traceroute uses wrong interface

[Andrew S Babb]

I have 4 network interfaces on my server, with lan0 being the public routable network, and lan1, lan2 & lan3 being private non-routable networks.

My network configuration is as follows:
# ifconfig lan0
lan0: flags=1843
inet 139.185.174.5 netmask ffffffc0 broadcast 139.185.174.63
# ifconfig lan1
lan1: flags=1843
inet 139.185.174.67 netmask ffffffe0 broadcast 139.185.174.95
# ifconfig lan2
lan2: flags=1843
inet 139.185.174.99 netmask ffffffe0 broadcast 139.185.174.127
# ifconfig lan3
lan3: flags=1843
inet 139.185.174.131 netmask fffffff0 broadcast 139.185.174.143

In addition, the netstat output is as follows:
# netstat -rna
Routing tables
Destination Gateway Flags Refs Interface Pmtu
127.0.0.1 127.0.0.1 UH 0 lo0 4136
139.185.174.5 139.185.174.5 UH 0 lan0 4136
139.185.174.67 139.185.174.67 UH 0 lan1 4136
139.185.174.99 139.185.174.99 UH 0 lan2 4136
139.185.174.131 139.185.174.131 UH 0 lan3 4136
139.185.174.128 139.185.174.131 U 2 lan3 1500
139.185.174.64 139.185.174.67 U 2 lan1 1500
139.185.174.96 139.185.174.99 U 2 lan2 1500
139.185.174.0 139.185.174.5 U 2 lan0 1500
127.0.0.0 127.0.0.1 U 0 lo0 0
default 139.185.174.1 UG 0 lan0 0

The gateway is 139.185.174.1 which is only accessible via lan0.

If I traceroute and force lan0, then all is OK
# traceroute -i lan0 -n 139.185.174.13
traceroute to 139.185.174.13 (139.185.174.13), 30 hops max, 40 byte packets
1 139.185.174.13 0.132 ms 0.063 ms 0.064 ms

However, if I do not specify an interface, then traceroute wants to use lan3 which is not routable.
traceroute -n 139.185.174.13
traceroute: Warning: Multiple interfaces found; using 139.185.174.131 @ lan3
traceroute to 139.185.174.13 (139.185.174.13), 30 hops max, 40 byte packets
1 * * *
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *


Any ideas why the system is wanting to route through lan3, and how I can get the system to route through the lan0 interface?

Thanks
Andrew

[Jeff Schussele]

Hi Andrew,

Use traceroute as follows:

traceroute -i lanX destination
or
traceroute -s xxx.xxx.xxx.xxx destination

where X=lan interface PPA
and xxx.xxx.xxx.xxx=IP of lanX or the NIC you want to originate the traceroute from.

HTH,
Jeff

[Andrew S Babb]

Thanks for the quick response, but is there a way to avoid this, and to have traceroute detect the lan0 as the default route?

[Ivan Ferreira]

It seems that the traceroute is not designed to check the subnet configurations and then, it thinks that you have more network interfaces on the same network segment.

I saw this problem on another posts and the solution suggested is to use the -i parameter.

[Ivan Ferreira · Accepted solution]

See this patch:

PHNE_34135

http://www2.itrc.hp.com/service/patch/patchDetail.do?patchid=PHNE_34135&sel={hpux:11.11,}&BC=main|search|

( SR:8606356802 CR:JAGaf17502 )

When neither '-i' nor '-s' option is specified, traceroute(1) uses the first interface address returned in the IP header of the outgoing probe packet without referring to the routing table. As a result, the response to this probe packet may not reach the host in case of a multihomed host.

[Josiah Henline]

This drove our networking folks nutts when we went from 11.0 to 11.11.

You can write a script that does a netstat -rn and looks for the default route.

netstat -i $(netstat -rn|grep ^default |awk '{print $5}') MACADD