libnet seems to write to the wrong interface

I'm trying to write a little program that acts like an IGMPv2 host, sending 
IGMP Reports at startup, listening for IGMP Query messages, and sending IGMP 
Reports in response after a random delay (unless another host beats it to the 
punch).  If I get it working -- and can figure out where to post it -- I'll be 
glad to add it to the libnet samples.

I've heavily adapted icmp_echo_cq.c and added code from a pcap sample to listen 
for IGMP messages then use libnet to respond.  I'm initializing libnet and 
libpcat with the same device string.  However, my system has two interfaces and 
I'm seeing Query messages on eth1 (the device I specified) but sending Report 
messages out eth0 (as confirmed by Ethereal).

Can someone explain to me why libnet would send on an interface other than the 
device passed to libnet_init()?  I'll be glad to show you my code but I don't 
see anyway to attach it here and 300 lines is a bit much to paste into this 
message.

TIA.

                                 Chris