Overview
========
The Stream5 preprocessor is a target-based TCP reassembly module
for Snort. It is intended to replace both the Stream4 and flow
preprocessors, and it is capable of tracking sessions for both
TCP and UDP. With Stream5, the rule 'flow' and 'flowbits' keywords
are usable with TCP as well as UDP traffic.
Since Stream5 replaces Stream4, both cannot be used simultaneously.
Remove the Stream4 and flow configurations from snort.conf when the
Stream5 configuration is added.
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
------------
Stream5, 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 Stream5 are the results of
extensive research with many target operating systems.
Stream API
----------
Stream5 fully supports the Stream API (partly supported by Stream4),
allowing other protocol normalizers/preprocessors to dynamically
configure reassembly behavior as required by the application layer
protocol, identify sessions that may be ignored (large data transfers,
etc), and update the identifying information about the session
(application protocol, direction, etc) that can later be used by rules.
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.
Rule Options
------------
Stream5 adds the 'stream_size' rule option. The 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;
Configuration
=============
Global Configuration
--------------------
Global settings for the Stream5 preprocessor
- Preprocessor name: stream5_global
- Options:
track_tcp <yes|no> - Track sessions for TCP. The default is "yes".
max_tcp <number> - Max concurrent sessions for TCP. The default
is "256000", maximum is "1052672", minimum is "1".
memcap <bytes> - Memcap for TCP packet storage. The default
is "8388608" (8MB), maximum is "1073741824" (1GB),
minimum is "32768" (32KB).
track_udp <yes|no> - Track sessions for UDP. The default is "yes".
max_udp <number> - Max concurrent sessions for UDP. The default
is "128000", maximum is "1052672", minimum is "1".
track_icmp <yes|no> - Track sessions for ICMP. The default is "yes".
max_icmp <number> - Max concurrent sessions for ICMP. The default
is "64000", maximum is "1052672", minimum is "1".
flush_on_alert - Backwards compatibility. Flush a TCP stream
when an alert is generated on that stream. The
default is set to off.
show_rebuilt_packets - Print/display packet after rebuilt (for
debugging). The default is set to off.
prune_log_max <bytes> - Print a message when a session terminates that
was consuming more than the specified number of
bytes. The default is "1048576" (1MB), minimum
is "0" (unlimited), maximum is not bounded, other
than by the memcap.
disabled - This optional keyword is allowed with any policy
to avoid packet processing. This option disables
the preprocessor. When the preprocessor is disabled
only the options memcap, max_tcp, max_udp and
max_icmp are applied when specified with the
configuration. The other options are parsed but
not used. Any valid configuration may have
"disabled" added to it.
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_addr> - 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 <policy_id> - 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 <number> - Limits number of overlapping packets.
The default is "0" (unlimited), the minimum is
"0", and the maximum is "255".
max_window <number> - 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
Stream5 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 [<number secs>]
- 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 <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 <num> - 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.
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
======
Stream5 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.
SID Description
--- -----------
1 SYN on established session
2 Data on SYN packet
3 Data sent on stream not accepting data
4 TCP Timestamp is outside of PAWS window
5 Bad segment, overlap adjusted size less than/equal 0
6 Window size (after scaling) larger than policy allows
7 Limit on number of overlapping TCP packets reached
8 Data after Reset packet
9 Possible Hijacked Client
10 Possible Hijacked Server
11 TCP packet with any control flags set
12 Limit on number of consecutive small segments reached
13 4-way handshake detected
14 Packet missing timestamp
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