What is `nc -z` used for?

Question

In a solution of an exercise I found this:

nc -z [serverip] [port]

What does it do?

On nc man page I found

-z zero-I/O mode [used for scanning]

not very explanatory... Searching on the web I found the Netcat Cheat Sheet which says:

-z: Zero-I/O mode (Don’t send any data, just emit a packet without payload)

So, why should I send a packet without anything? It's like a ping?

Answer 1

It can be more useful to think of -z option as meaning "immediately close the connection". My version of nc has this to say about port scanning:

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. Usually it's useful to turn on
 verbose output to stderr by use this option in conjunction with -v option.

 For example:

       $ nc -zv 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, and is scanned by increasing order (un‐
 less the -r flag is set).

 You can also specify a list of ports to scan, for example:

       $ nc -zv host.example.com http 20 22-23
       nc: connect to host.example.com 80 (tcp) failed: Connection refused
       nc: connect to host.example.com 20 (tcp) failed: Connection refused
       Connection to host.example.com port [tcp/ssh] succeeded!
       nc: connect to host.example.com 23 (tcp) failed: Connection refused

 The ports are scanned by the order you given (unless the -r flag is set).

 Alternatively, it might be useful to know which server software is running, and which versions.  This in‐
 formation 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

You can use tcpdump to see what nc sends with and without -z.

Without -z:

carbon# nc -v localhost 25
Connection to localhost 25 port [tcp/smtp] succeeded!
220 carbon.home ESMTP Postfix (Ubuntu)

tcpdump -i lo port 25:

15:59:07.956294 IP6 localhost.41584 > localhost.smtp: Flags [S], seq 717573315, win 65476, options [mss 65476,sackOK,TS val 4044858638 ecr 0,nop,wscale 7], length 0
15:59:07.956309 IP6 localhost.smtp > localhost.41584: Flags [S.], seq 3478976646, ack 717573316, win 65464, options [mss 65476,sackOK,TS val 4044858638 ecr 4044858638,nop,wscale 7], length 0
15:59:07.956320 IP6 localhost.41584 > localhost.smtp: Flags [.], ack 1, win 512, options [nop,nop,TS val 4044858638 ecr 4044858638], length 0
15:59:07.956536 IP6 localhost.smtp > localhost.41584: Flags [P.], seq 1:41, ack 1, win 512, options [nop,nop,TS val 4044858639 ecr 4044858638], length 40: SMTP: 220 carbon.home ESMTP Postfix (Ubuntu)
15:59:07.956548 IP6 localhost.41584 > localhost.smtp: Flags [.], ack 41, win 512, options [nop,nop,TS val 4044858639 ecr 4044858639], length 0
15:59:14.917615 IP6 localhost.41584 > localhost.smtp: Flags [F.], seq 1, ack 41, win 512, options [nop,nop,TS val 4044865599 ecr 4044858639], length 0
15:59:14.917754 IP6 localhost.smtp > localhost.41584: Flags [F.], seq 41, ack 2, win 512, options [nop,nop,TS val 4044865600 ecr 4044865599], length 0
15:59:14.917773 IP6 localhost.41584 > localhost.smtp: Flags [.], ack 42, win 512, options [nop,nop,TS val 4044865600 ecr 4044865600], length 0

With -z:

carbon# nc -zv localhost 25
Connection to localhost 25 port [tcp/smtp] succeeded!

tcpdump:

15:59:22.394593 IP6 localhost.41592 > localhost.smtp: Flags [S], seq 449578009, win 65476, options [mss 65476,sackOK,TS val 4044873076 ecr 0,nop,wscale 7], length 0
15:59:22.394605 IP6 localhost.smtp > localhost.41592: Flags [S.], seq 3916701833, ack 449578010, win 65464, options [mss 65476,sackOK,TS val 4044873076 ecr 4044873076,nop,wscale 7], length 0
15:59:22.394615 IP6 localhost.41592 > localhost.smtp: Flags [.], ack 1, win 512, options [nop,nop,TS val 4044873076 ecr 4044873076], length 0
15:59:22.394683 IP6 localhost.41592 > localhost.smtp: Flags [F.], seq 1, ack 1, win 512, options [nop,nop,TS val 4044873076 ecr 4044873076], length 0
15:59:22.394828 IP6 localhost.smtp > localhost.41592: Flags [P.], seq 1:41, ack 2, win 512, options [nop,nop,TS val 4044873077 ecr 4044873076], length 40: SMTP: 220 carbon.home ESMTP Postfix (Ubuntu)
15:59:22.394840 IP6 localhost.41592 > localhost.smtp: Flags [R], seq 449578011, win 0, length 0

You can see that the server still sent the greeting (220 carbon.home ESMTP Postfix (Ubuntu)) but nc did not print it (and presumably did not read it).