networking - 如何检查到端口的网络连接是否未打开，或者该端口没有监听？

Question

我总是使用 telnet [IP] [port] 检查网络连接。但是，有时连接会超时，因为没有任何东西在监听该端口，但到该端口的隧道是打开的。我如何检查差异，即 telnet 超时是因为隧道未打开还是因为没有东西在另一端监听该端口？

Answer 1

Probably by using tcptraceroute, from the man page

   This program attempts to trace the route an IP packet would  follow  to
   some internet host by launching probe packets with a small ttl (time to
   live) then listening for an ICMP "time exceeded" reply from a  gateway.
   We  start our probes with a ttl of one and increase by one until we get
   an ICMP "port unreachable" (or TCP reset)

set the max_ttl value to be appropriate for your firewall