garpd is a fast, simple daemon that listens for and reports gratuitous ARP
messages.
garpd is extremely simple. To compile it, just run:
gcc -ogarpd garpd.c
That will build an executable called garpd in your working directory,
which can be copied anywhere you want for installation. It does not
require linking with any libraries other than libc.
When you run garpd, it will create a Unix socket at /var/opt/garpd.sck.
Each time it finds a gratuitous ARP message it will write a JSON dictionary
into the socket, terminated by a newline. The format of the dictionary is:
{"ip": "<dotted decimal IP>", "mac": "<colon-separated MAC>"}
There are guaranteed to be no newlines in the body, so a simple parser can scan for newlines and split on them.