draft-ietf-tcpm-urgent-data-00.txt   draft-ietf-tcpm-urgent-data-01.txt 
TCP Maintenance and Minor F. Gont TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH Extensions (tcpm) UTN/FRH
Internet-Draft A. Yourtchenko Internet-Draft A. Yourtchenko
Intended status: Standards Track Cisco Intended status: Standards Track Cisco
Expires: November 21, 2009 May 20, 2009 Expires: May 14, 2010 November 10, 2009
On the implementation of TCP urgent data On the implementation of the TCP urgent mechanism
draft-ietf-tcpm-urgent-data-00.txt draft-ietf-tcpm-urgent-data-01.txt
Abstract
This document analyzes how current TCP implementations process TCP
urgent indications, and how the behavior of some widely-deployed
middle-boxes affect how urgent indications are processed by end
systems. This document updates the relevant specifications such that
they accommodate current practice in processing TCP urgent
indications, provides advice to applications that make use of the
urgent mechanism, and raises awareness about the reliability of TCP
urgent indications in the current Internet.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 33 skipping to change at page 1, line 44
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on November 21, 2009. This Internet-Draft will expire on May 14, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents
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Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
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Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
This document analyzes how current TCP implementations process TCP described in the BSD License.
urgent indications, and how the behavior of some widely-deployed
middle-boxes affect how urgent indications are processed by end
systems. This document updates the relevant specifications such that
they accommodate current practice in processing TCP urgent
indications, and raises awareness about the reliability of TCP urgent
indications in the current Internet.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of TCP urgent data . . . . . . . . . . . . . . . 3 2. Specification of the TCP urgent mechanism . . . . . . . . . . 3
2.1. Semantics of urgent inications . . . . . . . . . . . . . . 3 2.1. Semantics of urgent inications . . . . . . . . . . . . . . 3
2.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 4 2.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 4
2.3. Allowed length of urgent data . . . . . . . . . . . . . . 4 2.3. Allowed length of urgent data . . . . . . . . . . . . . . 4
3. Current implementation practice of TCP urgent data . . . . . . 4 3. Current implementation practice of TCP urgent data . . . . . . 4
3.1. Semantics of urgent indications . . . . . . . . . . . . . 4 3.1. Semantics of urgent indications . . . . . . . . . . . . . 4
3.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 5 3.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 5
3.3. Allowed length of urgent data . . . . . . . . . . . . . . 5 3.3. Allowed length of urgent data . . . . . . . . . . . . . . 5
3.4. Interaction of middle-boxes with urgent data . . . . . . . 6 3.4. Interaction of middle-boxes with TCP urgent indications . 6
4. Updating RFC 1122 . . . . . . . . . . . . . . . . . . . . . . 6 4. Updating RFC 1122 . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5. Advice to new applications employing TCP . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. Advice to applications that make use of the urgent
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Survey of the processing of urgent data by some 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
popular implementations . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Survey of the processing of TCP urgent
indications by some popular TCP implementations . . . 9
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 9 A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 9 A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 9 A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 . . . . . . . 9 A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 . . . . . . . 10
A.6. Microsoft Windows 2000, Service Pack 4 . . . . . . . . . . 10 A.6. Microsoft Windows 2000, Service Pack 4 . . . . . . . . . . 10
A.7. Microsoft Windows 2008 . . . . . . . . . . . . . . . . . . 10 A.7. Microsoft Windows 2008 . . . . . . . . . . . . . . . . . . 11
A.8. Microsoft Windows 95 . . . . . . . . . . . . . . . . . . . 10 A.8. Microsoft Windows 95 . . . . . . . . . . . . . . . . . . . 11
Appendix B. Changes from previous versions of the draft (to Appendix B. Changes from previous versions of the draft (to
be removed by the RFC Editor before publishing be removed by the RFC Editor before publishing
this document as an RFC) . . . . . . . . . . . . . . 10 this document as an RFC) . . . . . . . . . . . . . . 11
B.1. Changes from draft-gont-tcpm-urgent-data-01 . . . . . . . 10 B.1. Changes from draft-ietf-tcpm-urgent-data-00 . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 B.2. Changes from draft-gont-tcpm-urgent-data-01 . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
TCP incorporates a "urgent mechanism" that allows the sending user to TCP incorporates an "urgent mechanism" that allows the sending user
stimulate the receiving user to accept some urgent data and to permit to stimulate the receiving user to accept some "urgent data" and to
the receiving TCP to indicate to the receiving user when all the permit the receiving TCP to indicate to the receiving user when all
currently known urgent data has been received by the user. This the currently known urgent data has been received by the user. This
mechanism permits a point in the data stream to be designated as the mechanism permits a point in the data stream to be designated as the
end of urgent information. Whenever this point is in advance of the end of urgent information. Whenever this point is in advance of the
receive sequence number (RCV.NXT) at the receiving TCP, that TCP must receive sequence number (RCV.NXT) at the receiving TCP, that TCP must
tell the user to go into "urgent mode"; when the receive sequence tell the user to go into "urgent mode"; when the receive sequence
number catches up to the urgent pointer, the TCP must tell user to go number catches up to the urgent pointer, the TCP must tell user to go
into "normal mode" [RFC0793]. into "normal mode" [RFC0793].
The URG control flag indicates that the "Urgent Pointer" field is The URG control flag indicates that the "Urgent Pointer" field is
meaningful and must be added to the segment sequence number to yield meaningful and must be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that there is the urgent pointer. The absence of this flag indicates that there is
no urgent data outstanding [RFC0793]. no urgent data outstanding [RFC0793].
This document analyzes how current TCP implementations process TCP This document analyzes how current TCP implementations process TCP
urgent indications, and how the behavior of some widely-deployed urgent indications, and how the behavior of some widely-deployed
middle-boxes affect the processing of urgent indications by hosts. middle-boxes affect the processing of urgent indications by hosts.
This document updates the relevant specifications such that they This document updates RFC 1122 [RFC1122] such that IT accommodates
accommodate current practice in processing TCP urgent indications, current practice in processing TCP urgent indications, provides
and also raises awareness about the reliability of TCP urgent advice to applications using urgent the urgent mechanism, and raises
indications in the current Internet. awareness about the reliability of TCP urgent indications in the
current Internet.
Section 2 describes what the current IETF secifications state with Section 2 describes what the current IETF secifications state with
respect to TCP urgent indications. Section 3 describes how current respect to TCP urgent indications. Section 3 describes how current
TCP implementations actually process TCP urgent indications. TCP implementations actually process TCP urgent indications.
Section 4 updates RFC 1122 [RFC1122] such that it accommodates Section 4 updates RFC 1122 [RFC1122] such that it accommodates
current practice in processing TCP urgent indications. current practice in processing TCP urgent indications. Section 5
provides advice to to new applications employing TCP, with respect to
the TCP urgent mechanism. Section 6 provides advice to existing
applications that use or rely on the the TCP urgent mechanism.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Specification of TCP urgent data 2. Specification of the TCP urgent mechanism
2.1. Semantics of urgent inications 2.1. Semantics of urgent inications
As discussed in Section 1, the TCP urgent mechanism permits a point As discussed in Section 1, the TCP urgent mechanism permits a point
in the data stream to be designated as the end of urgent information. in the data stream to be designated as the end of urgent information.
Whenever this point is in advance of the receive sequence number Whenever this point is in advance of the receive sequence number
(RCV.NXT) at the receiving TCP, that TCP must tell the user to go (RCV.NXT) at the receiving TCP, that TCP must tell the user to go
into "urgent mode"; when the receive sequence number catches up to into "urgent mode"; when the receive sequence number catches up to
the urgent pointer, the TCP must tell user to go into "normal mode". the urgent pointer, the TCP must tell user to go into "normal mode".
This means, for example, that data that were received as "normal This means, for example, that data that were received as "normal
data" might become "urgent data" if an urgent indication is received data" might become "urgent data" if an urgent indication is received
in some successive TCP segment before those data are consumed by the in some successive TCP segment before those data are consumed by the
TCP user. TCP user.
The TCP urgent mechanism is NOT a mechanism for sending "out-of-band" The TCP urgent mechanism is NOT a mechanism for sending "out-of-band"
data: the "urgent data" should be delivered "in-line" to the TCP data: the so-called "urgent data" should be delivered "in-line" to
user. the TCP user.
2.2. Semantics of the Urgent Pointer 2.2. Semantics of the Urgent Pointer
There is some ambiguity in RFC 793 [RFC0793] with respect to the There is some ambiguity in RFC 793 [RFC0793] with respect to the
semantics of the Urgent Pointer. Section 3.1 (page 17) of RFC 793 semantics of the Urgent Pointer. Section 3.1 (page 17) of RFC 793
[RFC0793] states that the Urgent Pointer "communicates the current [RFC0793] states that the Urgent Pointer "communicates the current
value of the urgent pointer as a positive offset from the sequence value of the urgent pointer as a positive offset from the sequence
number in this segment. The urgent pointer points to the sequence number in this segment. The urgent pointer points to the sequence
number of the octet following the urgent data. This field is only be number of the octet following the urgent data. This field is only be
interpreted in segments with the URG control bit set." However, interpreted in segments with the URG control bit set". However,
Section 3.9 (page 56) of RFC 793 [RFC0793] states, when describing Section 3.9 (page 56) of RFC 793 [RFC0793] states, when describing
the processing of the SEND call in the ESTABLISHED and CLOSE-WAIT the processing of the SEND call in the ESTABLISHED and CLOSE-WAIT
states, that "If the urgent flag is set, then SND.UP <- SND.NXT-1 and states, that "If the urgent flag is set, then SND.UP <- SND.NXT-1 and
set the urgent pointer in the outgoing segments". set the urgent pointer in the outgoing segments".
RFC 961 [RFC0961] clarified this ambiguity in RFC 793 stating that RFC 961 [RFC0961] clarified this ambiguity in RFC 793 stating that
"Page 17 is wrong. The urgent pointer points to the last octet of "Page 17 is wrong. The urgent pointer points to the last octet of
urgent data (not to the first octet of non-urgent data)". RFC 1122 urgent data (not to the first octet of non-urgent data)". RFC 1122
[RFC1122] formally updated RFC 793 by stating, in Section 4.2.2.4 [RFC1122] formally updated RFC 793 by stating, in Section 4.2.2.4
(page 84), that "the urgent pointer points to the sequence number of (page 84), that "the urgent pointer points to the sequence number of
skipping to change at page 5, line 4 skipping to change at page 6, line 8
3. Current implementation practice of TCP urgent data 3. Current implementation practice of TCP urgent data
3.1. Semantics of urgent indications 3.1. Semantics of urgent indications
As discussed in Section 1, the TCP urgent mechanism simply permits a As discussed in Section 1, the TCP urgent mechanism simply permits a
point in the data stream to be designated as the end of urgent point in the data stream to be designated as the end of urgent
information, but does NOT provide a mechanism for sending out of band information, but does NOT provide a mechanism for sending out of band
data. data.
Unfortunately, virtually all TCP implementations process TCP urgent Unfortunately, virtually all TCP implementations process TCP urgent
data differently. By default, the "last byte of urgent data" is data differently. By default, the last byte of #urgent data" is
delivered "out of band" to the application. That is, it is not delivered "out of band" to the application. That is, it is not
delivered as part of the normal data stream. For example, the "out delivered as part of the normal data stream. For example, the "out
of band" byte is read by an application when a recv(2) system call of band" byte is read by an application when a recv(2) system call
with the MSG_OOB flag set is issued. with the MSG_OOB flag set is issued.
Most implementations provide a socket option (SO_OOBINLINE) that Most implementations provide a socket option (SO_OOBINLINE) that
allows an application to override the default processing of urgent allows an application to override the (broken) default processing of
data, so that they are delivered "in band" to the application, thus urgent data, so that they are delivered "in band" to the application,
providing the semantics intended by the IETF specifications. thus providing the semantics intended by the IETF specifications.
3.2. Semantics of the Urgent Pointer 3.2. Semantics of the Urgent Pointer
All the popular implementations that the authors of this document All the popular implementations that the authors of this document
have been able to test interpret the semantics of the TCP Urgent have been able to test interpret the semantics of the TCP Urgent
Pointer as specified in Section 3.1 of RFC 793. This means that even Pointer as specified in Section 3.1 of RFC 793. This means that even
when RFC 1122 officially updated RFC 793 to clarify the ambiguity in when RFC 1122 officially updated RFC 793 to clarify the ambiguity in
the semantics of the Urgent Pointer, this clarification never the semantics of the Urgent Pointer, this clarification never
reflected into actual implementations (i.e., virtually all reflected into actual implementations (i.e., virtually all
implementations default to the semantics of the urgent pointer implementations default to the semantics of the urgent pointer
specified in Section 3.1 of RFC 793). specified in Section 3.1 of RFC 793).
Some operating systems provide a system-wide toggle to override this Some operating systems provide a system-wide toggle to override this
behavior, and interpret the semantics of the Urgent Pointer as behavior, and interpret the semantics of the Urgent Pointer as
clarified in RFC 1122. However, this system-wide toggle has been clarified in RFC 1122. However, this system-wide toggle has been
found to be inconsistent. For example, Linux provides a the sysctl found to be inconsistent. For example, Linux provides the sysctl
"tcp_stdurg" (e.g., net.ivp4.tcp_stdurg) that, when set, supposedly "tcp_stdurg" (i.e., net.ivp4.tcp_stdurg) that, when set, supposedly
changes the system behavior to interpret the semantics of the TCP changes the system behavior to interpret the semantics of the TCP
Urgent Pointer as described in RFC 1122. However, this sysctl Urgent Pointer as specified in RFC 1122. However, this sysctl
changes the semantics of the Urgent Pointer only for incoming changes the semantics of the Urgent Pointer only for incoming
segments, but not for outgoing segments. This means that if this segments, but not for outgoing segments. This means that if this
sysctl is set, an application might be unable to interoperate with sysctl is set, an application might be unable to interoperate with
itself. itself if both the TCP sender and the TCP receiver are running on the
same host.
3.3. Allowed length of urgent data 3.3. Allowed length of urgent data
While Section 4.2.2.4 (page 84) of RFC 1122 explicitly states that "A While Section 4.2.2.4 (page 84) of RFC 1122 explicitly states that "A
TCP MUST support a sequence of urgent data of any length", in TCP MUST support a sequence of urgent data of any length", in
practice all those implementations that interpret TCP urgent practice all those implementations that interpret TCP urgent
indications as a mechanism for sending out-of-band data keep a buffer indications as a mechanism for sending out-of-band data keep a buffer
of a single byte for storing the "last byte of urgent data". Thus, of a single byte for storing the "last byte of urgent data". Thus,
if successive indications of urgent data are received before the if successive indications of urgent data are received before the
application reads the pending "out of band" byte, that pending byte application reads the pending "out of band" byte, that pending byte
skipping to change at page 6, line 10 skipping to change at page 7, line 15
In order to avoid urgent data from being discarded, some In order to avoid urgent data from being discarded, some
implementations queue each of the received "urgent bytes", so that implementations queue each of the received "urgent bytes", so that
even if another urgent indication is received before the pending even if another urgent indication is received before the pending
urgent data are consumed by the application, those bytes do not need urgent data are consumed by the application, those bytes do not need
to be discarded. Some of these implementations have been known to to be discarded. Some of these implementations have been known to
fail to enforce any limits on the amount of urgent data that they fail to enforce any limits on the amount of urgent data that they
queue, thus resulting vulnerable to trivial resource exhaustion queue, thus resulting vulnerable to trivial resource exhaustion
attacks [CPNI-TCP]. attacks [CPNI-TCP].
3.4. Interaction of middle-boxes with urgent data It should be reinforced that the aforementioned implementations are
broken. The TCP urgent mechanism is not a mechanism for delivering
out-of-band data.
3.4. Interaction of middle-boxes with TCP urgent indications
As a result of the publication of Network Intrusion Detection (NIDs) As a result of the publication of Network Intrusion Detection (NIDs)
evasion techniques based on urgent data [phrack] , some middle-boxes evasion techniques based on TCP urgent indications [phrack], some
modify the TCP data stream such that urgent data is put "in band", middle-boxes clear the urgent indications by clearing the URG flag
that is, they are accessible by the read(2) or recv(2) calls without and setting the Urgent Pointer to zero. This causes the "urgent
the MSG_OOB flag. Examples of such middle-boxes are Cisco PIX data" to become "in line" (that is, accessible by the read(2) call or
firewall [Cisco-PIX]. This should discourage applications to depend the recv(2) call without the MSG_OOB flag) in the case of those TCP
on urgent data for their corect operation, as urgent data may not be implementations that implement the urgent mechanism as out-of-band
not reliable in the current Internet. data (as described in Section 3.1). Examples of such middle-boxes
are Cisco PIX firewall [Cisco-PIX]. This should discourage
applications to depend on urgent indications for their correct
operation, as urgent indications may not be not reliable in the
current Internet.
4. Updating RFC 1122 4. Updating RFC 1122
Firstly, considering that as long as both the TCP sender and the TCP Considering that as long as both the TCP sender and the TCP receiver
receiver implement the same semantics for the Urgent Pointer there is implement the same semantics for the Urgent Pointer there is no
no functional difference in having the Urgent Pointer point to "the functional difference in having the Urgent Pointer point to "the
sequence number of the octet following the urgent data" vs. "the last sequence number of the octet following the urgent data" vs. "the last
octet of urgent data", and since all known implementations interpret octet of urgent data", and since all known implementations interpret
the semantics of the Urgent Pointer as pointing to "the sequence the semantics of the Urgent Pointer as pointing to "the sequence
number of the octet following the urgent data", we propose that RFC number of the octet following the urgent data", hereby we update RFC
1122 [RFC1122] be updated such that "the urgent pointer points to the 1122 [RFC1122] such that "the urgent pointer points to the sequence
sequence number of the octet following the urgent data" (in segments number of the octet following the urgent data" (in segments with the
with the URG control bit set), thus accommodating virtually all URG control bit set), thus accommodating virtually all existing TCP
existing TCP implementations. implementations.
Secondly, we strongly encourage applications that employ Sockets API 5. Advice to new applications employing TCP
to set the SO_OOBINLINE socket option, such that urgent data is
delivered inline, as intended by the IETF specifications.
Furthermore, we discourage the use of the MSG_OOB flag in recv(2)
calls to retrieve the "urgent data".
Finally, considering the discussion in Section 3.4, we discourage As a result of the issues discussed in Section 3.4, new applications
applications to depend on the TCP urgent mechanism for correct SHOULD NOT employ the TCP urgent mechanism. However, TCP
operation, as urgent data may not be reliable in the current implementations MUST still include support for the urgent mechanism
Internet. such that existing applications can still use it.
5. Security Considerations 6. Advice to applications that make use of the urgent mechanism
Applications that employ the Sockets API MUST set the SO_OOBINLINE
socket option, such that "urgent data" are delivered inline, as
intended by the IETF specifications.
7. Security Considerations
Given that there are two different interpretations of the semantics Given that there are two different interpretations of the semantics
of the Urgent Pointer in current implementations, and that either of the Urgent Pointer in current implementations (e.g., depnding on
middle-boxes (such as packet scrubbers) or the end-systems themselves the value of the tcp_stdurg sysctl), and that middle-boxes (such as
could cause the urgent data to be processed "in band", there exists packet scrubbers) or the end-systems themselves could cause the
ambiguity in how TCP urgent data sent by a TCP will be processed by urgent data to be processed "in band", there exists ambiguity in how
the intended recipient. This might make it difficult for a Network "urgent data" sent by a TCP will be processed by the intended
Intrusion Detection System (NIDS) to track the application-layer data recipient. This might make it difficult for a Network Intrusion
Detection System (NIDS) to track the application-layer data
transferred to the destination system, and thus lead to false transferred to the destination system, and thus lead to false
negatives or false positives in the NIDS [CPNI-TCP]. negatives or false positives in the NIDS [CPNI-TCP].
Probably the best way to avoid the security implications of TCP Probably the best way to avoid the security implications of TCP
urgent data is to avoid having application protocols depend on the urgent data is to avoid having applications use the TCP urgent
use of TCP urgent data altogether. Packet scrubbers could probably mechanism altogether. Packet scrubbers could probably be configured
be configured to clear the URG bit, and set the Urgent Pointer to to clear the URG bit, and set the Urgent Pointer to zero. This would
zero. This would basically cause the urgent data to be put "in basically cause the urgent data to be put "in band". However, this
band". However, this might cause interoperability problems or might cause interoperability problems or undesired behavior in the
undesired behavior in the applications running on top of TCP. applications running on top of TCP.
6. IANA Considerations 8. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
7. Acknowledgements 9. Acknowledgements
The authors of this document would like to thank (in alphabetical The authors of this document would like to thank (in alphabetical
order) David Borman, Alfred Hoenes, Carlos Pignataro, Anantha order) David Borman, Alfred Hoenes, Carlos Pignataro, Anantha
Ramaiah, Joe Touch, and Dan Wing for providing valuable feedback on Ramaiah, Joe Touch, and Dan Wing for providing valuable feedback on
earlier versions of this document. earlier versions of this document.
Additionally, Fernando would like to thank David Borman and Joe Touch Additionally, Fernando would like to thank David Borman and Joe Touch
for a fruitful discussion about TCP urgent mode at IETF 73 for a fruitful discussion about TCP urgent mode at IETF 73
(Minneapolis). (Minneapolis).
8. References 10. References
8.1. Normative References 10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References 10.2. Informative References
[CPNI-TCP] [CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", (to be published) . Protocol (TCP)", (to be published) .
[Cisco-PIX] [Cisco-PIX]
Cisco PIX, "http://www.cisco.com/en/US/docs/security/asa/ Cisco PIX, "http://www.cisco.com/en/US/docs/security/asa/
asa70/command/reference/tz.html#wp1288756". asa70/command/reference/tz.html#wp1288756".
[FreeBSD] The FreeBSD project, "http://www.freebsd.org". [FreeBSD] The FreeBSD project, "http://www.freebsd.org".
skipping to change at page 8, line 44 skipping to change at page 10, line 13
[Windows95] [Windows95]
Microsoft Windows 95, Microsoft Windows 95,
"ftp://ftp.demon.co.uk/pub/mirrors/win95netfaq/ "ftp://ftp.demon.co.uk/pub/mirrors/win95netfaq/
faq-c.html". faq-c.html".
[phrack] Ko, Y., Ko, S., and M. Ko, "NIDS Evasion Method named [phrack] Ko, Y., Ko, S., and M. Ko, "NIDS Evasion Method named
"SeolMa"", Phrack Magazine, Volume 0x0b, Issue 0x39, Phile "SeolMa"", Phrack Magazine, Volume 0x0b, Issue 0x39, Phile
#0x03 of 0x12 http://www.phrack.org/ #0x03 of 0x12 http://www.phrack.org/
issues.html?issue=57&id=3#article, 2001. issues.html?issue=57&id=3#article, 2001.
Appendix A. Survey of the processing of urgent data by some popular Appendix A. Survey of the processing of TCP urgent indications by some
implementations popular TCP implementations
A.1. FreeBSD A.1. FreeBSD
FreeBSD [FreeBSD] interprets the semantics of the urgent pointer as FreeBSD [FreeBSD] interprets the semantics of the urgent pointer as
specified in RFC 793. It does not provide any sysctl to override specified in Section 4 of this document. It does not provide any
this behavior. However, it provides the SO_OOBINLINE that when set sysctl to override this behavior.
causes TCP urgent data to be put "in band". That is, it will be
accessible by the read(2) or recv(2) calls without the MSG_OOB flag. FreeBSD provides the SO_OOBINLINE socket option that, when set,
causes TCP "urgent data" to remain "in band". That is, it will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
FreeBSD supports only one byte of urgent data. That is, only the FreeBSD supports only one byte of urgent data. That is, only the
byte preceding the Urgent Pointer is considered as "urgent data". byte preceding the Urgent Pointer is considered as "urgent data".
A.2. Linux A.2. Linux
Linux [Linux] interprets the semantics of the urgent pointer as Linux [Linux] interprets the semantics of the urgent pointer as
specified in RFC 793. It provides the net.ipv4.tcp_stdurg sysctl to specified in Section 4 of this document. It provides the
override this behavior to interpret the Urgent Pointer as specified net.ipv4.tcp_stdurg sysctl to override this behavior to interpret the
by RFC 1122. However, this sysctl only affects the processing of Urgent Pointer as specified in RFC 1122 [RFC1122]. However, this
incoming segments (the Urgent Pointer in outgoing segments will still sysctl only affects the processing of incoming segments (the Urgent
be set as specified in RFC 793). Pointer in outgoing segments will still be set as specified in
Section 4 of this document).
Linux provides the SO_OOBINLINE socket option that, when set, causes
TCP "urgent data" to remain "in band". That is, it will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
Linux supports only one byte of urgent data. That is, only the byte Linux supports only one byte of urgent data. That is, only the byte
preceding the Urgent Pointer is considered as "urgent data". preceding the Urgent Pointer is considered as "urgent data".
A.3. NetBSD A.3. NetBSD
NetBSD [NetBSD] interprets the semantics of the urgent pointer as NetBSD [NetBSD] interprets the semantics of the urgent pointer as
specified in RFC 793. It does not provide any sysctl to override specified in Section 4 of this document. It does not provide any
this behavior. However, it provides the SO_OOBINLINE that when set sysctl to override this behavior.
causes TCP urgent data to be put "in band". That is, it will be
accessible by the read(2) or recv(2) calls without the MSG_OOB flag. NetBSD provides the SO_OOBINLINE socket option that, when set, causes
TCP "urgent data" to remain "in band". That is, they will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
NetBSD supports only one byte of urgent data. That is, only the byte NetBSD supports only one byte of urgent data. That is, only the byte
preceding the Urgent Pointer is considered as "urgent data". preceding the Urgent Pointer is considered as "urgent data".
A.4. OpenBSD A.4. OpenBSD
OpenBSD [OpenBSD] interprets the semantics of the urgent pointer as OpenBSD [OpenBSD] interprets the semantics of the urgent pointer as
specified in RFC 793. It does not provide any sysctl to override specified in Section 4 of this document. It does not provide any
this behavior. However, it provides the SO_OOBINLINE that when set sysctl to override this behavior.
causes TCP urgent data to be put "in band". That is, it will be
OpenBSD provides the SO_OOBINLINE socket option that, when set,
causes TCP urgent data to remain "in band". That is, they will be
accessible by the read(2) or recv(2) calls without the MSG_OOB flag. accessible by the read(2) or recv(2) calls without the MSG_OOB flag.
OpenBSD supports only one byte of urgent data. That is, only the OpenBSD supports only one byte of urgent data. That is, only the
byte preceding the Urgent Pointer is considered as "urgent data". byte preceding the Urgent Pointer is considered as "urgent data".
A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7
Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 interpret the semantics Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 interpret the semantics
of the urgent pointer as specified in RFC 793. However, tests of the urgent pointer as specified in Section 4 of this document.
performed with an HTTP server running on Cisco IOS version
Tests performed with an HTTP server running on Cisco IOS version
12.2(18)SXF7 and 12.4(15)T7 suggest that urgent data is processed "in 12.2(18)SXF7 and 12.4(15)T7 suggest that urgent data is processed "in
band". That is, they are accessible together with "normal" data. band". That is, they are accessible together with "normal" data.
The TCP debugs on the Cisco IOS device do explicitly mention the The TCP debugs on the Cisco IOS device do explicitly mention the
presence of urgent data, and thus we infer that the behavior is presence of urgent data, and thus we infer that the behavior is
different depending on the application. different depending on the application.
A.6. Microsoft Windows 2000, Service Pack 4 A.6. Microsoft Windows 2000, Service Pack 4
Microsoft Windows 2000 [Windows2000] interprets the semantics of the Microsoft Windows 2000 [Windows2000] interprets the semantics of the
urgent pointer as specified in RFC 793. It provides the urgent pointer as specified in Section 4 of this document. It
TcpUseRFC1122UrgentPointer system-wide variable to override this provides the TcpUseRFC1122UrgentPointer system-wide variable to
behavior to interpret the Urgent Pointer as specified by RFC 1122. override this behavior, interpreting the Urgent Pointer as specified
However, the tests performed with the sample server application in RFC 1122 [RFC1122].
compiled using the cygwin environment, has shown that the default
behavior is to return the urgent data "in band". Tests performed with a sample server application compiled using the
cygwin environment, has shown that the default behavior is to return
the urgent data "in band".
A.7. Microsoft Windows 2008 A.7. Microsoft Windows 2008
Microsoft Windows 2008 interprets the semantics of the urgent pointer Microsoft Windows 2008 interprets the semantics of the urgent pointer
as specified in RFC 793. as specified in Section 4 of this document.
A.8. Microsoft Windows 95 A.8. Microsoft Windows 95
Microsoft Windows 95 interprets the semantics of the urgent pointer Microsoft Windows 95 interprets the semantics of the urgent pointer
as specified in RFC 793. It provides the BSDUrgent system-wide as specified in Section 4 of this document. It provides the
variable to override this behavior to interpret the Urgent Pointer as BSDUrgent system-wide variable to override this behavior,
specified by RFC 1122. Windows 95 supports only one byte of urgent interpreting the Urgent Pointer as specified in RFC 1122 [RFC1122].
data. That is, only the byte preceding the Urgent Pointer is Windows 95 supports only one byte of urgent data. That is, only the
considered as "urgent data". [Windows95] byte preceding the Urgent Pointer is considered as "urgent data".
[Windows95]
Appendix B. Changes from previous versions of the draft (to be removed Appendix B. Changes from previous versions of the draft (to be removed
by the RFC Editor before publishing this document as an by the RFC Editor before publishing this document as an
RFC) RFC)
B.1. Changes from draft-gont-tcpm-urgent-data-01 B.1. Changes from draft-ietf-tcpm-urgent-data-00
o Minor editorial changes.
o Incorporated the specific changes/advice stated in
http://www.ietf.org/mail-archive/web/tcpm/current/msg04548.html in
different sections (Section 4, Section 5, Section 6).
B.2. Changes from draft-gont-tcpm-urgent-data-01
o Draft resubmitted as draft-ietf, as a result of wg consensus on o Draft resubmitted as draft-ietf, as a result of wg consensus on
adopting the document as a tcpm wg item. adopting the document as a tcpm wg item.
Authors' Addresses Authors' Addresses
Fernando Gont Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644 Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706 Haedo, Provincia de Buenos Aires 1706
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