This loosens the ‘is this localhost?’ check to include *.localhost host
names.
This allows for clearer (hence better) names to be used in browsers,
e.g. when accessing a remote syncthing instance ‘foo’ using a ssh port
forward, one can use foo.localhost to remind oneself which one is which.
💡 Without these changes, Syncthing shows a ‘Host check error’ when
pointing a browser at http://foo.localhost/, and with these changes, the
interface loads as usual.
The .localhost top level domain is a reserved top-level domain (RFC 2606):
> The ".localhost" TLD has traditionally been statically defined in
> host DNS implementations as having an A record pointing to the
> loop back IP address and is reserved for such use. Any other use
> would conflict with widely deployed code which assumes this use.
> – https://tools.ietf.org/html/rfc2606
As Wikipedia puts it:
> This allows the use of these names for either documentation purposes
or in local testing scenarios. – https://en.wikipedia.org/wiki/.localhost
On Linux systems, systemd-resolved resolves *.localhost, on purpose:
https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html
See also #4815, #4816.
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