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Old 11-16-2015, 06:15 AM   #1
mod_dev_123
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telnet intermittent failure


I'm encountering a rather weird problem. My email hosting provider is often very slow, and my email client (Thunderbird on Linux) is unable to connect.

To get to the bottom of it, I tried performing telnet to port 25 of the server (which is what my email client would do). Sure enough, the telnet fails when the email client does, and succeeds when the email client gets through.

Note: the server is remote - i.e. at a 3rd party "distant" location.
The client is inside a corporate network, where there are many such clients trying to reach the same server to access email.

The weird part is the pattern of failure -
a) On an Xubuntu Linux client, telnet successfully connects sometimes, and hangs most of the time.
b) On a Windows client, telnet almost always succeeds.
c) On a couple of specific Xubuntu Linux clients, telnet always succeeds. This is true even if they are at the same location (in the network) as other 'failing' clients.

I've heard various theories why telnet would fail (firewalls, excessive network load, etc). However, none of these explain the intermittent failure pattern above, wherein some specific clients consistently work well, and other clients work occasionally.

Any ideas would be very much appreciated.
Thanks.
 
Old 11-16-2015, 07:41 AM   #2
berndbausch
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Try network tracing with tcpdump or wireshark. The latter runs on Windows and Linux.
 
  


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