DHCP Redundancy

Matt Jenkins matt at smarterbroadband.net
Thu Dec 2 09:08:16 UTC 2010

On 12/01/2010 11:48 PM, Simon Hobson wrote:
> Top posting fixed !
> Matt Jenkins wrote:
>> Would it be possible to have a distributed NFS directory and have 
>> many dhcp daemons read the same leases and configuration files? Does 
>> the DHCP Server re-read the leases file when it starts up so it knows 
>> about all existing leases?
> FYI the ISC server reads the leases file exactly once - when it starts 
> up. After that it is write only and the primary database is held 
> in-memory.
Bummer. I was hoping that the leases file was checked each time before 
handing out a new address. Is there a way to ask an active dhcp server 
for all issued leases (ip/mac pairs) and remaining time on each lease?
> Matt Jenkins wrote:
>> This is for a wireless ISP network built in a remote area where most 
>> of the systems are on solar power. It will not be unheard of for any 
>> given site to lose power for days or more. I need to be able to have 
>> dhcp servers at each location that all share the same leases 
>> information....
> A few questions about that :
> 1) Are they all on the same subnet ?
It is a giant layer 2 ring of sites, with a few other redundant paths. 
The terrain in the foothills makes it a bit of a hodpodge of links (all 
wireless and some is a mesh). There is also a mix of lots of different 
wireless equipment.
> 2) If the site is down, does it matter if clients trying to connect to 
> it can't get a lease ?
No but if the dhcp server were to be at the site that went down then the 
rest of the network would fail. With many feet of snow on the ground 
during most of the winter its very difficult to get to and fix any of 
these sites.
> 3) Does your setup require the client to get the same lease wherever 
> they are ?
I don't think so. Its very rare that a user would be at a different 
site, but some are hotspot type setups, others are AP-CPE setups.
> 4) If the latter, then can you use static assignments (eg client MAC 
> address) to assign addresses ?
The point is to get away from ever having to manually set anything 
again. The only reason to implement DHCP is so we no longer need a 
person managing anything except maintaining equipment. No one gets paid 
to maintain the system and there is less and less time for anyone to be 
"staff" anymore.

When there was 30 to 40 users of the system keeping up with IP addresses 
and making sure there were no conflicts was easy. Now that its starting 
to exceed 250 its becoming a nightmare. Seems like every week there is 
another IP conflict that someone has to track down...

I just need a stupid dhcp server that can works from a database table 
for lease information. I have spent the last few days coming up with 
nothing. I am shocked one doesn't exist. It could even work from a flat 
file, it just needs to reread the flat file every time before handing 
out a new address to ensure its actually available.

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