syslog Working Group J. Kelsey
Internet-Draft NIST
Intended status: Standards Track J. Callas
Expires: June 11, 2009 PGP Corporation
A. Clemm
Cisco Systems
December 8, 2008
Signed syslog Messages
draft-ietf-syslog-sign-24.txt
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Abstract
This document describes a mechanism to add origin authentication,
message integrity, replay resistance, message sequencing, and
detection of missing messages to the transmitted syslog messages.
This specification is intended to be used in conjunction with the
work defined in [RFC5424], "The syslog Protocol".
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in this Document . . . . . . . . . . . . . . 6
3. syslog Message Format . . . . . . . . . . . . . . . . . . . . 7
4. Signature Blocks . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. syslog Messages Containing a Signature Block . . . . . . . 8
4.2. Signature Block Format and Fields . . . . . . . . . . . . 8
4.2.1. Version . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Reboot Session ID . . . . . . . . . . . . . . . . . . 10
4.2.3. Signature Group and Signature Priority . . . . . . . . 11
4.2.4. Global Block Counter . . . . . . . . . . . . . . . . . 13
4.2.5. First Message Number . . . . . . . . . . . . . . . . . 13
4.2.6. Count . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.7. Hash Block . . . . . . . . . . . . . . . . . . . . . . 14
4.2.8. Signature . . . . . . . . . . . . . . . . . . . . . . 15
4.2.9. Example . . . . . . . . . . . . . . . . . . . . . . . 15
5. Payload and Certificate Blocks . . . . . . . . . . . . . . . . 16
5.1. Preliminaries: Key Management and Distribution Issues . . 16
5.2. Payload Block . . . . . . . . . . . . . . . . . . . . . . 17
5.3. Certificate Block . . . . . . . . . . . . . . . . . . . . 18
5.3.1. syslog Messages Containing a Certificate Block . . . . 18
5.3.2. Certificate Block Format and Fields . . . . . . . . . 19
6. Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 22
6.1. Configuration parameters . . . . . . . . . . . . . . . . . 22
6.1.1. Configuration Parameters for Certificate Blocks . . . 22
6.1.2. Configuration Parameters for Signature Blocks . . . . 23
6.2. Overlapping Signature Blocks . . . . . . . . . . . . . . . 23
7. Efficient Verification of Logs . . . . . . . . . . . . . . . . 24
7.1. Offline Review of Logs . . . . . . . . . . . . . . . . . . 24
7.2. Online Review of Logs . . . . . . . . . . . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8.1. Cryptographic Constraints . . . . . . . . . . . . . . . . 28
8.2. Packet Parameters . . . . . . . . . . . . . . . . . . . . 28
8.3. Message Authenticity . . . . . . . . . . . . . . . . . . . 29
8.4. Replaying . . . . . . . . . . . . . . . . . . . . . . . . 29
8.5. Reliable Delivery . . . . . . . . . . . . . . . . . . . . 29
8.6. Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 29
8.7. Message Integrity . . . . . . . . . . . . . . . . . . . . 30
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8.8. Message Observation . . . . . . . . . . . . . . . . . . . 30
8.9. Man In The Middle Attacks . . . . . . . . . . . . . . . . 30
8.10. Denial of Service . . . . . . . . . . . . . . . . . . . . 30
8.11. Covert Channels . . . . . . . . . . . . . . . . . . . . . 30
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
9.1. Structured Data and syslog messages . . . . . . . . . . . 31
9.2. Version Field . . . . . . . . . . . . . . . . . . . . . . 31
9.3. SG Field . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.4. Key Blob Type . . . . . . . . . . . . . . . . . . . . . . 33
10. Working Group . . . . . . . . . . . . . . . . . . . . . . . . 34
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
12.1. Normative References . . . . . . . . . . . . . . . . . . . 36
12.2. Informative References . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . . . . 39
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1. Introduction
This document describes a mechanism, called syslog-sign in this
document, that adds origin authentication, message integrity, replay
resistance, message sequencing, and detection of missing messages to
syslog. Essentially, this is accomplished by sending a special
syslog message. The contents of this syslog message is called a
Signature Block. Each Signature Block contains, in effect, a
detached signature on some number of previously sent messages. It is
cryptographically signed and contains the hashes of previously sent
syslog messages.
While most implementations of syslog involve only a single originator
and a single collector of each message, provisions need to be made to
cover situations in which messages are sent to multiple collectors.
This concerns, in particular, situations in which different messages
are sent to different collectors, which means that some messages are
sent to some collectors but not to others. The required
differentiation of messages is generally performed based on the
Priority value of the individual messages. For example, messages
from any Facility with a Severity value of 3, 2, 1, or 0 may be sent
to one collector while all messages of Facilities 4, 10, 13, and 14
may be sent to another collector. Appropriate syslog-sign messages
must be kept with their proper syslog messages. To address this,
syslog-sign uses a Signature Group. A Signature Group identifies a
group of messages that are all kept together for signing purposes by
the originator. A Signature Block always belongs to exactly one
Signature Group and always signs messages belonging only to that
Signature Group.
Additionally, an originator sends a Certificate Block to provide key
management information between the originator and the collector.
This Certificate Block has a field to denote the type of key material
which may be such things as a PKIX certificate, an OpenPGP
certificate, or even an indication that a key had been pre-
distributed. In the cases of certificates being sent, the
certificates may have to be split across multiple packets.
The collector of the previous messages may verify that the hash of
each received message matches the signed hash contained in the
Signature Block. A collector may process these Signature Blocks as
they arrive, building an authenticated log file. Alternatively, it
may store all the log messages in the order they were received. This
allows a network operator to authenticate the log file at the time
the logs are reviewed.
The mechanism described in this specification is intended to be used
in conjunction with the syslog protocol as defined in [RFC5424] as
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its message delivery mechanism and uses the concept of STRUCTURED-
DATA elements defined in that document. In fact, this specification
mandates implementation of syslog protocol. Nevertheless, it is
conceivable that the concepts underlying this mechanism could also be
used in conjunction with other message delivery mechanisms.
Designers of other efforts to define event notification mechanisms
are therefore encouraged to consider this specification in their
designs.
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2. Conventions Used in this Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. syslog Message Format
This specification is intended to be used in conjunction with the
syslog protocol as defined in [RFC5424]. The syslog protocol
therefore MUST be supported by implementations of this specification.
Because the originator generating the Signature Block message signs
each message in its entirety, the messages MUST NOT be changed in
transit. By the same token, the syslog-sign messages MUST NOT be
changed in transit. [RFC3164] describes relay behavior in which
syslog messages are altered. If such behavior were to occur in
conjunction with syslog-sign, it would render any signing invalid and
hence make the mechanism useless. Likewise, any truncation of
messages that occurs between sending and receiving renders the
mechanism useless. For this reason, syslog originator and collector
implementations implementing this specification MUST support messages
of up to and including 2048 octets in length, in order to minimize
the chance of truncation. While syslog originator and collector
implementations MAY support messages with a length larger than 2048
octets, implementors need to be aware that any message truncations
that occur render the mechanism useless.
This specification uses the syslog message format described in
[RFC5424]. Along with other fields, that document describes the
concept of Structured Data (SD). Structured Data is defined in terms
of SD ELEMENTS (SDEs). An SDE consists of a name and a set of
parameter name - value pairs. The SDE name is referred to as SD-ID.
The name-value pairs are referred to as SD-PARAM, or SD Parameters,
with the name constituting the SD-PARAM-NAME, and the value
constituting the SD-PARAM-VALUE.
The syslog messages defined in this document carry the signature and
certificate data as Structured Data. The special syslog messages
defined in this document include for this purpose definitions of SDEs
to convey parameters that relate to the signing of syslog messages.
The MSG part of the syslog messages defined in this document SHOULD
simply be empty -- the content of the messages is not intended for
interpretation by humans but by applications that use those messages
to build an authenticated log.
Because the syslog messages defined in this document adhere to the
format described in [RFC5424], they identify the machine that
originates the syslog message in the HOSTNAME field. Therefore, the
signature and certificate data do not need to include an additional
parameter to identify the machine that orginates the message.
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4. Signature Blocks
This section describes the format of the Signature Block and the
fields used within the Signature Block, as well as the syslog
messages used to carry the Signature Block.
4.1. syslog Messages Containing a Signature Block
There is a need to distinguish the Signature Block itself from the
syslog message that is used to carry a Signature Block. Signature
Blocks MUST be encompassed within completely formed syslog messages.
Syslog messages that contain a Signature Block are also referred to
as Signature Block messages.
A Signature Block message is identified by the presence of an SD
ELEMENT with an SD-ID with the value "ssign". In addition, a
Signature Block message MUST contain valid APP-NAME, PROCID, and
MSGID fields to be compliant with [RFC5424]. This specification does
not mandate particular values for these fields; however, for
consistency, originators MUST use the same values for APP-NAME,
PROCID, and MSGID fields for every Signature Block message that is
sent, whichever values are chosen. To allow for the possibility of
multiple originators per host, the combination of APP-NAME, PROCID,
and MSGID MUST be unique for each such originator. If an originator
daemon is restarted, it MAY use a new PROCID for what is otherwise
the same originator but MUST continue to use the same APP-NAME and
MSGID. It is RECOMMENDED (but not required) to use 110 as value for
the PRI field, corresponding to facility 13 and severity 6
(informational). The Signature Block is carried as Structured Data
within the Signature Block message, per the definitions that follow
in the next section. It is also RECOMMENDED (but not required) that
a Signature Block message carry no other Structured Data besides the
Structured Data of the Signature Block itself. The MSG part of a
Signature Block message SHOULD be empty.
The syslog messages defined as part of syslog-sign themselves
(Signature Block messages and Certificate Block messages) MUST NOT be
signed by a Signature Block. Collectors that implement syslog-sign
know to distinguish syslog messages that are associated with syslog-
sign from those that are subjected to signing and process them
differently. The intent of syslog-sign is to sign a stream of syslog
messages, not to alter it.
4.2. Signature Block Format and Fields
The content of a Signature Block message is the Signature Block. The
Signature Block MUST be encoded as an SD ELEMENT, as defined in
[RFC5424].
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The SD-ID MUST have the value of "ssign".
The SDE contains the fields of the Signature Block encoded as SD
Parameters, as specified in the following. The Signature Block is
composed of the following fields. The value of each field MUST be
printable ASCII, and any binary values MUST be base 64 encoded, as
defined in [RFC4648].
Field SD-PARAM-NAME Size in octets
----- ------------- ---- -- ------
Version VER 4
Reboot Session ID RSID 1-10
Signature Group SG 1
Signature Priority SPRI 1-3
Global Block Counter GBC 1-10
First Message Number FMN 1-10
Count CNT 1-2
Hash Block HB variable, size of hash
times the number of hashes
(base 64 encoded binary)
Signature SIGN variable
(base 64 encoded binary)
The fields MUST be provided in the order listed. Each SD parameter
MUST occur once and only once in the Signature Block. A Signature
Block is accordingly encoded as follows, where xxx denotes a
placeholder for the particular values:
[ssign VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" GBC="xxx" FMN="xxx"
CNT="xxx" HB="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence
enclosed in quotes, per [RFC5424]. The fields are separated by
single spaces and are described in the subsequent subsections.
4.2.1. Version
The Signature Block Version field is an alphanumeric value that has a
length of 4 octets, which may include leading zeroes. The first two
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octets and the last octet contain a decimal character in the range of
"0" to "9", whereas the third octet contains an alphanumeric
character in the range of "0" to "9", "a" to "z", or "A" to "Z". The
value in this field specifies the version of the syslog-sign
protocol. This is extensible to allow for different hash algorithms
and signature schemes to be used in the future. The value of this
field is the grouping of the protocol version (2 octets), the hash
algorithm (1 octet) and the signature scheme (1 octet).
Protocol Version - 2 octets, with "01" as the value for the
protocol version that is described in this document.
Hash Algorithm - 1 octet, where, in conjunction with Protocol
Version 01, a value of "1" denotes SHA1 and a value of "2" denotes
SHA256, as defined in [FIPS.180-2.2002]. (This is the octet that
can have a value of not just "0" to "9" but also "a" to "z" and
"A" to "Z".)
Signature Scheme - 1 octet, where, in conjunction with Protocol
Version 01, a value of "1" denotes OpenPGP DSA, defined in
[RFC4880] and [FIPS.186-2.2000].
The version, hash algorithm and signature scheme defined in this
document would accordingly be represented as "0111" (if SHA1 is used
as Hash Algorithm) and "0121" (if SHA256 is used as Hash Algorithm),
respectively (without the quotation marks).
The values of the Hash Algorithm and Signature Scheme are defined
relative to the Protocol Version. If the single-octet representation
of the values for Hash Algorithm and Signature Scheme were to ever
represent a limitation, this limitation could be overcome by defining
a new Protocol Version with additional Hash Algorithms and/or
Signature Schemes, and having implementations support both Protocol
Versions concurrently.
4.2.2. Reboot Session ID
The Reboot Session ID is a decimal value that has a length between 1
and 10 octets. The acceptable values for this are between 0 and
9999999999. Leading zeroes MUST be omitted. A Reboot Session ID is
expected to increase whenever an originator reboots in order to allow
collectors to distinguish messages and message signatures across
reboots. Hence, an originator needs to retain the previous Reboot
Session ID across reboots. In cases where an originator does not
support this capability, the Reboot Session ID MUST always be set to
a value of 0, which indicates that this capability is not supported.
Otherwise, it MUST increase whenever an originator reboots, starting
with a value of 1. If the value reaches 9999999999, then manual
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intervention may be required to subsequently reset it to 1.
Implementors MAY wish to consider using the snmpEngineBoots value as
a source for this counter as defined in [RFC3414].
If a reboot of an originator takes place, Signature Block messages
MAY use a new PROCID. However, Signature Block messages of the same
originator MUST continue to use the same APP-NAME and MSGID.
4.2.3. Signature Group and Signature Priority
The SG parameter may take any value from 0-3 inclusive. The SPRI
parameter may take any value from 0-191 inclusive. These fields
taken together allow network administrators to associate groupings of
syslog messages with appropriate Signature Blocks and Certificate
Blocks. Groupings of syslog messages that are signed together are
also called Signature Groups. A Signature Block contains only hashes
of those syslog messages that are part of the same Signature Group.
For example, in some cases, network administrators might have
originators send syslog messages of Facilities 0 through 15 to one
collector and those with Facilities 16 through 23 to another. In
such cases, associated Signature Blocks should likely be sent to the
corresponding collectors as well, signing the syslog messages that
are intended for each collector separately. This way, each collector
receives Signature Blocks for all syslog messages that it receives,
and only for those. The ability to associate different categories of
syslog messages with different Signature Groups, signed in separate
Signature Blocks, provides administrators with flexibility in this
regard.
Syslog-sign provides four options for handling Signature Groups,
linking them with PRI values so they may be routed to the destination
commensurate with the corresponding syslog messages. In all cases,
no more than 192 distinct Signature Groups (0-191) are permitted.
The Signature Group to which a Signature Block pertains is indicated
by the Signature Priority (SPRI) field. The Signature Group (SG)
field indicates how to interpret the Signature Priority field. (Note
that the SG field does not indicate the Signature Group itself, as
its name might suggest.) The SG field can have one of the following
values:
a. "0" -- There is only one Signature Group. In this case, the
administrators want all Signature Blocks to be sent to a single
destination; in all likelihood, all of the syslog messages will
also be going to that same destination. Signature Blocks sign
all messages regardless of their PRI value. This means that, in
effect, the Signature Block's SPRI value can be ignored.
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However, it is RECOMMENDED that a single SPRI value be used for
all Signature Blocks. Furthermore, it is RECOMMENDED to set that
value to the same value as the PRI field of the Signature Block
message. This way, the PRI of the Signature Block message
matches the SPRI of the Signature Block that it contains.
b. "1" -- Each PRI value is associated with its own Signature Group.
Signature Blocks for a given Signature Group have SPRI = PRI for
that Signature Group. In other words, the SPRI of the Signature
Block matches the PRI value of the syslog messages that are part
of the Signature Group and hence signed by the Signature Block.
An SG value of 1 can, for example, be used when the administrator
of an originator does not know where any of the syslog messages
will ultimately go but anticipates that messages with different
PRI values will be collected and processed separately. Having a
Signature Group per PRI value provides administrators with a
large degree of flexibility with regard to how to divide up the
processing of syslog messages and their signatures after they are
received, at the same time allowing Signature Blocks to follow
the corresponding syslog messages to their eventual destination.
c. "2" -- Each Signature Group contains a range of PRI values.
Signature Groups are assigned sequentially. A Signature Block
for a given Signature Group has its own SPRI value denoting the
highest PRI value of syslog messages in that Signature Group.
The lowest PRI value of syslog messages in that Signature Group
will be one larger than the SPRI value of the previous Signature
Group or "0" in case there is no other Signature Group with a
lower SPRI value. The specific Signature Groups and ranges they
are associated with are subject to configuration by a system
administrator.
d. "3" -- Signature Groups are not assigned with any of the above
relationships to PRI values of the syslog messages they sign.
Instead, another scheme is used, which is outside the scope of
this specification. There has to be some predefined arrangement
between the originator and the intended collectors as to which
syslog messages are to be included in which Signature Group,
requiring configuration by a system administrator. This provides
administrators also with the flexibility to group syslog messages
into Signature Groups according to criteria that are not tied to
the PRI value.
One reasonable way to configure some installations is to have only
one Signature Group, indicated with SG=0, and have the originator
send a copy of each Signature Block to each collector. In that case,
collectors that are not configured to receive every syslog message
will still receive signatures for every message, even ones they are
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not supposed to receive. While the collector will not be able to
detect gaps in the messages (because the presence of a signature of a
message that is missing does not tell the collector whether or not
the corresponding message would be of the collector's concern), it
does allow all messages that do arrive at each collector to be put
into the right order and to be verified. It also allows each
collector to detect duplicates. Likewise, configuring only one
Signature Group can be a reasonable way to configure installations
that involve relay chains, where one or more interim relays may or
may not relay all messages to the same destination.
4.2.4. Global Block Counter
The Global Block Counter is a decimal value representing the number
of Signature Blocks sent by syslog-sign before the current one, in
this reboot session. This takes at least 1 octet and at most 10
octets displayed as a decimal counter. The acceptable values for
this are between 0 and 9999999999, starting with 0. Leading zeroes
MUST be omitted. If the value of the Global Block Counter has
reached 9999999999 and the Reboot Session ID has a value other than 0
(indicating the fact that persistence of the Reboot Session ID is
supported), then the Reboot Session ID MUST be incremented by 1 and
the Global Block Counter resumes at 0. When the Reboot Session ID is
0 (i.e., persistent Reboot Session IDs are not supported) and the
Global Block Counter reaches its maximum value, then the Global Block
Counter is reset to 0 and the Reboot Session ID MUST remain at 0.
Note that the Global Block Counter crosses Signature Groups; it
allows one to roughly synchronize when two messages were sent, even
though they went to different collectors and are part of different
Signature Groups.
Because a reboot results in the start of a new reboot session, the
originator MUST reset the Global Block Counter to 0 after a reboot
occurs. Applications need to take into account the possibility that
a reboot occurred when authenticating a log, and situations in which
reboots occur frequently may result in losing the ability to verify
the proper sequence in which messages were sent, hence jeopardizing
the integrity of the log.
4.2.5. First Message Number
This is a decimal value between 1 and 10 octets, with leading zeroes
omitted. It contains the unique message number within this Signature
Group of the first message whose hash appears in this block. The
very first message of the reboot session is numbered "1". This
implies that when the Reboot Session ID increases, the message number
is reset to 1.
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For example, if this Signature Group has processed 1000 messages so
far and message number 1001 is the first message whose hash appears
in this Signature Block, then this field contains 1001. The message
number is relative to the Signature Group to which it belongs; hence,
a message number does not identify a message beyond its Signature
Group.
Should the message number reach 9999999999 within the same reboot
session and Signature Group, the message number subsequently restarts
at 1. In such event, the Global Block Counter will be vastly
different between two occurrences of the same message number.
4.2.6. Count
The count is a 1 or 2 octet field that indicates the number of
message hashes to follow. The valid values for this field are 1
through 99. The number of hashes included in the Signature Block
MUST be chosen such that the length of the resulting syslog message
does not exceed the maximum permissible syslog message length.
4.2.7. Hash Block
The hash block is a block of hashes, each separately encoded in base
64. Each hash in the hash block is the hash of the entire syslog
message represented by the hash, independent of the underlying
transport. Hashes are ordered from left to right in the order of
occurrence of the syslog messages that they represent. The space
character is used to separate the hashes. Note, the hash block
constitutes a single SD-Param; a Signature Block message MUST include
all its hashes in a single hash block and MUST NOT spread its hashes
across several hash blocks.
The "entire syslog message" refers to what is described as the syslog
message excluding transport parts that are described in [RFC5425] and
[RFC5426], and excluding other parts that may be defined in future
transports. The hash value will be the result of the hashing
algorithm run across the syslog message, starting with the "<" of the
PRI portion of the header part of the message. The hash algorithm
used and indicated by the Version field determines the size of each
hash, but the size MUST NOT be shorter than 160 bits without the use
of padding. It is base 64 encoded as per [RFC4648].
The number of hashes in a hash block SHOULD be chosen such that the
resulting Signature Block message does not exceed a length of 2048
octets in order to avoid the possibility that truncation occurs.
When more hashes need to be sent than fit inside a Signature Block
message, it is advisable to start a new Signature Block.
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4.2.8. Signature
This is a digital signature, encoded in base 64 per [RFC4648]. The
signature is calculated over the completely formatted Signature Block
message, excluding the signature field (SD Parameter Name and the
space before it [" SIGN"], "=", and corresponding value).
4.2.9. Example
An example of a Signature Block message is depicted below, broken
into lines to fit internet-draft publication rules. There is a space
at the end of each line, with the exception of the last line which
ends with "]" and the second-to-last line which ends with "ld6hg".
<110>1 2008-10-16T20:23:03+02:00 host.example.org syslogd 5660 -
[ssign VER="0111" RSID="1" SG="0" SPRI="0" GBC="1" FMN="1" CNT="15"
HB="W1knzOeMETXgCymaK7W8UAxDgP8= zTxfthW8WqmtFhOG4k/+ZxkirTA=
j9dubU1GNVp7qWShwph/w32nD08= XQDLZ/NuwirmLdMORtm84r9kIW4=
RNDFNCo7hiCsK/EKumsPBbFHNZA= ANiE3KbY948J6cEB640fAtWXuO4=
e2M/OqjHDfxLVUSPt1CsNJHm9wU= Y+racQst7F1gR8eEUh8O7o+M53s=
JAMULRxjMPbOO5EhhKbsUkAwbl0= pd+N5kmlnyQ0BoItELd/KWQrcMg=
dsMQSzPHIS6S3Vaa23/t7U8JAJ4= i4rE3x7N4qyQGTkmaWHsWDFP9SY=
qgTqV4EgfUFd3uZXNPvJ25erzBI= XW0YrME5kQEh+fxhg1fetnWxfIc=
7YPcRHsDwXWnQuGRWaJtFWw9hus=" SIGN="MC0CFQCEGQKze8v5Xde+ywQdzXUCBld6hg
IUcyWxzgIO7ouJcReGxHsPBhD+bBM="]
The message is of syslog-sign protocol version "01". It uses SHA1 as
hash algorithm and an OpenPGP DSA signature scheme. Its reboot
session ID is 1. Its Signature Group is 0 which means that all
syslog messages go to the same destination; its Signature Priority
(which can effectively be ignored because all syslog messages will be
signed regardless of their PRI value) is 0. Its Global Block Counter
is 1. The first message number is 1; the message contains 15 message
hashes.
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5. Payload and Certificate Blocks
Certificate Blocks and Payload Blocks provide key management for
syslog-sign. Their purpose is to support key management that uses
public key cryptosystems.
5.1. Preliminaries: Key Management and Distribution Issues
A Payload Block contains public key certificate information that is
to be conveyed to the collector. A Payload Block is sent at the
beginning of a new reboot session, carrying public key information in
effect for the reboot session. However, a Payload Block is not sent
directly, but in (one or more) fragments. Those fragments are termed
Certificate Blocks. Therefore, originators send at least one
Certificate Block at the beginning of a new reboot session.
There are three key points to understand about Certificate Blocks:
a. They handle a variable-sized payload, fragmenting it if necessary
and transmitting the fragments as legal syslog messages. This
payload is built (as described below) at the beginning of a
reboot session and is transmitted in pieces with each Certificate
Block carrying a piece. There is exactly one Payload Block per
reboot session.
b. The Certificate Blocks are digitally signed. The originator does
not sign the Payload Block, but the signatures on the Certificate
Blocks ensure its authenticity. Note that it may not even be
possible to verify the signature on the Certificate Blocks
without the information in the Payload Block; in this case the
Payload Block is reconstructed, the key is extracted, and then
the Certificate Blocks are verified. (This is necessary even
when the Payload Block carries a certificate, because some other
fields of the Payload Block are not otherwise verified.) In
practice, most installations keep the same public key over long
periods of time, so that most of the time, it is easy to verify
the signatures on the Certificate Blocks, and use the Payload
Block to provide other useful per-session information.
c. The kind of Payload Block that is expected is determined by what
kind of key material is on the collector that receives it. The
originator and collector (or offline log viewer) both have some
key material (such as a root public key or pre-distributed public
key) and an acceptable value for the Key Blob Type in the Payload
Block, below. The collector or offline log viewer MUST NOT
accept a Payload Block of the wrong type.
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5.2. Payload Block
The Payload Block is built when a new reboot session is started.
There is a one-to-one correspondence between reboot sessions and
Payload Blocks. An originator creates a new Payload Block after each
reboot. The Payload Block is used until the next reboot. A Payload
Block MUST have the following fields:
a. Full local time stamp for the originator at the time the reboot
session started. This must be in the time stamp format specified
in [RFC5424] (essentially, time stamp format per [RFC3339] with
some further restrictions).
b. Key Blob Type, a one-octet field containing one of five values:
1. 'C' -- a PKIX certificate.
2. 'P' -- an OpenPGP certificate.
3. 'K' -- the public key whose corresponding private key is
being used to sign these messages.
4. 'N' -- no key information sent; key is pre-distributed.
5. 'U' -- installation-specific key exchange information
c. The key blob, if any, base 64 encoded per [RFC4648] and
consisting of the raw key data.
The fields are separated by single space characters. Because a
Payload Block is not carried in a syslog message directly, only the
corresponding Certificate Blocks, it does not need to be encoded as
an SD ELEMENT. The Payload Block does not contain a field that
identifies the reboot session; instead, the reboot session can be
inferred from the Reboot Session ID parameter of the Certificate
Blocks that are used to carry the Payload Block.
When a PKIX certificate is used ("C" key blob type), it is the
certificate specified in ([RFC5280]). Per [RFC5425], syslog messages
may be transported over the TLS protocol, even where there is no PKI.
If that transport is used, then the device will already have a PKIX
certificate and it MAY use the private key associated with that
certificate to sign messages. In the case where there is no PKI, the
chain of trust of a PKIX certificate must still be established to
meet conventional security requirements. The methods for doing this
are described in [RFC5425].
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5.3. Certificate Block
This section describes the format of the Certificate Block and the
fields used within the Certificate Block, as well as the syslog
messages used to carry Certificate Blocks.
5.3.1. syslog Messages Containing a Certificate Block
Certificate Blocks are used to get the Payload Block to the
collector. As with a Signature Block, each Certificate Block is
carried in its own syslog message, called Certificate Block message.
Because certificates can legitimately be much longer than 2048
octets, the Payload Block can be split up into several pieces, with
each Certificate Block carrying a piece of the Payload Block. Note
that the originator MAY make the Certificate Blocks of any legal
length (that is, any length that keeps the entire Certificate Block
message within 2048 octets) that holds all the required fields.
Software that processes Certificate Blocks MUST deal correctly with
blocks of any legal length. The length of the fragment of the
Payload Block that a Certificate Block carries MUST be at least 1
octet. The length SHOULD be chosen such that the length of the
Certificate Block message does not exceed 2048 octets.
A Certificate Block message is identified by the presence of an SD
ELEMENT with an SD-ID with the value "ssign-cert". In addition, a
Certificate Block message MUST contain valid APP-NAME, PROCID, and
MSGID fields to be compliant with syslog protocol. Syslog-sign does
not mandate particular values for these fields; however, for
consistency, implementations MUST use the same value for APP-NAME,
PROCID, and MSGID fields for every Certificate Block message,
whichever values are chosen. To allow for the possibility of
multiple originators per host, the combination of APP-NAME, PROCID,
and MSGID MUST be unique for each such originator. If an originator
daemon is restarted, it MAY use a new PROCID for what is otherwise
the same originator. The combination of APP-NAME and PROCID MUST be
the same that is used for Signature Block messages of the same
originator; however, a different MSGID MAY be used. It is
RECOMMENDED to use 110 as value for the PRI field, corresponding to
facility 13 and severity 6 (informational). The Certificate Block is
carried as Structured Data within the Certificate Block message. It
is also RECOMMENDED (but not required) that a Certificate Block
message carry no other Structured Data besides the Structured Data of
the Certificate Block itself. The MSG part of a Certificate Block
message SHOULD be empty.
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5.3.2. Certificate Block Format and Fields
The contents of a Certificate Block message is the Certificate Block
itself. Like a Signature Block, the Certificate Block is encoded as
an SD ELEMENT. The SD-ID of the Certificate Block is "ssign-cert".
The Certificate Block is composed of the following fields, each of
which is encoded as an SD Parameter with parameter name as indicated.
Each field must be printable ASCII, and any binary values are base 64
encoded per [RFC4648].
Field SD-PARAM-NAME Size in octets
----- ------------- ---- -- ------
Version VER 4
Reboot Session ID RSID 1-10
Signature Group SG 1
Signature Priority SPRI 1-3
Total Payload Block Length TPBL 1-8
Index into Payload Block INDEX 1-8
Fragment Length FLEN 1-4
Payload Block Fragment FRAG variable
(base 64 encoded binary)
Signature SIGN variable
(base 64 encoded binary)
A Certificate Block is accordingly encoded as follows, where xxx
denotes a placeholder for the particular values:
[ssign-cert VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" TBPL="xxx"
INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence
enclosed in quotes, per [RFC5424]. The fields are separated by
single spaces and are described below. Each SD parameter MUST occur
once and only once.
5.3.2.1. Version
The Signature Group version field is 4 octets in length. This field
is identical in format and meaning to the Version field described in
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Section 4.2.1.
5.3.2.2. Reboot Session ID
The Reboot Session ID is identical in format and meaning to the RSID
field described in Section 4.2.2.
5.3.2.3. Signature Group and Signature Priority
The SIG field is identical in format and meaning to the SIG field
described in Section 4.2.3. The SPRI field is identical in format
and meaning to the SPRI field described there.
5.3.2.4. Total Payload Block Length
The Total Payload Block Length is a value representing the total
length of the Payload Block in octets, expressed as a decimal with
one to eight octets.
5.3.2.5. Index into Payload Block
This is a decimal value between 1 and 8 octets, with leading zeroes
omitted. It contains the number of octets into the Payload Block at
which this fragment starts. The first octet of the first fragment is
numbered "1". (Note, it is not numbered "0".)
5.3.2.6. Fragment Length
The total length of this fragment expressed as a decimal integer with
one to four octets. The fragment length must be at least 1.
5.3.2.7. Payload Block Fragment
The Payload Block Fragment contains a fragment of the payload block,
encoded in base 64, as per [RFC4648]. Its length must match the
indicated fragment length.
5.3.2.8. Signature
This is a digital signature, encoded in base 64, as per [RFC4648].
The Version field effectively specifies the original encoding of the
signature. The signature is calculated over the completely formatted
Certicificate Block message, excluding the signature field (SD
Parameter Name and the space before it [" SIGN"], "=", and
corresponding value).
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5.3.2.9. Example
An example of a Certificate Block message is is depicted below,
broken into lines to fit internet-draft publication rules. There are
no spaces at the end of the lines that contain the key blob and the
signature.
<110>1 2008-10-16T20:23:03+02:00 host.example.org syslogd 5660 -
[ssign-cert VER="0111" RSID="1" SG="0" SPRI="0" TBPL="620" INDEX="1"
FLEN="620" FRAG="2008-10-16T20:23:03+02:00 K MIIBtzCCASsGByqGSM44BAEwg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" SIGN="MCwCFFX85jQ0QK1aosxAH1lpgmEkSNspAhQ2elzq3h/wVU6u2CJ3KAD
uWsyzdg=="]
The message is of syslog-sign protocol version "01". It uses SHA1 as
hash algorithm and an OpenPGP DSA signature scheme. Its reboot
session ID is 1. Its Signature Group is 0; its Signature Priority is
0. The Total Payload Block Length is 620. The index into the
payload block is 1 (meaning this is the first fragment). The length
of the fragment is 620 (meaning that the Certificate Block message
contains the entire Payload Block). The Payload Block has the time
stamp 2008-10-16T20:23:03+02:00. The Key Blob Type is 'K', meaning
that it contains a public key whose corresponding private key is
being used to sign these messages.
Note that the Certificate Block message in this example has the same
time stamp as the Payload Block. This implies that this is the first
Certificate Block message sent in this reboot session; additional
Certificate Block messages can be sent later with a later time stamp,
which will carry the same Payload Block that will still contain the
same time stamp.
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6. Redundancy and Flexibility
As described in Section 8.5 of [RFC5424], a transport sender may
discard syslog messages. Likewise, when syslog messages are sent
over unreliable transport, they can be lost in transit. However, if
a collector does not receive Signature and Certificate Blocks, many
messages may not be able to be verified. The originator is allowed
to send Signature and Certificate Blocks multiple times. Sending
Signature and Certificate Blocks multiple times provides redundancy
with the intent to ensure that the collector or relay does get the
Signature Blocks and in particular the Payload Block at some point in
time. In the meantime, any online review of logs as described in
Section 7.2 is delayed until the needed blocks are received. The
collector MUST ignore Signature Blocks and Certificate Blocks it has
already received and authenticated. The originator can in principle
change its redundancy level for any reason, without communicating
this fact to the collector.
The originator does not need to queue up other messages while sending
redundant Certificate Block and Signature Block messages. It MAY
send redundant Certificate Block messages even after Signature Block
messages and regular syslog messages have been sent. By the same
token, it MAY send redundant Signature Block messages even after
newer syslog messages that are signed by a subsequent Signature Block
have been sent, or even after a subsequent Signature Block message.
In addition, the originator has flexibility in how many hashes to
include within a Signature Block. It is legitimate for an originator
to send short Signature Blocks to allow the collector to verify
messages with minimal delay.
6.1. Configuration parameters
Although the transport sender is not constrained in how it decides to
send redundant Signature and Certificate Blocks, or even in whether
it decides to send along multiple copies of normal syslog messages,
we define some redundancy parameters below which may be useful in
controlling redundant transmission from the transport sender to the
transport receiver, and which may be useful for administrators to
configure.
6.1.1. Configuration Parameters for Certificate Blocks
certInitialRepeat = number of times each Certificate Block should be
sent before the first message is sent.
certResendDelay = maximum time delay in seconds until resending the
Certificate Block.
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certResendCount = maximum number of other syslog messages to send
until resending the Certificate Block.
It is desirable to allow for configuration of the transport sender
such that Certificate Blocks are not sent at all after the first
normal syslog message has been sent. This could be expressed by
setting both certResendDelay and certResendCount to "0". However, it
is recommended to configure the transport sender to send redundant
Certificate Blocks even after the first message is sent when the UDP
transport [RFC5426] is used.
6.1.2. Configuration Parameters for Signature Blocks
sigNumberResends = number of times a Signature Block is resent. (It
is recommended so select a value of greater than "0" in particular
when the UDP transport [RFC5426] is used.)
sigResendDelay = maximum time delay in seconds from original sending
to next redundant sending.
sigResendCount = maximum number of sent messages to delay before next
redundant sending.
6.2. Overlapping Signature Blocks
Notwithstanding the fact that the originator is not constrained in
whether it decides to send redundant Signature Block messages,
Signature Blocks SHOULD NOT overlap. This facilitates their
processing by the receiving collector. This means that an originator
of Signature Block messages, after having sent a first message with
some First Message Number and a Count, SHOULD NOT send a second
message with the same First Message Number but a different Count. It
also means that an originator of Signature Block messages SHOULD NOT
send a second message whose First Message Number is greater than the
First Message Number, but smaller than the First Message Number plus
the Count indicated in the first message.
That said, the possibility of Signature Blocks that overlap does
provide additional flexibility with regards to redundancy; it
provides an additional option that may be desirable in some
deployments. Therefore collectors MUST be designed in a way that
they can cope with overlapping Signature Blocks when confronted with
them. The collector MUST ignore hashes of messages that it has
already received and validated.
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7. Efficient Verification of Logs
The logs secured with syslog-sign may be reviewed either online or
offline. Online review is somewhat more complicated and
computationally expensive, but not prohibitively so.
7.1. Offline Review of Logs
When the collector stores logs to be reviewed later, they can be
authenticated offline just before they are reviewed. Reviewing these
logs offline is simple and relatively inexpensive in terms of
resources used, so long as there is enough space available on the
reviewing machine. Here, we presume that the stored log files have
already been separated by originator, Reboot Session ID, and
Signature Group. This can be done easily with a script file. We
then do the following:
a. First, we go through the raw log file and split its contents into
three files. Each message in the raw log file is classified as a
normal message, a Signature Block message, or a Certificate Block
message. Signature Blocks and Certificate Blocks are then stored
in their own files. Normal messages are stored in a keyed file,
indexed on their hash values.
b. We sort the Certificate Block file by INDEX value, and check to
see whether we have a set of Certificate Blocks that can
reconstruct the Payload Block. If so, we reconstruct the Payload
Block, verify any key-identifying information, and then use this
to verify the signatures on the Certificate Blocks we have
received. When this is done, we have verified the reboot session
and key used for the rest of the process.
c. We sort the Signature Block file by First Message Number. We now
create an authenticated log file, which consists of some header
information and then a sequence of message number, message text
pairs. We next go through the Signature Block file. We
initialize a cursor for the last message number processed with
the number 0. For each Signature Block in the file, we do the
following:
1. Verify the signature on the Signature Block.
2. If the value of the First Message Number of the Signature
Block is less than or equal to the last message number
processed, skip the first (last message number processed
minus First Message Number plus 1) hashes.
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3. For each remaining hashed message in the Signature Block:
a. Look up the hash value in the keyed message file.
b. If the message is found, write (message number, message
text) to the authenticated log file.
4. Set the last message number processed to the value of the
First Message Number plus the Count of the Signature Block
minus 1.
5. Skip all other Signature Blocks with the same First Message
Number unless one with a larger Count is encountered.
The resulting authenticated log file contains all messages that
have been authenticated. In addition, it implicitly indicates
all gaps in the authenticated messages (specifically in the case
when all messages of the same Signature Group are sent to the
same collector), because their message numbers are missing.
One can see that, assuming sufficient space for building the keyed
file, this whole process is linear in the number of messages
(generally two seeks, one to write and the other to read, per normal
message received), and O(N lg N) in the number of Signature Blocks.
This estimate comes with two caveats: first, the Signature Blocks
arrive very nearly in sorted order, and so can probably be sorted
more cheaply on average than O(N lg N) steps. Second, the signature
verification on each Signature Block almost certainly is more
expensive than the sorting step in practice. We have not discussed
error-recovery, which may be necessary for the Certificate Blocks.
In practice, a simple error-recovery strategy is probably enough: if
the Payload Block is not valid, then we can just try alternate
instances of each Certificate Block, if such are available, until we
get the Payload Block right.
It is easy for an attacker to flood us with plausible-looking
messages, Signature Blocks, and Certificate Blocks.
7.2. Online Review of Logs
Some collector implementations may need to monitor log messages in
close to real-time. This can be done with syslog-sign, though it is
somewhat more complex than offline verification. This is done as
follows:
a. We have an authenticated message file, into which we write
(message number, message text) pairs which have been
authenticated. Again, we will assume that we are handling only
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one Signature Group and only one Reboot Session ID at any given
time.
b. We have three data structures: A queue in which (message number,
hash of message) pairs are kept in sorted order, a queue in which
(arrival sequence, hash of message) pairs are kept in sorted
order, and a hash table that stores (message text, count) pairs
indexed by hash value. In the hash table, count may be any
number greater than zero; when count is zero, the entry in the
hash table is cleared.
c. We must receive all the Certificate Blocks before any other
processing can really be done. (This is why they are sent
first.) Once that is done, any additional Certificate Block
message that arrives is discarded. Any syslog messages or
Signature Block messages that arrive before all Certificate
Blocks have been received need to be buffered. Once all
Certificate Blocks have been received, the messages in the buffer
can be retrieved and processed as if they were just arriving.
d. Whenever a normal message arrives, we add (arrival sequence, hash
of message) to our message queue. If our hash table has an entry
for the message's hash value, we increment its count by one;
otherwise, we create a new entry with count = 1. If the message
queue is full, we roll the oldest messages off the queue by
taking the oldest entry in the queue, and using it to index the
hash table. If that entry has count 1, we delete the entry from
the hash table; otherwise, we decrement its count. We then
delete the oldest entry in the queue.
e. Whenever a Signature Block message arrives, we first check to see
whether the First Message Number value is too old to still be of
interest, or if another Signature Block with that First Message
Number and the same Count or a greater Count has already been
received. If so, we discard the Signature Block. Otherwise, we
check its signature and discard it if the signature is not valid.
A Signature Block contains a sequence hashes, each of which is
associated with a message number, starting with the First Message
Number for the first hash and incrementing by one for each
subsequent hash. For each hash, we first check to see whether
the message hash is in the hash table. If this is the case, we
do the following:
A. We check if a message with the same message number is already
in the authenticated message queue.
B. If that is not the case, we write the (message number,
message text) into the authenticated message queue, otherwise
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the signed hash is a duplicate and we discard it.
Otherwise (the message hash is not in the hash table), we write
the (message number, message hash) to the message number queue.
This generally involves rolling the oldest entry out of this
queue: before this is done, that entry's hash value is again
looked up in the hash table. If a matching entry is found, a
check is made if the authenticated message file already contains
an entry with the same message number and if that is not the
case, the (message number, message text) pair is written to the
authenticated message. In either case, the oldest entry is then
discarded.
f. The result of this is a sequence of messages in the authenticated
message file, each of which has been authenticated, and which are
labeled with numbers showing their order of original
transmission.
One can see that this whole process is roughly linear in the number
of messages, and also in the number of Signature Blocks received.
The process is susceptible to flooding attacks; an attacker can send
enough normal messages that the messages roll off their queue before
their Signature Blocks can be processed.
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8. Security Considerations
Normal syslog event messages are unsigned and have most of the
security attributes described in Section 8 of [RFC5424]. This
document also describes Certificate Blocks and Signature Blocks,
which are signed syslog messages. The Signature Blocks contain
signature information for previously sent syslog event messages. All
of this information can be used to authenticate syslog messages and
to minimize or obviate many of the security concerns described in
[RFC5424].
The model for syslog-sign is a direct trust system where the
certificate transferred is its own trust anchor. If a transport
sender sends a stream of syslog messages that is signed using a
certificate, the operator or application will transfer to the
transport receiver the certificate that was used when signing. There
is no need for a certificate chain.
8.1. Cryptographic Constraints
As with any technology involving cryptography, it is advisable to
check the current literature to determine whether any algorithms used
here have been found to be vulnerable to attack.
This specification uses Public Key Cryptography technologies. The
proper party or parties have to control the private key portion of a
public-private key pair. Any party that controls a private key can
sign anything it pleases.
Certain operations in this specification involve the use of random
numbers. An appropriate entropy source SHOULD be used to generate
these numbers. See [RFC4086] and [NIST800.90].
8.2. Packet Parameters
As an originator, it is advisable to avoid message lengths exceeding
2048 octets. Various problems might result if an originator were to
send messages with a length greater than 2048 octets, because relays
MAY truncate messages with lengths greater than 2048 octets which
would make it impossible for collectors to validate a hash of the
packet. To increase the chance of interoperability, it tends to be
best to be conservative with what you send but liberal in what you
are able to receive.
Originators need to rigidly enforce the correctness of message
bodies. Problems may arise if the collector does not fully accept
the syslog packets sent from an originator, or if it has problems
with the format of the Certificate Block or Signature Block messages.
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Collectors are not to malfunction in case they receive malformed
syslog messages or messages containing characters other than those
specified in this document. In other words, they are to ignore such
messages and continue working.
8.3. Message Authenticity
Syslog does not strongly associate the message with the message
originator. That association is established by the collector upon
verification of the Signature Block. Before a Signature Block is
used to ascertain the authenticity of an event message, it might be
received, stored, and reviewed by a person or automated parser. It
is advisable not to assume a message is authentic until after a
message has been validated by checking the contents of the Signature
Block.
With the Signature Block checking, an attacker may only forge
messages if he or she can compromise the private key of the true
originator.
8.4. Replaying
Event messages might be recorded and replayed by an attacker. Using
the information contained in the Signature Blocks, a reviewer can
determine whether the received messages are the ones originally sent
by an originator. The reviewer can also identify messages that have
been replayed.
8.5. Reliable Delivery
Event messages sent over UDP might be lost in transit. [RFC5425] can
be used for the reliable delivery of syslog messages; however, it
does not protect against loss of syslog messages at the application
layer, for example if the TCP connection or TLS session has been
closed by the transport receiver for some reason. A reviewer can
pinpoint any messages sent by the originator but not received by the
collector by reviewing the Signature Block information. In addition,
the information in subsequent Signature Blocks allows a reviewer to
determine whether any Signature Block messages were lost in transit.
8.6. Sequenced Delivery
Syslog messages delivered over UDP might not only be lost, but also
arrive out of sequence. A reviewer can determine the original order
of syslog messages and identify which messages were delivered out of
order by examining the information in the Signature Block along with
any timestamp information in the message.
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8.7. Message Integrity
Syslog messages might be damaged in transit. A review of the
information in the Signature Block determines whether the received
message was the intended message sent by the originator. A damaged
Signature Block or Certificate Block is evident because the collector
will not be able to validate that it was signed by the originator.
8.8. Message Observation
Unless TLS is used as a secure transport [RFC5425], event messages,
Certificate Blocks, and Signature Blocks are all sent in plaintext.
This allows network administrators to read the message when sniffing
the wire. However, this also allows an attacker to see the contents
of event messages and perhaps to use that information for malicious
purposes.
8.9. Man In The Middle Attacks
It is conceivable that an attacker might intercept Certificate Block
messages and insert its own Certificate information. In that case,
the attacker would be able to receive event messages from the actual
originator and then relay modified messages, insert new messages, or
delete messages. It would then be able to construct a Signature
Block and sign it with its own private key. Network administrators
need to verify that the key contained in the Payload Block is indeed
the key being used on the actual originator. If that is the case,
then this MITM attack will not succeed. Methods for establishing a
chain of trust are also described in [RFC5425].
8.10. Denial of Service
An attacker might send invalid Signature Block messages to overwhelm
the collector's processing capability and consume all available
resources. For this reason, it can be appropriate to simply receive
the Signature Block messages and process them only as time permits.
An attacker might also just overwhelm a collector by sending more
messages to it than it can handle. Implementors are advised to
consider features that minimize this threat, such as only accepting
syslog messages from known IP addresses.
8.11. Covert Channels
Nothing in this protocol attempts to eliminate covert channels. In
fact, just about every aspect of syslog messages lends itself to the
conveyance of covert signals. For example, a collusionist could send
odd and even PRI values to indicate Morse Code dashes and dots.
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9. IANA Considerations
9.1. Structured Data and syslog messages
With regard to [RFC5424], IANA is requested to add the following
values to the registry entitled "syslog Structured Data id values":
SD-ID PARAM_NAME
----- ----------
ssign
VER
RSID
SG
SPRI
GBC
FMN
CNT
HB
SIGN
ssign-cert
VER
RSID
SG
SPRI
TBPL
INDEX
FLEN
FRAG
SIGN
In addition, several fields need to be controlled by the IANA in both
the Signature Block and the Certificate Block, as outlined in the
following sections.
9.2. Version Field
IANA is requested to create three registries, each associated with a
different subfield of the Version field of Signature Blocks and
Certificate Blocks, described in Section 4.2.1 and Section 5.3.2.1,
respectively.
The first registry that IANA is requested to create is entitled
"syslog-sign protocol version values". It is for the values of the
Protocol Version subfield. The Protocol Version subfield constitutes
the first 2 octets in the Version field. New values shall be
assigned by the IANA using the "IETF Consensus" policy defined in
[RFC5226]. Assigned numbers are to be increased by 1, up to a
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maximum value of "50". Protocol Version numbers of "51" through "99"
are vendor-specific; values in this range are not to be assigned by
the IANA.
IANA is requested to register the Protocol Version values shown
below.
VALUE PROTOCOL VERSION
----- ----------------
00 Reserved
01 Defined in RFC yyyy
The second registry that IANA is requested to create is entitled
"syslog-sign hash algorithm values". It is for the values of the
Hash Algorithm subfield. The Hash Algorithm subfield constitutes the
third octet in the Version field Signature Blocks and Certificate
Blocks. New values shall be assigned by the IANA using the "IETF
Consensus" policy defined in [RFC5226]. Assigned values are to be
increased by 1, up to a maximum value of "9". The values are
registered relative to the Protocol Version. This means that the
same Hash Algorithm value can be reserved for different Protocol
Versions, possibly referring to a different hash algorithm each time.
This makes it possible to deal with future scenarios in which the
single octet representation becomes a limitation, as more Hash
Algorithms can be supported by defining additional Protocol Versions
that implementations might support concurrently.
IANA is requested to register the Hash Algorithm values shown below.
VALUE PROTOCOL VERSION HASH ALGORITHM
----- ---------------- --------------
0 01 Reserved
1 01 SHA1
2 01 SHA256
The third registry that IANA is requested to create is entitled
"syslog-sign signature scheme values". It is for the values of the
Signature Scheme subfield. The Signature Scheme subfield constitutes
the fourth octet in the Version field of Signature Blocks and
Certificate Blocks. New values shall be assigned by the IANA using
the "IETF Consensus" policy defined in [RFC5226]. Assigned values
are to be increased by 1, up to a maximum value of "9". This means
that the same Signature Scheme value can be reserved for different
Protocol Versions, possibly in each case referring to a different
Signature Scheme each time. This makes it possible to deal with
future scenarios in which the single octet representation becomes a
limitation, as more Signature Schemes can be supported by defining
additional Protocol Versions that implementations might support
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concurrently.
IANA is requested to register the Signature Scheme values shown
below.
VALUE PROTOCOL VERSION SIGNATURE SCHEME
----- ---------------- ----------------
0 01 Reserved
1 01 OpenPGP DSA
9.3. SG Field
IANA is requested to create a registry entitled "syslog-sign sg field
values". It is for values of the SG Field as defined in
Section 4.2.3. New values shall be assigned by the IANA using the
"IETF Consensus" policy defined in [RFC5226]. Assigned values are to
be incremented by 1, up to a maximum value of "7". Values "8" and
"9" shall be left as vendor specific and shall not be assigned by the
IANA.
IANA is requested to register the SG Field values shown below.
VALUE MEANING
----- -------
0 per RFC yyyy
1 per RFC yyyy
2 per RFC yyyy
3 per RFC yyyy
9.4. Key Blob Type
IANA is requested to create a registry entitled "syslog-sign key blob
type values". It is to register one-character identifiers for the
key blob type, per Section 5.2. New values shall be assigned by the
IANA using the "IETF Consensus" policy defined in [RFC5226].
Uppercase letters may be assigned as values. Lowercase letters are
left as vendor specific and shall not be assigned by the IANA.
IANA is requested to register the key blob type values shown below.
VALUE KEY BLOB TYPE
----- ------------
'C' a PKIX certificate
'P' an OpenPGP certificate
'K' the public key whose corresponding private key is
used to sign the messages
'N' no key information sent, key is pre-distributed
'U' installation-specific key exchange information
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10. Working Group
The working group can be contacted via the mailing list:
syslog@ietf.org
The current Chairs of the Working Group can be contacted at:
Chris Lonvick
Cisco Systems
Email: clonvick@cisco.com
David Harrington
Huawei Technologies (USA)
Email: ietfdbh@comcast.net
dharrington@huawei.com
Tel: +1-603-436-8634
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11. Acknowledgements
The authors wish to thank Alex Brown, Chris Calabrese, Steve Chang,
Pasi Eronen, Carson Gaspar, Rainer Gerhards, Drew Gross, David
Harrington, Chris Lonvick, Albert Mietus, Darrin New, Marshall Rose,
Andrew Ross, Martin Schuette, Holt Sorenson, Rodney Thayer, and the
many Counterpane Internet Security engineering and operations people
who commented on various versions of this proposal.
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12. References
12.1. Normative References
[FIPS.186-2.2000]
National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-2, January 2000, <http:/
/csrc.nist.gov/publications/fips/fips186-2/
fips186-2-change1.pdf>.
[FIPS.180-2.2002]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[NIST800.90]
National Institute of Standards and Technology, "NIST
Special Publication 800-90: Recommendation for Random
Number Generation using Deterministic Random Bit
Generators", June 2006, <http://csrc.nist.gov/
publications/nistpubs/800-90/
SP800-90_DRBG-June2006-final.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", RFC 3414, December 2002.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 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,
December 2008.
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[RFC5425] Miao, F., Yuzhi, M., and J. Salowey, "TLS Transport
Mapping for syslog", RFC 5425, December 2008.
[RFC5426] Okmianski, A., "Transmission of syslog Messages over UDP",
RFC 5426, December 2008.
12.2. Informative References
[RFC3164] Lonvick, C., "The BSD syslog Protocol", RFC 3164,
August 2001.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", RFC 4086, June 2005.
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Authors' Addresses
John Kelsey
NIST
Email: john.kelsey@nist.gov
Jon Callas
PGP Corporation
Email: jon@callas.org
Alexander Clemm
Cisco Systems
Email: alex@cisco.com
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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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Kelsey, et al. Expires June 11, 2009 [Page 39]
