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I can define a secondary IP address on the same network card by adding the lines

auto eth0:1
iface eth0:1 inet static
        address 192.168.9.101
        ...

to /etc/network/interfaces.

This gives me an IP address that I can turn on or off using ifup and ifdown. However, by default it is on. From reading the man page, it seems that to have it off by default I just need to remove the line auto eth0:1. However, this makes ifup/down not work at all.

Is there a way to do this?

UPDATE: OK, so the problem was a lot simpler than I thought. Here's what I originally did:

root@dev003:~# cat /etc/network/interfaces 
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
    address 192.168.2.101
    netmask 255.255.0.0
    network 192.168.0.0
    broadcast 192.168.255.255
    gateway 192.168.1.1
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 192.168.1.1

auto eth0:1
iface eth0:1 inet static
    address 192.168.9.101
    netmask 255.255.0.0
    network 192.168.0.0
    broadcast 192.168.255.255
    gateway 192.168.1.1
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 192.168.1.1

root@dev003:~# /etc/init.d/networking restart
 * Reconfiguring network interfaces...                                                                                                                       ssh stop/waiting
ssh start/running, process 3339
ssh stop/waiting
ssh start/running, process 3373
                                                                                                                                                      [ OK ]
root@dev003:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:03:43:82  
          inet addr:192.168.2.101  Bcast:192.168.255.255  Mask:255.255.0.0
          inet6 addr: fe80::a00:27ff:fe03:4382/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2836310 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2806585 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:220309837 (220.3 MB)  TX bytes:187167315 (187.1 MB)

eth0:1    Link encap:Ethernet  HWaddr 08:00:27:03:43:82  
          inet addr:192.168.9.101  Bcast:192.168.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

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:1909267 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1909267 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:160262241 (160.2 MB)  TX bytes:160262241 (160.2 MB)

root@dev003:~# sed -i 's/auto eth0:1/#auto eth0:1/' /etc/network/interfaces 
root@dev003:~# /etc/init.d/networking restart
 * Reconfiguring network interfaces...                                                                                                                       SIOCDELRT: No such process
ssh stop/waiting
ssh start/running, process 3787
                                                                                                                                                      [ OK ]
root@dev003:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:03:43:82  
          inet addr:192.168.2.101  Bcast:192.168.255.255  Mask:255.255.0.0
          inet6 addr: fe80::a00:27ff:fe03:4382/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2840372 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2810267 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:220637496 (220.6 MB)  TX bytes:187451978 (187.4 MB)

eth0:1    Link encap:Ethernet  HWaddr 08:00:27:03:43:82  
          inet addr:192.168.9.101  Bcast:192.168.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

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:1911328 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1911328 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:160435069 (160.4 MB)  TX bytes:160435069 (160.4 MB)

root@dev003:~# ifdown eth0:1
ifdown: interface eth0:1 not configured

This is the point where I got stuck, and figured there's something wrong with using ifup/down in such a configuration. As it turns out, the solution is quite simple:

root@dev003:~# ifup eth0:1
ssh stop/waiting
ssh start/running, process 3829
root@dev003:~# ifdown eth0:1
root@dev003:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:03:43:82  
          inet addr:192.168.2.101  Bcast:192.168.255.255  Mask:255.255.0.0
          inet6 addr: fe80::a00:27ff:fe03:4382/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2840609 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2810473 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:220656625 (220.6 MB)  TX bytes:187469288 (187.4 MB)

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:1911447 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1911447 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:160445053 (160.4 MB)  TX bytes:160445053 (160.4 MB)

Thanks @Gilles for calling me out, and thanks @Marius for the alternate solution.

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What do you mean by “not work at all”: do they produce no visible effect? Are there any error messages? Post the complete contents of /etc/network/interfaces, the output of ifconfig -a before your attempts, and the exact ifup command(s) you tried. –  Gilles Nov 25 '10 at 21:24
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1 Answer

up vote 5 down vote accepted

I don't know if you can achieve this with ifup/ifdown, but you can add or remove an IP address from an interface at runtime by doing

ip addr add 192.168.9.101/32 dev eth0

and

ip addr del 192.168.9.101/32 dev eth0

instead.

You don't need to define any virtual 'eth0:1' interfaces for that (although you can, if you want, by adding label eth0:1 in front of dev eth0, I think. I never saw the point of that.).

Finally, if at some point you change your mind and decide this secondary IP to be available by default, you can do that in /etc/network/interfaces:

auto eth0
iface eth0 inet ...
    ... whatever you have there already for eth0 ...
    up ip addr add 192.168.9.101/32 dev eth0 
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