Network Working Group B. Kantor
Request for Comments: 1258 Univ. of Calif San Diego
September 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. 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
described below) 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.
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
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.
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