Question

With the following code

InetAddress.getLocalHost().getHostAddress();

it is possible to get the hosts address. But how does the JVM find that out?

The Java API only tells you that it returns it (API Reference), but is there a DNS-Server involved, and if yes, when is it called?

And if it is only called once how is the server name saved locally?

Answer 1

The actual implementation is done with JNI, in native code, so it is going to differ from platform to platform.

That said, there is no reason one needs to do DNS to lookup an ip address on a machine where the network cards are located. One can just read the ip information from the network cards.

The bad news: There is no way of knowing for certain if this will do a DNS lookup on any platform that Java runs upon, as it is native code, and the possibility of a machine doing a DNS lookup even when it is not actually necessary exists.

The good news: On my linux box, it doesn’t do a DNS lookup (confirmed via wireshark), which is exactly what I would expect. If you think it’s doing a lookup, there are a number of reasons why it may be doing a lookup (depending on how configurable your native bind client is) and if you install wireshark (or use a suitable network analyzer) you can quickly find out if you’re looking up yourself.

Edit: note that the name lookup would be in the .getLocalHost() portion of the chained calls, if it was to be looked-up at all.