Andrey,
thank you for your response.
I already checked the source based routing page before, however I wasn't really able to get it working.
I tried the following additional rule:
# ip rule show
0: from all lookup local
32765: from all to 192.168.255.0/24 lookup 123
32766: from all lookup main
32767: from all lookup default
# ip route show table 123
default dev eth1 scope link
Any hints?
Thank you,
Christoph