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
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
- 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