proper ip address aliasing

[Rick Stevens]

Ken Scott wrote:
> On Wed, 2005-02-02 at 12:59, Rick Stevens wrote:
> 
>>jdyke wrote:
>>
>>>I need to add a group of virutal ips to a single interface on FC3. I've 
>>>used
>>>`ip addr add 192.168.2.XX dev eth0` to add the ips which worked fine, 
>>>but what is the best/proper way to add them at boot up.  I'd assume i 
>>>could add them in rc.local which should work?  but is that right?
>>
>>The proper way is to create additional scripts in the
>>"/etc/sysconfig/network-scripts" directory.  They should be named
>>"ifcfg-eth0:x", with "x" being the alias number.  If you're adding 3
>>aliases to eth0, for example, the files would be:
>>
>>	ifcfg-eth0	(primary, leave it alone)
>>	ifcfg-eth0:1	(first alias)
>>	ifcfg-eth0:2	(second alias)
>>	ifcfg-eth0:3	(third alias)
>>
>>Inside those "ifcfg-eth0:x" scripts, you only need four lines:
>>
>>	DEVICE=eth0:x		(replace "x" with the alias number)
>>	IPADDR=xxx.xxx.xxx.xxx
>>	NETMASK=xxx.xxx.xxx.xxx
>>	ONBOOT=yes
>>
>>The first one, "ifcfg-eth0" should be left alone, as it sets up the
>>base device.
>>
>>That's the "official" way to do it and the way "system-config-network"
>>or "Fedora Icon->System Settings->Network" do it, which would allow you
>>to manage them via the GUI system.
> 
> 
> I have not had occasion to use this "virtual IP" technique but I
> appreciate the procedure.

Technically it's called "IP aliasing".

>                            Does this mean that I can take my development
> box (192.168.0.100) and assign it a virtual IP on a different subnet
> (say 192.168.2.100)  so that it could talk to a another box I  have on
> the local network temporarily (which I am working on for someone else)? 
> My machine only has a single network card in it.

Yes you can, but you have to be careful as your switch/router may not
allow you to cross subnet boundries.  For most people it'll work fine.

> That would be great if true, I could leave both my machine's IP alone
> and the other device's IP as well and simply change the virtual IP so
> that they could talk.

We do it a lot, but again, watch your routers and switches.  Also
remember that your _default_ route will remain the same.  It will add
another _network_ route, but not a default one.

> Seems so elegant that it probably does work that way (since it is after
> all Linux)!

Simple is elegant.  That's Unix/Linux in a nutshell.

> I know that I can change my internal IP to match the target machine but
> that solution doesn't allow concurrent web access.

This is the cheap solution.

Historical note:  Before hostheaders were invented in HTTP 1.1, this is
how you got a web server to handle multiple websites.  Each site got an
individual IP address and you aliased the NIC card out the wazoo.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-              Careful!  Ugly strikes 9 out of 10 people!            -
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