Routing

pf can route packets through arbitrary interfaces, overruling the routing table
OpenBSD supports only a single default gateway (multipath routing patches are available in KAME)
gateway with two uplinks, all outgoing packets go to the default route and get filtered there
filter rules can use 'route-to' to route matching packets to another interface (second uplink), based on arbitrary criteria (source based routing, protocols, ports)
works with stateful filtering, all packets matching a state created through a route-to rule will follow the initial packet (connection affinity)