[sclug] Irritating networking problem

David Given dg at cowlark.com
Tue Sep 4 09:16:26 UTC 2007

Hash: SHA1

Ah, I fondly remember the days, a mere couple of years ago, when I actually
*understood* Linux networking...

I have a desktop PC running Ubuntu on my internal network. It works fine. For
development purposes, I want to install an extra network card and bridge to
another device. This does not work fine, and I can't figure out why.

I load the module and verify that it appears on the list of devices; I
configure the address, netmask, gateway, etc; I ensure that the correct route
appears. But when I actually try to *use* it, I just get:

PING ( 56(84) bytes of data.
ping: sendmsg: Operation not permitted

...which is not something that I've ever seen before.

Investigation reveals that if I reboot the machine, it starts working, but
stops again if I change the IP address. This appears to point to firewalling
issues, and Ubuntu has, indeed, installed a bunch of iptables rules without me
asking it to.

Does this seem like a reasonable conclusion? Given that I already have a
perfectly good real firewall attached to my server, does anyone know how to
turn *off* a Linux firewall? Completely? Would this help?

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