Copyright © 2001-2005 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 ”.
2005-09-07
Table of Contents
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, the Shorewall Debian Maintainer:
“For more information about Shorewall usage on Debian system please look at /usr/share/doc/shorewall/README.Debian provided by [the] shorewall Debian package.”
If you install using the .deb, you will find that your /etc/shorewall directory is empty. This is
intentional. The released configuration file skeletons may be found on
your system in the directory /usr/share/doc/shorewall/default-config.
Simply copy the files you need from that directory to /etc/shorewall and modify the
copies.
Note that you must copy /usr/share/doc/shorewall/default-config/shorewall.conf
and /usr/share/doc/shorewall/default-config/modules
to /etc/shorewall even if you do
not modify those files.
Answer: Read the Installation Instructions!!!!!
Answer: The first example in the rules file documentation shows how to do port forwarding under Shorewall. The format of a port-forwarding rule to a local system is as follows:
#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:<local IP address>[:<local port>] <protocol> <port #>
So to forward UDP port 7777 to internal system 192.168.1.5, the rule is:
#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.5 udp 7777
If you want to forward requests directed to a particular address ( <external IP> ) on your firewall to an internal system:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:<local IP address>[:<local port>] <protocol> <port #> - <external IP>
Finally, if you need to forward a range of ports, in the DEST PORT column specify the range as <low-port>:<high-port>.
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 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.”).
You have a more basic problem with your local system (the one that you are trying to forward to) such as an incorrect default gateway (it should be set to the IP address of your firewall's internal interface).
Your ISP is blocking that particular port inbound.
You are running Mandrake Linux 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 “ iptables -t nat -Z ”. This clears the NetFilter counters in the nat table.
Try to connect to the redirected port from an external host.
As root type “ shorewall show nat ”
Locate the appropriate DNAT rule. It will be in a chain called <source zone>_dnat (“net_dnat” in the above examples).
Is the packet count in the first column non-zero? If so, the connection request is reaching the firewall and is being redirected to the server. In this case, the problem is usually a missing or incorrect default gateway setting on the local system (the system you are trying to forward to -- its default gateway should be the IP address of the firewall's interface to that system).
If the packet count is zero:
the connection request is not reaching your server (possibly it is being blocked by your ISP); or
you are trying to connect to a secondary IP address on your firewall and your rule is only redirecting the primary IP address (You need to specify the secondary IP address in the “ORIG. DEST.” column in your DNAT rule); or
your DNAT rule doesn't match the connection request in some other way. In that case, you may have to use a packet sniffer such as tcpdump or ethereal to further diagnose the problem.
In /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.3:22 tcp 1022
Answer: Let's assume the following:
External IP address is 206.124.146.176 on eth0.
Server's IP address is 192.168.2.4
You can enable access to the server from your local network using the firewall's external IP address by adding this rule:
#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL # PORT DEST DNAT loc dmz:192.168.2.4 tcp 80 - 206.124.146.176
If your external IP address is dynamic, then you must do the following:
In /etc/shorewall/init:
ETH0_IP=`find_interface_address eth0` For users of Shorewall 2.1.0 and later:
ETH0_IP=`find_first_interface_address eth0`and make your DNAT rule:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc dmz:192.168.2.4 tcp 80 - $ETH0_IP
Answer courtesy of Ryan: Assume that the IP address of your local firewall interface is 192.168.1.1. If you 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
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.
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 Bind Version 9 “views” (or using a separate DNS server for local clients) such that www.mydomain.com resolves to 130.141.100.69 externally and 192.168.1.5 internally. That's what I do here at shorewall.net for my local systems that use one-to-one NAT.
If you insist on a stupid IP solution to the accessibility problem rather than a more efficient DNS solution, then if you are running Shorewall 2.0.0 or 2.0.1 then please see the Shorewall 1.4 FAQ.
Otherwise, assuming that your external interface is eth0 and your internal interface is eth1 and that eth1 has IP address 192.168.1.254 with subnet 192.168.1.0/24, then:
All traffic redirected through use of this hack will look to the server as if it came from the firewall (192.168.1.254) rather than from the original client!
In /etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
loc eth1 detect routeback In /etc/shorewall/masq:
#INTERFACE SUBNET ADDRESS PROTO PORT(S) eth1:192.168.1.5 eth1 192.168.1.254 tcp www
In /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69
That rule only works of course if you have a static external
IP address. If you have a dynamic IP address and are running
Shorewall 1.3.4 through Shorewall 2.0.* then include this in
/etc/shorewall/init:
ETH0_IP=`find_interface_address eth0` For users of Shorewall 2.1.0 and later:
ETH0_IP=`find_first_interface_address eth0` and make your DNAT rule:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - $ETH0_IP
Using this technique, you will want to configure your DHCP/PPPoE client to automatically restart Shorewall each time that you get a new IP address.
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=0Answer: This is another problem that is best solved using Bind Version 9 “views”. It allows both external and internal clients to access a NATed host using the host's DNS name.
Another good way to approach this problem is to switch from one-to-one NAT to Proxy ARP. That way, the hosts in Z have non-RFC1918 addresses and can be accessed externally and internally using the same address.
If you don't like those solutions and prefer to stupidly route all Z->Z traffic through your firewall then:
Set the routeback option on the interface to Z.
Set the ALL INTERFACES column in the nat file to “Yes”.
Example 1. Example:
Zone: dmz Interface: eth2 Subnet: 192.168.2.0/24 Address: 192.168.2.254
In /etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
dmz eth2 192.168.2.255 routeback In /etc/shorewall/nat, be sure that you
have “Yes” in the ALL INTERFACES column.
In /etc/shorewall/masq:
#INTERFACE SUBNETS ADDRESS eth2 eth2 192.168.2.254
Like the idiotic hack in FAQ 2 above, this will make all dmz->dmz traffic appear to originate on the firewall.
Answer: Let's assume the following:
External IP address is 206.124.146.176 on eth0 (www.mydomain.com).
Server's IP address is 192.168.2.4
You can enable access to the server from your local network using the firewall's external IP address by adding this rule:
#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL # PORT DEST DNAT loc dmz:192.168.2.4 tcp 80 - 206.124.146.176
If your external IP address is dynamic, then you must do the following:
In /etc/shorewall/init:
ETH0_IP=`find_interface_address eth0`For users of Shorewall 2.1.0 and later:
ETH0_IP=`find_first_interface_address eth0` and make your DNAT rule:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc dmz:192.168.2.4 tcp 80 - $ETH0_IP
Answer: There is an H.323 connection tracking/NAT module that helps with Netmeeting. Note however that one of the Netfilter developers recently posted the following:
> I know PoM -ng is going to address this issue, but till it is ready, and > all the extras are ported to it, is there any way to use the h.323 > contrack module kernel patch with a 2.6 kernel? > Running 2.6.1 - no 2.4 kernel stuff on the system, so downgrade is not > an option... The module is not ported yet to 2.6, sorry. > Do I have any options besides a gatekeeper app (does not work in my > network) or a proxy (would prefer to avoid them)? I suggest everyone to setup a proxy (gatekeeper) instead: the module is really dumb and does not deserve to exist at all. It was an excellent tool to debug/develop the newnat interface.
Look here for a solution for MSN IM but be aware that there are significant security risks involved with this solution. Also check the Netfilter mailing list archives at http://www.netfilter.org.
Answer: (Shorewall versions prior to 2.0.0 only). 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, 139 and 445 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.
You can change the default behavior of Shorewall through use of an /etc/shorewall/common file. See the Extension Script Section.
Beginning with Shorewall 1.4.9, Shorewall no longer rejects the Windows SMB ports (135-139 and 445) by default and silently drops them instead.
Answer: (Shorewall versions 2.0.0
and later). The default Shorewall setup invokes the Drop action prior to enforcing a DROP policy and
the default policy to all zone from the internet is DROP. The Drop
action is defined in
/usr/share/shorewall/action.Drop which in turn
invokes the RejectAuth action (defined
in /usr/share/shorewall/action.RejectAuth). 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 firerwall will block that attempt.
Here's a writeup on a nice integration of Shorewall and PortSentry.
Answer: It depends…
If the application serving the port is running on the same system as Shorewall then add this rule:
#ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT net $FW <protocol> <port number>
Where <protocol> is either tcp or udp and <port number> is the port that you wish to "open".
If the application serving the port is running on one of the systems in your local network then please see FAQ 1.
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 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.
See the Shorewall and FTP page.
Answer: Most likely, you need to set CLAMPMSS=Yes in /etc/shorewall/shorewall.conf.
Answer: Add the routeback option to
br0 in /etc/shorewall/interfaces.
For more information on this type of configuration, see the Shorewall Simple Bridge documentation.
I have this entry in /etc/shorewall/tunnels:
# TYPE ZONE GATEWAY GATEWAY # ZONE openvpn:5000 net 69.145.71.133
Yet I am seeing this log message:
Oct 12 13:41:03 localhost kernel: Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:04:5a:7f:92:9f:00:b0:c2:89:68:e4:08:00 SRC=69.145.71.133 DST=216.187.138.18 LEN=42 TOS=0x00 PREC=0x00 TTL=46 ID=11 DF PROTO=UDP SPT=33120 DPT=5000 LEN=22
Answer: Shorewall's openvpn tunnel type assumes that OpenVPN will be using the same port (default 5000) for both the source and destination port. From the above message, it is clear that the remote client is using source port 33120. The solution is to replace your /etc/shorewall/tunnels entry with this one:
# TYPE ZONE GATEWAY GATEWAY # ZONE generic:udp:5000 net 69.145.71.133
I want to allow access from the local zone to the net except for two systems (192.168.100.101 and 192.168.100.115). I use the following rule but find that 192.168.100.115 can still access the net. Is this a bug?
#ACTION SOURCE DEST PROTO ACCEPT loc:!192.168.100.101,192.168.100.115 net
Answer: Shorewall is currently inconsistent as to where it correctly supports the "!" before a list of addresses. In some places, it works as you would expect and in other cases such as this one it does not. You will need to take a different approach to accomplish what you want. I recommend that you change your loc->net policy to ACCEPT and then use this rule:
#ACTION SOURCE DEST PROTO REJECT loc:192.168.100.101,192.168.100.115 net
Author's Note: I have looked several times at correcting this problem but it really isn't feasible until I muster the energy to rewrite the Shorewall rules parser. Sorry.
Answer: NetFilter uses the
kernel's equivalent of syslog (see “man syslog”) to log
messages. It always uses the LOG_KERN (kern) facility (see “man
openlog”) and you get to choose the log level (again, see
“man syslog”) in your policies and rules. The destination for
messages logged by syslog is controlled by
/etc/syslog.conf (see “man
syslog.conf”). When you have changed /etc/syslog.conf, be sure to
restart syslogd (on a RedHat system, “service syslog
restart”).
By default, older versions of Shorewall 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.
Answer: Here are several links that may be helpful:
http://www.shorewall.net/pub/shorewall/parsefw/
http://www.fireparse.com
http://cert.uni-stuttgart.de/projects/fwlogwatch
http://www.logwatch.org
http://gege.org/iptables
http://home.regit.org/ulogd-php.html
I personally use Logwatch. It emails me a report each day from my various systems with each report summarizing the logged activity on the corresponding system.
Temporarily add the following rule:
DROP net fw udp 10619
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)
Answer: If you are running Shorewall version 1.4.4 or 1.4.4a then check the errata. Otherwise:
Find where klogd is being started (it will be from one of the files in /etc/init.d -- sysklogd, klogd, ...). Modify that file or the appropriate configuration file so that klogd is started with “-c <n> ” where <n> is a log level of 5 or less; or
See the “dmesg” man page (“man dmesg”). You must add a suitable “dmesg” command to your startup scripts or place it in /etc/shorewall/start.
Under RedHat and Mandrake, the max log level that is sent to the console is specified in /etc/sysconfig/init in the LOGLEVEL variable. Set “LOGLEVEL=5” to suppress info (log level 6) messages on the console.
Under Debian, you can set KLOGD=“-c 5” in
/etc/init.d/klogd to suppress info (log level 6)
messages on the console.
Under SuSE, add “-c 5” to KLOGD_PARAMS in /etc/sysconfig/syslog to suppress info (log level 6) messages on the console.
Answer: Logging of dropped/rejected packets occurs out of a number of chains (as indicated in the log message) in Shorewall:
The destination address is listed in
/usr/share/shorewall/rfc1918 with a logdrop target -- see /usr/share/shorewall/rfc1918
.
The source or destination address is listed in
/usr/share/shorewall/rfc1918 with a logdrop target -- see /usr/share/shorewall/rfc1918
.
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.
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.
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 under the logunclean interface option.
The packet is being logged under the dropunclean interface option as
specified in the LOGUNCLEAN
setting in
/etc/shorewall/shorewall.conf
.
The packet is being logged because the source IP is
blacklisted in the /etc/shorewall/blacklist
file.
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
.
The packet has a source IP address that isn't in any of your
defined zones (“shorewall check” and look at the
printed zone definitions) or the chain is FORWARD and the
destination IP isn't in any of your defined zones. If the chain is
FORWARD and the IN and OUT interfaces are the same, then you
probably need the routeback
option on that interface in /etc/shorewall/interfaces
or you need the routeback option in the relevant entry in
/etc/shorewall/hosts
.
The packet has a destination IP address that isn't in any of your defined zones("shorewall check" and look at the printed zone definitions).
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=47Let'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.
the packet was sent by 192.168.2.2
the packet is destined for 192.168.1.3
UDP Protocol
The destination port is 53 (DNS)
For additional information about the log message, see http://logi.cc/linux/netfilter-log-format.php3.
In this case, 192.168.2.2 was in the “dmz” zone and 192.168.1.3 is in the “loc” zone. I was missing the rule:
ACCEPT dmz loc udp 53
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. When Netfilter displays these messages, the part before the "[" describes the ICMP packet and the part between the "[" and "]" describes the packet for which the ICMP is a response.
Here is my interpretation of what is happening -- to confirm this analysis, one would have to have packet sniffers placed a both ends of the connection.
Host 172.16.1.10 behind NAT gateway 206.124.146.179 sent a UDP DNS query to 192.0.2.3 and your DNS server tried to send a response (the response information is in the brackets -- note source port 53 which marks this as a DNS reply). When the response was returned to to 206.124.146.179, it rewrote the destination IP TO 172.16.1.10 and forwarded the packet to 172.16.1.10 who no longer had a connection on UDP port 2857. This causes a port unreachable (type 3, code 3) to be generated back to 192.0.2.3. As this packet is sent back through 206.124.146.179, that box correctly changes the source address in the packet to 206.124.146.179 but doesn't reset the DST IP in the original DNS response similarly. When the ICMP reaches your firewall (192.0.2.3), your firewall has no record of having sent a DNS reply to 172.16.1.10 so this ICMP doesn't appear to be related to anything that was sent. The final result is that the packet gets logged and dropped in the all2all chain. I have also seen cases where the source IP in the ICMP itself isn't set back to the external IP of the remote NAT gateway; that causes your firewall to log and drop the packet out of the rfc1918 chain because the source IP is reserved by RFC 1918.
Anyone with two Internet connections MUST read and understand this article on Shorewall and Routing. If you don't, you will be completely lost trying to make this work. And that article should be all that you need if you are running Shorewall 2.3.2 or later.
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
If you have masqueraded hosts, be sure to update
/etc/shorewall/masq to masquerade to both ISPs. For
example, if you masquerade all hosts connected to eth2 then:
#INTERFACE SUBNET ADDRESS eth0 eth2 eth1 eth2
Again, if you are running Shorewall 2.3.2 or later, please see this article for instructions for setting up the routing. Otherwise, follow the instructions that follow.
There was an article in SysAdmin covering the topic of setting up routing for this configuration. It may be found at http://www.samag.com/documents/s=1824/sam0201h/.
Stephen Carville has put together a Shorewall-specific writeup that covers this subject at http://www.heronforge.net/redhat/node17.html.
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.
The following was contributed by Martin Brown and is an excerpt from http://www.docum.org/stef.coene/qos/faq/cache/44.html .
Answer: This is usually the consequence of a one-to-one nat configuration blunder:
Specifying the primary IP address for an interface in the
EXTERNAL column of /etc/shorewall/nat even
though the documentation (and the comments in the file) warn you not
to do that.
Specifying ADD_IP_ALIASES=Yes and RETAIN_ALIASES=No in /etc/shorewall/shorewall.conf.
This combination causes Shorewall to delete the primary IP address from the network interface specified in the INTERFACE column which usually causes all routes out of that interface to be deleted. The solution is to not specify the primary IP address of an interface in the EXTERNAL column.
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.
Answer: The output you will see looks something like this:
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?) Perhaps iptables or your kernel needs to be upgraded.
This problem is usually corrected through the following sequence of commands
service ipchains stop
chkconfig --delete ipchains
rmmod ipchainsAlso, be sure to check the errata for problems concerning the version of iptables (v1.2.3) shipped with RH7.2.
Answer: This is usually cured by the sequence of commands shown above in the section called “(FAQ 8) When I try to start Shorewall on RedHat, I get messages about insmod failing -- what's wrong?”.
I just installed Shorewall and when I issue the start command, I see the following:
Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
Net Zone: eth0:0.0.0.0/0
Local Zone: eth1:0.0.0.0/0
Deleting user chains...
Creating input Chains...
...Why can't Shorewall detect my interfaces properly?
Answer: The above output is
perfectly normal. The Net zone is defined as all hosts that are
connected through eth0 and the local zone is defined as all hosts
connected through eth1. If you
are running Shorewall 1.4.10 or later, you can consider setting the
detectnets interface option on your local
interface (eth1 in the above
example). That will cause Shorewall to restrict the local zone to only
those networks routed through that interface.
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.
Using a light-weight shell such as ash can
dramatically decrease the time required to start or restart
Shorewall. See the SHOREWALL_SHELL variable in shorewall.conf .
Use a fast terminal emulator -- in particular the KDE konsole scrolls much faster than the Gnome terminal. Also use the '-q' option if you are restarting remotely or from a slow terminal (or redirect the output to a file as in shorewall restart > /dev/null).
Beginning with Shorewall version 2.0.2 Beta 1, Shorewall supports a fast start capability. To use this capability:
With Shorewall in the started state, run
shorewall save. This creates the script
/var/lib/shorewall/restore.
Use the -f option to the
start command (e.g., shorewall -f start). This
causes Shorewall to look for the
/var/lib/shorewall/restore script and if that
script exists, it is run. Running
/var/lib/shorewall/restore takes much less time
than a full shorewall start.
The /etc/init.d/shorewall script that is
run at boot time uses the -f
option.
The /var/lib/shorewall/restore script can
be run any time to restore the firewall. The script may be run
directly or it may be run indirectly using the shorewall
restore command.
If you change your Shorewall configuration, you must execute a shorewall start (without -f) or shorewall restart prior to doing another shorewall save. The shorewall save command saves the currently running configuration and not the one reflected in your updated configuration files.
Likewise, if you change your Shorewall configuration then once you
are satisfied that it is working properly, you must do another
shorewall save. Otherwise at the next reboot, you
will revert to the old configuration stored in
/var/lib/shorewall/restore.
Answer: iptables 1.2.9 is broken with respect to iptables-save and the connection tracking match extension. You must patch your iptables using the patch available from the Shorewall errata page.
When I start shorewall I got the following errors.
Oct 30 11:13:12 fwr modprobe: modprobe: Can't locate module ipt_conntrack Oct 30 11:13:17 fwr modprobe: modprobe: Can't locate module ipt_pkttype Oct 30 11:13:18 fwr modprobe: modprobe: Can't locate module ipt_pkttype Oct 30 11:13:57 fwr last message repeated 2 times Oct 30 11:14:06 fwr root: Shorewall Restarted
The "shorewall status" output seems complying with my rules set. Should I worry ? and is there any way to get rid of these errors ?
Answer: You are seeing two different things:
The normal checking that Shorewall does when it starts. Shorewall tries to determine the the capabilities of your 'iptables' and kernel and then taylors the ruleset accordingly.
A problem in Shorewall 2.0.3a through 2.0.5 whereby Shorewall tried to use the pkttype match feature each time that it wanted to generate a rule involving broadcast or multicast packets.
You can suppress the messages by aliasing the modules mentioned in the error messages to off in /etc/modules.conf. Just be sure to review these aliases each time that you do a kernel upgrade to be sure that you are not disabling a feature in your new kernel that you want to use.
alias ipt_conntrack off alias ipt_pkttype off
For users who don't have the pkttype match feature in their kernel, I also recommend upgrading to Shorewall 2.0.6 or later and then setting PKTTYPE=No in shorewall.conf.
Answer: When you install using the "rpm -U" command, Shorewall doesn't run your distribution's tool for configuring Shorewall startup. You will need to run that tool (insserv, chkconfig, run-level editor, …) to configure Shorewall to start in the run-levels that you run your firewall system at.
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".
Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.
Answer: See the Shorewall Feature List.
Answer: Yes. Shorewall support is included in Webmin 1.060 and later versions. See http://www.webmin.com
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.
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.
At the shell prompt, type:
/sbin/shorewall version Answer: Yes.
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: Shorewall can be configured to do that using the blacklisting facility. Shorewall versions 2.0.0 and later filter these packets under the nosmurfs interface option in /etc/shorewall/interfaces.
Answer: Yes, if the routefilter interface option is selected.
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.
The first release of Shorewall was in March of 2001. Shorewall 1.2.12 was released in May of 2002. It is now the year 2005 and Shorewall 2.2 is available. Shorewall 1.2.12 is poorly documented and is missing many of the features that Shorewall users find essential today and it is silly to continue to run it simply because it is bundled with an ancient Debian release.
Shorewall works with the 2.6 Kernels with a couple of caveats:
Netfilter/iptables doesn't fully support IPSEC in the 2.6 Kernels -- kernel and iptables patches are available and the details may be found at the Shorewall IPSEC-2.6 page.
The 2.6 Kernels do not provide support for the logunclean and
dropunclean options in
/etc/shorewall/interfaces. Note that support
for those options was also removed from Shorewall in version
2.0.0.
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, add the following to
/etc/shorewall/rfc1918
(Note: If you are running Shorewall 2.0.0 or later, you may need to
first copy /usr/share/shorewall/rfc1918 to
/etc/shorewall/rfc1918):
Be sure that you add the entry ABOVE the entry for 192.168.0.0/16.
#SUBNET TARGET 192.168.100.1 RETURN
If you add a second IP address to your external firewall interface to correspond to the modem address, you must also make an entry in /etc/shorewall/rfc1918 for that address. For example, if you configure the address 192.168.100.2 on your firewall, then you would add two entries to /etc/shorewall/rfc1918:
#SUBNET TARGET 192.168.100.1 RETURN 192.168.100.2 RETURN
The solution is the same as the section called “(FAQ 14) I'm connected via a cable modem and it has an internal web server that allows me to configure/monitor it but as expected if I enable rfc1918 blocking for my eth0 interface (the internet one), it also blocks the cable modems web server.” above. Simply substitute the IP address of your ISPs DHCP server.
I see the following in my log:
Mar 1 18:20:07 Mail kernel: Shorewall:OUTPUT:REJECT:IN= OUT=eth0 SRC=192.168.1.2 DST=192.168.1.1 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=26774 DF PROTO=TCP SPT=32797 DPT=80 WINDOW=5840 RES=0x00 SYN URGP=0
Answer: The fact that the message is being logged from the OUTPUT chain means that the destination IP address is not in any defined zone (see FAQ 17). You need to:
Add a zone for the modem in
/etc/shorewall/zones:
#ZONE DISPLAY COMMENTS modem ADSLModem Zone for modem
Define the zone to be associated with eth0 (or whatever interface connects
to your modem) in
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS modem eth0 detect
Allow web traffic to the modem in
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST PORT(S) ACCEPT fw modem tcp 80 ACCEPT loc modem tcp 80
Note that many of these ADSL/Cable Modems have no default gateway or their default gateway is at a fixed IP address that is different from the IP address you have assigned to your external interface. In either case, you may have problems browsing the modem from your local network even if you have the correct routes established on your firewall. This is usually solved by masquerading traffic from your local network to the modem.
/etc/shorewall/masq:
#INTERFACE SUBNET ADDRESS eth0 eth1 # eth1 = interface to local network
For an example of this when the ADSL/Cable modem is bridged, see my configuration. In that case, I masquerade using the IP address of my local interface!
Answer: Yes. See Shorewall and Aliased Interfaces.
You probably haven't set TC_ENABLED=Yes in /etc/shorewall/shorewall.conf so the contents of the tcrules file are simply being ignored.
Yes. Consult the QuickStart guide that you used during your initial setup for information about how to set up rules for your server.
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>,...
Edit /etc/shorewall/shorewall.conf and change “NEWNOTSYN=No” to “NEWNOTSYN=Yes” then restart Shorewall.
If you are running Shorewall 2.2.0 or later, set DROPINVALID=No in /etc/shorewall/shorewall.conf.
Otherwise, add this command to your /etc/shorewall/start file:
run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP 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.
Shorewall Bridging Firewall support is available — check here for details.
I tried this rule to block Google's Adsense that you'll find on everyone's site. Adsense is a Javascript that people add to their Web pages. So I entered the rule:
#ACTION SOURCE DEST PROTO REJECT fw net:pagead2.googlesyndication.com all
However, this also sometimes restricts access to "google.com". Why is that? Using dig, I found these IPs for domain googlesyndication.com:
216.239.37.99 216.239.39.99
And this for google.com:
216.239.37.99 216.239.39.99 216.239.57.99
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:216.239.37.99 all REJECT fw net:216.239.39.99 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 examiniation of protocol headers alone. While some protocols such as FTP require the firewall to examine and possibly modify packet payload, parsing the payload of individual packets doesn't always work because the application-level data stream can be split across packets in arbitrary ways. This is one of the weaknesses of the 'string match' Netfilter extension available in Patch-O-Matic. The only sure way to filter on packet content is to proxy the connections in question -- in the case of HTTP, this means running something like Squid. Proxying allows the proxy process to assemble complete application-level messages which can then be accurately parsed and decisions can be made based on the result.
Answer: Users running Shorewall 2.2.4 or later can simply use the shorewall show capabilities command at a root prompt.
For those running older versions, at a root prompt, enter the command shorewall check. There is a section near the top of the resulting output that gives you a synopsis of your kernel/iptables capabilities.
gateway:/etc/shorewall # shorewall check
Loading /usr/share/shorewall/functions...
Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf...
Loading Modules...
Notice: The 'check' command is unsupported and problem
reports complaining about errors that it didn't catch
will not be accepted
Shorewall has detected the following iptables/netfilter capabilities:
NAT: Available
Packet Mangling: Available
Multi-port Match: Available
Connection Tracking Match: Available
Packet Type Match: Not available
Policy Match: Available
Physdev Match: Available
IP range Match: Available
Verifying Configuration...
...