[Dnsmasq-discuss] Automatic DHCP client naming

[Simon Kelley]

Ferenc Wagner wrote:

> I can recall reading some warnings about long-running dhcp-scripts, that
> they block dnsmasq.  Did this change?

It has never been the case, dnsmasq will continue to function even if
the a dhcp never completes. The rules go like this: at most one instance
of the script is ever running (dnsmasq waits for the script to exit
before running the next). Changes to the lease database are which
require the script to be invoked are queued waiting running of the
script. The only time that a slow script can have any affect is if
multiple state-changes occur to a single lease before the script can be
run. In that case earlier states are discarded and the current state is
refelected when the script runs.

> 
> Anyway, wouldn't synchronous scripts be a worthy option, especially if
> given the possibility to communicate data back to dnsmasq?  It's also
> easy to fork off an async worker from a sync helper, if needed.

Strict asynchronous (ie block the daemon completely until the script
returns) would be a very bad idea since it would stop DNS requests as
well as DHCP. It's very difficult to write a script and be sure that it
never does DNS resolution, so deadlock would be an ever-present
possibility. Being a bit more clever and just blocking DHCP would be
possible, but still opens up the possibility of strange effects if a
script blocks. Making the DHCP server multi-task will turn dnsmasq into
 ISC dhcpd: a biggish program with a much bigger memory footprint and
make it much less useful on hardware-limited platforms.


> 
> A built-in solution (maybe in dhcp-generate-names) would be safer and
> more efficient, but somewhat cumbersome, because one may want to use the
> hardware address, the IP address, the client ID, the client provided
> hostname etc. as a source. 

It's also the case that of the members of that list, _only_ the IP
address is guaranteed always to be available. (though a proxy for the
MAC address is always around too.

 For my purposes, something like
> 
> dhcp-ignore-names
> dhcp-generate-names=hwaddr:y/:/-/
> 
> would be enough, and dnsmasq already has some regex machinery included,
> but I'm not sure this scheme is general enough, or whether the extra
> safety and efficiency is worth it.  Just brainstorming...

There's no regex machinery available.

something like

dhcp-generate-names=mac:<prefix>,<tag>

and

dhcp-generate-names=ip:<prefix>,<tag>

would be OK, both prefix and tags being optional.

Cheers,

Simon.