According to RFC3261, CANCEL requests have to be forwarded in the same way as INVITE requests. The code below is a shortcut. If an existing transaction exists, the CANCEL will be relayed automatically with the existing information about the transaction. The proxy needs to be operating in the stateful mode. In other words, using t_relay() instead of forward() to route the calls.

#CANCEL processing
if (is_method("CANCEL")) {
            if (t_check_trans()) t_relay();
            exit;
}

The t_check_trans() will take care of non-CANCEL and non-ACK requests belonging to a transaction, taking care of the retransmissions, if required. According to the documentation found at www.openser.org: non-CANCEL/non-ACK requests: if the request belongs to a transaction (it's a retransmision), the function will...