[clug] multiple network ports on one machine

Michael Cohen scudette at gmail.com
Mon Mar 30 11:53:35 GMT 2009

Hi Robert,
  Thats not exactly correct as its pretty easy to send a packet using
a raw socket on any interface you want bypassing any internal routing
tables. A good quick tool for this would be to use scapy:


For example:

scapy.sendp( Ether()/IP(dst="www.slashdot.org")/UDP(dport = 53)/"Some
data", iface="eth1")

May not be exactly right - just have a play.

Scapy is not quick but its great for rapid prototyping - i have some C
code for raw sockets around here somewhere if anyone wants (I had to
write a working multicast router a while ago).


2009/3/30 Robert Edwards <bob at cs.anu.edu.au>:
> Adrian wrote:
>> To do some testing I wish to send UDP packets out on one Ethernet
>> interface, through a switch, and back in on another interface of a 4
>> port machine.  What would be the basic server and client code to do this
>> or point me to some basic docs.
>> Adrian
> Not really all that easy from the set-up point of view as the (Linux?)
> IP routing code (UDP runs over IP) will detect that the outgoing
> datagram is destined for an IP address on the same kernel instance
> and so will route it directly internally instead of sending it out
> on the physical link. You may be able to do it with virtual machines
> etc. but then all the virtualisation overheads will hurt any
> performance measurements etc.
> Writing code to send/receive UDP datagrams is generally trivial. NetCat
> (nc) can do it for you with no code involved, otherwise see Beej's
> network programming guide for examples in C and elsewhere on the web
> for examples in Java, Python, Perl etc.
> Cheers,
> Bob Edwards.
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