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SubjectRe: Unplug ethernet cable, the route persists. Why?
On Wed, 24 Nov 2010 11:48:03 -0800 (PST)
Mike Caoco <caoco2002@yahoo.com> wrote:

> 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?
>

Because there is no reason for the kernel to not expect the link to come back.
It is up to user space to do routing policy. For desktop/laptop users this is
done typically with NetworkManager or Connman; for routers this is done with
Quagga; and for servers use other tools.

If the kernel automatically removed the route, it would cause routing daemons
to recompute the route table (and propagate the change) every time a cable
got pulled or NIC needed to be reset.


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