Routing question
pll at lanminds.com
pll at lanminds.com
Fri May 9 14:17:51 EDT 2003
In a message dated: Fri, 09 May 2003 13:32:05 EDT
bscott at ntisys.com said:
>On Fri, 9 May 2003, at 11:39am, pll at lanminds.com wrote:
>> I've got a system with 2 NICs in it on 2 separate subnets.
>
> Distro? Release? Kernel version? :)
RH7.3
# uname -a
Linux dell-1 2.4.18-3smp #1 SMP Thu Apr 18 07:27:31 EDT 2002 i686 unknown
> I assume, from the format of your config file contents, that you are
>running Red Hat or a derivative.
Ayup!
> It looks like, when "eth3" comes up, the "/sbin/ifup" script is seeing
>that a default route already exists, and thus does not attempt to process
>the "GATEWAY=" statement in the "ifcfg-eth3" config file. I know RHL 6.2
>and 7.3 do not do this, but it is quite possible it has been added in what
>you are running.
# cat /etc/redhat-release
Red Hat Linux release 7.3 (Valhalla)
Errr, are you sure about 7.3 :)
> Everything for which the system does not have a more specific route, yes.
>In particular, traffic for the 10.241.35.0/24 network should go out the eth0
>interface.
Right.
> Is "rp_filter" on? That can cause problems in asymmetric routing
>situations (which you've got). Turn if off. If this is a Red Hat box, see
>the "/etc/sysctl.conf" file. The generic method is:
>
> echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter
# cat /proc/sys/net/ipv4/conf/all/rp_filter
0
> Also, beware of source address filters on your gateways. If your network
>admin has configured your routes to only accept traffic from the subnet to
>which they are directly connected, you will have problems.
Grrrr, this I can neither verify nor change :(
>> If I add a second default route for the .37 subnet ... I then get no
>> response from either NIC.
>
> Weird. Have you tried putting a sniffer on the wire and seeing what is
>actually happening?
Not yet, I had to run off to anti-productivity session with some
management weenies. Now I'm reading all the e-mail which has piled
up since then :)
> Check the output of "arp -n". Does the system have ARP entries for the
>two gateways?
No, only the .37. It thinks it has no route for the .37:
# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.241.35.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
10.241.37.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 10.241.37.1 0.0.0.0 UG 40 0 0 eth3
0.0.0.0 10.241.35.1 0.0.0.0 UG 40 0 0 eth0
[root at dell-1 proc]# ping 10.241.37.1
PING 10.241.37.1 (10.241.37.1) from 10.241.37.70 : 56(84) bytes of data.
From 10.241.37.70 icmp_seq=1 Destination Host Unreachable
From 10.241.37.70 icmp_seq=2 Destination Host Unreachable
From 10.241.37.70 icmp_seq=3 Destination Host Unreachable
> You *can* ping each interface from the respective directly-connected
>subnet on each interface, correct?
Actually, no! Currently my routing table looks like this:
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.241.35.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
10.241.37.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 10.241.35.1 0.0.0.0 UG 40 0 0 eth0
And I can neither get traffic in or out of eth3. Interestingly, if I
reverse it, and make it look like:
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.241.35.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
10.241.37.0 0.0.0.0 255.255.255.0 U 40 0 0 eth3
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 10.241.37.1 0.0.0.0 UG 40 0 0 eth3
I still can't access eth3. The only thing I can think of is maybe
spanning tree is causing a problem on the switch/router ?
I don't know, I'm at a loss right now.
--
Seeya,
Paul
--
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