Change linux remote host *primary* IP address without losing control

Question

I was wondering about how one could change a remote host primary IP address without getting disconnected at all (without being in a "no IP addr" state).

The matter is poorly discussed on Internet (according to my research). The best resource I found is a little bit tricky.

EXAMPLE : change 10.0.0.11/24 to 10.0.0.15/24

1. ssh root@10.0.0.11
2. ip addr add 10.0.0.15/24 dev eth0 
3. logout

4. ssh root@10.0.0.15
5. ip addr del 10.0.0.11/24 dev eth0 

Problem: The last command removes both IP addresses and the connection is lost because 10.0.0.11 is primary, and it removes its secondary addresses (to which 10.0.0.15 belongs) when deleted.

I know I could "cheat" by adding 10.0.0.11/25 (instead of 24). However, I think it is theorically possible to do this properly.

What do you think?

Answer 1

You need to set the promote_secondaries option on the interface, or on all interfaces:

echo 1 > /proc/sys/net/ipv4/conf/eth0/promote_secondaries

or

sysctl net.ipv4.conf.eth0.promote_secondaries=1

Change eth0 to all to have it work on all interfaces.

This option has been in since 2.6.16 or so: http://www.mail-archive.com/netdev@vger.kernel.org/msg58796.html

I tested this with a dummy interface and it worked there.

Answer 2

If you want set primary address, you have to set this first.

As if you want do that remotely you may be in a strange situation...

As commands like ip may hang if network connexion is loose and/or break if STDOUT terminate (close). Simply bundling ip a del && ip a add on same commad line would not be enough. You have to ensure sanity of command's environment.

You could try something like this:

ssh root@10.0.0.11
echo 'sleep 1;ip a d 10.0.0.11/24 dev eth0;ip a a 10.0.0.15/24 dev eth0' |
    sh >/tmp/mycmd.log 2>/tmp/mycmd.err & exit

ssh root@10.0.0.15
ip a s dev eth0

Maybe the little function nohup could help you, but I'm not conviced about the need of them:

echo 'sleep 1;ip a d 10.0.0.11/24 dev eth0;ip a a 10.0.0.15/24 dev eth0' |
    nohup sh >/tmp/mycmd.log 2>/tmp/mycmd.err & exit

In this way, you will properly disconnect from old ip, for re-connecting via new ip.

CRON Alternative

If at and cron is installed on remote (target) host, you could;

echo 'ip a d 10.0.0.11/24 dev eth0;ip a a 10.0.0.15/24 dev eth0' |
    ssh root@10.0.0.11 batch