Introduction

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.

Configuration

IPsec

Assuming an external interface of eth0:

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

L2TP

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

Firewall

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 8.8.8.8
set vpn l2tp remote-access dns-servers server-2 8.8.4.4

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

And for 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