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PowerDNS / pdns

PowerDNS Authoritative, PowerDNS Recursor, dnsdist
https://www.powerdns.com/
GNU General Public License v2.0
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ALIAS Resolver in UDP mode: rework source port selection #13039

Open klaus-nicat opened 1 year ago

klaus-nicat commented 1 year ago

Short description

When PowerDNS resolves ALIAS targets in UDP mode, it uses https://github.com/PowerDNS/pdns/blob/master/pdns/dnsproxy.cc#L67 which chooses a random port and uses this for the PDNS lifetime. This works around the typical Linux ephmeral port selection and hence may cause conflicts with other local services running on high ports.

One solution would be to just let the OS select the ephmeral port, ie:

# sysctl net.ipv4.ip_local_port_range
net.ipv4.ip_local_port_range = 32768    60999

Environment

Habbie commented 1 year ago

Alternative outcome: we move the UDP path to stubDoResolve too.

klaus-nicat commented 1 year ago

A very fast workaround/fix until a clean solution:

local.sin4.sin_port = htons(32768+dns_random(28000));
zeha commented 2 months ago

Alternative outcome: we move the UDP path to stubDoResolve too.

I did the simple thing and made the port range configurable.

Moving the UDP path into stubDoResolve would imply having a thread pool, I guess? If I misunderstood, please explain what you had in mind there :)

Habbie commented 2 months ago

Moving the UDP path into stubDoResolve would imply having a thread pool, I guess? If I misunderstood, please explain what you had in mind there :)

It's already in a thread pool - the one holding database connections, which are not free. So a separate thread pool might be cool, but also is not easy.

However, meanwhile people solve this with LUA (which also runs in the thread holding a db connection) and they appear to be happy. So just calling stubDoResolve from the packet handler might just be fine. It would also simplify a few things, including DNSSEC.