dhclient dry-run - need to know the def gateway without installing the def route

alessandro macuz alessandro.macuz at gmail.com
Mon Apr 7 20:23:32 UTC 2014

2014-04-07 21:15 GMT+02:00 Simon Hobson <dhcp1 at thehobsons.co.uk>:

> alessandro macuz <alessandro.macuz at gmail.com> wrote:
> > I could deduce the gateway from the DHCP server IP address because
> normally they coincide but that would be my assumption.
> You've heard the expression that "assume" makes an "ass" out of "u" and
> "me", well your assumption is badly wrong. In home and small networks it is
> likely to be true (not mine though). But once you get to business networks
> then its more likely that the DHCP server is not at the same IP address as
> the router.
However, I'm still not quite sure why you are doing things the long way
> round. What's specifically wrong with using the route that comes with the
> lease ? Or more to the point, what's wrong with dealing with it when you
> get it ? And BTW - have you considered the possibility of getting different
> answers to your two queries ?

Sorry maybe I should have added more details but there is only one defaut
gateway on the network of that interface and hence the dry-run returns
always the same IP address. As to the second reply it will configure the

Why I want to run a dry-run? Because eventually I need to install some
routes pointing to the only gateway on the network. On the system there is
another interface that gets the IP dinamically and two default routes are
not OK

> For many clients (certainly the ISC one on Linux) you can supply your own
> script/modify the stock one. That means you can go and accept a lease, then
> use your own script to fiddle with the system. Or at least I thin that's
> the case.

I thought of this and delete the 2nd def route but even if for less than a
second packets would load-balanced between the two default routes. Now that
I'm writing I realize that the loadbalancing mechanism is in place only
when the adm distance is the same, so maybe I can play with that value if
the DHCP protocol allows that.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.isc.org/pipermail/dhcp-users/attachments/20140407/06bd6969/attachment.html>

More information about the dhcp-users mailing list