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diff --git a/doc/guix.texi b/doc/guix.texi index 5747484b20..ce1e5d075a 100644 --- a/doc/guix.texi +++ b/doc/guix.texi @@ -1242,6 +1242,56 @@ data in the right format. This is important because the locale data format used by different libc 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 @cindex fonts |