Shorewall FAQs

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 DESTINATION PROTOCOL PORT SOURCE PORT ORIG. DEST.

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 DESTINATION PROTOCOL PORT SOURCE PORT ORIG. DEST.

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 DESTINATION PROTOCOL PORT SOURCE PORT ORIG. 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 PORT column specify the range as low-port:high-port.

1a. Ok -- I followed those instructions but it doesn't work

Answer: That is usually the result of one of two things:

You are trying to test from inside your firewall (no, that won't work -- see FAQ #2).
You have a more basic problem with your local system such as an incorrect default gateway configured (it should be set to the IP address of your firewall's internal interface).

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 server (the server's default gateway should be the IP address of the firewall's interface to the server).
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.

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 static NAT.

If you insist on an IP solution to the accessibility problem rather than a DNS solution, then 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, in /etc/shorewall/rules, add:

ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIG. DEST.

DNAT loc:192.168.1.0/24 loc:192.168.1.5 tcp www - 130.151.100.69:192.168.1.254

That rule only works of course if you have a static external IP address. If you have a dynamic IP address and are running Shorewall 1.3.4 or later then include this in /etc/shorewall/init:

     ETH0_IP=`find_interface_address eth0`

and make your DNAT rule:

ACTION SOURCE DESTINATION PROTOCOL PORT SOURCE PORT ORIG. DEST.

DNAT loc:192.168.1.0/24 loc:192.168.1.5 tcp www - $ETH0_IP:192.168.1.254

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.

2a. I have a zone "Z" with an RFC1918 subnet and I use static 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.

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 static 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 routing all Z->Z traffic through your firewall then:

a) Set the Z->Z policy to ACCEPT.
b) Masquerade Z to itself.

Example:

Zone: dmz
Interface: eth2
Subnet: 192.168.2.0/24

In /etc/shorewall/interfaces:

ZONE INTERFACE BROADCAST OPTIONS

dmz eth2 192.168.2.255

In /etc/shorewall/policy:

SOURCE DESTINATION POLICY LIMIT:BURST

dmz dmz ACCEPT

In /etc/shorewall/masq:

INTERFACE SUBNET ADDRESS

eth2 192.168.2.0/24

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 may help with Netmeeting. 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.

4. I just used an online port scanner to check my firewall and it shows some ports as 'closed' rather than 'blocked'. Why?

Answer: The common.def included with version 1.3.x always rejects connection requests on TCP port 113 rather than dropping them. This is necessary to prevent outgoing connection problems to services that use the 'Auth' mechanism for identifying requesting users. Shorewall also rejects TCP ports 135, 137 and 139 as well as UDP ports 137-139. These are ports that are used by Windows (Windows can be configured to use the DCE cell locator on port 135). Rejecting these connection requests rather than dropping them cuts down slightly on the amount of Windows chatter on LAN segments connected to the Firewall.

If you are seeing port 80 being 'closed', that's probably your ISP preventing you from running a web server in violation of your Service Agreement.

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.

5. I've installed Shorewall and now I can't ping through the firewall

Answer: If you want your firewall to be totally open for "ping",

a) Create /etc/shorewall/common if it doesn't already exist.
b) Be sure that the first command in the file is ". /etc/shorewall/common.def"
c) Add the following to /etc/shorewall/common

run_iptables -A icmpdef -p ICMP --icmp-type echo-request -j ACCEPT

For a complete description of Shorewall 'ping' management, see this page.

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 messaged 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 ratelimited log messages through settings in /etc/shorewall/shorewall.conf -- If you want to log all messages, set:

     LOGLIMIT=""
     LOGBURST=""

Beginning with Shorewall version 1.3.12, you can set up Shorewall to log all of its messages to a separate file.

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

I personnaly use Logwatch. It emails me a report each day from my various systems with each report summarizing the logged activity on the corresponding system.

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

6c. All day long I get a steady flow of these DROP messages from port 53 to some high numbered port. They get dropped, but what the heck are they?

Jan  8 15:50:48 norcomix kernel: Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:40:c7:2e:09:c0:00:01:64:4a:70:00:08:00
                                                        SRC=208.138.130.16 DST=24.237.22.45 LEN=53 TOS=0x00 PREC=0x00
                                                        TTL=251 ID=8288 DF PROTO=UDP SPT=53 DPT=40275 LEN=33

Answer: There are two possibilities:

They are late-arriving replies to DNS queries.
They are corrupted reply packets.

You can distinguish the difference by setting the logunclean option (/etc/shorewall/interfaces) on your external interface (eth0 in the above example). If they get logged twice, they are corrupted. I solve this problem by using an /etc/shorewall/common file like this:

#
# Include the standard common.def file
#
. /etc/shorewall/common.def
#
# The following rule is non-standard and compensates for tardy
# DNS replies
#
run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP

The above file is also include in all of my sample configurations available in the Quick Start Guides and in the common.def file in Shorewall 1.4.0 and later.

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 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)

7. When I stop Shorewall using 'shorewall stop', 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.

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 is usually cured by 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.

9. Why can't Shorewall detect my interfaces properly?

I just installed Shorewall and when I issue the start command, I see the following:

     Processing /etc/shorewall/shorewall.conf ...
     Processing /etc/shorewall/params ...
     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

10. What Distributions does it work with?

Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.

11. What Features does it have?

Answer: See the Shorewall Feature List.

12. Is there a GUI?

Answer: Yes. Shorewall support is included in Webmin 1.060 and later versions. See http://www.webmin.com

13. Why do you call it "Shorewall"?

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 must more commonly used.

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: If you are running a version of Shorewall earlier than 1.3.1, create /etc/shorewall/start and in it, place the following:

     run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT

If you are running version 1.3.1 or later, simply add the following to /etc/shorewall/rfc1918:

SUBNET TARGET

192.168.100.1 RETURN

Be sure that you add the entry ABOVE the entry for 192.168.0.0/16.

Note: 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

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 FAQ 14 above. Simply substitute the IP address of your ISPs DHCP server.

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.

16. Shorewall is writing log messages all over my console making it unusable!

Answer: "man dmesg" -- add a suitable 'dmesg' command to your startup scripts or place it in /etc/shorewall/start. Under RedHat, the max log level that is sent to the console is specified in /etc/sysconfig/init in the LOGLEVEL variable.

17. How do I find out why this traffic is getting logged?

Answer: Logging occurs out of a number of chains (as indicated in the log message) in Shorewall:

man1918 - The destination address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918.
rfc1918 - The source address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918.
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.
<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.
newnotsyn - The packet is being logged because it is a TCP packet that is not part of any current connection yet it is not a syn packet. Options affecting the logging of such packets include NEWNOTSYN and LOGNEWNOTSYN in /etc/shorewall/shorewall.conf.
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.
logflags - The packet is being logged because it failed the checks implemented by the tcpflags interface option.

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.

19. I have added entries to /etc/shorewall/tcrules but they don't seem to do anything. Why?

You probably haven't set TC_ENABLED=Yes in /etc/shorewall/shorewall.conf so the contents of the tcrules file are simply being ignored.

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.

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 the internet. 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.

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.

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.

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.

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

25. How to I tell which version of Shorewall I am running?

At the shell prompt, type:

/sbin/shorewall version

Last updated 3/6/2003 - Tom Eastep

ACTION	SOURCE	DESTINATION	PROTOCOL	PORT	SOURCE PORT	ORIG. DEST.
DNAT	net	loc:<local IP address>[:<local port>]	<protocol>	<port #>

ZONE	INTERFACE	BROADCAST	OPTIONS
dmz	eth2	192.168.2.255

SOURCE	DESTINATION	POLICY	LIMIT:BURST
dmz	dmz	ACCEPT