Using alternate loopback addresses

Usually, when you use localhost, it’ll use the 127.0.0.1 loopback address. Did you know that the entire 127.x.y.z range is reserved for loopback? See RFC 990 and RFC 3330. This can be useful sometimes.

Load testing with multiple connections

One example is if you’re running load tests locally. Because each end of an established connection requires two port numbers, and there are only a limited number of ephemeral ports (~32K on Linux, by default), you’ll hit a limit with only ~16K test connections.

You can increase the limit (depending on OS), but port numbers are 16-bit (i.e. ~64K) and a large chunk of that range is reserved. That only gives you a little more breathing room.

Or you can bind your client connections to (e.g.) 127.0.0.2, meaning that you can now have ~32K connections (the server uses ~32K ports on 127.0.0.1 and the client uses ~32K ports on 127.0.0.2).

If you want to get clever, you can have your server listen on multiple loopback addresses, have your clients bind to different loopback addresses, and (somehow; this is the bit where you’ll have to get clever) spread them out to the point that you’ve got ~256M connections (back of the envelope, so possibly wrong: 127.0.0.0/8 is ~16.7M addresses, each with ~32K port numbers, divided by 2 for client/server). You’ll run out of some other resource before that point, though.

Identifying traffic in Wireshark

One other reason you might want to do this is if you’re capturing a network trace (in Wireshark, e.g.) and you’re trying to isolate a particular client. In my particular case, I’ve got a 3-node Kafka cluster running in Docker. Because of the replication and heartbeating between the nodes, there’s a lot of Kafka traffic. Because the cluster traffic uses the same ports as the client, I can’t easily use a filter in Wireshark to only see the client traffic.

On the other hand, if I can persuade the client to bind to 127.0.0.2, then I can use a filter to see just that traffic. The exact mechanism for this depends on the client.

And now I can use ip.addr==127.0.0.2 in Wireshark to see my client’s traffic, and no other.

macOS, though

On macOS, the lo0 loopback device is only bound to 127.0.0.1, so you’ll have to fix that first. See this Superuser question, but (tl;dr):

sudo ifconfig lo0 alias 127.0.0.2 up
sudo ifconfig lo0 alias 127.0.0.3 up
# ... repeat as many times as needed

To remove the aliases:

sudo ifconfig lo0 -alias 127.0.0.2
# ... and so on