背景：自己買了臺阿里雲公有云伺服器，準備在上面搭建L2TP ipsec vpn伺服器，vpn服務搭建號後撥號連線失敗。ipsec服務有用到500和4500的UDP埠，xl2tpd服務有用到1701tcp埠。然後就想著是不是埠沒有開放，查看了阿里雲安全策略組，如下圖沒有開放這些埠。

現在使用nc進行測試udp埠是否通常。在伺服器安裝nc服務：yum install -y nc;

a端：nc -ul 500(500為測試的埠)
b端：nc -u x.x.x.x 500(x.x.x.x為測試伺服器地址)

正常情況下在b端輸入字串可以在a端接受到。

以下為nc其他具體用法：
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~]# 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]