Syslog Working Group F. Miao
Internet-Draft M. Yuzhi
Intended status: Standards Track Huawei Technologies
Expires: November 13, 2007 May 12, 2007
TLS Transport Mapping for Syslog
draft-ietf-syslog-transport-tls-10.txt
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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 [2]
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" passes syslog messages to a specific transport
protocol.
o A "receiver" takes syslog messages from a specific transport
protocol.
o A "relay" forwards messages, accepting messages from originators
or other relays, and sending them to collectors or other relays.
o A "collector" gathers syslog content for further analysis.
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 the
receiver may modify an in-transit syslog message from the sender/
relay and then forward the message to the 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 contents 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 one or more
syslog message from a series of messages, replay a message, or
alter the delivery sequence. The 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 checking to counter modifications to a message on a hop-
by-hop basis;
o Server or mutual authentication to counter masquerade.
Note: This secure transport (i.e. TLS) only secures syslog in a hop-
by-hop manner, the threat of end-to-end message stream modification
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 certificates [3] to authenticate peers. If a client
authenticates a server it MUST validate the certificate.
Authentication in this specification means that the recipient of a
certificate must actually check the certificate rather than just
accept a 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 current practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by RFC3280
[3]. 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].
If the iPAddress subjectAltName is present in the certificate, it
must exactly match the IP address configured or resolved from the
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 any
other type subjectAltName for certificate verification, such as
ipAddress. If more than one identity of a given type is presented in
the certificate (e.g., more than one dNSName name), a match in any
one of the set is considered acceptable.
If the hostname does not match the identity in the certificate,
clients SHOULD log the error in some form or another (see next
paragraph), and SHOULD terminate the connection with a bad
certificate error. Clients MAY provide a configuration setting that
disables this check but MUST enable it by default.
The application developer must take some care to consider the case
when, for whatever reason, there is a problem with authenticating the
other end of the connection. Since this problem will prevent log
messages from being transmitted, each device having this problem
should use whatever means are available to inform the administrator
of the problem. This may include producing an error code on a
console, returning an error to a user (if there is one), or writing a
file to disk, being mindful that such writes should be rate limited
in the case of attacks.
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.
The subjectAltName may be the host name, IP address, MAC, device ID,
etc. SubjectAltName is not necessarily unique for different
certificates. For example, certificates for some types of printer
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might use the same 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.
4.2.3. Cryptographic Level
Syslog applications SHOULD 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 the
security requirement of the requested session to make sure that the
resumed session provides proper security.
4.3. Sending data
All syslog messages MUST be sent as TLS "application data". It is
possible that multiple syslog messages be contained in one TLS
record, or that a syslog message be 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.
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However, in order to establish a baseline for interoperability, this
specification requires that a receiver MUST be able to process
messages with a length up to and including 2048 octets. Receiver
SHOULD be able to process messages with lengths up to and including
8192 octets.
4.4. Closure
A TLS client MUST close the associated TLS connection if the
connection is not expected to deliver any syslog messages later. It
MUST send a TLS close_notify alert before closing the connection. A
client MAY choose to not 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 a 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 the
connection. The server MUST attempt to initiate an exchange of
close_notify alerts with the client before closing the connection.
Servers that 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 compliant with this specification
MUST support all three authentication modes for interoperability.
It is RECOMMENDED that mutual authentication 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 do 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.
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TLS authentication and the distribution of keys 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 authenticate the sender/
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 any mandatory cipher suite other than the one in the TLS
specification, and the cipher suite for TLS applies to this
specification for minimum interoperability purposes.
If there is an update to the TLS specification in the future, the
latest mandatory cipher suite in the update will apply to this
specification, too. The implementers 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
implementers 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
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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.
[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
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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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