[ale] Need help reading tcpdump output

Jim Philips jcphil at mindspring.com
Fri Feb 15 09:29:05 EST 2002


You didn't say what kernel and distro you're running, but when I was
having my problem here is what I learned: "Explicit congestion
notification" can cause a problem in some recent kernels. Guys on the
ppp list advised me to be sure it was turned off by adding the following
line to my rc.local:

echo "0" > /proc/sys/net/ipv4/tcp_ecn

This didn't really help in my case, but it is a known issue. What
finally did the trick was resetting my MTU like so:

ifconfig ppp0 mtu 1474

If turning off explicit congestion notification doesn't help, you might
try changing the MTU on ppp0 and ppp1 with your DSL connection.


On Fri, 2002-02-15 at 09:08, Gary MacKay wrote:
> > Could you post the output of ifconfig? That might show us all something
> > that didn't come up in the tcpdump.
> 
> ------------DSL line: (we0 internal, ne1 external)
> gw# ifconfig -a
> lo0: flags=8009<UP,LOOPBACK,MULTICAST> mtu 32972
>         inet6 fe80::1%lo0 prefixlen 64 scopeid 0x4
>         inet6 ::1 prefixlen 128
>         inet 127.0.0.1 netmask 0xff000000
> lo1: flags=8008<LOOPBACK,MULTICAST> mtu 32972
> ne1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu
> 1500
>         media: Ethernet autoselect (10baseT)
>         inet 207.87.222.229 netmask 0xfffffff0 broadcast 207.87.222.239
>         inet6 fe80::5054:40ff:fe20:4e7a%ne1 prefixlen 64 scopeid 0x1
> we0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu
> 1500
>         media: Ethernet 10base5
>         inet 192.168.0.254 netmask 0xffffff00 broadcast 192.168.0.255
>         inet6 fe80::200:c0ff:fef7:e7ec%we0 prefixlen 64 scopeid 0x2
> sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 296
> sl1: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 296
> ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
> ppp1: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
> tun0: flags=10<POINTOPOINT> mtu 3000
> tun1: flags=10<POINTOPOINT> mtu 3000
> enc0: flags=0<> mtu 1536
> bridge0: flags=0<> mtu 1500
> bridge1: flags=0<> mtu 1500
> gre0: flags=8010<POINTOPOINT,MULTICAST> mtu 1450
> 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
> 
> ----------- Cable modem:  (xl0 internal, ne1 external)
> gw2# ifconfig -a
> lo0: flags=8009<UP,LOOPBACK,MULTICAST> mtu 32972
>         inet6 fe80::1%lo0 prefixlen 64 scopeid 0x4
>         inet6 ::1 prefixlen 128
>         inet 127.0.0.1 netmask 0xff000000
> lo1: flags=8008<LOOPBACK,MULTICAST> mtu 32972
> xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
>         media: Ethernet 10baseT (10baseT half-duplex)
>         inet 192.168.0.253 netmask 0xffffff00 broadcast 192.168.0.255
>         inet6 fe80::201:2ff:fe73:bf7c%xl0 prefixlen 64 scopeid 0x1
> ne1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu
> 1500
>         media: Ethernet autoselect (10baseT)
>         inet6 fe80::5054:4cff:fe1d:2900%ne1 prefixlen 64 scopeid 0x2
>         inet 11.22.33.44 netmask 0xfffffc00 broadcast 255.255.255.255
> sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 296
> sl1: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 296
> ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
> ppp1: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
> tun0: flags=10<POINTOPOINT> mtu 3000
> tun1: flags=10<POINTOPOINT> mtu 3000
> enc0: flags=0<> mtu 1536
> bridge0: flags=0<> mtu 1500
> bridge1: flags=0<> mtu 1500
> gre0: flags=8010<POINTOPOINT,MULTICAST> mtu 1450
> 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
> 
> 
> - Gary



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