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L2TP: Fix UDP conversation handling. 

RFCs 2661 and 3931 say that L2TPv2 and L2TPv3 use a TFTP-like method
of selecting ports. The initiator picks a source port (which may or
may not be 1701, the IANA assigned L2TP port), and sends a message to
1701; the recipient picks a free port (which may or may not be 1701)
and replies to the initiator's chosen port and address, and the
conversation from then on uses the chosen ports.

In practice, due to NAT, firewalls, etc., most implementations just
use a symmetric predetermined L2TP port. To support both methods
we use one-sided conversations with one port omitted. Fix the lookup
of the reverse conversation. Part of #16565.
This commit is contained in:
John Thacker 2022-08-10 07:30:32 -04:00 committed by A Wireshark GitLab Utility
parent f1140dbc9c
commit e51916b54c
1 changed files with 9 additions and 3 deletions
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12
epan/dissectors/packet-l2tp.c
View File

@ -2933,12 +2933,18 @@ dissect_l2tp_udp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data
return 0;
}
/* RFCs 2661 and 3931 say that L2TPv2 and v3 use a TFTP-like method
* of each side choosing their own port and only using the L2TP port
* to establish the connection. In common practice, both parties use
* the assigned L2TP port the entire time, due to NAT, firewalls, etc.
* We support both methods by using conversations with no second port.
*/
conv = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst, ENDPOINT_UDP,
pinfo->srcport, pinfo->destport, NO_PORT_B);
if (conv == NULL) {
conv = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst, ENDPOINT_UDP,
pinfo->srcport, pinfo->destport, 0);
if (conv == NULL || (conversation_get_dissector(conv, pinfo->num) != l2tp_udp_handle)) {
conv = find_conversation(pinfo->num, &pinfo->dst, &pinfo->src, ENDPOINT_UDP,
pinfo->destport, pinfo->srcport, NO_PORT_B);
}
if ((conv == NULL) || (conversation_get_dissector(conv, pinfo->num) != l2tp_udp_handle)) {