The way HP-UX 11.23 chooses a route

[Victor_Makukha]

Dear all,

We have an Itanium machine with HP-UX 11.23.  The routing table is the following:

Destination           Gateway            Flags   Refs Interface  Pmtu
localhost             localhost          UH        0  lo0        4136
L5SVA                 L5SVA              UH        0  lan5       4136
L2SVA                 L2SVA              UH        0  lan2       4136
server1               server1            UH        0  lan0       4136
L1SVA                 L1SVA              UH        0  lan1       4136
L4SVA                 L4SVA              UH        0  lan4       4136
L3SVA                 L3SVA              UH        0  lan3       4136
172.30.136.182        172.30.157.60      UGH       0  lan0          0
172.30.133.180        172.30.157.60      UGH       0  lan0          0
172.30.149.14         172.30.157.60      UGH       0  lan0          0
172.30.149.3          172.30.157.60      UGH       0  lan0          0
192.168.1.0           L1SVA              U         2  lan1       1500
192.168.1.8           L2SVA              U         2  lan2       1500
192.168.1.16          L3SVA              U         2  lan3       1500
192.168.1.24          L4SVA              U         2  lan4       1500
192.168.1.32          L5SVA              U         2  lan5       1500
172.30.157.32         server1            U         2  lan0       1500
loopback              localhost          U         0  lo0           0
default               192.168.1.6        UG        0  lan1          0

But when we traceroute 172.30.149.3 it selects LAN3 somehow? 

server1,sys,root # traceroute 172.30.149.3
traceroute: Warning: Multiple interfaces found; using 192.168.1.17 @ lan3 traceroute to 172.30.149.3 (172.30.149.3), 30 hops max, 40 byte packets
 1  * * *
 2  * * *

Could you help to understand why???

Regards,

[Duncan Edmonstone]

A "netstat -rn" would be easier to interpret than just "netstat -r"

[Matti_Kurkela · Accepted solution]

The HP-UX traceroute does not use the routing table at all. If you don't use the one of the options "-i <interface>" or "-s <source-IP>", it simply uses the first network interface it sees.

"man traceroute" says:

Note: On a multi-homed host, if traceroute is invoked without the -s
or -i options, the source address used for the probe packets may not
be the same as the address of the interface through which they are
sent out.  As a result, the destination or the next hop may not be
able to reply to the probe and traceroute(1) may not display the route
to the destination properly.  Thus, in the multi-homed host case, use
the -s or -i options to explicitly specify the appropriate source
address for probes.

 So when you have a multi-homed host (two or more non-APA network interfaces) you should use the -i or -s options with traceroute to make sure it uses the right interface for the trace.