BSD Rlogin

Network Working Group                                          B. Kantor
Request for Comments: 1282                      Univ. of Calif San Diego
Obsoletes: RFC 1258                                        December 1991


                               BSD Rlogin

Status of this Memo

   This memo documents an existing protocol and common implementation
   that is extensively used on the Internet.  This memo provides
   information for the Internet community.  It does not specify an
   Internet standard.  Distribution of this memo is unlimited.

Protocol Description

   The rlogin facility provides a remote-echoed, locally flow-controlled
   virtual terminal with proper flushing of output [1].  It is widely
   used between Unix hosts because it provides transport of more of the
   Unix terminal environment semantics than does the Telnet protocol,
   and because on many Unix hosts it can be configured not to require
   user entry of passwords when connections originate from trusted
   hosts.

   The rlogin protocol requires the use of the TCP.  The contact port is
   513.  An eight-bit transparent stream is assumed.

Connection Establishment

   Upon connection establishment, the client sends four null-terminated
   strings to the server.  The first is an empty string (i.e., it
   consists solely of a single zero byte), followed by three non-null
   strings: the client username, the server username, and the terminal
   type and speed.  More explicitly:

        <null>
        client-user-name<null>
        server-user-name<null>
        terminal-type/speed<null>

        For example:

        <null>
        bostic<null>
        kbostic<null>
        vt100/9600<null>

   The server returns a zero byte to indicate that it has received these
   strings and is now in data transfer mode.  Window size negotiation
   may follow this initial exchange (see below).

From Client to Server (and Flow Control)

   Initially, the client begins operation in "cooked" (as opposed to
   to "raw") mode.  In this mode, the START and STOP (usually ASCII
   DC1,DC3) characters are intercepted and interpreted by the client to
   start and stop output from the remote server to the local terminal,
   whereas all other characters are transmitted to the remote host as
   they are received.  (But see below for the handling of the
   local-escape character.)

   In "raw" mode, the START and STOP characters are not processed
   locally, but are sent as any other character to the remote server.
   The server thus determines the semantics of the START and STOP
   characters when in "raw" mode; they may be used for flow control or
   have quite different meanings independent of their ordinary usage on
   the client.

Screen/Window Size

   The remote server indicates to the client that it can accept window
   size change information by requesting a window size message (as out
   of band data) just after connection establishment and user
   identification exchange.  The client should reply to this request
   with the current window size.

   If the remote server has indicated that it can accept client window
   size changes and the size of the client's window or screen dimensions
   changes, a 12-byte special sequence is sent to the remote server to
   indicate the current dimensions of the client's window, should the
   user process running on the server care to make use of that
   information.

   The window change control sequence is 12 bytes in length, consisting
   of a magic cookie (two consecutive bytes of hex FF), followed by two
   bytes containing lower-case ASCII "s", then 8 bytes containing the
   16-bit values for the number of character rows, the number of
   characters per row, the number of pixels in the X direction, and the
   number of pixels in the Y direction, in network byte order.  Thus:

        FF FF s s rr cc xp yp

   Other flags than "ss" may be used in future for other in-band control
   messages.  None are currently defined.

From Server to Client

   Data from the remote server is sent to the client as a stream of
   characters.  Normal data is simply sent to the client's display, but
   may be processed before actual display (tabs expanded, etc.).

   The server can imbed single-byte control messages in the data stream
   by inserting the control byte in the stream of data and pointing the
   TCP "urgent-data" pointer at the control byte.  When a TCP urgent-
   data pointer is received by the client, data in the TCP stream up to
   the urgent byte is buffered for possible display after the control
   byte is handled, and the control byte pointed to is received and
   interpreted as follows:

02   A control byte of hex 02 causes the client to discard all buffered
     data received from the server that has not yet been written to the
     client user's screen.

10   A control byte of hex 10 commands the client to switch to "raw"
     mode, where the START and STOP characters are no longer handled by
     the client, but are instead treated as plain data.

20   A control byte of hex 20 commands the client to resume interception
     and local processing of START and STOP flow control characters.

80   The client responds by sending the current window size as above.

   All other values of the urgent-data control byte are ignored.  In all
   cases, the byte pointed to by the urgent data pointer is NOT written
   to the client user's display.

Connection Closure

   When the TCP connection closes in either direction, the client or
   server process which notices the close should perform an orderly
   shut-down, restoring terminal modes and notifying the user or
   processes of the close before it closes the connection in the other
   direction.

Implementation Notes

   The client defines a client-escape character (customarily the tilde,
   "~"), which is handled specially only if it is the first character to
   be typed at the beginning of a line.  (The beginning of a line is
   defined to be the first character typed by the client user after a
   new-line [CR or LF] character, after a line-cancel character, after
   resumption of a suspended client session, or after initiation of the
   connection.)

   The client-escape character is not transmitted to the server until
   the character after it has been examined, and if that character is
   one of the defined client escape sequences, neither the client-escape
   nor the character following it are sent.  Otherwise, both the
   client-escape character and the character following it are sent to
   the server as ordinary user input.

   If the character following the client-escape character is the dot
   ".", or the client-defined end-of-file character (usually control-D),
   the connection is closed.  This is normally treated by the server as
   a disconnection, rather than an orderly logout.

   Other characters (client-defined, usually control-Z and control-Y)
   are used to temporarily suspend the rlogin client when the host has
   that ability.  One character suspends both remote input and output;
   the other suspends remote input but allows remote output to continue
   to be directed to the local client's terminal.

   Most client implementations have invocation switches that can defeat
   normal output processing on the client system, and which can force
   the client to remain in raw mode despite switching notification from
   the server.

A Cautionary Tale [2]

   The rlogin protocol (as commonly implemented) allows a user to set up
   a class of trusted users and/or hosts which will be allowed to log on
   as himself without the entry of a password.  While extremely
   convenient, this represents a weakening of security that has been
   successfully exploited in previous attacks on the internet.  If one
   wishes to use the password-bypass facilities of the rlogin service,
   it is essential to realize the compromises that may be possible
   thereby.

   Bypassing password authentication from trusted hosts opens ALL the
   systems so configured when just one is compromised.  Just as using
   the same password for all systems to which you have access lets a
   villain in everywhere you have access, allowing passwordless login
   among all your systems gives a marauder a wide playing field once he
   has entered any of your systems.  One compromise that many feel
   achieves a workable balance between convenience and security is to
   allow password bypass from only ONE workstation to the other systems
   you use, and NOT allow it between those systems.  With this measure,
   you may have reduced exposure to a workable minimum.

   The trusted host specification is ordinarily one of a host name.  It
   is possible, by compromise of your organization's domain name server,
   or compromise of your network itself, for a villain to make an
   untrusted host masquerade as a trusted system.  There is little that
   a user can do about this form of attack.  Luckily, so far such
   attacks have been rare, and often cause enough disruption of a
   network that attempts are quickly noticed.

   When the file containing a user's list of trusted logins is
   inadvertently left writeable by other users, untrustworthy additions
   may be made to it.

   Secure authentication extensions to the rlogin protocol (Kerberos,
   et al) can greatly reduce the possibility of compromise whilst still
   allowing the convenience of bypassing password entry.  As these
   become more widely deployed in the internet community, the hazards
   of rlogin will decrease.

References

      [1] Stevens, W., "UNIX Network Programming", ISBN 0-13-949876-1.

      [2] Garfinkel & Spafford, "Practical Unix Security",
          ISBN 0-937175-72-2.

Security Considerations

   See the "A Cautionary Tale" section above.

Author's Address

   Brian Kantor
   University of California at San Diego
   Network Operations C-024
   La Jolla, CA 92093-0214

   Phone: (619) 534-6865

   EMail: brian@UCSD.EDU