It's here: https://github.com/adhearsion/blather/blob/develop/lib/blather/client/dsl.rb#L131

I use `host` for two reasons: it's the host running the XMPP server; and "host" is the name of the bound variable in `Blather::DSL::setup`.

Looking at this line again, I think that `domain` is a somewhat misleading name.  This is actually the JID of the component.  (In the case of a component that's a domain name.)