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So I've got kind of a weird problem. In our classroom we have a small computer running Ubuntu Server and its main purpose is basically just a small FTP server and web-server for the classroom so that students can upload their files to it and go to its web-page to see any new assignments. Now the problem is, however, that the server will randomly not allow connections to it (be it over FTP or HTTP). It's completely random and sporadic and we don't know what causes it. I ran Nmap on another computer on the same network and during these spouts of down time it shows that the HTTP and FTP ports are not even open (when I'm pretty sure they are, on the server). Fixing it usually is just using /etc/init.d/networking and restarting that or sometimes restarting apache but it's extremely random. The server is also, because it's in a school, behind an FTP proxy but I don't know if that helps or means anything. Any ideas on what might cause this? Also is there a way in Ubuntu Server to set an FTP proxy (uses a pac file) so that it can have an outbound connection? Thanks.

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Any errors in any of the logs? messages? dmesg? kernel logs? What about the switch it is connected to? What about simply unplugging and plugging back in? –  Jon Angliss Aug 13 '10 at 1:03
    
Sounds like you have console access to that server. During one of these spasms, can you run "ps aux" and see whether the relevant processes are present? –  wolfgangsz Aug 13 '10 at 8:57

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Sounds like you may have a bad DNS entry. That would account for the issue being sporadic. If there are two entries, sometimes the good one is returned, sometimes the bad one.

Do a nslookup or dig on the hostname to see if 2 different IPs are returned.

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