Configure IPv6 on Ubuntu Server: Netplan Static Address, Gateway, DNS and Troubleshooting

IPv6 is the current Internet Protocol designed to provide a much larger address space than IPv4.

An IPv6 address is:

128 bits long

and is normally written as eight groups of hexadecimal values separated by colons.

Example:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Leading zeros inside each 16-bit group can be removed:

2001:db8:85a3:0:0:8a2e:370:7334

One continuous sequence of zero groups can also be compressed using :::

2001:db8:85a3::8a2e:370:7334

The :: abbreviation can appear only once in an IPv6 address.

Modern Ubuntu Server releases use Netplan to define network configuration.

This guide explains how to configure:

• Static IPv6 addressing
• IPv6 default route
• IPv6 DNS servers
• SLAAC and DHCPv6
• IPv6 connectivity
• Netplan safely on remote servers
• Common IPv6 configuration problems

Understand the IPv6 Configuration Before Making Changes

Before modifying Ubuntu networking, collect the required information.

For a static IPv6 server, you normally need:

• Interface name
• IPv6 address
• Prefix length
• Default router
• DNS servers
• Whether Router Advertisements should be accepted
• Whether DHCPv6 is being used

Example Network

This article uses the following documentation-only example:

Interface: ens18
IPv6 Address: 2001:db8:100:10::20/64
IPv6 Gateway: 2001:db8:100:10::1
Prefix Length: /64

The prefix:

2001:db8::/32

is reserved for documentation and examples.

Do not use it as a real production Internet prefix.

Replace the example values with addresses provided by your:

• ISP
• Hosting provider
• Cloud provider
• Network administrator

Common IPv6 Address Types

Global Unicast

Generally used for globally routable IPv6 communication.

Example documentation address:

2001:db8:100:10::20

Link-Local

IPv6 interfaces normally have a link-local address beginning with:

fe80::/10

Link-local addresses are used for communication on the local link and are important for mechanisms such as Neighbor Discovery.

Loopback

::1

This is the IPv6 equivalent of IPv4:

127.0.0.1

Prefix Length

A normal IPv6 LAN commonly uses:

/64

For example:

2001:db8:100:10::20/64

The prefix identifies the network portion associated with the address.

Do not randomly change the prefix length.

Use the prefix assigned by the network administrator or provider.

Identify the Network Interface

Use:

ip link show

A shorter view is:

ip -br link

Example:

lo               UNKNOWN
ens18            UP

In this example, the network interface is:

ens18

Interface names may instead look like:

• enp0s3
• ens33
• eno1
• eth0

Always use the actual interface name from your server.

Check Existing IPv6 Addresses

ip -6 addr show

For one interface:

ip -6 addr show dev ens18

You may already see a link-local address similar to:

inet6 fe80::20c:29ff:fe12:3456/64 scope link

Check Existing IPv6 Routes

ip -6 route show

Look for a default route similar to:

default via 2001:db8:100:10::1 dev ens18

Configure a Static IPv6 Address with Netplan

Modern Ubuntu Server systems normally store Netplan YAML files under:

/etc/netplan/

Step 1 — Find the Existing Netplan File

ls -l /etc/netplan/

Possible filenames include:

00-installer-config.yaml
01-netcfg.yaml
50-cloud-init.yaml
99_config.yaml

Do not assume the filename.

Inspect the actual files on the server.

Check the Current Netplan Configuration

sudo netplan get

On newer Netplan versions, you can also inspect runtime status:

sudo netplan status --all

Cloud-Init Warning

Cloud and virtual-machine installations may contain:

50-cloud-init.yaml

Inspect it:

sudo cat /etc/netplan/50-cloud-init.yaml

Some cloud-init-generated files explicitly warn that direct changes may not persist after reboot.

If the file is controlled by cloud-init, manage networking using the appropriate:

• Cloud platform
• Cloud-init configuration
• Provisioning system

or deliberately disable cloud-init network management before taking permanent manual control.

Step 2 — Back Up the Configuration

Example:

sudo cp /etc/netplan/00-installer-config.yaml /etc/netplan/00-installer-config.yaml.backup

Replace the filename with the actual file used by your server.

Step 3 — Configure the Static IPv6 Address

A complete IPv6-only static example is:

network:
  version: 2
  renderer: networkd

  ethernets:
    ens18:
      dhcp4: false
      dhcp6: false
      accept-ra: false

      addresses:
        - "2001:db8:100:10::20/64"

      routes:
        - to: default
          via: "2001:db8:100:10::1"

      nameservers:
        addresses:
          - "2001:4860:4860::8888"
          - "2001:4860:4860::8844"

Replace:

• ens18
• IPv6 address
• Prefix
• Gateway
• DNS servers

with values appropriate to your environment.

Understanding the Configuration

version: 2

Specifies the Netplan configuration format.

renderer: networkd

Uses systemd-networkd as the network backend.

This is common on Ubuntu Server.

dhcp4: false

Disables IPv4 DHCP for this example.

If the server also requires IPv4 DHCP, configure it according to the actual dual-stack environment.

dhcp6: false

Disables DHCPv6 because the server is using manually configured IPv6 information.

accept-ra: false

Prevents Router Advertisements from automatically adding IPv6 configuration.

This is appropriate in this example because the address and default route are explicitly configured.

Do not automatically disable RA if your network expects Router Advertisements to provide routing or SLAAC information.

addresses

Defines the server’s static IPv6 address.

routes

Creates the IPv6 default route.

Modern Netplan configuration should use:

routes:
  - to: default
    via: "2001:db8:100:10::1"

rather than the older:

gateway6:

which is deprecated in current Netplan documentation.

nameservers

Defines DNS resolvers.

The example uses Google’s public IPv6 DNS servers.

Enterprise servers should normally use the organization’s approved DNS resolvers.

Static Dual-Stack Example

If the server requires both IPv4 and IPv6:

network:
  version: 2
  renderer: networkd

  ethernets:
    ens18:
      dhcp4: false
      dhcp6: false
      accept-ra: false

      addresses:
        - "192.0.2.20/24"
        - "2001:db8:100:10::20/64"

      routes:
        - to: default
          via: "192.0.2.1"

        - to: default
          via: "2001:db8:100:10::1"

      nameservers:
        addresses:
          - "192.0.2.53"
          - "2001:4860:4860::8888"

Again, the documentation addresses must be replaced with real network values.

Step 4 — Validate the YAML

Before applying it:

sudo netplan generate

If there is a YAML or configuration error, correct it first.

YAML indentation matters.

Use spaces consistently.

Do not use tabs.

Step 5 — Test the Configuration Safely

For a remote server, use:

sudo netplan try

Netplan temporarily applies the configuration and waits for confirmation.

If you lose connectivity and do not confirm the configuration, Netplan is designed to roll it back after the timeout.

Step 6 — Apply the Configuration

If testing is successful:

sudo netplan apply

If netplan try was confirmed successfully, a separate netplan apply may not be necessary.

Configure SLAAC or DHCPv6 Instead of a Static Address

Not every Ubuntu server requires a manually configured IPv6 address.

IPv6 networks may use:

• SLAAC
• DHCPv6
• SLAAC with DHCPv6-provided options
• Static addressing

Router Advertisements

IPv6 hosts commonly learn important network information from Router Advertisements.

RA can provide information related to:

• Prefixes
• Default router
• SLAAC
• Whether DHCPv6 should be used

This is different from traditional IPv4 DHCP behavior.

DHCPv6 and Router Advertisements

DHCPv6 can provide:

• IPv6 address information
• DNS information
• Other configuration options

depending on the network design.

However:

DHCPv6 normally does not provide the IPv6 default router.

The default router is learned through Router Advertisements.

Dynamic IPv6 Netplan Example

A typical dynamically managed IPv6 server configuration may look like:

network:
  version: 2
  renderer: networkd

  ethernets:
    ens18:
      dhcp4: false
      dhcp6: true
      accept-ra: true

With this model, Netplan/systemd-networkd can honor Router Advertisements and use DHCPv6 when the network signals that it should.

The exact resulting behavior depends on:

• Router Advertisement flags
• DHCPv6 infrastructure
• DNS configuration
• Netplan version
• Network backend

Static Address with Router Advertisement Routing

Some environments manually configure a server’s IPv6 address but still learn the default router through Router Advertisements.

In that design, do not automatically configure:

accept-ra: false

The correct setting depends on the network architecture.

Check Whether an Address Is Dynamic

ip -6 addr show dev ens18

A dynamically generated address may show lifetime information rather than:

valid_lft forever preferred_lft forever

Temporary and Privacy Addresses

Netplan also supports IPv6 privacy addressing in appropriate environments.

However, servers usually benefit from predictable addresses for:

• DNS
• Monitoring
• Firewall policy
• Administration

Privacy addresses are more commonly useful on client endpoints than on infrastructure servers.

Verify IPv6 Addressing, Routing and DNS

After configuration, verify each component separately.

Check the Interface

ip -6 addr show dev ens18

Expected example:

inet6 2001:db8:100:10::20/64 scope global
   valid_lft forever preferred_lft forever

inet6 fe80::20c:29ff:fe12:3456/64 scope link
   valid_lft forever preferred_lft forever

Understand the Two Addresses

Global Address

2001:db8:100:10::20/64

Used for communication beyond the local link when routing permits it.

Link-Local Address

fe80::20c:29ff:fe12:3456/64

Used only on the local link.

The actual link-local interface identifier may differ from this example.

Check the Default Route

ip -6 route show

Look for:

default via 2001:db8:100:10::1 dev ens18

Test the Local Gateway

ping -6 -c 4 2001:db8:100:10::1

Replace the example address with the real router.

Test External IPv6 Connectivity

For example:

ping -6 -c 4 2606:4700:4700::1111

If the local gateway responds but an external IPv6 address does not, investigate:

• Upstream routing
• Default route
• Firewall policy
• Provider IPv6 connectivity

Test DNS Resolution

getent ahosts example.com

Or:

resolvectl query example.com

Then test IPv6 by hostname:

ping -6 -c 4 example.com

This tests more than simple IP connectivity because DNS resolution is also required.

Check DNS Configuration

resolvectl status

Review the DNS servers associated with the network interface.

Check Netplan Runtime State

On supported Netplan releases:

sudo netplan status --all

Verify the Selected Route

To determine how Linux intends to reach a particular IPv6 destination:

ip -6 route get 2606:4700:4700::1111

Example output may identify:

• Outgoing interface
• Gateway
• Selected source IPv6 address

This is extremely useful when a server has multiple IPv6 addresses or interfaces.

Troubleshoot and Secure the IPv6 Configuration

IPv6 configuration problems are often caused by a small number of issues.

Problem — Netplan YAML Error

Validate:

sudo netplan generate

Common causes include:

• Incorrect indentation
• Tabs instead of spaces
• Wrong interface name
• Incorrect YAML hierarchy

Problem — Correct Address but No Default Route

Check:

ip -6 route show

If no default route exists, review:

routes:
  - to: default
    via: "YOUR_IPV6_GATEWAY"

Do not add a random gateway.

It must be reachable according to the actual network design.

Problem — Gateway Cannot Be Reached

Check the neighbor table:

ip -6 neigh show

Look for states such as:

• REACHABLE
• STALE
• DELAY
• PROBE
• FAILED
• INCOMPLETE

FAILED or INCOMPLETE entries can indicate a local Layer 2 or Neighbor Discovery problem.

Problem — IPv6 Works by Address but Not by Name

If:

ping -6 2606:4700:4700::1111

works but hostname resolution fails, investigate DNS rather than IPv6 routing.

Check:

resolvectl status

Problem — Configuration Disappears After Reboot

Inspect:

ls -l /etc/netplan/

and:

sudo cat /etc/netplan/50-cloud-init.yaml

If cloud-init owns the network configuration, direct manual changes to its generated file may not be persistent.

Problem — Unexpected Extra IPv6 Address

Check whether:

accept-ra: true

or DHCPv6/SLAAC is enabled.

Linux can legitimately have multiple IPv6 addresses on one interface.

A server might have:

• Static global address
• SLAAC address
• Temporary address
• Link-local address

Determine which mechanisms are enabled before deleting addresses.

Problem — Remote SSH Disconnected After Netplan Change

Use console access if available.

Restore the backup configuration and validate it before reapplying.

This is why:

sudo netplan try

is preferable for remote changes.

Check IPv6 Firewall Policy

Configuring an IPv6 address can make services reachable through an additional protocol family.

Check listening services:

sudo ss -lntup

Review the host firewall:

sudo ufw status verbose

Do not assume an IPv4 firewall rule automatically provides the intended IPv6 security policy.

Do Not Block ICMPv6 Completely

IPv6 depends on ICMPv6 for important functionality including:

• Neighbor Discovery
• Router Discovery
• Path MTU Discovery
• Error reporting

A blanket ICMPv6 block can create connectivity problems.

Use appropriate IPv6-aware firewall rules.

Key Takeaways and Summary

Key Takeaways

1. IPv6 addresses are 128-bit identifiers represented using hexadecimal notation.
2. Modern Ubuntu Server networking is normally configured through Netplan YAML files under /etc/netplan/.
3. Always identify the actual network interface before editing the configuration.
4. Static IPv6 configuration normally requires an address, prefix, default route, and DNS information.
5. Current Netplan configurations should use routes: for the IPv6 default route instead of deprecated gateway6.
6. Use netplan generate to validate the configuration before applying it.
7. Use netplan try when changing networking remotely because it provides a rollback mechanism.
8. DHCPv6 and Router Advertisements perform different IPv6 configuration functions.
9. Verify the IPv6 address, route, gateway reachability, DNS, and selected source route separately.
10. After enabling IPv6, review firewall rules and listening services because the server may now be reachable through an additional network protocol.

Summary

Configuring IPv6 on Ubuntu Server is straightforward once the required network information is known.

For a static server, the basic process is:

Identify interface → Collect IPv6 information → Back up Netplan → Configure address → Configure default route → Configure DNS → Validate → Test safely → Verify connectivity

A current static Netplan configuration typically uses:

addresses:
  - "2001:db8:100:10::20/64"

routes:
  - to: default
    via: "2001:db8:100:10::1"

After applying the configuration, verify:

ip -6 addr show

ip -6 route show

resolvectl status

and test IPv6 connectivity using:

ping -6

Most importantly, treat networking changes carefully on remotely administered servers.

Validate the YAML and use:

sudo netplan try

before permanently committing potentially disruptive network changes.

IPv6 configuration is not complete until:

• Addressing works
• Routing works
• DNS works
• Firewall policy is correct
• Monitoring covers IPv6

That provides a reliable and secure foundation for operating Ubuntu Server in an IPv6 or dual-stack environment.

Useful Links and References

SanchitGurukul Static Resources

Official External References