One document matched: draft-tsou-behave-natx4-log-reduction-02.txt
Differences from draft-tsou-behave-natx4-log-reduction-01.txt
Behavior Engineering for Hindrance T. Tsou, Ed.
Avoidance Huawei Technologies
Internet-Draft W. Li, Ed.
Intended status: Informational China Telecom
Expires: April 3, 2011 T. Taylor
Huawei Technologies
September 30, 2010
Port Management To Reduce Logging In Large-Scale NATs
draft-tsou-behave-natx4-log-reduction-02
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 April 3, 2011.
Copyright Notice
Copyright (c) 2010 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
(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. 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
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. A Suggested Solution . . . . . . . . . . . . . . . . . . . . . 3
3. Issues Of Traceability . . . . . . . . . . . . . . . . . . . . 5
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
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. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. 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, converted source IP
address, converted source port, protocol (TCP/UDP), etc. If the
connection is ICMP, a mapping entry may include source IP address,
converted source IP address, source identifier, converted source
identifier, etc.
For the purpose of troubleshooting, and also as required by
regulations, operators must keep logs of network NAT mapping entries
for a period of time, e.g. 6 months or one year
[I-D.ietf-intarea-shared-addressing-issues], so the NAT device needs
to generate logs for mapping entries in addition to other
information. A traditional method is to generate a log for each
mapping entry. When a connection expires, the mapping entry will be
deleted, and the corresponding log is stored locally or sent to a log
storage server.
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.
To reduce the cost of log storage, [I-D.nishitani-cgn] proposes to
fix the port range for each user/CPE, and only one log will be
generated for each user. But this would significantly reduce the
number of subscribers that could share a public IP address, as
discussed in [I-D.softwire-dual-stack-lite].
1.1. Requirements Language
This draft includes no requirements language.
2. A Suggested Solution
We propose 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.
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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. assign 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, ports 3501~3800,
to the user's resource pool. Again , just one log needs to be
generated for this port block. A log may contain the following
information: source IP address, converted source IP address, port
range, start time, end time, and some other necessary information.
There is an alternative way of allocating port blocks
[I-D.bajko-pripaddrassign]. The ports in a block do not have to be
contiguous. Due to security concerns, the port numbers could be
worked out using some random algorithm along with some initial
parameters. The randomization algorithm would be applied at the NAT
when it generates a new mapping. The algorithm and initial
parameters together are required to define a discrete subset of the
entire available port range (1024 to 65335), such that it is possible
to assign the complete range to different internal addresses as
required by varying the initial parameters. When generating a log
message, these parameters instead of the upper and lower bounds of a
port range would be included in the log.
Suppose now that a given internal address has been assigned more than
one block of ports. Regardless of whether the ports within a block
are specified by a simple range or a random algorithm, it is clear
that the overall preference for port randomization will be better
achieved by spreading out new port assignments over all of the blocks
assigned to that internal address. That means that the NAT should
first select one of the assigned blocks pseudo-randomly before
applying any randomization algorithm within the block. Further
discussion of this point occurs below as part of the discussion of
block deallocation.
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. The deallocation may be logged when it
occurs, although some would view such logging as redundant.
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
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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.
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
[I-D.ietf-intarea-shared-addressing-issues] 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
The whole point of this proposal is to allow the NAT to support
regulatory requirements for traceability of usage. So it is only
right to verify that these requirements can be met with the proposal
made in the previous section. There are two cases:
1. the investigating authority requires a complete record of the
activities of a target individual;
2. the investigating authority is concerned with tracking down the
user responsible for wrongful behaviour at a specific end point
(e.g., server, individual user, enterpise network).
Assuming that per-sesssion logging at the NAT is to be avoided in
general (the whole point of this draft), the first requirement can
only be met by identifying a target device in advance and enabling
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per-session logging for the internal address assigned to that device
(... and variations for multi-address situations). This case is
basically out of scope of the draft.
Section 11 of [I-D.ietf-intarea-shared-addressing-issues] 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 [I-D.ietf-intarea-shared-addressing-issues], 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
[I-D.ietf-intarea-shared-addressing-issues] 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 the investigating authority would be able to
collect only the list of all internal addresses mapped to a given
public address during the period of time concerned. This has an
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impact on privacy as well as traceability, depending on the follow-up
actions taken by the investigating authority to achieve its
objectives.
4. Other Considerations
[I-D.ietf-intarea-shared-addressing-issues] 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 security considerations applicable to NAT operation for various
protocols as documented in, for example, [RFC4787] and [RFC5382] also
apply to this proposal.
7. Acknowledgements
Mohamed Boucadair reviewed the initial document and provided useful
comments to improve it. Reinaldo Penno, Joel Jaegli, and Dan Wing
provided comments on the subsequent version that resulted in major
revisions.
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.
Detail 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 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 disconnect
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);
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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
[I-D.bajko-pripaddrassign]
Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
"Port Restricted IP Address Assignment (Work in
progress)", October 2009.
[I-D.ietf-intarea-shared-addressing-issues]
Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing (Work in
progress)", June 2010.
9.2. Informative References
[APACHE_LOG_CONFIG]
"http://httpd.apache.org/docs/2.1/mod/
mod_log_config.html".
[I-D.nishitani-cgn]
Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida,
"Common requirements for IP address sharing schemes (Work
in progress)", July 2010.
[I-D.softwire-dual-stack-lite]
Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4 Exhaustion
(Work in progress)", July 2010.
[POSTFIX_LOG_CONFIG]
"http://www.postfix.org/postconf.5.html".
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
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[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[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".
Authors' Addresses
Tina Tsou (editor)
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Phone:
Email: tena@huawei.com
Weibo Li (editor)
China Telecom
109, Zhongshan Ave. West, Tianhe District
Guangzhou 510630
P.R. China
Phone:
Email: mweiboli@gmail.com
Tom Taylor
Huawei Technologies
1852 Lorraine Ave.
Ottawa K1H 6Z8
Canada
Phone:
Email: tom111.taylor@bell.net
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