374/376: Add section on SSH substituter

[Ludovic Courtès]

civodul pushed a commit to tag 1.8
in repository guix.

commit 2142f47c0606aac6f0b66bc3fe25fe79fc85e7d8
Author: Eelco Dolstra <address@hidden>
Date:   Sun Dec 14 03:19:15 2014 +0100

    Add section on SSH substituter
---
 doc/manual/expressions/simple-expression.xml       |    4 +-
 .../{sharing-packages.xml => copy-closure.xml}     |   27 +++-----
 doc/manual/packages/sharing-packages.xml           |   43 +-----------
 doc/manual/packages/ssh-substituter.xml            |   73 ++++++++++++++++++++
 4 files changed, 87 insertions(+), 60 deletions(-)

diff --git a/doc/manual/expressions/simple-expression.xml 
b/doc/manual/expressions/simple-expression.xml
index a8eb96f..29fd872 100644
--- a/doc/manual/expressions/simple-expression.xml
+++ b/doc/manual/expressions/simple-expression.xml
@@ -4,7 +4,7 @@
       version="5.0"
       xml:id="ch-simple-expression">
 
-<title>Simple Nix Expression Use-Case</title>
+<title>A Simple Nix Expression</title>
 
 <para>This section shows how to add and test the <link
 xlink:href='http://www.gnu.org/software/hello/hello.html'>GNU Hello
@@ -44,4 +44,4 @@ need to do three things:
 <xi:include href="simple-building-testing.xml" />
 <xi:include href="generic-builder.xml" />
 
-</chapter>
\ No newline at end of file
+</chapter>
diff --git a/doc/manual/packages/sharing-packages.xml 
b/doc/manual/packages/copy-closure.xml
similarity index 66%
copy from doc/manual/packages/sharing-packages.xml
copy to doc/manual/packages/copy-closure.xml
index 8fab15f..5ec7896 100644
--- a/doc/manual/packages/sharing-packages.xml
+++ b/doc/manual/packages/copy-closure.xml
@@ -1,16 +1,10 @@
-<chapter xmlns="http://docbook.org/ns/docbook";
-      xmlns:xlink="http://www.w3.org/1999/xlink";
-      xmlns:xi="http://www.w3.org/2001/XInclude";
-      version="5.0"
-      xml:id="sec-sharing-packages">
+<section xmlns="http://docbook.org/ns/docbook";
+         xmlns:xlink="http://www.w3.org/1999/xlink";
+         xmlns:xi="http://www.w3.org/2001/XInclude";
+         version="5.0"
+         xml:id="ssec-copy-closure">
 
-<title>Sharing Packages Between Machines</title>
-
-<para>Sometimes you want to copy a package from one machine to
-another.  Or, you want to install some packages and you know that
-another machine already has some or all of those packages or their
-dependencies.  In that case there are mechanisms to quickly copy
-packages between machines.</para>
+<title>Copying Closures</title>
 
 <para>The command <command
 linkend="sec-nix-copy-closure">nix-copy-closure</command> copies a Nix
@@ -49,9 +43,8 @@ another machine:
 $ nix-store --export $(nix-store -qR $(type -p firefox)) | bzip2 | \
     ssh address@hidden "bunzip2 | nix-store --import"</screen>
 
-But note that <command>nix-copy-closure</command> is generally more
-efficient in this example because it only copies paths that are not
-already present in the target Nix store.</para>
-
+However, <command>nix-copy-closure</command> is generally more
+efficient because it only copies paths that are not already present in
+the target Nix store.</para>
 
-</chapter>
+</section>
diff --git a/doc/manual/packages/sharing-packages.xml 
b/doc/manual/packages/sharing-packages.xml
index 8fab15f..586363b 100644
--- a/doc/manual/packages/sharing-packages.xml
+++ b/doc/manual/packages/sharing-packages.xml
@@ -12,46 +12,7 @@ another machine already has some or all of those packages or 
their
 dependencies.  In that case there are mechanisms to quickly copy
 packages between machines.</para>
 
-<para>The command <command
-linkend="sec-nix-copy-closure">nix-copy-closure</command> copies a Nix
-store path along with all its dependencies to or from another machine
-via the SSH protocol.  It doesn’t copy store paths that are already
-present on the target machine.  For example, the following command
-copies Firefox with all its dependencies:
-
-<screen>
-$ nix-copy-closure --to address@hidden $(type -p firefox)</screen>
-
-See <xref linkend='sec-nix-copy-closure' /> for details.</para>
-
-<para>With <command linkend='refsec-nix-store-export'>nix-store
---export</command> and <command
-linkend='refsec-nix-store-import'>nix-store --import</command> you can
-write the closure of a store path (that is, the path and all its
-dependencies) to a file, and then unpack that file into another Nix
-store.  For example,
-
-<screen>
-$ nix-store --export $(nix-store -qR $(type -p firefox)) > 
firefox.closure</screen>
-
-writes the closure of Firefox to a file.  You can then copy this file
-to another machine and install the closure:
-
-<screen>
-$ nix-store --import &lt; firefox.closure</screen>
-
-Any store paths in the closure that are already present in the target
-store are ignored.  It is also possible to pipe the export into
-another command, e.g. to copy and install a closure directly to/on
-another machine:
-
-<screen>
-$ nix-store --export $(nix-store -qR $(type -p firefox)) | bzip2 | \
-    ssh address@hidden "bunzip2 | nix-store --import"</screen>
-
-But note that <command>nix-copy-closure</command> is generally more
-efficient in this example because it only copies paths that are not
-already present in the target Nix store.</para>
-
+<xi:include href="copy-closure.xml" />
+<xi:include href="ssh-substituter.xml" />
 
 </chapter>
diff --git a/doc/manual/packages/ssh-substituter.xml 
b/doc/manual/packages/ssh-substituter.xml
new file mode 100644
index 0000000..f24f354
--- /dev/null
+++ b/doc/manual/packages/ssh-substituter.xml
@@ -0,0 +1,73 @@
+<section xmlns="http://docbook.org/ns/docbook";
+         xmlns:xlink="http://www.w3.org/1999/xlink";
+         xmlns:xi="http://www.w3.org/2001/XInclude";
+         version="5.0"
+         xml:id="ssec-ssh-substituter">
+
+<title>Serving a Nix store via SSH</title>
+
+<para>You can tell Nix to automatically fetch needed binaries from a
+remote Nix store via SSH. For example, the following installs Firefox,
+automatically fetching any store paths in Firefox’s closure if they
+are available on the server <literal>avalon</literal>:
+
+<screen>
+$ nix-env -i firefox --option ssh-substituter-hosts address@hidden
+</screen>
+
+This works similar to the binary cache substituter that Nix usually
+uses, only using SSH instead of HTTP: if a store path
+<literal>P</literal> is needed, Nix will first check if it’s available
+in the Nix store on <literal>avalon</literal>. If not, it will fall
+back to using the binary cache substituter, and then to building from
+source.</para>
+
+<note><para>The SSH substituter currently does not allow you to enter
+an SSH passphrase interactively. Therefore, you should use
+<command>ssh-add</command> to load the decrypted private key into
+<command>ssh-agent</command>.</para></note>
+
+<para>You can also copy the closure of some store path, without
+installing it into your profile, e.g.
+
+<screen>
+$ nix-store -r /nix/store/m85bxg…-firefox-34.0.5 --option 
ssh-substituter-hosts address@hidden
+</screen>
+
+This is essentially equivalent to doing
+
+<screen>
+$ nix-copy-closure --from address@hidden /nix/store/m85bxg…-firefox-34.0.5
+</screen>
+
+</para>
+
+<para>You can use SSH’s <emphasis>forced command</emphasis> feature to
+set up a restricted user account for SSH substituter access, allowing
+read-only access to the local Nix store, but nothing more. For
+example, add the following lines to <filename>sshd_config</filename>
+to restrict the user <literal>nix-ssh</literal>:
+
+<programlisting>
+Match User nix-ssh
+  AllowAgentForwarding no
+  AllowTcpForwarding no
+  PermitTTY no
+  PermitTunnel no
+  X11Forwarding no
+  ForceCommand nix-store --serve
+Match All
+</programlisting>
+
+On NixOS, you can accomplish the same by adding the following to your
+<filename>configuration.nix</filename>:
+
+<programlisting>
+nix.sshServe.enable = true;
+nix.sshServe.keys = [ "ssh-dss AAAAB3NzaC1k... address@hidden" ];
+</programlisting>
+
+where the latter line lists the public keys of users that are allowed
+to connect.</para>
+
+</section>