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How to Write a Protocol Analyzer
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Vlad Grigorescu
August 07, 2013
Programming
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170
How to Write a Protocol Analyzer
From the 2013 Bro Exchange
Vlad Grigorescu
August 07, 2013
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Transcript
How to Write a Protocol Analyzer Vlad Grigorescu 1 Friday,
October 18, 13
What Do Analyzers Do? • Parse the network traffic •
Generate events 2 • Handle the events • Generate logs Friday, October 18, 13
What Do Analyzers Do? • Parse the network traffic •
Generate events 2 • Handle the events • Generate logs Core Layer Script Layer Friday, October 18, 13
Parsing Traffic Goal: Convert packet payload to data structure 3
Friday, October 18, 13
Parsing Traffic Goal: Convert packet payload to data structure 3
<15>Mar 19 14:06:37 scobel-‐113 : debug... priority (8*facility + severity) message Friday, October 18, 13
4 Parsing Traffic struct syslog_data { int facility;
int severity; char* msg; }; /* Parsing code */ <15>Mar 19 14:06:37 scobel-113 : debug... Friday, October 18, 13
binpac • A domain-specific language for protocol parsing • Build
“types” to represent logical data structures • .pac files get processed into C++ source code 5 Friday, October 18, 13
6 Parsing Traffic type Syslog_Message = record {
PRI: Syslog_Priority; msg: bytestring &restofdata; } &byteorder = littleendian; <15>Mar 19 14:06:37 scobel-113 : debug... Friday, October 18, 13
type Syslog_Priority = record { lt
: uint8; tmppri: RE/[[:digit:]]+/; gt : uint8; } Parsing Traffic 7 <15>Mar 19 14:06:37 scobel-113 : debug... ; Friday, October 18, 13
type Syslog_Priority = record { lt
: uint8; tmppri: RE/[[:digit:]]+/; gt : uint8; } &let { pri: int=bytestring_to_int(tmppri, 10); Parsing Traffic 7 <15>Mar 19 14:06:37 scobel-113 : debug... }; Friday, October 18, 13
type Syslog_Priority = record { lt
: uint8; tmppri: RE/[[:digit:]]+/; gt : uint8; } &let { pri: int=bytestring_to_int(tmppri, 10); Parsing Traffic 7 <15>Mar 19 14:06:37 scobel-113 : debug... // pri == facility*8 + severity facility: int = pri / 8; severity: int = pri % 8; }; Friday, October 18, 13
8 Parsing Traffic √ syslog-‐protocol.pac - Defines the protocol Friday,
October 18, 13
Generating Events Whenever we see a syslog message, generate the
syslog_message event: syslog_message( conn facility severity msg ) 9 Friday, October 18, 13
Generating Events Whenever we see a syslog message, generate the
syslog_message event: syslog_message( conn facility severity msg ) 9 : connection, : int, : int, : string Friday, October 18, 13
10 Generating Events event syslog_message(%
c: connection, facility: int, severity: int, msg: string %); Friday, October 18, 13
10 Generating Events event syslog_message(%
c: connection, facility: int, severity: int, msg: string %); √ events.bif - Defines the events Friday, October 18, 13
11 Generating Events type Syslog_Message = record { PRI:
Syslog_Priority; msg: bytestring &restofdata; } &byteorder = littleendian; type Syslog_Priority = record { lt : uint8; tmppri: RE/[[:digit:]]+/; gt : uint8; } &let { pri: int = bytestring_to_int(val, 10); facility: int = pri / 8; severity: int = pri % 8; }; Friday, October 18, 13
function proc_syslog_msg(facility: count,
severity: count, msg: bytestring ): Generating Events 12 Friday, October 18, 13
13 Generating Events function proc_syslog_msg(facility: count,
severity: count, msg: bytestring ): bool %{ %} Friday, October 18, 13
14 Generating Events function proc_syslog_msg(facility: count,
severity: count, msg: bytestring ): bool %{ BifEvent::generate_syslog_message( connection()-‐>bro_analyzer(), connection()-‐>bro_analyzer()-‐>Conn(), facility, severity, bytestring_to_val(msg)); %} Friday, October 18, 13
15 Generating Events function proc_syslog_msg(facility: count,
severity: count, msg: bytestring ): bool %{ BifEvent::generate_syslog_message( connection()-‐>bro_analyzer(), connection()-‐>bro_analyzer()-‐>Conn(), facility, severity, bytestring_to_val(msg)); return true; %} Friday, October 18, 13
16 Connections and Flows Friday, October 18, 13
17 Connections and Flows Friday, October 18, 13
18 Connections and Flows Friday, October 18, 13
19 Generating Events refine flow Syslog_Flow += { function
proc_syslog_msg(facility: count, severity: count, msg: bytestring ): bool %{ BifEvent::generate_syslog_message( connection()-‐>bro_analyzer(), connection()-‐>bro_analyzer()-‐>Conn(), facility, severity, bytestring_to_val(msg)); return true; %} }; Friday, October 18, 13
refine typeattr Syslog_Message += &let { proc_syslog_message
= $context.flow.proc_syslog_msg(PRI.facility, PRI.severity, msg); }; Generating Events 20 Friday, October 18, 13
21 Generating Events refine flow Syslog_Flow += { function
proc_syslog_msg(facility: count, severity: count, msg: bytestring ): bool %{ BifEvent::generate_syslog_message( connection()-‐>bro_analyzer(), connection()-‐>bro_analyzer()-‐>Conn(), facility, severity, bytestring_to_val(msg)); return true; %} }; refine typeattr Syslog_Message += &let { proc_syslog_message = $context.flow.proc_syslog_msg(PRI.facility, PRI.severity, msg); }; Friday, October 18, 13
21 Generating Events refine flow Syslog_Flow += { function
proc_syslog_msg(facility: count, severity: count, msg: bytestring ): bool %{ BifEvent::generate_syslog_message( connection()-‐>bro_analyzer(), connection()-‐>bro_analyzer()-‐>Conn(), facility, severity, bytestring_to_val(msg)); return true; %} }; refine typeattr Syslog_Message += &let { proc_syslog_message = $context.flow.proc_syslog_msg(PRI.facility, PRI.severity, msg); }; √ syslog-‐analyzer.pac - Generates the events (and handles state) Friday, October 18, 13
22 What Do Analyzers Do? Parse the network traffic Generate
events • Handle the events • Generate logs Core Layer Script Layer Friday, October 18, 13
Handle Those Events! 23 module Syslog; const ports = {
514/udp }; redef likely_server_ports += { ports }; event bro_init() &priority=5 { Analyzer::register_for_ports( Analyzer::ANALYZER_SYSLOG, ports); } Friday, October 18, 13
24 Handle Those Events! event syslog_message(c: connection,
facility: count, severity: count, msg: string) &priority=5 { print(fmt(“I have a message! %s”, msg); } Friday, October 18, 13
Handle Those Events! 25 module Syslog; const ports = {
514/udp }; redef likely_server_ports += { ports }; event bro_init() &priority=5 { Analyzer::register_for_ports( Analyzer::ANALYZER_SYSLOG, ports); } event syslog_message(c: connection, facility: count, severity: count, msg: string) &priority=5 { print(fmt(“I have a message! %s”, msg); } Friday, October 18, 13
Handle Those Events! 25 module Syslog; const ports = {
514/udp }; redef likely_server_ports += { ports }; event bro_init() &priority=5 { Analyzer::register_for_ports( Analyzer::ANALYZER_SYSLOG, ports); } event syslog_message(c: connection, facility: count, severity: count, msg: string) &priority=5 { print(fmt(“I have a message! %s”, msg); } √ main.bro - Registers the analyzer, handles events, logs Friday, October 18, 13
26 Non-Standard Ports? const ports = { 514/udp }; Friday,
October 18, 13
26 Non-Standard Ports? signature dpd_syslog { ip-‐proto == udp
payload /^<[0-‐9][0-‐9]?[0-‐9]?>/ enable "syslog" } Friday, October 18, 13
26 Non-Standard Ports? signature dpd_syslog { ip-‐proto == udp
payload /^<[0-‐9][0-‐9]?[0-‐9]?>/ enable "syslog" } √ dpd.sig - Defines DPD signatures Friday, October 18, 13
Dealing with Errors 27 event protocol_violation(
c: connection, atype: Analyzer::Tag, aid: count, reason: string) Binpac exception: binpac exception: string mismatch at src/analyzer/protocol/sip/ sip-‐protocol.pac:61: expected pattern: "(([^: \t]+|\r\n))" actual data: "" Friday, October 18, 13
Bootstrapping Coming soon to a Github near you... 28 Friday,
October 18, 13
Bootstrapping Coming soon to a Github near you... 28 $
./start.py tftp “Trivial FTP Analyzer” ~/bro Files created. TODO: 1) src/analyzers/protocol/tftp/tftp-‐protocol.pac 2) src/analyzers/protocol/tftp/events.bif 3) src/analyzers/protocol/tftp/tftp-‐analyzer.pac 4) scripts/base/protocols/tftp/main.bro 5) scripts/base/protocols/tftp/dpd.sig Friday, October 18, 13
29 Extra Credit: State flow My_Flow(is_orig: bool) { %member{
bool server_hungry; %} %init{ server_hungry = T; %} function proc_error(msg: My_Type) { server_hungry = F; }; Friday, October 18, 13