I have a problem connecting to a pop3 server when connected over PPPoE
(MTU 1492), while everything works using an ISDN connection (MTU 1500).

I get bad tcp cksums for fragmented packets.

Forcing the MSS of the connection to 1412 fixes the problem, but a router
probably should not mess with MSSs negotiated by the peers, or do I
misunderstand this? (The rp-pppoe package has an otpion to clamp MSS.)

Does this ring a bell for someone? I'm suspecting a broken stack on the
remote router (probably a Linux system). The NetBSD client is running
NetBSD-current.

What options do I have to fix this?


Martin

tcpdump: listening on tlp0
22:42:33.319725 {netbsd-client}.62699 > {linux-server}.pop3: F [tcp sum ok] 1205370870:1205370870(0) ack 1090582912 win 17520 (DF) (ttl 64, id 50377)
22:42:33.928624 {linux-server}.pop3 > {netbsd-client}.62699: . [tcp sum ok] 1461:1461(0) ack 1 win 32735 (DF) (ttl 52, id 28229)
22:42:37.616032 {netbsd-client}.62688 > {linux-server}.pop3: S [tcp sum ok] 207863030:207863030(0) win 16384 <mss 1460> (DF) (ttl 64, id 50410)
22:42:37.769037 {linux-server}.pop3 > {netbsd-client}.62688: S [tcp sum ok] 222344672:222344672(0) ack 207863031 win 32736 <mss 1460> (ttl 52, id 65123)
22:42:37.769157 {netbsd-client}.62688 > {linux-server}.pop3: . [tcp sum ok] 1:1(0) ack 1 win 17520 (DF) (ttl 64, id 50413)
22:42:37.851589 {linux-server}.pop3 > {netbsd-client}.62688: P [tcp sum ok] 1:44(43) ack 1 win 32736 (DF) (ttl 52, id 65165)
22:42:37.852485 {netbsd-client}.62688 > {linux-server}.pop3: P [tcp sum ok] 1:21(20) ack 44 win 17520 (DF) (ttl 64, id 50417)
22:42:38.164842 {linux-server}.pop3 > {netbsd-client}.62688: P [tcp sum ok] 44:57(13) ack 21 win 32736 (DF) (ttl 52, id 65287)
22:42:38.169717 {netbsd-client}.62688 > {linux-server}.pop3: P [tcp sum ok] 21:36(15) ack 57 win 17520 (DF) (ttl 64, id 50420)
22:42:38.375061 {linux-server}.pop3 > {netbsd-client}.62688: P [tcp sum ok] 57:62(5) ack 36 win 32736 (DF) (ttl 52, id 65344)
22:42:38.428995 {netbsd-client}.62688 > {linux-server}.pop3: . [tcp sum ok] 36:36(0) ack 62 win 17520 (DF) (ttl 64, id 50427)
22:42:41.389218 {netbsd-client}.62688 > {linux-server}.pop3: P [tcp sum ok] 36:42(6) ack 62 win 17520 (DF) (ttl 64, id 50449)
22:42:41.468079 {linux-server}.pop3 > {netbsd-client}.62688: P [tcp sum ok] 62:71(9) ack 42 win 32736 (DF) (ttl 52, id 504)
22:42:41.482161 {netbsd-client}.62688 > {linux-server}.pop3: P [tcp sum ok] 42:48(6) ack 71 win 17520 (DF) (ttl 64, id 50450)
22:42:41.563073 {linux-server}.pop3 > {netbsd-client}.62688: P [tcp sum ok] 71:76(5) ack 48 win 32736 (DF) (ttl 52, id 519)
22:42:41.564226 {linux-server}.pop3 > {netbsd-client}.62688: F [tcp sum ok] 76:76(0) ack 48 win 32736 (ttl 52, id 520)
22:42:41.564298 {netbsd-client}.62688 > {linux-server}.pop3: . [tcp sum ok] 48:48(0) ack 77 win 17520 (DF) (ttl 64, id 50451)
22:42:41.564624 {netbsd-client}.62688 > {linux-server}.pop3: F [tcp sum ok] 48:48(0) ack 77 win 17520 (DF) (ttl 64, id 50452)
22:42:41.571255 {netbsd-client}.62686 > {linux-server}.pop3: S [tcp sum ok] 393771036:393771036(0) win 16384 <mss 1460> (DF) (ttl 64, id 50457)
22:42:41.648015 {linux-server}.pop3 > {netbsd-client}.62686: S [tcp sum ok] 1966123666:1966123666(0) ack 393771037 win 32736 <mss 1460> (ttl 53, id 54493)
22:42:41.648098 {netbsd-client}.62686 > {linux-server}.pop3: . [tcp sum ok] 1:1(0) ack 1 win 17520 (DF) (ttl 64, id 50458)
22:42:41.650525 {linux-server}.pop3 > {netbsd-client}.62688: . [tcp sum ok] 77:77(0) ack 49 win 32735 (DF) (ttl 52, id 538)
22:42:41.740156 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 1:44(43) ack 1 win 32736 (DF) (ttl 53, id 54512)
22:42:41.750485 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 1:21(20) ack 44 win 17520 (DF) (ttl 64, id 50461)
22:42:41.825081 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 44:57(13) ack 21 win 32736 (DF) (ttl 53, id 54529)
22:42:41.826658 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 21:36(15) ack 57 win 17520 (DF) (ttl 64, id 50462)
22:42:41.917726 {linux-server}.pop3 > {netbsd-client}.62686: . [tcp sum ok] 57:57(0) ack 36 win 32736 (DF) (ttl 53, id 54544)
22:42:42.814094 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 57:63(6) ack 36 win 32736 (DF) (ttl 53, id 54690)
22:42:42.829122 {netbsd-client}.62686 > {linux-server}.pop3: . [tcp sum ok] 36:36(0) ack 63 win 17520 (DF) (ttl 64, id 50463)
22:42:45.821614 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 36:42(6) ack 63 win 17520 (DF) (ttl 64, id 50464)
22:42:45.895607 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 63:77(14) ack 42 win 32736 (DF) (ttl 53, id 55210)
22:42:45.897490 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 42:48(6) ack 77 win 17520 (DF) (ttl 64, id 50465)
22:42:45.970313 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 77:84(7) ack 48 win 32736 (DF) (ttl 53, id 55220)
22:42:45.980738 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 48:54(6) ack 84 win 17520 (DF) (ttl 64, id 50466)
22:42:46.054656 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 84:90(6) ack 54 win 32736 (DF) (ttl 53, id 55235)
22:42:46.229177 {netbsd-client}.62686 > {linux-server}.pop3: . [tcp sum ok] 54:54(0) ack 90 win 17520 (DF) (ttl 64, id 50470)
22:42:46.247278 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 84:90(6) ack 54 win 32736 (DF) (ttl 53, id 55252)
22:42:46.247343 {netbsd-client}.62686 > {linux-server}.pop3: . [tcp sum ok] 54:54(0) ack 90 win 17520 (DF) (ttl 64, id 50471)
22:42:46.303728 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 90:211(121) ack 54 win 32736 (DF) (ttl 53, id 55260)
22:42:46.310168 {netbsd-client}.62686 > {linux-server}.pop3: P [tcp sum ok] 54:70(16) ack 211 win 17520 (DF) (ttl 64, id 50472)
22:42:46.384381 {linux-server}.pop3 > {netbsd-client}.62686: P [tcp sum ok] 211:228(17) ack 70 win 32736 (DF) (ttl 53, id 55270)
22:42:46.429166 {netbsd-client}.62686 > {linux-server}.pop3: . [tcp sum ok] 70:70(0) ack 228 win 17520 (DF) (ttl 64, id 50473)
22:42:46.665190 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 55317:1472@0+) (ttl 52)
22:42:47.144732 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 55374:1472@0+) (ttl 52)
22:42:47.720557 {linux-server}.pop3 > {netbsd-client}.62699: P [bad tcp cksum 4b3a!] 1:1453(1452) ack 1 win 32736 (frag 31523:1472@0+) (ttl 51)
22:42:48.104988 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 55529:1472@0+) (ttl 52)
22:42:50.025098 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 55858:1472@0+) (ttl 52)
22:42:53.865343 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 56521:1472@0+) (ttl 52)
22:43:01.546983 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 57866:1472@0+) (ttl 52)
22:43:16.905231 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 60170:1472@0+) (ttl 52)
22:43:47.625367 {linux-server}.pop3 > {netbsd-client}.62686: P [bad tcp cksum 560e!] 228:1680(1452) ack 70 win 32736 (frag 942:1472@0+) (ttl 52)
22:43:50.648441 {netbsd-client}.62686 > {linux-server}.pop3: F [tcp sum ok] 70:70(0) ack 228 win 17520 (DF) (ttl 64, id 50628)
22:43:50.721515 {linux-server}.pop3 > {netbsd-client}.62686: . [tcp sum ok] 1688:1688(0) ack 71 win 32735 (DF) (ttl 53, id 1866)

250 packets received by filter
0 packets dropped by kernel