ifconfig is not installed by default in Debian Stretch. However, it is available in package repository, so applications that depends on it can still be used.
Check if the application is not already installed.
$ sudo ifconfig sudo: ifconfig: command not found
This utility is provided by net-tools package.
$ dpkg-query -S /sbin/ifconfig net-tools: /sbin/ifconfig
Install net-tools package.
$ sudo apt-get install net-tools
Check ifconfig output.
$ sudo ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.170 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::5054:ff:fe02:4adc prefixlen 64 scopeid 0x20
ether 52:54:00:02:4a:dc txqueuelen 1000 (Ethernet)
RX packets 2180 bytes 4529382 (4.3 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 1793 bytes 140026 (136.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1 (Local Loopback)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
