Falls es noch jemanden interessiert, ich habe das Problem gelöst, indem ich einen Raspberry als CUPS Relay im anderen Subnetz eingerichtet habe. Man kann auch den Multicast routen, das ist aber unnötig umständlich, die folgende Antwort von stackexchange funktioniert aber.:
cups 1.6 used dnssd. This multicast to 224.0.0.251 port 5353. As router are not allowed to forward 224.0.0.251 the packets are stuck on the gateway border.
You could use avahi-daemon mDNS reflector feature on the gateway .
NB: If you nat at the gateway you will need to set NONAT for packets incoming from the parent network. In shorewall/masq file:
<external_interface> <parent_net_ip_range> NONAT
<external_interface> 224.0.0.0/4 NONAT
<external_interface>:!<parent_net_ip_range>,224.0.0.0/4 <subnet_ip_range> detect
avahi-daemon.conf on the router
[reflector]
enable-reflector=yes
avahi-daemon will reflect incoming mDNS requests to all local network interfaces, effectively allowing clients to browse mDNS/DNS-SD services on all networks connected to the gateway.
Make sure to not run multiple reflectors between the same networks, this might cause them to play Ping Pong with mDNS packets.
PS: One could need to mangle the packets to increase Time To Live. This is a topic that needs more investigations though here are the shorewall/start, raw iptables tweaks:
/sbin/iptables -t mangle -A PREROUTING -p all -s 224.0.0.0/4 -j TTL --ttl-inc <ttl_inc>
/sbin/iptables -t mangle -A PREROUTING -p all -d 224.0.0.0/4 -j TTL --ttl-inc <ttl_inc>
/sbin/iptables -t mangle -A PREROUTING -p all -s <parent_net_ip_range> -j TTL --ttl-inc <ttl_inc>
/sbin/iptables -t mangle -A PREROUTING -p all -d <parent_net_ip_range> -j TTL --ttl-inc <ttl_inc>
ttl_inc: any number starting from one. I use twenty locally though it is not set in stone. Mostly depends on the number of routers crossed in between printer and client station. Gnome control center "printers" here send a ttl f 255 (the maximum) so it is not required.
