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# Serving a Nix store via SSH

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 `avalon`:

    $ nix-env --install --attr nixpkgs.firefox --substituters ssh://alice@avalon

This works similar to the binary cache substituter that Nix usually uses, only using SSH instead of HTTP: if a store path `P` is needed, Nix will first check if it’s available in the Nix store on `avalon`. If not, it will fall back to using the binary cache substituter, and then to building from source.

> Note

> The SSH substituter currently does not allow you to enter an SSH passphrase interactively. Therefore, you should use `ssh-add` to load the decrypted private key into `ssh-agent`.

You can also copy the closure of some store path, without installing it into your profile, e.g.

    $ nix-store --realise /nix/store/m85bxg…-firefox-34.0.5 --substituters
    ssh://alice@avalon

This is essentially equivalent to doing

    $ nix-copy-closure --from alice@avalon
    /nix/store/m85bxg…-firefox-34.0.5

You can use SSH’s forced command 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 `sshd_config` to restrict the user `nix-ssh`:

    Match User nix-ssh
      AllowAgentForwarding no
      AllowTcpForwarding no
      PermitTTY no
      PermitTunnel no
      X11Forwarding no
      ForceCommand nix-store --serve
    Match All

On NixOS, you can accomplish the same by adding the following to your `configuration.nix`:

    nix.sshServe.enable = true;
    nix.sshServe.keys = [ "ssh-dss AAAAB3NzaC1k... bob@example.org" ];

where the latter line lists the public keys of users that are allowed to connect.