dhclient on a link shared with static IP addresses?

[Don Cohen]

Don Cohen writes:
 > I'm trying to use a single link to connect to two ISPs, one of
 > which has assigned me a static address.  For the other one I want
 > to run dhclient.  When dhclient gets the address it deletes my
 > static address!  Is there a way to tell it not to do that?
 > I don't see anything in the doc about that.
 > I can't be the first one who ever wanted to do this, can I?

After downloading and reading the source I think I've found a
solution.

I've changed dhclient-script to avoid using ifconfig.
  ifconfig $interface inet 0 down 
=>
  ip addr del $old_ip_address dev $interface
(I've not seen that one used yet)

and the normal case that was causing me grief
  ifconfig $interface inet $new_ip_address $new_subnet_arg $new_broadcast_arg
=> ip addr add ${new_ip_address}/$prefixlen $new_broadcast_arg dev $interface 

where I compute $prefixlen from $new_subnet_mask

Perhaps this is how it should be in the distribution?