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PostgreSQL: Don't require a lease to have expired if we reallocate it…

… to the previous client

The current default allocate_find query causes per-client IP allocation
instability that starts when allocate_clear rate limiting comes into effect.

With initial pool conditions the default alloc_find query selects from the pool
of all *expired* leases. It selects the least recently used IP address (i.e.
the one with the oldest expiry_time) unless a matching username and/or
callingstationid exists in the table in which case these are prioritised in
order to issue the client with their previous lease. Normally picking the least
recently used address helps to avoid issuing the address to a different client,
maximising the chance of stickiness, which is desirable.

However, if a user makes successive authentication attempts whilst
allocate_clear rate limiting is active they are always allocated a different IP
address because their existing address is unavailable (has not yet expired and
has not been explicitly expired by allocate_clear), with the effect that there
are now multiple rows in the table containing the same pool_key, username and
callingstationid.

The normal ordering then has the undesirable effect of ensuring that successive
future IP address selections for the client will flip-flop between addresses
because username + callingstationid are first prioritied then the *oldest
expiry_time* is chosen (rather than the most recent expiry relating to the most
recent lease). This behaviour persists once allocate_clear rate limiting is no
longer in effect.

We can avoid getting having multiple rows with the same client details by
amending the allocate_find query to include an existing row matching
nasipaddress and pool_key in the available set of IP addreses regardless of
whether the lease has expired or not (i.e. re-allocation of a current lease to
the same client).
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terryburton authored and alandekok committed Aug 28, 2019
1 parent 5891aee commit 362ef4362adacad91f922fb05ae341b85bbe7b43
Showing with 4 additions and 1 deletion.
  1. +4 −1 raddb/mods-config/sql/ippool/postgresql/queries.conf
@@ -13,7 +13,10 @@ allocate_find = "\
SELECT framedipaddress \
FROM ${ippool_table} \
WHERE pool_name = '%{control:${pool_name}}' \
AND expiry_time < 'now'::timestamp(0) \
AND ( \
expiry_time < 'now'::timestamp(0) \
OR ( nasipaddress = '%{NAS-IP-Address}' AND pool_key = '${pool_key}' ) \
) \
ORDER BY \
(username <> '%{SQL-User-Name}'), \
(callingstationid <> '%{Calling-Station-Id}'), \

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