# ifupdown
**ifupdown** is the traditional network manager on [[gnu-linux|GNU/Linux]] systems. It has since been replaced by [[systemd-networkd|systemd-networkd]]. 

It was especially commmon in Debian systems before Debian 12 (Bookworm).

The "if" part is short for "interface". "up" and "down" refer to `ifup` and `ifdown` commands, which are used to bring an interface link up and down + run some configuration scripts.


## Install
On Debian 10 and Debian 11:
```
sudo apt install ifupdown
```
Don't install ifupdown if you're using a different network manager, such as systemd-networkd!

## Manual

 - `man ifup`
 - `man ifdown`
 - `man ifquery`
 - `man interfaces`

## /etc/network/interfaces
All configuration lies in `/etc/network/` directory. The following is a list of files in that directory
```
 $ ls /etc/network
if-down.d  if-post-down.d  if-pre-up.d  if-up.d  interfaces  interfaces.d
```

The main configuration file is `/etc/network/interfaces`. The `*.d` directories contain auxiliary configuration and scripts.

The following is the minimal possible configuration. It only configures the loopback interface. Configuring the loopback interface gives you addresses `127.0.0.1` (IPv4) and `::1` (IPv6). These are home addresses so that you can ping yourself `ping 127.0.0.1` (hence why it's called a "loopback"). Lines that begin with '#' are comments. You can comment out or change any parts of the configuration file.
```bash
# /etc/network/interfaces

# Loopback
auto lo
iface lo inet loopback

# ... place your other config here ...
```

## Configuration examples

To configure a loopback device (the only device that's technically necessary)
```bash
# Loopback
auto lo
iface lo inet loopback
```

To configure an `eth0` interface dynamically (IPv4):
```bash
# IPv4 dynamic (DHCPv4)
allow-hotplug eth0
iface eth0 inet dhcp
```

To configure an `eth0` interface statically (IPv4):
```bash
# IPv4 static
auto eth0
iface eth0 inet static
    address 192.168.1.50
    netmask 255.255.255.0
    gateway 192.168.1.1
```

To configure an `eth0` interface dynamically (IPv6):  
The interface acquires IPv6 through SLAAC via Router Advertisements (RA).  
No DHCPv6 required.
```bash
# IPv6 dynamic
auto eth0
iface eth0 inet6 auto
    accept_ra
```

To configure an `eth0` interface statically (IPv6):
```bash
# IPv6 static
auto eth0
iface eth0 inet6 static
    address 2001:db8:1::50
    netmask 64
    gateway 2001:db8:1::1
```

To configure an `eth0` interface statically (IPv4 + IPv6): 
```bash
# IPv4 static block
iface eth0 inet static
    address 192.168.1.50
    netmask 255.255.255.0
    gateway 192.168.1.1

# IPv6 static block
iface eth0 inet6 static
    address 2001:db8:1::50
    netmask 64
    gateway 2001:db8:1::1
```

Full configuration (loopback + IPv4 + IPv6)
```bash
# /etc/network/interface

# Loopback
auto lo
iface lo inet loopback

# IPv4 static block
iface eth0 inet static
    address 192.168.1.50
    netmask 255.255.255.0
    gateway 192.168.1.1

# IPv6 static block
iface eth0 inet6 static
    address 2001:db8:1::50
    netmask 64
    gateway 2001:db8:1::1
```

## Commands (ifup ifdown ifquery)
There are three commands you should know about

 - `ifup` - Reads `/etc/network/interface` and brings an interface up
 - `ifdown` - Reads `/etc/network/interface` and brings an interface down
 - `ifquery` - Reads `/etc/network/interface` and tells you some information about the interface

Bringing an `eth0` interface up:
```
 # ifup eth0
```

Bringing all interfaces up
```
 # ifup -a
```

Bringing `eth0` interface down
```
 # if down eth0
```

These examples assume your network interface is named `eth0` by [[udev|udev]].

## Relation to ifconfig and route
The commands `ifconfig` and `route` seem related to `ifup`, `ifdown` and `ifquery`. However the former two are rather low-level commands that modify the kernel state. The latter three are high-level commands that sit on top of the former two.

The `ifconfig` and `route` commands are themselves considered legacy and replaced with [[ip-command|ip]] command (provided by iproute2 package) in a similar way systemd-networkd replaces ifupdown.

## Links

 - https://manpages.debian.org/unstable/ifupdown/
 - https://manpages.debian.org/unstable/ifupdown/interfaces.5.en.html
 - https://manpages.debian.org/unstable/ifupdown/ifquery.8.en.html
 - https://manpages.debian.org/unstable/ifupdown/ifdown.8.en.html
 - https://manpages.debian.org/unstable/ifupdown/ifup.8.en.html