Network Working Group J. Schoenwaelder
Request for Comments: 3179 TU Braunschweig
Obsoletes: 2593 J. Quittek
Category: EXPerimental NEC Europe Ltd.
October 2001
Script MIB Extensibility Protocol Version 1.1
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
The Script MIB extensibility protocol (SMX) defined in this memo
separates language specific runtime systems from language independent
Script MIB implementations. The IETF Script MIB defines an interface
for the delegation of management functions based on the Internet
management framework. A management script is a set of instrUCtions
that are executed by a language specific runtime system.
Table of Contents
1 Introduction ................................................. 2
2 Process Model and Communication Model ........................ 3
3 Security Profiles ............................................ 4
4 Start of Runtime Systems and Connection Establishment ........ 4
5 SMX Messages ................................................. 5
5.1 Common Definitions ......................................... 5
5.2 Commands ................................................... 7
5.3 Replies .................................................... 7
6 Elements of Procedure ........................................ 9
6.1 SMX Message Processing on the Runtime Systems .............. 9
6.1.1 Processing the `hello' Command ........................... 10
6.1.2 Processing the `start' Command ........................... 10
6.1.3 Processing the `suspend' Command ......................... 11
6.1.4 Processing the `resume' Command .......................... 12
6.1.5 Processing the `abort' Command ........................... 12
6.1.6 Processing the `status' Command .......................... 12
6.1.7 Generation of Asynchronous Notifications ................. 13
6.2 SMX Message Processing on the SNMP Agent ................... 13
6.2.1 Creating a Runtime System ................................ 14
6.2.2 Generating the `hello' Command ........................... 14
6.2.3 Generating the `start' Command ........................... 15
6.2.4 Generating the `suspend' Command ......................... 16
6.2.5 Generating the `resume' Command .......................... 16
6.2.6 Generating the `abort' Command ........................... 17
6.2.7 Generating the `status' Command .......................... 18
6.2.8 Processing Asynchronous Notifications .................... 19
7 Example SMX Message Flow ..................................... 20
8 Transport Mappings ........................................... 20
8.1 SMX over Bi-directional Pipes .............................. 21
8.2 SMX over TCP ............................................... 21
9 Security Considerations ...................................... 21
10 Changes from RFC2593 ....................................... 22
11 Acknowledgments ............................................. 23
12 References .................................................. 23
13 Authors' Addresses .......................................... 24
14 Full Copyright Statement .................................... 25
1. Introduction
The Script MIB [1] defines a standard interface for the delegation of
management functions based on the Internet management framework. In
particular, it provides the following capabilities:
1. Transfer of management scripts to a distributed manager.
2. Initiating, suspending, resuming and terminating management
scripts.
3. Transfer of arguments for management scripts.
4. Monitoring and control of running management scripts.
5. Transfer of results produced by management scripts.
A management script is a set of instructions executed by a language
specific runtime system. The Script MIB does not prescribe a
specific language. Instead, it allows to control scripts written in
different languages that are executing concurrently.
The Script MIB Extensibility protocol (SMX) defined in this memo can
be used to separate language specific runtime systems from the
runtime system independent Script MIB implementations. The
lightweight SMX protocol can be used to support different runtime
systems without any changes to the language neutral part of a Script
MIB implementation.
Examples of languages and runtime systems considered during the
design of the SMX protocol are the Java virtual machine [2] and the
Tool Command Language (Tcl) [3]. Other languages with comparable
features should be easy to integrate as well.
2. Process Model and Communication Model
Figure 1 shows the process and communication model underlying the SMX
protocol. The language and runtime system independent SNMP agent
implementing the Script MIB communicates with one ore more runtime
systems via the SMX protocol. A runtime system may be able to
execute one or multiple scripts simultaneously (multi-threading).
The SMX protocol supports multi-threading, but it does not require
multi-threaded runtime systems.
The SMX protocol uses a local storage device (usually implemented on
top of the local file system) to transfer scripts from the SNMP agent
to the runtime systems. The SNMP agent has read and write Access to
the script storage device while the runtime systems only need read
access. The SMX protocol passes the location of a script in the
local storage device to the runtime engines. It is then the
responsibility of the runtime engines to load the script from the
specified location.
runtime 1
+--------------+ SMX +---------+
<--------------> O O O <-+
SNMP Script MIB +---------+
<---------->
SNMP Agent runtime 2
SMX +---------+
<--------------> O
+--------------+ +---------+
^ ^
+---------+
script ----------+
+------> storage ------------------+
+---------+
Figure 1: SMX process and communication model
3. Security Profiles
Security profiles control what a running script is allowed to do. It
is useful to distinguish two different classes of security profiles:
- The operating system security profile specifies the set of
operating system services that can be used by the operating system
level process which executes a script. Under UNIX, this maps to
the effective user and group identity for the running process. In
addition, many UNIX versions allow to set other resource limits,
such as the number of open files or the maximum stack sizes.
Another mechanism in UNIX is the chroot() system call which
changes the file system root for a process. The chroot()
mechanism can be used to prevent runtime systems from accessing
any system files. It is suggested to make use of all applicable
operating system security mechanism in order to protect the
operating system from malicious scripts or runtime systems.
- Secure runtime systems provide fine grained control over the set
of services that can be used by a running script at a particular
point during script execution. A runtime security profile
specifying fine grained access control is runtime system
dependent. For a Java virtual machine, the runtime security
profile is interpreted by the SecurityManager and ClassLoader
classes[4]. For Tcl, the runtime security profile maps to the
interpreter's security profile [5].
The SMX protocol allows to execute scripts under different operating
system profiles and runtime system profiles. Multiple operating
system security profiles are realized by using multiple runtime
systems which execute in operating system processes with different
security profiles. Multiple runtime security profiles are supported
by passing a security profile name to a runtime system during script
invocation.
The Script MIB does not define how operating system or runtime system
security profiles are identified. This memo suggests that the
smLaunchOwner is mapped to an operating system security profile and a
runtime system security profile when a script is started.
4. Start of Runtime Systems and Connection Establishment
The SNMP agent starts runtime systems based on the static properties
of the runtime system (multi-threaded or single-threaded) and the
operating system security profiles. Starting a new runtime system
requires to create a process environment which matches the operating
system security profile.
In order to prevent SMX communication from untrusted peers the SNMP
agent has to choose a secure SMX transport. This memo defines two
transports in Section 8: (a) a bi-directional pipe using standard
input/output streams on the runtime engine side, and (b) a TCP
connection where the SNMP agent acts as a listening server that
accepts only connections from local runtime engines that authenticate
themselves with a secret shared between the agent and the runtime
engine.
5. SMX Messages
The message formats described below are defined using the Augmented
BNF (ABNF) defined in RFC2234 [6]. The definitions for `ALPHA',
`DIGIT', `HEXDIG', `WSP', `CRLF', `CR', `LF', `HTAB', `VCHAR' and
`DQUOTE' are imported from appendix A of RFC2234 and not repeated
here.
5.1. Common Definitions
The following ABNF definitions are used in subsequent sections to
define the SMX protocol messages.
Zero = %x30 ; the ASCII character '0'
ProfileChars = DIGIT / ALPHA / %x2D-2F / %x3A / %x5F
; digits, alphas, and the characters
; '-', '.', '/', ':', '_'
QuotedString = DQUOTE *(VCHAR / WSP) DQUOTE
HexString = 1*(HEXDIG HEXDIG)
Id = 1*DIGIT ; identifier for an SMX transaction
Script = QuotedString ; script file name
RunId = 1*DIGIT ; globally unique identifier for a
; running script (note, smRunIndex
; is only unique for a smLaunchOwner,
; smLaunchName pair)
Profile = 1*ProfileChars ; security profile name
RunState = "1" ; smRunState `initializing'
RunState =/ "2" ; smRunState `executing'
RunState =/ "3" ; smRunState `suspending'
RunState =/ "4" ; smRunState `suspended'
RunState =/ "5" ; smRunState `resuming'
RunState =/ "6" ; smRunState `aborting'
RunState =/ "7" ; smRunState `terminated'
ExitCode = "1" ; smRunExitCode `noError'
ExitCode =/ "2" ; smRunExitCode `halted'
ExitCode =/ "3" ; smRunExitCode `lifeTimeExceeded'
ExitCode =/ "4" ; smRunExitCode `noResourcesLeft'
ExitCode =/ "5" ; smRunExitCode `languageError'
ExitCode =/ "6" ; smRunExitCode `runtimeError'
ExitCode =/ "7" ; smRunExitCode `invalidArgument'
ExitCode =/ "8" ; smRunExitCode `securityViolation'
ExitCode =/ "9" ; smRunExitCode `genericError'
Authenticator = HexString ; authentication cookie
Version = "SMX/1.1" ; current version of the SMX protocol
Argument = HexString / QuotedString ; see smRunArgument
Result = HexString / QuotedString ; see smRunResult
ErrorMsg = HexString / QuotedString ; see smRunError
The definition of QuotedString requires further explanation. A
quoted string may contain special character sequences, all starting
with the backslash character (%x5C). The interpretation of these
sequences is as follows:
`\\' backslash character (`%x5C')
`\t' tab character (`HTAB')
`\n' newline character (`LF')
`\r' carriage-return character (`CR')
`\"' quote character (`DQUOTE')
In all other cases not listed above, the backslash is dropped and the
following character is treated as an ordinary character.
`Argument' and `Result' is either a QuotedString or a HexString. The
Script MIB defines script arguments and results as arbitrary octet
strings. The SMX protocol supports a binary and a human readable
representation since it is likely that printable argument and result
strings will be used frequently. However, an implementation must be
able to handle both formats in order to be compliant with the Script
MIB.
The `Authenticator' is a HexString which does not carry any semantics
other than being a random sequence of bytes. It is therefore not
necessary to have a human readable representation.
5.2. Commands
The following ABNF definitions define the set of SMX commands which
can be sent from the SNMP agent to a runtime system.
Command = "hello" WSP Id CRLF
Command =/ "start" WSP Id WSP RunId WSP Script WSP Profile
WSP Argument CRLF
Command =/ "suspend" WSP Id WSP RunId CRLF
Command =/ "resume" WSP Id WSP RunId CRLF
Command =/ "abort" WSP Id WSP RunId CRLF
Command =/ "status" WSP Id WSP RunId CRLF
The `hello' command is always the first command sent over a SMX
connection. It is used to identify and authenticate the runtime
system. The `start' command starts the execution of a script. The
`suspend', `resume' and `abort' commands can be used to change the
status of a running script. The `status' command is used to retrieve
status information for a running script.
There is no compile command. It is the responsibility of the SNMP
agent to perform any compilation steps as needed before using the SMX
`start' command. There is no SMX command to shutdown a runtime
system. Closing the connection must be interpreted as a request to
terminate all running scripts in that runtime system and to shutdown
the runtime system.
5.3. Replies
Every reply message starts with a three digit reply code and ends
with `CRLF'. The three digits in a reply code have a special
meaning. The first digit identifies the class of a reply message.
The following classes exist:
1yz transient positive response
2yz permanent positive response
3yz transient negative response
4yz permanent negative response
5yz asynchronous notification
The classes 1yz and 3yz are currently not used by SMX version 1.1.
They are defined only for future SMX extensions.
The second digit encodes the specific category. The following
categories exist:
x0z syntax errors that don't fit any other category
x1z replies for commands targeted at the whole runtime system
x2z replies for commands targeted at scripts
x3z replies for commands targeted at running instances of scripts
The third digit gives a finer gradation of meaning in each category
specified by the second digit. Below is the ABNF definition of all
reply messages and codes:
Reply = "211" WSP Id WSP Version *1(WSP Authenticator) CRLF
; identification of the
; runtime system
Reply =/ "231" WSP Id WSP RunState CRLF
; status of a running script
Reply =/ "232" WSP Id CRLF ; abort of a running script
Reply =/ "401" WSP Id CRLF ; syntax error in command
Reply =/ "402" WSP Id CRLF ; unknown command
Reply =/ "421" WSP Id CRLF ; unknown or illegal Script
Reply =/ "431" WSP Id CRLF ; unknown or illegal RunId
Reply =/ "432" WSP Id CRLF ; unknown or illegal Profile
Reply =/ "433" WSP Id CRLF ; illegal Argument
Reply =/ "434" WSP Id CRLF ; unable to change the status of
; a running script
Reply =/ "511" WSP Zero WSP QuotedString CRLF
; an arbitrary message send from
; the runtime system
Reply =/ "531" WSP Zero WSP RunId WSP RunState CRLF
; asynchronous running script
; status change
Reply =/ "532" WSP Zero WSP RunId WSP RunState WSP Result CRLF
; intermediate script result
Reply =/ "533" WSP Zero WSP RunId WSP RunState WSP Result CRLF
; intermediate script result that
; triggers an event report
Reply =/ "534" WSP Zero WSP RunId WSP Result CRLF
; normal script termination,
; deprecated
Reply =/ "535" WSP Zero WSP RunId WSP ExitCode WSP ErrorMsg CRLF
; abnormal script termination,
; deprecated
Reply =/ "536" WSP Zero WSP RunId WSP RunState WSP ErrorMsg CRLF
; script error
Reply =/ "537" WSP Zero WSP RunId WSP RunState WSP ErrorMsg CRLF
; script error that
; triggers an event report
Reply =/ "538" WSP Zero WSP RunId WSP ExitCode CRLF
; script termination
6. Elements of Procedure
This section describes in detail the processing steps performed by
the SNMP agent and the runtime system with regard to the SMX
protocol.
6.1. SMX Message Processing on the Runtime Systems
This section describes the processing of SMX command messages by a
runtime engine and the conditions under which asynchronous
notifications are generated.
When the runtime system receives a message, it first tries to
recognize a command consisting of the command string and the
transaction identifier. If the runtime system is not able to extract
both the command string and the transaction identifier, then the
message is discarded. An asynchronous `511' reply may be generated
in this case. Otherwise, the command string is checked to be valid,
i.e. to be one of the strings `hello', `start', `suspend', `resume',
`abort', or `status'. If the string is invalid, a `402' reply is
sent and processing of the message stops. If a valid command has
been detected, further processing of the message depends on the
command as described below.
The command specific processing describes several possible syntax
errors for which specific reply messages are generated. If the
runtime engine detects any syntax error which is not explicitly
mentioned or which cannot be identified uniquely, a generic `401'
reply is sent indicating that the command cannot be executed.
6.1.1. Processing the `hello' Command
When the runtime system receives a `hello' command, it processes it
as follows:
1. The runtime system sends a `211' reply. If the runtime system has
access to a shared secret, then the reply must contain the
optional `Authenticator', which is a function of the shared
secret.
6.1.2. Processing the `start' Command
When the runtime system receives a `start' command, it processes it
as follows:
1. The syntax of the arguments of the `start' command is checked.
The following four checks must be made:
(a) The syntax of the `RunId' parameter is checked and a `431'
reply is sent if any syntax error is detected.
(b) The syntax of the `Script' parameter is checked and a `421'
reply is sent if any syntax error is detected.
(c) The syntax of the `Profile' parameter is checked and a `432'
reply is sent if any syntax error is detected.
(d) If syntax of the `Argument' parameter is checked and a `433'
reply is sent if any syntax error is detected.
2. The runtime system checks whether the new `RunId' is already in
use. If yes, a `431' reply is sent and processing stops.
3. The runtime system checks whether the `Script' parameter is the
name of a file on the local storage device, that can be read. A
`421' reply is sent and processing stops if the file does not
exist or is not readable.
4. The runtime system checks whether the security profile is known
and sends a `432' reply and stops processing if not.
5. The runtime engine starts the script given by the script name.
When the script has been started, a `231' reply is sent including
the current run state.
Processing of the `start' command stops, when the script reaches the
state `running'. For each asynchronous state change of the running
script, a `531' reply is sent. Processing of the `start' command is
also stopped if an error occurs before the state `running' is
reached. In this case, the run is aborted and a `538' reply is
generated. An optional `536' reply can be send before the `538'
reply to report an error message.
If an `abort' command or a `suspend' command for the running script
is received before processing of the `start' command is complete,
then the processing of the `start' command may be stopped before the
state `running' is reached. In this case, the resulting status of
the running script is given by the respective reply to the `abort' or
`suspend' command, and no reply with the transaction identifier of
the `start' command is generated.
6.1.3. Processing the `suspend' Command
When the runtime system receives a `suspend' command, it processes it
as follows:
1. If there is a syntax error in the running script identifier or if
there is no running script matching the identifier, a `431' reply
is sent and processing of the command is stopped.
2. If the running script is already in the state `suspended', a `231'
reply is sent and processing of the command is stopped.
3. If the running script is in the state `running', it is suspended
and a `231' reply is sent after suspending. If suspending fails,
a `434' reply is sent and processing of the command is stopped.
4. If the running script has not yet reached the state `running' (the
`start' command still being processed), it may reach the state
`suspended' without having been in the state `running'. After
reaching the state `suspended', a `231' reply is sent.
5. If the running script is in any other state, a `434' reply is
sent.
6.1.4. Processing the `resume' Command
When the runtime system receives a `resume' command, it processes it
as follows:
1. If there is a syntax error in the running script identifier or if
there is no running script matching the identifier, a `431' reply
is sent and processing of the command is stopped.
2. If the running script is already in the state `running', a `231'
reply is sent and processing of the command is stopped.
3. If the running script is in the state `suspended', it is resumed
and a `231' reply is sent after resuming. If resuming fails, a
`434' reply is sent and processing of the command is stopped.
4. If the `start' command is still being processed for the script, a
`231' reply is sent when the state `running' has been reached.
5. If the running script is in any other state, a `434' reply is
sent.
6.1.5. Processing the `abort' Command
When the runtime system receives an `abort' command, it processes it
as follows:
1. If there is a syntax error in the running script identifier or if
there is no running script matching the identifier, a `431' reply
is sent and processing of the command is stopped.
2. If the running script is already aborted, a `232' reply is sent
and processing of the command is stopped.
3. The running script is aborted and a `232' reply is sent after
aborting. If aborting fails, a `434' reply is sent and processing
is stopped.
6.1.6. Processing the `status' Command
When the runtime system receives a `status' command, it processes it
as follows:
1. If there is a syntax error in the running script identifier or if
there is no running script matching the identifier, a `431' reply
is sent and processing of the command is stopped.
2. The status of the script is oBTained and a `231' reply is sent.
6.1.7. Generation of Asynchronous Notifications
The runtime system generates or may generate the following
notifications:
1. If a change of the status of a running script is observed by the
runtime system, a `531' reply is sent.
2. A `534' reply is sent if a running script terminates normally.
This reply is deprecated. You can emulate this reply with a
combination of a `532' reply and a `538' reply.
3. A `535' reply is sent if a running script terminates abnormally.
This reply is deprecated. You can emulate this reply with a
combination of a `536' reply and a `538' reply.
4. A `532' reply is sent if a script generates an intermediate
result.
5. A `533' reply is sent if a script generates an intermediate result
which causes the generation of a `smScriptResult' notification.
6. A `536' reply is sent if a running script produces an error. If
the error is fatal, the script execution will be terminated and a
538 reply will follow. Otherwise, if the error is non-fatal, the
script continues execution.
7. A `537' reply is sent if a running script produces an error which
should cause the generation of a `smScriptException' notification.
If the error is fatal, the script execution will be terminated and
a 538 reply will follow. Otherwise, if the error is non-fatal,
the script continues execution.
8. A `538' reply is sent if a running script terminates. The
ExitCode is used to distinguish between normal termination
(`noError') or abnormal termination.
9. Besides the notifications mentioned above, the runtime system may
generate arbitrary `511' replies, which are logged or displayed by
the SNMP agent.
6.2. SMX Message Processing on the SNMP Agent
This section describes the conditions under which an SNMP agent
implementing the Script MIB generates SMX commands. It also
describes how the SNMP agent processes replies to SMX commands.
6.2.1. Creating a Runtime System
New runtime systems are started by the SNMP agent while processing
set requests for a `smLaunchStart' variable. The SNMP agent first
searches for an already running runtime systems which matches the
security profiles associated with the `smLaunchStart' variable. If
no suitable runtime system is available, a new runtime system is
started by either
(a) starting the executable for the runtime system in a new process
which conforms to the operating system security profile, and
establishing a bi-directional pipe to the runtime systems
standard input/output streams to be used for SMX transport, or
(b) preparing the environment for the new runtime system and starting
the executable for the runtime system in a new process which
conforms to the operating system security profile. The SNMP
agent prepares to accept a connection from the new runtime
system.
The `smRunState' of all scripts that should be executed in the new
runtime system is set to `initializing'.
6.2.2. Generating the `hello' Command
The `hello' command is generated once an SMX connection is
established. The SNMP agent sends the `hello' command as defined in
section 5.2. The SNMP agent then expects a reply from the runtime
system within a reasonable timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the connection is closed and all data associated with it is
deleted. Any scripts that should be running in this runtime
system are aborted, the `smRunExitCode' is set to `genericError'
and `smRunError' is modified to describe the error situation.
2. If the received message can not be analyzed because it does not
have the required format, then the connection is closed and all
data associated with it is deleted. Any scripts that should be
running in this runtime system are aborted, the `smRunExitCode' is
set to `genericError' and `smRunError' is modified to describe the
error situation.
3. If the received message is a `211' reply, then the `Id' is checked
whether it matches the `Id' used in the `hello' command. If the
`Id' matches, then the `Version' is checked. If the `Version'
matches a supported SMX protocol version, then, if present, the
`Authenticator' is checked. If any of the tests fails or if the
SNMP agent requires an authenticator and it did not receive a
matching `Authenticator' with the `211' reply, then the connection
is closed and all data associated with this runtime system is
deleted. Any scripts that should be running in this runtime
system are aborted, the `smRunExitCode' is set to `genericError'
and `smRunError' is modified to describe the error situation.
4. Received messages are discarded if none of the previous rules
applies.
6.2.3. Generating the `start' Command
The `start' command is generated while processing set-requests for a
`smLaunchStart' variable. The `start' command assumes that the SNMP
agent already determined a runtime system suitable to execute the
script associated with the `smLaunchStart' variable. The SNMP agent
sends the `start' command as defined in section 5.2 to the selected
runtime system. The SNMP agent then expects a reply from the runtime
system within a reasonable timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the SNMP agent sends an `abort' command to abort the running
script and sets the `RunState' of the running script to
`terminated', the `smRunExitCode' to `genericError' and
`smRunError' is modified to describe the timeout situation.
2. If the received message can not be analyzed because it does not
have the required format, then the message is ignored. The SNMP
agent continues to wait for a valid reply message until the
timeout expires.
3. If the received message is a `4yz' reply and the `Id' matches the
`Id' of the `start' command, then the SNMP agent assumes that the
script can not be started. The `smRunState' of the running script
is set to `terminated', the `smRunExitCode' to `genericError' and
the `smRunError' is modified to contain a message describing the
error situation.
4. If the received message is a `231' reply and the `Id' matches the
`Id' of the `start' command, then the `smRunState' variable of the
running script is updated.
5. Received messages are discarded if none of the previous rules
applies.
6.2.4. Generating the `suspend' Command
The `suspend' command is generated while processing set-requests for
the `smLaunchControl' and `smRunControl' variables which change the
value to `suspend'. The SNMP agent sets the `smRunState' variable to
`suspending' and sends the `suspend' command as defined in section
5.2. The SNMP agent then expects a reply from the runtime system
within a reasonable timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the SNMP agent sends an `abort' command to abort the running
script and sets the `smRunState' of the running script to
`terminated', the `smRunExitCode' to `genericError' and
`smRunError' is modified to describe the timeout situation.
2. If the received message can not be analyzed because it does not
have the required format, then the message is ignored. The SNMP
agent continues to wait for a valid reply message until the
timeout expires.
3. If the received message is a `401', `402' or a `431' reply and the
`Id' matches the `Id' of the `suspend' command, then the runtime
systems is assumed to not provide the suspend/resume capability
and processing of the `suspend' command stops.
4. If the received message is a `231' reply and the `Id' matches the
`Id' of the `suspend' command, then the `smRunState' variable of
the running script is updated.
5. Received messages are discarded if none of the previous rules
applies.
6.2.5. Generating the `resume' Command
The `resume' command is generated while processing set-requests for
the `smLaunchControl' and `smRunControl' variables which change the
value to `resume'. The SNMP agent sets the `smRunState' variable to
`resuming' and sends the `resume' command as defined in section 5.2.
The SNMP agent then expects a reply from the runtime system within a
reasonable timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the SNMP agent sends an `abort' command to abort the running
script and sets the `smRunState' of the running script to
`terminated', the `smRunExitCode' to `genericError' and
`smRunError' is modified to describe the timeout situation.
2. If the received message can not be analyzed because it does not
have the required format, then the message is ignored. The SNMP
agent continues to wait for a valid reply message until the
timeout expires.
3. If the received message is a `401', `402' or a `431' reply and the
`Id' matches the `Id' of the `resume' command, then the runtime
systems is assumed to not provide the suspend/resume capability
and processing of the `resume' command stops.
4. If the received message is a `231' reply and the `Id' matches the
`Id' of the `resume' command, then the `smRunState' variable of
the running script is updated.
5. Received messages are discarded if none of the previous rules
applies.
6.2.6. Generating the `abort' Command
The `abort' command is generated while processing set-requests for
the `smLaunchControl' and `smRunControl' variables which change the
value to `abort'. In addition, the `abort' command is also generated
if the `smRunLifeTime' variable reaches the value 0. The SNMP agent
sends the `abort' command as defined in section 5.2. The SNMP agent
then expects a reply from the runtime system within a reasonable
timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the SNMP agent sets the `smRunState' of the running script to
`terminated', the `smRunExitCode' to `genericError' and
`smRunError' is modified to describe the timeout situation.
2. If the received message can not be analyzed because it does not
have the required format, then the message is ignored. The SNMP
agent continues to wait for a valid reply message until the
timeout expires.
3. If the received message is a `4yz' reply and the `Id' matches the
`Id' of the `abort' command, then the SNMP agent assumes that the
script can not be aborted. The `smRunState' of the running script
is set to `terminated', the `smRunExitCode' to `genericError' and
the `smRunResult' is modified to describe the error situation.
4. If the received message is a `232' reply and the `Id' matches the
`Id' of the `abort' command, then the `smRunExitCode' variable of
the terminated script is changed to either `halted' (when
processing a set-request for the `smLaunchControl' and
`smRunControl' variables) or `lifeTimeExceeded' (if the `abort'
command was generated because the `smRunLifeTime' variable reached
the value 0). The `smRunState' variable is changed to the value
`terminated'.
5. Received messages are discarded if none of the previous rules
applies.
6.2.7. Generating the `status' Command
The `status' command is generated either periodically or on demand by
the SNMP agent in order to retrieve status information from running
scripts. The SNMP agent sends the `status' command as defined in
5.2. The SNMP agent then expects a reply from the runtime system
within a reasonable timeout interval.
1. If the timeout expires before the SNMP agent received a reply,
then the SNMP agent sends an `abort' command to abort the running
script and sets the `smRunState' of the running script to
`terminated', the `smRunExitCode' to `genericError' and
`smRunError' is modified to describe the timeout situation.
2. If the received message can not be analyzed because it does not
have the required format, then the message is ignored. The SNMP
agent continues to wait for a valid reply message until the
timeout expires.
3. If the received message is a `4yz' reply and the `Id' matches the
`Id' of the `status' command, then the SNMP agent assumes that the
script status can not be read, which is a fatal error condition.
The SNMP agent sends an `abort' command to abort the running
script. The `smRunState' of the running script is set to
`terminated', the `smRunExitCode' to `genericError' and the
`smRunError' is modified to describe the error situation.
4. If the received message is a `231' reply and the `Id' matches the
`Id' of the `status' command, then the `smRunState' variable of
the running script is updated.
5. Received messages are discarded if none of the previous rules
applies.
6.2.8. Processing Asynchronous Notifications
The runtime system can send asynchronous status change notifications.
These `5yz' replies are processed as described below.
1. If the received message is a `511' reply, then the message is
displayed or logged appropriately and processing stops.
2. If the received message is a `531' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, the
`smRunState' is updated.
3. If the received message is a `532' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, `smRunState'
and `smRunResult' are updated.
4. If the received message is a `533' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, `smRunState'
and `smRunResult' are updated and the `smScriptResult'
notification is generated.
5. If the received message is a `534' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing stops if there is no running
script with the `RunId'. Otherwise, `smExitCode' is set to
`noError', `smRunState' is set to `terminated' and `smRunResult'
is updated.
6. If the received message is a `535' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing stops if there is no running
script with the `RunId'. Otherwise, `smRunState' is set to
`terminated' and `smExitCode' and `smRunError' are updated.
7. If the received message is a `536' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, `smRunState'
and `smRunError' are updated.
8. If the received message is a `537' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, `smRunState'
and `smRunError' are updated and the `smScriptException'
notification is generated.
9. If the received message is a `538' reply, then the SNMP agent
checks whether a running script with the given `RunId' exists in
the runtime system. Processing of the notification stops if there
is no running script with the `RunId'. Otherwise, `smRunState' is
set to `terminated' and the `smExitCode' is updated.
7. Example SMX Message Flow
Below is an example SMX message exchange. Messages sent from the
SNMP agent are marked with `>' while replies sent from the runtime
system are marked with `<'. Line terminators (`CRLF') are not shown
in order to make the example more readable.
> hello 1
< 211 1 SMX/1.1 0AF0BAED6F877FBC
> start 2 42 "/var/snmp/scripts/foo.jar" untrusted ""
> start 5 44 "/var/snmp/scripts/bar.jar" trusted "www.ietf.org"
< 231 2 2
> start 12 48 "/var/snmp/scripts/foo.jar" funny ""
< 231 5 2
< 532 0 44 2 "waiting for response"
> status 18 42
> status 19 44
< 432 12
< 231 19 2
< 231 18 2
> hello 578
< 211 578 SMX/1.1 0AF0BAED6F877FBC
> suspend 581 42
< 231 581 4
< 532 0 44 7 "test completed"
< 538 0 44 1
> abort 611 42
< 232 611
8. Transport Mappings
In order to prevent SMX communication from untrusted peers the SNMP
agent has to choose a secure SMX transport. This memo defines two
transports in Section 8: (a) a bi-directional pipe using standard
input/output streams on the runtime engine side, and (b) a TCP
connection where the SNMP agent acts as a listening server that
accepts only connections from local runtime engines that authenticate
themselves with a secret shared between the agent and the runtime
engine.
For simplicity and security reasons the transport over bi-directional
pipes is the preferred transport.
Further transports (e.g., UNIX domain sockets) are possible but not
defined at this point in time. The reason for choosing pipes and TCP
connections as the transport for SMX was that these IPC mechanisms
are supported by most potential runtime systems, while other
transports are not universally available.
8.1. SMX over Bi-directional Pipes
The SNMP agent first creates a bi-directional pipe. Then the agent
creates the runtime system process with its standard input and
standard output streams connected to the pipe. Further
authentication mechanisms are not required.
8.2. SMX over TCP
The SNMP agent first creates a listening TCP socket which accepts
connections from runtime systems. Then the agent creates the runtime
system process. It is then the responsibility of the runtime system
to establish a connection to the agent's TCP socket once it has been
started. The SNMP agent must ensure that only authorized runtime
systems establish a connection to the listening TCP socket. The
following rules are used for this purpose:
- The TCP connection must originate from the local host.
- The SNMP agent must check the `Authenticator' in the `211' reply
if authentication is required and it must close the TCP connection
if no valid response is received within a given time interval.
9. Security Considerations
The SMX protocol as specified in this memo runs over a bi-directional
pipe or over a local TCP connection between the agent and the runtime
system. Protocol messages never leave the local system. It is
therefore not possible to attack the message exchanges if the
underlying operating system protects bi-directional pipes and local
TCP connections from other users on the same machine.
The transport over a bi-directional pipe specifies that the pipe is
created and connected to the standard input/output stream of the
runtime engine by the agent before the runtime engine is started. It
is therefore not possible that an unauthorized process can exchange
SMX messages over the bi-directional pipe.
In case of the TCP transport, the only critical situation is the
connection establishment phase. The rules defined in section 8
ensure that only local connections are accepted and that a runtime
system has to authenticate itself with an authenticator if the agent
requires authentication. It is strongly suggested that agents
require authentication, especially on multiuser systems.
The SMX 1.0 specification in RFC2593 suggested a scheme where the
authenticator was passed to the runtime engines as part of the
process environment. This scheme relies on the protection of process
environments by the operating system against unauthorized access.
Some operating systems allow users to read the process environment of
arbitrary processes. Hence the scheme proposed in RFC2593 is
considered unsecure on these operating systems. This memo does not
dictate the mechanism by which the runtime obtains the shares secret.
It is the responsibility of implementors or administrators to select
a mechanism which is secure on the target platforms.
The SMX protocol assumes a local script storage area which is used to
pass script code from the SNMP agent to the runtime systems. The SMX
protocol passes file names from the agent to the runtime engines. It
is necessary that the script files in the local script storage area
are properly protected so that only the SNMP agent has write access.
Failure to properly protect write access to the local script storage
area can allow attackers to execute arbitrary code in runtime systems
that might have special privileges.
The SMX protocol allows to execute script under different operating
system and runtime system security profiles. The memo suggests to
map the smLaunchOwner value to an operating system and a runtime
system security profile. The operating system security profile is
enforced by the operating system by setting up a proper process
environment. The runtime security profile is enforced by a secure
runtime system (e.g., the Java virtual machine or a safe Tcl
interpreter) [7].
10. Changes from RFC2593
The following non-editorial changes have been made:
1. Added the `536' and `537' replies which may be generated
asynchronously by runtime engines to report error conditions.
2. Added the `538' reply which can be used to signal the (normal or
abnormal) termination of a running script. This new reply
replaces the `534' and `535' replies, which are now deprecated.
3. Relaxed the rules for ProfileChars to also include the characters
':' and '_', which are frequently used in namespaces and
identifiers.
4. Changed the SMX protocol version number from 1.0 to 1.1.
5. Added a second (and preferred) transport over a bi-directional
pipe due to security risks when a shared secret is passed through
an operating system's environment variable.
6. Made the `Authenticator' in the `211' reply optional.
11. Acknowledgments
The protocol described in this memo is the result of a joint project
between the Technical University of Braunschweig and C&C Research
Laboratories of NEC Europe Ltd. in Heidelberg. The authors like to
thank Matthias Bolz, Cornelia Kappler, Andreas Kind, Sven Mertens,
Jan Nicklisch, and Frank Strauss for their contributions to the
design and the implementation of the protocol described in this memo.
The authors also like to thank David Wallis for pointing out a
security risk in SMX 1.0 with passing a cookie via an operating
system environment variable.
12. References
[1] Levi, D. and J. Schoenwaelder, "Definitions of Managed Objects
for the Delegation of Management Scripts", RFC3165, September
2001.
[2] Lindholm, T., and F. Yellin, "The Java Virtual Machine
Specification", Addison Wesley, 1997.
[3] J.K. Ousterhout, "Tcl and the Tk Toolkit", Addison Wesley, 1994.
[4] Fritzinger, J.S., and M. Mueller, "Java Security", White Paper,
Sun Microsystems, Inc., 1996.
[5] Levy, J.Y., Demailly, L., Ousterhout, J.K., and B. Welch, "The
Safe-Tcl Security Model", Proc. USENIX Annual Technical
Conference, June 1998.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.
[7] Schoenwaelder, J., and J. Quittek, "Secure Internet Management
by Delegation", Computer Networks 35(1), January 2001.
13. Authors' Addresses
Juergen Schoenwaelder
TU Braunschweig
Bueltenweg 74/75
38106 Braunschweig
Germany
Phone: +49 531 391-3283
EMail: schoenw@ibr.cs.tu-bs.de
Juergen Quittek
NEC Europe Ltd.
C&C Research Laboratories
Adenauerplatz 6
69115 Heidelberg
Germany
Phone: +49 6221 90511-15
EMail: quittek@ccrle.nec.de
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