Add VPN chapter

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diff --git a/docs/ch09-vpn.rst b/docs/ch09-vpn.rst
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+VPN
+===
+
+OpenVPN
+-------
+
+Traditionally hardware routers implement IPsec exclusively due to relative
+ease of implementing it in hardware and insufficient CPU power for doing
+encryption in software. Since VyOS is a software router, this is less of a
+concern. OpenVPN has been widely used on UNIX platform for a long time and is
+a popular option for remote access VPN, though it's also capable of
+site-to-site connections.
+
+The advantages of OpenVPN are:
+* It uses a single TCP or UDP connection and does not rely on packet source
+  addresses, so it will work even through a double NAT: perfect for public
+  hotspots and such
+* It's easy to setup and offers very flexible split tunneling
+* There's a variety of client GUI frontends for any platform
+
+The disadvantages are:
+* It's slower than IPsec due to higher protocol overhead and the fact it runs
+  in user mode while IPsec, on Linux, is in kernel mode
+* None of the operating systems have client software installed by default
+
+In the VyOS CLI, a key point often overlooked is that rather than being
+configured using the `set vpn` stanza, OpenVPN is configured as a network
+interface using `set interfaces openvpn`.
+
+OpenVPN Site-To-Site
+^^^^^^^^^^^^^^^^^^^^
+
+While many are aware of OpenVPN as a Client VPN solution, it is often
+overlooked as a site-to-site VPN solution due to lack of support for this mode
+in many router platforms.
+
+Site-to-site mode supports x.509 but doesn't require it and can also work with
+static keys, which is simpler in many cases. In this example, we'll configure
+a simple site-to-site OpenVPN tunnel using a 2048-bit pre-shared key.
+
+First, one one of the systems generate the key using the operational command
+`generate openvpn key <filename>`. This will generate a key with the name
+provided in the `/config/auth/` directory. Once generated, you will need to
+copy this key to the remote router.
+
+In our example, we used the filename `openvpn-1.key` which we will reference
+in our configuration.
+
+* The public IP address of the local side of the VPN will be 198.51.100.10
+* The remote will be 203.0.113.11
+* The tunnel will use 10.255.1.1 for the local IP and 10.255.1.2 for the remote.
+* OpenVPN allows for either TCP or UDP. UDP will provide the lowest latency,
+  while TCP will work better for lossy connections; generally UDP is preferred
+  when possible.
+* The official port for OpenVPN is 1194, which we reserve for client VPN; we
+  will use 1195 for site-to-site VPN.
+* The `persistent-tunnel` directive will allow us to configure tunnel-related
+  attributes, such as firewall policy as we would on any normal network
+  interface.
+* If known, the IP of the remote router can be configured using the
+  `remote-host` directive; if unknown, it can be omitted. We will assume a
+  dynamic IP for our remote router.
+
+Local Configuration:
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun1 mode site-to-site
+  set interfaces openvpn vtun1 protocol udp
+  set interfaces openvpn vtun1 persistent-tunnel
+  set interfaces openvpn vtun1 local-host '198.51.100.10'
+  set interfaces openvpn vtun1 local-port '1195'
+  set interfaces openvpn vtun1 remote-port '1195'
+  set interfaces openvpn vtun1 shared-secret-key-file '/config/auth/openvpn-1.key'
+  set interfaces openvpn vtun1 local-address '10.255.1.1'
+  set interfaces openvpn vtun1 remote-address '10.255.1.2'
+
+Remote Configuration:
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun1 mode site-to-site
+  set interfaces openvpn vtun1 protocol udp
+  set interfaces openvpn vtun1 persistent-tunnel
+  set interfaces openvpn vtun1 remote-host '198.51.100.10'
+  set interfaces openvpn vtun1 local-port '1195'
+  set interfaces openvpn vtun1 remote-port '1195'
+  set interfaces openvpn vtun1 shared-secret-key-file '/config/auth/openvpn-1.key'
+  set interfaces openvpn vtun1 local-address '10.255.1.2'
+  set interfaces openvpn vtun1 remote-address '10.255.1.1'
+
+The configurations above will default to using 128-bit Blowfish in CBC mode
+for encryption and SHA-1 for HMAC authentication. These are both considered
+weak, but a number of other encryption and hashing algorithms are available:
+
+For Encryption:
+
+.. code-block:: sh
+
+  vyos@vyos# set interfaces openvpn vtun1 encryption
+  Possible completions:
+    des          DES algorithm
+    3des         DES algorithm with triple encryption
+    bf128        Blowfish algorithm with 128-bit key
+    bf256        Blowfish algorithm with 256-bit key
+    aes128       AES algorithm with 128-bit key
+    aes192       AES algorithm with 192-bit key
+    aes256       AES algorithm with 256-bit key
+
+For Hashing:
+
+.. code-block:: sh
+
+  vyos@vyos# set interfaces openvpn vtun1 hash
+  Possible completions:
+    md5          MD5 algorithm
+    sha1         SHA-1 algorithm
+    sha256       SHA-256 algorithm
+    sha512       SHA-512 algorithm
+
+If you change the default encryption and hashing algorithms, be sure that the
+local and remote ends have matching configurations, otherwise the tunnel will
+not come up.
+
+Static routes can be configured referencing the tunnel interface; for example,
+the local router will use a network of 10.0.0.0/16, while the remote has a
+network of 10.1.0.0/16:
+
+Local Configuration:
+
+.. code-block:: sh
+
+  set protocols static interface-route 10.1.0.0/16 next-hop-interface vtun1
+
+Remote Configuration:
+
+.. code-block:: sh
+
+  set protocols static interface-route 10.0.0.0/16 next-hop-interface vtun1
+
+Firewall policy can also be applied to the tunnel interface for `local`, `in`,
+and `out` directions and function identically to ethernet interfaces.
+
+If making use of multiple tunnels, OpenVPN must have a way to distinguish
+between different tunnels aside from the pre-shared-key. This is either by
+referencing IP address or port number. One option is to dedicate a public IP
+to each tunnel. Another option is to dedicate a port number to each tunnel
+(e.g. 1195,1196,1197...).
+
+OpenVPN status can be verified using the `show openvpn` operational commands.
+See the built-in help for a complete list of options.
+
+OpenVPN Server
+^^^^^^^^^^^^^^
+
+Multi-client server is the most popular OpenVPN mode on routers. It always uses
+x.509 authentication and therefore requires a PKI setup. This guide assumes you
+have already setup a PKI and have a CA certificate, a server certificate and
+key, a certificate revokation list, a Diffie-Hellman key exchange parameters
+file. You do not need client certificates and keys for the server setup.
+
+In this example we will use the most complicated case: a setup where each
+client is a router that has its own subnet (think HQ and branch offices), since
+simpler setups are subsets of it.
+
+Suppose you want to use 10.23.1.0/24 network for client tunnel endpoints and
+all client subnets belong to 10.23.0.0/20. All clients need access to the
+192.168.0.0/16 network.
+
+First we need to specify the basic settings. 1194/UDP is the default. The
+`persistent-tunnel` option is recommended, it prevents the TUN/TAP device from
+closing on connection resets or daemon reloads.
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun10 mode server
+  set interfaces openvpn vtun10 local-port 1194
+  set interfaces openvpn vtun10 persistent-tunnel
+  set interfaces openvpn vtun10 protocol udp
+
+Then we need to specify the location of the cryptographic materials. Suppose
+you keep the files in `/config/auth/openvpn`
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun10 tls ca-cert-file /config/auth/openvpn/ca.crt
+  set interfaces openvpn vtun10 tls cert-file /config/auth/openvpn/server.crt
+  set interfaces openvpn vtun10 tls key-file /config/auth/openvpn/server.key
+  set interfaces openvpn vtun10 tls crl-file /config/auth/openvpn/crl.pem
+  set interfaces openvpn vtun10 tls dh-file /config/auth/openvpn/dh2048.pem
+
+Now we need to specify the server network settings. In all cases we need to
+specify the subnet for client tunnel endpoints. Since we want clients to access
+a specific network behind out router, we will use a push-route option for
+installing that route on clients.
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun10 server push-route 192.168.0.0/16
+  set interfaces openvpn vtun10 server subnet 10.23.1.0/24
+
+Since it's a HQ and branch offices setup, we will want all clients to have
+fixed addresses and we will route traffic to specific subnets through them. We
+need configuration for each client to achieve this. Note that clients are
+identified by the CN field of their x.509 certificates, in this example the CN
+is client0:
+
+.. code-block:: sh
+
+  set interfaces openvpn vtun10 server client client0 ip 10.23.1.10
+  set interfaces openvpn vtun10 server client client0 subnet 10.23.2.0/25
+
+OpenVPN **will not** automatically create routes in the kernel for client
+subnets when they connect and will only use client-subnet association
+internally, so we need to create a route to the 10.23.0.0/20 network ourselves:
+
+.. code-block:: sh
+
+  set protocols static interface-route 10.23.0.0/20 next-hop-interface vtun10
+
+L2TP over IPsec
+---------------
+
+Example for configuring a simple L2TP over IPsec VPN for remote access (works
+with native Windows and Mac VPN clients):
+
+.. code-block:: sh
+
+  set vpn ipsec ipsec-interfaces interface eth0
+  set vpn ipsec nat-traversal enable
+  set vpn ipsec nat-networks allowed-network 0.0.0.0/0
+
+  set vpn l2tp remote-access outside-address 203.0.113.2
+  set vpn l2tp remote-access client-ip-pool start 192.168.255.1
+  set vpn l2tp remote-access client-ip-pool stop 192.168.255.254
+  set vpn l2tp remote-access ipsec-settings authentication mode pre-shared-secret
+  set vpn l2tp remote-access ipsec-settings authentication pre-shared-secret <secret>
+  set vpn l2tp remote-access authentication mode local
+  set vpn l2tp remote-access authentication local-users username <username> password <password>
+
+In the example above an external IP of 203.0.113.2 is assumed.
+
+If a local firewall policy is in place on your external interface you will need
+to open:
+
+* UDP port 500 (IKE)
+* IP protocol number 50 (ESP)
+* UDP port 1701 for IPsec
+
+In addition when NAT is detected by the VPN client ESP is encapsulated in UDP
+for NAT-traversal:
+
+* UDP port 4500 (NAT-T)
+
+Example:
+
+.. code-block:: sh
+
+  set firewall name OUTSIDE-LOCAL rule 40 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 40 destination port '50'
+  set firewall name OUTSIDE-LOCAL rule 40 protocol 'esp'
+  set firewall name OUTSIDE-LOCAL rule 41 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 41 destination port '500'
+  set firewall name OUTSIDE-LOCAL rule 41 protocol 'udp'
+  set firewall name OUTSIDE-LOCAL rule 42 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 42 destination port '4500'
+  set firewall name OUTSIDE-LOCAL rule 42 protocol 'udp'
+  set firewall name OUTSIDE-LOCAL rule 43 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 43 destination port '1701'
+  set firewall name OUTSIDE-LOCAL rule 43 ipsec 'match-ipsec'
+  set firewall name OUTSIDE-LOCAL rule 43 protocol 'udp'
+
+Also note that if you wish to allow the VPN to be used for external access you
+will need to add the appropriate source NAT rules to your configuration.
+
+.. code-block:: sh
+
+  set nat source rule 110 outbound-interface 'eth0'
+  set nat source rule 110 source address '192.168.255.0/24'
+  set nat source rule 110 translation address masquerade
+
+To be able to resolve when connected to the VPN, the following DNS rules are
+needed as well.
+
+  set vpn l2tp remote-access dns-servers server-1 '8.8.8.8'
+  set vpn l2tp remote-access dns-servers server-2 '8.8.4.4'
+
+Established sessions can be viewed using the **show vpn remote-access**
+operational command.
+
+.. code-block:: sh
+
+  vyos@vyos:~$ show vpn remote-access
+  Active remote access VPN sessions:
+  User            Proto Iface     Tunnel IP       TX byte RX byte  Time
+  ----            ----- -----     ---------       ------- -------  ----
+  vyos            L2TP  l2tp0     192.168.255.1      3.2K    8.0K  00h06m13s
+
+Site-to-Site IPsec
+------------------
+
+Example:
+* eth1 is WAN interface
+* left subnet: 192.168.0.0/24 #s ite1, server side (i.e. locality, actually
+  there is no client or server roles)
+* left local_ip: 1.1.1.1 # server side WAN IP
+* right subnet: 10.0.0.0/24  # site2,remote office side
+* right local_ip: 2.2.2.2 # remote office side WAN IP
+
+.. code-block:: sh
+
+  # server config
+  set vpn ipsec esp-group office-srv-esp compression 'disable'
+  set vpn ipsec esp-group office-srv-esp lifetime '1800'
+  set vpn ipsec esp-group office-srv-esp mode 'tunnel'
+  set vpn ipsec esp-group office-srv-esp pfs 'enable'
+  set vpn ipsec esp-group office-srv-esp proposal 1 encryption 'aes256'
+  set vpn ipsec esp-group office-srv-esp proposal 1 hash 'sha1'
+  set vpn ipsec ike-group office-srv-ike ikev2-reauth 'no'
+  set vpn ipsec ike-group office-srv-ike key-exchange 'ikev1'
+  set vpn ipsec ike-group office-srv-ike lifetime '3600'
+  set vpn ipsec ike-group office-srv-ike proposal 1 encryption 'aes256'
+  set vpn ipsec ike-group office-srv-ike proposal 1 hash 'sha1'
+  set vpn ipsec ipsec-interfaces interface 'eth1'
+  set vpn ipsec site-to-site peer 2.2.2.2 authentication mode 'pre-shared-secret'
+  set vpn ipsec site-to-site peer 2.2.2.2 authentication pre-shared-secret 'SomePreSharedKey'
+  set vpn ipsec site-to-site peer 2.2.2.2 ike-group 'office-srv-ike'
+  set vpn ipsec site-to-site peer 2.2.2.2 local-address '1.1.1.1'
+  set vpn ipsec site-to-site peer 2.2.2.2 tunnel 0 allow-nat-networks 'disable'
+  set vpn ipsec site-to-site peer 2.2.2.2 tunnel 0 allow-public-networks 'disable'
+  set vpn ipsec site-to-site peer 2.2.2.2 tunnel 0 esp-group 'office-srv-esp'
+  set vpn ipsec site-to-site peer 2.2.2.2 tunnel 0 local prefix '192.168.0.0/24'
+  set vpn ipsec site-to-site peer 2.2.2.2 tunnel 0 remote prefix '10.0.0.0/21'
+
+  # remote office config
+  set vpn ipsec esp-group office-srv-esp compression 'disable'
+  set vpn ipsec esp-group office-srv-esp lifetime '1800'
+  set vpn ipsec esp-group office-srv-esp mode 'tunnel'
+  set vpn ipsec esp-group office-srv-esp pfs 'enable'
+  set vpn ipsec esp-group office-srv-esp proposal 1 encryption 'aes256'
+  set vpn ipsec esp-group office-srv-esp proposal 1 hash 'sha1'
+  set vpn ipsec ike-group office-srv-ike ikev2-reauth 'no'
+  set vpn ipsec ike-group office-srv-ike key-exchange 'ikev1'
+  set vpn ipsec ike-group office-srv-ike lifetime '3600'
+  set vpn ipsec ike-group office-srv-ike proposal 1 encryption 'aes256'
+  set vpn ipsec ike-group office-srv-ike proposal 1 hash 'sha1'
+  set vpn ipsec ipsec-interfaces interface 'eth1'
+  set vpn ipsec site-to-site peer 1.1.1.1 authentication mode 'pre-shared-secret'
+  set vpn ipsec site-to-site peer 1.1.1.1 authentication pre-shared-secret 'SomePreSharedKey'
+  set vpn ipsec site-to-site peer 1.1.1.1 ike-group 'office-srv-ike'
+  set vpn ipsec site-to-site peer 1.1.1.1 local-address '2.2.2.2'
+  set vpn ipsec site-to-site peer 1.1.1.1 tunnel 0 allow-nat-networks 'disable'
+  set vpn ipsec site-to-site peer 1.1.1.1 tunnel 0 allow-public-networks 'disable'
+  set vpn ipsec site-to-site peer 1.1.1.1 tunnel 0 esp-group 'office-srv-esp'
+  set vpn ipsec site-to-site peer 1.1.1.1 tunnel 0 local prefix '10.0.0.0/21'
+  set vpn ipsec site-to-site peer 1.1.1.1 tunnel 0 remote prefix '192.168.0.0/24'
+
+Show status of new setup:
+
+.. code-block:: sh
+
+  vyos@srv-gw0:~$ show vpn ike sa
+  Peer ID / IP                            Local ID / IP
+  ------------                            -------------
+  2.2.2.2                                 1.1.1.1
+     State  Encrypt  Hash    D-H Grp  NAT-T  A-Time  L-Time
+     -----  -------  ----    -------  -----  ------  ------
+     up     aes256   sha1    5        no     734     3600
+
+  vyos@srv-gw0:~$ show vpn ipsec sa
+  Peer ID / IP                            Local ID / IP
+  ------------                            -------------
+  2.2.2.2                                 1.1.1.1
+     Tunnel  State  Bytes Out/In   Encrypt  Hash    NAT-T  A-Time  L-Time  Proto
+     ------  -----  -------------  -------  ----    -----  ------  ------  -----
+     0       up     7.5M/230.6K    aes256   sha1    no     567     1800    all
+
+If there is SNAT rules on eth1, need to add exclude rule
+
+.. code-block:: sh
+
+  # server side
+  set nat source rule 10 destination address '10.0.0.0/24'
+  set nat source rule 10 'exclude'
+  set nat source rule 10 outbound-interface 'eth1'
+  set nat source rule 10 source address '192.168.0.0/24'
+
+  # remote office side
+  set nat source rule 10 destination address '192.168.0.0/24'
+  set nat source rule 10 'exclude'
+  set nat source rule 10 outbound-interface 'eth1'
+  set nat source rule 10 source address '10.0.0.0/24'
+
+To allow traffic to pass through to clients, you need to add the following
+rules. (if you used the default configuration at the top of this page)
+
+.. code-block:: sh
+
+  # server side
+  set firewall name OUTSIDE-LOCAL rule 32 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 32 source address '10.0.0.0/24'
+
+  # remote office side
+  set firewall name OUTSIDE-LOCAL rule 32 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 32 source address '192.168.0.0/24'
+
+DMVPN
+-----
+
+Advanced DMVPN configuration examples are available on the [[DMVPN]] page.
+
diff --git a/docs/index.rst b/docs/index.rst
index 78e938e3..a38459bf 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -35,6 +35,8 @@ as a router and firewall platform for cloud deployments.
 
 .. include:: ch08-nat.rst
 
+.. include:: ch09-vpn.rst
+
 Indices and tables
 ==================