Internet DRAFT - draft-tsou-behave-natx4-log-reduction
draft-tsou-behave-natx4-log-reduction
Behavior Engineering for Hindrance T. Tsou
Avoidance Huawei Technologies (USA)
Internet-Draft W. Li
Intended status: Informational China Telecom
Expires: May 5, 2016 T. Taylor
J. Huang
Huawei Technologies
November 2, 2015
Port Management To Reduce Logging In Large-Scale NATs
draft-tsou-behave-natx4-log-reduction-06
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/.
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 May 5, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. A Suggested Solution . . . . . . . . . . . . . . . . . . . . . 3
3. Issues Of Traceability . . . . . . . . . . . . . . . . . . . . 5
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . . . . . . 8
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.
Due to legislation and law enforcement requirement, sometimes it is
necessary to log these mapping for a period of time, such as 6
months. The mapping information is highly privacy sensitive, if
possible, it 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
storage. According to a test done by
[I-D.ietf-sunset4-nat64-port-allocation], in a network with 20, 000
subscribers, over 60 days period, the raw log size can reach 42.5TB
if it is based on per-session log, while the log size will be 40.6 GB
if it is based on port blocks. Alghough compression technologies can
be used before the log storage, the log size is still big.
[RFC6888], REQ-13 suggest "maximize port utilization", REQ-14 suggest
"minimize log volume". However, it is difficult to achieve both,
there will be 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
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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.
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
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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
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
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
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
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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 [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.
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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].
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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
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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. Informative References
[APACHE_LOG_CONFIG]
The Apache Software Foundation,
"http://httpd.apache.org/docs/2.4/mod/
mod_log_config.html", 2013.
[I-D.ietf-sunset4-nat64-port-allocation]
Chen, G., Li, W., Tsou, T., Huang, J., Taylor, T., and J.
Tremblay, "Analysis of NAT64 Port Allocation Methods for
Shared IPv4 Addresses",
draft-ietf-sunset4-nat64-port-allocation-01 (work in
progress), July 2015.
[POSTFIX_LOG_CONFIG]
"http://www.postfix.org/postconf.5.html", 2013.
[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,
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RFC 5382, DOI 10.17487/RFC5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<http://www.rfc-editor.org/info/rfc6056>.
[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,
<http://www.rfc-editor.org/info/rfc6269>.
[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,
<http://www.rfc-editor.org/info/rfc6431>.
[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>.
[SENDMAIL_LOG_CONFIG]
O'Reilly, "Sendmail, 3rd Edition, Page 798",
December 2002.
[SSHD_LOG_CONFIG]
"http://www.openbsd.org/cgi-bin/
man.cgi?query=sshd_config&sektion=5", April 2013.
Authors' Addresses
Tina Tsou
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1 408 330 4424
Email: tina.tsou.zouting@huawei.com
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Weibo Li
China Telecom
109, Zhongshan Ave. West, Tianhe District
Guangzhou 510630
P.R. China
Phone:
Email: mweiboli@gmail.com
Tom Taylor
Huawei Technologies
Ottawa
Canada
Phone:
Email: tom.taylor.stds@gmail.com
James Huang
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Phone:
Email: James.huang@huawei.com
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