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I have 3 linux servers running Redhat 9.0 on my network. They all use GDM as their display manager. Users use the Cygwin X server to connect to each of the machines using XDMCP. The machines have been configured and working properly for approximately 6 weeks. Suddenly last week, when everyone came in in the morning, none of the remote connections worked to any of the machines. What appears to be happening is that GDM locks up when anyone tries to connect. You're even unable to log in at the console graphically (using GDM). Another symptom seems to be that ssh logins are slow. It takes approximately 10 seconds to return a password prompt after the initial connection. Once the user is logged in, the session seems to work normally. I've tried running netstat and lsof but there doesn't seem to be anything unusual. Rebooting the machines doesn't help. I've tried removing one of them from the network and connecting with a single pc over a direct connection and the problem still existed. Has anyone ever seen anything like this before? If so, please help.
For those interested, I've managed to solve the problem. I discovered that the DNS server running on our router here was malfunctioning. The linux machines are set up with static IP addresses and the only DNS server specified was the one running on the router. When an XDMCP client tried to connect, the Linux machine tried to reverse list the IP address. Since the DNS server didn't respond (even with a message saying it couldn't reverse list the IP address), GDM was sitting in a loop asking for the reverse list. When I power cycled the router and the on board DNS server started working again, the problem was fixed. Other machines on the network weren't affected by this problem because they use DHCP and our DHCP server species the router plus 2 other DNS servers hosted by our ISP. I've now added the extra DNS servers to the linux machines, so hopefully we won't see this problem again.