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Port Management To Reduce Logging In Large-Scale NATs

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7768.
Authors Tina Tsou (Ting ZOU) , Weibo Li , Tom Taylor , Jing Huang
Last updated 2015-10-14 (Latest revision 2015-04-26)
RFC stream Independent Submission
IETF conflict review conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction, conflict-review-tsou-behave-natx4-log-reduction
Stream ISE state In ISE Review
Revised I-D Needed
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Document shepherd Eliot Lear
Shepherd write-up Show Last changed 2015-05-04
IESG IESG state Became RFC 7768 (Informational)
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Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
Behavior Engineering for Hindrance                               T. Tsou
Avoidance                                      Huawei Technologies (USA)
Internet-Draft                                                     W. Li
Intended status: Informational                             China Telecom
Expires: October 29, 2015                                      T. Taylor
                                                                J. Huang
                                                     Huawei Technologies
                                                          April 27, 2015

         Port Management To Reduce Logging In Large-Scale NATs


   Various IPv6 transition strategies require the introduction of large-
   scale NATs (e.g.  AFTR, NAT64) to share the limited supply of IPv4
   addresses available in the network until transition is complete.
   There has recently been debate over how to manage the sharing of
   ports between different subscribers sharing the same IPv4 address.
   One factor in the discussion is the operational requirement to log
   the assignment of transport addresses to subscribers.  It has been
   argued that dynamic assignment of individual ports between
   subscribers requires the generation of an excessive volume of logs.
   This document suggests a way to achieve dynamic port sharing while
   keeping log volumes low.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 29, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  A Suggested Solution . . . . . . . . . . . . . . . . . . . . .  3
   3.  Issues Of Traceability . . . . . . . . . . . . . . . . . . . .  4
   4.  Other Considerations . . . . . . . . . . . . . . . . . . . . .  5
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Appendix A: Configure Server Software to Log Source Port . . .  7
     8.1.  Apache . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.2.  Postfix  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.3.  Sendmail . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.4.  sshd . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.5.  Cyrus IMAP and UW IMAP . . . . . . . . . . . . . . . . . .  9
   9.  Informative References . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   During the IPv6 transition period, some large-scale NAT devices may
   be introduced, e.g.  DS-Lite AFTR, NAT64.  When a NAT device needs to
   set up a new connection for a given internal address behind the NAT,
   it needs to create a new mapping entry for the new connection, which
   will contain source IP address, source port or ICMP identifier,
   converted source IP address, converted source port, protocol (TCP/
   UDP), etc.

   For various reasons it is necessary to log these mappings.  Some high
   performance NAT devices may need to create a large amount of new
   sessions per second.  If logs are generated for each mapping entry,
   the log traffic could reach tens of megabytes per second or more,
   which would be a problem for log generation, transmission and

   [RFC6888], REQ-13, REQ-14, and REQ-15 deal explicitly with port
   allocation schemes.  However, it is recognized that these are
   conflicting requirements, requiring a tradeoff between the efficiency
   with which ports are used and the rate of generation of log records.

2.  A Suggested Solution

   This document proposes a solution that allows dynamic sharing of port
   ranges between users while minimizing the number of logs that have to
   be generated.  Briefly, ports are allocated to the user in blocks.
   Logs are generated only when blocks are allocated or deallocated.
   This provides the necessary traceability while reducing log
   generation by a factor equal to the block size, as compared with
   fully dynamic port allocation.

   Here is how the proposal would work in greater detail.  When the user
   sends out the first packet, a port resource pool is allocated for the
   user, e.g., assigning ports 2001~2300 of a public IP address to the
   user's resource pool.  Only one log should be generated for this port
   block.  When the NAT needs to set up a new mapping entry for the
   user, it can use a port in the user's resource pool and the
   corresponding public IP address.  If the user needs more port
   resources, the NAT can allocate another port block, e.g., ports
   3501~3800, to the user's resource pool.  Again, just one log needs to
   be generated for this port block.

   [RFC6431] takes this idea further by allocating non-contiguous sets
   of ports using a pseudorandom function.  Scattering the allocated
   ports in this way provides a modest barrier to port guessing attacks.
   The use of randomization is discussed further in Section 6.

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   Suppose now that a given internal address has been assigned more than
   one block of ports.  The individual sessions using ports within a
   port block will start and end at different times.  If no ports in
   some port block are used for some configurable time, the NAT can
   remove the port block from the resource pool allocated to a given
   internal address, and make it available for other users.  In theory,
   it is unnecessary to log deallocations of blocks of ports, because
   the ports in deallocated blocks will not be used again until the
   blocks are reallocated.  However, the deallocation may be logged when
   it occurs add robustness to troubleshooting or other procedures.

   The deallocation procedure presents a number of difficulties in
   practice.  The first problem is the choice of timeout value for the
   block.  If idle timers are applied for the individual mappings
   (sessions) within the block, and these conform to the recommendations
   for NAT behaviour for the protocol concerned, then the additional
   time that might be configured as a guard for the block as a whole
   need not be more than a few minutes.  The block timer in this case
   serves only as a slightly more conservative extension of the
   individual session idle timers.  If, instead, a single idle timer is
   used for the whole block, it must itself conform to the
   recommendations for the protocol with which that block of ports is
   associated.  For example, REQ-5 of [RFC5382] requires an idle timer
   expiry duration of at least 2 hours and 4 minutes for TCP.

   The next issue with port block deallocation is the conflict between
   the desire to randomize port allocation and the desire to make unused
   resources available to other internal addresses.  As mentioned above,
   ideally port selection will take place over the entire set of blocks
   allocated to the internal address.  However, taken to its fullest
   extent, such a policy will minimize the probability that all ports in
   any given block are idle long enough for it to be released.

   As an alternative, it is suggested that when choosing which block to
   select a port from, the NAT should omit from its range of choice the
   block that has been idle the longest, unless no ports are available
   in any of the other blocks.  The expression "block that has been idle
   the longest" designates the block in which the time since the last
   packet was observed in any of its sessions, in either direction, is
   earlier than the corresponding time in any of the other blocks
   assigned to that internal address.

3.  Issues Of Traceability

   Section 11 of [RFC6269] provides a good discussion of the
   traceability issue.  Complete traceability given the NAT logging
   practices proposed in this draft requires that the remote destination

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   record the source port of a request along with the source address
   (and presumably protocol, if not implicit).  In addition, the logs at
   each end must be timestamped, and the clocks must be synchronized
   within a certain degree of accuracy.  Here is one reason for the
   guard timing on block release, to increase the tolerable level of
   clock skew between the two ends.

   The ability to configure various server applications to record source
   ports has been investigated, with the following results:

   o  Source port recording can be configured in Apache, Postfix,
      sendmail and sshd.  Please refer to the appendix for configuration

   o  Source port recording is not supported by IIS, Cyrus IMAP and UW
      IMAP.  But it should not be too difficult to get Cyrus IMAP and UW
      IMAP to support it by modifying the source code.

   Where source port logging can be enabled, this memo strongly urges
   the operators to do so.  Similarly, intrusion detection systems
   should capture source port as well as source address of suspect

   In some cases [RFC6269], a server may not record the source port of a
   connection.  To allow traceability, the NAT device needs to record
   the destination IP address of a connection.  As [RFC6269] points out,
   this will provide an incomplete solution to the issue of traceability
   because multiple users of the same shared public IP address may
   access the service at the same time.  From the point of view of this
   draft, in such situations the game is lost, so to speak, and port
   allocation at the NAT might as well be completely dynamic.

   The final possibility to consider is where the NAT does not do per-
   session logging even given the possibility that the remote end is
   failing to capture source ports.  In that case, the port allocation
   policy proposed in this draft can be used.  The impact on
   traceability is that analysis of the logs would yield only the list
   of all internal addresses mapped to a given public address during the
   period of time concerned.  This has an impact on privacy as well as
   traceability, depending on the follow-up actions taken.

4.  Other Considerations

   [RFC6269] notes several issues introduced by the use of dynamic as
   opposed to static port assignment.  For example, Section 12.2 of that
   document notes the effect on authentication procedures.  These issues
   must be resolved, but are not specific to the port allocation policy

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   described in this document.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   The discussion which follows addresses an issue that is particularly
   relevant to the proposal made in this document.  The security
   considerations applicable to NAT operation for various protocols as
   documented in, for example, [RFC4787] and [RFC5382] also apply to
   this proposal.

   [RFC6056] summarizes the TCP port-guessing attack, by means of which
   an attacker can hijack one end of a TCP connection.  One mitigating
   measure is to make the source port number used for a TCP connection
   less predictable.  [RFC6056] provides various algorithms for this

   As Section 3.1 of that RFC notes: "...provided adequate algorithms
   are in use, the larger the range from which ephemeral ports are
   selected, the smaller the chances of an attacker are to guess the
   selected port number."  Conversely, the reduced range sizes proposed
   by the present document increase the attacker's chances of guessing
   correctly.  This result cannot be totally avoided.  However,
   mitigating measures to improve this situation can be taken both at
   port block assignment time and when selecting individual ports from
   the blocks that have been allocated to a given user.

   At assignment time, one possibility is to assign ports as non-
   contiguous sets of values as proposed in [RFC6431].  However, this
   approach creates a lot of complexity for operations, and the pseudo
   randomization can create uncertainty when the accuracy of logs is
   important to protect someone's life or liberty.

   Alternatively, the NAT can assign blocks of contiguous ports.
   However, at assignment time the NAT could attempt to randomize its
   choice of which of the available idle blocks it would assign to a
   given user.  This strategy has to be traded off against the
   desirability of minimizing the chance of conflict between what
   [RFC6056] calls "transport protocol instances" by assigning the most-
   idle block, as suggested in Section 2.  A compromise policy might be
   to assign blocks only if they have been idle for a certain amount of
   time whenever possible, and select pseudorandomly between the blocks
   available according to this criterion.  In this case it is suggested

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   that the time value used be greater than the guard timing mentioned
   in Section 2, and that no block should ever be reassigned until it
   has been idle at least for the duration given by the guard timer.

   While the block assignment strategy can provide some mitigation of
   the port guessing attack, the largest contribution will come from
   pseudo randomization at port selection time.  [RFC6056] provides a
   number of algoriths for achieving this pseudorandomization.  When the
   available ports are contained in blocks which are not in general
   consecutive, the algorithms clearly need some adaptation.  The task
   is complicated by the fact that the number of blocks allocated to the
   user may vary over time.  Adaptation is left as an exercise for the

7.  Acknowledgements

   Mohamed Boucadair reviewed the initial document and provided useful
   comments to improve it.  Reinaldo Penno, Joel Jaeggli, and Dan Wing
   provided comments on the subsequent version that resulted in major
   revisions.  Serafim Petsis provided encouragement to publication
   after a hiatus of two years.

   The authors are grateful to Dan Wing for his help in moving this
   document forward, and in particular for his helpful comments on its

8.  Appendix A: Configure Server Software to Log Source Port

8.1.  Apache

   The user can use LogFormat command to define a customized log format
   and use CustomLog command to apply that log format. "%a" and
   "%{remote}p" can be used in the format string to require logging the
   client's IP address and source port respectively.  This feature is
   available since Apache version 2.1.

   A detailed configuration guide can be found at [APACHE_LOG_CONFIG].

8.2.  Postfix

   In order to log the client source port, macro
   smtpd_client_port_logging should be set to "yes" in the configuration

   This feature is available since Postfix version 2.5.

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8.3.  Sendmail

   Sendmail has a macro ${client_port} storing the client port.  To log
   the source port, the user can define some check rules.  Here is an
   example which should be in the .mc configuration macro

   Klog syslog

   R $* $@ $(log Port_Stat $&{client_addr} $&{client_port} $)

   This feature is available since version 8.10.

8.4.  sshd

   SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) NAME
   sshd_config - OpenSSH SSH daemon configuration file LogLevel Gives
   the verbosity level that is used when logging messages from sshd(8).
   The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG,
   DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.  DEBUG and DEBUG1
   are equivalent.  DEBUG2 and DEBUG3 each specify higher levels of
   debugging output.  Logging with a DEBUG level violates the privacy of
   users and is not recommended.  SyslogFacility Gives the facility code
   that is used when logging messages from sshd(8).  The possible values
   LOCAL5, LOCAL6, LOCAL7.  The default is AUTH.

   sshd supports logging the client IP address and client port when a
   client starts connection since version 1.2.2, here is the source code
   in sshd.c:

   verbose("Connection from %.500s port %d", remote_ip, remote_port);

   sshd supports logging the client IP address when a client
   disconnects, from version 1.2.2 to version 5.0.  Since version 5.1
   sshd supports logging the client IP address and source port.  Here is
   the source code in sshd.c:

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   /* from version 1.2.2 to 5.0*/
   verbose("Closing connection to %.100s", remote_ip);

   /* since version 5.1*/
   verbose("Closing connection to %.500s port %d",
   remote_ip, remote_port);

   In order to log the source port, the LogLevel should be set to
   VERBOSE [SSHD_LOG_CONFIG] in the configuration file:

   LogLevel    VERBOSE

8.5.  Cyrus IMAP and UW IMAP

   Cyrus IMAP and UW IMAP do not support logging the source port for the
   time being.  Both software use syslog to create logs; it should not
   be too difficult to get it supported by adding some new code.

9.  Informative References

              The Apache Software Foundation,
              mod_log_config.html", 2013.

              "", 2013.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              January 2011.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269,
              June 2011.

   [RFC6431]  Boucadair, M., Levis, P., Bajko, G., Savolainen, T., and

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              T. Tsou, "Huawei Port Range Configuration Options for PPP
              IP Control Protocol (IPCP)", RFC 6431, November 2011.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

              O'Reilly, "Sendmail, 3rd Edition, Page 798",
              December 2002.

              man.cgi?query=sshd_config&sektion=5", April 2013.

Authors' Addresses

   Tina Tsou
   Huawei Technologies (USA)
   2330 Central Expressway
   Santa Clara, CA  95050

   Phone: +1 408 330 4424

   Weibo Li
   China Telecom
   109, Zhongshan Ave. West, Tianhe District
   Guangzhou  510630
   P.R. China


   Tom Taylor
   Huawei Technologies


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   James Huang
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China


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