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X11 forwarding currently doesn't even attempt to authenticate with the local X server. In order for it to do so, PuTTY would have to know where to look for authentication data. It would be nice if this were to happen.
A simple approach might be for PuTTY just to pass on authentication
data supplied by the X11 client if it has no better ideas. The X
Protocol Manual suggests that servers should ignore any authentication
they aren't expecting, so this shouldn't break anything that currently
works, and it would allow the user to copy back PuTTY's cookie to their
real X server.
Ref: <web-71356@meredevice.com>
SGT, 2003-01-14: A couple of days ago I built in the
infrastructure for this, since X forwarding in the Unix port of
Plink is virtually useless without it. The platform-independent X
forwarding code now has the ability to talk either of
MIT-MAGIC-COOKIE-1 or XDM-AUTHORIZATION-1
to the local X server, provided a platform-specific function tells
it what authentication cookie to use. So it's still not actually
supported on Windows (since the Windows version of this function
does nothing), but the amount of work that would need to be done to
support it is now minimal.
SGT, 2008-11-04: I've just found this
piece of source code which contains a comment describing the
format of the .Xauthority file. So as not to lose it,
I'll reproduce the format here. The .Xauthority file is
an end-to-end concatenation of records consisting of a 2-byte
big-endian number followed by four strings. Each string consists of
a 2-byte big-endian number giving the length, followed by that many
bytes. The strings are, respectively: network address, display
number, authorisation method (e.g.
"MIT-MAGIC-COOKIE-1") and the authorisation data
itself. The initial bare number specifies the address family, i.e. how
to interpret the address string. Just to prove it works, the
following hacky piece of Perl successfully untangles the first
.Xauthority file I could lay my hands
on:
perl -e 'while (1) { read STDIN,$fam,2 or last; $fam = unpack "n", $fam; @strings = (); for $i (0..3) { read STDIN, $len, 2; $len = unpack "n", $len; $str=""; read STDIN, $str, $len; if ($i==1 or $i==2 or ($i==0 and $fam==256)) { $str = "\"" . $str . "\"" } else { $str = join "", "<", (map{sprintf"%02x",$_}unpack"C*",$str), ">"; } push @strings, $str; } printf "%s\n", join " ", $fam, @strings }' < ~/.XauthoritySGT, 2008-11-17: should now be done, at least for Windows X servers which provide X authority files in the standard format. There's a new config option on Windows which allows the user to point at the location of their X authority file, and PuTTY has the in-built ability to understand files in that format.