The flags Keyword Rule Options
3.6.13 The icode Keyword
In ICMP packets, the ICMP header comes after the IP header. It contains a code field, as shown in Appendix C and RFC 792 at http:www.rfc-editor.orgrfcrfc792.txt. The icode keyword is used to detect the code field in the ICMP packet header. The argu- ment to this field is a number and the general format is as follows: icode: ICMP_codee_number Table 3-3 ICMP type filed values Value Type of ICMP Packet Echo reply 3 Destination unreachable 4 Source quench 5 Redirect 8 Echo request 11 Time exceed 12 Parameter problem 13 Timestamp request 14 Timestamp reply 15 Information request 16 Information reply 100 Chapter 3 • Working with Snort Rules The type field in the ICMP header shows the type of ICMP message. The code field is used to explain the type in detail. For example, if the type field value is 5, the ICMP packet type is “ICMP redirect” packet. There may be many reasons for the gen- eration of an ICMP redirect packet. These reasons are defined by the code field as listed below: • If code field is 0, it is a network redirect ICMP packet. • If code field is 1, it is a host redirect packet. • If code is 2, the redirect is due to the type of service and network. • If code is 2, the redirect is due to type of service and host. The icode keyword in Snort rule options is used to find the code field value in the ICMP header. The following rule generates an alert for host redirect ICMP packets. alert icmp any any - any any itype: 5; \ icode: 1; msg: ICMP ID=100; Both itype and icode keywords are used. Using the icode keyword alone will not do the job because other ICMP types may also use the same code value.3.6.14 The id Keyword
The id keyword is used to match the fragment ID field of the IP packet header. Its purpose is to detect attacks that use a fixed ID number in the IP header of a packet. Its format is as follows: id: id_number If the value of the id field in the IP packet header is zero, it shows that this is the last fragment of an IP packet if the packet was fragmented. The value 0 also shows that it is the only fragment if the packet was not fragmented. The id keyword in the Snort rule can be used to determine the last fragment in an IP packet.3.6.15 The ipopts Keyword
A basic IPv4 header is 20 bytes long as described in Appendix C. You can add options to this IP header at the end. The length of the options part may be up to 40 bytes. IP options are used for different purposes, including: • Record Route rr • Time Stamps tsParts
» Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» Logs False Alarms Some Definitions
» Where IDS Should be Placed in Network Topology
» Honey Pots What is Intrusion Detection?
» Security Zones and Levels of Trust
» IDS Policy Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» Packet Decoder Preprocessors Components of Snort
» The Detection Engine Components of Snort
» Dealing with Switches Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» TCP Stream Follow Up Supported Platforms
» Snort on Stealth Interface Snort with no IP Address Interface
» References Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» Test Installation Snort Installation Scenarios
» Single Sensor Production IDS
» Multiple Snort Sensors with Centralized Database
» Download Install Installing Snort from the RPM Package
» Unpacking Installing Snort from Source Code
» Running the configure script. Running the
» Running the make install command.
» Create or copy the Snort configuration file in
» Create a directory After Installation Processes
» Generating Test Alerts Testing Snort
» Generating Test Alerts with Automatic Snort Startup
» Errors While Starting Snort Running Snort on a Non-Default Interface
» Automatic Startup and Shutdown
» Running Snort on Multiple Network Interfaces
» Logging Snort Data in Text Format
» Logging Snort in Binary Format
» Network Intrusion Detection Mode
» UNIX Socket Mode Snort Alert Modes
» Running Snort in Stealth Mode
» TCPIP Network Layers Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» CIDR Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» User Defined Actions Rule Actions
» Well-Known Port Numbers Port Number
» The ack Keyword The classtype Keyword
» The content Keyword Rule Options
» The offset Keyword Rule Options
» The depth Keyword The content-list Keyword
» The dsize Keyword Rule Options
» The flags Keyword Rule Options
» The fragbits Keyword Rule Options
» The itype Keyword Rule Options
» The icode Keyword Rule Options
» The id Keyword The ipopts Keyword
» The logto Keyword Rule Options
» The reference Keyword Rule Options
» The resp Keyword Rule Options
» The rev Keyword The rpc Keyword
» The session Keyword Rule Options
» The sid Keyword Rule Options
» The tag Keyword Rule Options
» The tos Keyword Rule Options
» The ttl Keyword Rule Options
» The uricontent Keyword Rule Options
» Using a List of Networks in Variables Using Interface Names in Variables
» The config Directives The Snort Configuration File
» Preprocessor Configuration Output Module Configuration
» Include Files The Snort Configuration File
» Order of Rules Based upon Action
» The Simple Method Automatically Updating Snort Rules
» The Sophisticated and Complex Method
» Writing Good Rules Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» The frag2 Module Preprocessors
» The stream4 Module Preprocessors
» The spade Module Preprocessors
» The alert_syslog Output Module
» The alert_smb Module The log_tcpdump Output Module
» Examples The XML Output Module
» Logging to Databases Output Modules
» CSV Output Module Output Modules
» Unified Logging Output Module SNMP Traps Output Module
» Using BPF Fileters Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» Creating Extra Tables Step 5: Creating Tables in the Snort Database
» Secure Logging to Remote Databases Securely Using Stunnel
» Archiving the Database Snort Database Maintenance
» Using Sledge Hammer: Drop the Database
» What is ACID? Installation and Configuration
» Listing Protocol Data Alert Details Searching
» Generating Graphs Archiving Snort Data
» SnortSnarf Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» Barnyard References Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» SnortSam Prentice.Hall – Intrusion.Detection.Systems.with.Snort
» IDS Policy Manager Prentice.Hall – Intrusion.Detection.Systems.with.Snort
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