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I run a Linux server in a subnet that is not under my control. From time to time, the server can not be reached from outside (the Internet) for a few seconds. I'm trying to trace why and started to have a look on the output of ip neigh show (written out to a file regularly with a cron job).

The next time it happened I looked at the file, and it read:

fe80::1 dev eth0 lladdr 00:22:64:b6:10:5c router STALE
192.168.14.1 dev eth0  FAILED

For me, this looks like the gateway (which is 192.168.14.1) does not respond on ARP requests made by the server. Is this correct?

I was trying to find more information, particularly in the iproute2 source code, but didn't find under which condition it would write out FAILED. But maybe that's because I'm not a C developer.

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2 Answers 2

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A FAILED output in the arp cache indicates your server was unable to reach the gateway. You can test this in your LAN but ping any PC in your LAN, check arp status, disconnect PC, then check arp status. You will notice the state change from REACHABLE to FAILED. Similarly, if you send an icmp request, and the gateway replies, the MAC address of the gateway will be included in the output when you run the ip neigh show or arp -a command. The state will be REACHABLE at first, but if there is a problem in connectivity, it might change state to FAILED. You might notice intermediate states of DELAY and PROBE as it tries to reach the gateway before labeling it as failed.

To identify the cause, you need to ping multiple hosts in your subnet and check your arp cache status as the connectivity problem occurs. If only the gateway is shown as failed, while the other hosts are ok, then the problem is between your server and the gateway. If all the hosts are shown as failed, the problem could the connectivity between your server to the switch, or just a cable issue.

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Thanks, but I think you meant "ping a PC in another subnet". Because the ICMP packet only gets through the gateway if it routes - which is not the case in the local LAN. Thanks for your answer, though. –  Daniel Feb 13 '13 at 15:53
    
No, the SAME subnet. As you said, the gateway is being unreachable for a few seconds, if so you won't be able to reach other subnets outside the gateway. We are troubleshooting now. I am assuming your server is connected to other servers in the same switch as well as same subnet, all using the same gateway. Is it just the gateway that you can't reach or you can't also reach the other servers in the same subnet or switch? –  Daniel t. Feb 13 '13 at 16:11
    
I don't need to reach any other servers in the same subnet. all are outside. so the server is connected to a layer2 switch, which is connected to a router, and this router is my gateway. if i would ping a host in the same subnet, it won't pass the gateway (given the fact that all hosts are connected to this l2 switch and there is only one upstream link to the router - which is the case in my scenario) –  Daniel Feb 13 '13 at 16:42

This may be a problem with your system, with the gateway, or with the connection itself. Can you reach other systems in that subnet? If they are reachable while the gateway is not this is a hint that something gets reloaded on the gateway (due to firewall / tc updates or whatever). Maybe reconfigurations of the switch (VLAN e.g.) can cause that, too, but then the connectivity to all systems should be affected.

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