ifup

Enable interface eth0

$ ifup [eth0]

$ ifup [[-a|--all]]

ifup interface [-v] [-i filename] [--allow type] [--no-act]

interface
    The name of the network interface to bring up (e.g., eth0, wlan0).

-v
    Verbose mode. Displays more detailed information about the actions being performed.

-i filename
    Specifies an alternative interfaces file to use instead of the default /etc/network/interfaces.

--allow type
    Only bring up interfaces of the specified type. This is useful for selectively activating interfaces based on their configuration type (e.g., 'hotplug').

--no-act
    Simulates the command execution without actually making any changes to the network configuration. It's useful for testing configurations.

The ifup command brings a network interface up, configuring it according to the settings specified in the /etc/network/interfaces file (on Debian-based systems) or similar configuration files depending on the distribution. It essentially activates a network interface, assigning it an IP address, netmask, gateway, and other necessary parameters for communication. It parses the configuration file to determine how the interface should be configured, and then uses other commands like ifconfig (deprecated, but still often used) or ip to actually bring the interface online. The command is designed to be idempotent; meaning it only tries to configure parameters that are not already active. The main purpose of ifup is to automate the network configuration process when a system boots or when a network connection needs to be re-established. If ifup fails, it usually means there's an error in the /etc/network/interfaces file or that a required dependency (like dhclient for DHCP) is not installed.

The behavior of ifup is highly dependent on the specific Linux distribution and its network management system. Modern systems often use NetworkManager or systemd-networkd instead of relying solely on ifup. In these cases, ifup might interact with those systems or be less relevant. When using ifup with NetworkManager, unpredictable behavior can result because NetworkManager has its own mechanisms for interface management. Using both simultaneously is generally discouraged.

The primary configuration file used by ifup is typically /etc/network/interfaces. This file contains definitions for each network interface, specifying its IP address, netmask, gateway, DNS servers, and other settings. The file format and options can vary slightly between distributions.

Example snippet from /etc/network/interfaces:
auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1

The ifup command has been a part of Linux network management for a long time, particularly in Debian-based systems and other distributions that traditionally rely on the /etc/network/interfaces file. Its development is intertwined with the evolution of network configuration methods in Linux. Over time, as Linux systems have become more complex and network management tools like NetworkManager have emerged, the role of ifup has slightly diminished in some environments, although it remains a core tool for manual network configuration and scripting, especially on older systems or in environments where more fine-grained control over network interfaces is required.

ifdown(8), ifconfig(8), ip(8), route(8), dhclient(8)