About NMAP

1. IP Network Scanning

2. 2
Outline
 What is IP network scanning?
 Concepts, motivation
 Example Tool
 nmap
 Scanning types
 Host discovery
 port scanning
 Version detection
 OS detection

3. 3
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)

4. 4
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)

5. 5
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

8. 8
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

14. 14
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