16.3. Pseudo-bridges with Proxy-ARP
If you just want to implement a Pseudo-bridge, skip down a few sections
to 'Implementing it', but it is wise to read a bit about how it works in
practice.
A Pseudo-bridge works a bit differently. By default, a bridge passes packets
unaltered from one interface to the other. It only looks at the hardware
address of packets to determine what goes where. This in turn means that you
can bridge traffic that Linux does not understand, as long as it has an
hardware address it does.
A 'Pseudo-bridge' works differently and looks more like a hidden router than
a bridge, but like a bridge, it has little impact on network design.
An advantage of the fact that it is not a bridge lies in the fact that
packets really pass through the kernel, and can be filtered, changed,
redirected or rerouted.
A real bridge can also be made to perform these feats, but it needs special
code, like the Ethernet Frame Diverter, or the above mentioned patch.
Another advantage of a pseudo-bridge is that it does not pass packets it
does not understand - thus cleaning your network of a lot of cruft. In cases
where you need this cruft (like SAP packets, or Netbeui), use a real bridge.
16.3.1. ARP & Proxy-ARP
When a host wants to talk to another host on the same physical network
segment, it sends out an Address Resolution Protocol packet, which, somewhat
simplified, reads like this 'who has 10.0.0.1, tell 10.0.0.7'. In response
to this, 10.0.0.1 replies with a short 'here' packet.
10.0.0.7 then sends packets to the hardware address mentioned in the 'here'
packet. It caches this hardware address for a relatively long time, and
after the cache expires, it re-asks the question.
When building a Pseudo-bridge, we instruct the bridge to reply to these ARP
packets, which causes the hosts in the network to send its packets to the
bridge. The bridge then processes these packets, and sends them to the
relevant interface.
So, in short, whenever a host on one side of the bridge asks for the
hardware address of a host on the other, the bridge replies with a packet
that says 'hand it to me'.
This way, all data traffic gets transmitted to the right place, and always
passes through the bridge.
16.3.2. Implementing it
In the bad old days, it used to be possible to instruct the Linux Kernel to
perform 'proxy-ARP' for just any subnet. So, to configure a pseudo-bridge,
you would have to specify both the proper routes to both sides of the bridge
AND create matching proxy-ARP rules. This is bad in that it requires a lot
of typing, but also because it easily allows you to make mistakes which make
your bridge respond to ARP queries for networks it does not know how to
route.
With Linux 2.4/2.5 (and possibly 2.2), this possibility has been withdrawn and
has been replaced by a flag in the /proc directory, called 'proxy_arp'. The
procedure for building a pseudo-bridge is then:
Assign an IP address to both interfaces, the 'left' and the 'right'
one
Create routes so your machine knows which hosts reside on the left,
and which on the right
Turn on proxy-ARP on both interfaces, echo 1 >
/proc/sys/net/ipv4/conf/ethL/proxy_arp, echo 1 >
/proc/sys/net/ipv4/conf/ethR/proxy_arp, where L and R stand for the numbers
of your interfaces on the left and on the right side
Also, do not forget to turn on the ip_forwarding flag! When converting from
a true bridge, you may find that this flag was turned off as it is not
needed when bridging.
Another thing you might note when converting is that you need to clear the
arp cache of computers in the network - the arp cache might contain old
pre-bridge hardware addresses which are no longer correct.
On a Cisco, this is done using the command 'clear arp-cache', under
Linux, use 'arp -d ip.address'. You can also wait for the cache to expire
manually, which can take rather long.
You can speed this up using the wonderful 'arping' tool, which on many
distributions is part of the 'iputils' package. Using 'arping' you can send
out unsolicited ARP messages so as to update remote arp caches.
This is a very powerful technique that is also used by 'black hats' to
subvert your routing!
 | On Linux 2.4, you may need to execute
'echo 1 > /proc/sys/net/ipv4/ip_nonlocal_bind' before being able to send
out unsolicited ARP messages! |
You may also discover that your network was misconfigured if you are/were of
the habit of specifying routes without netmasks. To explain, some versions
of route may have guessed your netmask right in the past, or guessed wrong
without you noticing. When doing surgical routing like described above, it
is *vital* that you check your netmasks!
