Shorewall FAQs
Shorewall Community
Tom
Eastep
2004-08-10
2001-2004
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
.
Installing Shorewall
Where do I find Step by Step Installation and Configuration
Instructions?
Answer: Check out the QuickStart Guides.
(FAQ 37) I just installed Shorewall on Debian and the
/etc/shorewall directory is empty!!!
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.
Port Forwarding (Port Redirection)
(FAQ 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 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 PORT
column specify the range as <low-port>:<high-port>.
(FAQ 1a) Ok -- I followed those instructions but it doesn't
work
Answer: That is usually the
result of one of four things:
You are trying to test from inside your firewall (no, that
won't work -- see ).
You have a more basic problem with your local system (the
one that you are trying to forward to) such as an incorrect
default gateway (it should be set to the IP address of your
firewall's internal interface).
Your ISP is blocking that particular port inbound.
You are running 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.
(FAQ 1b) I'm still having problems with port forwarding
Answer: To further diagnose
this problem:
As root, type iptables -t nat -Z
.
This clears the NetFilter counters in the nat table.
Try to connect to the redirected port from an external host.
As root type shorewall show nat
Locate the appropriate DNAT rule. It will be in a chain
called <source zone>_dnat (net_dnat
in the above examples).
Is the packet count in the first column non-zero? If so, the
connection request is reaching the firewall and is being
redirected to the server. In this case, the problem is usually a
missing or incorrect default gateway setting on the local system
(the system you are trying to forward to -- its default gateway
should be the IP address of the firewall's interface to that
system).
If the packet count is zero:
the connection request is not reaching your server
(possibly it is being blocked by your ISP); or
you are trying to connect to a secondary IP address on
your firewall and your rule is only redirecting the primary IP
address (You need to specify the secondary IP address in the
ORIG. DEST.
column in your DNAT rule); or
your DNAT rule doesn't match the connection request
in some other way. In that case, you may have to use a packet
sniffer such as tcpdump or ethereal to further diagnose the
problem.
(FAQ 1c) From the internet, I want to connect to port 1022 on
my firewall and have the firewall forward the connection to port 22 on
local system 192.168.1.3. How do I do that?
In /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:192.168.3:22 tcp 1022
(FAQ 30) I'm confused about when to use DNAT rules and when
to use ACCEPT rules.
It would be a good idea to review the QuickStart Guide
appropriate for your setup; the guides cover this topic in a tutorial
fashion. DNAT rules should be used for connections that need to go the
opposite direction from SNAT/MASQUERADE. So if you masquerade or use
SNAT from your local network to the internet then you will need to use
DNAT rules to allow connections from the internet to your local network.
In all other cases, you use ACCEPT unless you need to hijack connections
as they go through your firewall and handle them on the firewall box
itself; in that case, you use a REDIRECT rule.
(FAQ 38) Where can I find more information about DNAT?
Ian Allen has written a Paper about DNAT and Linux.
DNS and Port Forwarding/NAT
(FAQ 2) I port forward www requests to www.mydomain.com (IP
130.151.100.69) to system 192.168.1.5 in my local network. External
clients can browse http://www.mydomain.com but internal clients
can't.
Answer: I have two objections to
this setup.
Having an internet-accessible server in your local network is
like raising foxes in the corner of your hen house. If the server is
compromised, there's nothing between that server and your other
internal systems. For the cost of another NIC and a cross-over
cable, you can put your server in a DMZ such that it is isolated
from your local systems - assuming that the Server can be located
near the Firewall, of course :-)
The accessibility problem is best solved using Bind Version 9 views
(or using a separate DNS server for local clients) such that
www.mydomain.com resolves to 130.141.100.69 externally and
192.168.1.5 internally. That's what I do here at shorewall.net
for my local systems that use one-to-one NAT.
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:In this configuration, all loc->loc
traffic will look to the server as if it came from the firewall 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 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 or later then include this in /etc/shorewall/init:
ETH0_IP=`find_interface_address eth0`
and make your DNAT rule:
#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT loc loc:192.168.1.5 tcp www - $ETH0_IP
Using this technique, you will want to configure your
DHCP/PPPoE client to automatically restart Shorewall each time that
you get a new IP address.
(FAQ 2a) I have a zone <quote>Z</quote> with an RFC1918 subnet
and I use one-to-one NAT to assign non-RFC1918 addresses to hosts in
Z. Hosts in Z cannot communicate with each other using their external
(non-RFC1918 addresses) so they can't access each other using
their DNS names.
If the ALL INTERFACES column in /etc/shorewall/nat is empty or
contains Yes
, you will also see log messages like the
following when trying to access a host in Z from another host in Z
using the destination hosts's public address:
Oct 4 10:26:40 netgw kernel:
Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1 SRC=192.168.118.200
DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127 ID=1342 DF
PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0
Answer: This is another problem
that is best solved using Bind Version 9 views
. It
allows both external and internal clients to access a NATed host using
the host's DNS name.
Another good way to approach this problem is to switch from
one-to-one NAT to Proxy ARP. That way, the hosts in Z have non-RFC1918
addresses and can be accessed externally and internally using the same
address.
If you don't like those solutions and prefer routing all
Z->Z traffic through your firewall then:
Set the Z->Z policy to ACCEPT.
Masquerade Z to itself.
Set the routeback option on the interface to Z.
Set the ALL INTERFACES column in the nat file to
Yes
.
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
loc 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.
Netmeeting/MSN
(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
Shorewall. What do I do?
Answer: There is an H.323
connection tracking/NAT module that helps with Netmeeting. Note
however that one of the Netfilter developers recently posted the
following:
> I know PoM -ng is going to address this issue, but till it is ready, and
> all the extras are ported to it, is there any way to use the h.323
> contrack module kernel patch with a 2.6 kernel?
> Running 2.6.1 - no 2.4 kernel stuff on the system, so downgrade is not
> an option... The module is not ported yet to 2.6, sorry.
> Do I have any options besides a gatekeeper app (does not work in my
> network) or a proxy (would prefer to avoid them)?
I suggest everyone to setup a proxy (gatekeeper) instead: the module is
really dumb and does not deserve to exist at all. It was an excellent tool
to debug/develop the newnat interface.
Look here
for a solution for MSN IM but be aware that there are significant
security risks involved with this solution. Also check the Netfilter
mailing list archives at http://www.netfilter.org.
Open Ports
(FAQ 4) I just used an online port scanner to check my firewall
and it shows some ports as <quote>closed</quote> rather than
<quote>blocked</quote>. Why?
Answer: (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 /etc/shorewall/action.Drop
which in turn invokes the RejectAuth
action (defined in /etc/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.
(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
showed 100s of ports as open!!!!
Answer: Take a deep breath and
read the nmap man page section about UDP scans. If nmap gets nothing back from your firewall then it reports
the port as open. If you want to see which UDP ports are really open,
temporarily change your net->all policy to REJECT, restart
Shorewall and do the nmap UDP scan again.
(FAQ 4b) I have a port that I can't close no matter how I
change my rules.
I had a rule that allowed telnet from my local network to my
firewall; I removed that rule and restarted Shorewall but my telnet
session still works!!!
Answer: Rules only govern the
establishment of new connections. Once a connection is established
through the firewall it will be usable until disconnected (tcp) or
until it times out (other protocols). If you stop telnet and try to
establish a new session your firerwall will block that attempt.
(FAQ 4c) How to I use Shorewall with PortSentry?
Here's
a writeup on a nice integration of Shorewall and PortSentry.
Connection Problems
(FAQ 5) I've installed Shorewall and now I can't ping
through the firewall
Answer: For a complete
description of Shorewall ping
management, see this page.
(FAQ 15) My local systems can't see out to the net
Answer: Every time I read
systems can't see out to the net
, I wonder where the
poster bought computers with eyes and what those computers will
see
when things are working properly. That aside, the
most common causes of this problem are:
The default gateway on each local system isn't set to the
IP address of the local firewall interface.
The entry for the local network in the /etc/shorewall/masq
file is wrong or missing.
The DNS settings on the local systems are wrong or the user is
running a DNS server on the firewall and hasn't enabled UDP and
TCP port 53 from the firewall to the internet.
(FAQ 29) FTP Doesn't Work
See the Shorewall and FTP page.
(FAQ 33) From clients behind the firewall, connections to some
sites fail. Connections to the same sites from the firewall itself work
fine. What's wrong.
Answer: Most likely, you need to
set CLAMPMSS=Yes in /etc/shorewall/shorewall.conf.
(FAQ 35) I have two Ethernet interfaces to my local network which
I have bridged. When Shorewall is started, I'm unable to pass
traffic through the bridge. I have defined the bridge interface (br0) as
the local interface in /etc/shorewall/interfaces; the bridged Ethernet
interfaces are not defined to Shorewall. How do I tell Shorewall to
allow traffic through the bridge?
Answer: Add the routeback option to
br0 in /etc/shorewall/interfaces.
Logging
(FAQ 6) Where are the log messages written and how do I change
the destination?
Answer: NetFilter uses the
kernel's equivalent of syslog (see man syslog
) to log
messages. It always uses the LOG_KERN (kern) facility (see
man openlog
) and you get to choose the log level (again,
see man syslog
) in your policies and rules. The destination for
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.
(FAQ 6a) Are there any log parsers that work with Shorewall?
Answer: Here are several links
that may be helpful:
http://www.shorewall.net/pub/shorewall/parsefw/
http://www.fireparse.com
http://cert.uni-stuttgart.de/projects/fwlogwatch
http://www.logwatch.org
http://gege.org/iptables
http://home.regit.org/ulogd-php.html
I personally use Logwatch. It emails me a report each day from
my various systems with each report summarizing the logged activity on
the corresponding system.
(FAQ 2b) DROP messages on port 10619 are flooding the logs with
their connect requests. Can i exclude these error messages for this
port temporarily from logging in Shorewall?
Temporarily add the following rule:
DROP net fw udp 10619
(FAQ 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.
(FAQ 6d) Why is the MAC address in Shorewall log messages so
long? I thought MAC addresses were only 6 bytes in length.
What is labeled as the MAC address in a 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)
ExampleMAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00Destination
MAC address = 00:04:4c:dc:e2:28Source
MAC address = 00:b0:8e:cf:3c:4cEthernet
Frame Type = 08:00 (IP Version 4)
(FAQ 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:
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.
(FAQ 17) What does this log message mean?
Answer: Logging occurs out of a
number of chains (as indicated in the log message) in Shorewall:
man1918 or logdrop
The destination address is listed in /usr/share/shorewall/rfc1918
with a logdrop target -- see
/usr/share/shorewall/rfc1918.
rfc1918 or logdrop
The source or destination address is listed in
/usr/share/shorewall/rfc1918 with a logdrop target -- see /usr/share/shorewall/rfc1918.
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. Also see
for another cause of packets being logged
in the FORWARD chain.
logflags
The packet is being logged because it failed the checks
implemented by the tcpflags
interface option.
Here is an example:
Jun 27 15:37:56 gateway kernel:
Shorewall:all2all:REJECT:IN=eth2 OUT=eth1 SRC=192.168.2.2
DST=192.168.1.3 LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF PROTO=UDP
SPT=1803 DPT=53 LEN=47
Let's look at the important parts of this message:
all2all:REJECT
This packet was REJECTed out of the all2all
chain -- the packet was rejected under the all
->all
REJECT policy ( above).
IN=eth2
the packet entered the firewall via eth2. If you see
IN=
with no interface name, the packet originated
on the firewall itself.
OUT=eth1
if accepted, the packet would be sent on eth1. If you see
OUT=
with no interface name, the packet would be
processed by the firewall itself.
SRC=192.168.2.2
the packet was sent by 192.168.2.2
DST=192.168.1.3
the packet is destined for 192.168.1.3
PROTO=UDP
UDP Protocol
DPT=53
The destination port is 53 (DNS)
For additional information about the log message, see http://logi.cc/linux/netfilter-log-format.php3.
In this case, 192.168.2.2 was in the dmz
zone and
192.168.1.3 is in the loc
zone. I was missing the rule:
ACCEPT dmz loc udp 53
(FAQ 21) I see these strange log entries occasionally; what are
they?
Nov 25 18:58:52 linux kernel:
Shorewall:net2all:DROP:IN=eth1 OUT=
MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00 SRC=206.124.146.179
DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 PROTO=ICMP
TYPE=3 CODE=3 [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00
TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]
192.0.2.3 is external on my firewall... 172.16.0.0/24 is my
internal LAN
Answer: While most people
associate the Internet Control Message Protocol (ICMP) with
ping
, ICMP is a key piece of 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.
Routing
(FAQ 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
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
There was an article in SysAdmin covering this topic.
It may be found at http://www.samag.com/documents/s=1824/sam0201h/
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.
'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.
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.
The following was contributed by Martin Brown and is an excerpt
from http://www.docum.org/stef.coene/qos/faq/cache/44.html.
There are two issues requiring different handling when dealing
with multiple Internet providers on a given network. The below assumes
that the host which has multiple Internet connections is a
masquerading (or NATting) host and is at the chokepoint between the
internal and external networks. For the use of multiple inbound
connections to the same internal server (public IP A from ISP A and
public IP B from ISP B both get redirected to the same internal
server), the ideal solution involves using two private IP addresses on
the internal server. This leads to an end-to-end uniqueness of public
IP to private IP and can be easily accomplished by following the
directions here:
http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-inbound
For the use of multiple outbound links to the Internet, there
are a number of different techniques. The simplest is identified here:
http://linux-ip.net/html/adv-multi-internet.html#adv-multi-internet-outbound
Better (and more robust) techniques are available after a kernel
routing patch by Julian Anastasov. See the famous nano-howto.
http://www.ssi.bg/~ja/
Starting and Stopping
(FAQ 7) When I stop Shorewall using <quote>shorewall stop</quote>,
I can't connect to anything. Why doesn't that command work?
The stop
command is intended to
place your firewall into a safe state whereby only those hosts listed in
/etc/shorewall/routestopped' are activated. If
you want to totally open up your firewall, you must use the
shorewall clear
command.
(FAQ 8) When I try to start Shorewall on RedHat, I get messages
about insmod failing -- what's wrong?
Answer: The output you will see
looks something like this:
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed
iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.
This problem is usually corrected through the following sequence
of commands
service ipchains stop
chkconfig --delete ipchains
rmmod ipchains
Also, be sure to check the errata
for problems concerning the version of iptables (v1.2.3) shipped with
RH7.2.
(FAQ 8a) When I try to start Shorewall on RedHat I get a
message referring me to FAQ #8
Answer: This is usually cured
by the sequence of commands shown above in .
(FAQ 9) Why can't Shorewall detect my interfaces properly at
startup?
I just installed Shorewall and when I issue the start command, I
see the following:
Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
Net Zone: eth0:0.0.0.0/0
Local Zone: eth1:0.0.0.0/0
Deleting user chains...
Creating input Chains...
...
Why can't Shorewall detect my interfaces properly?
Answer: The above output is
perfectly normal. The Net zone is defined as all hosts that are
connected through eth0 and the local zone is defined as all hosts
connected through eth1. If you
are running Shorewall 1.4.10 or later, you can consider setting the
detectnets
interface option on your local interface (eth1 in the above example). That will
cause Shorewall to restrict the local zone to only those networks routed
through that interface.
(FAQ 22) I have some iptables commands that I want to run when
Shorewall starts. Which file do I put them in?
You can place these commands in one of the Shorewall Extension Scripts.
Be sure that you look at the contents of the chain(s) that you will be
modifying with your commands to be sure that the commands will do what
they are intended. Many iptables commands published in HOWTOs and other
instructional material use the -A command which adds the rules to the
end of the chain. Most chains that Shorewall constructs end with an
unconditional DROP, ACCEPT or REJECT rule and any rules that you add
after that will be ignored. Check man iptables
and look
at the -I (--insert) command.
(FAQ 34) How can I speed up start (restart)?
Using a light-weight shell such as ash can
dramatically decrease the time required to start
or restart Shorewall. See the
SHOREWALL_SHELL variable in shorewall.conf.
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.
(FAQ 34a) I get errors about a host or network not found when I
run<filename>/var/lib/shorewall/restore</filename>. The
<command>shorewall restore</command> and <command>shorewall -f start</command>
commands gives the same result.
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.
About Shorewall
(FAQ 10) What Distributions does it work with?
Shorewall works with any GNU/Linux distribution that includes the
proper prerequisites.
(FAQ 11) What Features does it have?
Answer: See the Shorewall Feature List.
(FAQ 12) Is there a GUI?
Answer: Yes. Shorewall support is
included in Webmin 1.060 and later versions. See http://www.webmin.com
(FAQ 13) Why do you call it <quote>Shorewall</quote>?
Answer: Shorewall is a
concatenation of Shoreline
(the city where I live) and
Firewall
. The full name of the
product is actually Shoreline Firewall
but
Shorewall
is must more commonly used.
(FAQ 23) Why do you use such ugly fonts on your web site?
The Shorewall web site is almost font neutral (it doesn't
explicitly specify fonts except on a few pages) so the fonts you see are
largely the default fonts configured in your browser. If you don't
like them then reconfigure your browser.
(FAQ 25) How to I tell which version of Shorewall I am running?
At the shell prompt, type:
/sbin/shorewall version
(FAQ 31) Does Shorewall provide protection against....
IP Spoofing: Sending packets over the WAN interface using an
internal LAP IP address as the source address?
Answer: Yes.
Tear Drop: Sending packets that contain overlapping fragments?
Answer: This is the responsibility of the IP stack, not the
Netfilter-based firewall since fragment reassembly occurs before
the stateful packet filter ever touches each packet.
Smurf and Fraggle: Sending packets that use the WAN or LAN
broadcast address as the source address?
Answer: Shorewall can be configured to do that using the
blacklisting
facility. Shorewall versions 2.0.0 and later filter these packets
under the nosmurfs interface option in
/etc/shorewall/interfaces.
Land Attack: Sending packets that use the same address as the
source and destination address?
Answer: Yes, if the routefilter interface option
is selected.
DOS: - SYN Dos - ICMP Dos - Per-host Dos protection
Answer: Shorewall has facilities for limiting SYN and ICMP
packets. Netfilter as included in standard Linux kernels
doesn't support per-remote-host limiting except by explicit
rule that specifies the host IP address; that form of limiting is
supported by Shorewall.
Given that the Debian Stable Release includes Shorewall 1.2.12,
how can you not support that version?
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 2004 and
Shorewall 2.0 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.
(FAQ 36) Does Shorewall Work with the 2.6 Linux Kernel?
Shorewall works with the 2.6 Kernels with a couple of caveats:
Netfilter/iptables doesn't fully support IPSEC in the 2.6
Kernels -- there are interim instructions linked from the Shorewall IPSEC page.
The 2.6 Kernels do not provide support for the logunclean and
dropunclean options in /etc/shorewall/interfaces.
Note that support for those options was also removed from Shorewall
in version 2.0.0.
RFC 1918
(FAQ 14) I'm connected via a cable modem and it has an
internal web server that allows me to configure/monitor it but as
expected if I enable rfc1918 blocking for my eth0 interface (the
internet one), it also blocks the cable modems web server.
Is there any way it can add a rule before the rfc1918 blocking
that will let all traffic to and from the 192.168.100.1 address of the
modem in/out but still block all other rfc1918 addresses?
Answer: 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
(FAQ 14a) Even though it assigns public IP addresses, my
ISP's DHCP server has an RFC 1918 address. If I enable RFC 1918
filtering on my external interface, my DHCP client cannot renew its
lease.
The solution is the same as above.
Simply substitute the IP address of your ISPs DHCP server.
Alias IP Addresses/Virtual Interfaces
(FAQ 18) Is there any way to use aliased ip addresses with
Shorewall, and maintain separate rulesets for different IPs?
Answer: Yes. See Shorewall and Aliased
Interfaces.
Miscellaneous
(FAQ 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.
(FAQ 20) I have just set up a server. Do I have to change
Shorewall to allow access to my server from the internet?
Yes. Consult the QuickStart
guide that you used during your initial setup for information
about how to set up rules for your server.
(FAQ 24) How can I allow conections to let's say the ssh port
only from specific IP Addresses on the internet?
In the SOURCE column of the rule, follow net
by a
colon and a list of the host/subnet addresses as a comma-separated list.
net:<ip1>,<ip2>,...
Example:
ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22
(FAQ 26) When I try to use any of the SYN options in nmap on or
behind the firewall, I get <quote>operation not permitted</quote>. How
can I use nmap with Shorewall?"
Edit /etc/shorewall/shorewall.conf and change NEWNOTSYN=No
to NEWNOTSYN=Yes
then restart Shorewall.
(FAQ 26a) When I try to use the <quote>-O</quote> option of
nmap from the firewall system, I get <quote>operation not permitted</quote>.
How do 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
(FAQ 27) I'm compiling a new kernel for my firewall. What
should I look out for?
First take a look at the Shorewall kernel
configuration page. You probably also want to be sure that you
have selected the NAT of local connections
(READ HELP)
on the Netfilter Configuration menu.
Otherwise, DNAT rules with your firewall as the source zone won't
work with your new kernel.
(FAQ 27a) I just built and installed a new kernel and now
Shorewall won't start. I know that my kernel options are correct.
The last few lines of a startup
trace are these:
+ run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ '[' 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE' = 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.
0/0 -j MASQUERADE' ']'
+ run_iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
iptables: Invalid argument
+ '[' -z '' ']'
+ stop_firewall
+ set +x
Answer: Your new kernel
contains headers that are incompatible with the ones used to compile
your iptables utility. You need to rebuild
iptables using your new kernel source.
(FAQ 28) How do I use Shorewall as a Bridging Firewall?
Experimental Shorewall Bridging Firewall support is available —
check here for details.
Revision History
1.282004-07-14TEInsert
link to Ian Allen's DNAT paper (FAQ 38)1.272004-06-18TECorrect
formatting in H323 quote.1.262004-05-18TEDelete
obsolete ping information.1.252004-05-18TEEmpty
/etc/shorewall on Debian.1.252004-05-08TEUpdate
for Shorewall 2.0.21.242004-04-25TEAdd
MA Brown's notes on multi-ISP routing.1.232004-04-22TERefined
SNAT rule in FAQ #2.1.222004-04-06TEAdded
FAQ 36.1.212004-03-05TEAdded
Bridging link.1.202004-02-27TEAdded
FAQ 35.1.192004-02-22TEAdded
mention of nosmurfs option under FAQ 31.1.182004-02-15TEAdded
FAQ 34.1.172004-02-11TEAdded
FAQ 33.1.162004-02-03TEUpdated
for Shorewall 2.0.1.152004-01-25TEUpdated
FAQ 32 to mention masquerading. Remove tables.1.142004-01-24TEAdded
FAQ 27a regarding kernel/iptables incompatibility.1.132004-01-24TEAdd
a note about the detectnets interface
option in FAQ 9.1.122004-01-20TEImprove
FAQ 16 answer.1.112004-01-14TECorrected
broken link1.102004-01-09TEAdded
a couple of more legacy FAQ numbers.1.92004-01-08TECorrected
typo in FAQ 26a. Added warning to FAQ 2 regarding source address of
redirected requests.1.82003-12-31TEAdditions
to FAQ 4.1.72003-12-30TERemove
dead link from FAQ 1.1.62003.12-18TEAdd
external link reference to FAQ 17.1.52003-12-16TEAdded
a link to a Sys Admin article about multiple internet interfaces. Added
Legal Notice. Moved "abstract" to the body of the document. Moved
Revision History to this Appendix.1.42003-12-13TECorrected
formatting problems1.32003-12-10TEChanged
the title of FAQ 171.22003-12-09TEAdded
Copyright and legacy FAQ numbers1.12003-12-04MNConverted
to Simplified DocBook XML1.02002-08-13TEInitial
revision