Overview

The Stream preprocessor is a target-based TCP reassembly module for Snort. It replaces both the Stream5 and the earlier Stream4 and flow preprocessors, and it is capable of tracking sessions fo"/>

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README.stream5

Overview

The Stream preprocessor is a target-based TCP reassembly module for Snort. It replaces both the Stream5 and the earlier Stream4 and flow preprocessors, and it is capable of tracking sessions for both TCP and UDP.

Since Stream replaces Stream5, both cannot be used simultaneously. Stream registers for the same configuration keywords as Stream5 with the exception of ‘stream5_global’, which is now processed by the Session preprocessor. Existing configuration files should continure to work with the Stream as long as there is only one ‘stream5_global’ and it is included with the global options of the base policy.

Transport Protocols

TCP sessions are identified via the classic TCP “connection”. UDP sessions are established as the result of a series of UDP packets from two end points via the same set of ports. ICMP messages are tracked for the purposes of checking for unreachable and service unavailable messages, which effectively terminate a TCP or UDP session.

Target-Based

Stream, like Frag3, introduces target-based actions for handling of overlapping data and other TCP anomalies. The methods for handling overlapping data, TCP Timestamps, Data on SYN, FIN and Reset sequence numbers, etc. and the policies supported by Stream are the results of extensive research with many target operating systems.

Stream API

Stream supports the modified Stream API that is now focused on functions specific to reassembly and protocol aware flushing operations. Session management functions have been moved to the Session API. The remaining API functions enable other protocol normalizers/preprocessors to dynamically configure reassembly behavior as required by the application layer protocol.

Anomaly Detection

TCP protocol anomalies, such as data on SYN packets, data received outside the TCP window, etc are configured via the detect_anomalies option to the TCP configuration. Some of these anomalies are detected on a per-target basis. For example, a few operating systems allow data in TCP SYN packets, while others do not.

Protocol Aware Flushing (PAF)

Protocol aware flushing of HTTP, SMB and DCE/RPC can be enabled with this option:

config paf_max:

where is between zero (off) and 63780. This allows Snort to statefully scan a stream and reassemble a complete PDU regardless of segmentation. For example, multiple PDUs within a single TCP segment, as well as one PDU spanning multiple TCP segments will be reassembled into one PDU per packet for each PDU. PDUs larger than the configured maximum will be split into multiple packets.

Rule Options

Stream adds support for a few rule options described below.

stream_size

The ‘stream_size’ rule option allows a rule to match traffic according to the number of bytes observed, as determined by the TCP sequence numbers.

stream_size takes a number of comma-separated arguments in the following format:

stream_size:<direction>,<operator>,<size>

Where direction is one of:

client  -   Client side traffic only
server  -   Sever side traffic only
both    -   Traffic from both sides
either  -   Traffic from either side

Valid operators are:

=
<
>
!=
<=
>=

For example:

stream_size:client,<,6;

stream_reassemble

The ‘stream_reassemble’ rule option allows a rule to enable or disable TCP stream reassembly on matching traffic.

stream_reassemble takes a number of comma-separated arguments in the following format:

stream_reassemble:<enable|disable>,<server|client|both> [,noalert] [,fastpath]
  • The optional noalert parameter causes the rule to not generate an alert when it matches.
  • The optional fastpath parameter causes Snort to ignore the rest of the connection.

For example:

To disable TCP reassembly for client traffic when we see a HTTP 200 Ok Response message:

alert tcp any 80 -> any any (flow:to_client,established;  content:"200 OK";
    stream_reassemble:disable,client,noalert;)

Configuration

Global Configuration ——————– Global settings prevousily processed by Stream5 are now handled by Session. Stream calls as Session API method to get a copy of these settings for its use. Refer to README.session for details on the global configuration options.

TCP Configuration

Provides a means on a per IP address target to configure a TCP policy. This can have multiple occurrences, per policy that is bound to an IP address or network. One default policy must be specified, and that policy is not bound to an IP address or network.

  • Preprocessor name: stream5_tcp
  • Options: bind_to - IP address for this policy. The default is set to any. timeout <number (secs)> - Session timeout. The default is "30", the minimum is "1", and the maximum is "86400" (approximately 1 day). policy - The Operating System policy for the target OS. The policy_id can be one the following: first - Favor first overlapped segment. last - Favor last overlapped segment. bsd - FreeBSD 4.x and newer NetBSD 2.x and newer OpenBSD 3.x and newer AIX linux - Linux 2.4 and 2.6 old-linux - Linux 2.2 and earlier windows - Windows 98, NT, 2000, XP (and others not specifically listed below) win2003 - Windows 2003 Server vista - Windows Vista solaris - Solaris 9.x and newer hpux10 - HPUX 10 hpux - HPUX 11 and newer irix - IRIX 6 and newer macos - MacOS 10.3 and newer The default is "bsd".

    overlap_limit - Limits number of overlapping packets. The default is "0" (unlimited), the minimum is "0", and the maximum is "255". max_window - Maximum allowed TCP window. The default is "0" (unlimited), the minimum is "0", and the maximum is "1073725440" (65535 left shift 14). That is the highest possible TCP window per RFCs. This option is intended to prevent a DoS against Stream by an attacker using an abnormally large window, so using a value near the maximum is discouraged. detect_anomalies - Detect TCP protocol anomalies. The default is set to off. require_3whs [] - Establish sessions only on completion of a SYN/SYN-ACK/ACK handshake. The default is set to off. The optional number of seconds specifies a startup timeout. This allows a grace period for existing sessions to be considered established during that interval immediately after Snort is started. The default is "0" (don't consider existing sessions established), the minimum is "0", and the maximum is "86400" (approximately 1 day). use_static_footprint_sizes - Emulate Stream4 behavior for flushing reassembled packets. The default is set to off. dont_store_large_packets - A performance improvement which does not queue large packets in reassembly buffer if set. Setting this option could result in missed packets. The default is set to off. check_session_hijacking - Check for TCP session hijacking. This check validates the hardware (MAC) address from both sides of the connect -- as established on the 3-way handshake against subsequent packets received on the session. If an ethernet layer is not part of the protocol stack received by Snort, there are no checks performed. Alerts are generated (per 'detect_anomalies' option) for either the client or server when the MAC address for one side or the other does not match. The default is set to off. dont_reassemble_async - Don't queue packets for reassembly if traffic has not been seen in both directions. The default is set to queue packets. max_queued_bytes - Limit the number of bytes queued for reassembly on a given TCP session to bytes. Default is "1048576" (1MB). A value of "0" means unlimited, with a non-zero minimum of "1024", and a maximum of "1073741824" (1GB). A message is written to console/syslog when this limit is enforced. max_queued_segs - Limit the number of segments queued for reassembly on a given TCP session. The default is "2621", derived based on an average size of 400 bytes. A value of "0" means unlimited, with a non-zero minimum of "2", and a maximum of "1073741824" (1GB). A message is written to console/syslog when this limit is enforced. small_segments bytes [ignore_ports port list] - Configure the maximum small segments queued. This feature requires that detect_anomalies be enabled. num1 is the number of consecutive segments that will trigger the detection rule. The default value is "0" (disabled),with a maximum of "2048". num2 is the minimum bytes for a segment to be considered "small". The default value is "0" (disabled), with a maximum of "2048". ignore_ports is optional, defines the list of ports in which will be ignored for this rule. The number of ports can be up to "65535". Example: small_segments 3 bytes 15 ignore_ports 33 44 55 A message is written to console/syslog when this limit is enforced. The generated alert is 129:12 ports <client|server|both> [all|space separated port list] - Specify the client, server, or both and list of ports in which to perform reassembly. This can appear more than once in a given config. For example: ports both 80 23 ports server 37 ports client 21 25 The default settings are: ports client 21 23 25 42 53 80 110 111 135 136 \ 137 139 143 445 513 514 1433 1521 2401 3306 The minimum port allowed is "1" and the maximum allowed is "65535". ignore_any_rules - Don't process any -> any (ports) rules for TCP that attempt to match payload if there are no port specific rules for the src or destination port. Rules that have flow or flowbits will never be ignored. This is a performance improvement, but may result in missed attacks. Using this does not affect rules that look at protocol headers, only those with content, PCRE, or byte test options. The default is "off". This option can be present only in default policy.

If no options are specified for a given TCP policy, that is the default TCP policy. If only a bind_to option is used with no other options that TCP policy uses all of the default values.

UDP Configuration

Configuration for UDP session tracking. Since there is no target based binding, there should be only one occurrence of the UDP configuration. - Preprocessor name: stream5_udp - Options: timeout <number (secs)> - Session timeout. The default is “30”, the minimum is “1”, and the maximum is “86400” (approximately 1 day). ignore_any_rules - Don’t process any -> any (ports) rules for UDP that attempt to match payload if there are no port specific rules for the src or destination port. Rules that have flow or flowbits will never be ignored. This is a performance improvement, but may result in missed attacks. Using this does not affect rules that look at protocol headers, only those with content, PCRE, or byte test options. The default is “off”.

NOTE: with the ignore_any_rules option, a UDP rule will be ignored except when there is another port specific rule that may be applied to the traffic. For example, if a UDP rule specifies destination port 53, the ‘ignored’ any -> any rule will be applied to traffic to/from port 53, but NOT to any other source or destination port. A list of rule SIDs affected by this option are printed at Snort’s startup.

NOTE: with the ignore_any_rules option, if a UDP rule that uses any -> any ports includes either flow or flowbits, the ignore_any_rules option is effectively pointless. Because of the potential impact of disabling a flowbits rule, the ignore_any_rules option will be disabled in this case.

ICMP Configuration

NOTE: ICMP is currently untested, in minimal code form and is NOT ready for use in production networks. It is not turned on by default.

Configuration for ICMP session tracking. Since there is no target based binding, there should be only one occurrence of the ICMP configuration. - Preprocessor name: stream5_icmp - Options: timeout <number (secs)> - Session timeout. The default is “30”, the minimum is “1”, and the maximum is “86400” (approximately 1 day).

Example Configurations

1) This example configuration emulates the behavior of Stream4 (with UDP support enabled).

preprocessor stream5_global: max_tcp 8192, track_tcp yes, \ track_udp yes, track_icmp no preprocessor stream5_tcp: policy first, use_static_footprint_sizes preprocessor stream5_udp: ignore_any_rules

2) This configuration maps two network segments to different reassembly policies, one for Windows, one for Linux, with all other traffic falling to the default policy Solaris.

preprocessor stream5_global: track_tcp yes preprocessor stream5_tcp: bind_to 192.168.1.0/24, policy windows preprocessor stream5_tcp: bind_to 10.1.1.0/24, policy linux preprocessor stream5_tcp: policy solaris

Alerts

Stream uses generator ID 129. It is capable of alerting on 10 anomalies, all of which relate to TCP anomalies. There are no anomaly detection capabilities for UDP or ICMP. Check etc/gen-msg.map for the current list of GID 129 alerts.