Port Management To Reduce Logging In Large-Scale NATs
draft-tsou-behave-natx4-log-reduction-04

Abstract

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 http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on January 03, 2014.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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

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 storage.

[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.

1.1. Requirements Language

This draft includes no requirements language.

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.

[I-D.bajko-pripaddrassign] 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.

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. As [RFC6269] points out, port randomization is just one security measure of several, and the loss of randomness incurred by the suggested procedure is justified by the increased utilization of port resources it allows.

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 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:

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

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 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 purpose.

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 [I-D.bajko-pripaddrassign]. 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 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 implementor.

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 content.

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 file. [POSTFIX_LOG_CONFIG]

This feature is available since Postfix version 2.5.

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 [SENDMAIL_LOG_CONFIG]:

LOCAL_CONFIG
Klog syslog

LOCAL_RULESETS
SLocal_check_mail
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 are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, 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:

...
/* 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. References

9.1. Normative References

9.2. Informative References

Authors' Addresses