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  1. Scanning

  2. Determining If The System Is Alive

  3. Determining If The System Is Alive • Network Ping Sweeps • Ping is traditionally used to send ICMP ECHO (Type 8) packets to a target system • Response is ICMP ECHO_REPLY (Type 0) indicating the target system is alive • Traffic can be ICMP, ARP, TCP, or UDP

  4. ARP Host Discovery • Cain

  5. ARP Host Discovery

  6. ARP Scan • Advantages • Operates at layer 2 • A firewall will not conceal a device from an ARP scan • Disadvantage • Must be on target’s network segment • Cannot scan through routers

  7. ARP Scanning Tools • arp-scan • Linux command-line tool • Nmap • -PR to do ARP scan • -sn to skip host discovery • Cain • Sniffer tab • Enable sniffer • Click +

  8. Nmap

  9. ICMP Packet Types • Message Type: 0 - Echo Reply • Message Type: 3 - Destination Unreachable • Message Type: 4 - Source Quench • Message Type: 5 - Redirect • Message Type: 8 – Echo Request • Message Type: 11 - Time Exceeded • Message Type: 12 - Parameter Problem • Message Type: 13 - Timestamp • Message Type: 14 - Timestamp Reply • Message Type: 15 - Information Request • Message Type: 16 - Information Reply • Message Type: 17 – Address Mask Request • Message Type: 18 – Address Mask Reply

  10. ICMP Queries • icmpquery uses ICMP type 13 (TIMESTAMP) to find the system time, which shows its timezone • ICMP type 17 (ADDRESS MASK REQUEST) shows the subnet mask

  11. Network Discovery Tools

  12. Nmap ICMP Options

  13. SuperScan • Windows freeware • Not so fast anymore • Does PING scanning, using several types of ICMP packets • Also does port scanning, banner grabbing, whois, and enumeration

  14. Unix Ping Detection Tools • Scanlogd • Courtney • Ippl • Protolog

  15. ICMP Blocking • ICMP is often blocked these days • Blocked by default in Win XP SP2, Win 2003 SP 1, and Vista • If ICMP is blocked, use port scanning • Slower than ping sweeping • SuperScan for Windows • Nmap for Linux, Unix, or Windows • Hping2 for Unix (can fragment packets)

  16. Ping Sweeps Countermeasures • Detecting Ping Sweeps • Network-based Intrusion Detection Systems like Snort detect ping sweeps • Ping scans will be in the host logs • Firewalls can detect ping scans

  17. Blocking ICMP • Routers may require some ICMP packets, but not all types • Safest procedure would be to allow ICMP only from your ISP, and only to public servers on your DMZ

  18. Other ICMP Threats • ICMP can be used for a Denial of Service attack • ICMP can be used as a covert channel with Loki • Allowing unauthorized data transfer • Such as control signals for a back-door trojan

  19. Determining Which Services are Running or Listening

  20. Port scanning • Now that we have a map with some hosts, let’s find out what ports are open on a target host • 65,535 TCP ports; 65,535 UDP ports • Web server: TCP port 80 • DNS server: UDP port 53 • Mail server: TCP port 25 • Port scanning tools can scan: • List of ports • Range of ports • All possible TCP and UDP ports • Attacker may scan a limited set of ports, to avoid detection

  21. TCP: Reset packet • If machine receives a TCP packet it is not expecting, it responds with TCP packet with RST bit set. • For example when no process is listening on destination port • For UDP, machine returns ICMP “port unreachable” instead

  22. Extremely popular usually run over linux rich feature set, exploiting raw sockets need root to use all features Ping sweeping over any range of IP addresses with ICMP, SYN, ACK OS determination Port scanning Over any range of ports Almost any type of TCP, UDP packet Source IP address spoofing Decoy scanning Packet fragmentation Timing Options Nmap (1) Excellent reference: Nmap man page

  23. Attacks Nmap (2) Input: • nmap [Scan Type] [Options] <target hosts> • Default for port scanning: ports 1-1024 plus ports listed in nmap service file Output: • open ports: syn/ack returned; port is open • unfiltered ports: RST returned: port is closed but not blocked by firewall • filtered ports: nothing returned; port is blocked by firewall

  24. Nmap (3): ping sweep Nmap –sP –v 116.27.38/24 • Sends ICMP echo request (ping) to 256 addresses • Can change options so that pings with SYNs, ACKs… • -sP = ping • -v = verbose

  25. Nmap (4): polite port scan • nmap –sT -v • Attempts to complete 3-way handshake with each target port • Sends SYN, waits for SYNACK, sends ACK, then sends FIN to close connection • If target port is closed, no SYNACK returned • Instead RST packet is typically returned • TCP connect scans are easy to detect • Target (e.g. Web server) may log completed connections • Gives away attacker’s IP address

  26. Nmap (5) : TCP SYN port scan • nmap –sS -v • Stealthier than polite scan • Send SYN, receive SYNACK, send RST • Send RST segment to avoid an accidental DoS attack • Stealthier: hosts do not record connection • But routers with logging enabled will record the SYN packet • Faster: don’t need to send FIN packet

  27. firewall ACK dest port 2031 ACK dest port 2032 RST Internal Network I learned port 2032 is open through the firewall Nmap (6): TCP ACK scans • Many filters (in firewalls and routers) only let internal systems hosts initiate TCP connections • Drop packets for which ACK=0 (ie SYN packet): no sessions initiated externally • To learn what ports are open through firewall, try an ACK scan (segments with ACK=1)

  28. Nmap (7): UDP port scans • UDP doesn’t have SYN, ACK, RST packets • nmap simply sends UDP packet to target port • ICMP Port Unreachable: interpret port closed • Nothing comes back: interpret port open • False positives common

  29. Nmap (8): Obscure source • Attacker can enter list of decoy source IP addresses into Nmap • For each packet it sends, Nmap also sends packets from decoy source IP addresses • For 4 decoy sources, send five packets • Attacker’s actual address must appear in at least one packet, to get a result • If there are 30 decoys, victim network will have to investigate 31 different sources!

  30. Nmap (9): TCP stack fingerprinting • In addition to determining open ports, attacker wants to know OS on targeted machine: • exploit machine’s known vulnerabilities • sophisticated hacker may set up lab environment similar to target network • TCP implementations in different OSes respond differently to illegal combinations of TCP flag bits.

  31. Nmap (10): Fingerprinting • Nmap sends • SYN to open port • NULL to open port (no flag bits set) • SYN/FIN/URG/PSH to open port • SYN to closed port • ACK to closed port • FIN/PSH/URG to closed port • UDP to closed port • Nmap includes a database of OS fingerprints for hundreds of platforms

  32. Nmap (11): examples • nmap -v • Scans all TCP default ports on; verbose mode • nmap -sS -O • First pings addresses in target network to find hosts that are up. Then scans default ports at these hosts; stealth mode (doesn’t complete the connections); tries to determine OS running on each scanned host • nmap -sX -p 22,53,110,143 198.116.*.1-127 • Sends an Xmas tree scan to the first half of each of the 255 possible subnets in the 198.116/16. Testing whether the systems run ssh, DNS, pop3, or imap • nmap -v -p 80 *.*.2.3-5 • finds all web servers on machines with IP addresses ending in .2.3, .2.4, or .2.5

  33. Defenses against network mapping • Filter using firewalls and packet-filtering capabilities of routers • Block incoming ICMP packets, except to the hosts that you want to be pingable • Filter Time Exceeded ICMP messages leaving your network • Close all unused ports • Scan your own systems to verify that unneeded ports are closed • Intrusion Detection Systems

  34. Windows-Based Port Scanners • SuperScan • Four different ICMP host-discovery techniques • Accurate UDP scan sending "Data+ICMP" • Banner grabbing • Many other tools • Nmap with the Zenmap GUI • Powerful, runs on Windows

  35. Command-line Scanners • Scanline • For Windows • netcat • For Windows and Linux • nmap • Can be run on the command line, on Windows or Linux

  36. Port Scanning Countermeasures • Snort ( is a great free IDS (Intrusion Detection System) • [**] spp_portscan: PORTSCAN DETECTED from [**] 05/22-18:48:53.681227 [**] spp_portscan: portscan status from 4 connections across 1 hosts: TCP(0), UDP(4) [**] 05/22-18:49:14.180505 [**] spp_portscan: End of portscan from [**] 05/22-18:49:34.180236

  37. Other Detection Tools • Scanlogd • Detects TCP Port Scans on Unix • Firewalls can detect port scans • Use threshold logging to limit the volume of email alerts sent by your firewall • That groups similar alerts into a single email • Attacker • Windows tool from Foundstone to detect port scans

  38. Preventing Port Scans • You can't stop the scans from coming in, but you can mimimize your attack surface • Disable unnecessary services

  39. Detecting the Operating System • Banner-Grabbing • Many services announce what they are in response to requests • Banner grabbers just collect those banners • But they could be spoofed

  40. Active Stack Fingerprinting • Details of the TCP Packets are used to identify the operating system • Nmap does this, using these probes: • FIN probe • Bogus Flag probe • Initial Sequence Number (ISN) sampling • "Don't fragment bit" monitoring • TCP initial window size • And many others

  41. Operating System Detection Countermeasures • IDS can detect operating system detection scans • Hacking the OS to change its TCP stack is dangerous, and not recommended • Best policy: Accept that your firewalls and proxy servers will be scanned and fingerprinted, and harden them against attackers who know the OS

  42. Passive Operating System Identification • Sniff traffic and guess the OS from that • Examine these features • TTL (time-to-live) • Window size • DF (Don't fragment bit) • siphon was the first tool to do this, it's out of date • p0f is a newer one

  43. p0f on Vista • Run p0f in a Command Prompt Window • Open a Web page • It fingerprints any OS it can see on the LAN

  44. Automated Discovery Tool: Cheops-ng • Combines Ping, Traceroute, Port Scans, and OS Detection to draw a network map • Windows 7's "Network Map" is similar