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I am attempting to place an Ubuntu server (11.04) on our Class C Active Directory (Server 2003 R2) network. I want this server to have a static IP, which I've selected from outside the DHCP pool, but I am not interested in joining the server to the domain properly; it just needs to be available to provide services to local clients that are pointed at it. I set up /etc/network/interfaces correctly, it responded to pings at that address, I rebooted and was able to subsequently ssh in from another client on the network. Great. At this time it was sitting in the same room as the other client (my workstation, on my workbench), going up to the server room and back down again, presumably. After taking it upstairs and plugging it directly in to two (2) of the switches in the server room (successively, not simultaneously), the lights on the port it was plugged into would consistently flicker during boot, but would go dead by the time the logon prompt has appeared, and would remain so thereafter, for any number of reboots (on both switches). So, I brought it back downstairs, plugged it back into my workbench, and...it's not picking up the network now, can't see other hosts (including the default gateway), can't see google, doesn't respond to pings... WHISKEY TANGO FOXTROT, MAN? I am learning a lot as I go in this job, and am definitely no CCNA, as this post almost certainly makes clear. But this seems all funhouse mirror to me, and I've spent the last couple of days trying to google up enough information to understand exactly what I should be googling to get the answers I need. It's not working.

Please forgive the length of this "question," which I suppose could be summarized as: A.) is there a clear, distinct problem domain that this issue lies in, which I could be focusing intently on to identify the solution, or B.) if not, what are the broader subject matters (not too broad; for goodness sake don't say 'networking') that I should be researching to narrow my problem down?

Thanks very much in advance for any direction you can provide.

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What's the output of the ifconfig command? Sounds like the network interface isn't coming up for some reason. –  Shane Madden Jul 22 '11 at 18:10
    
ifconfig shows eth1 is up, with the appropriate assigned inet addr, and 0 RX or TX packets of any kind. –  thump Jul 22 '11 at 18:37
    
Are there two network interfaces in the server? Which (or both?) is plugged in? –  Shane Madden Jul 22 '11 at 18:40
    
there's two, eth0 and eth1; /etc/network/interfaces is edited to reflect this (eth1 is the only connected interface and the only interface referenced in the configuration file). –  thump Jul 22 '11 at 18:54

2 Answers 2

2 things:
1. Is the network interface card faulty? You can check this via doing a cross connect to a laptop or something and assigning IPs in same subnet and pinging each other.
2. If network card is tested fine, then see if you have port security enabled in the network. With MAC based port security, that MAC might be locked out and might need to be cleared from the switch.

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I believe that the network card is fine, both because it had functioned without incident for years when this machine had been a Windows XP box in another life, as well as the fact that I tried this song and dance with eth0 when at first i didn't succeed, and got the exact same results. As far as MAC-based port security goes, does that originate in the switch or the domain controller? There are three switches in the closet, and this problem existed on two of them (didn't try the third, as there were no free ports and I was pretty sure at that point the switch wasn't the problem). –  thump Jul 22 '11 at 19:19
    
port security is switch based. Also, I would suggest, don't go with assumption that card is fine, it might have been fine before, but can fail now, so do test it and confirm the network cable is good as well. –  Raj J Jul 27 '11 at 0:59

I'd suggest connecting this machine to a standalone switch at your workbench, and connecting another test machine to that same switch. Configure the test machine to have an IP address on the same network that the Ubuntu server thinks it has, and then attempt to ping the server. If you can ping the server, then the problem is more likely to be with MAC filtering somewhere else on your network; if you cannot ping the server, then it's very likely the server's network port is bad (or that the server's network is misconfigured in some other way).

If the ping succeeds, I'd suggest walking this process up in baby steps; plug the server and your test machine into one of the switches in the closet and re-try the ping, then try moving the test machine back to your workbench, etc.

Basically, the way you troubleshoot a situation like this one is to make the simplest possible network you can and see if you can get it working there, and then add complexity systematically until it breaks again. If the simplest possible network doesn't work, then you have a very well-defined set of possible problems and you can start testing and replacing hardware until it does.

I'd also suggest running tcpdump on the Ubuntu server to see whether its network interface even thinks it's receiving traffic at all.

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