with.
VS1=fw:192.168.2.12 VS2=fw:192.168.2.13 VS3=fw:192.168.2.14
#ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT $VS1 net tcp 25 DNAT net $VS1 tcp 25 etc...
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.
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.
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.
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_sip
Then change the DONT_LOAD specification in your shorewall.conf to:
If your kernel version is 2.6.21 or later:
rmmod nf_nat_sip rmmod nf_conntrack_sip
Then change the DONT_LOAD specification in your shorewall.conf to:
If 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.
Answer: You have configured forwarding on the interface which disables autoconfiguration of the interface. To retain autoconfiguration on the interface when Shorewall6 starts, specify forwarding=0 in the OPTIONS column on the interface's entry in shorewall6-interfaces (5).
Answer: Yes. Consult the QuickStart guide that you used during your initial setup for information about how to set up rules for your server.
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.
Answer: Temporarily remove and
rejNotSyn, dropNotSyn and dropInvalid rules from
/etc/shorewall/rules and restart Shorewall.
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.
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.
Answer: Shorewall Bridging Firewall support is available — check here for details.
#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:
And this for google.com:
22.214.171.124 126.96.36.199 188.8.131.52
So my guess is that you are not actually blocking the domain, but rather the IP being called. So how in the world do you block an actual domain name?
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:184.108.40.206 all REJECT fw net:220.127.116.11 all
Given that name-based multiple hosting is a common practice (another example: lists.shorewall.net and www1.shorewall.net are both hosted on the same system with a single IP address), it is not possible to filter connections to a particular name by examination of protocol headers alone. While some protocols such as 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.
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:~#
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.