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I have turned my raspberry pi B+ into an AP (Wifi Access Point).

A TL-WN725N (wlan0) uses hostapd, setup by create_ap on archlinux arm (latest) to bridge the eth0 interface connected to the LAN via a switch connected to a PLC plug.

All is working fine. I usually have to ssh into the RPi to turn it off.

But today, I don't know why:

  • I can't ssh or even see the RPi on the LAN. It's not that the ssh server is not running, it's simply that an arp scan does not show the RPi anymore (or using Fing on android)! even a ping to the normal IP gives 'No route to host'. The router does not see it either
  • But devices connected on the AP can browse internet fine. And they do that by using eth0 somehow...

How is it possible that device connected to the AP can connect to internet while at the same time the eth0 interface is not visible from the LAN ?

I don't think it's the switch or the PLC plug. Other devices connected to the switch work as expected.

  • Can you ssh to the RPi when connected via wireless? Since you're bridging, I wouldn't expect iptables rules to be a problem. You're not using any, are you? Can you examine the arp tables on your Internet router? It surely must show up there! – bobstro Jul 23 '15 at 14:40
  • I can't see any arp table on the router. I can see current DHCP lease and there is none for the raspi. I can't ssh any way. wirelessly or from laptop connected to router via ethernet. I have configured the eth0 on raspi to fall back to link local ip But I can't see it either in the 169.254.x.x range either... – solsTiCe Jul 23 '15 at 14:53
  • Is the RPi using DHCP to get an IP address? Is it possible it simply got a new address? – bobstro Jul 23 '15 at 14:55
  • yes. I looked at 169.254.x.x – solsTiCe Jul 23 '15 at 14:56
  • So I am running an arp scan on every private network to look for it 172.16.0.0/12 and 10.0.0.0/8. Already done on 192.68.0.0/16. The green LED is blinkin on the eth port on the RPi during a scan. So somehow it works on the hardware level. – solsTiCe Jul 23 '15 at 15:22
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We're getting fussed at that our discussion is too long, and I think I've got enough detail to formulate an answer.

It is possible to bridge without having an IP interface up. This is exactly what most network switches do, in fact, unless they're inadvisably being managed on a user subnet.

What may have happened is that your RPi,for whatever reason, stopped responding to IP on the wired interface, but is still passing bridged traffic through. You don't need a Layer 3 address to pass Layer 2 traffic. Your computers connected to it are still able to talk to their default gateway on the other side, and even get DHCP addresses assigned from a server on the wired side.

If this "just happened", a restart might resolve your issue.

To be clear, were you able to ssh on the wireless side before this happened?

I won't have time to test this myself any time soon, unfortunately.

  • Yes, I was able to ssh from whatever side, wired or wireless. But no anymore on either side. To restart, it would have been better to ssh to the RPi. Because now I have to unplug it. There is no reboot button :-) – solsTiCe Jul 23 '15 at 16:31
  • So the systemd service, namely netctl-ifplugd@eth0.service, managing the interface failed and never auto-restarted as expected ? I tried to unplug/replug the eth cable but it changed nothing. – solsTiCe Jul 23 '15 at 16:40
  • An orderly shutdown is definitely preferred, but it doesn't sound like you have a lot of choice. With recent releases, I haven't had a lot of issues doing it unless something was doing a lot of disk access. I can't think of anything else to try, unfortunately. If you want to do more forensics before rebooting, you could plug in a monitor & keyboard and poke around. It sounds like the interface is up, but IP has gone away on both interfaces, but that's just a guess. – bobstro Jul 23 '15 at 16:59
  • wlan0 never had an IP. Well only internally but not visible outside as it is a bridge. – solsTiCe Jul 23 '15 at 17:08
  • Not reachable from either physical interface. – bobstro Jul 23 '15 at 17:12
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So it finally unfold that it was a bug in the DHCP server of the router that failed to renew the IP lease and let the interface without IP after the first DHCP lease.

http://dev.freebox.fr/bugs/task/20024

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