Messages in this thread | | | Date | Wed, 24 Nov 2010 12:18:26 -0800 | From | Stephen Hemminger <> | Subject | Re: Unplug ethernet cable, the route persists. Why? |
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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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