[Recursor] How to skip cache for a specific pattern of queries?

[AxelBomp]

Hi,

We're using PowerDNS Recursor 5.2.0 and we are trying to disable cache for certain request patterns. For example : when a client requests "blablabla.com", the recursor should not cache the answer.

So far we tried:

• The preresolve() function and "dq.variable = true", but it doesn't seem to work as we can't resolve our records anymore.

• The postresolve() function, like this (with and without the dq.variable // with a ttl set at 1 second):

function postresolve(dq)
pdnslog("postresolve called for "..dq.qname:toString())
local records = dq:getRecords()
for k,v in pairs(records) do
pdnslog(k.." "..v.name:toString().." "..v:getContent())
if v.type == pdns.TXT and dq.qname:toString():match("our_pattern") then
pdnslog("Changing ttl!")
v.ttl = 1
dq.variable = true
end
end
dq:setRecords(records)
return true
end

But it doesn't work either, the answer is still cached.

Any (simple) idea?

Thanks in advance!

[omoerbeek]

Note that setting variable only influences the packet cache. The record wil still be cached in the record cache.

If you say 'it doesn't work', please show exactly what you tried, what you expected and what you saw, including diagnostic commands used and their output. Also checks logs.

But to take one step back: what problem are you trying to solve?

[AxelBomp]

Thanks for your answer!

The problem we're trying to solve:

We have front DNSDist servers on which we have disabled caching for a specific DNS zone (with records that are often created and then deleted with the same name but different values).

These front servers are connected to two backend resolvers using PowerDNS Recursor in round-robin mode, where caching is enabled (by default).

If a record is cached on the first resolver, then deleted and recreated with a different value, and it gets cached by the second resolver, querying our front servers will yield one value at times and another value at other times.

We are addressing this by forcing a TTL of 1 second on these records, but we wanted to know if it is possible to disable caching for those records / this zone.

[omoerbeek]

Do note that some caching resolvers (also stub resolvers in clients) only regard a TTL as a hint. Relying on short lived DNS records is tricky at best. PowerDNS Recursor does respect TTLs, entries will be not be used if they have expired (unless server stale is enabled and the auth does not respond). Short TTLs do increase the load on the recursor and authoritative servers, so use with care.