linux环境下的nc（ncat的简写）命令用法和udp端口检测

有时我们的服务程序采用udp与客户端进行通讯，但是总是接收不到客户端的请求，或者客户端总是接收不到服务器的响应，这个时候，我们需要确认时网络不通导致的，还是我们的服务程序故障引起的，特别是服务程序对应的网络端口状态是否正常，这个时候，我们就可以借助于linux环境下的nc命令进行辅助分析了。

1. nc命令帮助输出

# nc -help
Ncat 7.50 ( https://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           Connect timeout
  -z                         Zero-I/O mode, report connection status only
      --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" or "socks5")
      --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
      --ssl-ciphers          Cipherlist containing SSL ciphers to use
      --version              Display Ncat's version information and exit
See the ncat(1) manpage for full options, descriptions and usage examples

2. 使用nc命令检测udp端口（正常端口）

# nc -uzv 192.168.0.180 1812       
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.0.180:1812.
Ncat: UDP packet sent successfully
Ncat: 1 bytes sent, 0 bytes received in 2.02 seconds.
 
#下面这样写也是一样的
# nc -u -z -v 192.168.0.180 1812
 

 可以看到这里检测结果是，udp包发送成功，发送了一个字节，没有收到数据。

这里的参数-u 表示udp协议。

-z表示不进行io，只报告连接状态，测试一次连接后，就退出了，不发送数据。

-v表示显示详细信息

3. 使用nc命令检测udp端口（没有开启的端口）

# nc -uzv 192.168.0.180 1813
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.0.180:1813.
Ncat: Connection refused.
 
 
#下面这样写也是一样的
# nc -u -z -v 192.168.0.180 1813

可以看到这里的结果，连接被拒绝。说明端口没有开启或者是网络不通。

4. 开启一个到udp端口的会话，可以持续发送消息

ncat -uv 192.168.0.180 1812
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.0.180:1812.

5. 最简单的方式，只需要指定一个协议参数

# ncat -u 192.168.0.180 1812