DCE/RPC 2 Preprocessor
================================================================================
The main purpose of the preprocessor is to perform SMB desegmentation and
DCE/RPC defragmentation to avoid rule evasion using these techniques.  SMB
desegmentation is performed for the following commands that can be used to
transport DCE/RPC requests and responses: Write, Write Block Raw,
Write and Close, Write AndX, Transaction, Transaction Secondary, Read,
Read Block Raw and Read AndX.  The following transports are supported for
DCE/RPC: SMB, TCP, UDP and RPC over HTTP v.1 proxy and server.  New rule
options have been implemented to improve performance, reduce false
positives and reduce the count and complexity of DCE/RPC based rules.

Sections:
    Dependency Requirements
    Target Based
    Configuration
    Events
    Rule Options


Dependency Requirements
================================================================================
For proper functioning of the preprocessor:
    The dcerpc preprocessor (the initial iteration) must be disabled.
    Stream session tracking must be enabled, i.e. stream5. The preprocessor
      requires a session tracker to keep its data.
    Stream reassembly must be performed for TCP sessions. If it is decided that
      a session is SMB or DCE/RPC, either through configured ports, servers or
      autodetecting, the dcerpc2 preprocessor will enable stream reassembly for
      that session if necessary.
    IP defragmentation should be enabled, i.e. the frag3 preprocessor should be
      enabled and configured.


Target Based
================================================================================
There are enough important differences between Windows and Samba versions that
a target based approach has been implemented. Some important differences:

Named pipe instance tracking
    A combination of valid login handle or UID, share handle or TID and
    file/named pipe handle or FID must be used to write data to a named pipe.
    The binding between these is dependent on OS/software version.
        Samba 3.0.22 and earlier
            Any valid UID and TID, along with a valid FID can be used to make
            a request, however, if the TID used in creating the FID is deleted
            (via a tree disconnect), the FID that was created using this TID
            becomes invalid, i.e. no more requests can be written to that named
            pipe instance.
        Samba greater than 3.0.22
            Any valid TID, along with a valid FID can be used to make a
            request. However, only the UID used in opening the named pipe can
            be used to make a request using the FID handle to the named pipe
            instance. If the TID used to create the FID is deleted (via a
            Tree Disconnect), the FID that was created using this TID becomes
            invalid, i.e. no more requests can be written to that named pipe
            instance. If the UID used to create the named pipe instance is
            deleted (via a Logoff AndX), since it is necessary in making a
            request to the named pipe, the FID becomes invalid.
        Windows 2003
        Windows XP
        Windows Vista
            These Windows versions require strict binding between the UID, TID
            and FID used to make a request to a named pipe instance. Both the
            UID and TID used to open the named pipe instance must be used when
            writing data to the same named pipe instance. Therefore, deleting
            either the UID or TID invalidates the FID.
        Windows 2000
            Windows 2000 is interesting in that the first request to a named
            pipe must use the same binding as that of the other Windows
            versions. However, requests after that follow the same binding as
            Samba 3.0.22 and earlier, i.e. no binding. It also follows Samba
            greater than 3.0.22 in that deleting the UID or TID used to create
            the named pipe instance also invalidates it.
Accepted SMB commands
    Samba in particular does not recognize certain commands under an IPC$ tree.
        Samba (all versions)
           Under an IPC$ tree, does not accept:
               Open
               Write And Close
               Read
               Read Block Raw
               Write Block Raw
        Windows (all versions)
           Accepts all of the above commands under an IPC$ tree.
AndX command chaining
    Windows is very strict in what command combinations it allows to be
    chained. Samba, on the other hand, is very lax and allows some nonsensical
    combinations, e.g. multiple logins and tree connects (only one place to
    return handles for these), login/logoff and tree connect/tree disconnect.
    Ultimately, we don't want to keep track of data that the server won't
    accept. An evasion possibility would be accepting a fragment in a request
    that the server won't accept that gets sandwiched between an exploit.
Transaction tracking
    The differences between a Transaction request and using one of the Write*
    commands to write data to a named pipe are that (1) a Transaction performs
    the operations of a write and a read from the named pipe, whereas in using
    the Write* commands, the client has to explicitly send one of the Read*
    requests to tell the server to send the response and (2) a Transaction
    request is not written to the named pipe until all of the data is received
    (via potential Transaction Secondary requests) whereas with the Write*
    commands, data is written to the named pipe as it is received by the
    server. Multiple Transaction requests can be made simultaneously to the
    same named pipe. These requests can also be segmented with
    Transaction Secondary commands. What distinguishes them (when the same
    named pipe is being written to, i.e. having the same FID) are fields in the
    SMB header representing a process id (PID) and multiplex id (MID).  The PID
    represents the process this request is a part of.  An MID represents
    different sub-processes within a process (or under a PID). Segments for
    each "thread" are stored separately and written to the named pipe when all
    segments are received. It is necessary to track this so as not to munge
    these requests together (which would be a potential evasion opportunity).
        Windows (all versions)
            Uses a combination of PID and MID to define a "thread".
        Samba (all versions)
            Uses just the MID to define a "thread".
Multliple Bind requests
    A Bind request is the first request that must be made in a
    connection-oriented DCE/RPC session in order to specify the
    interface/interfaces that one wants to communicate with.
        Windows (all versions)
            For all of the Windows versions, only one Bind can ever be made on
            a session whether or not it succeeds or fails. Any binding after
            that must use the Alter Context request.  If another Bind is made,
            all previous interface bindings are invalidated.
        Samba 3.0.20 and earlier
            Any amount of Bind requests can be made.
        Samba later than 3.0.20
            Another Bind request can be made if the first failed and no
            interfaces were successfully bound to. If a Bind after a
            successful Bind is made, all previous interface bindings are
            invalidated.
DCE/RPC Fragmented requests - Context ID
    Each fragment in a fragmented request carries the context id of the bound
    interface it wants to make the request to.
        Windows (all versions)
            The context id that is ultimately used for the request is contained
            in the first fragment.  The context id field in any other fragment
            can contain any value.
        Samba (all versions)
            The context id that is ultimately used for the request is contained
            in the last fragment.  The context id field in any other fragment
            can contain any value.
DCE/RPC Fragmented requests - Operation number
    Each fragment in a fragmented request carries an operation number (opnum)
    which is more or less a handle to a function offered by the interface.
        Samba (all versions)
        Windows 2000
        Windows 2003
        Windows XP
            The opnum that is ultimately used for the request is contained in
            the last fragment.  The opnum field in any other fragment can
            contain any value.
        Windows Vista
            The opnum that is ultimately used for the request is contained in
            the first fragment.  The opnum field in any other fragment can
            contain any value.
DCE/RPC Stub data byte order
    The byte order of the stub data is determined differently for Windows and
    Samba.
        Windows (all versions)
            The byte order of the stub data is that which was used in the Bind
            request.
        Samba (all versions)
            The byte order of the stub data is that which is used in the
            request carrying the stub data.


Configuration
================================================================================
The dcerpc2 preprocessor has a global configuration and one or more server
configurations.  The global preprocessor configuration name is dcerpc2 and the
server preprocessor configuration name is dcerpc2_server.

Global configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
preprocessor dcerpc2

The global dcerpc2 configuration is required.
Only one global dcerpc2 configuration can be specified.

Option syntax
    Option                  Argument        Required      Default
    ----------------------------------------------------------------------------
    memcap                  <memcap>              NO      memcap 102400
    disable_defrag          NONE                  NO      OFF
    max_frag_len            <max-frag-len>        NO      OFF
    events                  <events>              NO      events [smb, co, cl]
    reassemble_threshold    <re-thresh>           NO      OFF

    memcap           = 1024-4194303 (kilobytes)
    max-frag-len     = 1514-65535
    events           = pseudo-event | event | '[' event-list ']'
    pseudo-event     = "none" | "all"
    event-list       = event | event ',' event-list
    event            = "memcap" | "smb" | "co" | "cl"
    re-thresh        = 0-65535

Option explanations
    memcap
        Specifies the maximum amount of run-time memory that can be allocated.
        Run-time memory includes any memory allocated after configuration.
        Default is 100 MB.
    disable_defrag
        Tells the preprocessor not to do DCE/RPC defragmentation.  Default is
        to do defragmentation.
    max_frag_len
        Specifies the maximum fragment size that will be added to the
        defragmention module. If a fragment is greater than this size, it is
        truncated before being added to the defragmentation module.  Default is
        not set.
    events
        Specifies the classes of events to enable.  (See Events section for an
        enumeration and explanation of events.)
             memcap
                 Only one event. If the memcap is reached or exceeded, alert.
             smb
                 Alert on events related to SMB processing.
             co
                 Stands for connection-oriented DCE/RPC. Alert on events
                 related to connection-oriented DCE/RPC processing.
             cl
                 Stands for connectionless DCE/RPC. Alert on events related
                 to connectionless DCE/RPC processing.
        Defaults are smb, co and cl.
    reassemble_threshold
        Specifies a minimum number of bytes in the DCE/RPC desegmentation and
        defragmentation buffers before creating a reassembly packet to send to
        the detection engine. This option is useful in inline mode so as to
        potentially catch an exploit early before full defragmentation is done.
        A value of 0 supplied as an argument to this option will, in effect,
        disable this option.  Default is disabled.

Option examples
     memcap 30000
     max_frag_len 16840
     events none
     events all
     events smb
     events co
     events [co]
     events [smb, co]
     events [memcap, smb, co, cl]
     reassemble_threshold 500

Configuration examples
     preprocessor dcerpc2
     preprocessor dcerpc2: memcap 500000
     preprocessor dcerpc2: max_frag_len 16840, memcap 300000, events smb
     preprocessor dcerpc2: memcap 50000, events [memcap, smb, co, cl], \
                           max_frag_len 14440
     preprocessor dcerpc2: disable_defrag, events [memcap, smb]
     preprocessor dcerpc2: reassemble_threshold 500

Default configuration
     preprocessor dcerpc2: memcap 102400, events [smb, co, cl]

Server Configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
preprocessor dcerpc2_server

The dcerpc2_server configuration is optional.
A dcerpc2_server configuration must start with default or net options.
The default and net options are mutually exclusive.
At most one default configuration can be specified. If no default
    configuration is specified, default values will be used for the default
    configuration.
Zero or more net configurations can be specified.
For any dcerpc2_server configuration, if non-required options are not
    specified, the defaults will be used.
When processing DCE/RPC traffic, the default configuration is used if no net
    configurations match.
If a net configuration matches, it will override the default configuration. A
    net configuration matches if the packet's server IP address matches an IP
    address or net specified in the net configuration.
The net option supports IPv6 addresses.
Note that port and ip variables defined in snort.conf CANNOT be used.

Option syntax
    Option                 Argument  Required  Default
    ----------------------------------------------------------------------------
    default                    NONE       YES  NONE
    net                       <net>       YES  NONE
    policy                 <policy>        NO  policy WinXP
    detect                 <detect>        NO  detect \
                                                 [smb [139,445], \
                                                  tcp 135, udp 135, \
                                                  rpc-over-http-server 593]
    autodetect             <detect>        NO  autodetect \
                                                 [tcp 1025:, udp 1025:, \
                                                  rpc-over-http-server 1025:]
    no_autodetect_http_proxy_ports
                               NONE        NO  DISABLED (The preprocessor
                                                 autodetects on all proxy ports
                                                 by default)
    smb_invalid_shares     <shares>        NO  NONE
    smb_max_chain       <max-chain>        NO  smb_max_chain 3

    net          = ip | '[' ip-list ']'
    ip-list      = ip | ip ',' ip-list
    ip           = ip-addr | ip-addr '/' prefix | ip4-addr '/' netmask
    ip-addr      = ip4-addr | ip6-addr
    ip4-addr     = a valid IPv4 address
    ip6-addr     = a valid IPv6 address (can be compressed)
    prefix       = a valid CIDR
    netmask      = a valid netmask
    policy       = "Win2000" | "Win2003" | "WinXP" | "WinVista" |
                   "Samba" | "Samba-3.0.22" | "Samba-3.0.20"
    detect       = "none" | detect-opt | '[' detect-list ']'
    detect-list  = detect-opt | detect-opt ',' detect-list
    detect-opt   = transport | transport port-item | 
                   transport '[' port-list ']'
    transport    = "smb" | "tcp" | "udp" | "rpc-over-http-proxy" | 
                   "rpc-over-http-server"
    port-list    = port-item | port-item ',' port-list
    port-item    = port | port-range
    port-range   = ':' port | port ':' | port ':' port
    port         = 0-65535
    shares       = share | '[' share-list ']'
    share-list   = share | share ',' share-list
    share        = word | '"' word '"' | '"' var-word '"'
    word         = graphical ascii characters except ',' '"' ']' '[' '$'
    var-word     = graphical ascii characters except ',' '"' ']' '['

    Because the Snort main parser treats '$' as the start of a variable and
    tries to expand it, shares with '$' must be enclosed quotes.

    max-chain    = 0-255

Option explanations
   default
        Specifies that this configuration is for the default server
        configuration.
   net
        Specifies that this configuration is an IP or net specific
        configuration. The configuration will only apply to the IP addresses
        and nets supplied as an argument.
   policy
        Specifies the target-based policy to use when processing.  Default is
        "WinXP".
   detect
        Specifies the DCE/RPC transport and server ports that should be
        detected on for the transport.  Defaults are ports 139 and 445 for SMB,
        135 for TCP and UDP, 593 for RPC over HTTP server and 80 for RPC over
        HTTP proxy. 
   autodetect
        Specifies the DCE/RPC transport and server ports that the preprocessor
        should attempt to autodetect on for the transport. The autodetect ports
        are only queried if no detect transport/ports match the packet. The
        order in which the preprocessor will attempt to autodetect will be -
        TCP/UDP, RPC over HTTP server, RPC over HTTP proxy and lastly SMB.
        Note that most dynamic DCE/RPC ports are above 1024 and ride directly
        over TCP or UDP.  It would be very uncommon to see SMB on anything
        other than ports 139 and 445.  Defaults are 1025-65535 for TCP, UDP and
        RPC over HTTP server.
   no_autodetect_http_proxy_ports
        By default, the preprocessor will always attempt to autodetect for
        ports specified in the detect configuration for rpc-over-http-proxy.
        This is because the proxy is likely a web server and the preprocessor
        should not look at all web traffic. This option is useful if the RPC
        over HTTP proxy configured with the detect option is only used to proxy
        DCE/RPC traffic.  Default is to autodetect on RPC over HTTP proxy
        detect ports.
   smb_invalid_shares
        Specifies SMB shares that the preprocessor should alert on if an
        attempt is made to connect to them via a Tree Connect or
        Tree Connect AndX.  Default is empty.
   smb_max_chain
        Specifies the maximum amount of AndX command chaining that is allowed
        before an alert is generated.  Default maximum is 3 chained commands.
        A value of 0 disables this option.

Option examples
   net 192.168.0.10
   net 192.168.0.0/24
   net [192.168.0.0/24]
   net 192.168.0.0/255.255.255.0
   net feab:45b3:ab92:8ac4:d322:007f:e5aa:7845
   net feab:45b3:ab92:8ac4:d322:007f:e5aa:7845/128
   net feab:45b3::/32
   net [192.168.0.10, feab:45b3::/32]
   net [192.168.0.0/24, feab:45b3:ab92:8ac4:d322:007f:e5aa:7845]
   policy Win2000
   policy Samba-3.0.22
   detect none
   detect smb
   detect [smb]
   detect smb 445
   detect [smb 445]
   detect smb [139,445]
   detect [smb [139,445]]
   detect [smb, tcp]
   detect [smb 139, tcp [135,2103]]
   detect [smb [139,445], tcp 135, udp 135, \
           rpc-over-http-server [593,6002:6004]]
   autodetect none
   autodetect tcp
   autodetect [tcp]
   autodetect tcp 2025:
   autodetect [tcp 2025:]
   autodetect tcp [2025:3001,3003:]
   autodetect [tcp [2025:3001,3003:]]
   autodetect [tcp, udp]
   autodetect [tcp 2025:, udp 2025:]
   autodetect [tcp 2025:, udp, rpc-over-http-server [1025:6001,6005:]]
   smb_invalid_shares private
   smb_invalid_shares "private"
   smb_invalid_shares "C$"
   smb_invalid_shares [private, "C$"]
   smb_invalid_shares ["private", "C$"]
   smb_max_chain 1

Configuration examples
   preprocessor dcerpc2_server: default
   preprocessor dcerpc2_server: default, policy Win2000
   preprocessor dcerpc2_server: default, policy Win2000, detect [smb, tcp], \
     autodetect tcp 1025:, smb_invalid_shares ["C$", "D$", "ADMIN$"]
   preprocessor dcerpc2_server: net 10.4.10.0/24, policy Win2000
   preprocessor dcerpc2_server: net [10.4.10.0/24,feab:45b3::/126], \
     policy WinVista, smb_max_chain 1
   preprocessor dcerpc2_server: net [10.4.10.0/24,feab:45b3::/126], \
     policy WinVista, detect [smb, tcp, rpc-over-http-proxy 8081], \
     autodetect [tcp, rpc-over-http-proxy [1025:6001,6005:]], \
     smb_invalid_shares ["C$", "ADMIN$"], no_autodetect_http_proxy_ports
   preprocessor dcerpc2_server: net [10.4.11.56,10.4.11.57], policy Samba, \
     detect smb, autodetect none

Default configuration
   preprocessor dcerpc2_server: default, policy WinXP, \
     detect [smb [139,445], tcp 135, udp 135, rpc-over-http-server 593], \
     autodetect [tcp 1025:, udp 1025:, rpc-over-http-server 1025:], \
     smb_max_chain 3

Complete dcerpc2 default configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   preprocessor dcerpc2: memcap 102400, events [smb, co, cl]
   preprocessor dcerpc2_server: default, policy WinXP, \
     detect [smb [139,445], tcp 135, udp 135, rpc-over-http-server 593], \
     autodetect [tcp 1025:, udp 1025:, rpc-over-http-server 1025:], \
     smb_max_chain 3


Events
================================================================================
The preprocessor uses GID 133 to register events.

Memcap events
 SID   Description 
--------------------------------------------------------------------------------
   1   If the memory cap is reached and the preprocessor is configured to
       alert.

SMB events
 SID   Description 
--------------------------------------------------------------------------------
   2   An invalid NetBIOS Session Service type was specified in the header.
       Valid types are: Message, Request (only from client), Positive Response
       (only from server), Negative Response (only from server), Retarget
       Response (only from server) and Keep Alive.
   3   An SMB message type was specified in the header. Either a request was
       made by the server or a response was given by the client.
   4   The SMB id does not equal "\xffSMB". Note that since the preprocessor
       does not yet support SMB2, id of "\xfeSMB" is turned away before an
       eventable point is reached.
   5   The word count of the command header is invalid. SMB commands have
       pretty specific word counts and if the preprocessor sees a command with
       a word count that doesn't jive with that command, the preprocessor will
       alert.
   6   Some commands require a minimum number of bytes after the command
       header. If a command requires this and the byte count is less than the
       minimum required byte count for that command, the preprocessor will
       alert.
   7   Some commands, especially the commands from the SMB Core implementation
       require a data format field that specifies the kind of data that will be
       coming next. Some commands require a specific format for the data. The
       preprocessor will alert if the format is not that which is expected for
       that command.
   8   Many SMB commands have a field containing an offset from the beginning
       of the SMB header to where the data the command is carrying starts. If
       this offset puts us before data that has already been processed or after
       the end of payload, the preprocessor will alert.
   9   Some SMB commands, such as Transaction, have a field containing the
       total amount of data to be transmitted. If this field is zero, the
       preprocessor will alert.
  10   The preprocessor will alert if the NetBIOS Session Service length field
       contains a value less than the size of an SMB header.
  11   The preprocessor will alert if the remaining NetBIOS packet length is
       less than the size of the SMB command header to be decoded.
  12   The preprocessor will alert if the remaining NetBIOS packet length is
       less than the size of the SMB command byte count specified in the
       command header.
  13   The preprocessor will alert if the remaining NetBIOS packet length is
       less than the size of the SMB command data size specified in the command
       header.
  14   The preprocessor will alert if the total data count specified in the
       SMB command header is less than the data size specified in the SMB
       command header. (Total data count must always be greater than or equal
       to current data size.)
  15   The preprocessor will alert if the total amount of data sent in a
       transaction is greater than the total data count specified in the SMB
       command header.
  16   The preprocessor will alert if the byte count specified in the SMB
       command header is less than the data size specified in the SMB command.
       (The byte count must always be greater than or equal to the data size.)
  17   Some of the Core Protocol commands (from the initial SMB implementation)
       require that the byte count be some value greater than the data size
       exactly. The preprocessor will alert if the byte count minus a
       predetermined amount based on the SMB command is not equal to the data
       size.
  18   For the Tree Connect command (and not the Tree Connect AndX command),
       the preprocessor has to queue the requests up and wait for a server
       response to determine whether or not an IPC share was successfully
       connected to (which is what the preprocessor is interested in). Unlike
       the Tree Connect AndX response, there is no indication in the
       Tree Connect response as to whether the share is IPC or not. There
       should be under normal circumstances no more than a few pending tree
       connects at a time and the preprocessor will alert if this number is
       excessive.
  19   After a client is done writing data using the Write* commands, it issues
       a Read* command to the server to tell it to send a response to the data
       it has written. In this case the preprocessor is concerned with the
       server response. The Read* request contains the file id associated with
       a named pipe instance that the preprocessor will ultimately send the
       data to. The server response, however, does not contain this file id, so
       it need to be queued with the request and dequeued with the response. If
       multiple Read* requests are sent to the server, they are responded to in
       the order they were sent. There should be under normal circumstances no
       more than a few pending Read* requests at a time and the preprocessor
       will alert if this number is excessive.
  20   The preprocessor will alert if the number of chained commands in a
       single request is greater than or equal to the configured amount
       (default is 3).
  21   With AndX command chaining it is possible to chain multiple
       Session Setup AndX commands within the same request. There is, however,
       only one place in the SMB header to return a login handle (or Uid).
       Windows does not allow this behavior, however Samba does. This is
       anomalous behavior and the preprocessor will alert if it happens.
  22   With AndX command chaining it is possible to chain multiple
       Tree Connect AndX commands within the same request. There is, however,
       only one place in the SMB header to return a tree handle (or Tid).
       Windows does not allow this behavior, however Samba does. This is
       anomalous behavior and the preprocessor will alert if it happens.
  23   When a Session Setup AndX request is sent to the server, the server
       responds (if the client successfully authenticates) which a user id or
       login handle. This is used by the client in subsequent requests to
       indicate that it has authenticated. A Logoff AndX request is sent by the
       client to indicate it wants to end the session and invalidate the login
       handle. With commands that are chained after a Session Setup AndX
       request, the login handle returned by the server is used for the
       subsequent chained commands. The combination of a Session Setup AndX
       command with a chained Logoff AndX command, essentially logins in and
       logs off in the same request and is anomalous behavior. The preprocessor
       will alert if it sees this.
  24   A Tree Connect AndX command is used to connect to a share. The
       Tree Disconnect command is used to disconnect from that share. The
       combination of a Tree Connect AndX command with a chained
       Tree Disconnect command, essentially connects to a share and disconnects
       from the same share in the same request and is anomalous behavior. The
       preprocessor will alert if it sees this.
  25   An Open AndX or Nt Create AndX command is used to open/create a file or
       named pipe. (The preprocessor is only interested in named pipes as this
       is where DCE/RPC requests are written to.) The Close command is used to
       close that file or named pipe. The combination of a Open AndX or
       Nt Create AndX command with a chained Close command, essentially opens
       and closes the named pipe in the same request and is anomalous behavior.
       The preprocessor will alert if it sees this.
  26   The preprocessor will alert if it sees any of the invalid SMB shares
       configured. It looks for a Tree Connect or Tree Connect AndX to the
       share.

Connection-oriented DCE/RPC events
 SID   Description 
--------------------------------------------------------------------------------
  27   The preprocessor will alert if the connection-oriented DCE/RPC major
       version contained in the header is not equal to 5.
  28   The preprocessor will alert if the connection-oriented DCE/RPC minor
       version contained in the header is not equal to 0.
  29   The preprocessor will alert if the connection-oriented DCE/RPC PDU type
       contained in the header is not a valid PDU type.
  30   The preprocessor will alert if the fragment length defined in the header
       is less than the size of the header.
  31   The preprocessor will alert if the remaining fragment length is less
       than the remaining packet size.
  32   The preprocessor will alert if in a Bind or Alter Context request, there
       are no context items specified.
  33   The preprocessor will alert if in a Bind or Alter Context request, there
       are no transfer syntaxes to go with the requested interface.
  34   The preprocessor will alert if a non-last fragment is less than the size
       of the negotiated maximum fragment length. Most evasion techniques try
       to fragment the data as much as possible and usually each fragment comes
       well below the negotiated transmit size.
  35   The preprocessor will alert if a fragment is larger than the maximum
       negotiated fragment length.
  36   The byte order of the request data is determined by the Bind in
       connection-oriented DCE/RPC for Windows.  It is anomalous behavior to 
       attempt to change the byte order mid-session.
  37   The call id for a set of fragments in a fragmented request should stay
       the same (it is incremented for each complete request). The preprocessor
       will alert if it changes in a fragment mid-request.
  38   The operation number specifies which function the request is calling on
       the bound interface. If a request is fragmented, this number should stay
       the same for all fragments.  The preprocessor will alert if the opnum
       changes in a fragment mid-request.
  39   The context id is a handle to a interface that was bound to. If a
       request if fragmented, this number should stay the same for all
       fragments. The preprocessor will alert if the context id changes in a
       fragment mid-request.

Connectionless DCE/RPC events
 SID   Description 
--------------------------------------------------------------------------------
  40   The preprocessor will alert if the connectionless DCE/RPC major version
       is not equal to 4.
  41   The preprocessor will alert if the connectionless DCE/RPC pdu type is
       not a valid pdu type.
  42   The preprocessor will alert if the packet data length is less than the
       size of the connectionless header.
  43   The preprocessor will alert if the sequence number uses in a request is
       the same or less than a previously used sequence number on the session.
       In testing, wrapping the sequence number space produces strange behavior
       from the server, so this should be considered anomalous behavior.


Rule Options
================================================================================
New rule options are supported by enabling the dcerpc2 preprocessor:

dce_iface
dce_opnum
dce_stub_data

New modifiers to existing byte_test and byte_jump rule options:

byte_test: dce
byte_jump: dce


dce_iface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For DCE/RPC based rules it has been necessary to set flow-bits based on a client
bind to a service to avoid false positives. It is necessary for a client to bind
to a service before being able to make a call to it. When a client sendsa bind
request to the server, it can, however, specify one or more service interfaces
to bind to.  Each interface is represented by a UUID. Each interface UUID is
paired with a unique index (or context id) that future requests can use to
reference the service that the client is making a call to. The server will
respond with the interface UUIDs it accepts as valid and will allow the client
to make requests to those services.  When a client makes a request, it will
specify the context id so the server knows what service the client is making a
request to. Instead of using flow-bits, a rule can simply ask the preprocessor,
using this rule option, whether or not the client has bound to a specific
interface UUID and whether or not this client request is making a request to it.
This can eliminate false positives where more than one service is bound to
successfully since the preprocessor can correlate the bind UUID to the context
id used in the request.  A DCE/RPC request can specify whether numbers are
represented as big endian or little endian. The representation of the interface
UUID is different depending on the endianness specified in the DCE/RPC
previously requiring two rules - one for big endian and one for little endian.
The preprocessor eliminates the need for two rules by normalizing the UUID.
An interface contains a version. Some versions of an interface may not be
vulnerable to a certain exploit.  Also, a DCE/RPC request can be broken up into
1 or more fragments. Flags (and a field in the connectionless header) are set in
the DCE/RPC header to indicate whether the fragment is the first, a middle or
the last fragment. Many checks for data in the DCE/RPC request are only relevant
if the DCE/RPC request is a first fragment (or full request), since subsequent
fragments will contain data deeper into the DCE/RPC request. A rule which is
looking for data, say 5 bytes into the request (maybe it's a length field), will
be looking at the wrong data on a fragment other than the first, since the
beginning of subsequent fragments are already offset some length from the
beginning of the request. This can be a source of false positives in fragmented
DCE/RPC traffic. By default it is reasonable to only evaluate if the request is
a first fragment (or full request). However, if the "any_frag" option is used to
specify evaluating on all fragments.

Syntax
    <uuid> [ ',' <operator> <version> ] [ ',' "any_frag" ]

    uuid       = hexlong '-' hexshort '-' hexshort '-' 2hexbyte '-' 6hexbyte
    hexlong    = 4hexbyte
    hexshort   = 2hexbyte
    hexbyte    = 2HEXDIGIT
    operator   = '<' | '>' | '=' | '!'
    version    = 0-65535

Examples
    dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188;
    dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188,<2;
    dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188,any_frag;
    dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188,=1,any_frag;

This option is used to specify an interface UUID. Optional arguments are an
interface version and operator to specify that the version be less than ('<'),
greater than ('>'), equal to ('=') or not equal to ('!') the version specified.
Also, by default the rule will only be evaluated for a first fragment (or full
request, i.e. not a fragment) since most rules are written to start at the
beginning of a request. The "any_frag" argument says to evaluate for middle and
last fragments as well.  This option requires tracking client Bind and
Alter Context requests as well as server Bind Ack and Alter Context responses
for connection-oriented DCE/RPC in the preprocessor. For each Bind and
Alter Context request, the client specifies a list of interface UUIDs along
with a handle (or context id) for each interface UUID that will be used during
the DCE/RPC session to reference the interface.  The server response indicates
which interfaces it will allow the client to make requests to - it either
accepts or rejects the client's wish to bind to a certain interface. This
tracking is required so that when a request is processed, the context id used
in the request can be correlated with the interface UUID it is a handle for.

hexlong and hexshort will be specified and interpreted to be in big endian
order (this is usually the default way an interface UUID will be seen and
represented). As an example, the following Messenger interface UUID as taken
off the wire from a little endian Bind request:
    |f8 91 7b 5a 00 ff d0 11 a9 b2 00 c0 4f b6 e6 fc|
must be written as:
    5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc
The same UUID taken off the wire from a big endian Bind request:
    |5a 7b 91 f8 ff 00 11 d0 a9 b2 00 c0 4f b6 e6 fc|
must be written the same way:
    5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc

This option matches if the specified interface UUID matches the interface UUID
(as referred to by the context id) of the DCE/RPC request and if supplied, the
version operation is true. This option will not match if the fragment is not a
first fragment (or full request) unless the "any_frag" option is supplied in
which case only the interface UUID and version need match.  Note that a
defragmented DCE/RPC request will be considered a full request.

dce_opnum
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The opnum represents a specific function call to an interface. After is has
been determined that a client has bound to a specific interface and is making
a request to it (see above - dce_iface) usually we want to know what function
call it is making to that service. It is likely that an exploit lies in the
particular DCE/RPC function call.

Syntax
    <opnum-list>

    opnum-list   = opnum-item | opnum-item ',' opnum-list
    opnum-item   = opnum | opnum-range
    opnum-range  = opnum '-' opnum
    opnum        = 0-65535

Examples
    dce_opnum: 15;
    dce_opnum: 15-18;
    dce_opnum: 15,18-20;
    dce_opnum: 15,17,20-22;

This option is used to specify an opnum (or operation number), opnum range or
list containing either or both opnum and/or opnum-range. The opnum of a
DCE/RPC request will be matched against the opnums specified with this option.
This option matches if any one of the opnums specified match the opnum of the
DCE/RPC request.

dce_stub_data
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since most DCE/RPC based rules had to do protocol decoding only to get to the
DCE/RPC stub data, i.e. the remote procedure call or function call data, this
option will alleviate this need and place the cursor at the beginning of the
DCE/RPC stub data. This reduces the number of rule option checks and the
complexity of the rule.

This option takes no arguments.

Example:
    dce_stub_data;

This option is used to place the cursor (used to walk the packet payload in
rules processing) at the beginning of the DCE/RPC stub data, regardless of
preceding rule options. There are no arguments to this option.  This option
matches if there is DCE/RPC stub data.

byte_test and byte_jump
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A DCE/RPC request can specify whether numbers are represented in big or little
endian. These rule options will take as a new argument "dce" and will work
basically the same as the normal byte_test/byte_jump, but since the DCE/RPC
preprocessor will know the endianness of the request, it will be able to do
the correct conversion.

byte_test
---------
Syntax
    <convert> ',' [ '!' ] <operator> ',' <value> \
        [ ',' <offset> [ ',' "relative" ]] ',' "dce"

    convert    = 1 | 2 | 4
    operator   = '<' | '=' | '>' | '&' | '^'
    value      = 0-4294967295
    offset     = -65535 to 65535

Examples
    byte_test: 4,>,35000,0,relative,dce;
    byte_test: 2,!=,2280,-10,relative,dce;

When using the "dce" argument to a byte_test, the following normal byte_test
arguments will not be allowed: "big", "little", "string", "hex", "dec" and
"oct".

byte_jump
---------
Syntax
    <convert> ',' <offset> [ ',' "relative" ] \
        [ ',' "multiplier" <mult-value> ] [ ',' "align" ] \
        [ ',' "post_offset" <adjustment-value> ] ',' "dce"

    convert           = 1 | 2 | 4
    offset            = -65535 to 65535
    mult-value        = 0-65535
    adjustment-value  = -65535 to 65535

Examples
    byte_jump:4,-4,relative,align,multiplier 2,post_offset -4,dce;

When using the dce argument to a byte_jump, the following normal byte_jump
arguments will not be allowed: "big", "little", "string", "hex", "dec", "oct"
and "from_beginning".

Example of rule complexity reduction
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The following two rules using the new rule options replace 64 (set and isset
flowbit) rules that are necessary if the new rule options are not used:

alert tcp $EXTERNAL_NET any -> $HOME_NET [135,139,445,593,1024:] (msg:"dns R_Dnssrv funcs2 overflow attempt"; flow:established,to_server; dce_iface:50abc2a4-574d-40b3-9d66-ee4fd5fba076; dce_opnum:0-11; dce_stub_data; pcre:"/^.{12}(\x00\x00\x00\x00|.{12})/sR"; byte_jump:4,-4,relative,align,dce; byte_test:4,>,256,4,relative,dce; reference:bugtraq,23470; reference:cve,2007-1748; classtype:attempted-admin; sid:1000068;)

alert udp $EXTERNAL_NET any -> $HOME_NET [135,1024:] (msg:"dns R_Dnssrv funcs2 overflow attempt"; flow:established,to_server; dce_iface:50abc2a4-574d-40b3-9d66-ee4fd5fba076; dce_opnum:0-11; dce_stub_data; pcre:"/^.{12}(\x00\x00\x00\x00|.{12})/sR"; byte_jump:4,-4,relative,align,dce; byte_test:4,>,256,4,relative,dce; reference:bugtraq,23470; reference:cve,2007-1748; classtype:attempted-admin; sid:1000069;)

