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“nc -z”有什么用？

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“nc -z”有什么用？

在一个练习的解决方案中我发现了这个：

nc -z [serverip] [port]

它有什么作用？

在nc手册页上我发现

-z 零 I/O 模式 [用于扫描]

不太解释...在网上搜索我发现了Netcat 备忘单其中说：

-z：零I/O模式（不发送任何数据，仅发出没有有效负载的数据包）

那么，为什么我要发送一个没有任何内容的数据包呢？就像ping一样？

答案1

-z将选项视为“立即关闭连接”可能更有用。我的版本nc对端口扫描有这样的说法：

端口扫描

 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

您可以使用tcpdump来查看nc带有和不带有 的发送内容-z。

没有-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

和-z：

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

tcp转储：

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

您可以看到服务器仍然发送了问候语 ( 220 carbon.home ESMTP Postfix (Ubuntu))，但nc没有打印它（并且可能没有读取它）。

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