One document matched: draft-tsou-behave-natx4-log-reduction-03.txt
Differences from draft-tsou-behave-natx4-log-reduction-02.txt
Behavior Engineering for Hindrance Avoidance T. Tsou
Internet-Draft Huawei Technologies (USA)
Intended status: Informational W. Li
Expires: November 11, 2013 China Telecom
T. Taylor
Huawei Technologies
May 10, 2013
Port Management To Reduce Logging In Large-Scale NATs
draft-tsou-behave-natx4-log-reduction-03
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 November 11, 2013.
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Copyright Notice
Copyright (c) 2013 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. A Suggested Solution . . . . . . . . . . . . . . . . . . . . 3
3. Issues Of Traceability . . . . . . . . . . . . . . . . . . . 5
4. Other Considerations . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.3. Sendmail . . . . . . . . . . . . . . . . . . . . . . . . 7
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
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.
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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 [RFC6269], 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.
[I-D.behave-lsn-requirements], 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
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.
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.
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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
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,
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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
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, enterprise network).
Assuming that per-session logging at the NAT is to be avoided in
general (the whole point of this document), the first requirement can
only be met by identifying a target device in advance and enabling
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 this document.
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.
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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 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 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
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
[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.
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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 Jaeggli, and Dan Wing
provided comments on the subsequent version that resulted in major
revisions. James Huang performed the research contributed in the
Appendix. Serafim Petsis provided encouragement to publication after
a hiatus of two years.
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]:
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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);
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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
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", RFC 6269, June
2011.
9.2. Informative References
[APACHE_LOG_CONFIG]
The Apache Software Foundation,
"http://httpd.apache.org/docs/2.4/mod/mod_log_config.html
", 2013.
[I-D.bajko-pripaddrassign]
Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
"Port Restricted IP Address Assignment (Work in
progress)", April 2012.
[I-D.behave-lsn-requirements]
Perrault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for Carrier Grade NATs
(CGNs) (Work in progress)", December 2012.
[POSTFIX_LOG_CONFIG]
, "http://www.postfix.org/postconf.5.html ", 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.
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[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
Weibo Li
China Telecom
109, Zhongshan Ave. West, Tianhe District
Guangzhou 510630
P.R. China
Email: mweiboli@gmail.com
Tom Taylor
Huawei Technologies
Ottawa
Canada
Email: tom.taylor.stds@gmail.com
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