Network Working Group D. Zimmerman

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

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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.

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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.

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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>

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{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>.

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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

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RFC 1288 Finger December 1991

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.

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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:

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RFC 1288 Finger December 1991

- 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.

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RFC 1288 Finger December 1991

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

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RFC 1288 Finger December 1991

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

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RFC 1288 Finger December 1991

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: [email protected]

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