Using bridges under NixOS

This article has been nominated for deletion.

Reason: The related bug has been fixed and the contents of this page no longer apply to a modern NixOS, which uses systemd-networkd. (Discuss in Talk:Using bridges under NixOS#)
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In order to use macvtap bridges in KVM, LXC, Xen or another hypervisor on NixOS, you have to configure your DHCP server to not assign an IP address to that interface.

By default, the DHCP server on NixOS configures every interface with an IP.

Configuration

If you are running dhcpcd, then you have to define a denyInterfaces parameter:

configuration.nix

{
  networking.dhcpcd.denyInterfaces = [ "macvtap0@*" ];
}

Tip

In order to affect your NixOS system by your nix-language-specific changes you must first evaluate it:

$ nixos-rebuild switch --use-remote-sudo

Issue #67966 is relevant for DHCP and bridging.