For your interface collection - from a NetBSD list

[Hal Murray]

Subject: Re: bind reacts badly to dhcpcd losing/regaining connectivity
From: Robert Elz <kre at munnari.OZ.AU>
Date: Sat, 15 Apr 2017 07:59:12 +0700
To: Rhialto <rhialto at falu.nl>
Cc: netbsd-users at netbsd.org

    Date:        Sat, 15 Apr 2017 01:41:17 +0200
    From:        Rhialto <rhialto at falu.nl>
    Message-ID:  <20170414234117.GA18315 at falu.nl>


  | Why does named not succeed in using the interface when it gets an
  | address again? What to do about it? I noticed partly because my dns data
  | seemed to have dropped out of caching name servers elsewhere.

This will be a side-effect of the non-root version of named.

Named binds to port 53 on each address it can find, rather than
just port 53 (any address) as typical daemons do, as it is required
to send its replies (UDP replies) from the same address as they
were sent to (part of the DNS spec.)

[These days, I think there's an interface to allow a UDP socket
to be told which (local) addr a packet was sent to, but when bind
was created there was no such thing, so it does it the way that
works everywhere.]

Binding to port 53 requires root permissions - when named first starts
it binds to all addresses, and then drops privs.

Later, when an addr goes away, it will close the socket bound to that
addr - if the addr comes back (or a new address appears) it (attempts
to) bind to port 53 on that addr - but without root privs any more, it
cannot (EPERM).

Solutions to this are just to always run as root, or to recode the
receive code to use the new way to receive the dest addr of incoming
packets, and to set the source addr of outgoing ones (so just one
UDP socket is needed), or perhaps to have named simply re-exec itself
whenever a new addr appears, if not running as root.

kre


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