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our office is on a VPN connection to our customer's big UNIX servers to provide tech support, and we use telnet client, such as putty or xshell on windows XP&7.

One month before, we changed our ISP from company A to B, and some of us changed our network ipv4 setting's DNS server setting during the switching process,from the default automatically assigned one, to some public one, such as, and changed it back.

Now, any machines with its DNS setting touched, can still telnet to client's UNIX machine, but even a "ps -ef" or "ls -l" which may return more than 1 screen of content get stuck there, until over the idle time and closed by remote servers. All clients, such as telnet, putty, xshell all work the same way.

Has anybody experienced that before? And how to settle it?

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The root cause is the default MTU size changed by switching ISP from A to B, thus the long response message will be truncated, that's why telnet will stuck. To fix the problem, i followed instructions as the steps in this post:

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1200 store=persistent

The size before is 1500. The reason why it is triggered after DNS setting, I guess the network setting will be updated if you change any of the setting including DNS.

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