The ifconfig command has been deprecated in favour of the iproute2 (the ip command), therefore missing by default on Debian 9. Similarly, the arp , route and netstat commands are also deprecated. The announcement was made back in 2009.
If you would like to continue using ifconfig you can install it with the following command
1 | sudo apt-get install net-tools |
The new and recommend alternative network tool on Debian Stretch is ip

The command syntax is similar.
- Show network devices and configuration: ip a instead of ifconfig
- Enable an interface: ip link set eth0 up instead of ifconfig eth0 up
- Disable an interface: ip link set eth0 down instead of ifconfig eth0 down
Other examples.
- Display information about all IPv4 network interfaces: ip -4 a
- Display information about all IPv6 network interfaces: ip -6 a
- Display information about a particular interface: ip a show eth0