On Tue, 16 May 2000, Kevin M. Myer wrote:
> I have come across what appears to be a slight problem in the
> implimentation of IP routing when IP aliasing is used in the 2.2.X kernel
> series. Basically, the problem is this:
> (snip)
> This problem doesn't appear to exist in 2.0.X, as a packet sent to one
> interface returns through that interface. I have not tested 2.3.X.
>
> By the way, this problem was seen with Red Hat Linux 6.1 (2.2.X) but not
> with Red Hat Linux 5.2 (2.0.X kernel). I only have access to Red Hat
> boxes so it would be nice if someone could test this on another
> distribution to confirm that this is really a kernel problem and not an
> init script problem with route setup.
I came across this (or something very like it) a while ago and, after a bit of
head scratching decided it to be a userspace problem. I had a machine with a
similar setup and suddenly couldnt get out of the two local subnets. Debugging
RH's ifup I found that it ran the following two commands when bringing the
aliased interface up:
adding route add -host 195.102.69.253 eth0:1
adding route add default 195.166.20.3 eth0:1
(195.166.20.3 being the default gateway for the network on eth0). Now I
had a routing table that looked like this (with bits snipped):
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
195.102.69.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
195.166.20.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 195.166.20.3 0.0.0.0 UG 0 0 0 eth0
0.0.0.0 195.166.20.3 0.0.0.0 UG 0 0 0 eth0
/sbin/route and /proc/net/route don't know about eth0:1, only eth0
and packets started coming out of the wrong interfaces. You can trivially fix
it in RedHat by setting GATEWAYDEV in /etc/sysconfig/network to the correct
interface or deleting the spurious second default gateway.
If there is any issue with the kernel, I think its only in the way that the
parsing of the routing table has changed since 2.0 - previously it would match
the first default route and exit, now it parses the whole tree.
cheers
john
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