Troubleshoot IPv6 Problems
Last modified: July 13, 2022
Overview
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.
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 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
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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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:
- CentOS-virt mailing list’s Issues with routing IPv6 to KVM Guests thread
- Patchwork’s Patchwork bridge is not forwarding ICMP6 neighbor solicitation to KVM guest post
- Launchpad’s Multicast snooping on bridge breaks ICMP6 neighbor solicitation for KVM guests post
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.
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