Date: Wed, 1 Nov 2000 21:20:53 -0800 From: "Timothy L. Robertson" <timothyr@timothyr.com> To: <freebsd-questions@freebsd.org> Subject: Flaky IP Connection Message-ID: <NEBBKDBFKKIDPAPJPFOPGELPCGAA.timothyr@timothyr.com>
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Hi Everyone,
I've been having problems with my PPPoE connection. It works fine
most of the time, but sometimes connections hang. On repeatable time
seems to be when I cvsup, but it also seems to hang when sendmail
exchanges certain messages with my ISP's SMTP server. I've found
that this icmp packet seems to suggest the problem:
tcpdump -vvv -X -i tun0 -s 2000 icmp
tcpdump: listening on tun0
20:06:30.673201 165.247.202.104 > 165.247.202.104: icmp:
165.247.202.104 unreachable - need to frag (mtu 1480) for
165.247.202.104.iad2 > 165.247.202.104.cvsup: [|tcp] (DF) (ttl 64, id
1920, bad cksum 0!) (ttl 255, id 1921)
0x0000 4500 0038 0781 0000 ff01 d383 a5f7 ca68
E..8...........h
0x0010 a5f7 ca68 0304 7dc7 0000 05c8 4500 05dc
...h..}.....E...
0x0020 0780 4000 4006 0000 a5f7 ca68 a5f7 ca68
..@.@......h...h
0x0030 0407 176f 150c 95c6
This confuses me a little, because my MTUs are set to 1492, not 1480:
bash-2.03$ ifconfig
fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1492
inet 192.168.1.1 netmask 0xffffff00 broadcast 192.168.1.255
inet6 fe80::202:b3ff:fe03:aae1%fxp0 prefixlen 64 scopeid 0x1
ether 00:02:b3:03:aa:e1
media: autoselect (100baseTX) status: active
supported media: autoselect 100baseTX <full-duplex> 100baseTX
10baseT/UTP <full-duplex> 10baseT/UTP
xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1492
inet 10.0.0.2 netmask 0xffffff00 broadcast 10.0.0.255
inet6 fe80::201:2ff:fe48:ad91%xl0 prefixlen 64 scopeid 0x2
ether 00:01:02:48:ad:91
media: 10baseT/UTP (10baseT/UTP <half-duplex>)
supported media: 10baseT/UTP <full-duplex> 10baseT/UTP
<half-duplex> 10baseT/UTP
lp0: flags=8810<POINTOPOINT,SIMPLEX,MULTICAST> mtu 1500
ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 552
faith0: flags=8000<MULTICAST> mtu 1500
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
gif1: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
gif2: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
gif3: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 fe80::1%lo0 prefixlen 64 scopeid 0xb
inet6 ::1 prefixlen 128
inet 127.0.0.1 netmask 0xff000000
tun0: flags=8151<UP,POINTOPOINT,RUNNING,PROMISC,MULTICAST> mtu 1492
inet6 fe80::202:b3ff:fe03:aae1%tun0 --> :: prefixlen 64
scopeid 0xc
inet 165.247.202.104 --> 165.247.202.1 netmask 0xffffffff
Opened by PID 107
My best guess about what's happening is that sometimes a packet of
length > 1480 bytes gets sent out, somehow gets diverted back to my
own interface (by NAT or ipfw?) and then gets stuck by this 1480 MTU.
Is this a bug or do I have something misconfigured? Why does
tcpdump report a different MTU than ifconfig? Any help appreciated.
Thanks,
- - -Tim
timothyr@timothyr.com
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