Network Working Group F. da Cruz
Request for Comments: 2839 J. Altman
Category: Informational Columbia University
May 2000
Internet Kermit Service
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
ABSTRACT
This document describes a new file transfer service for the Internet
based on Telnet Protocol for option negotiation and Kermit Protocol
for file transfer and management. The Internet Kermit Service
provides access to both authenticated and anonymous users. The use
of Kermit protocol over a Telnet connection provides several
advantages over FTP, including easy traversal of firewalls, transfers
over multiple transports, and security via a combination of supported
Telnet authentication and encryption option negotiations, plus
significant functional benefits. While this document describes a new
service for the Internet, the clients for this service already exist
on most platforms in the form of Telnet clients that support the
Kermit file transfer protocol. These clients are available not only
from Columbia University's Kermit Project but also numerous third
parties.
TABLE OF CONTENTS
1. INTRODUCTION ................................................ 2
2. BACKGROUND .................................................. 3
2.1. History ................................................... 3
2.2. Motivation ................................................ 4
3. THE INTERNET KERMIT SERVICE MODEL ........................... 7
3.1. Server-Side Kermit Server ................................. 7
3.2. Client-Side Kermit Server ................................. 8
3.3. Loosely Coupled Operation ................................. 9
4. SECURITY CONSIDERATIONS .....................................10
4.1. AUTHENTICATION ............................................10
4.1.1. Telnet Authentication ...................................10
4.1.2. Telnet over TLS option ..................................11
da Cruz & Altman Informational [Page 1]
RFC 2839 Internet Kermit Service May 2000
4.1.3. Plaintext Authentication via Kermit REMOTE LOGIN ........11
4.1.4. Plaintext Authentication via Command Prompt .............11
4.1.5. Anonymous Login .........................................12
4.2. ENCRYPTION (PRIVACY) ......................................12
4.2.1 Telnet Encryption .......................................12
4.2.2 Telnet Start_TLS ........................................12
5. SERVICES ....................................................13
5.1. Features for System Administrators ........................13
5.2. Features for Users ........................................14
5.3. User Interface ............................................16
6. REFERENCES ..................................................18
7. AUTHORS' ADDRESSES ..........................................19
8. Full Copyright Statement ....................................20
PREFACE
This document describes an Internet Kermit Service (IKS) which
provides an alternative to FTP for the transfer of files. This
service is based upon both the TELNET protocol and the Kermit file
transfer protocol.
1. INTRODUCTION
The Internet Kermit Service:
1. Provides direct access to Kermit file transfer and management
services without requiring the user to first login to a shell
account;
2. Provides Kermit file transfer and management services to anonymous
users;
3. Provides services to all Telnet clients that support Kermit file
transfer protocol via a simple, predictable, scriptable, and
well-documented textual interface;
4. Provides direct and tightly-coupled access to a Kermit server when
requested via the Telnet Kermit Option [TKO].
This memo assumes knowledge of Transmission Control Protocol, the
Telnet Protocol [TEL], the Kermit File Transfer Protocol [KER,PRF],
Telnet Kermit Option [TKO], and the commands and features of Kermit
software [CKB,CMG,K95].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [BCP].
da Cruz & Altman Informational [Page 2]
RFC 2839 Internet Kermit Service May 2000
Definitions:
Kermit server
A software program that is ready to accept and act upon commands
in the form of well-defined Kermit packets [KER].
Kermit client
A software program that receives requests through its user
interface from a human user (or a script or other source) and
translates them to command packets, which it sends to a Kermit
server, thus initiating a Kermit protocol transaction such as the
transfer of one or more files.
2. BACKGROUND
2.1. History
"Kermit" is the name of an extensible platform- and medium-
independent file transfer and management protocol [KER,PRF] and of a
suite of communications software programs that implement it and
integrate it with other communications functions [CMG,CKB,K95].
The Kermit protocol was first developed at Columbia University in New
York City in 1981 for transferring files without errors between
diverse types of computers over potentially hostile communication
links. Since 1981, the Kermit Project at Columbia University has
expanded the protocol, developed communications software that
implements it upon key platforms, and worked with volunteer
programmers at other sites adapting Kermit protocol to other
platforms or communication methods. The Kermit Project also serves
as the central point of Kermit software development, support,
information, and distribution throughout the world.
Kermit software is now available for nearly every computer and
operating system in existence. The major features of the most
popular Kermit programs are:
- Connection establishment and maintenance for a variety of
connection methods including direct serial, dialup, TCP/IP, X.25,
DECnet, and NETBIOS.
- Terminal emulation.
- Error-free transfer of both text and binary files, individually or
in groups.
- Character-set translation during both terminal emulation and
text-mode file transfer -- a unique feature of Kermit software.
da Cruz & Altman Informational [Page 3]
RFC 2839 Internet Kermit Service May 2000
- Remote file management through the client/server protocol.
- A powerful and portable scripting language allowing complete
automation of any task that can be performed manually.
Kermit's command and script language is consistent across all
platforms and communication methods, thus offering a unified method
for accomplishing a wide range of communication tasks manually or
under script control.
A single Kermit program combines the functions of many different
programs such as uucp, cu, tip, telnet, rlogin, ftp, iconv, and
expect: it is a Telnet and Rlogin client that can also transfer
files; it is a file transfer program that can also convert character
sets; it is a dialout program that can use dialing directories and
understands country codes and area codes; it is fully scriptable; it
offers both client/server and interactive modes of operation. In its
desktop versions (particularly for DOS, Windows, and OS/2) it offers
all the features of communications software that are usually lacking
from Internet client software (key mapping, colors, scrollback, mouse
functions, printer control, etc)
Kermit software is widely used throughout the academic, government,
and corporate spheres, both in the USA and internationally.
In addition to the Kermit software developed and/or distributed by
the Kermit Project at Columbia University, hundreds of other software
products -- commercial, shareware, and freeware -- also include some
level of support for the Kermit protocol. Thus there are hundreds,
perhaps thousands, of independent and interoperable Kermit protocol
implementations based upon the open Kermit protocol specification
[KER].
The Internet has formed the primary mechanism by which users and
developers of Kermit software have collaborated to produce feature
and command sets that continually evolve to meet their needs as
technology changes.
2.2. Motivation.
Kermit protocol and software makes connections from one computer to
another and transfers data between them. Countless people "live" in
Kermit all day long; as a customizable Telnet or Rlogin (or serial
communication) client with a wide selection of terminal emulations
and convenience features, it is their window onto the Internet.
da Cruz & Altman Informational [Page 4]
RFC 2839 Internet Kermit Service May 2000
Others use it in more creative ways, including some that involve key
parts of the Internet, e.g. in batch or cron jobs that update news or
Web servers or fetch email, or to monitor routers, terminal servers,
and hubs and dial pagers when faults are detected. It is used by
vendors of telecommunications equipment for remote diagnosis,
patching, and updates. Telecom managers often use Kermit scripts to
configure PBXs, muxes, routers, or terminal servers. In the world of
commerce, Kermit is widely used for financial transactions, EDI,
medical claim submission, and so forth. It is used with mobile
barcode readers in warehousing and inventory applications. It is
found in US Postal Service sorting and scanning equipment. It
connects many of the logistics and supply systems throughout the
military. It is found in fast-food restaurant cash registers,
milling and die-cutting machines, textile looms and cutters, printing
presses, and medical diagnostic equipment. It was the communications
backbone of the 1994 Brazilian national election -- the largest in
history.
And yet there has never been a strong, explicit connection of Kermit
with the Internet. In the early years, Kermit acted as a kind of
do-it-yourself network, enabling ordinary users to make connections
that were not already there, and for some years was the predominant
method of connecting a personal computer to the ARPAnet (e.g. by
dialing a TAC).
Nowadays, however, with so many of the world's computers on the
Internet, the role of Kermit software and protocol is changing.
Kermit users on the network would like to have the features,
functions, and interface they are accustomed to -- especially the
automation features -- available for use in settings where presently
only tools like FTP are available -- and even more so in situations
where standard software like FTP can't be used.
An Internet Kermit Service can fill this role, and augment the data
transfer power and flexibility of other Internet applications such as
Web browsers:
- Like FTP, Kermit provides a service that can be accessed from many
different platforms with a consistent set of commands, but unlike
FTP, these commands include programming constructions such as
variables, arrays, looping and selection mechanisms, and local and
remote procedure calls.
- Like FTP, Kermit provides both text- and binary-mode data
transfer, as well as file management capabilities. But Kermit
also offers numerous features lacking from FTP, such as
da Cruz & Altman Informational [Page 5]
RFC 2839 Internet Kermit Service May 2000
character-set translation, flexible file selection mechanisms,
attribute preservation, and so on (see Section 5.3 for a longer
list).
- Unlike standard FTP, Kermit can transfer data through multiple
firewalls, proxies, and network address translators (NATs) on a
single port.
- Unlike FTP, Kermit can transfer data across a combination of
transports (e.g. dial-up to a terminal server and thence to an
Internet host).
- Authentication and data transfer can take place over secure
connections (mutually authenticated and encrypted) using
established Telnet authentication and encryption options.
- Unlike traditional Kermit use over Telnet, anonymous access is
possible, and the considerable overhead of the intervening Telnet
server and pseudoterminal service is eliminated.
Until now the primary obstacles to an Internet Kermit Service have
been:
- Issues of authentication, privacy, and anonymous access. These
have been addressed in our implementation, as described Section 4
of this document.
- Issues of coordination and control. A Kermit software program can
be in any of several "modes": at its command prompt or menu,
awaiting commands from the user; in terminal mode, in which the
user's keystrokes are sent to the remote computer or service; or
in protocol mode, in which two Kermit programs communicate via
well-defined Kermit packets [KER]. Commands or operations valid
in one mode do not necessarily work in another. Until now, it has
been the user's responsibility to switch modes at one or both ends
of the connection as needed. A companion document [TKO] to this
one specifies a mechanism to closely couple the client and server
via Telnet protocol negotiations, allowing each to know the
other's state and to switch to the appropriate mode automatically
so a valid and useful relationship obtains at all times.
- Lack of a standard TCP port. The "registered" port 1649 was
assigned by IANA for this purpose (27 September 1995) and is named
"Kermit". (renamed from "Inspect".)
da Cruz & Altman Informational [Page 6]
RFC 2839 Internet Kermit Service May 2000
3. THE INTERNET KERMIT SERVICE MODEL
The Internet Kermit Service (IKS) uses a standard Telnet [TEL]
connection, in which all Telnet rules apply. Unlike FTP, which
requires additional TCP connections, IKS uses a single channel for
both signaling and data transfer. The connection is multiplexed via
(a) Telnet options, and (b) Kermit protocol messages. This allows
existing Telnet clients that also support the Kermit protocol,
whether or not they support the Telnet Kermit Option [TKO], to use
the IKS and take advantage of all relevant Telnet options including
authentication and encryption.
The system Internet services daemon (e.g. inetd) waits for a
connection on the Kermit socket (1649) and then starts the IKS on the
new connection. The IKS performs the familiar Telnet negotiations
including the Telnet Kermit option. Unlike a standard Telnet server,
the IKS does not support the ability to present the user with an
interactive system shell. The Kermit socket is used only for file
transfer and management functions provided by Kermit file transfer
protocol and the Kermit script language.
Once the connection is established, the Telnet Kermit Option is
negotiated in both directions. The results determine which of the
following configurations is used by the Telnet client and Server:
. Server-side Kermit Server (SKS)
. Client-side Kermit Server (CKS)
. No Kermit Server (NKS)
Different procedures and functions apply to each configuration. The
configuration may be changed at any time by Telnet Kermit Option
subnegotiations, which assure that the Telnet client and server are
always in compatible states.
The three configurations are described in the following sections.
3.1. Server-Side Kermit Server
In the Server-Side Kermit Server (SKS) configuration, the Telnet
server is the Kermit server and the Telnet client is the Kermit
client. This configuration is used when both Telnet client and IKS
support the Telnet Kermit Option and the IKS sends WILL KERMIT to the
Telnet client and receives DO KERMIT from the Telnet client [TKO].
In this case, the IKS immediately starts a Kermit server and reports
this to the Telnet client with a Telnet KERMIT START-SERVER
subnegotiation.
da Cruz & Altman Informational [Page 7]
RFC 2839 Internet Kermit Service May 2000
The SKS configuration is appropriate when the user wishes to interact
only with the Telnet client's commands or menus.
If authentication was not performed with one of the Telnet
Authentication Option protocols, the Kermit server rejects all Kermit
protocol operations (except REMOTE LOGIN, REMOTE HELP, REMOTE EXIT,
BYE, or FINISH -- that is, the ones that request help, that log in,
that close the connection, or that change the status of the
connection) until:
- A Kermit REMOTE LOGIN command successfully authenticates the user;
- The login retry limit is reached;
- A Kermit BYE or REMOTE EXIT command is received, which closes the
connection;
- A Kermit FINISH command or a Telnet KERMIT REQ-KERMIT-STOP
subnegotiation is received to request the IKS exit from Kermit
server mode. At this point, the IKS can either exit and close the
connection or issue an interactive login prompt, depending on how
it was started or configured by the system administrator.
Once the user is authenticated:
- The Telnet client configures itself for Kermit client/server
operation, with itself as the Kermit client, communicating with
the server only by Kermit packets, and optionally adjusting its
menus or commands to eliminate functions (such as terminal
emulation) that make no sense in this context.
- The relationship persists until the Telnet client and IKS agree to
terminate the Kermit server via Kermit protocol commands (BYE,
FINISH, or REMOTE EXIT), or by Telnet Kermit Option
subnegotiation, or by closing the connection.
3.2. Client-Side Kermit Server
In the Client-Side Kermit Server (CKS) configuration, the Telnet
server is the Kermit client, and the Telnet client is the Kermit
server. This configuration is used when the IKS has sent WONT KERMIT
or SB KERMIT STOP-SERVER, and the Telnet Client has sent WILL KERMIT
and SB KERMIT START-SERVER, indicating that it is prepared to accept
and process Kermit protocol packets.
In the CKS configuration, the Telnet client assumes the role of
Kermit server by virtue of its ability to recognize and process
Kermit protocol packets in its terminal emulator. Thus the Telnet
da Cruz & Altman Informational [Page 8]
RFC 2839 Internet Kermit Service May 2000
client must not send WILL KERMIT or the KERMIT START-SERVER
subnegotiation unless its terminal emulator is capable of recognizing
Kermit packets.
If the IKS is at top command level (as opposed to executing a
script), or when it reaches top level after finishing a script, it
issues its interactive command prompt.
At this point, the user may type commands or send scripted commands
to the IKS command prompt. When a data-transfer command (such as
SEND) is issued by the user at the IKS prompt, a Kermit packet is
transmitted and recognized by the Telnet client, causing it to
automatically perform the requested action (e.g. receive a file), and
then resume its previous mode (terminal emulation or script
execution) when the data transfer is complete.
Thus, in the CKS configuration, data transfers are initiated by the
IKS rather than by the Telnet client. This configuration is useful
when the user prefers the command interface or repertoire of the
server to that of the client.
If the IKS sends a Telnet KERMIT START-SERVER subnegotiation, the
relationship switches automatically to Server-Side Kermit Server
(Section 3.1), in which the Telnet client is the Kermit client and
the Telnet server is the Kermit server.
If the Telnet client sends a KERMIT STOP-SERVER subnegotiation, the
connection switches to No Kermit Server (Section 3.3) and the IKS
issues its command prompt. At this point, neither side is a Kermit
server, and both sides may optionally disable Kermit protocol
commands. Subsequent user action can designate one side or the other
as the Kermit server, as desired.
3.3. No Kermit Server
If both Telnet client and IKS send WONT KERMIT or SB KERMIT STOP-
SERVER, or if the Kermit client and server are connected across
multiple hosts or transports, thus precluding end-to-end Telnet
negotiation, a Kermit server is not known to be available. In the
KERMIT STOP-SERVER case, the Kermit partners can later switch back to
SKS or CKS, but in the other two cases, there is no such signaling
and loose coupling characterizes the entire session.
In the No Kermit Server (NKS) configuration, the IKS presents a
command prompt to the Telnet client. As in the Client-Side Kermit
Server configuration, plain-text commands are issued to the IKS.
da Cruz & Altman Informational [Page 9]
RFC 2839 Internet Kermit Service May 2000
In the loosely coupled NKS configuration, the Telnet client does not
know the state of the Telnet server, and so can not automatically
adjust its commands and menus to present only valid choices, or
automatically change its state to complement the server's; it is the
user's responsibility to assure that the "mode" (command prompt,
terminal emulation, server command wait) of each Kermit partner is
appropriate for each action. Thus an Internet Kermit Server appears
as an ordinary remote Kermit program to any Telnet client that does
not implement the Telnet Kermit Option, or in which this feature is
disabled or can not be used.
The NKS configuration allows successful manual operation of the IKS
through Telnet clients that do not support the Telnet Kermit Option.
The Telnet client might or might not support Kermit "autodownload"
and "autoupload"; if it does not, then the user is forced to manually
issue command on both sides of the connection in the traditional and
familiar manner [CKB,CMG,K95].
4. SECURITY CONSIDERATIONS
4.1. AUTHENTICATION
Authentication is provided via one or more of the following methods:
- The Telnet AUTHENTICATION option;
- The Telnet START_TLS option;
- Plaintext userid/password verification.
4.1.1. Telnet Authentication option
The use of one of the many Telnet authentication option methods
removes the need to transmit passwords in plaintext across public
networks. In addition, the exchange of user authentication
information often provides a shared secret that can be used with the
Telnet Encryption Option protocols to encrypt the connection in one
or both directions.
Telnet authentication may also be used in conjunction with the Telnet
START_TLS option to negotiate end user identity over the encrypted
and host authenticated TLS channel.
The IKS currently supports Kerberos 4, Kerberos 5, Secure Remote
Password and Microsoft NTLM authentication methods via the Telnet
AUTH option.
da Cruz & Altman Informational [Page 10]
RFC 2839 Internet Kermit Service May 2000
4.1.2. Telnet over TLS option
The Telnet START_TLS option provides for the negotiation and
establishment of a TLS version 1 session after the initial telnet
connection. The TLS connection provides host to client
authentication via the use of X.509 certificate chains. TLS also
supports optional client to host authentication using host verified
X.509 certificates which may be used to authenticate a userid
provided by the client or be mapped to a userid based upon properties
of the certificate.
4.1.3. Plaintext Authentication via Kermit REMOTE LOGIN
In the Server-Side Kermit Server configuration, if the client is not
yet authenticated, the client must log in using a REMOTE LOGIN
command, in which a Kermit packet containing user ID and password in
clear text is sent from the Telnet client to the Telnet server, which
then calls upon local mechanisms to authenticate the user. Any
packets other than login (or REMOTE HELP, REMOTE EXIT, FINISH, or
BYE) packets are rejected (returned with an error message) until the
user is authenticated. If the number of unsuccessful login attempts
exceeds the limit, the connection is closed. Many Kermit client
programs support this login method already.
This method should be avoided whenever possible. If plaintext
passwords are used, they should only be sent after the Telnet START-
TLS option has been negotiated (see 4.2.2). Otherwise, passwords are
open to packet sniffing.
4.1.4. Plaintext Authentication via Command Prompt
In the Client-Side Kermit Server and No Kermit Server configurations,
the server presents the user with a plain-text interactive interface
that begins with the server issuing "Username:" and "Password:"
prompts, just as if the user were logging in to a multiuser
timesharing system such as VMS or UNIX. When a password is not
required an empty response can be given. Invalid username-password
combinations result in a new series of prompts up to the login retry
limit, and then disconnection.
This method should be avoided whenever possible. If plaintext
passwords are used, they should only be sent after the Telnet START-
TLS option has been negotiated (see 4.2.2). Otherwise, passwords are
open to packet sniffing.
da Cruz & Altman Informational [Page 11]
RFC 2839 Internet Kermit Service May 2000
4.1.5. Anonymous Login
When the username is "anonymous" or "ftp", the IKS behaves like an
anonymous ftp server, in a manner appropriate to the underlying
platform. In UNIX, for example, access is restricted to a designated
area of the file system. A password might or might not be required,
according to the preference of the site administrator.
If privacy is desired the Telnet START-TLS option should be used (see
4.2.2).
4.2. ENCRYPTION (PRIVACY)
As the Internet becomes ever more public and susceptible to
eavesdropping, it becomes increasingly necessary to provide methods
for private access to services. Telnet provides two such mechanisms:
. Telnet Encryption option
. Telnet START-TLS option
4.2.1. Telnet Encryption option
The Telnet Encryption option, although it has never achieved RFC
status, has been used for years in conjunction with the Telnet Auth
option in Telnet clients and servers that support Kerberos 4,
Kerberos 5, Secure Remote Password, and others. The IKS currently
supports the following encryption methods under the Telnet Encryption
option:
. cast128_ofb64
. cast5_40_ofb64
. des_ofb64
. cast128_cfb64
. cast5_40_cfb64
. des_cfb64
4.2.2. Telnet over TLS option
Transport Layer Security (TLS), the successor to Secure Sockets Layer
(SSL), provides methods to implement Server authentication, Client
authentication, and Transport Layer encryption. Unlike Telnet
Encryption, Start-TLS does not require the use of Telnet
Authentication in order to provide a private channel. This means
that it can be used in conjunction with plaintext passwords and
anonymous connections.
da Cruz & Altman Informational [Page 12]
RFC 2839 Internet Kermit Service May 2000
5. SERVICES
The Internet Kermit Service includes features for both users and
system administrators. The IKS is incorporated into the 7.0 release
of Columbia University's C-Kermit software, which is the "master"
Kermit software program in terms of features and command language.
An overview of C-Kermit can be found at:
http://www.columbia.edu/kermit/ckermit.html
http://www.kermit-project.org/ckermit.html
When C-Kermit is employed as an Internet Kermit Service, it may offer
all its functions to "real" users (those who are authenticated as
specific users), and a safe subset of its functions to anonymous
users.
The Internet Kermit Service resembles an FTP server in that it
performs its own authentication and uses a well-defined protocol to
communicate with its client, but differs from the FTP server by also
offering (at the system manager's discretion) an interactive user
interface to the Telnet client when it is in terminal mode. It also
differs from FTP in restricting all protocol messages and data
transfer to a single socket connection.
An IKS has been deployed at Columbia University for worldwide public
access to the Kermit FTP site:
telnet://kermit.columbia.edu:1649/
telnet://ftp.kermit-project.org:1649/
5.1. Features for System Administrators
The system administrator can supply IKS configuration parameters as
command-line options or in a configuration file, or both in
combination. Such parameters include:
. Whether anonymous logins are allowed.
. The file system or root directory to which anonymous users are
restricted.
. Specification of permissions and other attributes to be assigned
to files uploaded by anonymous users.
. Whether to make session entries in system logs.
da Cruz & Altman Informational [Page 13]
RFC 2839 Internet Kermit Service May 2000
. Specific services to disable: reception of files, sending of
files, sending of email, printing, changing of directories,
getting directory listings, deleting files, etc (see next
section).
. Whether access to the interactive command prompt is allowed.
5.2. Features for Users
The IKS supports a wide range of services, including, but not limited
to, the following:
. Authentication as a real user or anonymously.
. Transmission of files to which read access is allowed.
. Reception of files into directories or devices to which write
access is allowed.
. The ability to display a file on the client's screen.
. Ability to list files.
. Ability to change its working (default) directory.
. Ability to delete files to which write or delete access is
allowed.
. Ability to rename and copy files
. Ability to create and remove directories.
. The ability to route received files to a specified printer, or to
send them as email to a specified address list.
. Client control of server parameter settings, within limits
established by the server system administrator.
. Transmission of variables from client to server or vice versa.
. Remote and local script execution.
. Remote and local procedure execution.
da Cruz & Altman Informational [Page 14]
RFC 2839 Internet Kermit Service May 2000
File transfer features include:
. Kermit text-mode transfers incorporate not only record-format
conversion, but also character-set translation;
. Kermit can switch automatically between text and binary mode on a
per-file basis when sending groups of files.
. A selection of file collision options, including "make backup copy
of existing file and accept incoming file", "reject incoming
file", "accept incoming file only if newer than existing file",
etc.
. Numerous methods for selecting the files to be transferred,
including pattern matching, lists of filenames (or patterns),
exception lists, date and/or size ranges, etc.
. Filename conversion and file renaming.
. Automatic directory creation if elected and enabled.
. Standard mechanisms for directory traversal, allowing transmission
of entire directory trees or other file hierarchies even between
unlike file systems such as VMS, UNIX, and Windows.
. Atomic file movement: optionally, the source file can be deleted
(or renamed, or moved) when and only when it has been transferred
successfully.
. Kermit can retain file attributes including time stamps and
permissions (at the user's or system administrator's discretion),
even between unlike platforms;
. Recovery of interrupted transfers from the point of failure.
. File-transfer pipes and filters.
Script programming features include:
. Macros with parameter substitution.
. Built-in and user-defined variables and arrays, with global or
local scope.
da Cruz & Altman Informational [Page 15]
RFC 2839 Internet Kermit Service May 2000
. Built-in and user-defined functions. Built-in functions include:
- String functions
- Arithmetic functions
- Date / time functions
- File functions
. Input search for multiple simultaneous targets.
. IF-ELSE, WHILE, FOR, SWITCH, GOTO, C-like block structure.
. Every command returns a completion status that may be tested and
used as a basis for subsequent actions.
5.3. User Interface
The Internet Kermit Service uses the Kermit command and script
language, as implemented in Columbia University's C-Kermit
communication software [CKB]. This program and its command language
are portable to all known varieties of UNIX, as well as to Windows
95/98/NT, OS/2, Digital (Open)VMS, Stratus VOS, Data General AOS/VS,
Plan 9, OS-9, QNX, the Commodore Amiga, and other platforms. The
C-Kermit command language is a superset of that of other Kermit
software programs including MS-DOS Kermit for DOS and Windows 3.x,
IBM Mainframe Kermit for VM/CMS, MVS/TSO, CICS, and MUSIC, PDP-11
Kermit for RT-11, RSTS/E, RSX-11, and IAS, and dozens of other Kermit
programs.
It is far beyond the scope of this document to enumerate, let alone
describe, the commands and services of C-Kermit; this is the subject
of a 600-page book [CKB], augmented by hundreds of pages of online
material. A brief overview is included here.
Commands are based on English words. There is no plan at present to
support other natural languages (Italian, Portuguese, Norwegian,
Russian, Hebrew, Japanese, Cherokee, etc) as alternative bases for
command words, since this would reduce the portability of scripts.
However, since the command language includes a macro capability,
macros may be defined to provide selected commands in different
languages if desired.
Certain commands can apply either locally or remotely, for example
"CD" (Change Directory). The convention is to prefix the command
with the word REMOTE if it is to apply remotely. Example: "cd foo"
changes to the "foo" directory on the computer where the command was
given; "remote cd foo" sends a Kermit packet to the Kermit server
requesting it to change its directory to "foo". The commands in this
category include:
da Cruz & Altman Informational [Page 16]
RFC 2839 Internet Kermit Service May 2000
ASSIGN <variable> <value> Assign a value to a variable.
CD <directory> Change working directory.
COPY <files> <destination> Copy file(s)
DELETE <files> Delete file(s)
DIRECTORY [ <pattern> ] List file(s)
EXIT Exit
HELP [ <topic-or-command> ] Display help text
MKDIR <directory> Create a directory
PRINT <files> Print file(s)
PWD Print working directory
RENAME <old> <new> Rename file(s)
RMDIR <directory> Remove a directory
SET <parameter> <value> Change a parameter's value
TYPE <file> Display the contents of a file
As a convenience, REMOTE commands also have short synonyms: RASSIGN,
RCD, RCOPY, RDELETE, and so forth.
The basic file transfer commands are:
SEND [ modifiers ] <files> Send file(s) (to server)
GET [ modifiers ] <files> Get file(s) (from server)
These commands take a file name, pattern, or list, plus various
optional modifiers, including transfer mode specifiers (text,
binary), file selectors (date, size, exception list), aliasing, name
and path options, disposition specifiers, and so on.
In addition to the commands listed above, the following commands are
sent by the client to the server:
REMOTE QUERY Get value of variable or procedure
BYE Log out and close the connection
FINISH Request the server leave server mode
Like all Kermit client/server commands, these can be disabled if
desired.
Of course there are numerous other commands with purely local effect,
such as the many scripting commands. These, plus all the commands
above, are fully documented in [CKB]. The repertoire grows over
time, but never in a way that invalidates existing scripts.
The system administrator can allow or forbid access to any of these
features, and to the command language as a whole. In the latter
case, the IKS may be accessed only as a Kermit server, by giving
commands to the client.
da Cruz & Altman Informational [Page 17]
RFC 2839 Internet Kermit Service May 2000
6. REFERENCES
[TKO] Altman, J. and F. da Cruz, "Telnet Kermit Option", RFC 2840,
May 2000.
[BCP] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[KER] da Cruz, Frank, "Kermit, A File Transfer Protocol", Digital
Press/ Butterworth Heinemann, Newton, MA (1987). 379 pages,
ISBN 0-932376-88-6.
[CKB] da Cruz, Frank, and Christine M. Gianone, "Using C-Kermit",
Second Edition, Digital Press / Butterworth-Heinemann, Woburn,
MA (1997). 622 pages, ISBN 1-55558-164-1.
[CMG] Gianone, Christine M., "Using MS-DOS Kermit", Second Edition,
Digital Press / Butterworth-Heinemann, Woburn, MA (1992). 345
pages, ISBN 1-55558-082-3.
[K95] Gianone, Christine M., and Frank da Cruz, "Kermit 95", Manning
Publications, Greenwich CT, (1996). 88 pages, ISBN 1-884777-
14-7.
[PRF] Huggins, James K., "Kermit Protocol - Formal Specification and
Verification", in Boerger, E., "Specification and Validation
Methods", Oxford University Press (1995). ISBN 0-19-853854-5.
[FTP] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD
9, RFC 959, October 1985.
[TEL] Postel, J. and J. Reynolds, "Telnet Protocol Specification",
STD 8, RFC854, May 1983, et seq.; "Telnet Option
Specification", STD 8, RFC855, May 1983, et seq.
[IAN] Internet Assigned Numbers Authority:
http://www.iana.org/numbers.html
http://www.iana.org/assignment/port-numbers
da Cruz & Altman Informational [Page 18]
RFC 2839 Internet Kermit Service May 2000
7. AUTHORS' ADDRESSES
Frank da Cruz
EMail: fdc@columbia.edu
Jeffrey E. Altman
EMail:jaltman@columbia.edu
The Kermit Project
Columbia University
612 West 115th Street
New York NY 10025-7799
USA
http://www.columbia.edu/kermit/
http://www.kermit-project.org/
da Cruz & Altman Informational [Page 19]
RFC 2839 Internet Kermit Service May 2000
8. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
da Cruz & Altman Informational [Page 20]