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doc: Document NSS incompatibility issues on foreign distros.

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* doc/guix.texi (Application Setup)[Name Service Switch]: New
subsection.
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Ludovic Courtès 2016-11-28 16:19:04 +01:00
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@ -1242,6 +1242,56 @@ data in the right format.
This is important because the locale data format used by different libc This is important because the locale data format used by different libc
versions may be incompatible. versions may be incompatible.
@subsection Name Service Switch
@cindex name service switch, glibc
@cindex NSS (name service switch), glibc
@cindex nscd (name service caching daemon)
@cindex name service caching daemon (nscd)
When using Guix on a foreign distro, we @emph{strongly recommend} that
the system run the GNU C library's @dfn{name service cache daemon},
@command{nscd}, which should be listening on the
@file{/var/run/nscd/socket} socket. Failing to do that, applications
installed with Guix may fail to look up host names or user accounts, or
may even crash. The next paragraphs explain why.
@cindex @file{nsswitch.conf}
The GNU C library implements a @dfn{name service switch} (NSS), which is
an extensible mechanism for ``name lookups'' in general: host name
resolution, user accounts, and more (@pxref{Name Service Switch,,, libc,
The GNU C Library Reference Manual}).
@cindex Network information service (NIS)
@cindex NIS (Network information service)
Being extensible, the NSS supports @dfn{plugins}, which provide new name
lookup implementations: for example, the @code{nss-mdns} plugin allow
resolution of @code{.local} host names, the @code{nis} plugin allows
user account lookup using the Network information service (NIS), and so
on. These extra ``lookup services'' are configured system-wide in
@file{/etc/nsswitch.conf}, and all the programs running on the system
honor those settings (@pxref{NSS Configuration File,,, libc, The GNU C
Reference Manual}).
When they perform a name lookup---for instance by calling the
@code{getaddrinfo} function in C---applications first try to connect to
the nscd; on success, nscd performs name lookups on their behalf. If
the nscd is not running, then they perform the name lookup by
themselves, by loading the name lookup services into their own address
space and running it. These name lookup services---the
@file{libnss_*.so} files---are @code{dlopen}'d, but they may come from
the host system's C library, rather than from the C library the
application is linked against (the C library coming from Guix).
And this is where the problem is: if your application is linked against
Guix's C library (say, glibc 2.24) and tries to load NSS plugins from
another C library (say, @code{libnss_mdns.so} for glibc 2.22), it will
likely crash or have its name lookups fail unexpectedly.
Running @command{nscd} on the system, among other advantages, eliminates
this binary incompatibility problem because those @code{libnss_*.so}
files are loaded in the @command{nscd} process, not in applications
themselves.
@subsection X11 Fonts @subsection X11 Fonts
@cindex fonts @cindex fonts