snix/doc/manual/installation.xml

<?xml version="1.0" encoding="utf-8"?>
<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-installation">

<title>Installation</title>


<section><title>Supported platforms</title>

<para>Nix is currently supported on the following platforms:

<itemizedlist>

  <listitem><para>Linux (particularly on x86, x86_64, and
  PowerPC).</para></listitem>

  <listitem><para>Mac OS X, both on Intel and
  PowerPC.</para></listitem>

  <listitem><para>FreeBSD (only tested on Intel).</para></listitem>

  <listitem><para>Windows through <link
  xlink:href="http://www.cygwin.com/">Cygwin</link>.</para>

  <warning><para>On Cygwin, Nix <emphasis>must</emphasis> be installed
  on an NTFS partition.  It will not work correctly on a FAT
  partition.</para></warning>

  </listitem>

</itemizedlist>

</para>

<para>Nix is pretty portable, so it should work on most other Unix
platforms as well.</para>

</section>


<section><title>Obtaining Nix</title>

<para>The easiest way to obtain Nix is to download a <link
xlink:href="http://nixos.org/">source distribution</link>.  RPMs
for Red Hat, SuSE, and Fedora Core are also available.</para>

<para>Alternatively, the most recent sources of Nix can be obtained
from its <link
xlink:href="https://svn.nixos.org/repos/nix/nix/trunk">Subversion
repository</link>.  For example, the following command will check out
the latest revision into a directory called
<filename>nix</filename>:</para>

<screen>
$ svn checkout https://svn.nixos.org/repos/nix/nix/trunk nix</screen>

<para>Likewise, specific releases can be obtained from the <link
xlink:href="https://svn.nixos.org/repos/nix/nix/tags">tags
directory</link> of the repository.</para>

</section>


<section><title>Prerequisites</title>

<para><emphasis>The following prerequisites only apply when you build
from source</emphasis>.  Binary releases (e.g., RPMs) have no
prerequisites.</para>

<para>A fairly recent version of GCC/G++ is required.  Version 2.95
and higher should work.</para>

<para>To build this manual and the man-pages you need the
<command>xmllint</command> and <command>xsltproc</command> programs,
which are part of the <literal>libxml2</literal> and
<literal>libxslt</literal> packages, respectively.  You also need the
<link
xlink:href="http://docbook.sourceforge.net/projects/xsl/">DocBook XSL
stylesheets</link> and optionally the <link
xlink:href="http://www.docbook.org/schemas/5x"> DocBook 5.0 RELAX NG
schemas</link>.  Note that these are only required if you modify the
manual sources or when you are building from the Subversion
repository.</para>

<para>To build the parser, very <emphasis>recent</emphasis> versions
of Bison and Flex are required.  (This is because Nix needs GLR
support in Bison and reentrancy support in Flex.)  For Bison, you need
version 2.3 or higher (1.875 does <emphasis>not</emphasis> work),
which can be obtained from
the <link xlink:href="ftp://alpha.gnu.org/pub/gnu/bison">GNU FTP
server</link>.  For Flex, you need version 2.5.33, which is available
on <link xlink:href="http://lex.sourceforge.net/">SourceForge</link>.
Slightly older versions may also work, but ancient versions like the
ubiquitous 2.5.4a won't.  Note that these are only required if you
modify the parser or when you are building from the Subversion
repository.</para>

<para>Nix uses Sleepycat's Berkeley DB, CWI's ATerm library and the
bzip2 compressor (including the bzip2 library).  These are included in
the Nix source distribution.  If you build from the Subversion
repository, you must download them yourself and place them in the
<filename>externals/</filename> directory.  See
<filename>externals/Makefile.am</filename> for the precise URLs of
these packages.  Alternatively, if you already have them installed,
you can use <command>configure</command>'s
<option>--with-bdb</option>, <option>--with-aterm</option> and
<option>--with-bzip2</option> options to point to their respective
locations.  Note that Berkeley DB <emphasis>must</emphasis> be version
4.5; other versions may not have compatible database formats.</para>

</section>


<section><title>Building Nix from source</title>

<para>After unpacking or checking out the Nix sources, issue the
following commands:

<screen>
$ ./configure <replaceable>options...</replaceable>
$ make
$ make install</screen>

</para>

<para>When building from the Subversion repository, these should be
preceded by the command:

<screen>
$ ./bootstrap</screen>

</para>

<para>The installation path can be specified by passing the
<option>--prefix=<replaceable>prefix</replaceable></option> to
<command>configure</command>.  The default installation directory is
<filename>/nix</filename>.  You can change this to any location you
like.  You must have write permission to the
<replaceable>prefix</replaceable> path.</para>

<warning><para>It is best <emphasis>not</emphasis> to change the
installation prefix from its default, since doing so makes it
impossible to use pre-built binaries from the standard Nixpkgs
channels.</para></warning>

<para>If you want to rebuilt the documentation, pass the full path to
the DocBook RELAX NG schemas and to the DocBook XSL stylesheets using
the
<option>--with-docbook-rng=<replaceable>path</replaceable></option>
and
<option>--with-docbook-xsl=<replaceable>path</replaceable></option>
options.</para>

</section>


<section><title>Installing from RPMs</title>

<para>RPM packages of Nix can be downloaded from <link
xlink:href="http://nixos.org/" />.  These RPMs should work for most
fairly recent releases of SuSE and Red Hat Linux.  They have been
known to work work on SuSE Linux 8.1 and 9.0, and Red Hat 9.0.  In
fact, it should work on any RPM-based Linux distribution based on
<literal>glibc</literal> 2.3 or later.</para>

<para>Once downloaded, the RPMs can be installed or upgraded using
<command>rpm -U</command>.  For example,

<screen>
$ rpm -U nix-0.5pre664-1.i386.rpm</screen>

</para>

<para>The RPMs install into the directory <filename>/nix</filename>.
Nix can be uninstalled using <command>rpm -e nix</command>.  After
this it will be necessary to manually remove the Nix store and other
auxiliary data:

<screen>
$ rm -rf /nix/store
$ rm -rf /nix/var</screen>

</para>

</section>


<section><title>Upgrading Nix through Nix</title>

<para>You can install the latest stable version of Nix through Nix
itself by subscribing to the channel <link
xlink:href="http://nixos.org/releases/nix/channels/nix-stable" />,
or the latest unstable version by subscribing to the channel <link
xlink:href="http://nixos.org/releases/nix/channels/nix-unstable" />.
You can also do a <link linkend="sec-one-click">one-click
installation</link> by clicking on the package links at <link
xlink:href="http://nixos.org/releases/full-index-nix.html" />.</para>

</section>


<section><title>Security</title>

<para>Nix has two basic security models.  First, it can be used in
“single-user mode”, which is similar to what most other package
management tools do: there is a single user (typically <systemitem
class="username">root</systemitem>) who performs all package
management operations.  All other users can then use the installed
packages, but they cannot perform package management operations
themselves.</para>

<para>Alternatively, you can configure Nix in “multi-user mode”.  In
this model, all users can perform package management operations — for
instance, every user can install software without requiring root
privileges.  Nix ensures that this is secure.  For instance, it’s not
possible for one user to overwrite a package used by another user with
a Trojan horse.</para>


<section><title>Single-user mode</title>
  
<para>In single-user mode, all Nix operations that access the database
in <filename><replaceable>prefix</replaceable>/var/nix/db</filename>
or modify the Nix store in
<filename><replaceable>prefix</replaceable>/store</filename> must be
performed under the user ID that owns those directories.  This is
typically <systemitem class="username">root</systemitem>.  (If you
install from RPM packages, that’s in fact the default ownership.)
However, on single-user machines, it is often convenient to
<command>chown</command> those directories to your normal user account
so that you don’t have to <command>su</command> to <systemitem
class="username">root</systemitem> all the time.</para>

</section>


<section xml:id="ssec-multi-user"><title>Multi-user mode</title>

<para>To allow a Nix store to be shared safely among multiple users,
it is important that users are not able to run builders that modify
the Nix store or database in arbitrary ways, or that interfere with
builds started by other users.  If they could do so, they could
install a Trojan horse in some package and compromise the accounts of
other users.</para>

<para>To prevent this, the Nix store and database are owned by some
privileged user (usually <literal>root</literal>) and builders are
executed under special user accounts (usually named
<literal>nixbld1</literal>, <literal>nixbld2</literal>, etc.).  When a
unprivileged user runs a Nix command, actions that operate on the Nix
store (such as builds) are forwarded to a <emphasis>Nix
daemon</emphasis> running under the owner of the Nix store/database
that performs the operation.</para>

<note><para>Multi-user mode has one important limitation: only
<systemitem class="username">root</systemitem> can run <command
linkend="sec-nix-pull">nix-pull</command> to register the availability
of pre-built binaries.  However, those registrations are shared by all
users, so they still get the benefit from <command>nix-pull</command>s
done by <systemitem class="username">root</systemitem>.</para></note>


<section><title>Setting up the build users</title>

<para>The <emphasis>build users</emphasis> are the special UIDs under
which builds are performed.  They should all be members of the
<emphasis>build users group</emphasis> (usually called
<literal>nixbld</literal>).  This group should have no other members.
The build users should not be members of any other group.</para>

<para>Here is a typical <filename>/etc/group</filename> definition of
the build users group with 10 build users:

<programlisting>
nixbld:!:30000:nixbld1,nixbld2,nixbld3,nixbld4,nixbld5,nixbld6,nixbld7,nixbld8,nixbld9,nixbld10
</programlisting>

In this example the <literal>nixbld</literal> group has UID 30000, but
of course it can be anything that doesn’t collide with an existing
group.</para>

<para>Here is the corresponding part of
<filename>/etc/passwd</filename>:

<programlisting>
nixbld1:x:30001:65534:Nix build user 1:/var/empty:/noshell
nixbld2:x:30002:65534:Nix build user 2:/var/empty:/noshell
nixbld3:x:30003:65534:Nix build user 3:/var/empty:/noshell
...
nixbld10:x:30010:65534:Nix build user 10:/var/empty:/noshell
</programlisting>

The home directory of the build users should not exist or should be an
empty directory to which they do not have write access.</para>

<para>The build users should have write access to the Nix store, but
they should not have the right to delete files.  Thus the Nix store’s
group should be the build users group, and it should have the sticky
bit turned on (like <filename>/tmp</filename>):

<screen>
$ chgrp nixbld /nix/store
$ chmod 1777 /nix/store
</screen>

</para>

<para>Finally, you should tell Nix to use the build users by
specifying the build users group in the <link
linkend="conf-build-users-group"><literal>build-users-group</literal>
option</link> in the <link linkend="sec-conf-file">Nix configuration
file</link> (<literal>/nix/etc/nix/nix.conf</literal>):

<programlisting>
build-users-group = nixbld
</programlisting>

</para>

</section>


<section><title>Nix store/database owned by root</title>

<para>The simplest setup is to let <literal>root</literal> own the Nix
store and database.  I.e.,

<screen>
$ chown -R root /nix/store /nix/var/nix</screen>

</para>

<para>The Nix daemon should be started as follows (as
<literal>root</literal>):

<screen>
$ nix-worker --daemon</screen>

You’ll want to put that line somewhere in your system’s boot
scripts.</para>

<para>To let unprivileged users use the daemon, they should set the
<link linkend="envar-remote"><envar>NIX_REMOTE</envar> environment
variable</link> to <literal>daemon</literal>.  So you should put a
line like

<programlisting>
export NIX_REMOTE=daemon</programlisting>

into the users’ login scripts.</para>

</section>


<section><title>Nix store/database not owned by root</title>

<para>It is also possible to let the Nix store and database be owned
by a non-root user, which should be more secure<footnote><para>Note
however that even when the Nix daemon runs as root, not
<emphasis>that</emphasis> much code is executed as root: Nix
expression evaluation is performed by the calling (unprivileged) user,
and builds are performed under the special build user accounts.  So
only the code that accesses the database and starts builds is executed
as <literal>root</literal>.</para></footnote>.  Typically, this user
is a special account called <literal>nix</literal>, but it can be
named anything.  It should own the Nix store and database:

<screen>
$ chown -R root /nix/store /nix/var/nix</screen>

and of course <command>nix-worker --daemon</command> should be started
under that user, e.g.,

<screen>
$ su - nix -c "exec /nix/bin/nix-worker --daemon"</screen>

</para>

<para>There is a catch, though: non-<literal>root</literal> users
cannot start builds under the build user accounts, since the
<function>setuid</function> system call is obviously privileged.  To
allow a non-<literal>root</literal> Nix daemon to use the build user
feature, it calls a setuid-root helper program,
<command>nix-setuid-helper</command>.  This program is installed in
<filename><replaceable>prefix</replaceable>/libexec/nix-setuid-helper</filename>.
To set the permissions properly (Nix’s <command>make install</command>
doesn’t do this, since we don’t want to ship setuid-root programs
out-of-the-box):

<screen>
$ chown root.root /nix/libexec/nix-setuid-helper
$ chmod 4755 /nix/libexec/nix-setuid-helper
</screen>

(This example assumes that the Nix binaries are installed in
<filename>/nix</filename>.)</para>

<para>Of course, the <command>nix-setuid-helper</command> command
should not be usable by just anybody, since then anybody could run
commands under the Nix build user accounts.  For that reason there is
a configuration file <filename>/etc/nix-setuid.conf</filename> that
restricts the use of the helper.  This file should be a text file
containing precisely two lines, the first being the Nix daemon user
and the second being the build users group, e.g.,

<programlisting>
nix
nixbld
</programlisting>

The setuid-helper barfs if it is called by a user other than the one
specified on the first line, or if it is asked to execute a build
under a user who is not a member of the group specified on the second
line.  The file <filename>/etc/nix-setuid.conf</filename> must be
owned by root, and must not be group- or world-writable.  The
setuid-helper barfs if this is not the case.</para>

</section>


<section><title>Restricting access</title>

<para>To limit which users can perform Nix operations, you can use the
permissions on the directory
<filename>/nix/var/nix/daemon-socket</filename>.  For instance, if you
want to restrict the use of Nix to the members of a group called
<literal>nix-users</literal>, do

<screen>
$ chgrp nix-users /nix/var/nix/daemon-socket
$ chmod ug=rwx,o= /nix/var/nix/daemon-socket
</screen>

This way, users who are not in the <literal>nix-users</literal> group
cannot connect to the Unix domain socket
<filename>/nix/var/nix/daemon-socket/socket</filename>, so they cannot
perform Nix operations.</para>

</section>


</section> <!-- end of multi-user -->


</section> <!-- end of security -->


<section><title>Using Nix</title>

<para>To use Nix, some environment variables should be set.  In
particular, <envar>PATH</envar> should contain the directories
<filename><replaceable>prefix</replaceable>/bin</filename> and
<filename>~/.nix-profile/bin</filename>.  The first directory contains
the Nix tools themselves, while <filename>~/.nix-profile</filename> is
a symbolic link to the current <emphasis>user environment</emphasis>
(an automatically generated package consisting of symlinks to
installed packages).  The simplest way to set the required environment
variables is to include the file
<filename><replaceable>prefix</replaceable>/etc/profile.d/nix.sh</filename>
in your <filename>~/.bashrc</filename> (or similar), like this:</para>

<screen>
source <replaceable>prefix</replaceable>/etc/profile.d/nix.sh</screen>

</section>


</chapter>