force DDNS update

[Simon Hobson]

Carl Karsten wrote:

>  > Yes, simply delete the stale records using nsupdate*
>
>That just dumps the 'incorrect' entry, but won't add the correct one, right?
>guessing the easiest way to fix is force the client to do another 
>dchp request?

Correct

>Is there some way to get dhcpd to do this?

No

>  > Thw DHCP server will NOT replace or remove an A record that does not
>  > have the correct TXT record to go with it. The TXT record has a hash
>>  of several bits of information that allows the server to determine
>>  that it wasn't something else that put the record there. This is a
>>  safety feature - otherwise someone could name their client "server"
>>  and the DHCP server would happily replace the A record for you
>>  important server of the same name with one that points to the client,
>>  with the obvious effects on the network !
>
>I see your point.  But I think we can get the best of both worlds.
>
>In my case, dhcpd is the only thing with the key.
>
>The host name comes from the following:
>
>Client supplied
>
>host sahara {
>          hardware ethernet 00:40:ca:11:3c:6c ;
>          option host-name "sahara" ;
>          fixed-address 192.168.1.3 ; }
>
>option host-name=concat("dhcp", binary-to-ascii(10, 8, "-",
>suffix(leased-address,1) ) ) ;
>
>What determines the precedence order,
>and is there a way to ignore the client supplied one?

ddns-hostname="something";

>  > The TXT record has a hash
>>  of several bits of information that allows the server to determine
>>  that it wasn't something else that put the record there.
>
>Which server? (dhcp or bind?)

DHCP

>This has me wondering:
>Box1 does DHCPREQUEST and gets a lease.
>Could a Box2 construct a DHCPRELEASE that looks like it came from Box1 so that
>the dhcp server doesn't know that Box1 is still using the IP?

Yes, it is almost trivial to do. Just create a DHCP-Release with the 
other machines MAC address and send it to the server. This might not 
actually get your too far though, the server will not give it to 
another client for two reasons :

1) It will not be chosen for reuse until other, less recently used, 
addresses have been exhausted.

2) It will gte abandoned when the server does a "ping before offer" 
check - assuming of course that the client doesn't have a firewall 
blocking pings (which IMHO is a  stupid thing to do !)