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Raw TCP Connection

What is it?

Since WebSSH 33.41, a Telnet connection can be switched to a raw connection: a plain TCP socket with no Telnet protocol on top. What you type is sent byte for byte, what the device sends is displayed byte for byte. This is the equivalent of nc host port, of PuTTY's Raw connection type or of socat - TCP:host:port, and it is free.

When do you need it?

Many devices listen on a TCP port without being Telnet servers:

  • a serial port published on the network by a serial-to-IP bridge (ser2net in raw mode, ESP-Link, USR-TCP232, a Moxa NPort in TCP server mode…)
  • retro computers and single-board machines (RC2014, Z80 or 6502 boards, CP/M monitors…) reached through such a bridge
  • microcontroller consoles, lab instruments, GPS receivers and anything you would normally test with nc

A Telnet client talks to them in a language they do not speak. The usual symptoms are:

  • missing characters, or a device that seems to swallow part of what it sends
  • a line feed added after every Enter, which a ROM monitor may read as one more command
  • stray characters received by the device right after it sent some binary data
  • everything you type shown twice

Does it work with netcat?

If nc host port or PuTTY Raw works with your device and Telnet does not, a raw connection is what you need.

Telnet or raw?

Telnet Raw connection
Options negotiation Answers the server's requests (echo, terminal type…) Nothing is negotiated, nothing is sent until you type
Byte 0xFF received Start of a Telnet command, the bytes that follow are interpreted A byte like any other, displayed as it is
Enter Always one CR LF, the Telnet end of line Exactly the Carriage Return of the Terminal Settings (\r by default)
Local Echo set to Auto On until the server says it echoes Off, the device echoes or not

A bridge configured in Telnet mode (ser2net telnet, RFC 2217 serial port control) is a real Telnet server: leave the raw connection off for it, see Telnet Connections.

Turn it on

  1. Edit your server settings (or add a new server)
  2. Open Roles and enable Telnet
  3. In the Telnet section, type the Port of your device
  4. Turn Raw connection on
  5. Save, then launch the server as usual

The setting belongs to the server and follows it through iCloud. Turn it off to go back to Telnet, nothing else changes.

What is sent and what is displayed

  • Enter sends the Carriage Return chosen in the connection's Terminal Settings: \r (the default, what most serial monitors expect), \n or \r\n. Nothing is added.
  • Control keys are plain bytes: Ctrl + C sends 0x03, Esc sends 0x1B. They reach the device, they do not close the connection.
  • Text is sent as UTF-8. Plain ASCII, which is all most serial devices understand, is one byte per character.
  • What the device sends is displayed without any filtering: UTF-8 when it is valid UTF-8, one character per byte otherwise. Set the Remote Character Set to CP437 for a device that draws boxes with the IBM PC character set.
  • Local Echo (Terminal Settings): leave it on Auto or Disabled when the device echoes what it receives, choose Enabled when you do not see what you type.
You are used to In WebSSH
nc 10.0.20.20 4196 Raw connection on
PuTTY Raw, with Local echo and Local line editing forced off Raw connection on, Local Echo on Auto or Disabled
socat -,rawer,echo=0 TCP:10.0.20.20:4196 Raw connection on, Local Echo on Auto or Disabled

Pasting a file or a listing

A raw connection removes the bytes Telnet was adding, it does not slow anything down. A device without flow control still loses characters when a whole file arrives at once: see Paste Slowly to send an Intel HEX file or a BASIC listing in small chunks.

Troubleshooting

How can I see exactly what WebSSH sends?

On a Mac, listen on a port with netcat and print every byte:

nc -l 4196 | xxd -c 1

Then connect WebSSH to 127.0.0.1, port 4196, with the raw connection on. Typing a then Enter prints 61 then 0d, and nothing else. With the raw connection off, Enter prints 0d then 0a.

Everything I type appears twice

WebSSH and the device both echo. Set Local Echo to Disabled in the connection's Terminal Settings.

I do not see what I type

The device does not echo. Set Local Echo to Enabled.

The device ignores Enter, or runs every command twice

It expects another end of line. Change Carriage Return in the Terminal Settings: \r for most monitors and modems, \n for Unix-like consoles, \r\n for a few line-oriented services.

Characters are still lost when I paste

The device cannot keep up with a burst. Use Paste Slowly, or make it the default Paste Mode of the connection.

No encryption

Like Telnet, a raw connection is not encrypted and not authenticated. Keep it on a network you trust, or reach the device through a VPN or an SSH Port Forwarding.

  1. The raw connection requires WebSSH 33.4 or later. ↩

Last update: November 14, 2026