IP Aliasing

Siraj Shaikh siraj.shaikh at gmail.com
Thu Feb 21 12:07:11 UTC 2008

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 netmask 0xffffffff"
ifconfig_ed0_alias1="inet netmask 0xffffffff"
ifconfig_ed0_alias2="inet netmask 0xffffffff"

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?


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