Munin is cool. However, by default, it sends everything in plaintext, and relies on silly schemes like subnet whitelisting for authentication. However, it can use TLS.
Most of the info is here: http://munin-monitoring.org/wiki/MuninTLSSetup. Read that first.
The munin wiki assumes you know how to make openssl certs, though, which I don't. Here's the cheat-sheet for that (this all assumes you are using tls paranoid):
First, set up 'vars' file the way you want. This should be in any fairly standard easy-rsa tutorial. I set mine up to use a non-standard 'keys' directory, because I already had OpenVPN keys in the default one. I have my munin-specific easy-rsa vars file in 'vars-munin':
$ . vars-munin $ ./clean-all # this creates index.txt # default answers to everything: $ ./build-ca # default answers, then sign=yes, commit=yes $ ./build-key $munin_master $ ./build-key-server $munin_node
The key is that the you use build-key (a "client" certificate) for the master, and build-key-server (a "server" certificate) for the node.
You can check which one a given cert is with:
$ openssl x509 -in some-cert-name.crt -text -noout | grep -A 1 "Netscape Cert Type:"
(The difference is the value of nsCertType, I believe. I know very, very little about this. There is some explanation here.)
If things aren't working, see the instructions on http://munin-monitoring.org/wiki/Debugging_Munin_plugins for debugging a single plugin on a single host -- that will let you test the TLS authentication in a simpler, faster way.
[Addendum: I see, according to these debconf13 slides, munin 2.0 introduces ssh as a transport. That would have been way better. D'oh! Well, munin 2.0 isn't in ubuntu 12.04 anyway, so I learned something.]