1. RFC 7768
Independent Submission                                           T. Tsou
Request for Comments: 7768                              Philips Lighting
Category: Informational                                            W. Li
ISSN: 2070-1721                                            China Telecom
                                                               T. Taylor
                                                                J. Huang
                                                     Huawei Technologies
                                                            January 2016

         Port Management to Reduce Logging in Large-Scale NATs


   Various IPv6 transition strategies require the introduction of large-
   scale NATs (e.g., AFTR and 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  A Suggested Solution  . . . . . . . . . . . . . . . . . . . .   3
   3.  Issues Of Traceability  . . . . . . . . . . . . . . . . . . .   4
   4.  Other Considerations  . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Configure Server Software to Log Source Port . . . .   9
     A.1.  Apache  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.2.  Postfix . . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.3.  Sendmail  . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.4.  sshd  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     A.5.  Cyrus IMAP and UW IMAP  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   During the IPv6 transition period, some large-scale NAT devices may
   be introduced, e.g., Dual-Stack Lite (DS-Lite), Address Family
   Transition Router (AFTR), and 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.

   Due to legislation and law enforcement requirement, sometimes it is
   necessary to log these mappings for a period of time, such as 6
   months.  The mapping information is highly privacy sensitive; if
   possible, the information should be deleted as soon as possible.
   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

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   storage.  According to a test discussed in "Analysis of NAT64 Port
   Allocation Methods for Shared IPv4 Addresses" [ALLOC-METHODS], in a
   network with 20,000 subscribers, over a 60-day period, the raw log
   size can reach 42.5 TB if it is based on per-session log, while the
   log size will be 40.6 GB if it is based on port blocks.  Although
   compression technologies can be used before log storage, the log size
   is still big.

   [RFC6888], REQ-13 suggests "maximize port utilization" and REQ-14
   suggests "minimize log volume".  However, it is difficult to achieve
   both; there will be a trade-off 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.

   This is how the proposal works 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 5.

   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

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   the ports in deallocated blocks will not be used again until the
   blocks are reallocated.  However, the deallocation may be logged when
   it occurs adding 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 behavior 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 12 of [RFC6269] provides a good discussion of the
   traceability issue.  Complete traceability given the NAT-logging
   practices proposed in this document requires that the remote
   destination 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.

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   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 the
      configuration guide.

   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
   document, 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 document 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 13.2 of
   that document notes the effect on authentication procedures.  These
   issues must be resolved, but are not specific to the port allocation
   policy described in this document.

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5.  Security Considerations

   The discussion that 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
   pseudorandomization 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, and select pseudorandomly between the blocks available
   according to this criterion.  In this case, it is suggested 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
   pseudorandomization at port-selection time.  [RFC6056] provides a
   number of algorithms for achieving this pseudorandomization.  When

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   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

6.  Informative References

              Chen, G., Li, W., Tsou, T., Huang, J., Taylor, T., and J.
              Tremblay, "Analysis of NAT64 Port Allocation Methods for
              Shared IPv4 Addresses", Work in Progress, draft-ietf-
              sunset4-nat64-port-allocation-02, January 2016.

              The Apache Software Foundation, "Apache Module
              mod_log_config", <http://httpd.apache.org/docs/2.4/mod/

              "Postfix Configuration Parameters",

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <http://www.rfc-editor.org/info/rfc4787>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6431]  Boucadair, M., Levis, P., Bajko, G., Savolainen, T., and
              T. Tsou, "Huawei Port Range Configuration Options for PPP
              IP Control Protocol (IPCP)", RFC 6431,
              DOI 10.17487/RFC6431, November 2011,

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   [RFC6888]  Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
              A., and H. Ashida, "Common Requirements for Carrier-Grade
              NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
              April 2013, <http://www.rfc-editor.org/info/rfc6888>.

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

              "sshd_config OpenSSH SSH daemon configuration file",

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Appendix A.  Configure Server Software to Log Source Port

A.1.  Apache

   The user can use the LogFormat command to define a customized log
   format and use the 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
   has been available since Apache version 2.1.

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

A.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 has been available since Postfix version 2.5.

A.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 that should be in the .mc configuration macro

   Klog syslog

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

   This feature has been available since version 8.10.

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A.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, and 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
   in 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:

   /* 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

A.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.

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   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 draft version that resulted in
   major revisions.  Serafim Petsis provided encouragement to publish
   the document 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

Authors' Addresses

   Tina Tsou
   Philips Lighting
   3 Burlington Woods Dr #4t
   Burlington, MA  01803
   United States

   Email: tina.tsou@philips.com

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

   Email: mweiboli@gmail.com

   Tom Taylor
   Huawei Technologies

   Email: tom.taylor.stds@gmail.com

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

   Email: James.huang@huawei.com

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  1. RFC 7768