mirror.hydra.gnu.org server is a front-end to an official build farm
that builds packages from Guix continuously for some
architectures, and makes them available as substitutes. 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 HTTPS. 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.
Substitutes from the official build farm are enabled by default when using the Guix System Distribution (see GNU Distribution). However, they are disabled by default when using Guix on a foreign distribution, unless you have explicitly enabled them via one of the recommended installation steps (see Installation). The following paragraphs describe how to enable or disable substitutes for the official build farm; the same procedure can also be used to enable substitutes for any other substitute server.