IP network scanning involves gathering information about devices on a network such as which hosts are active and which services and ports are open. The document discusses common scanning techniques including ping sweeps to discover active hosts, port scanning to identify open ports, and methods for detecting operating systems and software versions running on remote hosts. It provides examples using the free and open-source nmap tool, which is considered the standard for port scanning and network discovery.

1. IP Network Scanning

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Outline
 What is IP network scanning?
 Concepts, motivation
 Example Tool
 nmap
 Scanning types
 Host discovery
 port scanning
 Version detection
 OS detection

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What is Scanning?
 Method to gather information regarding the
devices running on the network
 Typically to discover services or servers on a network
 Which hosts are up?
 Which services are offering?
 Do not confuse with “host vulnerability scanner”
which further explore a computer by testing for
common vulnerabilities (nessus, SAINT)

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Why Scanning?
 Network Security assessment
 Evaluation and Auditing the security
 Firewall Penetration Test (Policy auditing)
 IDS proof/evaluation
 Identifying unexpected new servers
 Identifying open ports for
 proactively protect the network (Network and security
admin)
 attacking it (Hackers)

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nmap
 A well known and free security scanner written
by Fyodor (http://insecure.org/nmap/)
 First released Sept 1, 1997 in Phrack 51 “The Art of Port
Scanning” (http://www.phrack.org/issues.html?issue=51
 Many updates since then:
 OS Detection (http://www.phrack.org/issues.html?issue=54&id=9#article)
 Version scanning
 ARP Scanning
 Version 5.00 as of this doc
 Usage:
nmap [scan types] [options] <host or net …>

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Why nmap
 An excellent tool
 Long history of development and support
 Continuous development and improvements
 “Industry Standard” port scanner

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nmap features
 Host Discovery: Which host is alive?
 Identifying computers on a network, for example listing the computers
which respond to pings (Ping Sweeps)
 Port Scanning : What services are available?
 Enumerating the open ports on one or more target computers
 Service and Version Detection : Which version is running?
 Determine the application name and version number
 OS Detection: What platforms are served?
 Remotely determining the OS and some hardware characteristics of
network devices

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Host Discovery
 Querying multiple hosts using this method is
referred to as ping sweeps
 The most basic step in mapping out a network.
 Several Sweeps technique
 ICMP Sweeps
 Broadcast ICMP
 NON Echo ICMP
 TCP sweep
 UDP sweep

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Host Discovery : ICMP Sweeps
 Technique
 sending an ICMP ECHO request (ICMP type 8)
 If an ICMP ECHO reply (ICMP type 0) is received : target is alive;
 No response: target is down
 Pros & Cons
 easy to implement
 fairly slow, easy to be blocked
Scanner Target
ICMP ECHO request
ICMO ECHO reply
Scanner Target
a host is alive
a host is down/filtered
ICMP ECHO request
No response

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Host Discovery : Broadcast ICMP
 Sending ICMP ECHO request to the network
and/or broadcast addresses
 Windows ignored this
 Most routers blocked this

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Host Discovery : Non-ECHO ICMP
 ICMP type 13 messages (TIMESTAMP),
 Query Current Time
 ICMP type 17 messages (ADDRESS MASK REQUEST)
 diskless systems to obtain its subnet mask at bootstrap time
(RFC 792)

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Host Discovery : TCP Sweeps
 Sending TCP ACK or TCK SYN packets
 The port number can be selected to avoid blocking
by firewall
 Usually a good pick would be 21 / 22 / 23 / 25 / 80
 But.. firewalls can spoof a RESET packet for an IP
address, so TCP Sweeps may not be reliable.

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Host Discovery : UDP Sweeps
 Relies on the ICMP PORT UNREACHABLE
 Assume the port is opened if no ICMP PORT
UNREACHABLE message is received after sending a UDP
datagram
 Cons:
 Routers can drop UDP packets as they cross the Internet
 Many UDP services do not respond when correctly probed
 Firewalls are usually configured to drop UDP packets (except for
DNS)
 UDP sweep relies on the fact that a non-active UDP port will
respond with an ICMP PORT UNREACHABLE message

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nmap Host Discovery summary
 sL: List Scan - simply list targets to scan
 -sP: Ping Scan - go no further than determining if host is online
 -PN: Treat all hosts as online -- skip host discovery
 -PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
 -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery
probes
 -PO [protocol list]: IP Protocol Ping
 -n/-R: Never do DNS resolution/Always resolve [default: sometimes]
 --dns-servers <serv1[,serv2],...>: Specify custom DNS servers
 --system-dns: Use OS's DNS resolver
 -sU: UDP Scan

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Port Scanning
 To determine what services are running or in a
LISTENING
 Some well known types
 TCP Connect Scan
 TCP SYN scan
 Stealth scan
 FTP bounce scan

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Port Scanning : TCP Connect Scan
 Use basic TCP connection establishment mechanism;
complete 3-ways handshake
 Easily to detect by inspecting the system log
Scanner Target
SYN
SYN/ACK
ACK
Scanner Target
SYN
RST/ACK
a port is opened
a port is closed

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Port Scanning : TCP SYN scan
 Do not establish a complete connection (Half Open
scanning)
 send a SYN packet and wait for a response
 If an SYN/ACK is received=> the port is LISTENING
 immediately tear down the connection by sending a RESET
 If an RST/ACK is received =>a non-LISTENING port.
Scanner Target
SYN
SYN/ACK
Scanner Target
SYN
RST/ACK
a port is closed
a port is opened
RST

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Port Scanning : Stealth Scan
 To gather information about target sites while
avoiding detection
 Try to hide themselves among normal network traffic
 Not to be logged by logging mechanism (stealth)
 Techniques
 Flag Probe packets (Also called “Inverse mapping”)
 Response is sent back only by closed port
 By determining what services do not exist, an intruder can infer
what service do exist
 Slow scans rate
 difficult to detect =>need long history log
CERT reported this technique in CERT® Incident Note IN-98.04
http://www.cert.org/incident_notes/IN-98.04.html

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Port Scanning : Stealth Mapping
 RFC793: to handle wrong state packets
 closed ports : reply with a RESET packet to wrong state packets
 opened ports : ignore any packet in question
 Technique
 A RST scan
 A FIN probe with the FIN TCP flag set
 An XMAS probe with : set FIN, URG, ACK, SYN, RST, PUSH flags set
 A NULL probe with no TCP flags set
Scanner Target
Probe packet
No response
Scanner Target
RST/ACK
a port is closed
a port is opened
Probe packet

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Port Scanning : FTP Bounce scanning
 Connects to an FTP server, and establishes a control
communication connection, ask the FTP server to initiate an
active server data transfer process
 Rather slow
 Some FTP servers disable the “Proxy” feature, but there are still
many who do not
PORT 10,0,0,5,0,22
(10.0.0.4) (10.0.0.5)
TCP SYN
RST
425 Cannot build data connection
Scanner FTP Target

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Port Scanning with nmap
 SCAN TECHNIQUES:
 -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
 -sN/sF/sX: TCP Null, FIN, and Xmas scans
 -b <FTP relay host>: FTP bounce scan
 PORT SPECIFICATION AND SCAN ORDER:
 -p <port ranges>: Only scan specified ports
 Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
 -F: Fast mode - Scan fewer ports than the default scan
 -r: Scan ports consecutively - don't randomize
 --top-ports <number>: Scan <number> most common ports
 --port-ratio <ratio>: Scan ports more common than <ratio>

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Services and Versions Detection
 The nmap-service-probes database contains
probes for querying various services and match
expressions to recognize and parse responses

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Operating System Detection
 Banner, DNS HINFO and …
 TCP/IP fingerprinting (IP stack
implementation will response differently)
 FIN probe, Bogus Flag probe
 TCP initial sequence number sampling, TCP initial
window, ACK value
 ICMP error quenching, message quoting, ICMP echo
integrity
 IP: DF, TOS, Fragmentation

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OS Detection : Examples
 ACK : sending FIN|PSH|URG to a closed port
 most OS : ACK with the same sequence number
 Windows: ACK with sequence number+1
 Type of Service: Probing with an ICMP_PORT_UNREACHABLE
message
 most OS : TOS = 0
 Linux : TOS= 0xC0

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Version and OS Detection with nmap
 SERVICE/VERSION DETECTION:
 -sV: Probe open ports to determine service/version info
 --version-intensity <level>: Set from 0 (light) to 9 (try all probes)
 --version-light: Limit to most likely probes (intensity 2)
 --version-all: Try every single probe (intensity 9)
 --version-trace: Show detailed version scan activity (for
debugging)
 OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively

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Port Scanning Detection
 For Administrators to detect scanning
 Logs suspicious packets
 Identifies connections not properly terminated
 Records ports usage

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Port Scanning Detection
 For hacker to dodge from detection
 Randomize the sequence of ports to prevent detection
 Slow scan: exceed the site detection threshold in IDS, 2
packets/day/site!
 Decoy: spoofed address in attack
 Coordinated Scans: multiple scanners probe the same
host or network