[CentOS] Wierd Networking Problem

Shawn Everett shawn at tandac.com
Wed Oct 31 07:24:22 UTC 2007

Hi Guys,

I'm 100% sure this is not a Linux issue but I want a second opinion...

I have a Linux server running CentOS.  It was been absolutely perfect 
without any errors or problems.

Since Monday it has been unable to send email.  It is configured to act as 
a smart host.  All email gets forwarded to smtp.isp.net.

The server is on a network behind a firewall completely nat'd.  No ports 
are forwarded to it.

telnet smtp.isp.net 25 generates the following:
Trying a.b.c.d...
Connected to smtp.isp.net.
Escape character is '^]'.

And then nothing.  All other machines can connect to smtp.isp.net 25 
without a problem.  They also use that host for their outgong mail server.

Using links to surf the web has a similar result.  It just hangs

traceroute and nslookups work correctly.

There are no errors anywhere.

The network in question has various routing and vlan configurations, proxy 
servers and such.  I tend to suspect they're the problem.  They're also 
not in my control.

The firewall does show the Linux box attempting to make SMTP connections 
but nothing else.

Any ideas or suggestions would be appreciated.  I have not yet rebooted the 


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