Most operating systems support a feature called multicast DNS that
lets you discover other devices on your local network.  Unfortunately,
a few operating systems do not support this correctly.

mDNSRelay was written specifically to work around these deficiencies.

At a high level, mDNSRelay is a DNS server that listens for DNS
requests, ignores everything but the host part, and looks up the
host part using the operating system's built-in resolver.  By running
this on a computer on your network, it therefore becomes possible to
look up mDNS hostnames as though they were normal DNS names.

To use this, you need two things:

1.  A computer on the network with a static IP that can properly
    resolve .local addresses.  This computer, for obvious reasons,
    must not already be a DNS server.

2.  A domain name that you fully control and a working nameserver for
    that domain.

After you have those things:

1.  Create a nameserver record in your domain, such as the following:

    localnameserver     IN A    192.168.1.253
    local               IN NS   localnameserver

    Replace 192.168.1.253 with the static IP of the device you're
    going to run this tool on.

2.  Compile and run this tool on that machine (ideally on startup).

That's it.  After that, when you visit a hostname within that local
subdomain, e.g. internalmdnsname.local.yourdomain.com, your public
nameserver will redirect the client to your local nameserver, which
will then look up internalmdnsname.local and return its IP address
with an authoritative response.

This tool should "just work" on almost any operating system as long
as the OS is configured correctly for looking up .local domains.

Basically, if you can ping or SSH into a .local hostname from the
command line, your OS is properly configured.  If not, figure out
how to make that work first.

This code should run on almost any operating system.  It has been
tested on Linux (Ubuntu Buster on a first-generation Raspberry Pi)
and macOS (Sonoma).  It should work without modification on any
remotely POSIX-like operating system, including *BSD, and possibly
even Windows.  It's that simple and low-level.  It uses no
libraries whatsoever other than libc.