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<div class="left"><a href="https://utcc.utoronto.ca/~cks/">Chris Siebenmann</a> ::
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FirefoxDBusRemoteControl</span></div>
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<div class="wikitext titlehack"><h2>Firefox now implements its remote control partly over D-Bus</h2>
<div class="posttitle" style="opacity: 0.6;"><small>August 3, 2018</small></div>
</div>
<div class="wikitext"><p>On Unix, Firefox has had a long standing feature where <a href="https://utcc.utoronto.ca/~cks/space/blog/unix/WeirdFirefoxRemoteControl">you could
remote control a running Firefox instance</a>.
This has traditionally worked through X properties (with <a href="https://utcc.utoronto.ca/~cks/space/blog/programming/GoLowLevelX">two
generations of protocols</a>), which has
the nice advantage that it works from remote machines as well as
your local one, provided that you're forwarding X. Since I read my
mail through <a href="http://beedub.com/exmh/">exmh</a> that's running on one
of our servers, not my desktop, this is pretty useful for me; I can
click on a link in mail in exmh, and it opens in my desktop Firefox.
However, working through X properties also has the disadvantage
that it naturally doesn't work at all on <a href="https://en.wikipedia.org/wiki/Wayland_(display_server_protocol)">Wayland</a>. Since
Wayland is increasingly important, last November or so the Mozilla
people fixed this by adding a new D-Bus based protocol (it landed
in <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1360560">bug 1360560</a>
and <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1360566">bug 1360566</a>
but has evolved in various ways since then).</p>
<p>On current versions of Firefox, you will find this service on the
session bus under the name org.mozilla.firefox.<something>, where
the <something> is often 'ZGVmYXVsdA__'. In general this weird
thing is the base64 encoded name of your Firefox profile with a few
special characters turned into _, and that particular name is,
well:</p>
<blockquote><pre style="white-space: pre-wrap;">
; echo -n default | base64
ZGVmYXVsdA==
</pre>
</blockquote>
<p>Because this directly encodes the profile name in something that
you have to get right, the D-Bus based version of Firefox remote
control will reliably restrict itself to talking to a running Firefox
that's using the same profile; the X properties based version doesn't
always (or didn't always, at any rate). You can force a new Firefox
to not try to talk to an existing Firefox by using <code>--new-instance</code>,
as before.</p>
<p>(One case where you might need this is if you're testing an
alternate version of Firefox by manually setting your <code>$HOME</code>
to, eg, <code>/tmp/ffox-test</code>.)</p>
<p>It turns out that which protocol Firefox uses when is a bit tangled.
If Firefox is built with D-Bus support, a running Firefox on X will
be listening for incoming requests using both D-Bus and the X
properties based protocol; you can talk to this Firefox with either.
In the current Firefox code, if you built with both D-Bus and Wayland
support, the client Firefox always uses D-Bus to try to talk to the
running 'server' Firefox; it doesn't fall back to X properties if
there's no D-Bus available. If you built Firefox without Wayland
support, it always uses the X properties based protocol (even if
you built with D-Bus, and so the running Firefox is listening there).
You can see this sausage being made in <code>StartRemoteClient()</code> <a href="https://hg.mozilla.org/mozilla-central/file/tip/toolkit/xre/nsAppRunner.cpp#l1725">here</a>.</p>
<p>This logic was introduced in the change for <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1465371">bug 1465371</a>. Before then
Firefox tried to use the X properties based remote control if it
was running on X, and fell back to the D-Bus protocol otherwise.
In thinking about it I've come to believe that the logic here is
sound, because in a Wayland session you may have some programs that
think they're running in X and then pass this view on to things run
from them. D-Bus is more session type agnostic, although it only
works on the local machine.</p>
<p>Note that this implies that you can no longer use Firefox itself
as a client on a second machine, at least not if your second machine
Firefox is a modern one that was built with Wayland support; it'll
try to talk D-Bus and fail because your running Firefox isn't on
that machine. If you want to remote control Firefox from a second
machine, you now want a dedicated client like <a href="https://github.com/siebenmann/ffox-remote">my <code>ffox-remote</code>
program</a>.</p>
<p>(Hopefully Mozilla will leave the X properties based protocol there
for many years to come, so my cross-machine remote control will still
keep working.)</p>
<h3>Sidebar: some D-Bus protocol details</h3>
<p>The D-Bus object path is /org/mozilla/firefox/Remote, which has one
org.mozilla.firefox method, <code>OpenURL()</code>, all of which you can see
by using a D-Bus browsing program such as <a href="https://wiki.gnome.org/Apps/DFeet">d-feet</a>. In the Firefox source code,
what you want to look at is widget/xremoteclient/DBusRemoteClient.cpp
(the client side, ie the <code>firefox</code> command you just ran that is
going to pass your URL or whatever to the currently running one)
and toolkit/components/remote/nsDBusRemoteService.cpp (the server
side, ie the running Firefox).</p>
<p>Despite the fact that D-Feet will tell you that the argument to
<code>OpenURL()</code> is a string, in actuality it's an entire command line
encoded in the same annoying binary encoding that is used in the
current X property based protocol, which you can read a concise
description of in <a href="https://hg.mozilla.org/mozilla-central/file/tip/toolkit/components/remote/nsRemoteService.cpp#l165">nsRemoteService.cpp</a>.
Presumably this minimizes code changes, although it's not the most
natural D-Bus interface. This encoding does mean that you're going
to need some moderately tangled code to remote-control Firefox over
D-Bus; you can't fire up just any old D-Bus client program for it.</p>
<p>The client code for this is in <a href="https://hg.mozilla.org/mozilla-central/file/tip/toolkit/xre/nsAppRunner.cpp">toolkit/xre/nsAppRunner.cpp</a>,
in the <code>StartRemoteClient()</code> function.</p>
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<h3> <a name="comments">Comments</a> on this page: </h3>
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<div class="comment">
<div class="entryid"> By Tüdelbüdel at <a name="guest-20180804070451">2018-08-04 07:04:51</a>: </div>
<blockquote> <div class="wikitext"><p>Ah yes, the remote control feature, because it is SO FUNNY if you want to open /home/tuedel/localinfo.html and the request gets sent to a totally different computer. </p>
<p>That was literally the only time I was confronted with that feature. It took a while to figure out what was even happening and then how to avoid it. Even with my current knowledge, I would be hard pressed to find the relevant section in the firefox user documentation.</p>
<p>And this seems to be a general problem with complex software (browsers, *office, gimp etc.): You can start using them without reading any documentation, but when strange complex things happen or when you need more in-depth information on some feature, the answer will more likely be found in a stackoverflow posting or something, not the built in documentation.</p>
<p>Of course it would be a huge effort to always write and maintain such comprehensive and in-depth documentation, so I am not complaining.</p>
</div> </blockquote> </div>
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<small>Written on <a href="https://utcc.utoronto.ca/~cks/space/blog/2018/08/03/">03</a> <a href="https://utcc.utoronto.ca/~cks/space/blog/2018/08/">August</a> <a href="https://utcc.utoronto.ca/~cks/space/blog/2018/">2018</a>.</small> <br>
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