minimdnsd (MDNS server)

Extremely simple MDNS server for Linux. Much smaller and lighter weight than Avahi.

⚠️ This has not been battle hardened, or even thoroughly checked ⚠️

Primarily MDNS Hostname responder - i.e. run this, and any computer on your network can say ping your_hostname.local and it will resolve to your PC. Specifically, it uses whatever name is in your /etc/hostname

• Uses no CPU unless event requested.
• Only needs around 32kB RAM.
• Compiles to between 15-45kB
• Can run as a user or root.
• Zero config + Watches for /etc/hostname changes. (Optionally: Can use -h to also watch for a host alias)
• Works on IPv6

⚠️ Caveats ⚠️

• This tool only replies to hostnames, so you can use hostname.local but not services, so you can't use it to find your printer.
• This tool assumes UNICAST-RESPONSE - whether true or not. I am unaware of any MDNS clients that don't know how to accept it and it's tidier to just force UNICAST-RESPONSE.

General Motivation

1. To obviate need for avahi-daemon
2. To provide an MDNS server on very simple systems
3. To act as an educational tool for the following items

To demonstrate the following:

1. Use of inotify to detect changes of /etc/hostname
2. Use of getifaddrs to iterate through all available interfaces
3. Use of NETLINK_ROUTE and RTMGRP_IPV4_IFADDR and RTMGRP_IPV6_IFADDR to monitor for any new network interfaces or addresses.
4. Use of multicast in IPv4 and IPv6 to join a multicast group
5. Use of recvmsg to get the interface and address that a UDP packet is received on
6. Use of optarg to handle command-line parameters.
7. Use of fork() and wait3() to handle workers. (Only used in DNS forwarding mode)

And for general housekeeping:

7. Use of github workflows to build and test tool on Linux
8. Makefile example for building .deb files
9. Github workflow example for generating releases on new tags.
10. How to use/install a basic man page

Building

Prerequisits

• build-essential (make + GCC + system headers)

Build process

• make
• or, optionally make install to install it to /usr/local/bin/minimdnsd, and install the initd service

Long-Term

• Allow response to services (see original MDNS server here: https://github.com/cnlohr/esp82xx/blob/master/fwsrc/mdns.c)
• Keep it under or around 1k LoC
• Keep it < 32kB .text

Things I learned

IPv6 is pain. this would have been a tiny fraction of its size if it weren't for IPv6.

Also IPv6 with MDNS is in a really sorry state, so I was barely able to test this with IPv6.

https://superuser.com/questions/1086954/how-do-i-use-mdns-for-ssh-6

And ping6 has a similar issue.

Thanks

For code reviews: @GPSBabelDeveloper, @probonopd