Policy routing problem

Hello,

I am running a DHCP-Server that serves multiple subnets. The server has
an IP address in all of these subnets, and its primary IP address in a
subnet that is not served by DHCP. Every IP address has its own VLAN
Ethernet interface, eth0.102, eth0.104, etc. In this setup, the DHCP
server often does not send its unicast replies on the interface where it
received the corresponding request, but on the interface of its primary
IP address, and with that IP. My first thought how to change this was by
setting routes depending on destination addresses, but this would cause
big problems with other services running on the same machine, so I tried
to combine iproute2 and iptables, like this:

iptables -t mangle -A OUTPUT -p udp --source-port bootps \
   -d xxx.xxx.22.0/24 -j MARK --set-mark 122

ip route add to xxx.xxx.22.0/24 dev eth0.122 table 122

ip rule add fwmark 122 table 122

Though I can see in my logs that iptables really sets that mark, routing
does not work as expected and the server still uses its primary IP address
for sending unicast DHCP replies.

What am I doing wrong and what must be done to achieve the desired
behaviour?

Regards
  Christoph