== IPv6 with Lighttpd on Linux

The situation with lighttpd is sufficiently complicated that I can't
call it a [[brief note SomeIpv6SupportNotes]], so here's what I've
worked out. This is not necessarily valid for all distributions,
since I'm working on Fedora 11.

Lighttpd is fully IPv6 enabled, but normally only binds its listening
socket to the IPv4 address. Some distributions override this by setting
the _server.use-ipv6_ configuration option if IPv6 is enabled on the
machine.

Now, _server.use-ipv6_ is somewhat underdocumented; the lighttpd
documentation says only 'bind to the IPv6 socket'. What it actually does
at the moment is make lighttpd *always* create IPv6 sockets, regardless
of what the socket is supposed to be bound to. (I am not sure how well
this works if you are binding to specific IPv4 addresses, even on a
[[dual binding Ipv6DualBinding]] machine with _bindv6only_ turned off.)

Since lighttpd fully supports IPv6, you do not need to turn
_server.use-ipv6_ on in order to bind to an IPv6 socket; instead,
you simply tell lighttpd to do so by giving it an IPv6 address as
_server.bind_ or whatever. So if you have turned _bindv6only_ on to turn
off dual-binding and thus need to bind separately to both the IPv4 and
the IPv6 sockets, you just need to tell lighttpd to do this by adding
the following configuration bit:

> _$SERVER["socket"] == "[::]:80" { }_

(As the lighttpd documentation dryly notes, 'note that it's perfectly
valid to leave the body of the [_$SERVER_] conditional empty'. Lighttpd
configuration is sometimes full of these magic side effects.)

This is sadly a lot more complicated than the Apache way, which can
be described as 'it all just works, you don't have to worry about it'
(cf [[the brief note SomeIpv6SupportNotes]]).