Shorewall and Routing
Tom
Eastep
2005
2006
2007
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
.
Routing vs. Firewalling.
One of the most misunderstood aspects of Shorewall is its
relationship with routing. This article attempts to clear some of the fog
that surrounds this issue.
As a general principle:
Routing determines where packets are to be sent.
Once routing determines where the packet is to go, the firewall
(Shorewall) determines if the packet is allowed to go there.
There are ways that Shorewall can affect routing which are described
in the following sections.
Routing and Netfilter
The following diagram shows the relationship between routing
decisions and Netfilter.
The light blue boxes indicate where routing decisions are made. Upon
exit from one of these boxes, if the packet is being sent to another
system then the interface and the next hop have been uniquely
determined.
The green boxes show where Netfilter processing takes place (as
directed by Shorewall). You will notice that there are two different paths
through this maze, depending on where the packet originates. We will look
at each of these separately.
Packets Entering the Firewall from Outside
When a packet arrives from outside, it first undergoes Netfilter
PREROUTING processing. In Shorewall terms:
Packets may be marked using entries in the /etc/shorewall/tcrules file. Entries in that file
containing ":P" in the mark column are applied here as are rules
that default to the MARK_IN_FORWARD_CHAIN=No setting in
/etc/shorewall/shorewall.conf. These marks may
be used to specify that the packet should be routed using an
alternate routing table; see the Shorewall Squid
documentation for examples.
Marking packets then using the fwmark
selector in your "ip rule add"
commands should NOT be your first choice. In most cases, you can
use the from or dev
selector instead.
The destination IP address may be rewritten as a consequence
of:
DNAT[-] rules.
REDIRECT[-] rules.
Entries in /etc/shorewall/nat.
So the only influence that Shorewall has over where these packets
go is via NAT or by marking them so that they may be routed using an
alternate routing table.
Packets Originating on the Firewall
Processing of packets that originate on the firewall itself are
initially routed using the default routing table then passed through the
OUTPUT chains. Shorewall can influence what happens here:
Packets may be marked using entries in the /etc/shorewall/tcrules file (rules with "$FW" in
the SOURCE column). These marks may be used to specify that the
packet should be re-routed using an alternate routing table.
The destination IP address may be rewritten as a consequence
of:
DNAT[-] rules that specify $FW as the SOURCE.
Entries in /etc/shorewall/nat that
have "Yes" in LOCAL column.
So again in this case, the only influence that Shorewall has over
the packet destination is NAT or marking.
Alternate Routing Table Configuration
The Shorewall 2.x Shorewall
Squid documentation shows how alternate routing tables can be
created and used. That documentation shows how you can use logic in
/etc/shorewall/init to create and populate an
alternate table and to add a routing rule for its use. It is fine to use
that technique so long as you understand that you are basically just using
the Shorewall init script (/etc/init.d/shorewall) to
configure your alternate routing table at boot time and that other than as described in the previous section, there is no
connection between Shorewall and routing when using Shorewall versions
prior to 2.3.2.
Routing and Proxy ARP
There is one instance where Shorewall creates main routing table
entries. When an entry in /etc/shorewall/proxyarp
contains "No" in the HAVEROUTE column then Shorewall will create a host
route to the IP address listed in the ADDRESS column through the interface
named in the INTERFACE column. This is the only case
where Shorewall directly manipulates the main routing
table.
Example:
/etc/shorewall/proxyarp:
#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT
206.124.146.177 eth1 eth0 No
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
The above entry will cause Shorewall to execute the following
command:
ip route add 206.124.146.177 dev eth1
Multiple Internet Connection Support in Shorewall 2.4.2 and
Later
Beginning with Shorewall 2.3.2, support is included for multiple
Internet connections. If you wish to use this feature, we recommend
strongly that you upgrade to version 2.4.2 or later.
Shorewall multi-ISP support is now covered in a separate article.