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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`:
```console
$ nix-env -iA 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.
```console
$ nix-store -r /nix/store/m85bxg…-firefox-34.0.5 --substituters
ssh://alice@avalon
```
This is essentially equivalent to doing
```console
$ 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
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.
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