Configure IP addresses for virtual interfaces only

Question

In my current CentOS ifconfig, I have eth0 and the range of virtual interfaces eth0:0 - eth0:3 each taking up a unique address in the subnet 69.xx.yy.194/29 (gateway = .193, broadcast = .199)

Now I only get one shot at this because I access the server via remote terminal, so I thought I'd ask here to double check what I'm about to do.

I'm guessing if I perform ifdown eth0, it will take everything, including the interfaces down. What I would like is to be able to take down any one of my 5 services running on the machine by plugging them each into their own virtual interface (note, I only have 4 interfaces right now)

Therefore, is it possible to not have an IP address assigned to eth0, and rather put it on eth0:0 so I end up with 5 interfaces each with a separate address?

(Will this have unexpected side-effects for my PuTTY connection?)

score 1 · Answer 1 · answered Nov 08 '13 at 10:44

You're correct that taking down eth0 will take down all cirtual interfaces based on eth0.

Instead of taking the interface down, you could reconfigure it with the address 0.0.0.0 - this will keep the interface up but without an IP address. I just verified that the following sequence worked perfectly:

The server originally has the address 172.17.8.10 on eth0, default gateway is 172.17.8.1

draal:~ # ifconfig eth0:0 172.17.8.11
draal:~ # ifconfig eth0 0.0.0.0
draal:~ # ping 172.17.8.1                                    
PING 172.17.8.1 (172.17.8.1) 56(84) bytes of data.           
64 bytes from 172.17.8.1: icmp_seq=1 ttl=255 time=0.119 ms   
64 bytes from 172.17.8.1: icmp_seq=2 ttl=255 time=0.244 ms   
^C                                                           
--- 172.17.8.1 ping statistics ---                           
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.119/0.181/0.244/0.063 ms            
draal:~ # ifconfig -a
eth0      Link encap:Ethernet  HWaddr 08:00:27:8C:98:44  
          inet6 addr: fe80::a00:27ff:fe8c:9844/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11 errors:0 dropped:0 overruns:0 frame:0
          TX packets:24 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000                         
          RX bytes:1002 (1002.0 b)  TX bytes:1940 (1.8 Kb)     

eth0:0    Link encap:Ethernet  HWaddr 08:00:27:8C:98:44  
          inet addr:172.17.8.11  Bcast:172.17.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

When you're doing this, it's highly likely that your SSH connection will be interrupted. I would start by SSH:ing to one of the virtual interfaces; as long as you don't do ifcfg eth0 down, that session should not be interrupted.

I would also advice you to setup an at job or cron job that resets all interfaces to their previous state after 5 minutes. If your new setup works, you can cancel the at job, but if it doesn't, you'll be able to get back in after a few minutes.

This doesn't appear to work for me. What is your ifconfig-eth0 file like? — Chris Watts, Nov 09 '13 at 13:30
eth0 is not brought back online if its IP address is 0.0.0.0 I can't really provide much more than this as it requires me to contact the datacenter to get them to attach a KVM — Chris Watts, Nov 12 '13 at 22:18

Configure IP addresses for virtual interfaces only

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