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To run OpenVPN over IPv6 and carry IPv6 traffic through the tunnel, you must configure two separate paths: an IPv6 transport path to the server (proto udp6) and an IPv6 address pool, routes, forwarding policy, and upstream reachability for clients. A working listener alone does not give connected clients IPv6 internet access.
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This guide builds a routed TUN server on Ubuntu, using a provider-routed IPv6 block, certificate authentication, kernel forwarding, firewall rules, and layered verification. Replace every documentation prefix such as 2001:db8::/64 with the real prefix routed to your host.
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The finished design has four independent pieces:
- Transport: the client reaches the OpenVPN daemon over IPv6 UDP, normally with
proto udp6. - Tunnel addressing: OpenVPN assigns each client an address from a dedicated IPv6 network.
- Routing policy: the server pushes either selected IPv6 routes (split tunnel) or all IPv6 unicast traffic (full tunnel).
- Forwarding and return path: Ubuntu forwards packets between the TUN interface and its IPv6 uplink, while the upstream network knows how to return traffic to the client pool, or a deliberate SNAT design translates it.
The OpenVPN Community IPv6 guide says IPv6 support inside the tunnel and IPv6 as the transport protocol are optional, separate capabilities. It also states that a routed setup must use a unique routed network range rather than reusing the server’s on-link LAN prefix. That distinction explains most “the client connects but has no IPv6” failures.
Prerequisites and design decisions
Host and provider requirements
- An Ubuntu server with a globally usable IPv6 address.
- A working default IPv6 route on that server.
- IPv6 support on every client that will connect.
- A provider-routed IPv6 block that reaches the Ubuntu host. The community guide recommends a separate
/64for the VPN subnet; obtaining and routing that block is a provider or ISP task, not an OpenVPN setting. - Permission to open a UDP port in both the host firewall and any cloud, VPS, or edge firewall.
Do not assign the same on-link LAN prefix to both the physical interface and the VPN pool. In a routed design, the provider must route the VPN prefix toward the server’s existing IPv6 address. If your provider cannot route an additional public prefix, use a private or unique-local pool with IPv6 SNAT only when that is appropriate for your network policy.
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Choose transport, payload, and traffic scope
| Decision | Option | When to use it |
|---|---|---|
| OpenVPN transport | udp6 |
The client must reach the daemon through IPv6. OpenVPN 2.6 defines this as IPv6-only UDP transport. |
| OpenVPN transport | udp4 |
Use when clients cannot reach the server over IPv6. The tunnel can still carry IPv6 payload traffic if its routing is configured correctly. |
| Client traffic | Split tunnel | Push only the IPv6 networks clients need; ordinary internet traffic continues through the client’s local connection. |
| Client traffic | Full tunnel | Push the IPv6 default/unicast routes so internet IPv6 traffic exits through the Ubuntu host. |
| Tunnel topology | Routed TUN | The normal IPv6 design described by the community guide; it scales without bridging a layer-2 network. |
| Tunnel topology | Bridged TAP | A specialist case for layer-2 requirements, not the generally recommended IPv6 approach. |
Decide this before writing configuration. A server can listen on IPv6 while still giving clients no usable IPv6 egress, and a tunnel can carry IPv6 while its control connection arrives over IPv4.
Install OpenVPN and create certificates
Install the packages
- Update Ubuntu’s package index and install OpenVPN plus Easy-RSA:
sudo apt update sudo apt install openvpn easy-rsa - Create a working Easy-RSA directory using the version installed by Ubuntu. A typical setup is:
make-cadir ~/openvpn-ca cd ~/openvpn-ca ./easyrsa init-pki ./easyrsa build-caFollow the prompts for the certificate authority name and protect the CA key.
- Create and sign a server certificate, then generate Diffie-Hellman parameters:
./easyrsa gen-req server nopass ./easyrsa sign-req server server ./easyrsa gen-dhUse a passphrase-protected server key instead of
nopassif your service-start process can unlock it securely. - Create one client certificate and private key per device. For example:
./easyrsa gen-req client1 nopass ./easyrsa sign-req client client1Never reuse a client private key on multiple devices; revoking one shared key would affect every device using it.
Create the TLS protection key
Ubuntu’s guide uses OpenVPN’s static TLS key and references it with tls-crypt:
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sudo openvpn --genkey secret ta.keyCopy the generated key, the CA certificate, the issued server certificate and key, and the Diffie-Hellman file into the directory used by the server configuration. OpenVPN 2.4 introduced tls-crypt; systems older than Ubuntu 18.04 require the older tls-auth form instead. Keep private keys and ta.key readable only by the appropriate administrator or service account.
Write the IPv6 server configuration
Choose a real routed pool
Use the separate prefix supplied by your provider. The following uses documentation-only space to show the shape of the file; 2001:db8::/64 is not globally routable and must never be deployed:
port 1194
proto udp6
dev tun
topology subnet
server-ipv6 2001:db8:1234:5678::/64
# Split tunnel example: push only the network you need
# push "route-ipv6 2001:db8:abcd::/48"
# Full-tunnel alternative: send IPv6 unicast traffic through this server
# push "route-ipv6 2000::/3"
keepalive 10 120
persist-key
persist-tun
ca ca.crt
cert server.crt
key server.key
dh dh.pem
tls-crypt ta.key
user nobody
group nogroup
verb 3The exact IPv6 pool directive depends on the OpenVPN release and the sample configuration shipped with your Ubuntu version. Current releases commonly use server-ipv6; older examples may use lower-level IPv6 interface and pool directives. Start with Ubuntu’s sample and retain the syntax supported by the installed binary.
Push only the routes you intend to use
For split tunneling, add one push "route-ipv6 ..." line for each internal or public network clients should reach. A client using the normal client directive accepts pushed routes automatically unless its profile contains route-nopull.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesFor a full tunnel, push the IPv6 unicast range (commonly 2000::/3) or use the IPv6 redirect behavior supported by your installed OpenVPN version. Ensure the VPN server itself has a route to the upstream gateway before testing; pushing a route cannot create provider reachability that does not exist.
Enable IPv6 forwarding in Ubuntu
The Linux kernel must route packets between the TUN interface and the physical IPv6 interface. Create a persistent sysctl drop-in:
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sudo tee /etc/sysctl.d/50-enable-ipv6-forwarding.conf <<'EOF'
net.ipv6.conf.all.forwarding = 1
EOF
sudo sysctl --systemVerify the effective value:
sysctl net.ipv6.conf.all.forwardingIt should report net.ipv6.conf.all.forwarding = 1. If this host will also route IPv4 traffic, Ubuntu documents the equivalent setting and reload command:
echo "net.ipv4.ip_forward = 1" | sudo tee /etc/sysctl.d/50-enable-ipv4-forwarding.conf
sudo sysctl -p /etc/sysctl.d/50-enable-ipv4-forwarding.confForwarding is a kernel function, not an OpenVPN option. Restarting OpenVPN without enabling it leaves the tunnel connected but unable to pass packets.
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Configure firewall forwarding and upstream routing
Open the listener
Allow the selected UDP port, commonly 1194, in every firewall between the client and server. With UFW, the basic host rule is:
sudo ufw allow 1194/udpCloud security groups, provider firewalls, and an IPv6 edge router may need an equivalent rule. Confirm that UFW is managing IPv6 (IPV6=yes in its configuration) before assuming an IPv4 rule also applies to IPv6.
Permit the routed data path
Permit forwarding from the TUN interface to the IPv6 uplink and allow established return traffic. Interface names differ: the tunnel may be tun0, while the uplink might be ens3, eth0, or another name. A UFW route rule therefore has to use your actual interfaces, for example:
sudo ufw route allow in on tun0 out on ens3Do not copy that command unchanged. Check ip link and your firewall’s active policy first, then add the reverse or stateful return rule required by your firewall framework. nftables, iptables, and UFW have different persistence mechanisms; make the rule in the tool that owns your host policy.
Choose routing or SNAT
Public routed pool: the provider routes the VPN prefix to the Ubuntu host. Clients retain their public IPv6 addresses, and no translation is required. This is the cleanest design when your provider supports it.
Private or unique-local pool with SNAT: the server translates client addresses to its own global IPv6 address on egress. This can work when the provider will not route an additional public prefix, but it changes inbound reachability and logging semantics. The masquerade rule must name the correct IPv6 egress interface and must be accompanied by forwarding policy.
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OpenVPN Access Server documentation distinguishes public routed pools, local pools with SNAT, and NAT-disabled routing. Whichever model you select, verify that the return path exists. A client may receive an address and send packets successfully while replies disappear because the upstream network has no route back to that pool.
Start the Ubuntu service
Ubuntu uses templated systemd units. If your configuration file is /etc/openvpn/server.conf, the documented unit name is based on the configuration filename:
sudo systemctl start openvpn@server
sudo systemctl enable openvpn@server
sudo systemctl status openvpn@serverThe generic openvpn unit does not select and start a file named server.conf. If your Ubuntu package uses a different directory layout, list the available OpenVPN units and follow the template provided by that package, but keep the instance name aligned with the configuration file.
Read the service log immediately after starting:
sudo journalctl -u openvpn@server -bCertificate errors, missing key files, unsupported directives, and bind failures are much easier to fix before a client is involved.
Build a matching client profile
Install OpenVPN on the client and transfer its own CA certificate, client certificate, private key, and ta.key through a secure channel. A minimal profile looks like this:
client
dev tun
proto udp6
remote [2001:db8:feed::10] 1194
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
verb 3
ca ca.crt
cert client1.crt
key client1.key
tls-crypt ta.keyUse brackets around a literal IPv6 address in the remote line. If you use a hostname, make sure it has an AAAA record and that the client has working IPv6 DNS and routing. The client’s proto must match the server’s transport choice: udp6 for an IPv6-only control path, or udp4 when the server is reached over IPv4.
Do not place a shared client key in a profile distributed to an entire team. Issue and revoke certificates per person or device, and protect profile files because they contain private credentials.
Verify the connection in layers
1. Confirm authentication and tunnel completion
Start the client and look for certificate verification followed by Initialization Sequence Completed. On the server, inspect:
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sudo journalctl -u openvpn@server -fIf the log stops before initialization, fix transport, certificate, key, or TLS errors before investigating routing.
2. Inspect addresses and routes
On the server and client, inspect the tunnel interface and IPv6 table:
ip addr show tun0
ip -6 addr show dev tun0
ip -6 routeThe client should have an address from the dedicated VPN prefix and routes corresponding to the policy you pushed. If the TUN interface has no IPv6 address, the server’s IPv6 pool directive or certificate-side configuration is wrong. If the address exists but no route was installed, check the pushed route and whether the profile uses route-nopull.
3. Test the tunnel gateway
From the client, ping or otherwise connect to the IPv6 address assigned to the VPN gateway. This isolates tunnel addressing and local forwarding from internet routing. A failure here points to the OpenVPN configuration, interface firewall, or kernel forwarding.
4. Test external IPv6 egress
Use a trusted external IPv6 test endpoint or public-address service from the connected client. In a routed design, the observed address should belong to the provider-routed VPN prefix. In an SNAT design, it will normally be the server’s global IPv6 address. If the test works for the gateway but not externally, investigate forwarding, upstream routes, and SNAT rather than certificates.
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- Check prefix separation. Confirm that the VPN pool is a distinct routed network, not the server’s on-link LAN prefix.
- Check kernel forwarding. Verify
net.ipv6.conf.all.forwardingis1after reboot. - Check firewall forwarding. Confirm the TUN-to-uplink rule, return traffic, and any provider security-group rule.
- Check the provider route. Ask the host provider to route the VPN prefix to the server’s IPv6 address, or confirm that your SNAT design is intentional and active.
- Check pushed routes. Inspect the client log and
ip -6 route. A full-tunnel profile must actually install the IPv6 unicast/default routes. - Check the egress interface in NAT rules. A masquerade rule bound to the wrong interface silently produces a connected tunnel with no internet replies.
- Check address and route conflicts. Remove duplicate prefixes, stale routes, and client-side VPN software that may install a competing default route.
- Check MTU and filtering only after routing. If small pings work but larger HTTPS connections stall, investigate path-MTU behavior and ICMPv6 filtering. IPv6 depends on ICMPv6 for packet-too-big messages; blocking it can break otherwise valid paths.
Operational and security considerations
Keep credentials and keys private
The CA key should remain offline or on a tightly controlled administration system. The server key, each client key, and ta.key should have restrictive permissions. Remove temporary Easy-RSA working copies from shared machines and revoke a client certificate when a device is lost.
Use least-privilege traffic policy
Full-tunnel IPv6 sends every IPv6 destination through your server, increasing bandwidth use and making the server a single egress point. Split tunneling limits exposure and cost but leaves ordinary IPv6 traffic on each client’s local network. Document which policy you selected so users understand what the VPN does and does not protect.
Monitor after upgrades
OpenVPN syntax and Ubuntu service layouts evolve. After an Ubuntu or OpenVPN upgrade, validate the service unit, supported IPv6 directives, certificate paths, and firewall persistence. Keep a copy of the known-good configuration and record the routed prefix, interface names, and provider next-hop details.
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Answers to common questions
Can a client use an IPv4 internet connection to reach an IPv6-only OpenVPN listener?
No. proto udp6 requires IPv6 connectivity to the server’s listening address. If some clients are IPv4-only, provide a separate IPv4 transport profile (or a dual-transport design) while keeping the tunnel’s IPv6 routing policy independent.
Is a /64 mathematically required for every OpenVPN IPv6 pool?
No, but the OpenVPN Community guide recommends a /64 for the VPN subnet, and many IPv6 tools and provider policies assume that size. Use the prefix your provider actually routes and the pool size supported by your installed OpenVPN version; do not invent a /64 that is not routed to the host.
Why is bridging usually avoided for this setup?
Bridged TAP extends a layer-2 segment and introduces broadcast, neighbor-discovery, and operational complexity. The community IPv6 guidance presents routed TUN as the normal approach, where explicit prefixes and routes are easier to audit.
What does tls-crypt protect?
It uses the static TLS key to protect and authenticate the OpenVPN control channel in addition to certificate authentication. It does not replace the CA, server certificate, client certificate, firewall, or IPv6 routing requirements.
Frequently Asked Questions
Can I keep IPv4 transport and still route IPv6 through the tunnel?
Yes. The transport protocol controls how the client reaches OpenVPN; the tunnel’s IPv6 pool and route directives control payload traffic. Use matching udp4 settings for an IPv4 control path, then configure IPv6 addressing, forwarding, and routes separately.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWhat should I do if my hosting provider will not route an extra IPv6 block?
Use a provider-approved private or unique-local pool and an IPv6 SNAT design, or move the server to a provider that delegates a routed prefix. Do not reuse the host’s on-link prefix as a substitute for routing.
How can I tell whether the failure is OpenVPN or the upstream network?
Test in order: service log, client initialization, TUN address, VPN-gateway reachability, installed IPv6 routes, and finally an external IPv6 endpoint. The first failed layer identifies where to troubleshoot.
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