Advanced IP Network Scanning Methods - TCP Sequence and IP ID Incrementation
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If TCP sequence numbers are generated in a predictable way by the target host, then blind spoofing and hijacking can occur (although this is usually limited to internal network spaces). Older Windows operating platforms suffer from this because the sequence numbers are simply incremented instead of randomly generated.
If the IP ID value is incremental, the host can be used as a third party to perform IP ID header scanning. IP ID header scanning requires the ID values returned from the third party to be incremental so that accurate scan results can be gathered.
Example 4-13 shows Nmap being run in verbose mode (-v) with TCP/IP fingerprinting (-O). Setting both options shows the results of both TCP and IP ID sequence number predictability tests.
Example 4-13. Using Nmap to test TCP and IP ID sequences
$ nmap -v -sS -O 192.168.102.251
Starting Nmap 4.10 ( http://www.insecure.org/nmap/ ) at 2007-04-01 23:26 UTC
Interesting ports on cartman (192.168.102.251):
(The 1524 ports scanned but not shown below are in state: closed)
Port State Service
25/tcp open smtp
53/tcp open domain
8080/tcp open http-proxy
Remote OS guesses: Windows 2000 RC1 through final release
TCP Sequence Prediction: Class=random positive increments
Difficulty=15269 (Worthy challenge)
IPID Sequence Generation: Incremental
Network Scanning Recap
Different IP network scanning methods allow you to test and effectively identify vulnerable network components. Here is a list of effective network scanning techniques and their applications:
ICMP scanning and probing
By launching an ICMP ping sweep, you can effectively identify poorly protected hosts (as security-conscious administrators filter inbound ICMP messages) and perform a degree of operating system fingerprinting and reconnaissance by analyzing responses to the ICMP probes.
Half-open SYN flag TCP port scanning
A SYN port scan is often the most effective type of port scan to launch directly against a target IP network space. SYN scanning is extremely fast, allowing you to scan large networks quickly.
Inverse TCP port scanning
Inverse scanning types (particularly FIN, XMAS, and NULL) take advantage of idiosyncrasies in certain TCP/IP stack implementations. This scanning type isn’t effective when scanning large network spaces, although it is useful when testing and investigating the security of specific hosts and small network segments.
Third-party TCP port scanning
Using a combination of vulnerable network components and TCP spoofing, third-party TCP port scans can be effectively launched. Scanning in this fashion has two benefits: hiding the true source of a TCP scan and assessing the filters and levels of trust between hosts. Although time-consuming to undertake, third-party scanning is extremely useful when applied correctly.
UDP port scanning
Identifying accessible UDP services can be undertaken easily only if ICMP type 3 code 3 (“Destination port unreachable”) messages are allowed back through filtering mechanisms that protect target systems. UDP services can sometimes be used to gather useful data or directly compromise hosts (the DNS, SNMP, TFTP, and BOOTP services in particular).
IDS evasion and filter circumvention
Intrusion detection systems and other security mechanisms can be rendered ineffective by using multiple spoofed decoy hosts when scanning or by fragmenting probe packets using Nmap or fragroute. Filters such as firewalls, routers, and even software (including the Microsoft IPsec filter) can sometimes be bypassed using specific source TCP or UDP ports, source routing, or stateful attacks.
Next: Network Scanning Countermeasures >>
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 This article is excerpted from chapter four of Network Security Assessment, Second Edition, written by Chris McNab (O'Reilly, 2007; ISBN: 0596510306). Check it out today at your favorite bookstore. Buy this book now.
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