TLS Transport Mapping for Syslog
draft-ietf-syslog-transport-tls-12.txt

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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.

Table of Contents

1.  Introduction
    1.1.  Terminology
2.  Security Requirements for Syslog
3.  TLS to Secure Syslog
4.  Protocol Elements
    4.1.  Port Assignment
    4.2.  Initiation
        4.2.1.  Server Authentication and Basic Authorization
        4.2.2.  Client Authentication and Authorization
        4.2.3.  Certificate Fingerprints
        4.2.4.  Cryptographic Level
    4.3.  Sending data
        4.3.1.  Message Length
    4.4.  Closure
5.  Security Considerations
    5.1.  Authentication and Certificates
    5.2.  Cipher Suites
6.  IANA Considerations
    6.1.  Port Number
7.  Acknowledgments
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

This document describes the use of Transport Layer Security (TLS (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.) [I‑D.ietf‑tls‑rfc4346‑bis]) to provide a secure connection for the transport of syslog (Gerhards, R., “The syslog Protocol,” September 2007.) [I‑D.ietf‑syslog‑protocol] 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:

• An "originator" generates syslog content to be carried in a message.
• A "collector" gathers syslog content for further analysis.
• A "relay" forwards messages, accepting messages from originators or other relays, and sending them to collectors or other relays.
• A "transport sender" passes syslog messages to a specific transport protocol.
• A "transport receiver" takes syslog messages from a specific transport protocol.
• A "TLS client" is an application that can initiate a TLS connection by sending a Client Hello to a peer.
• 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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [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:

• Masquerade. An unauthorized transport sender may send messages to a legitimate transport receiver, or an unauthorized transport receiver tries to deceive a legitimate transport sender into sending syslog messages to it.
• 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.
• 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:

• 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:

• Denial of Service
• 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:

• Confidentiality to counter disclosure of the message contents;
• Integrity checking to counter modifications to a message on a hop-by-hop basis;
• 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.

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 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 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 (Cooper, D., “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” February 2008.) [I‑D.ietf‑pkix‑rfc3280bis] to authenticate peers. Authentication and authorization of the TLS server (transport receiver) is described in Section 4.2.1 (Server Authentication and Basic Authorization); authentication and authorization of the TLS client (transport sender) is described in Section 4.2.2 (Client Authentication and Authorization).

4.2.1.  Server Authentication and Basic Authorization

[Editor's Note: Text referencing RFC2818 was removed, it is not clear that it is all the relevant to SYSLOG.]

[Editor's Note: Option for certificate fingerprint added]

The transport sender (TLS client) has three different options for authenticating and authorizing the transport receiver (TLS server).

• Certification path validation and subject name verification: the client is configured with one or more trust anchors, and for each transport receiver, the name to be matched against the certificate for authorization. This option MUST be supported.
• Certificate fingerprints: For each transport receiver, the client is configured with a fingerprint of the server's certificate (which can be self-signed). This option MUST be supported.
• No server authentication/authorization: The client is configured to accept any certificate from the server. This option MAY be supported, but MUST NOT be enabled by default.

For certification path validation, client implementations MUST provide a mechanism for configuring one or more trust anchors, and MUST perform certification path validation as specified in [I‑D.ietf‑pkix‑rfc3280bis] (Cooper, D., “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” February 2008.).

For subject name verification, client implementations MUST support configuring, for each transport receiver, the name to be matched against the certificate. This name may be the host name or IP address used when opening the TCP connection. Implementations MUST support checking the hostname against a SubjectAltName field with a type of dNSName and SHOULD support checking hostname against the Common Name portion of the Subject Distinguished Name. Matching for certificate credentials is performed using the matching rules specified by [I‑D.ietf‑pkix‑rfc3280bis] (Cooper, D., “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” February 2008.). 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. Implementations MAY support other authorization processes matching against other fields in a certificate. Implementations also MAY support wildcards to match a range of values. For example, names to be matched against a certificate 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.

[Editor's Note: How useful is it to match against IP address? Do we expect deployments to issue certificates with IP addresses in them? Are IP addresses typically used in configuration? ]

To support certificate fingerprints, client implementations MUST support configuring, for each transport receiver, a fingerprint of the server certificate. See Section 4.2.3 (Certificate Fingerprints) for details.

If the certificate fails authorization or validity checks, clients SHOULD log the error in some form or another (see next paragraph), and SHOULD terminate the connection with a bad certificate error.

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 Authentication and Authorization

The transport receiver (TLS server) has three different options for authenticating and authorizing the transport sender (TLS client).

[Editor's note: At least one one authorization mechanism should be mandatory to implement to protect against the threat of masquerade listed above.]

• Certification path validation and subject name verification: the server is configured with one or more trust anchors, and a set of names (or wildcard patterns) to be matched against the certificates. This option MUST be supported.
• Certificate fingerprints: The server is configured with the fingerprints of client certificates. This option MUST be supported.
• No client authentication or authorization (or authorization based only on connection source IP address): This option MAY be supported, but MUST NOT be enabled by default.

Certification path validation and subject name verification work as described in Section Section 4.2.2 (Client Authentication and Authorization) above. A server may allow wild card names to match against the certificate which will result in a large number of clients with valid certificates to be authorized. Servers SHOULD provide a mechanism to log the identity and issuer of an accepted certificate to enable messages received from the client to be associated with an authenticated entity.

To support certificate fingerprints, server implementations MUST support configuring with a list of fingerprints of authorized certificates. See Section Section 4.2.3 (Certificate Fingerprints) for details.

[Editor's note: Removed section on using the same name for more that one host as it seems implementation specific. Removed section on discussion on who issues the certificate since it is out of scope.]

4.2.3.  Certificate Fingerprints

Both client and server implementations MUST make the certificate fingerprint available through a management interface. If no other certificate is configured, both client and server implementations MUST support generating a key pair and self-signed certificate.

The RECOMMENDED mechanism to generate a fingerprint is to take the SHA-1 hash of the certificate and convert the 20 byte result into 20 colon separated, hexadecimal bytes, each represented by 2 uppercase ASCII characters. When a fingerprint value is displayed or configured the algorithm used to generate the fingerprint SHOULD be indicated.

4.2.4.  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 (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) [RFC5234] 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 (Gerhards, R., “The syslog Protocol,” September 2007.) [I‑D.ietf‑syslog‑protocol] protocol.

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 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 and Certificates

In security sensitive environments, it is recommended that mutual authentication be deployed as that will prevent masquerade attacks, modification of the messages, and disclosure of the contents of the messages. Mutual authentication means the TLS client and server are provisioned with necessary trust anchors and must perform certificate validation.

[Editor's Note: added text on self-signed certificate validation and removed text on caching.]

The use of self-signed certificates with certificate fingerprint authorization lists provides more protection from masquerade and man-in-the-middle attacks than forgoing certificate validation and authorization.

[Editor's Note: It may be useful to suggest some operational practice that facilitates the deployment of self-signed certificates. For example, in order to initially populate an authorization list a client or server can display a certificate finger-print through a user interface to an administrator to be authorized and added to the authorization list.]

If the client or server choose to forgo certificate validation then the threats listed in Section 2 (Security Requirements for Syslog) may not be appropriately mitigated. Malicious entities may masquerade as the client or server, or they may insert themselves as a man-in-the middle of the conversation. This may result in modification and disclosure of data. While this may be acceptable in a security insensitive environment, it is recommended that server and client are configured with certificates and validate received certificate against provisioned trust anchors and authorization lists.

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 server authenticate the client to ensure that information received is authentic.

5.2.  Cipher Suites

This specification specifies the following cipher suite required for all compliant implementation for minimum interoperability purposes:

TLS_RSA_WITH_AES_128_CBC_SHA

Operators MAY choose to disable cipher suites for TLS that are regarded as too weak for the environment in which this specification is being used, especially older cipher suites. This MAY lead to a reduction of interoperability. It is likely that, in time, the cipher suite specified here will become subject to attack and the use of it will too be deprecated. This allows the future update of the specification to change mandatory-to-implement cipher requirement for interoperability. This also allows the TLS community to change its recommendations, and operators to follow those recommendations.

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 (Orman, H. and P. Hoffman, “Determining Strengths For Public Keys Used For Exchanging Symmetric Keys,” April 2004.) [RFC3766]. 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 and Pasi Eronen for his contributions to certificate authentication and authorization sections.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses

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