Starting with macOS High Sierra, Apple removed telnet from macOS. nc or netcat can be used for remote connectivity testing.

NC(1)                     BSD General Commands Manual                    NC(1)

NAME
     nc -- arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc [-46AcDCdhklnrtUuvz] [-b boundif] [-i interval] [-p source_port] [-s source_ip_address] [-w timeout] [-X proxy_protocol]
        [-x proxy_address[:port]] [--apple-delegate-pid pid] [--apple-delegate-uuid uuid] [--apple-ext-bk-idle]
        [--apple-nowakefromsleep] [--apple-ecn mode] [hostname] [port[s]]

DESCRIPTION
     The nc (or netcat) utility is used for just about anything under the sun involving TCP or UDP.  It can open TCP connections,
     send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6.  Unlike
     telnet(1), nc scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as
     telnet(1) does with some.

     Common uses include:

           o   simple TCP proxies
           o   shell-script based HTTP clients and servers
           o   network daemon testing
           o   a SOCKS or HTTP ProxyCommand for ssh(1)
           o   and much, much more

     The options are as follows:

     -4      Forces nc to use IPv4 addresses only.

     -6      Forces nc to use IPv6 addresses only.

     -A      Set SO_RECV_ANYIF on socket.

     -b boundif
             Specifies the interface to bind the socket to.

     -c      Send CRLF as line-ending

     -D      Enable debugging on the socket.

     -C      Forces nc not to use cellular data context.

     -d      Do not attempt to read from stdin.

     -h      Prints out nc help.

     -i interval
             Specifies a delay time interval between lines of text sent and received.  Also causes a delay time between connections
             to multiple ports.

     -G conntimeout
             TCP connection timeout in seconds.

     -H keepidle
             Initial TCP keep alive timeout in seconds.

     -I keepintvl
             Interval for repeating TCP keep alive timeouts in seconds.

     -J keepcnt
             Number of times to repeat TCP keep alive packets.

     -k      Forces nc to stay listening for another connection after its current connection is completed.  It is an error to use
             this option without the -l option.

     -l      Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host.
             It is an error to use this option in conjunction with the -p, -s, or -z options.  Additionally, any timeouts specified
             with the -w option are ignored.

     -L num_probes
             Number of probes to send to the peer before declaring that the peer is not reachable and generating an adpative time-
             out read/write event.

     -n      Do not do any DNS or service lookups on any specified addresses, hostnames or ports.

     -p source_port
             Specifies the source port nc should use, subject to privilege restrictions and availability.  It is an error to use
             this option in conjunction with the -l option.

     -r      Specifies that source and/or destination ports should be chosen randomly instead of sequentially within a range or in
             the order that the system assigns them.

     -s source_ip_address
             Specifies the IP of the interface which is used to send the packets.  It is an error to use this option in conjunction
             with the -l option.

     -t      Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.  This makes it possible to use nc
             to script telnet sessions.

     -U      Specifies to use Unix Domain Sockets.

     -u      Use UDP instead of the default option of TCP.

     -v      Have nc give more verbose output.

     -w timeout
             If a connection and stdin are idle for more than timeout seconds, then the connection is silently closed.  The -w flag
             has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag.  The
             default is no timeout.

     -X proxy_version
             Requests that nc should use the specified protocol when talking to the proxy server.  Supported protocols are ``4''
             (SOCKS v.4), ``5'' (SOCKS v.5) and ``connect'' (HTTPS proxy).  If the protocol is not specified, SOCKS version 5 is
             used.

     -x proxy_address[:port]
             Requests that nc should connect to hostname using a proxy at proxy_address and port.  If port is not specified, the
             well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS).

     -z      Specifies that nc should just scan for listening daemons, without sending any data to them.  It is an error to use
             this option in conjunction with the -l option.

     --apple-delegate-pid
             Requests that nc should delegate the socket for the specified PID.  It is an error to use this option in conjunction
             with the --apple-delegate-uuid option.

     --apple-delegate-uuid
             Requests that nc should delegate the socket for the specified UUID.  It is an error to use this option in conjunction
             with the --apple-delegate-pid option.

     --apple-ext-bk-idle
             Requests that nc marks its socket for extended background idle time when the process becomes suspended.

     --apple-nowakefromsleep
             Requests that nc marks its socket to exlude the local port from the list of opened ports that is queried by drivers
             when the system goes to sleep.

     --ecn   Requests that nc marks to use the socket option TCP_ECN_MODE to set the ECN mode (default, enable, disable)

     hostname can be a numerical IP address or a symbolic hostname (unless the -n option is given).  In general, a hostname must be
     specified, unless the -l option is given (in which case the local host is used).

     port[s] can be single integers or ranges.  Ranges are in the form nn-mm.  In general, a destination port must be specified,
     unless the -U option is given (in which case a socket must be specified).

CLIENT/SERVER MODEL
     It is quite simple to build a very basic client/server model using nc.  On one console, start nc listening on a specific port
     for a connection.  For example:

           $ nc -l 1234

     nc is now listening on port 1234 for a connection.  On a second console (or a second machine), connect to the machine and port
     being listened on:

           $ nc 127.0.0.1 1234

     There should now be a connection between the ports.  Anything typed at the second console will be concatenated to the first,
     and vice-versa.  After the connection has been set up, nc does not really care which side is being used as a `server' and
     which side is being used as a `client'.  The connection may be terminated using an EOF (`^D').

DATA TRANSFER
     The example in the previous section can be expanded to build a basic data transfer model.  Any information input into one end
     of the connection will be output to the other end, and input and output can be easily captured in order to emulate file trans-
     fer.

     Start by using nc to listen on a specific port, with output captured into a file:

           $ nc -l 1234> filename.out

     Using a second machine, connect to the listening nc process, feeding it the file which is to be transferred:

           $ nc host.example.com 1234 <filename.in

     After the file has been transferred, the connection will close automatically.

TALKING TO SERVERS
     It is sometimes useful to talk to servers ``by hand'' rather than through a user interface.  It can aid in troubleshooting,
     when it might be necessary to verify what data a server is sending in response to commands issued by the client.  For example,
     to retrieve the home page of a web site:

           $ echo -n "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80

     Note that this also displays the headers sent by the web server.  They can be filtered, using a tool such as sed(1), if neces-
     sary.

     More complicated examples can be built up when the user knows the format of requests required by the server.  As another exam-
     ple, an email may be submitted to an SMTP server using:

           $ nc localhost 25 <<EOF
           HELO host.example.com
           MAIL FROM: <user@host.example.com>
           RCPT TO: <user2@host.example.com>
           DATA
           Body of email.
           .
           QUIT
           EOF

PORT SCANNING
     It may be useful to know which ports are open and running services on a target machine.  The -z flag can be used to tell nc to
     report open ports, rather than initiate a connection.  For example:

           $ nc -z host.example.com 20-30
           Connection to host.example.com 22 port [tcp/ssh] succeeded!
           Connection to host.example.com 25 port [tcp/smtp] succeeded!

     The port range was specified to limit the search to ports 20 - 30.

     Alternatively, it might be useful to know which server software is running, and which versions.  This information is often
     contained within the greeting banners.  In order to retrieve these, it is necessary to first make a connection, and then break
     the connection when the banner has been retrieved.  This can be accomplished by specifying a small timeout with the -w flag,
     or perhaps by issuing a "QUIT" command to the server:

           $ echo "QUIT" | nc host.example.com 20-30
           SSH-1.99-OpenSSH_3.6.1p2
           Protocol mismatch.
           220 host.example.com IMS SMTP Receiver Version 0.84 Ready

EXAMPLES
     Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds:

           $ nc -p 31337 -w 5 host.example.com 42

     Open a UDP connection to port 53 of host.example.com:

           $ nc -u host.example.com 53

     Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection:

           $ nc -s 10.1.2.3 host.example.com 42

     Create and listen on a Unix Domain Socket:

           $ nc -lU /var/tmp/dsocket

     Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080.  This example could also be used by ssh(1);
     see the ProxyCommand directive in ssh_config(5) for more information.

           $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

SEE ALSO
     cat(1), ssh(1)

AUTHORS
     Original implementation by *Hobbit* <hobbit@avian.org>.
     Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.

CAVEATS
     UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.

BSD                              June 25, 2001                             BSD
(END)

To test port 80 on a remote web server:

nc -vz 10.200.2.37 80

Output:

Connection to 10.200.2.37 port 80 [tcp/http] succeeded!