How to diagnostic UDP discovery failed situation

Wed Jun 21 02:22:48 CEST 2017

Hi, experts

for example, the below case:

You can see a lot of back and forth MTU probe packets been exchanged between tinc nodes, but it’s weird that, from the debug log, one line shows "No response to MTU probes from node1”, but it indeed received a lot of MTU probe response, and finally it get the conclusion of "Packet for node1 (1.1.1.1 port 443) larger than minimum MTU”.

2017-06-21 08:12:05 tinc.myvpn[18854]: Got MTU probe length 1341 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:05 tinc.myvpn[18854]: Got MTU probe length 619 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Got MTU probe length 396 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Sending MTU probe length 77 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Sending MTU probe length 1033 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Sending MTU probe length 798 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Got MTU probe length 607 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:06 tinc.myvpn[18854]: Got MTU probe length 902 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Got MTU probe length 143 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Sending MTU probe length 1156 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Sending MTU probe length 723 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Sending MTU probe length 617 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Got MTU probe length 993 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:07 tinc.myvpn[18854]: Got MTU probe length 546 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Got MTU probe length 901 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Sending MTU probe length 1246 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Sending MTU probe length 786 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Sending MTU probe length 221 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Got MTU probe length 910 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:08 tinc.myvpn[18854]: Got MTU probe length 649 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Got MTU probe length 218 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Sending MTU probe length 526 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Sending MTU probe length 353 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Sending MTU probe length 547 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Got MTU probe length 602 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:09 tinc.myvpn[18854]: Got MTU probe length 201 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Got MTU probe length 543 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Sending MTU probe length 141 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Sending MTU probe length 445 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Sending MTU probe length 806 to node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Got MTU probe length 1418 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:10 tinc.myvpn[18854]: Got MTU probe length 309 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:11 tinc.myvpn[18854]: Got MTU probe length 192 from node1 (1.1.1.1 port 443)

2017-06-21 08:12:11 tinc.myvpn[18854]: No response to MTU probes from node1 (1.1.1.1 port 443)

2017-06-21 08:12:11 tinc.myvpn[18854]: Got MTU probe length 1247 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:11 tinc.myvpn[18854]: Got MTU probe length 1104 from node1 (1.1.1.1 port 443)
2017-06-21 08:12:38 tinc.myvpn[18854]: Packet for node1 (1.1.1.1 port 443) larger than minimum MTU, forwarding via TCP
2017-06-21 08:12:53 tinc.myvpn[18854]: Packet for node1 (1.1.1.1 port 443) larger than minimum MTU, forwarding via TCP
2017-06-21 08:13:04 tinc.myvpn[18854]: Packet for node1 (1.1.1.1 port 443) larger than minimum MTU, forwarding via TCP
2017-06-21 08:13:05 tinc.myvpn[18854]: Packet for node1 (1.1.1.1 port 443) larger than minimum MTU, forwarding via TCP
2017-06-21 08:13:08 tinc.myvpn[18854]: Packet for node1 (1.1.1.1 port 443) larger than minimum MTU, forwarding via TCP

Also you can see from tcpdump that, 192.168.31.114 received the MTU probe response on it’s port 8201

08:14:21.497863 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 980
08:14:22.529725 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 72
08:14:22.529805 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 868
08:14:22.530085 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 1353
08:14:22.531425 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 76
08:14:22.532885 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 876
08:14:22.534025 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 1356
08:15:31.904410 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 868
08:15:31.905610 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 868
08:15:31.907070 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 1340
08:15:32.209491 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 113
08:15:32.209631 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 549
08:15:32.209651 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 637
08:15:32.210451 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 116
08:15:32.211271 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 556
08:15:32.212111 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 644
08:16:41.634229 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 588
08:16:41.634909 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 76
08:16:41.635909 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 620
08:16:42.173050 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 433
08:16:42.173210 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 447
08:16:42.173250 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 1209
08:16:42.174150 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 436
08:16:42.174970 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 452
08:16:42.175890 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 1212
08:17:51.201088 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 324
08:17:51.202368 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 1020
08:17:51.203788 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 980
08:17:52.251311 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 369
08:17:52.251451 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 784
08:17:52.251511 IP 1.1.1.1.https > 192.168.31.114.8201: UDP, length 981
08:17:52.252351 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 372
08:17:52.253511 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 788
08:17:52.254471 IP 192.168.31.114.8201 > 1.1.1.1.https: UDP, length 988