SSH SMTP Tunneling problem

Discussion in 'Linux Networking' started by Lione, Jul 14, 2003.

  1. Lione

    Lione Guest

    Hi All I have the following net config


    There is a firewall between the two machines than allow ssh connections
    I set up this tunnen

    linux1% ssh -L 1025:smtpserver:25 ...other tunnel stuff... linux2

    POP and IMAP seems to work but I have problems sending mail

    If I send a 2 or tree line mail the sending is ok but if my is a 10 lines
    mail the connection goes in time out

    What could be the problem?
    Lione, Jul 14, 2003
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  2. It almost sounds like an mtu path discovery problem. I have never
    experienced that with an ssh tunnel, but have not tried sending mail
    through my home smtp via ssh tunnel (I just ssh in and use pine).

    My home is connected with PPPoE which has an 8 byte header, so its max mtu
    is 1492. Since I am using a hardware NAT gateway, I could not set its LAN
    mtu, so I had to set the LAN nic of my smtp server to mtu 1492 to receive
    mail larger than small test messages (otherwise "timeout waiting for data
    transfer" in mail logs with default 1500 mtu).

    But in your case I am not sure where you might need to reduce the mtu.
    If Linux2 is on on adsl and Linux1 is on dialup or company network, maybe
    reducing it on your end would help (ppp mtu if dialup, or nic mtu if on
    some other network). I have noticed that on dialup, my default mtu
    sometimes ends up 1524.
    David Efflandt, Jul 15, 2003
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  3. Lione

    Lione Guest

    Linux2 has an interface to an ADSL-NAT router that is it's default gateway.
    I dont have for the moment access to the Router configuration interface to
    reduce the MTU.
    I've tried reducing the MTU of the Linux2 ethernet interface to 576 and
    seems to work even there are still some problem while sending large (Mbytes)

    May thanks
    Lione, Jul 15, 2003
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