centos 上我们可以直接用telnet进行连接，测试是否可以
如果没有telnet命令，可以使用yum 先安装telnet
yum install telnet
[root@VM_0_14_centos /]# telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
macos 下面使用一下命令
van@van ~$ nc -vz -w 2 94.19.**.218 80
found 0 associations
found 1 connections:
1: flags=82
outif en0
src 192.168.10.51 port 59344
dst 94.191.120.218 port 80
rank info not available
TCP aux info available
Connection to 94.19.**.218 port 80 [tcp/http] succeeded!
van@van ~$ nc -help
usage: nc [-46AacCDdEFhklMnOortUuvz] [-K tc] [-b boundif] [-i interval] [-p source_port] [--apple-delegate-pid pid] [--apple-delegate-uuid uuid]
[-s source_ip_address] [-w timeout] [-X proxy_version]
[-x proxy_address[:port]] [hostname] [port[s]]
Command Summary:
-4 Use IPv4
-6 Use IPv6
-A Set SO_RECV_ANYIF on socket
-a Set SO_AWDL_UNRESTRICTED on socket
-b ifbound Bind socket to interface
-c Send CRLF as line-ending
-C Don't use cellular connection
-D Enable the debug socket option
-d Detach from stdin
-E Don't use expensive interfaces
-F Do not use flow advisory (flow adv enabled by default)
-G conntimo Connection timeout in seconds
-h This help text
-H keepidle Initial idle timeout in seconds
-I keepintvl Interval for repeating idle timeouts in seconds
-i secs Delay interval for lines sent, ports scanned
-J keepcnt Number of times to repeat idle timeout
-k Keep inbound sockets open for multiple connects
-K tclass Specify traffic class
-l Listen mode, for inbound connects
-L num_probes Number of probes to send before generating a read timeout event
-m Set SO_INTCOPROC_ALLOW on socket
-n Suppress name/port resolutions
-M Use MULTIPATH domain socket
-N num_probes Number of probes to send before generating a write timeout event
-O Use old-style connect instead of connectx
-p port Specify local port for remote connects (cannot use with -l)
-r Randomize remote ports
-s addr Local source address
-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]
-o Issue socket options after connect/bind
--apple-delegate-pid pid Set socket as delegate using pid
Port numbers can be individual or ranges: lo-hi [inclusive]
