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<chapter xml:id="remoting">
	<title>Remoting / Forwarding</title>

	<para>Smartcards or hardware security modules (HSM) are technologies to
	keep private keys on devices physically isolated to a device only available
	to the user. That way only the intended user can use that device to authenticate,
	authorize or perform other functions that involve the private keys. These come
	usually in the form of a USB device or token which is plugged into the local computer.
	</para>
	<para>In modern "cloud" computing, it is often desirable to virtually transfer such
	a device on remote servers. For example, one can sign software or documents on a remote
	server, use the local smart card to authorize itself to Kerberos, or any other
	possible use. There are various approaches to tackle that problem, and on different
	levels of the smart card application stack. It is possible to forward the USB
	device holding the smart card, or forward the lower-level PC/SC protocol which
	some smart cards talk, or forward the high-level interface used to communicate
	with smart cards, the PKCS#11 interface.</para>

	<para>To address that problem, in p11-kit, we allow the forwarding of
	the higher level smart card interface, PKCS#11. In the following paragraphs
	we describe the approach and tools needed to perform that forwarding over SSH
	secure communication channels.</para>

<refsect1 id="remoting-scenario">
        <title>Scenario</title>

	<para>We assume having a local workstation, and a remote server. On the local
	computer we have inserted a smart card, in our examples we use a Nitrokey
	card with the OpenSC drivers. We will forward the card
	from the workstation to the remote server.</para>
</refsect1>

<refsect1 id="remoting-setup">
        <title>Setting up the PKCS#11 forwarding server on a local client</title>

	<para>To forward a smartcard to a remote server, we first need to identify which
	smartcards are available. To list the smartcards currently attached to the local
	computer, use the p11tool command from the gnutls package. For example:
	</para>

<programlisting>
$ p11tool --list-tokens
...
Token 6:
        URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=www.CardContact.de;serial=DENK0000000;token=UserPIN%20%28Daiki%27s%20token%29
        Label: UserPIN (Daiki's token)
        Type: Hardware token
        Manufacturer: www.CardContact.de
        Model: PKCS#15 emulated
        Serial: DENK0000000
        Module: opensc-pkcs11.so
...
</programlisting>

	<para>This is the entry for the card we'd like to forward to remote system. The important
	pieces are the 'pkcs11:' URL listed above, and the module name. Once we determine which
	smartcard to forward, we expose it to a local Unix domain socket, with the following
	p11-kit server command.
	</para>
<programlisting>
$ p11-kit server --provider /usr/lib64/pkcs11/opensc-pkcs11.so "pkcs11:model=PKCS%2315%20emulated;manufacturer=www.CardContact.de;serial=DENK0000000;token=UserPIN%20%28Daiki%27s%20token%29"
</programlisting>

	<para>Here we provide to the server the module location (optional) with the --provider
	option, as well as the URL of the card. We copied the values from the Module and URL
	lines of the p11tool output above. When the p11-kit server command starts, it will
	print the address of the PKCS#11 unix domain socket and the process ID of the server.
	</para>

<programlisting>
P11_KIT_SERVER_ADDRESS=unix:path=/run/user/12345/p11-kit/pkcs11-12345
P11_KIT_SERVER_PID=12345
</programlisting>

	<para>For later use, set the variables output by the tool on your shell prompt
	(e.g., copy and paste them or call the above p11-kit server command line with
	<literal>eval $(p11-kit server ...)</literal>).
	</para>

</refsect1>

<refsect1 id="remoting-forwarding-socket">
        <title>Forwarding and using the PKCS#11 Unix socket on the remote server</title>

	<para>On the remote server, we will initially forward the previously generated PKCS#11
	unix socket, and then access the smart card through it. To access the forwarded socket
	as if it were a smart card, a dedicated PKCS#11 module p11-kit-client.so is provided as
	part of the p11-kit-server package.
	</para>
</refsect1>

<refsect1 id="remoting-forwarding-socket-prep">
        <title>Preparing the remote system for PKCS#11 socket forwarding</title>

	<para>One important detail you should be aware of, is the file system location of the
	forwarded socket. By convention, the p11-kit-client.so module utilizes the "user runtime
	directory", managed by systemd; the directory is created when a user logs in, and removed
	upon logout, so that the user doesn't need to manually clean up the socket file.
	</para>

	<para>To locate your user runtime directory, do:
	</para>

<programlisting>
$ systemd-path user-runtime
/run/user/1000
</programlisting>

	<para>The <literal>p11-kit-client.so</literal> module looks for the socket file under a
	subdirectory (<literal>/run/user/1000/p11-kit</literal> in this example). To enable
	auto-creation of the directory, do the following.
	</para>

<programlisting>
$ systemctl --user enable p11-kit-client.service
</programlisting>
</refsect1>

<refsect1 id="remoting-finally-forwarding">
        <title>Forwarding the PKCS#11 socket</title>

	<para>We will use ssh to forward the local PKCS#11 unix socket to the remote server.
	Following the p11-kit-client convention, we will forward the socket to the remote user
	run-time path so that cleaning up on disconnect is not required. The remote location
	of the run-time path can be obtained as follows.
	</para>

<programlisting>
$ ssh [user]@[remotehost] systemd-path user-runtime
/run/user/1000
</programlisting>

	<para>The number at the end of the path above is your user ID in that system
	(and thus will vary from user to user). You can now forward the Unix domain socket
	with the -R option of the ssh command, after replacing the example path with the
	actual run-time path.
	</para>

<programlisting>
$ ssh -R /run/user/[userID]/p11-kit/pkcs11:${P11_KIT_SERVER_ADDRESS#*=} [user]@[remotehost]
</programlisting>

	<para>After successfully logging in to the remote host, you can use the forwarded
	smartcard as if it were directly connected to the server using the
	<literal>p11-kit-client.so</literal>. Note that if any error occurs during the forwarding
	setup, you will see something like this on your terminal:
	</para>

<programlisting>
Warning: remote port forwarding failed for listen path /run/user/...
</programlisting>
</refsect1>

<refsect1 id="remoting-using">
        <title>Using the forwarded PKCS#11 socket</title>

	<para>Let's first make sure the smart card works on the remote system, by listing it:
	</para>

<programlisting>
$ ls -l /run/user/1000/p11-kit/pkcs11

$ p11tool --provider /usr/lib64/pkcs11/p11-kit-client.so --list-tokens
...
Token 0:
        URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=www.CardContact.de;serial=DENK0000000;token=UserPIN%20%28Daiki%27s%20token%29
        Label: UserPIN (Daiki's token)
        Type: Hardware token
        Manufacturer: www.CardContact.de
        Model: PKCS#15 emulated
        Serial: DENK0000000
        Module: (null)
...
</programlisting>

	<para>We can similarly generate, copy objects or test certificates to the card using
	the same command. Any applications which support PKCS#11 can perform cryptographic
	operations through the client module.
	</para>
</refsect1>

<refsect1 id="remoting-registering">
        <title>Registering the client module for use with OpenSSL and GnuTLS apps</title>

	<para>To utilize the p11-kit-client module with OpenSSL (via engine_pkcs11 provided
	by the libp11 package) and GnuTLS applications, you have to register it in
	p11-kit. To do it for the current user, use the following commands:
	</para>

<programlisting>
$ mkdir .config/pkcs11/modules/
$ echo "module: /usr/lib64/pkcs11/p11-kit-client.so" >.config/pkcs11/modules/p11-kit-client.module
</programlisting>

	<para>Once this is done both OpenSSL and GnuTLS applications should work, for example:
	</para>

<programlisting>
$ URL="pkcs11:model=PKCS%2315%20emulated;manufacturer=www.CardContact.de;serial=DENK0000000;token=UserPIN%20%28Daiki%27s%20token%29"

# Generate a key using gnutls’ p11tool
$ p11tool --generate-ecc --login --label test-key "$URL"

# generate a certificate request with the previous key using openssl
$ openssl req -engine pkcs11 -new -key "$URL;;object=test-key;type=private;pin-value=XXXX" \
         -keyform engine -out req.pem -text -subj "/CN=Test user"
</programlisting>

	<para>Note that the token URL remains the same in the forwarded system as in the original one.
	</para>

</refsect1>

<refsect1 id="remoting-ssh">
        <title>Using the client module with OpenSSH</title>

	<para>To re-use the already forwarded smartcard for authentication with another remote host, you can run ssh and provide the -I option with p11-kit-client.so. For example:
	</para>

<programlisting>
$ ssh -I /usr/lib64/pkcs11/p11-kit-client.so [user]@[anotherhost]
</programlisting>
</refsect1>

<refsect1 id="remoting-nss">
        <title>Using the client module with NSS applications</title>

	<para>To register the forwarded smartcard in NSS applications, you can set it up with
	the modutil command, as follows.
	</para>

<programlisting>
$ sudo modutil -dbdir /etc/pki/nssdb -add p11-kit-client -libfile /usr/lib64/pkcs11/p11-kit-client.so

$ modutil -dbdir /etc/pki/nssdb -list
...
  3. p11-kit-client
    library name: /usr/lib64/pkcs11/p11-kit-client.so
       uri: pkcs11:library-manufacturer=OpenSC%20Project;library-description=OpenSC%20smartcard%20framework;library-version=0.17
     slots: 1 slot attached
    status: loaded

     slot: Nitrokey Nitrokey HSM (010000000000000000000000) 00 00
    token: UserPIN (Daiki's token)
      uri: pkcs11:token=UserPIN%20(Daiki's%20token);manufacturer=www.CardContact.de;serial=DENK0000000;model=PKCS%2315%20emulated
</programlisting>
</refsect1>


</chapter>
