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I have a Perle IOLAN device like this, and one configuration option is for a serial terminal.

The corresponding configuration screen.

As you can see, one option is to configure the terminal to connect to a Telnet server. Can somebody explain how it is possible for the telnet application protocol to be transported over a serial link (from the terminal to the IOLAN)? My understanding was that Telnet is exclusive to TCP.

A follow up question is how to configure a Linux host as a terminal for the purposes of connecting to an IOLAN (i.e. emulate a terminal).

tavr
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how it is possible for the telnet application protocol to be transported over a serial link

That isn't what happens. You use telnet to connect to the terminal server. The 'telnet' session does not get transmitted over serial. Instead, when you connect to the terminal server via telnet a process is started within that session that connects to the serial device.

It is more or less the same as what would happen if you used SSH to connect to a linux box, then use GNU screen to connect to some serial device that was connected to that remote system.

Linux host as a terminal for the purposes of connecting to an IOLAN

There are lots of guides about running a Linux box with a serial console. It usually invokes configuring your bootloader (grub) to listen on the serial port, and configuring a getty via in your init daemon to listen on the serial port.

If the device you are connecting to is already configured to work on a serial port (like console ports for network equipment) then you probably don't have to do anything.

Zoredache
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  • I think what you're describing is the [console configuration](https://i.stack.imgur.com/hB9Kz.png), where the remote client is initiating a telnet session to the IOLAN, then the IOLAN connects to the serial device. Under the terminal configuration, it is the serial device itself that is initiating the connection, and I guess the IOLAN then starts a client session to the telnet server. But yeah, your answer does clarify what is happening as far as telnet is concerned. – tavr Jan 24 '18 at 07:14
  • Given that I'm trying to set up a terminal configuration, in which my linux box will connect to the IOLAN and spawn a telnet session to a remote host, do I still need to set up the linux machine as a serial server, as described [here](https://help.ubuntu.com/community/SerialConsoleHowto), or is it enough to use minicom to connect to the IOLAN over serial port. When I try through minicom, the screen just shows a bunch of periodically refreshing garbage characters, but that may be because my minicom isn't configured properly for IOLAN access. – tavr Jan 24 '18 at 07:24
  • Btw, I'ws using a cisco db9-rj45 console cable. Judging by this [document](https://www.perle.com/support_services/cabling/documents/iolan%20-%20rj45%20cable%20diagrams.pdf), that might not be the right pinout to use. – tavr Jan 24 '18 at 08:44
  • I really don't understand what you are trying to do. You may need to update your question with some kind of diagram/drawing or something identifying all the devices invoked, and the a description of which devices will be initiating the connection. – Zoredache Jan 24 '18 at 08:52
  • There is a diagram in my original question (behind the "serial terminal" link). This is exactly my situation, except that my 'terminal' is a linux box which is booted in single-user mode without network interfaces. My objective is to reach another machine via ssh/telnet from this computer. Which is what the IOLAN is for. Anyhow, just wanted to report that this works now that I made a cable with the right pinout. – tavr Jan 24 '18 at 21:33