It is possible to control which source address is put in to ARP headers using the ARP_ANNOUNCE option in the kernel.
It takes the following values.
0 (default) Any local address 1 Use address from the same subnet as the target address 2 prefer primary address.
This is worth knowing because the default can give some very strange results when routing packets with linux.
To test this you can run the following command
# sysctl -w net.ipv4.conf.all.arp_announce=1
To make it a permanent solution add a line like this to /etc/sysctl.conf
net.ipv4.conf.all.arp_announce = 1