Messages in this thread | | | Date | Wed, 24 Nov 2010 11:48:03 -0800 (PST) | From | Mike Caoco <> | Subject | Unplug ethernet cable, the route persists. Why? |
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Hello,
This may have been discussed, but all search engines couldn't give me a good answer...
I notice that when an interface is up/running, a local route is in the routing table:
$ ifconfig eth1 eth1 Link encap:Ethernet HWaddr 00:13:20:0e:2f:ed inet addr:192.168.1.125 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::213:20ff:fe0e:2fed/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:35984995 errors:0 dropped:0 overruns:0 frame:0 TX packets:7409151 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:3252413825 (3.2 GB) TX bytes:1340077250 (1.3 GB)
$ ip route 192.168.20.0/24 dev eth0 proto kernel scope link src 192.168.20.120 192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.125 default via 192.168.20.254 dev eth1 metric 100
After I unplug the cable from eth1, the RUNNING flag disappears, but the route is still there:
$ ifconfig eth1 eth1 Link encap:Ethernet HWaddr 00:13:20:0e:2f:ed inet addr:192.168.1.125 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::213:20ff:fe0e:2fed/64 Scope:Link UP BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:35985023 errors:0 dropped:0 overruns:0 frame:0 TX packets:7409151 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:3252415633 (3.2 GB) TX bytes:1340077250 (1.3 GB)
$ ip route 192.168.20.0/24 dev eth0 proto kernel scope link src 192.168.20.120 192.168.1.0/24 dev eth1 proto kernel scope link src 192.168.1.125 default via 192.168.20.254 dev eth1 metric 100
And that *prevents* from using the default route to reach 192.168.1/24 subnet after eth1 is out.
I looked at the code, it seems the IFF_RUNNING flag change is ignored in dev_change_flags():
void __dev_notify_flags(struct net_device *dev, unsigned int old_flags) { ..... if (dev->flags & IFF_UP && (changes & ~(IFF_UP | IFF_PROMISC | IFF_ALLMULTI | IFF_VOLATILE))) call_netdevice_notifiers(NETDEV_CHANGE, dev); }
I searched in the Internet, and saw some people suggest using an application listener (eg, netplug) to remove the route.
My question is why cannot the kernel remove the route automatically when the link becomes down? Why should this complexity be pushed to the user to find a program to do that?
Thanks, Joe
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