Internet Engineering Task Force                     Dominique Brezinski
INTERNET-DRAFT                                                 In-Q-Tel
Valid for six months                                       Tom Killalea
                                                          November 2001

            Guidelines for Evidence Collection and Archiving



Status of this Memo

   This document is an Internet-Draft and is in full conformance with
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Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   A "security incident" as defined in [RFC2828] is a security-relevant
   system event in which the system's security policy is disobeyed or
   otherwise breached.  The purpose of this document is to provide
   System Administrators with guidelines on the collection and archiving
   of evidence relevant to such a security incident.

   If evidence collection is done correctly, it is much more useful in
   apprehending the attacker, and stands a much greater chance of being
   admissible in the event of a prosecution.

Table of Contents

   1 Introduction
     1.1 Conventions Used in this Document

   2 Guiding Principles during Evidence Collection
     2.1 Order of Volatility
     2.2 Things to avoid
     2.3 Privacy Considerations
     2.4 Legal Considerations

   3 The Collection Procedure
     3.1 Transparency
     3.2 Collection Steps

   4 The Archiving Procedure
     4.1 Chain of Custody
     4.2 The Archive

   5 Tools you'll need

   6 References

   7 Acknowledgements

   8 Security Considerations

   9 Authors' Addresses

   10 Full Copyright Statement

1 Introduction

   A "security incident" as defined in [RFC2828] is a security-relevant
   system event in which the system's security policy is disobeyed or
   otherwise breached.  The purpose of this document is to provide
   System Administrators with guidelines on the collection and archiving
   of evidence relevant to such a security incident.  It's not our
   intention to insist that all System Administrators rigidly follow
   these guidelines every time they have a security incident.  Rather,
   we want to provide guidance on what they should do if they elect to
   collect and protect information relating to an intrusion.

   Such collection represents a considerable effort on the part of the
   System Administrator.  Great progress has been made in recent years
   to speed up the re-installation of the Operating System and to
   facilitate the reversion of a system to a 'known' state, thus making
   the 'easy option' even more attractive.  Meanwhile little has been
   done to provide easy ways of archiving evidence (the difficult
   option).  Further, increasing disk and memory capacities and the more
   widespread use of stealth and cover-your-tracks tactics by attackers
   have exacerbated the problem.

   If evidence collection is done correctly, it is much more useful in
   apprehending the attacker, and stands a much greater chance of being
   admissible in the event of a prosecution.

   You should use these guidelines as a basis for formulating your
   site's evidence collection procedures, and should incorporate your
   site's procedures into your Incident Handling documentation.  The
   guidelines in this document may not be appropriate under all
   jurisdictions.  Once you've formulated your site's evidence
   collection procedures, you should have law enforcement for your
   jurisdiction confirm that they're adequate.

1.1 Conventions Used in this Document

   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   and "MAY" in this document are to be interpreted as described in "Key
   words for use in RFCs to Indicate Requirement Levels" [RFC2119].

2 Guiding Principles during Evidence Collection

     - Adhere to your site's Security Policy and engage the appropriate
       Incident Handling and Law Enforcement personnel.

     - Capture as accurate a picture of the system as possible.

     - Keep detailed notes.  These should include dates and times.
       If possible generate an automatic transcript.
       (e.g., On Unix systems the 'script' program can be used, however
       the output file it generates should not be to media that is part
       of the evidence).  Notes and print-outs should be signed and

     - Note the difference between the system clock and UTC.  For
       each timestamp provided, indicate whether UTC or local time is

     - Be prepared to testify (perhaps years later) outlining all
       actions you took and at what times.  Detailed notes will be

     - Minimise changes to the data as you are collecting it.  This is
       not limited to content changes; you should avoid updating file or
       directory access times.

     - Remove external avenues for change.

     - When confronted with a choice between collection and analysis you
       should do collection first and analysis later.

     - Though it hardly needs stating, your procedures should be
       implementable.  As with any aspect of an incident response
       policy, procedures should be tested to ensure feasibility,
       particularly in a crisis.  If possible procedures should be
       automated for reasons of speed and accuracy.  Be methodical.

     - For each device, a methodical approach should be adopted which
       follows the guidelines laid down in your collection procedure.
       Speed will often be critical so where there are a number of
       devices requiring examination it may be appropriate to spread the
       work among your team to collect the evidence in parallel.
       However on a single given system collection should be done step
       by step.

     - Proceed from the volatile to the less volatile (see the Order of
       Volatility below).

     - You should make a bit-level copy of the system's media.  If you
       wish to do forensics analysis you should make a bit-level copy of
       your evidence copy for that purpose, as your analysis will almost
       certainly alter file access times.  Avoid doing forensics on the
       evidence copy.

2.1 Order of Volatility

   When collecting evidence you should proceed from the volatile to the
   less volatile.  Here is an example order of volatility for a typical

     - registers, cache

     - routing table, arp cache, process table, kernel statistics,

     - temporary file systems
     - disk

     - remote logging and monitoring data that is relevant to the system
       in question

     - physical configuration, network topology

     - archival media

2.2 Things to avoid

   It's all too easy to destroy evidence, however inadvertently.

     - Don't shutdown until you've completed evidence collection.  Much
       evidence may be lost and the attacker may have altered the
       startup/shutdown scripts/services to destroy evidence.

     - Don't trust the programs on the system.  Run your evidence
       gathering programs from appropriately protected media (see

     - Don't run programs that modify the access time of all files on
       the system (e.g., 'tar' or 'xcopy').

     - When removing external avenues for change note that simply
       disconnecting or filtering from the network may trigger "deadman
       switches" that detect when they're off the net and wipe evidence.

2.3 Privacy Considerations

     - Respect the privacy rules and guidelines of your company and
       your legal jurisdiction.  In particular, make sure no information
       collected along with the evidence you are searching for is
       available to anyone who would not normally have access to this
       information.  This includes access to log files (which may reveal
       patterns of user behaviour) as well as personal data files.

     - Do not intrude on people's privacy without strong justification.
       In particular, do not collect information from areas you do not
       normally have reason to access (such as personal file stores)
       unless you have sufficient indication that there is a real

     - Make sure you have the backing of your company's established
       procedures in taking the steps you do to collect evidence of an

2.4 Legal Considerations

   Computer evidence needs to be

      - Admissible:  It must conform to certain legal rules before it
        can be put before a court.

      - Authentic:  It must be possible to positively tie evidentiary
        material to the incident.

      - Complete:  It must tell the whole story and not just a
        particular perspective.

      - Reliable:  There must be nothing about how the evidence was
        collected and subsequently handled that casts doubt about its
        authenticity and veracity.

      - Believable:  It must be readily believable and understandable by
        a court.

3 The Collection Procedure

   Your collection procedures should be as detailed as possible.  As is
   the case with your overall Incident Handling procedures, they should
   be unambiguous, and should minimise the amount of decision-making
   needed during the collection process.

3.1 Transparency

   The methods used to collect evidence should be transparent and
   reproducible.  You should be prepared to reproduce precisely the
   methods you used, and have those methods tested by independent

3.2 Collection Steps

     - Where is the evidence ?  List what systems were involved in the
       incident and from which evidence will be collected.

     - Establish what is likely to be relevant and admissible.  When in
       doubt err on the side of collecting too much rather than not

     - For each system, obtain the relevant order of volatility.

     - Remove external avenues for change.

     - Following the order of volatility, collect the evidence with
       tools as discussed in Section 5.

     - Record the extent of the system's clock drift.

     - Question what else may be evidence as you work through the
       collection steps.

     - Document each step.

     - Don't forget the people involved.  Make notes of who was there
       and what were they doing, what they observed and how they

   Where feasible you should consider generating checksums and
   cryptographically signing the collected evidence, as this may make it
   easier to preserve a strong chain of evidence.  In doing so you must
   not alter the evidence.

4 The Archiving Procedure

   Evidence must be strictly secured.  In addition, the Chain of Custody
   needs to be clearly documented.

4.1 Chain of Custody

   You should be able to clearly describe how the evidence was found,
   how it was handled and everything that happened to it.

   The following need to be documented

     - Where, when and by whom was the evidence discovered and

     - Where, when and by whom was the evidence handled or examined.

     - Who had custody of the evidence, during what period.  How was it

     - When the evidence changed custody, when and how did the transfer
       occur (include shipping numbers, etc.).

4.2 Where and how to Archive
   If possible commonly used media (rather than some obscure storage
   media) should be used for archiving.

   Access to evidence should be extremely restricted, and should be
   clearly documented.  It should be possible to detect unauthorised

5 Tools you'll need

   You should have the programs you need to do evidence collection and
   forensics on read-only media (e.g., a CD).  You should have prepared
   such a set of tools for each of the Operating Systems that you manage
   in advance of having to use it.

   Your set of tools should include the following

     - a program for examining processes (e.g., 'ps').

     - programs for examining system state (e.g., 'showrev', 'ifconfig',
       'netstat', 'arp').

     - a program for doing bit-to-bit copies (e.g., 'dd'). 'dd', 'SafeBack').

     - programs for generating checksums and signatures (e.g.,
       'sha1sum', a checksum-enabled 'dd', 'SafeBack', 'pgp').

     - programs for generating core images and for examining them (e.g, (e.g.,
       'gcore', 'gdb').

     - scripts to automate evidence collection (e.g., The Coroner's
       Toolkit [FAR1999]).

   The programs in your set of tools should be statically linked, and
   should not require the use of any libraries other than those on the
   read-only media.  Even then, since modern rootkits may be installed
   through loadable kernel modules, you should consider that your tools
   might not be giving you a full picture of the system.

   You should be prepared to testify to the authenticity and reliability
   of the tools that you use.

6 References

     Farmer, D., and W Venema, "Computer Forensics Analysis Class
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, March 1997.

   [RFC2196] Fraser, B., "Site Security Handbook", RFC 2196, September

   [RFC2350] Brownlee, N., and  E. Guttman, "Expectations for Computer
     Security Incident Response", RFC 2350, June 1998.

   [RFC2828] Shirey, R., "Internet Security Glossary", RFC 2828, May

7 Acknowledgements

   We gratefully acknowledge the constructive comments received from
   Harald Alvestrand, Byron Collie, Barbara Y. Fraser, Gordon Lennox,
   Andrew Rees, Steve Romig and Floyd Short.

8 Security Considerations

   This entire document discusses security issues.

9 Authors' Addresses

   Dominique Brezinski
   1000 Wilson Blvd., Ste. 2900
   Arlington, VA 22209


   Tom Killalea
   Lisi/n na Bro/n
   Be/al A/tha na Muice
   Co. Mhaigh Eo

   Phone: +1 206 266-2196

10 Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   This document and the information contained herein is provided on an

This document expires May 5, 15, 2002.