telnet: first content

added a blank tftp

SUMMARY.md

@@ -67,6 +67,7 @@
     * [Sending email](usingcurl/smtp.md)
     * [MQTT](usingcurl/mqtt.md)
     * [TELNET](usingcurl/telnet.md)
+    * [TFTP](usingcurl/tftp.md)
     * [TLS](usingcurl/tls.md)
         * [SSLKEYLOGFILE](usingcurl/tls/sslkeylogfile.md)
     * [Copy as curl](usingcurl/copyas.md)

usingcurl/telnet.md

@@ -1,3 +1,64 @@
-## TELNET
+# TELNET
+
+Telnet is an ancient application protocol for bidirectional **clear-text**
+communication. It was designed for interactive text-oriented communications
+and *there is no encrypted or secure version* of Telnet.
+
+TELNET is not a very good match for curl. The protocol is not done to handle
+plain uploads or downloads so the usual curl paradigms have had to be
+stretched somewhat to make curl deal with it appropriately.
+
+curl sends received data to stdout and it reads input to ssend on stdin. The
+transfer is complete when the connection is dropped or when the user presses
+control-c.
+
+## Historic TELNET
+
+Once upon the time, systems provided telnet access for login. Then you could
+connect to a server and login to it, much like how you would do it with SSH
+today. That practice have fortunately now mostly been moved into the museum
+cabinets due to the insecure nature of the protocol.
+
+The default port number for telnet is 23.
+
+## Debugging with TELNET
+
+The fact that TELNET is basically just a simple clear-text TCP connnection to
+the target host and port makes it somewhat useful to debug other protocols and
+services at times.
+
+Example, connect to your local HTTP server on port 80 and send a (broken)
+request to it by manually entering `GET /` and press return twice:
+
+    curl telnet://localhost:80
+
+Your web server will most probably return something like this back:
+~~~
+HTTP/1.1 400 Bad Request
+Date: Tue, 07 Dec 2021 07:41:16 GMT
+Server: softeare/7.8.9
+Content-Length: 31
+Connection: close
+Content-Type: text/html
+
+[message]
+~~~
+
+## Options
+
+When curl sets up a TELNET connection to a server, you can ask it to pass on
+options. You do this with `--telnet-option` (or `-t`), and there are three
+options available to use:
+
+- `TTYPE=<term>` sets the "terminal type" for the session to be `<term>`.
+- `XDISPLOC=<X display>` sets the X display location
+- `NEW_ENV=<var,val>` sets the environment variable `var` to the value `val`
+  in the remote session
+
+Login to your local machine's telnet server and tell it you use a vt100
+terminal:
+
+    curl --telnet-option TTYPE=vt100 telnet://localhost
+
+You need to manually enter your name and password when prompted.
 
-TBD

usingcurl/tftp.md

@@ -0,0 +1,3 @@
+# TFTP
+
+TBD