From: Christopher L. Barnard (cbar44@tsg.cbot.com)
Date: Thu Aug 15 2002 - 11:28:11 EDT
I asked:
> I have a server with two interfaces: hme0 to talk to most servers, and
> hme1 to do backups and talk to the backup server. My problem is getting
> this server to use hme1 when talking to a server on that segment.
>
> details:
> 30 cms1!/ >> ifconfig -a
> lo0: flags=849<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
> inet 127.0.0.1 netmask ff000000
> hme0: flags=863<UP,BROADCAST,NOTRAILERS,RUNNING,MULTICAST> mtu 1500
> inet 172.31.2.5 netmask ffffff00 broadcast 172.31.2.255
> ether a:0:20:1f:2:5
> hme1: flags=843<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
> inet 172.31.14.84 netmask ffffff00 broadcast 172.31.14.255
> ether a:0:20:1f:e:54
>
> The 172.31.2 interface is the default; the 172.31.14 interface is only to
> talk to boxes on the 172.31.14 network. As the ifconfig shows, the netmasks
> are correct.
>
> 31 cms1!/ >> netstat -nr
>
> Routing Table:
> Destination Gateway Flags Ref Use Interface
> -------------------- -------------------- ----- ----- ------ ---------
> 172.31.2.0 172.31.2.5 U 3 10986 hme0
> 172.31.14.0 172.31.14.84 U 2 0 hme1
> 224.0.0.0 172.31.2.5 U 3 0 hme0
> default 172.31.2.7 UG 0 415611
> 127.0.0.1 127.0.0.1 UH 0 1134 lo0
>
> A netstat shows that local traffic to the 172.31.2 network goes out the
> 172.31.2 interface, local traffic to the 172.31.14 network goes out the
> 172.31.14 interface, and everything else goes out the 172.31.2 interface
> to the default router.
>
> I am using IP numbers on traceroute to show that this is not a DNS issue.
>
> When I do a traceroute to a machine on the 172.31.2 network, everything
> is fine.
> 37 cms1!/ >> traceroute 172.31.2.2
> traceroute: Warning: Multiple interfaces found; using 172.31.2.5 @ hme0
> traceroute to 172.31.2.2 (172.31.2.2), 30 hops max, 40 byte packets
> 1 cmstst.cbot.com (172.31.2.2) 1.215 ms 0.291 ms 0.318 ms
>
> When I do a tracetroute to a machine that is not on any directly-connected
> network, packets go to the default router as they should.
> 39 cms1!/ >> traceroute 164.74.19.193
> traceroute: Warning: Multiple interfaces found; using 172.31.2.5 @ hme0
> traceroute to 164.74.19.193 (164.74.19.193), 30 hops max, 40 byte packets
> 1 Router-172-31-2.infra.cbot.com (172.31.2.7) 0.625 ms 0.428 ms 0.415 ms
> 2 192.168.130.2 (192.168.130.2) 0.477 ms 0.448 ms 0.433 ms
> 3 192.168.13.11 (192.168.13.11) 0.537 ms 0.469 ms 0.445 ms
> 4 shalmaneser.tsg.cbot.com (164.74.19.193) 0.547 ms 0.527 ms 0.462 ms
>
> However, when I do a traceroute to a machine on the 172.31.14 interface,
> the machine is *still* telling me that it is using the 172.31.2 interface!
> 38 cms1!/ >> traceroute 172.31.14.11
> traceroute: Warning: Multiple interfaces found; using 172.31.2.5 @ hme0
> traceroute to 172.31.14.11 (172.31.14.11), 30 hops max, 40 byte packets
> 1 netbacker.cbot.com (172.31.14.11) 0.565 ms 0.318 ms 0.405 ms
>
> Does anyone have any suggestions of what else to try to get this machine to
> use its dedicated 172.31.14 interface for machines on the 172.31.14 network?
> TIA, and I will summarize.
The answer:
Traceroute is doing just what it is supposed to do. It is using the
dedicated interface as needed. On a multihomed host, traceroute just uses
in its output the name of the first interface in the list that ifconfig
generates. It is still using the correct interface, however. A series of
snoops in different windows confirms that it is doing what I want. So there
was never a problem, just my misinterpreting the output of traceroute.
Thanks to:
"Safdar Mirza" <SMRZ44@cbot.com>
Very Pinguin <dt1649651@yahoo.com>
Al Hopper <al@logical-approach.com>
Steve Sandau <ssandau@bath.tmac.com>
Roy Culley <tgdcuro1@gd2.swissptt.ch>
Michael Maciolek <mikem@world.std.com>
Simon-Bernard Drolet <sxi@sympatico.ca>
David Morley <dmorley@boothnewspapers.com>
Mike van der Velden <mvanderv@redback.com>
Rahul Parasnis <rahul.parasnis@creditlyonnais.fr>
"McAlister, Bruce" <Bruce.McAlister@irl.xerox.com>
+-----------------------------------------------------------------------+
| Christopher L. Barnard O When I was a boy I was told that |
| cbarnard@tsg.cbot.com / \ anybody could become president. |
| (312) 347-4901 O---O Now I'm beginning to believe it. |
| http://www.cs.uchicago.edu/~cbarnard --Clarence Darrow |
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