Shorewall FAQs

Looking for Step by Step Configuration Instructions? Check out the QuickStart Guides.

PORT FORWARDING

1. I want to forward UDP port 7777 to my my personal PC with IP address 192.168.1.5. I've looked everywhere and can't find how to do it.

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

1b. I'm still having problems with port forwarding

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?

30. I'm confused about when to use DNAT rules and when to use ACCEPT rules.

DNS and PORT FORWARDING/NAT

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.

2a. I have a zone "Z" 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.

NETMEETING/MSN

3. I want to use Netmeeting or MSN Instant Messenger with Shorewall. What do I do?

OPEN PORTS

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

4a. I just ran an nmap UDP scan of my firewall and it showed 100s of ports as open!!!!

4b. I have a port that I can't close no matter how I change my rules.

4c. How to I use Shorewall with PortSentry?

CONNECTION PROBLEMS

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

15. My local systems can't see out to the net

29. FTP Doesn't Work

LOGGING

6. Where are the log messages written and how do I change the destination?

6a. Are there any log parsers that work with Shorewall?

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?

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?

6d. Why is the MAC address in Shorewall log messages so long? I thought MAC addresses were only 6 bytes in length.

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

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

21. I see these strange log entries occasionally; what are they?

ROUTING

32. My firewall has two connections to the internet from two different ISPs. How do I set this up in Shorewall?

STARTING AND STOPPING

7. When I stop Shorewall using 'shorewall stop', I can't connect to anything. Why doesn't that command work?

8. When I try to start Shorewall on RedHat I get messages about insmod failing -- what's wrong?

8a. When I try to start Shorewall on RedHat I get a message referring me to FAQ #8

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

22. I have some iptables commands that I want to run when Shorewall starts. Which file do I put them in?

ABOUT SHOREWALL

10. What distributions does it work with?

11. What features does it support?

12. Is there a GUI?

13. Why do you call it "Shorewall"?

23. Why do you use such ugly fonts on your web site?

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

31. Does Shorewall provide protection against...

RFC 1918

14. I'm connected via a cable modem and it has an internel web server that allows me to configure/monitor it but as expected if I enable rfc1918 blocking for my eth0 interface, it also blocks the cable modems web server.

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.

ALIAS IP ADDRESSES/VIRTUAL INTERFACES

18. Is there any way to use aliased ip addresses with Shorewall, and maintain separate rulesets for different IPs?

MISCELLANEOUS

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

20. I have just set up a server. Do I have to change Shorewall to allow access to my server from the internet?

24. How can I allow conections to let's say the ssh port only from specific IP Addresses on the internet?

26. When I try to use any of the SYN options in nmap on or behind the firewall, I get "operation not permitted". How can I use nmap with Shorewall?"

26a.  When I try to use the "-O" option of nmap from the firewall system, I get "operation not permitted". How to I allow this option?

27. I am compiling a new kernel for my firewall. What should I look out for?

28. How do I use Shorewall as a Bridging Firewall?

1. I want to forward UDP port 7777 to my 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 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 three things:

1b. I'm still having problems with port forwarding

Answer: To further diagnose this problem:

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 DESTINATION PROTOCOL PORT SOURCE PORT ORIG. DEST.
DNAT net
 loc:192.168.1.3:22 tcp 1022

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.

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.

If you are running Shorewall 1.4.0 or earlier see the 1.3 FAQ for instructions suitable for those releases.

If you are running Shorewall 1.4.1 or Shorewall 1.4.1a, please upgrade to Shorewall 1.4.2 or later.

Otherwise:

ZONE
 INTERFACE
 BROADCAST
 OPTIONS
loc
 eth1
 detect
 routeback
ACTION SOURCE DEST PROTO DEST
PORT(S)		 SOURCE
PORT(S)		 ORIGINAL
DEST
DNAT
 loc web: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 web: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 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.

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

a) Set the Z->Z policy to ACCEPT.
b) Masquerade Z to itself.
c) Set the routeback option on the interface to Z.
d) Set the ALL INTERFACES column in the nat file to "Yes".

WARNING: In this configuration, all Z->Z traffic will look to the server as if it came from the firewall rather than from the original client! I DO NOT RECOMMEND THIS SETUP.

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 routeback

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
In /etc/shorewall/nat, be sure that you have "Yes" in the ALL INTERFACES column.

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. 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.

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.

4c. How to I use Shorewall with PortSentry?

Here's a writeup on a nice integration of Shorewall and PortSentry.

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
http://home.regit.org/ulogd-php.html

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:
  1. They are late-arriving replies to DNS queries.
  2. 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:
Example:

MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00

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.

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 #8

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

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:

  1. The default gateway on each local system isn't set to the IP address of the local firewall interface.

  2. The entry for the local network in the /etc/shorewall/masq file is wrong or missing.

  3. 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: If you are running Shorewall version 1.4.4 or 1.4.4a then check the errata. Otherwise, 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, 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:
  1. man1918 or logdrop - The destination address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918.
  2. rfc1918 or logdrop - The source address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918.
  3. 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.
  4. <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.
  5. <interface>_mac - The packet is being logged under the maclist interface option.
  6. logpkt - The packet is being logged under the logunclean interface option.
  7. badpkt - The packet is being logged under the dropunclean interface option as specified in the LOGUNCLEAN setting in /etc/shorewall/shorewall.conf.
  8. blacklst - The packet is being logged because the source IP is blacklisted in the /etc/shorewall/blacklist file.
  9. 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.
  10. 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. Also see FAQ 2a for another cause of packets being logged in the FORWARD chain.
  11. 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:

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

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

26. When I try to use any of the SYN options in nmap on or behind the firewall, I get "operation not permitted". How can I use nmap with Shorewall?"

Edit /etc/shorewall/shorewall.conf and change "NEWNOTSYN=No" to "NEWNOTSYN=Yes" then restart Shorewall.

26a.  When I try to use the "-O" option of nmap from the firewall system, I get "operation not permitted". How to I allow this option?

Add this command to your /etc/shorewall/start file:
run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP

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.

28. How do I use Shorewall as a Bridging Firewall?

Basically, you don't. While there are kernel patches that allow you to route bridge traffic through Netfilter, the environment is so different from the Layer 3 firewalling environment that very little of Shorewall works. In fact, so much of Shorewall doesn't work that my official position is that "Shorewall doesn't work with Layer 2 Bridging".

29. FTP Doesn't Work

See the Shorewall and FTP page.

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.

31. Does Shorewall provide protection against....

  1. IP Spoofing: Sending packets over the WAN interface using an internal LAP IP address as the source address? Answer: Yes.
  2. 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.
  3. 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.
  4. Land Attack: Sending packets that use the same address as the source and destination address? Answer: Yes, if the routefilter interface option is selected.
  5. 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.

32. My firewall has two connections to the internet from two different ISPs. How do I set this up in Shorewall?

Setting this up in Shorewall is easy; setting up the routing is a bit harder.

Assuming that eth0 and eth1 are the interfaces to the two ISPs then:

/etc/shorewall/interfaces:
ZONE INTERFACE BROADCAST OPTIONS
net
 eth0 detect
 ...
net
 eth1
 detect
 ...
/etc/shorewall/policy:
SOURCE DESTINATION POLICY LIMIT:BURST
net
 net
 DROP
The following information regarding setting up routing for this configuration is reproduced from the LARTC HOWTO and has not been verified by the author. If you have questions or problems with the instructions given below, please post to the LARTC mailing list.
A common configuration is the following, in which there are two providers that connect a local network (or even a single machine) to the big Internet. 
                                                                 ________
                                          +------------+        /
                                          |            |       |
                            +-------------+ Provider 1 +-------
        __                  |             |            |     /
    ___/  \_         +------+-------+     +------------+    |
  _/        \__      |     if1      |                      /
 /             \     |              |                      |
| Local network -----+ Linux router |                      |     Internet
 \_           __/    |              |                      |
   \__     __/       |     if2      |                      \
      \___/          +------+-------+     +------------+    |
                            |             |            |     \
                            +-------------+ Provider 2 +-------
                                          |            |       |
                                          +------------+        \________

There are usually two questions given this setup.

Split access

The first is how to route answers to packets coming in over a particular provider, say Provider 1, back out again over that same provider.

Let us first set some symbolical names. Let $IF1 be the name of the first interface (if1 in the picture above) and $IF2 the name of the second interface. Then let $IP1 be the IP address associated with $IF1 and $IP2 the IP address associated with $IF2. Next, let $P1 be the IP address of the gateway at Provider 1, and $P2 the IP address of the gateway at provider 2. Finally, let $P1_NET be the IP network $P1 is in, and $P2_NET the IP network $P2 is in.

One creates two additional routing tables, say T1 and T2. These are added in /etc/iproute2/rt_tables. Then you set up routing in these tables as follows: 

	  ip route add $P1_NET dev $IF1 src $IP1 table T1
	  ip route add default via $P1 table T1
	  ip route add $P2_NET dev $IF2 src $IP2 table T2
	  ip route add default via $P2 table T2
Nothing spectacular, just build a route to the gateway and build a default route via that gateway, as you would do in the case of a single upstream provider, but put the routes in a separate table per provider. Note that the network route suffices, as it tells you how to find any host in that network, which includes the gateway, as specified above.

Next you set up the main routing table. It is a good idea to route things to the direct neighbour through the interface connected to that neighbour. Note the `src' arguments, they make sure the right outgoing IP address is chosen. 

	    ip route add $P1_NET dev $IF1 src $IP1
	    ip route add $P2_NET dev $IF2 src $IP2
Then, your preference for default route: 
	    ip route add default via $P1
Next, you set up the routing rules. These actually choose what routing table to route with. You want to make sure that you route out a given interface if you already have the corresponding source address: 
	    ip rule add from $IP1 table T1
	    ip rule add from $IP2 table T2
This set of commands makes sure all answers to traffic coming in on a particular interface get answered from that interface.

Warning

Reader Rod Roark notes: 'If $P0_NET is the local network and $IF0 is its interface, the following additional entries are desirable: 

ip route add $P0_NET     dev $IF0 table T1
ip route add $P2_NET     dev $IF2 table T1
ip route add 127.0.0.0/8 dev lo   table T1
ip route add $P0_NET     dev $IF0 table T2
ip route add $P1_NET     dev $IF1 table T2
ip route add 127.0.0.0/8 dev lo   table T2
'

Now, this is just the very basic setup. It will work for all processes running on the router itself, and for the local network, if it is masqueraded. If it is not, then you either have IP space from both providers or you are going to want to masquerade to one of the two providers. In both cases you will want to add rules selecting which provider to route out from based on the IP address of the machine in the local network.

Load balancing

The second question is how to balance traffic going out over the two providers. This is actually not hard if you already have set up split access as above.

Instead of choosing one of the two providers as your default route, you now set up the default route to be a multipath route. In the default kernel this will balance routes over the two providers. It is done as follows (once more building on the example in the section on split-access): 

	    ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \
	    nexthop via $P2 dev $IF2 weight 1
This will balance the routes over both providers. The weight parameters can be tweaked to favor one provider over the other.

Note that balancing will not be perfect, as it is route based, and routes are cached. This means that routes to often-used sites will always be over the same provider.

Furthermore, if you really want to do this, you probably also want to look at Julian Anastasov's patches at http://www.ssi.bg/~ja/#routes , Julian's route patch page. They will make things nicer to work with.

End of information reproduced from the LARTC HOWTO. If you have questions or problems with the instructions given above, please post to the LARTC mailing list.
Last updated 11/20/2003 - Tom Eastep

Copyright © 2001, 2002, 2003 Thomas M. Eastep.