Guix supports transparent source/binary deployment, which means that it can either build things locally, or download pre-built items from a server. We call these pre-built items substitutes—they are substitutes for local build results. In many cases, downloading a substitute is much faster than building things locally.
Substitutes can be anything resulting from a derivation build (see Derivations). Of course, in the common case, they are pre-built package binaries, but source tarballs, for instance, which also result from derivation builds, can be available as substitutes.
hydra.gnu.org server is a front-end to a build farm that
builds packages from the GNU distribution continuously for some
architectures, and makes them available as substitutes (see Emacs Hydra, for information on how to query the continuous integration
server). This is the
default source of substitutes; it can be overridden by passing the
--substitute-urls option either to
or to client tools such as
(see client --substitute-urls
Substitute URLs can be either HTTP or HTTPS4 HTTPS is recommended because communications are encrypted; conversely, using HTTP makes all communications visible to an eavesdropper, who could use the information gathered to determine, for instance, whether your system has unpatched security vulnerabilities.
To allow Guix to download substitutes from
hydra.gnu.org or a
mirror thereof, you
must add its public key to the access control list (ACL) of archive
imports, using the
guix archive command (see Invoking guix archive). Doing so implies that you trust
hydra.gnu.org to not
be compromised and to serve genuine substitutes.
This public key is installed along with Guix, in
prefix/share/guix/hydra.gnu.org.pub, where prefix is
the installation prefix of Guix. If you installed Guix from source,
make sure you checked the GPG signature of
guix-0.10.0.tar.gz, which contains this public key file.
Then, you can run something like this:
# guix archive --authorize < hydra.gnu.org.pub
Once this is in place, the output of a command like
should change from something like:
$ guix build emacs --dry-run The following derivations would be built: /gnu/store/yr7bnx8xwcayd6j95r2clmkdl1qh688w-emacs-24.3.drv /gnu/store/x8qsh1hlhgjx6cwsjyvybnfv2i37z23w-dbus-1.6.4.tar.gz.drv /gnu/store/1ixwp12fl950d15h2cj11c73733jay0z-alsa-lib-184.108.40.206.tar.bz2.drv /gnu/store/nlma1pw0p603fpfiqy7kn4zm105r5dmw-util-linux-2.21.drv …
to something like:
$ guix build emacs --dry-run The following files would be downloaded: /gnu/store/pk3n22lbq6ydamyymqkkz7i69wiwjiwi-emacs-24.3 /gnu/store/2ygn4ncnhrpr61rssa6z0d9x22si0va3-libjpeg-8d /gnu/store/71yz6lgx4dazma9dwn2mcjxaah9w77jq-cairo-1.12.16 /gnu/store/7zdhgp0n1518lvfn8mb96sxqfmvqrl7v-libxrender-0.9.7 …
This indicates that substitutes from
hydra.gnu.org are usable and
will be downloaded, when possible, for future builds.
Guix ignores substitutes that are not signed, or that are not signed by one of the keys listed in the ACL. It also detects and raises an error when attempting to use a substitute that has been tampered with.
Substitutes are downloaded over HTTP or HTTPS.
variable can be set in the environment of
guix-daemon and is
honored for downloads of substitutes. Note that the value of
http_proxy in the environment where
guix package, and other client commands are run has
absolutely no effect.
When using HTTPS, the server’s X.509 certificate is not validated (in other words, the server is not authenticated), contrary to what HTTPS clients such as Web browsers usually do. This is because Guix authenticates substitute information itself, as explained above, which is what we care about (whereas X.509 certificates are about authenticating bindings between domain names and public keys.)
The substitute mechanism can be disabled globally by running
--no-substitutes (see Invoking guix-daemon). It can also be disabled temporarily by passing the
--no-substitutes option to
build, and other command-line tools.
Today, each individual’s control over their own computing is at the
mercy of institutions, corporations, and groups with enough power and
determination to subvert the computing infrastructure and exploit its
weaknesses. While using
hydra.gnu.org substitutes can be
convenient, we encourage users to also build on their own, or even run
their own build farm, such that
hydra.gnu.org is less of an
interesting target. One way to help is by publishing the software you
guix publish so that others have one more choice
of server to download substitutes from (see Invoking guix publish).
Guix has the foundations to maximize build reproducibility
(see Features). In most cases, independent builds of a given
package or derivation should yield bit-identical results. Thus, through
a diverse set of independent package builds, we can strengthen the
integrity of our systems. The
guix challenge command aims to
help users assess substitute servers, and to assist developers in
finding out about non-deterministic package builds (see Invoking guix challenge). Similarly, the --check option of
build allows users to check whether previously-installed substitutes
are genuine by rebuilding them locally (see
guix build --check).
In the future, we want Guix to have support to publish and retrieve binaries to/from other users, in a peer-to-peer fashion. If you would like to discuss this project, join us on email@example.com.