Chapter 4 -IP Network Scanning
This chapter focuses on the technical execution of IP network scanning. After undertaking initial reconnaissance to identify IP address spaces of interest, network scanning builds a clearer picture of accessible hosts and their network services. Network scanning and reconnaissance is the real data gathering exercise of an Internet-based security assessment. The rationale behind IP network scanning is to gain insight into the following elements of a given network:
• ICMP message types that generate responses from target hosts
• Accessible TCP and UDP network services running on the target hosts
• Operating platforms of target hosts and their configuration
• Areas of vulnerability within target host IP stack implementations (including sequence number predictability for TCP spoofing and session hijacking)
• Configuration of filtering and security systems (including firewalls, border routers, switches, and IDS sensors)
Performing both network scanning and reconnaissance tasks paints a clear picture of the network topology and its security mechanisms. Before penetrating the target network, further assessment steps involve gathering specific information about the TCP and UDP network services that are running, including their versions and enabled options.
ICMP Probing
The Internet Control Message Protocol (ICMP) identifies potentially weak and poorly protected networks. ICMP is a short messaging protocol that’s used by systems administrators and end users for continuity testing of networks (e.g., using the ping or traceroute commands). From a network scanning and probing perspective, the following types of ICMP messages are useful:
Type 8 (echo request)
Echo request messages are also known as ping packets. You can use a scanning tool such as nmap to perform ping sweeping and easily identify hosts that are accessible.
Type 13 (timestamp request)
A timestamp request message requests system time information from the target host. The response is in a decimal format and is the number of milliseconds elapsed since midnight GMT.
Type 15 (information request)
The ICMP information request message was intended to support self-configuring systems such as diskless workstations at boot time, to allow them to discover their network address. Protocols such as RARP, BOOTP, or DHCP do so more robustly, so type 15 messages are rarely used.
Type 17 (subnet address mask request)
An address mask request message reveals the subnet mask used by the target host. This information is useful when mapping networks and identifying the size of subnets and network spaces used by organizations.
Firewalls of security-conscious organizations often blanket-filter inbound ICMP messages and so ICMP probing isn’t effective; however, ICMP isn’t filtered in most networks because ICMP messages are often useful for network troubleshooting purposes.
There are a handful of other ICMP message types that have relevant security applications
(such as ICMP type 5 redirect messages sent by routers), but they aren’t related
to network scanning.
Table 4-1 outlines popular operating systems and their responses to certain types of
direct ICMP query messages.
Indirect ICMP query messages can be sent to the broadcast address of a given subnet (such as 192.168.0.255 in a 192.168.0.0/24 network). Operating systems respond in different ways to indirect queries issued to a broadcast address, as shown in Table 4-2.
Ofir Arkin of the Sys-Security Group has undertaken a lot of research into ICMP over recent years, publishing white papers dedicated entirely to the use of ICMP probes for OS fingerprinting. For quality in-depth details of ICMP probing techniques, please consult his research available from his web site.
--This text refers to an out of print or unavailable edition of this title.
