Tutorial: Assign IPv6 IP Addresses to VPN Clients From A Global Pool

Abstract

Assign IPv6 IP addresses to your Access Server VPN clients from a global pool. OpenVPN tutorial

Overview

This tutorial explains enabling and configuring IPv6 in Access Server to assign IP addresses to VPN clients from a global IPv6 pool. Although Access Server requires IPv4 for incoming connections, it supports IPv6 routing inside the VPN tunnel. This means that clients connected over IPv4 can send and receive IPv6 packets through the VPN.

Note

The IPv6 address 2001:db8 used in our examples is reserved for documentation use.

Replace it with a valid IPv6 address for your network.

Prerequisites

An installed Access Server.
An IPv6 interface and IPv6 default gateway configured on your Access Server host machine.
A valid IPv6 address range to assign to VPN clients.

Step 1: Enable IPv6 routing

Connect to the console and get root privileges.
Switch to the scripts directory:
```
cd /usr/local/openvpn_as/scripts/
```

Turn on IPv6 routing:

./sacli --key "vpn.routing6.enable" --value "true" ConfigPut

Step 2: Route all IPv6 traffic through the tunnel (full tunnel)

Turn on full tunneling to route all IPv6 traffic through the tunnel:

./sacli --key "vpn.client.routing6.reroute_gw" --value "true" ConfigPut

Step 3: Define the default IPv6 address pool

Create a default IPv6 address pool to assign to your VPN clients:
```
./sacli --key "vpn.server.daemon.vpn_network6.0" --value "2001:db8:b84b:2::/112" ConfigPut
```
- In this example, we use "2001:db8:b84b:2::/112". This is a unique local IPv6 address (IPv4 private IP equivalent), which means this IP is not routable over the internet.

Step 4: Configure SNAT for IPv6 internet traffic

Configure SNAT so IPv6 traffic from the VPN client uses it to go to the internet:

./sacli --key "vpn.server.routing6.snat_source.0" --value "eth0:2001:db8::963::1" ConfigPut

Step 5: Save and apply the configuration

Save and apply the settings by restarting the server:
```
./sacli start
```

Step 6: Test the setup

From a client machine, connect to the VPN (we recommend using OpenVPN Connect).
- The client should receive an IPv6 address, such as 2001:db8:b84b:2:://112, from the global pool.

Ping an external IPv6 site:

ping ipv6.google.com

Output from a Windows PC:

PS C:\Users\AUser> ping ipv6.google.com

Reply from 2607:f8b0:4006:81f::200e: bytes=1500 time=30ms TTL=54
Reply from 2607:f8b0:4006:81f::200e: bytes=1500 time=30ms TTL=54
Reply from 2607:f8b0:4006:81f::200e: bytes=1500 time=29ms TTL=54
Reply from 2607:f8b0:4006:81f::200e: bytes=1500 time=30ms TTL=54
Reply from 2607:f8b0:4006:81f::200e: bytes=1500 time=31ms TTL=54
Ping statistics for 2607:f8b0:4006:81f::200e:
Packets: Sent = 5, Received = 5, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 29ms, Maximum = 31ms, Average = 30ms

Monitor the traffic on the server using tcpdump:

tcpdump -eni any host ipv6.google.com

Here's an example output showing how the IPv6 traffic is processed and redirected using SNAT:

root@openvpn-access-server-AUser:/usr/local/openvpn_as/scripts# tcpdump -eni any host ipv6.google.com
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes

10:47:31.114998 In ethertype IPv6 (0x86dd), length 96: 2001:db8:b84b:2::8002 > 2607:f8b0:4006:81f::200e: ICMP6, echo request, seq 9, length 40
10:47:31.115031 Out d6:77:ce:97:8c:7c ethertype IPv6 (0x86dd), length 96: 2604:a880:400:d0::189e:6005 > 2607:f8b0:4006:81f::200e: ICMP6, echo request, seq 9, length 40
10:47:31.116589 In fe:00:00:00:01:01 ethertype IPv6 (0x86dd), length 96: 2607:f8b0:4006:81f::200e > 2604:a880:400:d0::189e:6005: ICMP6 echo reply, id 1, seq 9, length 40
10:47:31.116610 Out ethertype IPv6 (0x86dd), length 96: 2607:f8b0:4006:81f::200e > 2001:db8:b84b:2::8002: ICMP6 echo reply, id 1, seq 9, length 40

IPv6 configuration keys

Use the following configuration keys to work with IPv6 in Access Server.

Configuration Key	Type	Description
vpn.routing6.enable	bool	Enable IPv6 routing.
vpn.server.nat6	bool	Enable IPv6 NAT.
vpn.server.nat6.masquerade	bool	Enable IPv6 masquerade.
vpn.client.routing6.reroute_gw	bool	Route all IPv6 traffic through the tunnel.
vpn.server.daemon.vpn_network6	list of subnets	Default IPv6 VPN subnets to be subdivided among OpenVPN daemons: These are used by clients as VPN routing gateways and allocated to non-group clients.
vpn.client.routing6.inter_client	bool	Enable client-to-client IPv6 traffic.
vpn.server.routing6.private_access	string	Controls how to route private traffic: nat route none
vpn.server.routing6.gateway_access	bool (default=true)	If true, clients may access the server-side tun gateway IPv6 address.
vpn.server.routing6.allow_private_nets_to_clients	bool	If true, all IPv6 addresses in vpn.server.routing6.private_network will be allowed to initiate client connections.
vpn.server.routing6.private_network	list of subnets	Access granted to private server-side subnets.
vpn.server.routing6.incoming_network	list of subnets	IPv6 addresses within this range may initiate connections with VPN clients.
vpn.server.routing6.routed_subnets	list of subnets	Subnets that should be routed rather than NATed (when NAT is enabled).
vpn.server.group_pool6	list of subnets	Optional pool of VPN IPv6 addresses to be subdivided across groups that don't define group_subnets6 or group_range6.
vpn.server.routing6.snat_source	list of SNA spec strings	This defines how to perform Source Network Address Translation (SNAT) for outgoing IPv6 packets. When NAT is enabled, SNAT ensures that VPN client traffic uses a specific IPv6 address or range for outgoing traffic. Specify a range of IPv6 addresses for SNAT on each outgoing network interface.
vpn.server.custom_snat6_chain	string	Define a custom ip6tables chain to handle all outgoing NAT.

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