Troubleshoot IPv6 Problems

Last modified: July 13, 2022

Overview

Warning:

Due to networking requirements, you cannot run an IPv6-only cPanel & WHM server. You must have at least one IPv4 address.

This documentation lists common IPv6 problems, and steps to resolve them.

Confirm that IPv6 functions on the server

To confirm whether IPv6 functions on your server, use the ping6 command to ping another server, and to ping the IPv6 server.

Note:

If the ping6 command fails, we recommend that you use the traceroute command to begin your investigation. This command displays detailed information about the path that information travels between one server and another, and lists the associated routers.

Ping another server from the IPv6 server

To confirm that your server can reach other servers via IPv6, run the following command as the root user on your IPv6 server:

ping6 ipv6.google.com

If IPv6 functions on your server, this command’s output resembles the following example:

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PING ipv6.google.com(2001:4860:b002::68) 56 data bytes
64 bytes from 2001:4860:b002::68: icmp_seq=0 ttl=59 time=58.4 ms
64 bytes from 2001:4860:b002::68: icmp_seq=1 ttl=59 time=56.4 ms
64 bytes from 2001:4860:b002::68: icmp_seq=2 ttl=59 time=62.1 ms

Important:

If the command’s output does not resemble this example, IPv6 does not function on your server. You must resolve this issue before you can use cPanel & WHM with IPv6.

Ping the IPv6 server from another server

To confirm that other servers can reach your server via IPv6, run the following command as the root user on another server:

ping6 2001:db8:10fe::5000

Note:

In this example, replace 2001:db8:10fe::5000 with your server’s IPv6 address.

If IPv6 functions on your server, this command’s output resembles the following example:

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PING 2001:0db8:85a3::8a2e:0370:7334 56 data bytes
64 bytes from 2001:db8:10fe::5000: icmp_seq=1 ttl=62 time=0.847 ms
64 bytes from 2001:db8:10fe::5000: icmp_seq=2 ttl=62 time=0.994 ms
64 bytes from 2001:db8:10fe::5000: icmp_seq=3 ttl=62 time=0.909 ms

Important:

If the command’s output does not resemble this example, IPv6 does not function on your server. You must resolve this issue before you can use cPanel & WHM with IPv6.

Netmask misconfiguration

IPv6 uses the same netmask as IPv4, but expresses it in CIDR notation. Typically, interfaces display this netmask and the IPv6 address together.

Make certain that you enter the correct netmask for your network when you add IPv6 addresses to your server. For example, IPv6 networks can use a /128 CIDR mask to route single addresses and ranges, but recommended rollout procedures typically use the /64 netmask instead. If you enter an incorrect netmask for your network, your server may not function correctly.

Neighbor Delivery Protocol problems

IPv6 replaces IPv4’s Address Resolution Protocol (ARP) and Internet Control Message Protocol (ICMP) with the Neighbor Discovery Protocol (NDP). NDP uses multicast to solicit responses from the router and other network devices. Some configurations of hardware, VLANs, and software bridges can interfere with NDP and may cause network issues. For example, KVM-based virtualization uses the Linux software bridge, which filters superfluous traffic and may therefore interfere with NDP.

To resolve these issues, upgrade to a newer Linux kernel. If the problems persist, you may need to make additional changes to your configuration for IPv6 to function correctly.

For steps to resolve this issue on a KVM server, read the following posts:

Check the IPv6 addresses on your server

In some cases, you may need to check the assigned IPv6 addresses on your server. To do this, use the following command:

ip -6 addr show

This command displays all of the server’s IPv6 addresses, network devices, and their configurations.

Note:

This command’s output includes local link and non-global addresses. To display global addresses, run the following command: ip -6 addr show scope global