Network Working Group F. Miao, Ed.
Request for Comments: 5425 Y. Ma, Ed.
Category: Standards Track Huawei Technologies
J. Salowey, Ed.
Cisco Systems, Inc.
March 2009 Transport Layer Security (TLS) Transport Mapping for Syslog
Status of This Memo
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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.
This document describes the use of Transport Layer Security (TLS
[RFC5246]) to provide a secure connection for the transport of syslog
[RFC5424] messages. This document describes the security threats to
syslog and how TLS can be used to counter such threats.
The following definitions are used in this document:
o An "originator" generates syslog content to be carried in a
o A "collector" gathers syslog content for further analysis.
o A "relay" forwards messages, accepting messages from originators
or other relays, and sending them to collectors or other relays.
o A "transport sender" passes syslog messages to a specific
o A "transport receiver" takes syslog messages from a specific
o A "TLS client" is an application that can initiate a TLS
connection by sending a Client Hello to a server.
o A "TLS server" is an application that can receive a Client Hello
from a client 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 [RFC2119].
2. Security Requirements for Syslog
Syslog messages may transit several hops to arrive at the intended
collector. Some intermediary networks may not be trusted by the
originator, relay, or receiver because the network is in a different
security domain or at a different security level from the originator,
relay, or collector. Another security concern is that the
originator, 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 transport sender may send messages to
a legitimate transport receiver, or an unauthorized transport
receiver may try to deceive a legitimate transport sender into
sending syslog messages to it.
o Modification. An attacker between the transport sender and the
transport receiver may modify an in-transit syslog message and
then forward the message to the transport receiver. Such
modification may make the transport receiver misunderstand the
message or cause it 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 messages from a series of messages, replay a message, or
alter the delivery sequence. The syslog protocol itself is not
based on message order. However, 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. Using TLS to Secure Syslog
TLS can be used as a secure transport to counter all the primary
threats to syslog described above:
o Confidentiality to counter disclosure of the message contents.
o Integrity-checking to counter modifications to a message on a hop-
o Server or mutual authentication to counter masquerade.
Note: This secure transport (i.e., TLS) only secures syslog transport
in a hop-by-hop manner, and is not concerned with the contents of
syslog messages. In particular, the authenticated identity of the
transport sender (e.g., subject name in the certificate) is not
necessarily related to the HOSTNAME field of the syslog message.
When authentication of syslog message origin is required, [SYS-SIGN]
can be used.
4. Protocol Elements
4.1. Port Assignment
A syslog transport sender is always a TLS client and a transport
receiver is always a TLS server.
The TCP port 6514 has been allocated as the default port for syslog
over TLS, as defined in this document.
The transport sender should initiate a connection to the transport
receiver and then send the TLS Client Hello to begin the TLS
handshake. When the TLS handshake has finished, the transport sender
MAY then send the first syslog message.
TLS typically uses certificates [RFC5280] to authenticate peers.
Implementations MUST support TLS 1.2 [RFC5246] and are REQUIRED to
support the mandatory to implement cipher suite, which is
TLS_RSA_WITH_AES_128_CBC_SHA. This document is assumed to apply to
future versions of TLS, in which case the mandatory to implement
cipher suite for the implemented version MUST be supported.
4.2.1. Certificate-Based Authentication
Both syslog transport sender (TLS client) and syslog transport
receiver (TLS server) MUST implement certificate-based
authentication. This consists of validating the certificate and
verifying that the peer has the corresponding private key. The
latter part is performed by TLS. To ensure interoperability between
clients and servers, the following methods for certificate validation
SHALL be implemented:
o Certification path validation: The TLS peer is configured with one
or more trust anchors (typically root CA (certification authority)
certificates), which allow it to verify a binding between the
subject name and the public key. Additional policy controls
needed for authorizing the syslog transport sender and receiver
(i.e., verifying that the subject name represents an authorized
party) are described in Section 5. Certification path validation
is performed as defined in [RFC5280]. This method is useful where
there is a Public Key Infrastructure (PKI) deployment.
o End-entity certificate matching: The transport sender or receiver
is configured with information necessary to identify the valid
end-entity certificates of its authorized peers. The end-entity
certificates can be self-signed, and no certification path
validation is needed. Implementations MUST support certificate
fingerprints in Section 4.2.2 and MAY allow other formats for
end-entity certificates such as a DER-encoded certificate. This
method provides an alternative to a PKI that is simple to deploy
and still maintains a reasonable level of security.
Both transport receiver and transport sender implementations MUST
provide means to generate a key pair and self-signed certificate in
the case that a key pair and certificate are not available through
The transport receiver and transport sender SHOULD provide mechanisms
to record the end-entity certificate for the purpose of correlating
it with the sent or received data.
4.2.2. Certificate Fingerprints
Both client and server implementations MUST make the certificate
fingerprints for their certificate available through a management
interface. The labels for the algorithms are taken from the textual
names of the hash functions as defined in the IANA registry "Hash
Function Textual Names" allocated in [RFC4572].
The mechanism to generate a fingerprint is to take the hash of the
DER-encoded certificate using a cryptographically strong algorithm,
and convert the result into colon-separated, hexadecimal bytes, each
represented by 2 uppercase ASCII characters. When a fingerprint
value is displayed or configured, the fingerprint is prepended with
an ASCII label identifying the hash function followed by a colon.
Implementations MUST support SHA-1 as the hash algorithm and use the
ASCII label "sha-1" to identify the SHA-1 algorithm. The length of a
SHA-1 hash is 20 bytes and the length of the corresponding
fingerprint string is 65 characters. An example certificate
During validation the hash is extracted from the fingerprint and
compared against the hash calculated over the received certificate.
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 requirements 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 for multiple syslog messages to be contained in one TLS
record or for a single syslog message to be transferred in multiple
TLS records. The application data is defined with the following ABNF
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 the syslog protocol [RFC5424].
4.3.1. Message Length
The message length is the octet count of the SYSLOG-MSG in the
SYSLOG-FRAME. A transport 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, this specification requires that a transport
receiver MUST be able to process messages with a length up to and
including 2048 octets. Transport receivers SHOULD be able to process
messages with lengths up to and including 8192 octets.
A transport sender 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
transport sender (TLS client) MAY choose to not wait for the
transport receiver's close_notify alert and simply close the
connection, thus generating an incomplete close on the transport
receiver (TLS server) side. Once the transport receiver gets a
close_notify from the transport sender, it MUST reply with a
close_notify unless it becomes aware that the connection has already
been closed by the transport sender (e.g., the closure was indicated
When no data is received from a connection for a long time (where the
application decides what "long" means), a transport receiver MAY
close the connection. The transport receiver (TLS server) MUST
attempt to initiate an exchange of close_notify alerts with the
transport sender before closing the connection. Transport receivers
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 transport sender side.
5. Security Policies
Different environments have different security requirements and
therefore would deploy different security policies. This section
discusses some of the security policies that may be implemented by
syslog transport receivers and syslog transport senders. The
security policies describe the requirements for authentication and
authorization. The list of policies in this section is not
exhaustive and other policies MAY be implemented.
If the peer does not meet the requirements of the security policy,
the TLS handshake MUST be aborted with an appropriate TLS alert.
5.1. End-Entity Certificate Based Authorization
In the simplest case, the transport sender and receiver are
configured with information necessary to identity the valid
end-entity certificates of its authorized peers.
Implementations MUST support specifying the authorized peers using
certificate fingerprints, as described in Section 4.2.1 and
5.2. Subject Name Authorization
Implementations MUST support certification path validation [RFC5280].
In addition, they MUST support specifying the authorized peers using
locally configured host names and matching the name against the
certificate as follows.
o Implementations MUST support matching the locally configured host
name against a dNSName in the subjectAltName extension field and
SHOULD support checking the name against the common name portion
of the subject distinguished name.
o The '*' (ASCII 42) wildcard character is allowed in the dNSName of
the subjectAltName extension (and in common name, if used to store
the host name), but only as the left-most (least significant) DNS
label in that value. This wildcard matches any left-most DNS
label in the server name. That is, the subject *.example.com
matches the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations MUST
support wildcards in certificates as specified above, but MAY
provide a configuration option to disable them.
o Locally configured names MAY contain the wildcard character to
match a range of values. The types of wildcards supported MAY be
more flexible than those allowed in subject names, making it
possible to support various policies for different environments.
For example, a policy could allow for a trust-root-based
authorization where all credentials issued by a particular CA
trust root are authorized.
o If the locally configured name is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons, as
specified in Section 7 of [RFC5280].
o Implementations MAY support matching a locally configured IP
address against an iPAddress stored in the subjectAltName
extension. In this case, the locally configured IP address is
converted to an octet string as specified in [RFC5280], Section
220.127.116.11. A match occurs if this octet string is equal to the
value of iPAddress in the subjectAltName extension.
5.3. Unauthenticated Transport Sender
In some environments the authenticity of syslog data is not important
or is verifiable by other means, so transport receivers may accept
data from any transport sender. To achieve this, the transport
receiver can skip transport sender authentication (by not requesting
client authentication in TLS or by accepting any certificate). In
this case, the transport receiver is authenticated and authorized,
however this policy does not protect against the threat of transport
sender masquerade described in Section 2. The use of this policy is
generally NOT RECOMMENDED for this reason.
5.4. Unauthenticated Transport Receiver
In some environments the confidentiality of syslog data is not
important, so messages are sent to any transport receiver. To
achieve this, the transport sender can skip transport receiver
authentication (by accepting any certificate). While this policy
does authenticate and authorize the transport sender, it does not
protect against the threat of transport receiver masquerade described
in Section 2, leaving the data sent vulnerable to disclosure and
modification. The use of this policy is generally NOT RECOMMENDED
for this reason.
5.5. Unauthenticated Transport Receiver and Sender
In environments where security is not a concern at all, both the
transport receiver and transport sender can skip authentication (as
described in Sections 5.3 and 5.4). This policy does not protect
against any of the threats described in Section 2 and is therefore
6. Security Considerations
This section describes security considerations in addition to those
6.1. Authentication and Authorization Policies
Section 5 discusses various security policies that may be deployed.
The threats in Section 2 are mitigated only if both the transport
sender and transport receiver are properly authenticated and
authorized, as described in Sections 5.1 and 5.2. These are the
RECOMMENDED configurations for a default policy.
If the transport receiver does not authenticate the transport sender,
it may accept data from an attacker. Unless it has another way of
authenticating the source of the data, the data should not be
trusted. This is especially important if the syslog data is going to
be used to detect and react to security incidents. The transport
receiver may also increase its vulnerability to denial of service,
resource consumption, and other attacks if it does not authenticate
the transport sender. Because of the increased vulnerability to
attack, this type of configuration is NOT RECOMMENDED.
If the transport sender does not authenticate the syslog transport
receiver, then it may send data to an attacker. This may disclose
sensitive data within the log information that is useful to an
attacker, resulting in further compromises within the system. If a
transport sender is operated in this mode, the data sent SHOULD be
limited to data that is not valuable to an attacker. In practice
this is very difficult to achieve, so this type of configuration is
Forgoing authentication and authorization on both sides allows for
man-in-the-middle, masquerade, and other types of attacks that can
completely compromise integrity and confidentiality of the data.
This type of configuration is NOT RECOMMENDED.
6.2. Name Validation
The subject name authorization policy authorizes the subject in the
certificate against a locally configured name. It is generally not
appropriate to obtain this name through other means, such as DNS
lookup, since this introduces additional security vulnerabilities.
It should be noted that the syslog transport specified in this
document does not use application-layer acknowledgments. TCP uses
retransmissions to provide protection against some forms of data
loss. However, if the TCP connection (or TLS session) is broken for
some reason (or closed by the transport receiver), the syslog
transport sender cannot always know what messages were successfully
delivered to the syslog application at the other end.
7. IANA Considerations
7.1. Port Number
IANA assigned TCP port number 6514 in the "Registered Port Numbers"
range with the keyword "syslog-tls". This port will be the default
port for syslog over TLS, as defined in this document.
Authors appreciate Eric Rescorla, Rainer Gerhards, Tom Petch, Anton
Okmianski, Balazs Scheidler, Bert Wijnen, Martin Schuette, Chris
Lonvick, and members of the syslog working group for their effort on
issues resolving discussion. Authors would also like to thank 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 and Pasi Eronen for his contributions to certificate
authentication and authorization sections.
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March
9.2. Informative References
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[SYS-SIGN] Kelsey, J., "Signed syslog Messages", Work in Progress,
Fuyou Miao (editor)
No. 3, Xinxi Rd
Shangdi Information Industry Base
Haidian District, Beijing 100085
P. R. China
Phone: +86 10 8288 2008
Yuzhi Ma (editor)
No. 3, Xinxi Rd
Shangdi Information Industry Base
Haidian District, Beijing 100085
P. R. China
Phone: +86 10 8288 2008
Joseph Salowey (editor)
Cisco Systems, Inc.
2901 3rd. Ave
Seattle, WA 98121