Note that the idle-session expiry routine is never run more frequently than every 30 seconds, so the timeout is approximate. The default value for timeout is 600 seconds (10 minutes.)
pppoe-relay listens for incoming PPPoE PADI frames on all interfaces specified with -B or -C options. When a PADI frame appears, pppoe-relay adds a Relay-Session-ID tag and broadcasts the PADI on all interfaces specified with -B or -S options (except the interface on which the frame arrived.)
Any PADO frames received are relayed back to the client which sent the PADI (assuming they contain valid Relay-Session-ID tags.) Likewise, PADR frames from clients are relayed back to the matching access concentrator.
When a PADS frame is received, pppoe-relay enters the two peers' MAC addresses and session-ID's into a hash table. (The session-ID seen by the access concentrator may be different from that seen by the client; pppoe-relay must renumber sessions to avoid the possibility of duplicate session-ID's.) Whenever either peer sends a session frame, pppoe-relay looks up the session entry in the hash table and relays the frame to the correct peer.
When a PADT frame is received, pppoe-relay relays it to the peer and deletes the session entry from its hash table.
If a client and server crash (or frames are lost), PADT frames may never be sent, and pppoe-relay's hash table can fill up with stale sessions. Therefore, a session-cleaning routine runs periodically, and removes old sessions from the hash table. A session is considered "old" if no traffic has been seen within timeout seconds. When a session is deleted because of a timeout, a PADT frame is sent to each peer to make certain that they are aware the session has been killed.
pppoe-relay -C eth0 -S eth1
The example above relays frames between PPPoE clients on the eth0 network and PPPoE servers on the eth1 network.
pppoe-relay -B eth0 -B eth1
This example is a transparent relay -- frames are relayed between any mix of clients and servers on the eth0 and eth1 networks.
pppoe-relay -S eth0 -C eth1 -C eth2 -C eth3
This example relays frames between servers on the eth0 network and clients on the eth1, eth2 and eth3 networks.
The pppoe home page is https://dianne.skoll.ca/projects/rp-pppoe/.