1. Linux Networking and Security
Chapter 12
Network Intrusion Detection

2. Network Intrusion Detection
Use network scanning and packet-sniffing utilities
Understand basic intrusion detection systems
Perform automated security audits of your Linux system

3. Scanners and Sniffers
Cracker can employ the following techniques in order to gain access to a Linux system:
Port scanning, in which packets are sent to a host to gain information about it based on its response
Packet sniffing, in which every packet on the network has its header and data examined
Network administrators also use these techniques to check for security weaknesses, and though some feel their use is illegitimate, it is important to stay ahead of crackers

4. Port Scanning
A port scan enables someone to identify a network’s operating system and any services that could potentially allow greater access
Port scans typically use the TCP protocol and its associated flags to gather information about the host and its network services
Some port scanners use ICMP and UDP packets, which do not provide as much data as TCP, but can offer some information that TCP cannot

5. Port Scanning

6. Port Scanning
The most widely used port-scanning utility is nmap, the network mapper
nmap is a command-line utility that uses a variety of scanning methods
nmap allows for fingerprinting hosts, greater output, and configuration of timing policy
nmap can also perform a Ping scan, which reports hosts that are reachable using ICMP echo packets

7. Port Scanning

8. Port Scanning

9. Port Scanning

10. Packet Sniffing
A packet sniffer allows for the examination of any or all of the traffic passing through a network cable or wireless space
An Ethernet card can enable packet sniffing only if it is operating in promiscuous mode
Users must be logged in as root to use this mode, so packet sniffers require root access
If encryption technologies such as SSH, GPG, and stunnel are used, packet data is more secure

11. Packet Sniffing Three popular Linux utilities are:
IPTraf displays individual network connections, with protocol and other data for each one, and it also displays statistics by protocols, certain host names, or certain IP addresses
tcpdump provides information similar to IPTraf, but it also includes more detailed information about network packets
Ethereal takes tcpdump a step farther in that it is a graphical network analysis tool

12. Packet Sniffing

13. Packet Sniffing

14. Packet Sniffing

15. Packet Sniffing

16. Packet Sniffing

17. Packet Sniffing

18. Packet Sniffing

19. Packet Sniffing

20. Packet Sniffing

21. Packet Sniffing

22. Using Intrusion Detection Software
Intrusion detection is the process of noticing when someone is trying to break into (or has already broken into) a system
This category of software is called intrusion detection systems (IDS)
PortSentry, by Psionic, watches network ports for packets that appear to be port scans
A more complex tool than PortSentry is Linux IDS, or LIDS, which can alter the Linux kernel

23. Using Intrusion Detection Software
Big Brother provides a different level of intrusion detection than LIDS and it uses a client/server model similar to SNMP
Big Brother includes a server that gathers data from clients on each network host and displays that data as a Web page
Some of the 26 standard services Big Brother will manage are DNS, FTP, HTTP, POP3, SSH, Telnet, disk space and memory usage

24. Using Intrusion Detection Software

25. Using Intrusion Detection Software
Suggested use of intrusion detection tools:
Use nmap to scan the system after configuration to check for security holes
Next use PortSentry to watch for outside hosts trying to port scan the server
Use LIDS to secure your file system and processes so that anyone who is able to gain unauthorized access will have very limited power
Use Big Brother to keep a constant eye on services that are provided on network servers

26. System Security Audits
The best way to test confidence in the security of a Linux system is to perform a security audit
Security audits are reviews or tests of how secure the system is and what needs to be done to improve its security
A security audit could take the form of:
A careful review of the security policy
Use of special security-auditing software

27. System Security Audits
One of the first security-auditing programs was called Security Administrator Tool for Analyzing Networks (SATAN)
The Security Administrator’s Integrated Network Tool (SAINT) replaced SATAN
SAINT uses a Web browser interface to manage an “attack” on a network and report vulnerabilities found
Other security audit tools are Tiger and SARA

28. System Security Audits

29. System Security Audits

30. System Security Audits

31. Chapter Summary
Port-scanning software lets anyone learn about the potentially vulnerable network access points on any networked computer
Port scanners use various combinations of TCP flags, UDP packets, and Ping packets to elicit responses that inform the scanner about the services running on the targeted host
When a host detects that someone is using a port scanner, software such as PortSentry can take action to prevent the completion of the port scan and block all future access by the host performing the scan

32. Chapter Summary
The most used port-scanning software is nmap and graphical utilities are available as nmap front ends
Packet sniffers use the promiscuous mode of a NIC to capture all data passing through that node of the network, including all headers and payloads; Ethereal is a powerful and popular graphical packet sniffer
Packet sniffing is just one type - though the most comprehensive - of network traffic analysis; other programs such as IPTraf help network administrators analyze network traffic patterns based on protocol, point of origin or destination, and other factors

33. Chapter Summary
The tcpdump program is a very popular network traffic analysis program that captures detailed information about network packets
Intrusion detection systems (IDS) are an important part of modern network security and they watch for signs of intruders trying to access your servers and help you respond appropriately
PortSentry is one piece of IDS software that detects port scans from programs like nmap; A more comprehensive package is LIDS, which alters the Linux kernel so that the root user has limited access

34. Chapter Summary
Big Brother is a simpler IDS that watches the status of network services on multiple servers through a Web page interface
Security audits using security policies or specialized software can help network administrators see potential security problems and fix them before someone else finds them
One popular security-auditing software tool is SAINT and many others are available