dhcp1 at thehobsons.co.uk
Wed Sep 9 07:09:29 UTC 2009
Peter Laws wrote:
>With the students back (all of whom apparently have iPhones!) we're
>getting very close to the WiFi pool's max size.
>While we work on a long-term fix for that, what can I do to get that
>cleanup process to run more frequently so that we aren't as likely
>to run out of addresses?
As Glenn says, this is only a cleanup of the leases file, it does
nothing whatsoever to the number of available leases. The only way to
have more available leases is to change your address range and/or
Examples of changes you might make :
1) The most obvious is to add more addresses, though I realise this
may be difficult.
2) Shorten lease times. If you have a high churn rate, then it's
quite likely that you have a lot of leases that are active but the
client has walked away. There is nothing the server can do about this
- it cannot reclaim the lease before it expires*. However, if you
have a long lease time then shortening it will mean that unused
leases will expire more quickly and be available for re-use - at the
expense of active clients renewing more often.
To put this in perspective, if you have 8 hour lease times, then
every person that walks through with their iPhone set to
automatically connect will then tie up a lease for anywhere up to 8
hours after it's no longer on the network. The minimum 'dead time'
should be about 4 hours since clients will normally renew at half the
lease time. In practical terms then, it means leases tied up all day.
If you cut lease times to (say) 2 hours, then you cut that dead time
down to between 1 and 2 hours. That means you have a lease available
for up to four different people who walk through during a day
(assuming this is a 'working day', yeah I know, students and 'working
day' ...). Potentially this means doing with one address what was
previously done with four. OK, this is a bit contrived and extreme,
but it illustrates the point.
* You wrote :
>it appears that this is DHCPD going through and expiring leases that
>it otherwise assumed were in use
You must erase this idea from your head, lease expiration is an
ongoing task. When a lease reaches it's end time then it's expired -
that's it (apart from some extra stuff if running failover, but we'll
ignore that). The server cannot look around, decide a client has gone
away, and expire it's lease - the client could come back at any time
and continue using the lease it's got. The server and client have a
'contract', the server is not allowed to break that. This does mean
that you can have 'unused' address tied up for days or weeks when a
client goes away - that's part of the tradeoff between stability from
longer leases and quick reuse from short leases.
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