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Why packets from local interface to local interface get in lo0?


I am currently testing dhcping 1.2 on OpenBSD 3.5 and have some problems
to get it to work on the same machine (works fine when dhcpinging other
machines). The dhcpd is on the machine on interface em0 IP
I do:

# dhcping -v -c -s -h 00:D0:59:83:A0:62

And I got: no answer.

When I sniff with tcpdump -n -i em0... I see nothing!!! If I do tpcdump
-n -i lo0... I see the dhcp request coming but no answer (the dhcpd is
configured for em0 only).

1. Is this normal that OpenBSD send the packet through lo0 when I use
   an IP address of another interface?

2. Is there a way to make it recognised by dhcpd to be coming from em0?

I thought it was special packet crafting it this case that was the problem but
I tried with nc (netcat) with a source address of em0 and got the same thing:
it pass through lo0 with address.

Thanks for your help.

Sylvain Falardeau