Interesting networking problem

The Problem

Hello again, I have another interesting computer networking issue I resolved lately and thought I would share it with you.


The client has a network of about 20 computers and is on a DSL Internet connection. We swapped out their old Linksys router which needed to be rebooted more than once a week. At my recommendation, the client purchased a SonicWall TZ 190 firewall to take its place. After the swap the client could get everywhere on the Internet except their most important site which was hosted at Their email was not downloading either (also hosted at


The strange thing with the email was that it could connect to the server. Outlook would connect but then sit there for 10 minutes trying to download 23Kb of mail before it would time out.

Web site:

The website where their software was installed would not connect at all.

I did a speed test and they were getting 1.3 Mb down and 359Kb up, just what you would expect from the DSL line they were using (AT&T). They could get to every other site I tried on the Internet except the one at

I could telnet to their website on port 80.

I could telnet into their POP3 server on port 110.

When we tried to connect to either they would appear to connect but the data got hung up while transferring.


I put the Linksys router back in and all was well.

We upgraded the firmware on the SonicWall to the latest and greatest - still same problem.

I reset the SonicWall to factory defaults and re-configured it - still same problem.

I swapped out the DSL modem - still same problem.

I started thinking maybe it was a fragmentation or packet size problem so I pinged the server with 5k packets and it failed (ping "ipaddress" -l 5000).

This failed so I started digging a little deeper on Google.

SonicWall Firewalls

I found on Google that the SonicWall had an MTU problem while using the VPN. While we were not using a VPN the symptoms were similar.

I found where to configure the MTU setting for the DSL connection on the SonicWall and changed it from the default 1500 to 1460 and everything started working.

The funny thing is I went to put the setting back to 1500 to make sure it broke again and the SonicWall gave me a warning message saying " Are you sure you want to change the MTU to 1500 as some DSL lines have problem with an MTU of 1500" - Go FIGURE.

They set the default to 1500 but if you try to change it to 1500 they warn you it might not work.

Anyhow all is well now and I though I would post this in hopes that it may save you some time troubleshooting a similar problem

Good luck.

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