Configuring Linux DNS

Configuring DNS (Domain Name System) on Linux can be complex due to the various ways it can be set up.

In this context, Tailscale is a service that aims to work alongside the existing DNS configuration on a Linux system.

However, some configurations may not be fully compatible with Tailscale's cooperative management of the host's DNS settings.

Here's a detailed explanation of the two common problems mentioned:

NetworkManager + systemd-resolved

• NetworkManager is a service that manages network connections on many Linux distributions.

• systemd-resolved is a system service that provides network name resolution to local applications.

• In distributions that use both NetworkManager and systemd-resolved, it's important to ensure that the /etc/resolv.conf file is a symbolic link (symlink) pointing to /run/systemd/resolve/stub-resolv.conf.

• The /etc/resolv.conf file is a configuration file that specifies the DNS servers and search domains used by the system for name resolution.

• By default, /etc/resolv.conf should be a symlink to /run/systemd/resolve/stub-resolv.conf. If it's not, you can create the symlink manually using the command:

sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

• When NetworkManager detects the presence of this symlink, it assumes that systemd-resolved is being used for DNS resolution and avoids overwriting the resolv.conf file.

• After fixing the symlink, it's recommended to restart the relevant services to ensure the changes take effect:

sudo systemctl restart systemd-resolved
sudo systemctl restart NetworkManager
sudo systemctl restart tailscaled

DHCP dhclient overwriting /etc/resolv.conf

• In the absence of a DNS management system, DHCP clients like dhclient and programs like tailscaled have no choice but to directly modify the /etc/resolv.conf file.

• DHCP (Dynamic Host Configuration Protocol) is a network protocol that automatically assigns IP addresses and other network configuration parameters to devices on a network.

• When a DHCP client like dhclient obtains network configuration from a DHCP server, it may overwrite the /etc/resolv.conf file with the DNS servers provided by the DHCP server.

• Similarly, tailscaled, the Tailscale daemon, may also attempt to modify the /etc/resolv.conf file to configure its own DNS settings.

• If both dhclient and tailscaled are modifying the resolv.conf file independently, it can lead to conflicts and loss of functionality. For example, a DHCP renewal may overwrite the resolv.conf file, causing the loss of Tailscale's MagicDNS functionality.

To address these issues, it's recommended to use a centralised DNS management system that can handle the coordination between different components.

By using a compatible DNS management system, such as systemd-resolved or dnsmasq, you can ensure that the DNS configuration is managed consistently and avoid conflicts between different programs modifying the /etc/resolv.conf file directly.

It's important to note that the specific steps and configurations may vary depending on the Linux distribution and the DNS management system being used.

It's always a good idea to refer to the documentation and guidelines provided by your distribution and the respective services to ensure proper configuration and compatibility.