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When a TELNET session is initialised using PuTTY, I see that PuTTY is sending the following characters to the server at the beginning of the connection:

..... .....'.........

I was able to capture this using WireShark. The hex characters are as follows:

ff fb 1f ff fb 20 ff fb 18 ff fb 27 ff fd 01 ff fb 03 ff fd 03

Can you please help me understand why PuTTY sends these characters? I assumed it is something with the connection, but then, these characters are found in the DATA category of the WireShark capture.

Here is a video: https://www.youtube.com/watch?v=WGZru4KPmsM

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Thanks to Some programmer dude and Elmi.

These extra chunk of characters are the option negotiating sequences from PuTTY (client) to the server. Different telnet clients (For example: ZOC Terminal or native macOS terminal's telnet) use different options. Negotiations allow the client to have an enriched experience.

A list of negotiation sequences in telnet protocol and their explanation is given here

If you are using PuTTY and do not need these sequences, go to Connection > Telnet and set the Telnet negotiation mode to Passive

In the words of Some programmer dude:

Perhaps there's settings in the other client to not start with sending these option negotiating sequences? Or perhaps PuTTY is more telnet-compliant than the other clients? There are unfortunately many so-called "telnet clients" which only handles sending plain text and just don't care about the rich telnet protocol.

Here is a link to Elmi's post

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  • Note that many "telnet clients" change their behavior depending on what port you use. For example, if you connect to a server on port 23 (the regular Telnet port) they perform active negotiation, but if you connect on some other port (e.g. SMTP or HTTP) they accept passive negotiation but do not send anything on their own. This can be a source of confusion. Oct 31, 2019 at 21:34
  • And if you use PuTTY to connect to a non-Telnet server... just don't use the Telnet mode at all. That's what the "Raw" mode in PuTTY is for! Oct 31, 2019 at 21:35

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