Munin and IPv6

Okay here it comes another post about Munin for those who are using this awesome monitoring solution (okay I think I’ve been involved in upstream development more than I expected when Jeremy pointed me at it). While the main topic of this post is going to be IPv6 support, I’d like first to spend a few words for context of what’s going on.

Munin in Gentoo has been slightly patched in the 2.0 series — most of the patches were sent upstream the moment when they were introduced, and most of them have been merged in for the following release. Some of them though, including the one bringing my FreeIPMI plugin to replace the OpenIPMI plugins, or at least the first version of it, and those dealing with changes that wouldn’t have been kosher for other distributions (namely, Debian) at this point, were also not merged in the 2.0 branch upstream.

But now Steve opened a new branch for 2.0, which means that the development branch (Munin does not use the master branch, for a simple logistic reason of having a master/ directory in GIT I suppose) is directed toward the 2.1 series instead. This meant not only that I can finally push some of my recent plugin rewrites but also that I could make some more deep changes to it, including rewriting the seven asterisk plugins into a single one, and work hard on the HTTP-based plugins (for web servers and web services) so that they use a shared backend, like SNMP. This actually completely solved an issue that, in Gentoo, we solved only partially before — my ModSecurity ruleset blacklists the default libwww-perl user agent, so with the partial and complete fix, Munin advertises itself in the request; with the new code it includes also the plugin that is currently making the request so that it’s possible to know which requests belongs to what).

Speaking of Asterisk, by the way, I have to thank Sysadminman for lending me a test server for working on said plugins — this not only got us the current new Asterisk plugin (7-in-1!) but also let me modify just a tad said seven plugins, so that instead of using Net::Telnet, I could just use IO::Socket::INET. This has been merged for 2.0, which in turn means that the next ebuild will have one less dependency, and one less USE flag — the asterisk flag for said ebuild only added the Net::Telnet dependency.

To the main topic — how did I get to IPv6 in Munin? Well, I was looking at which other plugins need to be converted to “modernity” – which to me means re-using as much code possible, collapse multiple plugins in one through multigraph, and support virtual-nodes – and I found the squid plugins. This was interesting to me because I actually have one squid instance running, on the tinderbox host to avoid direct connection to the network from the tinderboxes themselves. These plugins do not use libwww-perl like the other HTTP plugins, I suppose (but I can’t be sure, for what I’m going to explain in a moment) because the cache://objects request that has to be done might or might not work with the noted library. Since as I said I have a squid instance, and these (multiple) plugins look exactly like the kind of target that I was looking for to rewrite, I started looking into them.

But once I started, I had a nasty surprise: my Squid instance only replies over IPv6, and that’s intended (the tinderboxes are only assigned IPv6 addresses, which makes it easier for me to access them, and have no NAT to the outside as I want to make sure that all network access is filtered through said proxy). Unfortunately, by default, libwww-perl does not support accessing IPv6. And indeed, neither do most of the other plugins, including the Asterisk I just rewrote, since they use IO::Socket::INET (instead of IO::Socket::INET6). A quick searching around, and this article turned up — although then this also turned up that relates to IPv6 support in Perl core itself.

Unfortunately, even with the core itself supporting IPv6, libwww-perl seems to be of different ideas, and that is a showstopper for me I’m afraid. At least, I need to find a way to get libwww-perl to play nicely if I want to use it over IPv6 (yes I’m going to work this around for the moment and just write the new squid plugins against the IPv4). On the other hand, using IO::Socket::IP would probably solve the issue for the remaining parts of the node and that will for sure at least give us some better support. Even better, it might be possible to abstract and have a Munin::Plugin::Socket that will fall-back to whatever we need. As it is, right now it’s a big question mark of what we can do there.

So what can be said about the current status of IPv6 support in Munin? Well, the Node uses Net::Server, and that in turn is not using IO::Socket::IP, but rather IO::Socket::INET or INET6 if installed — that basically means that the node itself will support IPv6 as long as INET6 is installed, and would call for using it as well, instead of using IO::Socket::IP — but the latter is the future and, for most people, will be part of the system anyway… The async support, in 2.0, will always use IPv4 to connect to the local node. This is not much of a problem, as Steve is working on merging the node and the async daemon in a single entity, which makes the most sense. Basically it means that in 2.1, all nodes will be spooled, instead of what we have right now.

The master, of course, also uses IPv6 — via IO::Socket::INET6 – yet another nail in the coffin of IO::Socket::IP? Maybe. – this covers all the communication between the two main components of Munin, and could be enough to declare it fully IPv6 compatible — and that’s what 2.0 is saying. But alas, this is not the case yet. On an interesting note, the fact that right now Munin supports arbitrary commands as transports, as long as they provide an I/O interface to the socket, make the fact that it supports IPv6 quite moot. Not only you just need an IPv6-capable SSH to handle it, but you can probably use SCTP instead of TCP simply by using a hacked up netcat! I’m not sure if monitoring would get any improvement of using SCTP, although I guess it might overcome some of the overhead related to establishing the connection, but.. well it’s a different story.

Of course, Munin’s own framework is only half of what has to support IPv6 for it to be properly supported; the heart of Munin is the plugins, which means that if they don’t support IPv6, we’re dead in the water. Perl plugins, as noted above, have quite a few issues with finding the right combination of modules for supporting IPv6. Bash plugins, and indeed any other language that could be used, would support IPv6 as good as the underlying tools — indeed, even though libwww-perl does not work with IPv6, plugins written with wget would work out of the box, on an IPv6-capable wget… but of course, the gains we have by using Perl are major enough that you don’t want to go that route.

All in all, I think what’s going to happen is that as soon as I’m done with the weekend’s work (which is quite a bit since the Friday was filled with a couple of server failures, and me finding out that one of my backups was not working as intended) I’ll prepare a branch and see how much of IO::Socket::IP we can leverage, and whether wrapping around that would help us with the new plugins. So we’ll see where this is going to lead us, maybe 2.1 will really be 100% IPv6 compatible…

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