SSH SMTP Tunneling problem

Hi All I have the following net config

Linux1------------Linux2---NAT---SMTPserver

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?

On Mon, 14 Jul 2003 10:23:43 GMT, Lione <(E-Mail Removed)> wrote:
> Hi All I have the following net config
>
> Linux1------------Linux2---NAT---SMTPserver
>
> 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?

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.

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> >
> > Linux1------------Linux2---NAT---SMTPserver
> >

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)
files.

May thanks


> 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.
>
> --