Kermit

Kermit is one of those things I’m fond of that’s really hard to describe. It is:

  • A file transfer protocol for running over serial lines, Modems, or TCP/IP. The protocol is quite flexible, supporting everything from tiny embedded devices with 90-byte packets to streaming over ssh.
  • A FTP- or SFTP-like system-agnostic protocol for looking at directories of files on remote systems, renaming files, deleting them, etc. with a standard process.
  • Capable of operating under extremely challenging conditions, including 7-bit connections and connections that otherwise “eat” certain characters.
  • A modem dialer.
  • A fully-functional standalone TCP server.
  • A system for inband communication over a single connection.
  • On Windows, a terminal emulator. (On Linux, it runs within a terminal emulator already, so doesn’t need to provide that functionality).
  • A thin wrapper around ssh providing new features.
  • A scripting engine, which can be used to automate terminal interaction, file transfer, and more. There are some pretty neat kermit scripts out there - everything from renaming JPEGs based on embedded metadata to budgeting.
  • It’s also a Telnet, SSH, FTP, and HTTP(S) client.

You can download kermit for dozens of platforms, many of which are decades out of date. They all interoperate. It is rather impressive. In fact, a new version of Kermit for TOPS-20 was released in June 2023! (TOPS-20 was active between 1976 and 1988).

There are many Kermit implementations; this page is primarily about CKermit, the most up-to-date and featureful implementation for *nix. For many years, CKermit wasn’t fully open-source, but since 2011, has been released under a BSD license.

I operate the quux.org Kermit Server - open to the public for you to experiment with!

The author’s About Kermit page gives a lot of detail about the program.

Kermit is an example of Old and Small Technology. I wrote about it in Try the Last Internet Kermit Server.

Quick tour: as a ssh wrapper

You can very easily use something like this:

kermit -C 'ssh user@host'

Now, you are logged in to the remote host, using the same OpenSSH you always use. You can happily do all your work as usual. Now let’s say you’re at the shell, you’ve been editing a file on the remote end, and you’d like to copy it to your local machine.

In a traditional setup, you’d open a new local window, then have to copy the full path name and run scp to bring it across. With Kermit, you can just type this at your shell prompt:

kermit -Iis filename.tar.gz

It will send a special byte sequence that your local kermit will pick up on, and enter transfer mode. The file will be transferred, complete with integrity checking. Then you’ll be right back at your shell.

You can even send directories:

cd /usr/share/doc
kermit -Ii -C 'send /recursive libthreadar-dev, exit'

Here we gave a short Kermit script with -C. send is a Kermit command, and this tells it to send the file, then exit.

Kermit Modes

Kermit has these modes:

  • Command mode. This is a command-line interface and should be familiar to people that are used to the traditional FTP client, or to CLIs on things like routers and firewalls, or shells. You can use commands like cd and dir to view files on your local system, or remote cd (aka rcd), rdir, etc. to view files on the local system. Commands like get and send transfer files. This is similar to the mode you get when you press Ctrl-] in telnet. If you just invoke kermit with no parameters, you are in command mode.
  • Connect mode. In this mode, you are connected to the remote system. When you are in command mode and run ssh somehost, you will automatically be put into connect mode. This is very much like the mode you are in when you type ssh somehost at the shell.
  • Server mode. In this mode, kermit is waiting for Kermit protocol commands from the remote. These commands might include things like listing directories, transferring files, etc.
  • Transfer mode. This mode happens when a file is actively being transferred.

You can flip between modes:

  • When in command mode, if you have an active connection, just type c and hit return to go back to connect mode. Or, use a command like ssh, telnet, set host, etc. to establish a connection.
  • When in connect mode, press Ctrl-Backslash and then hit c to return to command mode. Ctrl-Backslash in kermit is very similar to the RET ~ in OpenSSH; you can press Ctrl-\ ? to see other connect-mode actions.
  • You can explicitly enter server mode by running kermit -x, typing the server command while in command mode.

Kermit also has a feature where it will automatically start server mode on the remote. Here’s an example:

jgoerzen@hephaestus:/tmp/t$ kermit
C-Kermit 10.0 Beta.08, 15 Dec 2022, for Linux+SSL (64-bit)
 Copyright (C) 1985, 2022,
  Trustees of Columbia University in the City of New York.
  Open Source 3-clause BSD license since 2011.
Type ? or HELP for help.
(/tmp/t/) C-Kermit>

OK, I’ve launched kermit. Here I am in command mode with my local user. Let’s ssh to the remote; that will put me into connect mode:

(/tmp/t/) C-Kermit>ssh auxuser@localhost
Connecting via command "ssh -e none auxuser@localhost"
 Escape character: Ctrl-\ (ASCII 28, FS): enabled
Type the escape character followed by C to get back,
or followed by ? to see other options.
----------------------------------------------------
auxuser@hephaestus:~$

OK, now I’m in connect mode. I can send a file:

auxuser@hephaestus:~$ kermit -Iis /bin/sh
Return to your local Kermit and give a RECEIVE command.

KERMIT READY TO SEND...
----------------------------------------------------
----------------------------------------------------
 SENT: [/usr/bin/dash] To: [/tmp/t/sh] (OK)

auxuser@hephaestus:~$

I didn’t actually have to give the receive command, because my local kermit saw the special sequence to initiate a transfer, and automatically jumped into transfer mode.

Now let’s go into command mode and do some stuff. I’ll hit Ctrl-\ and then press c.

(/tmp/t/) C-Kermit>rpwd
/home/auxuser

So we see the current working directory on the remote. But wait a minute, rpwd (aka remote pwd) uses the kermit protocol, but at the remote I was just at the shell. What happened?

Kermit was smart enough to know there wasn’t a kermit connection running, so it just sent kermit -x to the remote, placing it in server mode. Clever!

I can even do something like this:

(/tmp/t/) C-Kermit>get /usr/bin/bzip2

Since all these transfers are on my local machine, they’re so fast I don’t even see the transfer-in-progress status screen. Rest assured it does exist and you’ll see it on slower connections.

Now, I could type connect (or just c) to return to the remote. But the remote is in server mode, so I’d be greeted by silence if I did that. I have two ways to get the remote out of server mode:

  1. I could just connect and send a few Ctrl-Cs
  2. I can send the finish (or just f) comand before returning.

Let’s do the second:

(/tmp/t/) C-Kermit>finish
(/tmp/t/) C-Kermit>c
Connecting via command "ssh -e none auxuser@localhost"
 Escape character: Ctrl-\ (ASCII 28, FS): enabled
Type the escape character followed by C to get back,
or followed by ? to see other options.
----------------------------------------------------

C-Kermit server done

auxuser@hephaestus:~$ history | tail -n 3
  799  kermit -Iis /bin/sh
  800  kermit -x
  801  history | tail -n 3

Look at that - it’s the kermit -x invocation right there in history even.

Kermit in Kermit

You can of course do something like this:

  1. Fire up local kermit
  2. Run ssh hostname
  3. While you’re in ssh over there, fire up kermit on the remote. Now, when you’re in connect mode, you’re sending commands to the remote kermit, and when you’re in command mode, you’re sending commands to the local kermit. Potentially confusing – and also potentially useful.

This is what you will encounter if you use the quux.org Kermit Server - try it out!

Performance

These settings make kermit fast over a reliable (ssh, etc) connection. You can add them to your ~/.kermrc if you never need to use unreliable serial links:

set reliable on
set clearchannel on
set receive packet-length 9000
set window 32
set control unprefix all
set transfer slow-start off

Security

The kermit protocol, by default, can work in either direction.

If you connect to untrusted remote systems, I recommend running disable all to prevent the remote from doing much to your local system other than sending files. For instance, rcd is enabled by default and allows the remote to change the directory for you to receive files.

Further reading

Sometimes we want better-than-firewall security for things. For instance:

This page will describe how to run an Internet Kermit server, like the quux.org Kermit Server that was featured in my article Try the Last Internet Kermit Server.

What is this mysterious protocol? Who uses it and what is its story?

This is a Kermit server maintained by me, John Goerzen.

The old sense of the word “modem” in computing referred to a device that would let two computers communicate over telephone lines. It was quite slow and unreliable by modern standards. Nowadays, the term is more broad.

Old technology is any tech that’s, well… old.