brctl

brctl is a user-space utility for configuring Ethernet bridges in Linux. It is part of the bridge-utils package and provides commands to create bridge devices, attach or detach interfaces, and inspect bridge state. Linux bridging operates at layer 2, forwarding traffic between interfaces that are members of the same bridge, a setup commonly used for virtual machines, containers, and multi-homed hosts.

Common tasks include: brctl addbr BRIDGE to create a bridge; brctl delbr BRIDGE to delete it; brctl addif BRIDGE INTERFACE and brctl delif BRIDGE INTERFACE to manage ports; brctl show to list bridges and ports; brctl showmacs BRIDGE to view learned addresses.

For spanning-tree and port behavior, brctl stp BRIDGE on|off enables or disables Spanning Tree Protocol; brctl setfd BRIDGE seconds, brctl sethello BRIDGE seconds, and brctl setmaxage BRIDGE seconds adjust STP timing. Hairpin mode on a port can be configured with brctl sethairpin INTERFACE on|off.

brctl communicates with the Linux bridge kernel module via netlink and ioctl interfaces to create bridges, add or remove ports, and modify bridge options. In recent years, some distributions have shifted toward iproute2-based tooling for bridge configuration, and brctl is increasingly described as legacy or compatibility software. Nevertheless, brctl remains widely available for compatibility and is still used in many virtualization and legacy environments.

Typical use cases include connecting virtual network interfaces to a host bridge, enabling guests to obtain network access through the bridge, and sharing a physical NIC among multiple virtual guests.