[dnsdist] Per-domain cache and CNAME records

[Carlo Rengo]

Hello everyone,

I have a PowerDNS Authoritative server, where some domains starting with a
specific pattern end up in a "pipe" backend (instead of going to the main,
MySQL-powered backend). The pipe command returns a different result based
on some conditions, so I found myself forced to disable the cache globally.

Given that I want to put dnsdist in front of PowerDNS Authoritative (for
various reasons) I was thinking to create two pools pointing to the same
backend server, associate a packet cache only to one of them, then send the
"pattern" queries to the non-cached backend. So I made this:

    newServer({address='127.0.0.1:5301'})
    newServer({address='127.0.0.1:5301', pool="noCache"})

    pc = newPacketCache(20000000, {maxTTL=86400, minTTL=0,
temporaryFailureTTL=60, staleTTL=3600, dontAge=false})
    getPool(""):setCache(pc)

    special_domains = newSuffixMatchNode()
    special_domains:add(newDNSName("pattern-a.mydomain.com."))
    special_domains:add(newDNSName("pattern-b.mydomain.com."))

    addAction(SuffixMatchNodeRule(special_domains), PoolAction("noCache"))

which works perfectly, but there's a catch. We have plenty of records
outside of those patterns that are CNAMEs to those patterns, i.e. "
www.mydomain.com" is a CNAME for "www.pattern-a.mydomain.com". Therefore,
queries like these end up being cached, which is not what I want to achieve.

Is there anything I can do to avoid that?

Thanks,
Carlo
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