ifup

Enable interface eth0

$ ifup [eth0]

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

ifup [-vnafh] [-i interfaces-file] [--allow=class] [--force] [--no-scripts] [--state=state-file] interface

-a, --all
    Bring up all interfaces marked 'auto'

-v, --verbose
    Print verbose log messages

-n, --noact, --dryrun
    Do not actually configure, just test

-h, --help
    Show help and exit

--force
    Force up even if already up

--no-scripts
    Skip running pre-up/up/post-up scripts

--allow=<class>
    Only allow interfaces in specified class

-i <file>, --interfaces=<file>
    Use alternative interfaces file

--state=<file>
    Use alternative state file

The ifup command brings up (activates) network interfaces defined in configuration files, typically /etc/network/interfaces. It is part of the ifupdown suite, commonly used in Debian-based systems like Ubuntu for managing network settings via scripts.

Upon invocation, ifup reads the interfaces file, parses the stanza for the specified interface (or all if -a is used), and executes hooks: pre-up, up, and post-up scripts. It handles IP assignment (static or DHCP via dhclient/udhcpc), routing, and other settings like MTU or wireless ESSID.

For example, a config might specify:
auto eth0
iface eth0 inet dhcp
Running ifup eth0 activates it. It supports logical interfaces (aliases, VLANs via vconfig), bonding, bridges, and tunnels.

Primarily for servers or scripts; desktop users often prefer NetworkManager. Requires root privileges. Logs to syslog with -v. Dry-run mode (-n) simulates without changes.

Requires root; deprecated in systemd-heavy distros favoring systemd-networkd or NetworkManager. May conflict with other managers. Interfaces file syntax errors halt execution.

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1

Run as root: sudo ifup -a at boot via /etc/rc.local or systemd service.

Developed for Debian's ifupdown package in the 1990s by Anthony Towns and Bernd Eckenfels. Integral to Debian sysadmin tools; ported to other distros. Persists in Ubuntu server editions despite shift to netplan/systemd.

ifdown(8), ifquery(8), ifstatus(8), ip(8), ifconfig(8)