使用nc命令可以探测目标主机的端口。但是在Centos 6 和 CentOS 7中这个命令的使用有所不同，甚至可以说功能已经不同，下面分别是CentOS 6 和 CentOS 7中对nc命令的解释：

CentOS 6：nc - arbitrary TCP and UDP connections and listens

CentOS 7：ncat - Concatenate and redirect sockets

CentOS 6 中nc命令的使用

# nc -h

usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]

[-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]

[-x proxy_address[:port]] [hostname] [port[s]]

Command Summary:

-4 Use IPv4

-6 Use IPv6

-D Enable the debug socket option

-d Detach from stdin

-h This help text

-i secs Delay interval for lines sent, ports scanned

-k Keep inbound sockets open for multiple connects

-l Listen mode, for inbound connects

-n Suppress name/port resolutions

-p port Specify local port for remote connects

-r Randomize remote ports

-S Enable the TCP MD5 signature option

-s addr Local source address

-T ToS Set IP Type of Service

-C Send CRLF as line-ending

-t Answer TELNET negotiation

-U Use UNIX domain socket

-u UDP mode

-v Verbose

-w secs Timeout for connects and final net reads

-X proto Proxy protocol: "4", "5" (SOCKS) or "connect"

-x addr[:port] Specify proxy address and port

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

Port numbers can be individual or ranges: lo-hi [inclusive]

参数说明：

-h 在线帮助。
-i<延迟秒数> 设置时间间隔，以便传送信息及扫描通信端口。
-l 使用监听模式，管控传入的资料。
-n 直接使用IP地址，而不通过域名服务器。
-o<输出文件> 指定文件名称，把往来传输的数据以16进制字码倾倒成该文件保存。
-p<通信端口> 设置本地主机使用的通信端口。
-r 随机指定本地与远端主机的通信端口。
-s<来源位址> 设置本地主机送出数据包的IP地址。
-u 使用UDP传输协议。
-v 显示指令执行过程。
-w<超时秒数> 设置等待连线的时间。
-z 使用0输入/输出模式，只在扫描通信端口时使用。

示例

1.扫描TCP端口，端口范围是 20-2000

# nc -rz -w 2 10.252.32.122 20-2000

Connection to 10.252.32.122 111 port [tcp/sunrpc] succeeded!

Connection to 10.252.32.122 22 port [tcp/ssh] succeeded!

2.扫描UDP端口，端口范围是 20-2000

1	# nc -z -u -w 2 10.252.32.122 20-2000

3.扫描指定端口，扫描 111端口

1 2	# nc -nvv -w 2 10.252.32.122 111 Connection to 10.252.32.122 111 port [tcp/*] succeeded!

CentOS 7 中nc命令的使用

# nc -h

Ncat 6.40 ( http://nmap.org/ncat )

Usage: ncat [options] [hostname] [port]

Options taking a time assume seconds. Append 'ms' for milliseconds,

's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).

-4 Use IPv4 only

-6 Use IPv6 only

-U, --unixsock Use Unix domain sockets only

-C, --crlf Use CRLF for EOL sequence

-c, --sh-exec <command> Executes the given command via /bin/sh

-e, --exec <command> Executes the given command

--lua-exec <filename> Executes the given Lua script

-g hop1[,hop2,...] Loose source routing hop points (8 max)

-G <n> Loose source routing hop pointer (4, 8, 12, ...)

-m, --max-conns <n> Maximum <n> simultaneous connections

-h, --help Display this help screen

-d, --delay <time> Wait between read/writes

-o, --output <filename> Dump session data to a file

-x, --hex-dump <filename> Dump session data as hex to a file

-i, --idle-timeout <time> Idle read/write timeout

-p, --source-port port Specify source port to use

-s, --source addr Specify source address to use (doesn't affect -l)

-l, --listen Bind and listen for incoming connections

-k, --keep-open Accept multiple connections in listen mode

-n, --nodns Do not resolve hostnames via DNS

-t, --telnet Answer Telnet negotiations

-u, --udp Use UDP instead of default TCP

--sctp Use SCTP instead of default TCP

-v, --verbose Set verbosity level (can be used several times)

-w, --wait <time> Connect timeout

--append-output Append rather than clobber specified output files

--send-only Only send data, ignoring received; quit on EOF

--recv-only Only receive data, never send anything

--allow Allow only given hosts to connect to Ncat

--allowfile A file of hosts allowed to connect to Ncat

--deny Deny given hosts from connecting to Ncat

--denyfile A file of hosts denied from connecting to Ncat

--broker Enable Ncat's connection brokering mode

--chat Start a simple Ncat chat server

--proxy <addr[:port]> Specify address of host to proxy through

--proxy-type <type> Specify proxy type ("http" or "socks4")

--proxy-auth <auth> Authenticate with HTTP or SOCKS proxy server

--ssl Connect or listen with SSL

--ssl-cert Specify SSL certificate file (PEM) for listening

--ssl-key Specify SSL private key (PEM) for listening

--ssl-verify Verify trust and domain name of certificates

--ssl-trustfile PEM file containing trusted SSL certificates

--version Display Ncat's version information and exit

See the ncat(1) manpage for full options, descriptions and usage examples

可以看到centos 6中nc 原有的 -r,-z选项在centos 7中都已经没有了，那么如何测试端口连通性呢？可以使用如下格式来测试端口的连通性

1	nc -w 1 IP地址端口 < /dev/null && echo "tcp port ok"

示例

tcp端口测试

# nc 10.252.35.37 111 < /dev/null && echo "tcp port ok"

tcp port ok

# nc 10.252.35.37 21 < /dev/null && echo "tcp port ok"

Ncat: Connection refused.

参考：

https://www.cnblogs.com/panzhicheng/p/5704561.html

https://www.cnblogs.com/richerdyoung/p/8336893.html

CentOS 使用nc命令进行端口扫描

CentOS 6 中nc命令的使用

CentOS 7 中nc命令的使用

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