Telnet has been removed from Mac OS High Sierra, but can still be useful for developers to check ports are open etc.
If you need to enable it, you can do so with Homebrew:
brew tap theeternalsw0rd/telnet brew install telnet
Update (10/10/17): I received an email from David Waitzman in which he notes you can get telnet from an old Time Machine backup and then copy it into /usr/local/bin – I’d recommend that options too if you have a backup.
Update (25/11/17): I received a second email from Michael Bazik which details how you can install the telnet client directly from GNU provided tarballs:
curl http://ftp.gnu.org/gnu/inetutils/inetutils-1.9.4.tar.gz -o inetutils-1.9.4.tar.gz tar xvzf inetutils-1.9.4.tar.gz cd inetutils-1.9.4 ./configure make sudo make install
Update (05/01/18): Jonny Bradley emailed in to address an issue with installing non homebrew telnet. If you encounter the error:
ping: Lacking privilege for raw socket.
then you’ll need to chmod ping to allow it permission to run with:
sudo chmod a+s /usr/local/bin/ping
Update (08/02/18): Matthew Ward got in touch to say that you can compile telnet without all of the other gunk that still comes with Mac OS, using the following:
# out of the "inet-utils" bundle from GNU, the only utilities it provides # that Mac doesn't "natively" are: # # ftp the ftp client # telnet the telnet client # # you don't need to build the whole set. use the following "configure" # command line: ./configure --disable-servers --disable-dnsdomainname \ --disable-hostname --disable-ping --disable-ping6 --disable-rcp \ --disable-rexec --disable-rlogin --disable-rsh --disable-logger \ --disable-talk --disable-tftp --disable-whois --disable-ifconfig \ --disable-traceroute