IP Aliasing

[Jerry McAllister]

On Thu, Feb 21, 2008 at 12:07:08PM +0000, Siraj Shaikh wrote:

> On 29/01/2008, Olivier Nicole <on at cs.ait.ac.th> wrote:
> > > 1) is there an upper limit to configuring a number of alias addresses?
> >
> > I have a machine with 200+ IP without any problem.
> >
> >  > 2) if an interface is configured with an alias address, then what
> >  > address is shown on the traffic leaving this interface? So, for
> >  > example, if I were to ping this machine on its primary address, I
> >  > expect to get a response from the primary address of the interface.
> >  > What happens if I ping an alias address, would I get a response from
> >
> > By default exiting traffic is using the primary address (the one
> >  defined with no keyword alias in the ifconfig). I think there is a way
> >  to choose the exiting IP.
> >
> >  When a paket is responding, it use the same IP that was used in the
> >  query (else any firewall would be confused in the way).
> >
> >
> >  > 3) In the above scenario, all traffic leaving the interface
> >  > (regardless of the source IP on it) will have the same MAC address
> >  > (the one of the interface) - is that right?
> >
> >
> > Right except maybe some NIC that allow several MAC addresses? That
> >  could be used in hi availability?
> >
> >
> >  > 4) Does anyone know if there are there any other network
> >  > characteristics or behaviour by which we can distinguish a machine
> >  > having more than one IP address (primary plus alias) configued on one
> >  > of its interface?
> >
> >
> > Once you cross a router, you don't see the MAC of the machine anymore,
> >  MAC is local to your LAN anyway.
> >
> >  Olivier
> >
> >
> >
> 
> One last thing I wanted to know (sorry to email after a long delay),
> in order for me to add aliases that I want to remain configured on the
> machine at every boot, I can simply add, for exmaple, the following
> lines to the rc.conf file?
> 
> ifconfig_ed0_alias0="inet 127.0.0.251 netmask 0xffffffff"
> ifconfig_ed0_alias1="inet 127.0.0.252 netmask 0xffffffff"
> ifconfig_ed0_alias2="inet 127.0.0.253 netmask 0xffffffff"
> 

Looks right.  The main nasty thing is in the aliasnn, the nn must
start at 0 and be sequential - like you have it here.  But, you can't
just take one out of the middle without moving the others up to fill in.

> Just want to know, as I want to configure about 253 addresses as an
> alias on a single machine (along with the primary address, this will
> be 254 address, a whole C-class subnet) - and would like these entries
> to hold when I boot. Also, is there any shortcut to adding a range of
> net/host address or would I have to add a line for each address?

Not that I know of.
But, maybe someone has written something.

////jerry

> 
> Thanks
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