Internet DRAFT - draft-gerhards-syslog-transport-ssh
draft-gerhards-syslog-transport-ssh
SYSLOG Working Group R. Gerhards
Internet-Draft Adiscon GmbH
Expires: January 20, 2007 July 19, 2006
SSH transport mapping for SYSLOG
draft-gerhards-syslog-transport-ssh-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
This document describes a method for invoking and running the SYSLOG
protocol within a Secure Shell (SSH) session as an SSH subsystem.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Requirements for SYSLOG . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . . 4
4. Starting SYSLOG over SSH . . . . . . . . . . . . . . . . . . . 4
5. Using SYSLOG over SSH . . . . . . . . . . . . . . . . . . . . 5
5.1. framing . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Exiting the SYSLOG Subsystem . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. Notes to the RFC Editor . . . . . . . . . . . . . . . . . . . 7
12. Normative . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
The SYSLOG protocol [9] is a text-based protocol used to convey event
information. SYSLOG is defined to be session-layer and transport
independent, allowing mappings to be defined for multiple session-
layer or transport protocols. This document defines how SYSLOG can
be used within a Secure Shell (SSH) session, using the SSH connection
protocol RFC4254 [8] over the SSH transport protocol RFC4253 [7].
This mapping will allow SYSLOG to be executed from a secure shell
session by a user or application. Throughout this document, the
terms "client" and "server" are used to refer to the two ends of the
SSH transport connection. The client actively opens the SSH
connection, and the server passively listens for the incoming SSH
connection. The terms "sender" and "receiver" are used to refer to
the two ends of the SYSLOG protocol session and are consistent with
the definitions in SYSLOG-protocol. When SYSLOG is run over SSH
using the mapping defined in this document, the client is always the
sender, and the server is always the receiver. This document
describes a layered architecture for SYSLOG. The goal of this
architecture is to separate message content from message transport
while enabling easy extensibility for each layer.
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 or the receiver or both because the network is in a different
security domain or at a different security level from the receiver or
sender. Another security concern is that the sender 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 may send messages to a
legitimate receiver, or an unauthorized receiver tries to deceive
a legitimate sender into sending SYSLOG messages to it.
o Modification. An attacker between the sender and receiver may
modify an in-transit SYSLOG message from the sender and then
forward the message to receiver. Such modification may make the
receiver misunderstands the message or causes 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.
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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. Conventions Used in This Document
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].
4. Starting SYSLOG over SSH
To run SYSLOG over SSH, the client will first establish an SSH
transport connection using the SSH transport protocol, and the client
and server will exchange keys for message integrity and encryption.
The client will then invoke the "ssh-userauth" service to
authenticate the user, as described in the SSH authentication
protocol RFC4252 [6]. Once the user has been successfully
authenticated, the client will invoke the "ssh-connection" service,
also known as the SSH connection protocol.
After the ssh-connection service is established, the client will open
a channel of type "session", which will result in an SSH session.
Once the SSH session has been established, the user (or application)
will invoke SYSLOG as an SSH subsystem called "syslog". Subsystem
support is a feature of SSH version 2 (SSHv2) and is not included in
SSHv1. Running SYSLOG as a SSH subsystem avoids the need for the
script to recognize shell prompts or skip over extraneous
information, such as a system message that is sent at shell start-up.
However, if a subsystem cannot be used, it should be possible for a
client to skip over any system messages that are sent at shell
start-up by searching for a SYSLOG <hello> element. Note that this
may not avoid problems if system messages are recieved later in the
session.
In order to allow SYSLOG traffic to be easily identified and filtered
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by firewalls and other network devices, SYSLOG servers MUST default
to providing access to the "syslog" SSH subsystem only when the SSH
session is established using the IANA-assigned TCP port <TBD>.
Servers SHOULD be configurable to allow access to the syslog SSH
subsystem over other ports.
A user (or application), could use the following command line to
invoke SYSLOG as an SSH subsystem on the IANA-assigned port:
[user@client]$ ssh -s server.example.org -p <TBD> syslog
Note that the -s option causes the command ("syslog") to be invoked
as an SSH subsystem.
5. Using SYSLOG over SSH
A SYSLOG over SSH session consists of the client sending a continous
stream of syslog frames to the receiver. The receiver does not
acknowledge frames.
5.1. framing
The SYSLOG frame has the following ABNF [2] definition:
SYSLOG-FRAME = HEADER SP SYSLOG-MSG TRAILER
HEADER = ENTITY SP FRAME-LEN
ENTITY = "MSG"
FRAME-LEN = NONZERO-DIGIT 0*DIGIT
SP = %d32
DIGIT = %d48 / NONZERO-DIGIT
NONZERO-DIGIT = %d49-57
TRAILER = CRLF
Figure 1
SYSLOG-MSG is defined in RFCXXXX [9].
[This text needs to be edited once the specific framing has been
selected. This eventually happens in a separate document.]
6. Exiting the SYSLOG Subsystem
Exiting SYSLOG is accomplished using the "CLOSE" operation verb on
the frame stream. If the server receivers the "CLOSE" operation, it
will return an "ACK" and terminate the connetion.
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The server MAY decide to terminate a session at its discretion. In
this case, the underlying SSH connection is terminated. No
notification other than the SSH error occurs to the server.
[This text needs to be edited once the specific framing has been
selected. This eventually happens in a separate document.]
7. Security Considerations
SYSLOG is used to convey potentially sensitive information, so the
ability to access this protocol should be limited to users and
systems that are authorized to view this information.
The identity of the server MUST be verified and authenticated by the
client according to local policy before password-based authentication
data or any configuration or state data is sent to or received from
the server. The identity of the client MUST also be verified and
authenticated by the server according to local policy to ensure that
the incoming client request is legitimate before any configuration or
state data is sent to or received from the client. Neither side
should establish a syslog over SSH connection with an unknown,
unexpected or incorrect identity on the opposite side.
SYSLOG messages may include sensitive information, such as usernames
or security keys. So, SYSLOG should only be used over communications
channels that provide strong encryption for data privacy. This
document defines a SYSLOG over SSH mapping which provides for support
of strong encryption and authentication.
This document requires that servers default to allowing access to the
"syslog" SSH subsystem only when using a specific TCP port assigned
by IANA for this purpose. This will allow SYSLOG over SSH traffic to
be easily identified and filtered by firewalls and other network
nodes. However, it will also allow SYSLOG over SSH traffic to be
more easily identified by attackers.
This document also recommends that servers be configurable to allow
access to the "syslog" SSH subsystem over other ports. Use of that
configuration option without corresponding changes to firewall or
network device configuration may unintentionally result in the
ability for nodes outside of the firewall or other administrative
boundary to gain access to "syslog" SSH subsystem.
8. Authors
The author of this draft is:
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Rainer Gerhards
Email: rgerhards@adiscon.com
Phone: +49-9349-92880
Fax: +49-9349-928820
Adiscon GmbH
Mozartstrasse 21
97950 Grossrinderfeld
Germany
9. IANA Considerations
IANA is requested to assign a TCP port number which will be the
default port for SYSLOG over SSH sessions as defined in this
document.
IANA has assigned port <TBD> for this purpose.
IANA is also requested to assign "syslog" as an SSH Service Name as
defined in RFC 4250 [5] as follows:
Service Name Reference
------------- ---------
syslog [This Document]
10. Acknowledgments
This document was written using the xml2rfc tool described in RFC2629
[4].
The authors wish to thank Chris Lonvick, Anton Okmianski, David
Harrington, Tom Petch, and all other people who commented on various
versions of this proposal.
11. Notes to the RFC Editor
These are notes to the RFC editor. Please delete this section after
the notes have been followed.
Please replace the instances of <TBD> the port number assigned by
IANA.
This ID is submitted along with draft-ietf-syslog-protocol. When a
RFC number is determined for draft-ietf-syslog-protocol, replace XXXX
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in RFCXXXX with the proper RFC number.
12. Normative
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[4] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[5] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol
Assigned Numbers", RFC 4250, January 2006.
[6] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[7] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport
Layer Protocol", RFC 4253, January 2006.
[8] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection
Protocol", RFC 4254, January 2006.
[9] Gerhards, R., "The syslog Protocol",
draft-ietf-syslog-protocol-17 (work in progress), June 2006.
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Author's Address
Rainer Gerhards
Adiscon GmbH
Mozartstrasse 21
Grossrinderfeld, BW 97950
Germany
Email: rgerhards@adiscon.com
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