UDP port 137 open for communication using vi-server

[alex b.]

Hi,

I am using vi-server to share the interface of a server application to be control by a client. It works fine in our offices. Now, I transfered it to a client, and it works fine as well. But there is a security issue on their side. They told me that when the communication is initiated from the client, the server application is opening a communication channel on UDP port 137. They have to open this port through the firewall. Is it normal? I configured the communication to be on the port 3363. I attached the .ini file of the server application.

Thank you

Alexandre

[altenbach]

This has nothing to do with your code, it is a windows "feature". 😞

I has to do with the way windows resolves hosts.  Does you code by any chance try to do a reverse lookup (e.g. find the hostname given the IP address).

Windows will first query the configured nameserver. However, if the DNS lookup fails (e.g. timeout, a private or unlisted IP or there is no DNS server configured), windows machines will do a direct probe on port 137 of the target machine with the idea of obtaining the netbios name.

There are many solutions, e.g. configure a local DNS server, allow/ignore these packets, use local hosts or lmhosts files, etc.

[altenbach]

Here is a more detailed explanation. A very nice writeup by Robert Graham from many years ago:

http://www.linuxsecurity.com/resource_files/firewalls/firewall-seen.html#10

[alex b.]

Thank you! I will forwar this information, and I am sure it will be a great help.