systemd-nspawn: disabling link-local addressing

Posted on August 6, 2017

I recently migrated my various bespoke web services from a bunch of AWS EC2 instances to a single DigitalOcean droplet. The reason is simple: I ran out of AWS credits. In an effort to expedite any future migrations, I decided to containerize all my services. Rather than go with a full-blown container solution like Docker or LXC, I opted to roll my own using systemd-nspawn.

Everything was set up and working well, until I noticed that it would take my blog a full three seconds to load, whereas it should be pretty much instantaneous. Let’s debug this.

Measuring connection times

Accessing the blog works like this: a local machine connects to the nginx instance running on which performs TLS termination and reverse-proxies the request into a container named blog. This machine has its own dead-simple nginx instance running to serve up the static blog files.

First, let’s check the speed of a no-frills TCP connection from my local host to the remote host. The easy, quick-and-dirty was to accomplish this is with netcat.

netcat -lp 5000 # on the remote host
time netcat -z 5000 # on the local host

The -z switch sets netcat into the “zero I/O” mode, which makes it immediately shut down the connection after establishing it. This can useful (as it is here) for simple network diagnostics.

This took about 20 milliseconds, so the problem isn’t with the connection to the remote host.

Next let’s try the connection from the remote host to the container. Using the same measurement technique, I found that this took about three seconds. There’s the problem. But why?

What’s stranger is that some containers can be connected to instantly, but the blog container has this flat three second penalty on any new connection.

I should mention at this point that I’m using the NSS mymachines plugin which enables glibc’s gethostbyname to resolve container names to the IP addresses assigned to them internally. So to connect to the blog container, I just needed to write blog rather than an IP address that changes on every reboot due to DHCP.

I was at a loss about where this three second penalty could possibly be coming from, until I tried telnet. In the container I set up another simple server with netcat, and on the host I connected to it with telnet. Telnet tells you something very very useful when it’s establishing a connection, and that’s the order in which it tries IP addresses for the host you’re trying to connect to. Here’s what I saw:

$ telnet blog 5000
Connected to blog.
Escape character is '^]'.

And guess what? Trying the link-local address took about three seconds!

For comparison with the containers that don’t have the mysterious three second penalty, if I telnet into those, then I see that the private IP address is tried first, which immediately succeeds. That’s why those containers could be connected to in a reasonable time.

At this point, rather than continue my investigation of why it takes three seconds for the client to decide that the link-local address is no good, I simply decided to eliminate link-local addressing from my containers.

The beautiful thing about systemd-nspawn containers is that the networking is almost like magic. All you need to do is use machinectl start blog and tadaa! The container is now behind NAT. If you want to forward ports from the host directly into a container, it’s easy to do so by specifying Port=2233:22 for instance in the blog.nspawn configuration file for the container.

There’s one problem with magic though, and that’s that nobody actually knows how it works. All this stuff is taken care of by systemd-networkd and it’s unclear how exactly it does these things.

Luckily I stumbed onto a GitHub issue that made mention of the files /lib/systemd/network/ and /lib/systemd/network/ These files respectively configure the host and guest systemd-networkd instances to work together to provide DHCP. And guess what I found in these files? LinkLocalAddressing=yes.

It suffices to copy these files into into /etc/systemd/network/ on the host and into /etc/systemd/network/ on the guest and set LinkLocalAddressing=no in both to completely eliminate link-local addressing from the network setup.

At last, connection times were normal again, and everybody lived happily every after.