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Snort Rules: Application

Paul Ritchey,

Jacob and Sundstrom, Inc.

[email protected]

V1.0.0

Welcome to the class titled Snort Rules: Application. The purpose of this class is to take the

material you learned in the previous section, Snort Rules: Syntax and Keywords. This section will

take those individual keywords, values and syntax to form complete rules. You will also learn how

to analyze existing rules piece by piece to determine what the rule is looking for.


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Agenda

! Rule Analysis! Simple Rules

! Difficult Rules

! Advanced Rules

! Writing Rules! Simple Rules

! Difficult Rules

! Advanced Rules! Tying It All Together

The first half of this presentation will examine rules of increasing complexity. You will be taught

how to analyze an existing rule to determine what it is looking for. This ability is key to

understanding how to piece together a complete rule from scratch that matches the signature of an

attack.

The second half of the presentation will ask you to write rules from scratch of increasing difficulty.The process of creating these rules will be covered in a step by step process. This will show you a

possible methodology you can use when creating rules on your own.

The very last section will tie together everything you have learned so far, showing you a few of the

options available for Snort output.

This presentation covers Snort version 1.7. If you are using a newer version of Snort, please

remember that new features may have been added or existing features may have been modified after

this presentation was assembled.


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Rule Analysis: Simple Rules

This section will show you how to analyze simple rules, step by step. The analysis skills learned

here will be built upon in later sections to analyze rules of increasing difficulty. This will help you

later when you will be required to write rules from scratch.


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Rule Analysis: Simple Rules

! Learn to analyze simple rules.! Signature based on rule header.

! Examples taken from snort.org ruleset and www.whitehats.com.

! Use logical approach! Analyze rule header first

Determine source and destinationaddresses and ports

Snort uses this section first.

! Analyze rule options next

In this section you will learn how to analyze simple rules. The rules were chosen because they do

not incorporate packet attributes which can make some rules difficult to analyze. These are real life

rules, taken directly from the rule set available from the snort.org web site and www.whitehats.com.

This means that its possible to do further research on the exploits that the rules are designed to

detect to fully round out your understanding of rules.

This section will start with teaching you how to analyze rules based on a logical approach. The first

step is to analyze the rule header. This determines what hosts, ports, protocols and traffic flow must

be involved before Snort even starts to examine the rest of the rule this allows Snort to quickly

determine if it should completely analyze the rule against the options section, saving valuable time.

Later sections will combine the analysis of the rule header with the options section for more

complicated rules.


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Simple Rule #1:Back Orifice

! Background:

! Trojan

! Allows remote control of infectedmachine

! Rule:! alert UDP $EXTERNAL any -> $INTERNAL 31337 \

(msg: "IDS188/trojan-probe-back-orifice";)

The first rule we are going to examine is one that looks for attempts at connecting Back Orifice

trojans. This particular exploit works by means of a trojan that is somehow installed on the target

machine. The trojan can be installed accidentally by end users running executables attached to email

messages, downloading the trojan masquerading as a useful utility, etc. Once installed, the trojan

opens a port and makes itself available for control from a remote host.

Further information on this particular trojan can be obtained any of the major online security web

sites. In depth analysis of this trojan is beyond the scope of this course.


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Simple Rule #1:Back Orifice (cont.)

! Examine the rule header:! Will alert when triggered.

! Applies only to UDP traffic.

! Source defined by variable $EXTERNAL = !$HOME_NET

! Destination defined by variable $HOME_NET = your network

alert UDP $EXTERNAL any -> $INTERNAL 31337 \


For this simple rule, the contents of the signature is completely contained in the rule header. This

rule, when it is triggered, will execute the action alert. Alert means Snort will write an entry to the

alert file and an entry to the logs unless they are overridden by command line options or other

means.

This rule only applies to UDP traffic. If snort the traffic Snort is examining is from another protocol,this rule will not be tested against them.

The destination address is defined as a variable, HOME_NET. This variable is typically defined at

the top of the rules file being used, and is set to the addresses Snort is monitoring. In this particular

rule, the source address is also defined as a variable, named EXTERNAL. Typically, this is set to

!$HOME_NET, meaning that the source address should be outside of the network address space

Snort is monitoring.

The UDP packet can, however, be originating from any of the possible ports on the source host, but

must be destined specifically for the port 31337 (otherwise known as eleet) on the destination

machine.


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Simple Rule #1:Back Orifice (cont.)

! Examine rule options.

! No packet attributes are examined.

! Only includes message.

! Possibility of false-positives:

! Low likelihood of occurrence.! High likelihood of false-positives.

alert UDP $EXTERNAL any -> $INTERNAL 31337 \


Examining the rule options section, it is seen that the only option being used is the message option.

This option provides a string that is used to tag alert and log entries, making it easier to determine

what a log or alert entry represents.

This rule is very simple. The only thing limiting the rule down to a specific subset of UDP traffic is

the destination port. Since no packet attributes or options are specified, it is very likely that detects,although not often, may very well be false-positives. Any traffic, such as streaming audio or video,

that happens to be destined for destination port 31337 will trigger this rule. Care must be taken when

analyzing any available data to validate that the packet was truly a probe for Back Orifice or the

master program contacting a Back Orifice client.


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Simple Rule #2:Deep Throat Trojan

! Background

! Trojan

! Allows remote control of infected host.

! Rule:! alert udp any 2140 -> $HOME_NET 60000 \

(msg:"IDS106 - BACKDOOR SIGNATURE - DeepThroat3.1 Server Active on Network";)

The next simple rule we will examine is one that detects Deep Throat trojans. Deep Throat is

another trojan that can be accidentally installed by users who unknowingly execute attachments or

download the software by accident. Once installed, the trojan opens a port that allows remote hosts

to control the infected machine.


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Simple Rule #2:Deep Throat (cont.)

! Examine the rule header:

! Will alert when triggered.

! Applies only to UDP traffic.

! Source specified as anyAny matches all possible IP addresses, including

internal addresses.

! Destination defined by variable

$HOME_NET = internal network

alert udp any 2140 -> $HOME_NET 60000 \

(msg:"IDS106 - BACKDOOR SIGNATURE - \

DeepThroat 3.1 Server Active on Network";)

For this simple rule, the contents of the signature is again completely contained in the rule header. This

rule, when it is triggered, will execute the action alert. Alert means Snort will write an entry to the

alert file and an entry to the logs unless they are overridden by command line options or other means.

This rule only applies to UDP traffic. If snort the traffic Snort is examining is from another protocol,

this rule will not be tested against them.

Now the rule deviates from the previous example. Instead of specifying a variable for the source IP

address, the keyword any is specified. This means that the packet can originate from any possible IP

address, there are no restrictions. However, the packet must originate from a specific port 2140. The

packet must be destined for the network the variable HOME_NET is set to, and to the specific port

6000. If the packet meets all of the above criteria, it will trigger the rule and will be logged to the alert

file and logs with the message specified in the rule options section.

I would like take a second to discuss the keyword any that was specified for the source address. Snort

is typically installed on a machine that resides in a DMZ. The DMZ sites outside of your internal

network, and sees all traffic in bound from the internet to your network, or outbound from your network

to the internet. It does not and should not see your internal traffic. Because of this, it would have beenjust as effective to replace the keyword any with !$HOME_NET.


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Simple Rule #2:Deep Throat (cont.)

! Examine rule options.

! No packet attributes are examined.

! Only includes message.

! Possibility of false-positives:

! Low likelihood of occurrence.

! Likelihood of detect being a false-positives.

alert udp any 2140 -> $HOME_NET 60000 \

(msg:"IDS106 - BACKDOOR SIGNATURE - \

DeepThroat 3.1 Server Active on Network";)

Examining the rule options section, we again that this rule like the previous example is only

specifying the message option. This option provides a string that is used to tag alert and log entries,

making it easier to determine what a log or alert entry represents.

This rule is very simple. The only real limiting factors are the source and destination ports. Both

ports are ephemeral ports, meaning they are out of the reserved range. Although unlikely, itspossible that this port combination could be used during the course of a valid connection, and

because there are no other criteria for the rule false-positive detects may be made. Most virus

software should be capable of detecting this trojan if properly installed and used regularly. This

increases the chances that a detect is a false-positive so care must be taken to fully resolve any

detects.


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Rule Analysis: Complex Rules

In this section the rules presented for analysis are a little more complicated than the previous

examples. Essentially they provide additional information about packets that are considered hostile

beyond source and destination IPs and ports.


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Rule Analysis: Complex Rules

! Learn to analyze complex rules.! Signature based on rule header.

! Signature also based on rule options.

! Examples taken from www.snort.org ruleset and www.whitehats.com.


! Analyze rule options next Specifies specific packet attributes

Can increase accuracy decrease false positives

This section concentrates on analyzing more complicated rules those containing packet attributes

in the rule options section. In these rules, the signature doesnt just consist of the contents of the rule

header. It consists of the rule header and additional information specified in the rule options.

This section will continue to build on the rule analysis technique that was used in the first section.

Interpretation of the rule option section with different kinds of packet attributes will be introducedhere. By adding packet attributes (such as TCP flags) to the rule options section, its possible to

make rules more accurate, which can potentially reduce the number of false positives.

The example rules used in this section are real world rules. They have been taken from the rule sets

available at the www.snort.org web site and from the www.whitehats.com web site.


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Complex Rule #1:NetMetro

! Background:

! Trojan

! Allows remote control of infectedmachine

! Rule:! alert tcp $EXTERNAL_NET 5031 -> $HOME_NET !53:80 \

(msg:"IDS79 - BACKDOOR SIGNATURE NetMetro

Incoming Traffic"; flags:PA;)

The rule we are going to examine next is one that detects the NetMetro trojan. NetMetro is another

trojan that when installed allows remote control of the infected machine. Again, this trojan like any

other can be accidentally installed by executing attachments to email messages, or downloading the

trojan as it masquerades as a useful utility or game. Most virus detection software should detect this

trojan as long as the signatures are properly maintained.


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Complex Rule #1:NetMetro (cont.)


! Applies only to TCP traffic.

! Source defined by variable $EXTERNAL_NET = !$HOME_NET


alert tcp $EXTERNAL_NET 5031 -> $HOME_NET !53:80 \(msg:"IDS79 - BACKDOOR SIGNATURE - NetMetro Incoming

Traffic"; flags:PA;)

This rule when triggered will alert meaning it will create an entry in the alerts file and create a log

file, unless these options are overridden by command line options. It also only applies to TCP traffic

that meets the criteria of the rest of the signature.

The source address is specified by the variable EXTERNAL_NET. In most cases

EXTERNAL_NET is set to !$HOME_NET, which means that the source address can be any IPaddress except the IPs belonging to your network. The destination address is specified by the

variable HOME_NET. This variable is set to the IP addresses your sensor is to monitor. Both of

these variables are typically defined at the top of a rules file, but may also be set by command line

options.

The source port the traffic must originate from is port 5031. If the source port is anything but 5031,

it will not match the rule header information and this rule will not be triggered. The destination port

setting is more interesting. It specifies that the destination port can be any port except ports 53

through 80, inclusive.


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Complex Rule #1:NetMetro (cont.)

! Examine the rule options:

! TCP flags PUSH and ACK must be set.

! No other packet attributes examined.

! Likelihood of false positives:

! Low likelihood of occurrence.! High likelihood of being false positive.

alert tcp $EXTERNAL_NET 5031 -> $HOME_NET !53:80 \(msg:"IDS79 - BACKDOOR SIGNATURE - NetMetro Incoming\Traffic"; flags:PA;)

This rule is the first example of packet attributes being used in the rule options section. The attribute

being tested is the TCP flags setting. In this case, the TCP flags PUSH and ACK must be set. Other

flags, such as SYN, FIN, URG and the two reserved bits must NOT be set. No other packet attributes

are examined beyond the TCP flag setting.

For this particular rule, there is a low likelihood of false positives, although they will happen. The falsepositives are limited because the source port must exactly match 5031, and the destination port must be

outside the specified range. The addition of packet attributes (in this case TCP flags) to the rule options

section aids in reducing the possibility of false positives because it helps to narrow the possibility of

matches somewhat.

To rule out the possibility of a detect being a false positive, additional data possibly beyond what Snort

provides may need to be examined. For example, if an outside user telnets in to a server in your

network, its possible this rule may be triggered. The source port 5031 is an ephemeral port, meaning

that is not a reserved port and available for anyone and any application to use. If the port 5031 is used

by the person connecting to your telnet server, the rule will be triggered as soon as the TCP three way

handshake is completed and the first packed with a payload is sent inbound to your network. Telnet

runs on port 23, outside the range specified by the destination port setting that specifies what ports it

cannot be.


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Complex Rule #2:Myscan

! Background:

! Port scanner.

! Allows remote detection of availableservices and OS fingerprinting.

! Rule:! alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan - myscan"; ttl: >220; ack: 0; \flags: S;)

The second difficult rule to be examined detects a particular tool used for scanning. This particular

scanner can allow an attacker to easily determine what services are available on a host. Combined

with the ability to determine the OS, and the hacker now has enough information to launch an

effective attack.

For this scanner certain packet attributes are hard coded in the original source code. This allowingan accurate rule to be written that can easily detect scans from this software. It can also allow the

rule to be tuned to help eliminate false positives, increasing the accuracy.


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Complex Rule #2:Myscan (cont.)





alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan -myscan"; ttl: >220; \

ack: 0; flags: S;)






variable HOME_NET. This variable is set to the IP address range your sensor is to monitor. Both of


options.

The source port the traffic must originate from is port 10101. If the source port is anything but

10101, it will not match the rule header information and this rule will not be triggered. The

destination port can be anything, specified by the keyword any. This means the rule does not care

what port is used on the destination host.


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Complex Rule #2:Myscan (cont.)


! Time to live value must be greater than 220.

! Acknowledgement number must be zero (0).

! TCP flag SYN must be set.


! Low likelihood of occurrence.

! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan -myscan"; ttl: >220; \

ack: 0; flags: S;)

In this rules option section, the packet attributes time to live (ttl), acknowledgement number (ack)

and the TCP flag settings are examined. The first attribute that is examined, time to live, must have

a value greater than or equal to 220. The second attribute, the acknowledgement number, must be

zero (0). The last attribute, TCP flags, must have the SYN flag set.

For this rule, there is a low likelihood that the rule will be triggered, but a very high likelihood that ifit is triggered that it is NOT a false positive. There are many key items that lead to this conclusion

and show that this rule is a very well written one. The next slide will show you the individual parts

that combined together make this happen.


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Rule Analysis: Advanced Rules

This section provides analysis of advanced rules those using more sophisticated packet attributes to

examine the packets payload. These rules are the most difficult to write because they require close

analysis of an attacks signature and of the source code of the attack application if available.

These types of rules also have the lowest likelihood of false positives because of the completeness of

the examination of the packets. They are also the easiest to avoid triggering by making slightalterations in the applications source code.


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Rule Analysis: Advanced Rules

! Learn to analyze difficult rules.! Signature based on rule header.

! Signature also based on rule options.

! Examples taken from www.snort.org ruleset and www.whitehats.com.


! Analyze rule options next Specifies specific packet attributes

Can increase accuracy decrease false positives

This section concentrates on analyzing more complicated rules those containing packet attributes

in the rule options section. In these rules, the signature doesnt just consist of the contents of the rule

header. It consists of the rule header and additional information specified in the rule options.

This section will continue to build on the rule analysis technique that was used in the first section.

Interpretation of the rule option section with different kinds of packet attributes will be introducedhere. By adding packet attributes (such as TCP flags) to the rule options section, its possible to

make rules more accurate, which can potentially reduce the number of false positives.

The example rules used in this section are real world rules. They have been taken from the rule sets

available at the www.snort.org web site and from the www.whitehats.com web site.


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Advanced Rule #1:Wu-FTP Exploit

! Background:

! Exploits a bug in wu-ftp daemon.

! Allows instant root access.

! Rule:! alert tcp $EXTERNAL_NET any -> $HOME_NET 21 \

(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \4389 D941 B03F CD80|";)

The first advanced rule we will examine is one that exploits a bug in an ftp daemon provided by

www.wu-ftpd.org that is used as a replacement for many native ftp daemons on some flavors of

Unix, as well as coming native in many Linux distributions. In this case the exploit is known as the

wuftp2600.c exploit which was originally distributed in a broken form. If the exploit is successful,

the attacker is instantly granted root access on a high numbered port that is opened up.


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Advanced Rule #1:Wu-FTP Exploit (cont.)





alert tcp $EXTERNAL_NET any -> $HOME_NET 21 \

(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \

content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \

43 89D941 B03F CD80|";)








options.

The source port is set to the keyword any, meaning that the TCP packet can originate from any

possible port on the source host. However, the packet must be destined for port 21. Port 21 is a well

known reserved port that is used to provide FTP services.


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Advanced Rule #1:Wu-FTP Exploit (cont.)



! Examines payload for specific values.


! Low likelihood of occurrence.! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET any -> $HOME_NET 21 \

(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \4389 D941 B03F CD80|";)

For this rule, two packet attributes are examined in order to detect the exploit. The first attribute is

the TCP flag settings. For this rule, the PUSH and ACK TCP flags must be set. The second attribute

specified examines the packets payload. The examination of a packets payload is triggered by

specifying the keyword content. In this example the content that is being searched for is given in

hex values, which is denoted by the enclosing pipe (|) symbols.

This rule is tuned ever so slightly by the TCP flags attribute. Only packets with a payload should be

applied against this rule. These packets will have the PUSH flag set indicating that data is being

sent. Its possible to have other flag settings without the PUSH flag set and still have a payload,

however these are typically other exploits which this rule doesnt apply to and should have a

different rule written to detect them. The exploit detected here works by initiating a proper TCP

connection, more specifically an anonymous FTP session and initiating a buffer overflow.

For this rule, detects will very rarely occur primarily because of the very specific content that is

being searched for. When detects do occur, its very likely that it is a positive detect. The content

value, although possible, is very unlikely to occur during a normal FTP session, hence this rules

high level of accuracy.


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Advanced Rule #2:cgitest.exe Exploit

! Background:

! Web exploit.

! Allows arbitrary execution of code onserver.

! Rule:! alert tcp $EXTERNAL_NET any -> $HOME_NET 80 \

(msg:"IDS265 - Web cgi cgitest"; \ content:"cgitest.exe|0d0a|user"; nocase; flags: AP; \

offset:4;)

The second advanced rule we will examine is a web based exploit. The cgitest.exe is a CGI that if

it is left installed on a particular web server can allow a remote attacker to execute arbitrary code on

the web server. The exploit works because of a buffer overflow vulnerability, which is one of the

more lethal types of attacks an attacker can use. The web daemon affected by this vulnerability runs

on Windows 95, which limits the possible ramifications of a successful attack that might exist on a

Unix or Windows NT machine.


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Advanced Rule #2:cgitest.exe Exploit (cont.)





alert tcp $EXTERNAL_NET any -> $HOME_NET 80 \(msg:"IDS265 - Web cgi cgitest"; \content: "cgitest.exe|0d0a|user"; nocase; \

flags: AP; offset:4;)








options.

The source port is set to the keyword any, meaning that the TCP packet can originate from any

possible port on the source host. However, the packet must be destined for port 80. Port 80 is one of

the most common ports used for web daemons. If there are web daemons used on your network

using alternative ports, the rule should be duplicated for each of the ports being used.


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Advanced Rule #2:cgitest.exe Exploit (cont.)



! Examines payload for specific values.


! Low likelihood of occurrence.! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET any -> $HOME_NET 80 \(msg:"IDS265 - Web cgi cgitest"; \

content: "cgitest.exe|0d0a|user"; nocase; \flags: AP; offset:4;)

For this rule, two packet attributes are examined in order to detect the exploit. The first attribute is the

TCP flag settings. For this rule, the PUSH and ACK TCP flags must be set. The second attribute

specified examines the packets payload. The examination of a packets payload is triggered by

specifying the keyword content. In this example the content that is being searched for is a

combination of two sections of ASCII data and one section of hex values. This example shows how

ASCII and hex values can be combined to form a payload signature, and can be interspersed between

each other. Note the use of the nocase option. This informs Snort that for the ASCII content beingsearched for, it can appear in any possible combination of upper and lower case letters possible.

This rule is tuned ever so slightly by the TCP flags attribute. Only packets with a payload should be

applied against this rule. These packets will have the PUSH flag set indicating that data is being sent.

Its possible to have other flag settings without the PUSH flag set and still have a payload, however

these are typically other exploits which this rule doesnt apply to and should have a different rule

written to detect them. The exploit detected here works by initiating a proper TCP connection to a web

server, and then executing the cgitest.ext CGI on that server and causing a buffer overflow.

The content attribute can be a very resource intensive attribute to use. To help reduce the overhead of

processing that must take place, it can be tuned by specifying the offset and depth options. These

options reduce the amount of a packets payload that must be inspected by Snort. In this rule, only the

offset option is used. This rule tells Snort to start examining the payload 4 bytes in, effectively

ignoring the first 3 bytes. This may not seem like a lot, but by ignoring 3 bytes of every packet on avery busy network can quickly add up.

For this rule, detects will very rarely occur primarily because of the very specific content that is being

searched for. When detects do occur, its very likely that it is a positive detect. The content value,

although possible, is very unlikely to occur during a normal web sessions by chance.


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Writing Rules

In this section will demonstrate how to write a few rules from scratch of increasing difficulty. A

specification for a needed rule will be provided, followed by a possible solution. Keep in mind that

for some types of rules there may be several possible answers, all of which may be correct.


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Writing Rules: Simple Rule

! Your boss wants to know about allICMP echo requests (pings) cominginto your network. Write the ruleusing the variable HOME_NET torepresent your network address space.The rule should both alert and log.The alert message should contain the

textInbound Ping.

Your boss is concerned about inbound ICMP echo requests from outside addresses. He would like to

have Snort record this packets for future analysis and to see if there are any trends. Write the rule,

using the variable HOME_NET to represent your network address space.

Please briefly pause this presentation now and resume it when you have written the rule.


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Writing Rules: Simple Rule(cont.)

! Possible Answer:

alert icmp !$HOME_NET any -> $HOME_NET any \

(msg:Inbound Ping"; itype: 8;)

According to the specification given on the previous slide, the rule is to both alert and log. To do

this, the rules action field must be set to the value alert. We were also told to only record inbound

ICMP echo requests. Therefore the protocol field is set to ICMP, and the source address field is set

to !$HOME_NET. We were told that the variable HOME_NET would represent our internal

network, so specifying the not sign (!) with the HOME_NET variable represents all addresses

except those in your network.

Snort rules always require a port to be specified, but ICMP does not use ports so we used the

keyword any as a placeholder. We could have used any value for this field, it will be ignored by

Snort when evaluating a packet against this rule. It is needed only to satisfy the rule parser when

Snort reads and process the rules file on startup.

In the rules option section we set the message option to the appropriate value as requested. We also

used the itype attribute with a value of 8 to limit the rule to only record echo requests otherwise

known as pings.


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Writing Rules: Simple Rule #2

! Corporate headquarters routinely runsa scan of all IPs owned by thecompany, including satellite offices.Write a rule that will cause Snort toignore all inbound TCP packets fromthe scanning machine, 192.168.1.1.The address space at the satelliteoffice you work at is the Class B

10.1.x.x. What command line optionmust also be included?

In order to try to keep a step ahead of the hackers, corporate headquarters decided to run a periodic

scan against all IP addresses the company owns, including the satellite office you work at. Tired of

filtering through the false positives caused by this routine scanning, you decided to write a rule to

ignore inbound packets from this scanning box. Also list the command line option that must be

included for this rule to be effective.



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Writing Rules: Simple Rule #2(cont.)

! Possible Answer:! pass tcp 192.168.1.1/32 any -> 10.1.0.0/16 any

! Snort Command Line:

! Snort c snortrules -o

This is a simple rule to write, but also has a special requirement that must not be forgotten. To

ignore packets, the rules action field must be set to the value pass. This tells Snort to drop the

packet being inspected when the rule is triggered. This can be a useful capability in order to reduce

false positives or to ignore traffic from a particular host.

You were told this rule should ignore TCP traffic, so the protocol field in the rule was set to thevalue TCP. The source address was set to the specific host from corporate headquarters,

192.168.1.1. Since the source port can vary, the keyword any was specified, indicating that we

dont care what the source port is, it can be any in the entire range possible. The destination address

field is set to the proper CIDR notation for the satellite office, 10.1.0.0/16. The destination port is set

to the same value as the source port, the keyword any.

In order for this rule to be effective, Snort must be told to process the pass rules first. By default

Snort processes alert and log rules first, then the pass rules last. This effectively ignores pass rules.

To reverse this order, you must specify the -o option. This causes Snort to process pass rules first,

then alert and log rules.


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Writing Rules: Difficult Rule

! Odd behavior has been detected onyour anonymous FTP server. Writea rule to log all activity to thisserver (192.168.1.2) to a singlefile. The source of the possibleanomalous behavior is the 10.1.1.0class C address space.

During routine monitoring of your logs on your anonymous FTP server, you have detected some

behavior that just doesnt seem normal. In order to investigate this matter further, you have decided

to log all FTP activity to this server to a separate log file so you can see the full session. Write a

Snort rule that will accomplish this.



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Writing Rules: Difficult Rule(cont.)

! Possible Answer:

log 10.1.1.0/24 any -> 192.168.1.2/32 21 \

(msg: FTP activity to anonymous FTP server; \session: printable; logto: anonftp;)

For this rule, we specified the log action. We dont really want to have every packets header

written to the alerts file, we really dont care about having them. For the source IP we specified the

class C where the potentially hostile traffic is originating from using CIDR notation. Since the

source port can be any of the ephemeral ports, we decided to specify the keyword any. We could

have specified the range of ports from 1,024 and up, but just in case the traffic is hostile and the

attacker tries to use a reserved port we decided to use any instead.

For the destination address we specified its full IP address in CIDR notation, along with the

destination port of 21. Port 21 is the control port for FTP sessions where we can record the

commands and responses of the user and server.

To record the activity, we have specified the session option which will record all printable (ASCII)

information. We have also redirected the output to the file anonftp by using the logto option.

This will conveniently log all of the activity to a single file making it easy to review any activity that

is recorded.


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Writing Rules: Advanced Rule

! A new (fictitious) probe has been detectedfrom a new scanner called pr0b3z. Thescan originates from port 53 to port 53 andhas a TCP sequence number of123456789 for every packet. The packetsalso include the payload Boo!and haveonly the SYN TCP flag set. The network

being monitored is the class C address

space of192.168.1.x. Write a rule that willboth alert and log.

The next rule to write is one for a new fictitious scan that has been seen recently. This particular

scan use port 53 for both the source and destination ports, and each packet has the same sequence

number. Oddly enough there is a payload of varying length that always contains the string Boo!

imbedded somewhere. Although there is a payload, the PUSH flag is not set. The only TCP flag set

is the SYN flag.



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Writing Rules: Advanced Rule(cont.)

! Possible Answer:

alert any 53 -> 192.168.1.0/24 53 \

(msg: Inbound Scan: Pr0b3z; \ seq: 123456789; flags: S; content: Boo!;)

For this rule we set the action field to the standard alert action. We want this activity to be written to

both the alert file and the log file especially if we later run SnortSnarf on these files which we use

during our analysis work. These scans can originate anywhere so we have specified the keyword any

as the source IP. Since both the source and destination ports use 53, we have set both in the rule to that

number. For the destination addresses we specified our network using standard CIDR notation.

In the rules option section we specified on output message thats descriptive and will mean something to

us when we review the alerts file and log data later. From the description we have been given the

sequence number is the same for all packets. We used the seq packet attribute to specify the sequence

number.

The content option was used to search packets for a payload that contains the ASCII string Boo!.

According to the description this string can appear anywhere in the payload, so we cant specify the

offset or depth options to limit the amount of processing Snort will have to do. But, the description

given to use said that the packets only have the SYN flag set, and no other flags. Well specify this in

the rule using the flags attribute and this will indirectly limit the amount of payload processing Snort

will have to do because although it is possible to have a payload in a SYN packet it is a rare occurrence.


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Tying It All Together

You have learned how to write rules and all of the syntax and keywords that go along with it. This

last section will what you have learned and tie it all together showing how those rules would be used

in real world situations. Sample Snort output is supplied as well, showing how the detects being

monitored for are provided to you when they are detected.


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Specifying Rules File(cont.)

snort -c snort-lib

cd 192.168.5.5; ls -l

total 12

-rw------- 1 root root 232 Mar 22 06:58 TCP:12345-2985



cat TCP:12345-2985

[**] Netbus/GabanBus [**]

03/21-13:33:54.275350 192.168.5.5:12345 -> 1.2.3.4:2985TCP TTL:64 TOS:0x0 ID:9173 DF

**S***** Seq: 0x9C9B544A Ack: 0x0 Win: 0x7D78

TCP Options => MSS: 1460 SackOK TS: 9306314 0 NOP WS: 0

Continuing from the previous slide, we will examine one of the log subdirectories found in/var/log/snort. Changing directories to one of the ones listed - 192.168.5.5 - we see

additional files that house all logged activity originating from that IP. For instance, the file

TCP:12345-2985 represents activity from the host 192.168.5.5 that used the TCP protocol and has a

source port of 12345 and a destination port of 2985.

Examining the contents of that file you see that source IP 192.168.5.5 sent IP 1.2.3.4 traffic to

destination port 2985. The reason this traffic was logged there is because a rule fired that checks for

traffic to or from port 12345. A well known trojan named Netbus uses this port.


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Log Alerts to Directory ./log

snort -c snort-lib -l ./log

" Output placed in directory ./log

" alerts file contains alerts generated by Snort

" IP subdirectories with logged payloads

ls -l ./log

drwx------ 2 root root 4096 Mar 22 08:16 1.2.3.4

drwx------ 2 root root 4096 Mar 22 08:16

192.168.5.5

-rw-------- 1 root root 2512 Mar 22 08:16 alerts

By default, Snort places the logs and alerts in /var/log/snort. You can specify a default

directory by using the -l option and the name of the directory where you want the information

placed. In this case, we have created a log file in the current directory and want the activity recorded

there.

Snort will then record all alerts to an alert file in this directory, as well as creating the loggingdirectories and log files here. This is the same as if everything was written to the default directory

/var/log/snort. Remember the directory you tell Snort to write the alert and logs to must already

exist. Snort will not create this directory on its own.


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Logging Options

Default: Full logging to default Snortdirectory

Binary: tcpdump binary output to asingle log file

None: Disable logging

Database: Log packets to SQL database

XML: Log packets in portable XML format

The default method of logging is to capture the output generated from Snort detects and store it inthe default Snort directory /var/log/snort. Depending on other command line options or rules

options you use, this will log the traffic that triggered the scan in some kind of human readable

format.

Alternatively, you can take any detect that is discovered and log in tcpdump raw output binaryformat. This is often done with high traffic volume so as not to bog down Snort with the logging

process.

There are also output plugins available to log packets to a XML formatted file as well as a variety of

SQL databases (e.g. MySQL, PostgreSQL, Oracle)

Finally, logging can be totally disabled if not desired. This is only recommended if you are not

interested in the payload of packets that trigger rules. The log files (or file if you are logging in

binary format) are the only place the FULL packet will be written out to including the payload.


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Alerting Options

Full: writes alert message and headerinformation to alert file (default)

Fast: writes alert message and condensedheader to alert file

None: disable alerts

Syslog: send alert messages to syslog

SMB: send WinPopup messages to Windowshosts via smbclient

Database: Send alerts to SQL database

XML: Write alerts in a portable XML format

Alerts are an abbreviated format of capturing the detect. The default method is to capture the detectin the file /var/log/snort/alert. The fast method writes partial information to the alert file.

None will disable alerting all together.

Full alerting is Snorts default behaviour. When using this level of alerting, the message (if any) in

the rule is written to the alert file first, followed by a date/timestamp and full packet headerinformation. Fast alerting on the other hand writes the message (if any) in the rule is again written

first, followed only by a data/timestamp and source and destination ports and addresses. Fast

alerting does not include the full packet header information. Syslog alerts send messages in a format

similar to the fast alerts, but write them to the syslog facilities. There are also options available to

send alerts to a database, Windows host via SMB alerts, as well as to an XML formatted file.


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Alert and LoggingDifferences

Alerts are all contained in one file

Alerts are decoded through transportlayer only

Logging produces multiple files

Logging creates a directory structure byIP numbers

Subdirectories contain activity - possibly

decoded through application layer

You may be wondering what the difference between logging and alerting is. Logging will create

multiple files under multiple directories based on the IP number of the source host. The directory

name (IP number) indicates the source IP that triggered the logging activity, and the contents are

files named according to the protocol and ports involved. The actual contents of the files record the

payload of the packet(s) involved.

Alerts are more abbreviated captures of the detect that can all be found in a single file. This is a

better overview of what is happening on the network versus the more detailed captures for logging.

It also provides a convenient one stop place to do quick searches for items that may be of interest,

such as specific exploits or specific hosts.

Logging and alerting are conceptually different in a few ways. Alerts exist to let the user know that

something has happened and to give that user enough information to decide whether the alert

warrants further investigation immediately. Logs exist to allow the user to analyse the exact packets

that caused an alert in addition to any other packets that are possibly related to the alert event. The

log files are there to allow follow forensic analysis of events, the alert files exist merely to give the

user a single place to monitor for Snort events.


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[**] IDS249 - SMTP Relaying Denied [**]

10/09-02:34:59.775359 FF:FF:FF:FF:FF:FF -> FF:FF:FF:FF:FF:FF type:0x800

len:0x71

192.168.1.2:25 -> 192.168.200.2:25432 TCP TTL:255 TOS:0x0 ID:24915 DF

*****PA* Seq: 0x30AC5391 Ack: 0x1E3E4A55 Win: 0x2238

35 35 30 20 35 2E 37 2E 31 20 3C FF FF FF FF 2D 550 5.7.1 ... Relayi

6E 67 20 64 65 6E 69 65 64 0D 0A ng denied..

Alert and Logging Format

Alert/Log Message Text

Date, Timestamp

Packet Header (varies)

Ethernet (optional)

Packet Payload, Hex and ASCII (optional, log file only)

This slide shows you the general format of the alert and log files. Alert and log records are identical,

with the exception of the packet payload which can be optionally included in the log files. If the

packet payload is included in the logs, both the hex representation of the payload and the ASCII

printable characters will be displayed.

Both log and alert messages start with the message text included in the rule. This essentially labelseach detect as it is written to disk, making it easy to determine why the packet was logged. The next

item written is a date and timestamp. The date and timestamp represent the time on the sensor when

the detect was made. Optionally following the date and timestamp is the ethernet information. Next,

the source and destination addresses and ports involved in the detect appear, followed immediately

by the packets decoded header information which can vary depending on the protocol of the packet.


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Logging/alert Examples

The following rule will be used to testvarious options to log and alert:

alert tcp any any > 192.168.143.0/24 21 \(msg: "anonymous FTP attempt"; flags: PA; \Content: "anonymous"; nocase)

Place the above rule in rules file

snortrules.

In the next several slides, different options will be shown to explain various logging and alerting

choices. The above rule will be used for most of the detects.

This rule says that we want to alert if any ftp connection is generated to the 192.168.143 network

that has the PUSH and ACK flags set and has a content of 'anonymous' in the payload. We will put

this single rule in the rules file name snortrules to simplify the logging and alert messagesgenerated.


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Alert and Log

snort -l logdir -c snortrules

In directory logdir you will find a file named alert.

[**] Anonymous FTP attempt [**]

04/28-11:58:06.350754 192.168.143.15:1536-> 192.168.143.16:21

TCP TTL:64 TOS:0x10 ID:18558 DF

*****PA* Seq: 0x7C451B73 Ack: 0x7DC44632 Win: 0x7D78

TCP Options => NOP NOP TS: 27713449 92831

In this first example, we specify that we want to use a default logging directory of logdir. This has to

be an existing directory. We run Snort using our one rule found in snortrules. Next, what you

don't see above is we attempt to ftp to a host on the 192.168.143 network using a username of

anonymous. That triggers a detect and causes Snort to create an entry in the alerts file.

If we examine the contents of logdir directory, we notice that there is a file named alert. Itcontains the output that was generated for the detect, in this case since we did not specify the alert

level Snort will default to full. It contains the packet information decoded through the TCP

transport layer as can be seen by examining the file.


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Alert and Log WithDecode

snort -l logdir -c snortrules -d

In directory logdir the file alert has the

following contents:


04/28-12:03:59.888357 192.168.143.15:1537 -> 192.168.143.16:21


*****PA* Seq: 0x7C451B73 Ack: 0x7DC44632 Win: 0x7D78TCP Options => NOP NOP TS: 27713449 92831

Now we add the -d option to the same command used previously, which says to decode the

application layer. We follow the same process as before and discover that we have an alert file inlogdir which is the same as before. Note that the contents of the alert file have not changed

from what would normally be recorded.

The alert file still contains the message from the triggered rule, the date/timestamp and the hostsinvolved. It also still contains the full packet header information, still written in a human readable

format.


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Alert and Log WithDecode (Cont)

The directory logdir contains the subdirectory192.168.143.15.

The directory 192.168.143.15 contains file named:

TCP:1537-21


04/28-12:03:59.888357 192.168.143.15:1537 ->192.168.143.16:21


*****PA* Seq: 0x94102D52 Ack: 0x94529A7B Win: 0x7D78


55 53 45 52 20 61 6E 6F 6E 79 6D 6F 75 73 0D 0A USER anonymous..

If you now look at the log file, you will not only see the information contained in the alerts file, but

now the actual payload of the packet at the bottom of the alert. The output of the packets payload is

broken into two parts. The left portion contains the hex values of the payload, while the right portion

contains the ASCII representation.


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Alert and Log in Binary

snort -l logdir -c snortrules -d -b

The directory logdir contains the file alert.


04/28-11:58:06.350754 192.168.143.15:1536 -> 192.168.143.16:21


*****PA* Seq: 0x7C451B73 Ack: 0x7DC44632 Win: 0x7D78


The -b option allows you to log the packets to a tcpdump file instead of the normal decoded ASCII

files. This creates a single binary file instead of creating many subdirectories with files in them that

may contain only one packet.

If you are deploying Snort on high capacity networks or Snort starts to drop packets, log in binary

format. Logging using the binary format is much more efficient than having Snort write out acompletely decoded packet in an ASCII format. It also relieves Snort from having to create

directories and constantly opening and closing files to write out the information in ASCII format.

Instead Snort can open one file and continuously write to that file for the entire duration Snort is

running.


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Alert and Log in Binary(Cont)

In directory logdir we find the

following file:

[email protected]

This is a tcpdump binary output ofentire packet.

In the logdir directory, we find a file [email protected] which is a tcpdump raw binary

output file of the detect that was captured. The name has the date of the capture (0428 - April 28th)and the time of the capture (11:58 AM). This can be read either using Snort with the -r option or

with tcpdump with the -r option. This requires less work of Snort to capture and is used when

there is a lot of traffic on the network and there is a concern for packets being dropped.


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Reading Tcpdump Files

snort -vd -r tcpdump.raw.data

Initializing Network Interface...

snaplen = 144

Entering readback mode....

03/21-13:33:51.960219 1.2.3.4:1398 -> 192.168.5.5:2307


**S***** Seq: 0x9C857C3C Ack: 0x0 Win: 0x7D78


03/21-13:33:51.960269 1.2.3.4:1399 -> 192.168.5.5:693TCP TTL:64 TOS:0x0 ID:7570 DF

**S***** Seq: 0x9C55968F Ack: 0x0 Win: 0x7D78


Another useful ability of Snort is the -r command line option. This option instructs Snort to read

from tcpdump binary files instead of the network interface. This can be done if you've collected data

using tcpdump from another sensor or other tcpdump compatible software, or instructed Snort to log

in binary. In the example shown here, we are using Snort in verbose mode, sending the data to thescreen, but this time we use the -r switch to tell Snort to read its input from

tcpdump.raw.data instead of from the network interface. This assumes that we have collected

tcpdump.raw.data earlier and it contains tcpdump binary data.

Note the Entering readback mode. This is Snorts way of informing you that it is reading from a

file and not from the network interface. Also, note the snaplen = 144. That is the tcpdump

snapshot length, indicating that the software used to create the dump file collected 144 bytes for each

packet collected.

Readback mode can be especially useful for busy networks where full and constant processing on the

sensor itself may not be feasible. In that case you can pull the data back periodically and run Snort

on the retrieved data without using extra CPU cycles on the sensor itself.


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Snortsnarf.Pl

perl script to take alerts:

Formats Snort alert and log files into html output

Places output in following files for drill down:

Overall summary of detected alerts (index.html)

Alert wrap-up html files

Specific source/destination alert html files

Optionally linked to log files for packet inspection

Located in snort directory contrib subdirectory

http://www.silicondefense.com/snortsnarf/

snortsnarf.pl /var/log/snort/alert

There are tools available that will help you with the analysis of the alert and log files. One tool thathas proven to be popular as well as being very useful is SnortSnarf. The SnortSnarf program is

intended to help you view your Snort alerts in an orderly fashion using a web browser. This is easier

than trying to assess what is happening by looking at the alerts file, and allows you to drill down

from the general list of alerts to the specific packet that triggered the alert (providing logging was

turned on and the decode option was specified).

At the top level, SnortSnarf creates an index.html file containing a summarized list of alerts. At

the next level down, an html file is created containing each of the same alerts in a single file. Iflogging was turned on in Snort, and you provide the directory the logs are located in, SnortSnarf

will allow you to drill down to the packet that triggered a specific alert.


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Snortsnarf: Snort signatures in snort.alert et al7 alerts processed.

Files included:

snort.alert

snort_portscan.log

Earliest alert at 02:34:59.775359 on 10/09

Latest alert at 03:00:36 on 10/9

Signa tu re (click for def in ition) # Aler ts # Sources # Destinations Detail l ink

IDS249 - SMTP Relaying Denied 1 1 1 Summary

UDP scan 6 1 1 Summary

Generated by Snortsnarfv100400.1 (Jim HoaglandandStuart Staniford)

SnortSnarf Output

This is a sample index page created by SnortSnarf. This page contains a summarized list of alerts

that were triggered during the time period listed. It lists the signatures that were detected, number of

times it was triggered, and the total number of source and destination hosts involved. To see

additional details, click on the summary link which will display a page containing information about

the selected alerts.

For some signatures, such as the SMTP Relaying Denied example shown here, you can click on the

signature name and a page displaying information about the signature will be displayed. The

information displayed can include a sample rule that would detect the traffic, sample packets, and

further explanation of the exploit.


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All 1 alerts from 192.168.1.2 in snort.alert et alLooking in files:

snort.alert

snort_portscan.logEarliest: 02:34:59.775359 on 10/09

Latest:02:34:59

.775359 on 10/091 different signatures are present for 206.181.216.216as a source

1 instances ofIDS249 - SMTP Relaying DeniedThere are 1 distinct destination IPs in the alerts of the type on this page.

192.168.1.2

Whois lookup at: ARIN RIPE APNIC Geektools

DNS lookup at: Amenesi Riherds Princeton

[**] IDS249 - SMTP Relaying Denied [**]

3 1 3 7 snort rules application 7406

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