Add section on SSH substituter

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diff --git a/doc/manual/packages/ssh-substituter.xml b/doc/manual/packages/ssh-substituter.xml
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+<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 alice@avalon
+</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 alice@avalon
+</screen>
+
+This is essentially equivalent to doing
+
+<screen>
+$ nix-copy-closure --from alice@avalon /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... bob@example.org" ];
+</programlisting>
+
+where the latter line lists the public keys of users that are allowed
+to connect.</para>
+
+</section>