DVREF check fails with LSTN packet.

[ak7an]

Feature Request: Support MREFD LSTN listen-only connection packet for RefCheck.Radio compatibility.

M17 LSTN Packet Compatibility

Environment:

• URFD 3.1.4-dht
• UDP port 17000 open and externally reachable
• Normal M17 CONN/ACKN/DISC exchange succeeds

While validating the reflector, URFD receives an M17 packet beginning with LSTN but logs it as an unknown packet.

Journal output:

Jul 26 07:04:04 ed start-urfd-tcd.sh[2720697]: Unknown M17 packet from 74.208.60.144:
Jul 26 07:04:04 ed start-urfd-tcd.sh[2720697]: 0000:  4C 53 54 4E 01 AB 52 3F 35 F4 42                   *LSTN..R?5.B*

Jul 26 07:04:05 ed start-urfd-tcd.sh[2720697]: Unknown M17 packet from 66.179.211.91:
Jul 26 07:04:05 ed start-urfd-tcd.sh[2720697]: 0000:  4C 53 54 4E 0D 97 14 3F 35 F4 42                   *LSTN...?5.B*

Immediately afterward, a normal M17 connection succeeds:

Jul 26 07:04:10 ed start-urfd-tcd.sh[2720697]: M17 connect packet for module A from DVREF A B at 66.179.211.91:39319
Jul 26 07:04:10 ed start-urfd-tcd.sh[2720697]: New client DVREF A B at 66.179.211.91:39319 added with protocol M17 on module A
Jul 26 07:04:10 ed start-urfd-tcd.sh[2720697]: M17 disconnect packet from DVREF A B at 66.179.211.91:39319
Jul 26 07:04:10 ed start-urfd-tcd.sh[2720697]: Client DVREF A B at 66.179.211.91:39319 removed with protocol M17 on module A

tcpdump confirms the same sequence:

74.208.60.144 -> UDP/17000 : LSTN
(no reply)

66.179.211.91 -> UDP/17000 : LSTN
(no reply)

66.179.211.91 -> UDP/17000 : CONN
URFD -> ACKN
66.179.211.91 -> DISC
URFD -> DISC

Question:

Is the LSTN packet intentionally unsupported in URFD, or would support for this packet improve interoperability with newer M17 reflector clients and validation tools?