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From my windows 10 machine (172.18.164.50) if I do a telnet 172.20.187.10 443 It shows a blank screen as if it is connected.

It is totally unespected because in theory a firewall is blocking the traffic to port 443.

Looking in Message Analyser (filtering network card with source or destination 172.20.187.10) I see SYN message and Retransmit SYN, with no ACK-SYN reponse.

If I try a telnet 172.20.187.10 666 it shows typical message "Could not open connection to..." In this case it is expected because there is nothing on the server listening on port 666.

Again looking in Message Analyser I see the very same SYN messages (with different port number) with no ACK-SYN.

Why is telnet acting so differently if TCP messages look the same?

Daniele Santi
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user506420
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  • To test please use openssl - openssl s_client -connect 172.18.164.50:443 This will initiate the TLS handshake and tell you if the port is actually accessible & listening (as well as if it's configured properly for HTTPS if it is). – TheFiddlerWins Jan 22 '19 at 15:50
  • https://serverfault.com/questions/157375/reject-vs-drop-when-using-iptables – Sven Jan 22 '19 at 15:52
  • Thanks for the link. From the TCP client side how can I see if a REJECT is receveid? Is the server sending a new message? Which kind of message is, TCP, UDP? – Eduard Jan 23 '19 at 08:32

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