If I "ifconfig eth1 down", a rogue route disappears.
If I "ifconfig eth1 up" or "ip link set dev eth1 up", the rogue route
comes back.
The route in question is for 10.3/16, and it's breaking some of my
communications that should be handled by my default route.
I'm not accustomed to ifconfig changing the routing table. Is this
normal?
The kernel in question is 2.6.9-42.0.8.plus.c4smp. It's a CentOSplus
kernel.
The relevant syscalls in an strace of the above ip link command (which a
very short strace, BTW) appear to be:
9992 socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
9992 ioctl(3, SIOCGIFFLAGS, {ifr_name="eth1", ifr_flags=IFF_BROADCAST|
IFF_MULTICAST}) = 0
9992 ioctl(3, SIOCSIFFLAGS, 0x7fbffff510) = 0
9992 close(3) = 0
The strace of ifconfig eth1 up is pretty similar, albeit with a slightly
different 3rd argument to the SIOCSIFFLAGS.
I tracked the call chain to dev_change_flags(), but then things seemed to
start branching out pretty fast, so I thought it might be more productive
to ask around before continuing.
Is anyone on the list familiar with ifconfig up and/or ip link up
changing a routing table? How does the kernel decide what route to add
in such a circumstance?
Thanks!
From: Dan Stromberg <[email protected]>
Date: Fri, 31 Aug 2007 03:55:53 +0000 (UTC)
> I'm not accustomed to ifconfig changing the routing table. Is this
> normal?
Every time you enable an IP address on an interface, the
kernel creates a route to that subnet which gets likewise
deleted when you remove that IP address.
This is why you can ping hosts on your local subnet even
before you add any explicit routes.
It's been like this since ancient times and it's correct.
On Fri, Aug 31, 2007 at 03:55:53AM +0000, Dan Stromberg wrote:
> Is anyone on the list familiar with ifconfig up and/or ip link up
> changing a routing table? How does the kernel decide what route to add
> in such a circumstance?
By the parameteres you supplied for the interface configuration.
So, if you're getting a connected route for 10.3.0.0/16 out of dev
eth0, it's because you assigned some IP in that subnet to that device,
with a netmask of 255.255.0.0
Check that your netmask is correct.
--
Ross Vandegrift
[email protected]
"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37