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I have a strange problem win an Ubuntu Server 12.04 box. I've just added a wireless card to the box, and configured it. The wireless is working fine but the wired connection stops responding.

If I disable the new wirless (ifdown wlan0) then the old eth0 immediately starts responding. If I bring the wireless back up (ifup wlan0) then the old eth0 stops responding.

To be clear of what I mean by "stops responding"; I can not find any indication on the server that eth0 is being disabled, just any attempt to contact that IP (including a ping) will fail.

The wireless is on a separate subset to the wired this is for routing reasons. The wireless is on 192.168.10.X and the wired is on 192.168.20.X

My /etc/network/interfaces file

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
auto wlan0
iface wlan0 inet dhcp
    wpa-ssid Somewhere Over The Rainbow
    wpa-psk foo bar

iface eth0 inet dhcp
    post-up /sbin/ethtool -s eth0 wol g

The result if ifconfig

eth0      Link encap:Ethernet  HWaddr 00:01:2e:2d:64:03  
          inet addr:192.168.20.46  Bcast:192.168.20.255  Mask:255.255.255.0
          inet6 addr: fe80::201:2eff:fe2d:6403/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2504 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1616 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:263710 (263.7 KB)  TX bytes:146295 (146.2 KB)
          Interrupt:16 Base address:0x6c00 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:300 errors:0 dropped:0 overruns:0 frame:0
          TX packets:300 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:32604 (32.6 KB)  TX bytes:32604 (32.6 KB)

wlan0     Link encap:Ethernet  HWaddr 1c:4b:d6:c3:2b:53  
          inet addr:192.168.10.14  Bcast:192.168.10.255  Mask:255.255.255.0
          inet6 addr: fe80::1e4b:d6ff:fec3:2b53/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:962 errors:0 dropped:0 overruns:0 frame:0
          TX packets:806 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:100025 (100.0 KB)  TX bytes:104869 (104.8 KB)

Can anyone see why this wont let both interfaces work at the same time, and how to fix this?

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1 Answer 1

Given that they're on separate subnets, this is probably a routing issue. It sounds like packets that should go out directly on eth0 to the 192.168.20.0/24 subnet are instead going to the default gateway on (presumably) 192.168.10.x, which then doesn't understand how to route them.

Also, I need clarification: are you saying that other machines on the LAN can no longer successfully ping the server, or are you saying that the server can no longer ping other machines on the LAN?

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Hmmm detecting this is going to be interesting, I'll have to look up a good packet sniffer. I've been testing this as other machines being unable to ping the server. –  couling Nov 27 '12 at 16:36
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