Syslog Working Group F. Miao
Internet-Draft M. Yuzhi
Intended status: Standards Track Huawei Technologies
Expires: September 30, 2007 March 29, 2007
TLS Transport Mapping for Syslog
draft-ietf-syslog-transport-tls-07.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes the use of Transport Layer Security (TLS) to
provide a secure connection for the transport of Syslog messages.
This document describes the security threats to Syslog and how TLS
can be used to counter such threats.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Requirements for Syslog . . . . . . . . . . . . . . . 3
3. TLS to Secure Syslog . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Port Assignment . . . . . . . . . . . . . . . . . . . . . 5
4.2. Initiation . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Server Identity . . . . . . . . . . . . . . . . . . . 5
4.2.2. Client Identity . . . . . . . . . . . . . . . . . . . 6
4.2.3. Cryptographic Level . . . . . . . . . . . . . . . . . 7
4.3. Sending data . . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. Message Length . . . . . . . . . . . . . . . . . . . . 7
4.4. Closure . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5.1. Authentication . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
This document describes the use of Transport Layer Security (TLS [6])
to provide a secure connection for the transport of Syslog messages.
This document describes the security threats to Syslog and how TLS
can be used to counter such threats.
1.1. Terminology
The following definitions are used in this document:
o A sender is an application that can generate and send a Syslog [2]
message to another application.
o A receiver is an application that can receive a Syslog message.
o A relay is an application that can receive Syslog messages and
forward them to another receiver.
o A collector is an application that can receive messages but does
not relay them to any other receiver.
o A TLS client is an application that can initiate a TLS connection
by sending a Client Hello to a peer.
o A TLS server is an application that can receive a Client Hello
from a peer and reply with a Server Hello.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2. Security Requirements for Syslog
Syslog messages may pass several hops to arrive at the intended
receiver. Some intermediary networks may not be trusted by the
sender/relay, receiver, or all because the network is in a different
security domain or at a different security level from the receiver,
relay, or sender. Another security concern is that the sender/relay,
or receiver itself is in an insecure network.
There are several threats to be addressed for Syslog security. The
primary threats are:
o Masquerade. An unauthorized sender/relay may send messages to a
legitimate receiver, or an unauthorized receiver tries to deceive
a legitimate sender/relay into sending Syslog messages to it.
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o Modification. An attacker between the sender/relay and receiver
may modify an in-transit Syslog message from the sender/relay and
then forward the message to receiver. Such modification may make
the receiver misunderstand the message or cause the receiver to
behave in undesirable ways.
o Disclosure. An unauthorized entity may examine the content of the
Syslog messages, gaining unauthorized access to the information.
Some data in Syslog messages is sensitive and may be useful to an
attacker, such as the password of an authorized administrator or
user.
The secondary threat is:
o Message stream modification. An attacker may delete a Syslog
message from a series of messages, replay a message or alter the
delivery sequence. Syslog protocol itself is not based on message
order, but an event in a Syslog message may relate semantically to
events in other messages, so message ordering may be important to
understanding a sequence of events.
The following threats are deemed to be of lesser importance for
Syslog, and are not addressed in this document:
o Denial of Service
o Traffic Analysis
3. TLS to Secure Syslog
TLS can be used as a secure transport to counter all the primary
threats to Syslog described in section 2:
o Confidentiality to counter disclosure of the message contents;
o Integrity check to counter modifications to a message on a hop-to-
hop basis;
o Server or mutual authentication to counter masquerade.
Note: Secure transport (i.e. TLS) only secures syslog in a hop by
hop manner, end to end message stream modificationis threat is not
addressed in this document.
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4. Protocol Elements
4.1. Port Assignment
A Syslog sender/relay is always a TLS client and a Syslog receiver is
always a TLS server.
The TCP port NNN has been allocated as the default port for Syslog
over TLS, as defined in this document.
Note to RFC Editor: please replace NNN with the IANA-assigned value,
and remove this note.
4.2. Initiation
The sender/relay should initiate a connection to the receiver and
then send the TLS Client Hello to begin the TLS handshake. When the
TLS handshake has finished the sender/relay may then send the first
Syslog message.
TLS uses certificate [3] to authenticate the peers. If a client
authenticates a server it MUST validate the certificate.
Authentication in the specification means that it must actually check
the certificate other than just exchange the certificate.
4.2.1. Server Identity
A procedure similar to RFC2818 [7] is used to check the server's
identity in the certificate.
In general, the client is configured with the hostname or IP address
of the TLS server. As a consequence, the hostname or IP address for
the server is known to the client. If the hostname (or IP address)
is available, the client MUST check it against the server's identity
as presented in the server's Certificate message, in order to prevent
man-in-the-middle attacks.
If the client has external information as to the expected identity of
the server, the hostname (or IP address) check MAY be omitted. (For
instance, a client may be connecting to a machine whose address and
hostname are dynamic but the client knows the certificate that the
server will present.) In such cases, it is important to narrow the
scope of acceptable certificates as much as possible in order to
prevent man in the middle attacks. In special cases, it may be
appropriate for the client to simply ignore the server's identity,
but it must be understood that this leaves the connection open to
active attack.
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If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by RFC3280.
Names may contain the wildcard character * which is considered to
match any single domain name component or component fragment. E.g.,
*.a.com matches foo.a.com but not bar.foo.a.com. f*.com matches
foo.com but not bar.com. If the client is configured with IP address
of the server, the hostname should be got first through a trusted
mechanism such as a preconfigured hosts table or DNSSEC [8].
In some cases, the iPAddress subjectAltName presents in the
certificate, it must exactly match the IP address configured or
resolved from configured hostname through a trusted mechanism such as
a preconfigured hosts table or DNSSEC.
It is recommended to use dNSName in the certificate rather than other
type subjectAltName for certification verification, such as
ipAddress. If more than one identity of a given type presents in the
certificate (e.g., more than one dNSName name), a match in any one of
the set is considered acceptable.
If the hostname (or IP address) does not match the identity in the
certificate, the clients MUST terminate the connection with a bad
certificate error. Clients MAY log the error to an appropriate audit
log (if available) and SHOULD terminate the connection (with a bad
certificate error).
4.2.2. Client Identity
If a server authenticates a client and the client presents a
certificate to the server, the server MUST validate the certificate.
A client's certificate must be associated with a unique private key .
Private keys MUST NOT be shared between clients. The subjectAltName
may be host name, IP address, MAC, or device ID etc. SubjectAltName
is not necessarily unique for different certificate, for example,
certificates for some type printer shares same type code as
subjectAltName.
A client certificate may be issued by an operator when a device/
application is being provisioned or by a vendor when the device/
application is manufactured. This document does not define how the
client certificate is issued.
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4.2.3. Cryptographic Level
Syslog applications MUST be implemented in a manner that permits
administrators, as a matter of local policy, to select the
cryptographic level and authentication options they desire.
TLS permits the resumption of an earlier TLS session or the use of
another active session when a new session is requested, in order to
save the expense of another full TLS handshake. The security
parameters of the resumed session are reused for the requested
session. The security parameters SHOULD be checked against security
requirement of requested session to make sure the resumed session
provides proper security.
4.3. Sending data
All Syslog messages MUST be sent as TLS "application data". It is
possible that there are multiple Syslog messages in one TLS record,
or a Syslog message is transferred in multiple TLS records. The
application data is defined with the following ABNF [5] expression:
APPLICATION-DATA = 1*SYSLOG-FRAME
SYSLOG-FRAME = MSG-LEN SP SYSLOG-MSG
MSG-LEN = NONZERO-DIGIT *DIGIT
SP = %d32
NONZERO-DIGIT = %d49-57
DIGIT = %d48 / NONZERO-DIGIT
SYSLOG-MSG is defined in Syslog [2] protocol.
4.3.1. Message Length
The message length is the octet count of the SYSLOG-MSG in the
SYSLOG-FRAME. A receiver MUST use the message length to delimit a
Syslog message. There is no upper limit for a message length per se.
However, in order to establish a baseline for interoperability, the
specification requires that a receiver MUST be able to process
message with size up to and including 2048 octets. Receiver SHOULD
be able to process message with size up to and including 8192 octets.
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4.4. Closure
A TLS client MUST close the associated TLS connection if the
connection is not expected to deliver Syslog message later. It MUST
send a TLS close_notify alert before closing the connection. A
client MAY choose not to wait for the server's close_notify alert and
simply close the connection, thus generating an incomplete close on
the server side. Once the server gets close_notify from the client,
it MUST reply with a close_notify unless it becomes aware that the
connection has already been closed by the client (e.g., the closure
was indicated by TCP).
When no data is received from a connection for a long time (where the
application decides what "long" means), a server MAY close a
connection. The server MUST attempt to initiate an exchange of
close_notify alerts with the client before closing the connection.
Servers those are unprepared to receive any more data MAY close the
connection after sending the close_notify alert, thus generating an
incomplete close on the client side. When the client has received
the close_notify alert from the server and still has pending data to
send, it SHOULD send the pending data before sending the close_notify
alert.
5. Security Considerations
5.1. Authentication
TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and
total anonymity. An implementation of this specification MUST
support all three authentication modes for interoperability.
It is RECOMMENDED that mutual authentication should be deployed in
all cases as that will prevent masquerade attacks, modification of
the messages, and disclosure of the contents of the messages. Server
authentication does not prevent masquerade attacks but does prevent
modification and disclosure. Unauthenticated TLS sessions does not
address any of the threats as an unauthenticated TLS session is
susceptible to a man in the middle attack, deploying Syslog over TLS
with total anonymity is NOT RECOMMENDED.
TLS authentication and the establishment of secrets is based on
certificates and asymmetric cryptography. This makes TLS transport
more expensive than non-TLS plain transport. An attacker may
initialize many TLS connections to a receiver as a denial of service
attack. Since a receiver may act upon received data, for Syslog over
TLS, it is recommended that the receiver authenticates the sender/
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relay to ensure that information received is authentic.
5.2. Cipher Suites
TLS [6] specifies a mandatory cipher suite to enable minimum
interoperability for TLS implementation. This specification does not
specify a mandatory cipher suite other than the one in TLS
specification, and the one for TLS applies to this specification for
minimum interoperability purpose.
If there is update to TLS specification in the future, the latest
mandatory cipher suite in the update will apply to this
specification, too. The implementors and deployers should be aware
of the strengths of the public keys algorithm in the suite for
exchanging symmetric keys, which is elaborated in BCP86 [4]. The
implementors and deployers should also be aware of the latest TLS and
other IETF cryptography standards including BCP86.
6. IANA Considerations
6.1. Port Number
IANA is requested to assign a TCP port number in the range 1..1023 in
the http://www.iana.org/assignments/port-numbers registry which will
be the default port for Syslog over TLS, as defined in this document.
7. Acknowledgments
Authors appreciate Eric Rescorla, Rainer Gerhards, Tom Petch, Anton
Okmianski, Balazs Scheidler, Bert Wijnen, and Chris Lonvick for their
effort on issues resolving discussion. Authors would also like to
appreciate Balazs Scheidler, Tom Petch and other persons for their
input on security threats of Syslog. The authors would like to
acknowledge David Harrington for his detailed reviews of the content
and grammar of the document.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Gerhards, R., "The syslog Protocol",
draft-ietf-syslog-protocol-19 (work in progress), November 2006.
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[3] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3280, April 2002.
[4] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys
Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004.
[5] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[6] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.1", RFC 4346, April 2006.
8.2. Informative References
[7] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[8] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
Authors' Addresses
Miao Fuyou
Huawei Technologies
No. 3, Xinxi Rd
Shangdi Information Industry Base
Haidian District, Beijing 100085
P. R. China
Phone: +86 10 8288 2008
Email: miaofy@huawei.com
URI: www.huawei.com
Ma Yuzhi
Huawei Technologies
No. 3, Xinxi Rd
Shangdi Information Industry Base
Haidian District, Beijing 100085
P. R. China
Phone: +86 10 8288 2008
Email: myz@huawei.com
URI: www.huawei.com
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