CIT 480: Securing Computer Systems

Scanning

Topics 1. Port Scanning 2. Stealth Scanning 3. Version Identification 4. OS Fingerprinting

Port Scanning Port scanning is a method of discovering potential input channels on a host by proving the TCP and UDP ports on which services may be listening.

nmap TCP connect() scan > nmap -sT scanme.nmap.org Starting Nmap 6.40 ( http://nmap.org ) at 2013-10-26 Nmap scan report for scanme.nmap.org (74.207.244.221) Host is up (0.11s latency). Not shown: 996 closed ports PORT STATE SERVICE 22/tcp open ssh 80/tcp open http 1720/tcp filtered H.323/Q.931 9929/tcp open nping-echo done: 1 IP address (1 host up) scanned in 9.92 seconds

Scanning Techniques 1. TCP connect() scan 2. TCP SYN scan 3. TCP FIN scan 4. TCP Xmas scan 5. TCP Null scan 6. TCP ACK scan 7. Fragmentation Scan 8. FTP bounce scan 9. Idle Scan 10. UDP scan

TCP connect() scan • Use connect() system call on each port,

following normal TCP connection protocol (3-way handshake).

• connect() will succeed if port is listening. • Advantages: fast, requires no privileges • Disadvantages: easily detectable and

blockable.

TCP SYN Scan • Send SYN packet and wait for response

– SYN+ACK • Port is open • Send RST to tear down connection

– RST • Port is closed

• Advantage: less likely to be logged or blocked • Disadvantage: requires root privilege

TCP FIN scan • Send TCP FIN packet and wait for response

– No response • Port is open

– RST • Port is closed.

• Advantages: more stealthy than SYN scan • Disadvantages: MS Windows doesn’t follow

standard (RFC 793) and responds with RST in both cases, requires root privilege.

Xmas and Null Scans • Similar to FIN scan with different flag

settings. • Xmas Scan: Sets FIN, URG, and PUSH

flags. • Null Scan: Turns off all TCP flags.

TCP ACK Scan • Does not identify open ports • Used to determine firewall type

– Packet filter (identifies responses by ACK bit) – Stateful

• Send TCP ACK packet to specified port – RST

• Port is unfiltered (packet got through) – No response or ICMP unreachable

• Port is filtered

Fragmentation Scan • Modify TCP stealth scan (SYN, FIN, Xmas,

NULL) to use tiny fragmented IP datagrams. • Advantages: increases difficulty of scan

detection and blocking. • Disadvantages: does not work on all Oses,

and may crash some firewalls/sniffers.

FTP Bounce Scan FTP protocol supports proxy ftp

– Client requests server send file to another IP, port. – If server can open connection, port is open.

Advantages: – Hide identity of scanning host. – Bypass firewalls by using ftp server behind firewall.

Disadvantages: – Most ftp servers no longer support proxying. – Printer ftp servers often do still support.

Idle Scan Use intermediate idle host to do scan.

– Idle host must increment IP ID for each packet. – Idle host must not receive traffic from anyone other than attacker.

Scan Process 1. Attacker connects to idle host to obtain initial IP ID X. 2. Send SYN packet to port Y of target with spoofed IP of idle host. 3. If port is open, target host will send SYN+ACK to idle host. 4. Idle host with send RST packet with IP ID X+1 to target. 5. Attacker connects with SYN to idle host to obtain updated IP ID. 6. Idle host sends back SYN+ACK to attacker.

• Note that this action will increment IP ID by 1. • If IP ID is X+2, then port Y on target is open.

Advantages: hides scanner IP address from target.

UDP Scans Send 0-byte UDP packet to each UDP port

UDP packet returned • Port is open

ICMP port unreachable • Port is closed

Nothing • Port listed as open|filtered • Could be that packet was lost. • Could be that server only returns UDP on valid input.

Disadvantages: – ICMP error rate throttled to a few packets/second (RFC 1812),

making UDP scans of all 65535 ports very slow. – MS Windows doesn’t implement rate limiting.

Version Scanning • Port scanning reveals which ports are open

– Guess services on well-known ports. • How can we do better?

– Find what server: vendor and version – telnet/netcat to port and check for banner – Version scanning

Banner Checking with netcat > nc www.nku.edu 80 GET / HTTP/1.1 HTTP/1.1 400 Bad Request Date: Sun, 07 Oct 2007 19:27:08 GMT Server: Apache/1.3.34 (Unix) mod_perl/1.29 PHP/4.4.1 mod_ssl/2.8.25 OpenSSL/0.9.7a Connection: close Transfer-Encoding: chunked Content-Type: text/html; charset=iso-8859-1 127 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <HTML><HEAD> <TITLE>400 Bad Request</TITLE> </HEAD><BODY> <H1>Bad Request</H1> Your browser sent a request that this server could not understand.<P> client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /<P> </BODY></HTML>

Version Scanning 1. If port is TCP, open connection. 2. Wait for service to identify self with banner. 3. If no identification or port is UDP,

1. Send probe string based on well-known service. 2. Check response against db of known results.

4. If no match, test all probe strings in list.

nmap version scan > nmap -sV scanme.nmap.org Starting Nmap 6.40 ( http://nmap.org ) at 2013-10-26 17:11 EDT Nmap scan report for scanme.nmap.org (74.207.244.221) Host is up (0.10s latency). Not shown: 996 closed ports PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0) 80/tcp open http Apache httpd 2.2.14 ((Ubuntu)) 1720/tcp filtered H.323/Q.931 9929/tcp open nping-echo Nping echo Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

More nmap Tools Set source port

– Bypass firewall by using allowed source port. – Use port 80 for TCP, port 53 for UDP scans.

Decoys – Send additional scans from list of decoys. – Spoof IP addresses of decoy hosts. – Defender has to investigate decoys + attacker.

Defences Prevention

– Disable unnecessary services. – Block ports at firewall. – Use a stateful firewall instead of packet filter.

Detection – Network Intrusion Detection Systems. – Port scans often have distinct signatures. – IPS can react to scan by blocking IP address.

OS Fingerprinting Identify OS by specific features of its

TCP/IP network stack implementation. – Explore TCP/IP differences between OSes. – Build database of OS TCP/IP fingerprints. – Send set of specially tailored packets to host – Match results to identical fingerprint in db to

identify operating system type and version.

nmap OS fingerprint examples > sudo nmap -O scanme.nmap.org Device type: general purpose Running: Linux 2.6.X|3.X OS CPE: cpe:/o:linux:linux_kernel:2.6 OS details: Linux 2.6.38 - 3.0 Uptime guess: 12.224 day TCP Sequence Prediction: Difficulty=202 (Good luck!) IP ID Sequence Generation: All zeros

> sudo nmap –v -O 192.168.1.1 Device type: general purpose Running: Linux 2.4.X OS CPE: cpe:/o:linux:linux_kernel:2.4 OS details: Linux 2.4.18 - 2.4.35 (likely embedded) Uptime guess: 29.789 days TCP Sequence Prediction: Difficulty=196 (Good luck!) IP ID Sequence Generation: All zeros

OS Fingerprinting Techniques FIN probe

– RFC 793 requires no response – MS Windows, BSDI, Cisco IOS send RST

Bogus flag probe – Bit 7 of TCP flags unused – Linux <2.0.35 keeps flag set in response

TCP ISN sampling – Different algorithms for TCP ISNs

IP Identification – Different algorithms for incrementing IPID

Passive Fingerprinting • Identify OSes of hosts on network by sniffing

packets sent by each host. • Use similar characteristics as active techniques:

– TTL – MSS – Initial Window Size – Don’t Fragment bit

• Tools: p0f

Fingerprinting Defences Detection

– NIDS Blocking

– Firewalling – Some probes can’t be blocked.

Deception – IPpersonality changes Linux TCP/IP stack

signature to that of another OS in nmap db.

Tools Summary Information Tool IP addresses of hosts ping, nmap -sP Network topology traceroute, lft Open ports nmap -sT -sU Service versions nmap -sV OS nmap –O, p0f Vulnerabilities Nessus, OpenVAS

References 1. Fyodor, NMAP documentation, http://nmap.org/docs.html. 2. Fyodor, “Remote OS detection via TCP/IP Stack

FingerPrinting,” Phrack 54, http://www.insecure.org/nmap/nmap-fingerprinting-article.html

3. Gordon “Fyodor” Lyon, Nmap Network Scanning, 2008. 4. Ed Skoudis, Counter Hack Reloaded, Prentice Hall, 2006.

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