Layer 2 Tunnel Protocol (L2TP) over IPsec is a very common way of configuring remote access via VPN. This article shows an example of the configuration process in VyOS.
Assuming an external interface of eth0:
set vpn ipsec ipsec-interfaces
interfaceeth0 set vpn ipsec nat-traversal enable set vpn ipsec nat-networks allowed-network 0.0.0.0/0
Assuming a public IP of
203.0.113.2 and an address pool for VPN clients of
192.168.255.2 - 192.168.255.254:
set vpn l2tp remote-access outside-address 203.0.113.2 set vpn l2tp remote-access client-ip-pool start 192.168.255.2 set vpn l2tp remote-access client-ip-pool stop 192.168.255.254
Authentication may be configured either using a pre-shared-secret (a text password given to all clients) or by using X.509 certificates.
Client authentication for L2TP may be configured either using a username/password combination, or by using a RADIUS server. For simplicity, we will use a pre-shared-secret and basic username/password authentication;
not-so-secret for the secret,
alice for the user, and
notsecure for the user's password:
set vpn l2tp remote-access ipsec-settings authentication mode pre-shared-secret set vpn l2tp remote-access ipsec-settings authentication pre-shared-secret "not-so-secret" set vpn l2tp remote-access authentication mode local set vpn l2tp remote-access authentication local-users username alice password notsecure
Additional configuration may be needed if you have a firewall policy on the external interface.
The following ports will need to be open:
- UDP port 500 for IKE
- IP protocol number 50 (ESP)
- UDP port 1701 for IPsec
- UDP port 4500 for ESP NAT traversal
When NAT is detected by the client's VPN software, ESP is encapsulated in UDP for NAT traversal, hence UDP port 4500.
Allow clients to reach external hosts
If you want the VPN to be used for external access (that is, allow clients connected to reach external hosts from the VPN server), SNAT will need to be properly configured:
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
Additionally, clients will need their DNS servers configured (this example uses Google's public DNS servers; replace with your organization's if desired):
set vpn l2tp remote-access dns-servers server-1 18.104.22.168 set vpn l2tp remote-access dns-servers server-2 22.214.171.124
Additional Configuration Options
A full list of configuration options for L2TP can be seen by hitting the
tab key after typing
set vpn l2tp remote-access:
[email protected]# set vpn l2tp remote-access Possible completions: > authentication Authentication for remote access L2TP VPN > client-ip-pool Pool of IP address to be assigned to remote clients description Description for L2TP remote-access settings dhcp-interface DHCP interface to listen on > dns-servers Domain Name Service (DNS) server > ipsec-settings Internet Protocol Security (IPsec) for remote access L2TP VPN mtu Maximum Transmission Unit (MTU) outside-address Outside IP address to which VPN clients will connect outside-nexthop Nexthop IP address for reaching the VPN clients > wins-servers Windows Inernet Name Service (WINS) server settings
set vpn ipsec:
[email protected]# set vpn ipsec Possible completions: auto-update Set auto-update interval for IPsec daemon. disable-uniqreqids Option to disable requirement for unique IDs in the Security Database +> esp-group Name of Encapsulating Security Payload (ESP) group +> ike-group Name of Internet Key Exchange (IKE) group > ipsec-interfaces Interface to use for VPN [REQUIRED] > logging IPsec logging > nat-networks Network Address Translation (NAT) networks nat-traversal Network Address Translation (NAT) traversal +> profile VPN IPSec Profile > site-to-site Site to site VPN
Tweak these options and their sub-options as needed/desired.
Viewing VPN Status
Currently connected clients may be viewed through the following operational mode command:
[email protected]:~$ show vpn remote-access Active remote access VPN sessions: User Proto Iface Tunnel IP TX byte RX byte Time ---- ----- ----- --------- ------- ------- ---- alice L2TP l2tp0 192.168.255.2 3.2K 8.0K 00h06m13s