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.

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

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.

Site-To-Site

../../_images/openvpn_site2site_diagram.jpg

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 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 public IP address of the remote side of the VPN 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.
The local site will have a subnet of 10.0.0.0/16.
The remote site will have a subnet of 10.1.0.0/16.
Static Routing or other dynamic routing protocols can be used over the vtun interface
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:

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 '203.0.113.11'
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'

Local Configuration - Annotated:

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 '203.0.113.11'                         # Pub IP of other site
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'                         # Local IP of vtun interface
set interfaces openvpn vtun1 remote-address '10.255.1.2'                        # Remote IP of vtun interface

Remote Configuration:

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'

Remote Configuration - Annotated:

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'                         # Pub IP of other site
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'                          # Local IP of vtun interface
set interfaces openvpn vtun1 remote-address '10.255.1.1'                         # Remote IP of vtun interface

Firewall Exceptions

For the OpenVPN traffic to pass through the WAN interface, you must create a firewall exception.

set firewall name OUTSIDE_LOCAL rule 10 action accept
set firewall name OUTSIDE_LOCAL rule 10 description 'Allow established/related'
set firewall name OUTSIDE_LOCAL rule 10 state established enable
set firewall name OUTSIDE_LOCAL rule 10 state related enable
set firewall name OUTSIDE_LOCAL rule 20 action accept
set firewall name OUTSIDE_LOCAL rule 20 description OpenVPN_IN
set firewall name OUTSIDE_LOCAL rule 20 destination port 1195
set firewall name OUTSIDE_LOCAL rule 20 log enable
set firewall name OUTSIDE_LOCAL rule 20 protocol udp
set firewall name OUTSIDE_LOCAL rule 20 source

You should also ensure that the OUTISDE_LOCAL firewall group is applied to the WAN interface and a direction (local).

set interfaces ethernet eth0 firewall local name 'OUTSIDE-LOCAL'

Static Routing:

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:

set protocols static interface-route 10.1.0.0/16 next-hop-interface vtun1

Remote Configuration:

set protocols static interface-route 10.0.0.0/16 next-hop-interface vtun1

The configurations above will default to using 256-bit AES in GCM mode for encryption (if both sides support NCP) and SHA-1 for HMAC authentication. SHA-1 is considered weak, but other hashing algorithms are available, as are encryption algorithms:

For Encryption:

This sets the cipher when NCP (Negotiable Crypto Parameters) is disabled or OpenVPN version < 2.4.0.

vyos@vyos# set interfaces openvpn vtun1 encryption cipher
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 CBC
  aes128gcm    AES algorithm with 128-bit key GCM
  aes192       AES algorithm with 192-bit key CBC
  aes192gcm    AES algorithm with 192-bit key GCM
  aes256       AES algorithm with 256-bit key CBC
  aes256gcm    AES algorithm with 256-bit key GCM

This sets the accepted ciphers to use when version => 2.4.0 and NCP is enabled (which is the default). Default NCP cipher for versions >= 2.4.0 is aes256gcm. The first cipher in this list is what server pushes to clients.

vyos@vyos# set int open vtun0 encryption ncp-ciphers
Possible completions:
  des          DES algorithm
  3des         DES algorithm with triple encryption
  aes128       AES algorithm with 128-bit key CBC
  aes128gcm    AES algorithm with 128-bit key GCM
  aes192       AES algorithm with 192-bit key CBC
  aes192gcm    AES algorithm with 192-bit key GCM
  aes256       AES algorithm with 256-bit key CBC
  aes256gcm    AES algorithm with 256-bit key GCM

For Hashing:

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.

Firewall policy can also be applied to the tunnel interface for local, in, and out directions and functions 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.

Server

Multi-client server is the most popular OpenVPN mode on routers. It always uses x.509 authentication and therefore requires a PKI setup. Refer this section Generate X.509 Certificate and Keys to generate a CA certificate, a server certificate and key, a certificate revocation 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.

Note

Using openvpn-option -reneg-sec can be tricky. This option is used to renegotiate data channel after n seconds. When used at both server and client, the lower value will trigger the renegotiation. If you set it to 0 on one side of the connection (to disable it), the chosen value on the other side will determine when the renegotiation will occur.

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

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.

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

Clients are identified by the CN field of their x.509 certificates, in this example the CN is client0:

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:

set protocols static interface-route 10.23.0.0/20 next-hop-interface vtun10

Generate X.509 Certificate and Keys

OpenVPN ships with a set of scripts called Easy-RSA that can generate the appropriate files needed for an OpenVPN setup using X.509 certificates. Easy-RSA comes installed by default on VyOS routers.

Copy the Easy-RSA scripts to a new directory to modify the values.

cp -r /usr/share/easy-rsa/ /config/my-easy-rsa-config
cd /config/my-easy-rsa-config

To ensure the consistent use of values when generating the PKI, set default values to be used by the PKI generating scripts. Rename the vars.example filename to vars

mv vars.example vars

Following is the instance of the file after editing. You may also change other values in the file at your discretion/need, though for most cases the defaults should be just fine. (do not leave any of these parameters blank)

set_var EASYRSA_DN      "org"
set_var EASYRSA_REQ_COUNTRY     "US"
set_var EASYRSA_REQ_PROVINCE    "California"
set_var EASYRSA_REQ_CITY        "San Francisco"
set_var EASYRSA_REQ_ORG "Copyleft Certificate Co"
set_var EASYRSA_REQ_EMAIL       "[email protected]"
set_var EASYRSA_REQ_OU          "My Organizational Unit"
set_var EASYRSA_KEY_SIZE        2048

init-pki option will create a new pki directory or will delete any previously generated certificates stored in that folder. The term ‘central’ is used to refer server and ‘branch’ for client

Note

Remember the “CA Key Passphrase” prompted in build-ca command, as it will be asked in signing the server/client certificate.

vyos@vyos:/config/my-easy-rsa-config$./easyrsa init-pki
vyos@vyos:/config/my-easy-rsa-config$./easyrsa build-ca
vyos@vyos:/config/my-easy-rsa-config$./easyrsa gen-req central nopass
vyos@vyos:/config/my-easy-rsa-config$./easyrsa sign-req server central
vyos@vyos:/config/my-easy-rsa-config$./easyrsa gen-dh
vyos@vyos:/config/my-easy-rsa-config$./easyrsa build-client-full branch1 nopass

To generate a certificate revocation list for any client, execute these commands:

vyos@vyos:/config/my-easy-rsa-config$./easyrsa revoke client1
vyos@vyos:/config/my-easy-rsa-config$ ./easyrsa gen-crl

Copy the files to /config/auth/openvpn/ to use in OpenVPN tunnel creation

vyos@vyos:/config/my-easy-rsa-config$ sudo mkdir /config/auth/openvpn
vyos@vyos:/config/my-easy-rsa-config$ sudo cp pki/ca.crt /config/auth/openvpn
vyos@vyos:/config/my-easy-rsa-config$ sudo cp pki/dh.pem  /config/auth/openvpn
vyos@vyos:/config/my-easy-rsa-config$ sudo cp pki/private/central.key /config/auth/openvpn
vyos@vyos:/config/my-easy-rsa-config$ sudo cp pki/issued/central.crt  /config/auth/openvpn
vyos@vyos:/config/my-easy-rsa-config$ sudo cp pki/crl.pem /config/auth/openvpn

Additionally, each client needs a copy of ca.crt and its own client key and cert files. The files are plaintext so they may be copied either manually, or through a remote file transfer tool like scp. Whichever method you use, the files need to end up in the proper location on each router. For example, Branch 1’s router might have the following files:

vyos@branch1-rtr:$ ls /config/auth/openvpn
ca.crt branch1.crt branch1.key

Client Authentication

LDAP

Enterprise installations usually ship a kind of directory service which is used to have a single password store for all employees. VyOS and OpenVPN support using LDAP/AD as single user backend.

Authentication is done by using the openvpn-auth-ldap.so plugin which is shipped with every VyOS installation. A dedicated configuration file is required. It is best practise to store it in /config to survive image updates

set interfaces openvpn vtun0 openvpn-option "--plugin /usr/lib/openvpn/openvpn-auth-ldap.so /config/auth/ldap-auth.config"

The required config file may look like this:

<LDAP>
# LDAP server URL
URL             ldap://ldap.example.com
# Bind DN (If your LDAP server doesn't support anonymous binds)
BindDN          cn=LDAPUser,dc=example,dc=com
# Bind Password password
Password        S3cr3t
# Network timeout (in seconds)
Timeout         15
</LDAP>

<Authorization>
# Base DN
BaseDN          "ou=people,dc=example,dc=com"
# User Search Filter
SearchFilter    "(&(uid=%u)(objectClass=shadowAccount))"
# Require Group Membership - allow all users
RequireGroup    false
</Authorization>

Active Directory

Despite the fact that AD is a superset of LDAP

<LDAP>
  # LDAP server URL
  URL ldap://dc01.example.com
  # Bind DN (If your LDAP server doesn’t support anonymous binds)
  BindDN CN=LDAPUser,DC=example,DC=com
  # Bind Password
  Password mysecretpassword
  # Network timeout (in seconds)
  Timeout  15
  # Enable Start TLS
  TLSEnable no
  # Follow LDAP Referrals (anonymously)
  FollowReferrals no
</LDAP>

<Authorization>
  # Base DN
  BaseDN        "DC=example,DC=com"
  # User Search Filter, user must be a member of the VPN AD group
  SearchFilter  "(&(sAMAccountName=%u)(memberOf=CN=VPN,OU=Groups,DC=example,DC=com))"
  # Require Group Membership
  RequireGroup    false # already handled by SearchFilter
  <Group>
    BaseDN        "OU=Groups,DC=example,DC=com"
    SearchFilter  "(|(cn=VPN))"
    MemberAttribute  memberOf
  </Group>
</Authorization>

If you only want to check if the user account is enabled and can authenticate (against the primary group) the following snipped is sufficient:

<LDAP>
  URL ldap://dc01.example.com
  BindDN CN=SA_OPENVPN,OU=ServiceAccounts,DC=example,DC=com
  Password ThisIsTopSecret
  Timeout  15
  TLSEnable no
  FollowReferrals no
</LDAP>

<Authorization>
  BaseDN          "DC=example,DC=com"
  SearchFilter    "sAMAccountName=%u"
  RequireGroup    false
</Authorization>

A complete LDAP auth OpenVPN configuration could look like the following example:

vyos@vyos# show interfaces openvpn
 openvpn vtun0 {
     mode server
     openvpn-option "--tun-mtu 1500 --fragment 1300 --mssfix"
     openvpn-option "--plugin /usr/lib/openvpn/openvpn-auth-ldap.so /config/auth/ldap-auth.config"
     openvpn-option "--push redirect-gateway"
     openvpn-option --duplicate-cn
     openvpn-option --client-cert-not-required
     openvpn-option --comp-lzo
     openvpn-option --persist-key
     openvpn-option --persist-tun
     server {
         domain-name example.com
         max-connections 5
         name-server 203.0.113.0.10
         name-server 198.51.100.3
         subnet 172.18.100.128/29
     }
     tls {
         ca-cert-file /config/auth/ca.crt
         cert-file /config/auth/server.crt
         dh-file /config/auth/dh1024.pem
         key-file /config/auth/server.key
     }
 }

Client

VyOS can not only act as an OpenVPN site-to-site or server for multiple clients. You can indeed also configure any VyOS OpenVPN interface as an OpenVPN client connecting to a VyOS OpenVPN server or any other OpenVPN server.

Given the following example we have one VyOS router acting as OpenVPN server and another VyOS router acting as OpenVPN client. The server also pushes a static client IP address to the OpenVPN client. Remember, clients are identified using their CN attribute in the SSL certificate.

Server

set interfaces openvpn vtun10 encryption cipher 'aes256'
set interfaces openvpn vtun10 hash 'sha512'
set interfaces openvpn vtun10 local-host '172.18.201.10'
set interfaces openvpn vtun10 local-port '1194'
set interfaces openvpn vtun10 mode 'server'
set interfaces openvpn vtun10 persistent-tunnel
set interfaces openvpn vtun10 protocol 'udp'
set interfaces openvpn vtun10 server client client1 ip '10.10.0.10'
set interfaces openvpn vtun10 server domain-name 'vyos.net'
set interfaces openvpn vtun10 server max-connections '250'
set interfaces openvpn vtun10 server name-server '172.16.254.30'
set interfaces openvpn vtun10 server subnet '10.10.0.0/24'
set interfaces openvpn vtun10 server topology 'subnet'
set interfaces openvpn vtun10 tls ca-cert-file '/config/auth/ca.crt'
set interfaces openvpn vtun10 tls cert-file '/config/auth/server.crt'
set interfaces openvpn vtun10 tls dh-file '/config/auth/dh.pem'
set interfaces openvpn vtun10 tls key-file '/config/auth/server.key'
set interfaces openvpn vtun10 use-lzo-compression

Client

set interfaces openvpn vtun10 encryption cipher 'aes256'
set interfaces openvpn vtun10 hash 'sha512'
set interfaces openvpn vtun10 mode 'client'
set interfaces openvpn vtun10 persistent-tunnel
set interfaces openvpn vtun10 protocol 'udp'
set interfaces openvpn vtun10 remote-host '172.18.201.10'
set interfaces openvpn vtun10 remote-port '1194'
set interfaces openvpn vtun10 tls ca-cert-file '/config/auth/ca.crt'
set interfaces openvpn vtun10 tls cert-file '/config/auth/client1.crt'
set interfaces openvpn vtun10 tls key-file '/config/auth/client1.key'
set interfaces openvpn vtun10 use-lzo-compression

Options

We do not have CLI nodes for every single OpenVPN option. If an option is missing, a feature request should be opened at Phabricator so all users can benefit from it (see Issues/Feature requests).

If you are a hacker or want to try on your own we support passing raw OpenVPN options to OpenVPN.

set interfaces openvpn vtun10 openvpn-option ‘persistent-key’

Will add persistent-key at the end of the generated OpenVPN configuration. Please use this only as last resort - things might break and OpenVPN won’t start if you pass invalid options/syntax.

set interfaces openvpn vtun10 openvpn-option ‘push "keepalive 1 10"’

Will add push "keepalive 1 10" to the generated OpenVPN config file.

Note

Sometimes option lines in the generated OpenVPN configuration require quotes. This is done through a hack on our config generator. You can pass quotes using the " statement.

Troubleshooting

VyOS provides some operational commands on OpenVPN.

Check status

The following commands let you check tunnel status.

show openvpn client

Use this command to check the tunnel status for OpenVPN client interfaces.

show openvpn server

Use this command to check the tunnel status for OpenVPN server interfaces.

show openvpn site-to-site

Use this command to check the tunnel status for OpenVPN site-to-site interfaces.

Reset OpenVPN

The following commands let you reset OpenVPN.

reset openvpn client <text>

Use this command to reset the specified OpenVPN client.

reset openvpn interface <interface>

Use this command to reset the OpenVPN process on a specific interface.