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What is Kermit?

Frank da Cruz, Columbia University,
125th Street and Broadway NYC
Most recent update: Wed Aug 17 07:40:55 2011


Effective 1 July 2011, the Kermit Project at Columbia ceased to exist. This is not, however, a death knell for Kermit software: There is every reason to believe that, freed of its corporate ties and requirement to sustain itself by revenue, and with its new license, Kermit will enjoy a renaissance.

Announcement   Transition roadmap   Kermit 95   C-Kermit   E-Kermit   Other Kermit software


Kermit is the name of a file-transfer and -management protocol and a suite of computer programs for many types of computers that implements that protocol as well as other communication functions ranging from terminal emulation to automation of communications tasks through a high-level cross-platform scripting language. The software is transport-independent, operating over TCP/IP connections in traditional clear-text mode or secured by SSH, SSL/TLS, or Kerberos IV or V, as well as over serial-port connections, modems, and other communication methods (X.25, DECnet, various LAN protocols such as NETBIOS and LAT, parallel ports, etc, on particular platforms).

The Kermit Project was founded at the Columbia University Computer Center (now CUIT) in 1981 to meet a specific need, and until the mid- to late 1990s, Kermit was Columbia's standard desktop connectivity software, used universally by students, faculty, and staff to connect from desktop microcomputers, PCs, Macintoshes, and Unix workstations to the central computing facilities: the IBM mainframes (1963-present), the DECSYSTEM-20s (1977-1988), CLIO (Columbia's first online library information system, 1984-2003), and Cunix (our Unix-based servers, 1986-present), and to departmental VAXes, PDP-11s, Suns, and other minicomputers. In the early days of microcomputers and PCs but before widespread deployment of local area networks and desktop workstations that connected to them, Kermit software linked the desktop to e-mail, bulletin boards, file sharing, text processing, messaging, and other aspects of the new on-line culture that is now taken for granted, long before the experience was available at most other institutions. At Columbia, the DEC-20s and the departmental minicomputers are long gone and the IBM mainframes are now only for backoffice use, but Kermit software is still used for SSH sessions from the desktop to CUNIX, and by the technical staff for system and network administration tasks; for example, configuring racks full of HP blade servers as they arrive, management of the University's telephone system, CGI scripting, alpha paging of on-call staff, and so on.

Over the years, the Kermit Project grew into a worldwide cooperative nonprofit software development and distribution effort, headquartered at and coordinated from Columbia University, as Kermit software was ported to or developed for more and more computers and operating systems (see list). The Kermit Project is dedicated to production of cross-platform, long-lasting, stable, standards-conformant, interoperable communications software, and has been actively engaged in the standards process. Kermit software is used all over the world in every sector of the economy: national government, state and local government, academic, medicine and health care, engineering, aerospace, nonprofit, and commercial.

EM-APEX ocean float Although terminal emulation has been largely supplanted by the Web for online access, Kermit software continues to play a role in other applications such as remote sensing and data collection, management and troubleshooting of networking and telecommunications equipment, back office work, cargo and inventory management, medical insurance claim submission, electronic funds transfer, and online filing of income tax returns. Kermit software is embedded in network routers and switches, in cell-phone towers, in medical diagnostic and monitoring equipment, in programmable die-cutting and stamping presses and woodworking tools, even in cardiac pacemakers, not to mention the cash registers of quite a few big-name "big box" retailers. In 2002 Kermit flew on the International Space Station, and Kermit software is the communication method used by EM APEX ocean floats (left) supplying realtime data to hurricane researchers and trackers to this day (the hurricane project entered a new expanded phase in 2010 based on a new version of Embedded Kermit).

In the 1990s Kermit software was used in US Post Office automation, it played an key role in the 1994 Brazilian national election (the biggest in the history of the world up to that time), and it was central to the UN relief mission to Bosnia, “linking the entire spectrum of the project operation, from mainframe, minicomputer, PCs, to handheld devices and barcode readers.”

USSR Kermit sweatshirt In the 1980s the robustness of the Kermit protocol suited it ideally for service in the Green Revolution in Africa, the joint European-USSR Giotto space mission, and perhaps most notably in reestablishing data communication between US research stations in Antarctica and the mainland after they were cut off in 1986 in a computer mishap during the 9-month Antarctic winter. In 1988 an international conference on Kermit was hosted in Moscow, USSR, and Kermit sessions were featured at other conferences throughout the 1980s in Tokyo, Bern, Paris, Nashville, and elsewhere.

Muppets Calendar page from May 1981 The Kermit protocol and software are named after Kermit the Frog, star of the television series, The Muppet Show; the name Kermit is used by permission of Henson Associates, Inc. Why is it named after Kermit the Frog? In May of 1981 we already had first implementations of the protocol working, but we didn't have a name for the protocol or the software yet. A group of us was discussing it (me, Bill Catchings, Bill Schilit, Jeff Damens, I think that was the group), without actually caring too much since we never expected the software to spread all over the world and last more than 30 years. I happened to be facing the wall that had a Muppets calendar on it, and since my children were such big fans of the Muppet Show I said, How about Kermit?  Thirty years later (May 2011) I found the calendar page that I was looking at when I said that, you can see it on the left and you can click on it to see a bigger image.


Kermit software has been written for hundreds of different computers and operating systems, some of it by volunteer programmers all over the world, some of it by the Kermit Project professional staff. The major features of the most popular Kermit programs are:
  • Connection establishment and maintenance for a wide variety of connection methods (TCP/IP, X.25, LAN, serial port, modem, etc).
  • Terminal emulation.
  • Error-free file transfer.
  • Internet protocols including Telnet, Rlogin, FTP, and HTTP.
  • Internet security methods including Kerberos, SSL/TLS, SSH, and SRP.
  • Character-set conversion during both terminal emulation and file transfer – a unique feature of Kermit software.
  • Numeric and alphanumeric paging.
  • Script programming to automate complicated or repetitive tasks.
Kermit's user interface and script programming language are consistent across platforms and communication methods, allowing the investment in learning to pay off time and again as you move from one platform to another, one communication method to another.

Our premiere Kermit software implementations are:

C-Kermit and IBM Mainframe Kermit are host-based packages with an unequaled range of versatility. Kermit 95 and MS-DOS Kermit are full-featured desktop communication software programs rivaling the quality of anything else on (or off) the market, except perhaps in flashiness of user interface: Kermit programs follow the text-mode prompt-and-command style of yesteryear, which is baffling to some people until they realize the advantages:

  • The command set is reasonably consistent across all platforms, and almost totally consistent across modern platforms such as Windows, Mac OS X, Linux, and VMS. Learn it once, use it everywhere.
  • Commands may be combined into "macros" or "programs" to automate any task that can be done by hand, as described here. In fact in C-Kermit and Kermit 95, the command language is a full-blown programming language with variables, control structures, functions, "subroutines", plus a few surprises.
  • You don't have know the commands in advance nor type them out in full. The command style is called “context-sensitive menu on demand” (you see the available choices when you type a question mark), and keywords can be abbreviated. There is plenty of built-in help, and plenty more help to be found on the Kermit website; for example the C-Kermit tutorial and the the Kermit 95 tutorial, just for starters.
  • Touch typists can work faster when they don't have to move their hands away from the home keys, and they suffer less repetitive strain injury.
  • Certain things just can't be done efficiently or at all using a GUI interface. Here's a completely random example, but it makes the point:

    On a PC I have directory that contains thousands of images, together with their thumbnails. For every image xxx.jpg there is a thumbnail xxx-t.jpg. I want to load all the thumbnails into Photoshop. Using the mouse, this would take all day. With Kermit you can do it like this:

    Kermit> mkdir thumbnails
    Kermit> rename *-t.jpg thumbnails/

    And then in the thumbnails subdirectory, Ctrl-A to "select all" and drag to Photoshop (and then, if desired, drag the thumbnails back to the original directory with one mouse motion, or rename them back with one Kermit command).

The Kermit 95 2.1 shrinkwrapped retail package Kermit 95 was developed not only to meet Columbia's need for connectivity from Windows 95 (and later) to the central text-based services, but also to raise money to support the Kermit Project. Unlike other Kermit programs, K95 was strictly commercial, available in both a retail shrinkwrapped version (right) and in bulk right-to-copy licenses. From its release in 1995 until mid-2011, over a quarter million bulk license seats were purchased in over 1000 licenses licenses ranging in size from 100 seats to 10,000. About 30,000 shrinkwrapped copies were sold, many thousands more purchased for download from e-academy, and K95 was been site-licensed by over 100 universities as well as by entire statewide university systems such as SUNY (64 campuses with about 400,000 students).

The Kermit Project was put on a self-funding basis in 1984, and from then until its cancellation in 2011, it realized $8,894,912.00 in revenue for the University, plus an equipment grant (the Hermit Project) valued at $3,000,000.00.

Kermit books   Kermit 95   C-Kermit   E-Kermit   G-Kermit   Current software versions


Since its inception in 1981, the Kermit protocol has developed into a sophisticated and powerful transport-independent tool for file transfer and management, incorporating, among other things:

Kermit protocol uses well-defined, sequenced, error-checked packets in each direction to effect a file-transfer session, following standard rules of protocol layering. Packets are designed for maximum transparency, so they can pass though any communication medium, no matter how restrictive. Half-duplex (stop and wait), full-duplex (sliding windows with selective retransmission), and continuous streaming transport can be used to adapt to any connection.

The feature that distinguishes Kermit protocol from most others is its wide range of settings to allow adaptation to any kind and quality of connection between any two kinds of computer — packet length, packet encoding, window size, character set, error-detection method, timeouts, pauses. Most other protocols are designed to work only on certain kinds or qualities of connections, and/or between certain kinds of computers or like file systems, and therefore work poorly (or not at all) elsewhere and offer few if any methods to adapt to unplanned-for situations. Kermit, on the other hand, allows you to achieve successful file transfer and the highest possible performance on any given connection.

Unlike FTP or X-, Y-, and ZMODEM (the other protocols with which Kermit is most often compared) Kermit protocol does not assume or require:

  • a full-duplex connection;
  • a connection that is transparent to control characters;
  • an 8-bit connection;
  • a clean connection;
  • big buffers all along the communication path;
  • physical-link-layer flow control.

(although Kermit does not require any of these conditions, it can take advantage of them when they are available). A feature article on Kermit protocol by Tim Kientzle in the February 1996 issue of Dr. Dobb's Journal noted that “Kermit's windowing approach is faster than protocols such as XModem and YModem . . . What many people don't realize is that under less-than-ideal conditions, Kermit's windowing approach is significantly faster than ZModem, a protocol with a well-deserved reputation for fast transfers over good-quality lines.” The efficiency of the Kermit protocol is analyzed in depth here and here.

Thus Kermit transfers work "out of the box" almost every time. And at a higher level, the Kermit command language allows all sorts of handy file selection criteria to be used in any combination, for example:

  • Wildcards and patterns to match filenames
  • Selection by date ranges
  • Selection by size ranges
  • Only text files
  • Only binary files
  • Only files that don't exist on the other end or that are newer
  • Exception lists and patterns
to accomplish almost any grouping you can imagine. In transit, a file can have its character-set converted, it can be passed through a filter, etc, and upon successful transfer, the source file can be deleted or renamed, the destination file can be renamed or mailed, and so on.

The original Kermit book The Kermit file transfer protocol specification is given in the book, Kermit, A File Transfer Protocol by Frank da Cruz, with a foreword by Donald Knuth.

A formal specification and verification of the Kermit protocol was published by James Huggins of the University of Michigan in 1995.


Let's look the common case where you have a Windows desktop computer with a connection — any kind of connection (modem, serial port, regular Telnet, secure Telnet, rlogin, secure rlogin, SSH) — to a shell session on a Unix server ("Unix" = Linux, Mac OS X, FreeBSD, Solaris, AIX, HP-UX, etc) and you want to transfer a file between your PC and the Unix server. Your terminal emulator on Windows is Kermit 95 and the Unix server has C-Kermit or G-Kermit installed, which can be invoked simply by typing “kermit” at the shell prompt (or perhaps “ckermit” or “gkermit”).

To download a file, say, message.txt, you type the following command at the shell prompt:

kermit -s message.txt
The file is sent to Kermit 95's current directory on your PC (or to its DOWNLOAD DIRECTORY if you have defined one). It doesn't matter if the file is text or binary; Kermit figures it out and transfers it automatically in the appropriate mode.

Similarly if you want to transfer a group of files, say, all the files whose names start with “daily.”:

kermit -s daily.*
Kermit sends each file that matches, switching automatically between text and binary mode as appropriate for each file (daily.jpg, daily.xls, daily.txt, ...)

Uploading a file from your PC to Unix is just as easy. Suppose you have a file called “budget.xls” in Kermit 95's current directory on your PC. To upload it to UNIX, type this at the Unix shell prompt:

kermit -g budget.xls
Those are the basics; there are many variations and refinements; for example:
  • Only transfer files that are newer than the counterparts on the other end.
  • Convert character sets of text files appropriately (e.g. between ISO 8859-1 and Unicode UTF-8).
  • Recover a partial transfer from the point of failure (binary mode only).

To save yourself some typing, you can define aliases on Unix (in your shell profile):

alias s="kermit -Ys"
alias g="kermit -Yg"
(s for Send, g for Get). And then:

s message.txt
g budget.xls
It's worth noting that you are transferring your files over the same connection you already have; thus there is no need make a new connection, re-authenticate yourself, or similar bureaucracy. If the connection is secured by SSH, Kerberos, SSL, TLS, or SRP, then the file transfer is also secure.

This marks an unparalleled degree of convenience. When you tell C-Kermit on Unix to send or get a file, its first file-transfer packet is recognized automatically by Kermit 95's terminal emulator and K95 pops into either receive mode or server mode, depending on the direction, and when the transfer is finished, K95 returns to its terminal emulation screen. If there is an error (for example, if you do not have write permission in the destination directory) K95 remains in its file-transfer screen so you can see what the problem was.

Unfortunately, most information about Kermit software and protocol that is found on external websites is inaccurate or severely dated. Even the relatively favorable Wikipedia article focuses on details and long-forgotten trivia from 30 years ago and mostly ignores the steady progress that has been made since then.

What Is Kermit? / Frank da Cruz / / Most recent update: 16 August 2011