The Finger User Information Protocol

Network Working Group                                       D. Zimmerman
Request for Comments: 1288           Center for Discrete Mathematics and
Obsoletes: RFCs 1196, 1194, 742             Theoretical Computer Science
                                                           December 1991


                  The Finger User Information Protocol

Status of this Memo

   This memo defines a protocol for the exchange of user information.
   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo describes the Finger user information protocol.  This is a
   simple protocol which provides an interface to a remote user
   information program.

   Based on RFC 742, a description of the original Finger protocol, this
   memo attempts to clarify the expected communication between the two
   ends of a Finger connection.  It also tries not to invalidate the
   many existing implementations or add unnecessary restrictions to the
   original protocol definition.

   This edition corrects and clarifies RFC 1196.

   Table of Contents

   1.      Introduction  ...........................................   2
     1.1.    Intent  ...............................................   2
     1.2.    History  ..............................................   3
     1.3.    Requirements  .........................................   3
     1.4.    Updates  ..............................................   3
   2.      Use of the protocol  ....................................   4
     2.1.    Flow of events  .......................................   4
     2.2.    Data format  ..........................................   4
     2.3.    Query specifications  .................................   4
     2.4.    RUIP {Q2} behavior  ...................................   5
     2.5.    Expected RUIP response  ...............................   6
       2.5.1.  {C} query  ..........................................   6
       2.5.2.  {U}{C} query  .......................................   6
       2.5.3.  {U} ambiguity  ......................................   7
       2.5.4.  /W query token  .....................................   7
       2.5.5.  Vending machines  ...................................   7
   3.      Security  ...............................................   7
     3.1.    Implementation security  ..............................   7
     3.2.    RUIP security  ........................................   8
       3.2.1.  {Q2} refusal  .......................................   8
       3.2.2.  {C} refusal  ........................................   8
       3.2.3.  Atomic discharge  ...................................   8
       3.2.4.  User information files  .............................   9
       3.2.5.  Execution of user programs  .........................   9
       3.2.6.  {U} ambiguity  ......................................   9
       3.2.7.  Audit trails  .......................................   9
     3.3.    Client security  ......................................   9
   4.      Examples  ...............................................  10
     4.1.    Example with a null command line ({C})  ...............  10
     4.2.    Example with name specified ({U}{C})  .................  10
     4.3.    Example with ambiguous name specified ({U}{C})  .......  11
     4.4.    Example of query type {Q2} ({U}{H}{H}{C})  ............  11
   5.      Acknowledgments  ........................................  12
   6.      Security Considerations  ................................  12
   7.      Author's Address  .......................................  12

1.  Introduction

1.1.  Intent

   This memo describes the Finger user information protocol.  This is a
   simple protocol which provides an interface to a remote user
   information program (RUIP).

   Based on RFC 742, a description of the original Finger protocol, this
   memo attempts to clarify the expected communication between the two
   ends of a Finger connection.  It also tries not to invalidate the
   many current implementations or add unnecessary restrictions to the
   original protocol definition.

   The most prevalent implementations of Finger today seem to be
   primarily derived from the BSD UNIX work at the University of
   California, Berkeley.  Thus, this memo is based around the BSD
   version's behavior.

   However, the BSD version provides few options to tailor the Finger
   RUIP for a particular site's security policy, or to protect the user
   from dangerous data.  Furthermore, there are MANY potential security
   holes that implementors and administrators need to be aware of,
   particularly since the purpose of this protocol is to return
   information about a system's users, a sensitive issue at best.
   Therefore, this memo makes a number of important security comments
   and recommendations.

1.2.  History

   The FINGER program at SAIL, written by Les Earnest, was the
   inspiration for the NAME program on ITS.  Earl Killian at MIT and
   Brian Harvey at SAIL were jointly responsible for implementing the
   original protocol.

   Ken Harrenstien is the author of RFC 742, "Name/Finger", which this
   memo began life as.

1.3.  Requirements

   In this document, the words that are used to define the significance
   of each particular requirement are capitalized.  These words are:

   * "MUST"

      This word or the adjective "REQUIRED" means that the item is an
      absolute requirement of the specification.

   * "SHOULD"

      This word or the adjective "RECOMMENDED" means that there may
      exist valid reasons in particular circumstances to ignore this
      item, but the full implications should be understood and the case
      carefully weighed before choosing a different course.

   * "MAY"

      This word or the adjective "OPTIONAL" means that this item is
      truly optional.  One vendor may choose to include the item because
      a particular marketplace requires it or because it enhances the
      product, for example; another vendor may omit the same item.

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST requirements.  An implementation that satisfies all the
   MUST and all the SHOULD requirements is said to be "unconditionally
   compliant"; one that satisfies all the MUST requirements but not all
   the SHOULD requirements is said to be "conditionally compliant".

1.4.  Updates

   The differences of note between RFC 1196 and this memo are:

      o the optional /W switch in the Finger query specification was
        mistakenly placed at the end of the line.  The 4.3BSD Finger
        specifies it at the beginning, where this memo now also puts
        it.

      o the BNF in the Finger query specification was not clear on the
        treatment of blank space.  This memo is more exacting by
        including an explicit token for it.

      o The flow of events in a Finger connection is now better
        defined on the topic of the close of the Finger connection.

2.  Use of the protocol

2.1.  Flow of events

   Finger is based on the Transmission Control Protocol, using TCP port
   79 decimal (117 octal).  The local host opens a TCP connection to a
   remote host on the Finger port.  An RUIP becomes available on the
   remote end of the connection to process the request.  The local host
   sends the RUIP a one line query based upon the Finger query
   specification, and waits for the RUIP to respond.  The RUIP receives
   and processes the query, returns an answer, then initiates the close
   of the connection.  The local host receives the answer and the close
   signal, then proceeds closing its end of the connection.

2.2.  Data format

   Any data transferred MUST be in ASCII format, with no parity, and
   with lines ending in CRLF (ASCII 13 followed by ASCII 10).  This
   excludes other character formats such as EBCDIC, etc.  This also
   means that any characters between ASCII 128 and ASCII 255 should
   truly be international data, not 7-bit ASCII with the parity bit set.

2.3.  Query specifications

   An RUIP MUST accept the entire Finger query specification.

   The Finger query specification is defined:

        {Q1}    ::= [{W}|{W}{S}{U}]{C}

        {Q2}    ::= [{W}{S}][{U}]{H}{C}

        {U}     ::= username

        {H}     ::= @hostname | @hostname{H}

        {W}     ::= /W

        {S}     ::= <SP> | <SP>{S}

        {C}     ::= <CRLF>

   {H}, being recursive, means that there is no arbitrary limit on the
   number of @hostname tokens in the query.  In examples of the {Q2}
   request specification, the number of @hostname tokens is limited to
   two, simply for brevity.

   Be aware that {Q1} and {Q2} do not refer to a user typing "finger
   user@host" from an operating system prompt.  It refers to the line
   that an RUIP actually receives.  So, if a user types "finger
   user@host<CRLF>", the RUIP on the remote host receives "user<CRLF>",
   which corresponds to {Q1}.

   As with anything in the IP protocol suite, "be liberal in what you
   accept".

2.4.  RUIP {Q2} behavior

   A query of {Q2} is a request to forward a query to another RUIP.  An
   RUIP MUST either provide or actively refuse this forwarding service
   (see section 3.2.1).  If an RUIP provides this service, it MUST
   conform to the following behavior:

   Given that:

         Host <H1> opens a Finger connection <F1-2> to an RUIP on host
         <H2>.

         <H1> gives the <H2> RUIP a query <Q1-2> of type {Q2}
         (e.g., FOO@HOST1@HOST2).

   It should be derived that:

         Host <H3> is the right-most host in <Q1-2> (i.e., HOST2)

         Query <Q2-3> is the remainder of <Q1-2> after removing the
         right-most "@hostname" token in the query (i.e., FOO@HOST1)

   And so:

         The <H2> RUIP then must itself open a Finger connection <F2-3>
         to <H3>, using <Q2-3>.

         The <H2> RUIP must return any information received from <F2-3>
         to <H1> via <F1-2>.

         The <H2> RUIP must close <F1-2> in normal circumstances only
         when the <H3> RUIP closes <F2-3>.

2.5.  Expected RUIP response

   For the most part, the output of an RUIP doesn't follow a strict
   specification, since it is designed to be read by people instead of
   programs.  It should mainly strive to be informative.

   Output of ANY query is subject to the discussion in the security
   section.

2.5.1.  {C} query

   A query of {C} is a request for a list of all online users.  An RUIP
   MUST either answer or actively refuse (see section 3.2.2).  If it
   answers, then it MUST provide at least the user's full name.  The
   system administrator SHOULD be allowed to include other useful
   information (per section 3.2.3), such as:

      -    terminal location
      -    office location
      -    office phone number
      -    job name
      -    idle time (number of minutes since last typed input, or
           since last job activity).

2.5.2.  {U}{C} query

   A query of {U}{C} is a request for in-depth status of a specified
   user {U}.  If you really want to refuse this service, you probably
   don't want to be running Finger in the first place.

   An answer MUST include at least the full name of the user.  If the
   user is logged in, at least the same amount of information returned
   by {C} for that user MUST also be returned by {U}{C}.

   Since this is a query for information on a specific user, the system
   administrator SHOULD be allowed to choose to return additional useful
   information (per section 3.2.3), such as:

            -    office location
            -    office phone number
            -    home phone number
            -    status of login (not logged in, logout time, etc)
            -    user information file

   A user information file is a feature wherein a user may leave a short
   message that will be included in the response to Finger requests.
   (This is sometimes called a "plan" file.)  This is easily implemented
   by (for example) having the program look for a specially named text
   file in the user's home directory or some common area; the exact
   method is left to the implementor.  The system administrator SHOULD
   be allowed to specifically turn this feature on and off.  See section
   3.2.4 for caveats.

   There MAY be a way for the user to run a program in response to a
   Finger query.  If this feature exists, the system administrator
   SHOULD be allowed to specifically turn it on and off.  See section
   3.2.5 for caveats.

2.5.3.  {U} ambiguity

   Allowable "names" in the command line MUST include "user names" or
   "login names" as defined by the system.  If a name is ambiguous, the
   system administrator SHOULD be allowed to choose whether or not all
   possible derivations should be returned in some fashion (per section
   3.2.6).

2.5.4.  /W query token

   The token /W in the {Q1} or {Q2} query types SHOULD at best be
   interpreted at the last RUIP to signify a higher level of verbosity
   in the user information output, or at worst be ignored.

2.5.5.  Vending machines

   Vending machines SHOULD respond to a {C} request with a list of all
   items currently available for purchase and possible consumption.
   Vending machines SHOULD respond to a {U}{C} request with a detailed
   count or list of the particular product or product slot.  Vending
   machines should NEVER NEVER EVER eat money.

3.  Security

3.1.  Implementation security

   Sound implementation of Finger is of the utmost importance.
   Implementations should be tested against various forms of attack.  In
   particular, an RUIP SHOULD protect itself against malformed inputs.
   Vendors providing Finger with the operating system or network
   software should subject their implementations to penetration testing.

   Finger is one of the avenues for direct penetration, as the Morris
   worm pointed out quite vividly.  Like Telnet, FTP and SMTP, Finger is
   one of the protocols at the security perimeter of a host.
   Accordingly, the soundness of the implementation is paramount.  The
   implementation should receive just as much security scrutiny during
   design, implementation, and testing as Telnet, FTP, or SMTP.

3.2.  RUIP security

   Warning!!  Finger discloses information about users; moreover, such
   information may be considered sensitive.  Security administrators
   should make explicit decisions about whether to run Finger and what
   information should be provided in responses.  One existing
   implementation provides the time the user last logged in, the time he
   last read mail, whether unread mail was waiting for him, and who the
   most recent unread mail was from!  This makes it possible to track
   conversations in progress and see where someone's attention was
   focused.  Sites that are information-security conscious should not
   run Finger without an explicit understanding of how much information
   it is giving away.

3.2.1.  {Q2} refusal

   For individual site security concerns, the system administrator
   SHOULD be given an option to individually turn on or off RUIP
   processing of {Q2}.  If RUIP processing of {Q2} is turned off, the
   RUIP MUST return a service refusal message of some sort.  "Finger
   forwarding service denied" is adequate.  The purpose of this is to
   allow individual hosts to choose to not forward Finger requests, but
   if they do choose to, to do so consistently.

   Overall, there are few cases which would warrant processing of {Q2}
   at all, and they are far outweighed by the number of cases for
   refusing to process {Q2}.  In particular, be aware that if a machine
   is part of security perimeter (that is, it is a gateway from the
   outside world to some set of interior machines), then turning {Q2} on
   provides a path through that security perimeter.  Therefore, it is
   RECOMMENDED that the default of the {Q2} processing option be to
   refuse processing.  It certainly should not be enabled in gateway
   machines without careful consideration of the security implications.

3.2.2.  {C} refusal

   For individual site security concerns, the system administrator
   SHOULD be given an option to individually turn on or off RUIP
   acceptance of {C}.  If RUIP processing of {C} is turned off, the RUIP
   MUST return a service refusal message of some sort.  "Finger online
   user list denied" is adequate.  The purpose of this is to allow
   individual hosts to choose to not list the users currently online.

3.2.3.  Atomic discharge

   All implementations of Finger SHOULD allow individual system
   administrators to tailor what atoms of information are returned to a
   query.  For example:

      -    Administrator A should be allowed to specifically choose to
           return office location, office phone number, home phone
           number, and logged in/logout time.

      -    Administrator B should be allowed to specifically choose to
           return only office location, and office phone number.

      -    Administrator C should be allowed to specifically choose to
           return the minimum amount of required information, which is
           the person's full name.

3.2.4.  User information files

   Allowing an RUIP to return information out of a user-modifiable file
   should be seen as equivalent to allowing any information about your
   system to be freely distributed.  That is, it is potentially the same
   as turning on all specifiable options.  This information security
   breach can be done in a number of ways, some cleverly, others
   straightforwardly.  This should disturb the sleep of system
   administrators who wish to control the returned information.

3.2.5.  Execution of user programs

   Allowing an RUIP to run a user program in response to a Finger query
   is potentially dangerous.  BE CAREFUL!! -- the RUIP MUST NOT allow
   system security to be compromised by that program.  Implementing this
   feature may be more trouble than it is worth, since there are always
   bugs in operating systems, which could be exploited via this type of
   mechanism.

3.2.6.  {U} ambiguity

   Be aware that a malicious user's clever and/or persistent use of this
   feature can result in a list of most of the usernames on a system.
   Refusal of {U} ambiguity should be considered in the same vein as
   refusal of {C} requests (see section 3.2.2).

3.2.7.  Audit trails

   Implementations SHOULD allow system administrators to log Finger
   queries.

3.3.  Client security

   It is expected that there will normally be some client program that
   the user runs to query the initial RUIP.  By default, this program
   SHOULD filter any unprintable data, leaving only printable 7-bit
   characters (ASCII 32 through ASCII 126), tabs (ASCII 9), and CRLFs.

   This is to protect against people playing with terminal escape codes,
   changing other peoples' X window names, or committing other dastardly
   or confusing deeds.  Two separate user options SHOULD be considered
   to modify this behavior, so that users may choose to view
   international or control characters:

      -    one to allow all characters less than ASCII 32

      -    another to allow all characters greater than ASCII 126

   For environments that live and breathe international data, the system
   administrator SHOULD be given a mechanism to enable the latter option
   by default for all users on a particular system.  This can be done
   via a global environment variable or similar mechanism.

4.  Examples

4.1.  Example with a null command line ({C})

Site: elbereth.rutgers.edu
Command line: <CRLF>

Login       Name               TTY Idle    When            Office
rinehart Mark J. Rinehart      p0  1:11 Mon 12:15  019 Hill       x3166
greenfie Stephen J. Greenfiel  p1       Mon 15:46  542 Hill       x3074
rapatel  Rocky - Rakesh Patel  p3    4d Thu 00:58  028 Hill       x2287
pleasant Mel Pleasant          p4    3d Thu 21:32  019 Hill    908-932-
dphillip Dave Phillips         p5  021: Sun 18:24  265 Hill       x3792
dmk      David Katinsky        p6    2d Thu 14:11  028 Hill       x2492
cherniss Cary Cherniss         p7    5  Mon 15:42  127 Psychol    x2008
harnaga  Doug Harnaga          p8  2:01 Mon 10:15  055 Hill       x2351
brisco   Thomas P. Brisco      pe  2:09 Mon 13:37  h055           x2351
laidlaw  Angus Laidlaw         q0  1:55 Mon 11:26  E313C       648-5592
cje      Chris Jarocha-Ernst   q1    8  Mon 13:43  259 Hill       x2413

4.2.  Example with name specified ({U}{C})

Site: dimacs.rutgers.edu
Command line: pirmann<CRLF>
Login name: pirmann                     In real life: David Pirmann
Office: 016 Hill, x2443                 Home phone: 989-8482
Directory: /dimacs/u1/pirmann           Shell: /bin/tcsh
Last login Sat Jun 23 10:47 on ttyp0 from romulus.rutgers.
No unread mail
Project:
Plan:
                      Work Schedule, Summer 1990
                 Rutgers LCSR Operations, 908-932-2443

                        Monday       5pm - 12am
                        Tuesday      5pm - 12am
                        Wednesday    9am -  5pm
                        Thursday     9am -  5pm
                        Saturday     9am -  5pm

                           larf larf hoo hoo

4.3.  Example with ambiguous name specified ({U}{C})

Site: elbereth.rutgers.edu
Command line: ron<CRLF>
Login name: spinner                     In real life: Ron Spinner
Office: Ops Cubby,    x2443             Home phone: 463-7358
Directory: /u1/spinner                  Shell: /bin/tcsh
Last login Mon May  7 16:38 on ttyq7
Plan:
            ught i
          ca      n
        m           a
       '      ...     t
      I      .   .     i
             !         m
      !       !       e
       p       !pool
        l
         e
          H

Login name: surak                       In real life: Ron Surak
Office: 000 OMB Dou,    x9256
Directory: /u2/surak                    Shell: /bin/tcsh
Last login Fri Jul 27 09:55 on ttyq3
No Plan.

Login name: etter                       In real life: Ron Etter
Directory: /u2/etter                    Shell: /bin/tcsh
Never logged in.
No Plan.

4.4.  Example of query type {Q2} ({U}{H}{H}{C})

Site: dimacs.rutgers.edu
Command line: hedrick@math.rutgers.edu@pilot.njin.net<CRLF>
[pilot.njin.net]
[math.rutgers.edu]
Login name: hedrick                     In real life: Charles Hedrick
Office: 484 Hill, x3088

Directory: /math/u2/hedrick             Shell: /bin/tcsh
Last login Sun Jun 24 00:08 on ttyp1 from monster-gw.rutge
No unread mail
No Plan.

5.  Acknowledgments

   Thanks to everyone in the Internet Engineering Task Force for their
   comments.  Special thanks to Steve Crocker for his security
   recommendations and prose.

6.  Security Considerations

   Security issues are discussed in Section 3.

7.  Author's Address

   David Paul Zimmerman
   Center for Discrete Mathematics and
   Theoretical Computer Science (DIMACS)
   Rutgers University
   P.O. Box 1179
   Piscataway, NJ 08855-1179

   Phone: (908)932-4592

   EMail: dpz@dimacs.rutgers.edu