Written by Ethical Hacker
Sunday, 17 February 2008 17:30

Footprinting and Scanning is the first basis of hacking. Information gathering has many phases like profiling your target. Whois, ARIN can reveal public information of a domain that can be leveraged further. Traceroute and mail tracking can be used to target specific IP and later for spoofing. Nslookup can reveal specific users and zone transfers can compromise DNS security. Footprinting is necessary to systematically and methodically ensure that all pieces of information related to the aforementioned technologies are identified.

Without a sound methodology for performing this type of reconnaissance, you are likely to miss key pieces of information related to a specific technology or organization. Footprinting is often the most arduous task of trying to determine the security posture of an entity; however, it is one of the most important.
Footprinting must be performed accurately and in a controlled fashion. This is the reconnaissance step before anything is done. Tools like Nmap will be deployed to scan the target and get any available information possible. Information warfare is not without its battle plans or surveillance techniques. In this context, a strategic map used in a battle would be a close analogy to a footprint.

Note that through this course, we use the term 'organization' to represent a target system. This includes discussion pertaining to a single system as well. Footprinting therefore, needs to be carried out precisely and in an organized manner. The information unveiled at various network levels can include details of domain name, network blocks, network services and applications, system architecture, intrusion detection systems, specific IP addresses, access control mechanisms and related lists, phone numbers, contact addresses, authentication mechanisms and system enumeration. This listing may include more information depending on how various security aspects are addressed by the organization.

Information gathered during the Footprinting phase can be used as a springboard in narrowing down the attack methodology and also in assessing its merit. One dubious aspect of the information gathering phase is that most of it can be sought within legal bindings and from publicly available information. It is to be noted that though the Internet originated from the efforts of the defense department and many of the protocols were established to serve the purpose of communicating information reliably, completely and dependably; the speed with which it would penetrate the common world was unpredicted, and so were the security concerns that would arise from the increased networked environment.

One of the best Hack Tool to gather information is Google! Google Hacking if most popular among Ethical Hackers and Black Hat Hackers. When using scanning tools the purpose is to detect 'live' systems on target network. Discovering services running/ listening on target systems. Understanding port scanning techniques. Identifying TCP and UDP services running on target network.
Discovering the operating system. Understanding active and passive fingerprinting. Automated discovery tools.
There are various scan types - SYN, FIN, Connect, ACK, RPC, Inverse Mapping, FTP Bounce, Idle Host etc. The use of a particular scan type depends on the objective at hand. Port Scanning is one of the most popular reconnaissance techniques used by hackers to discover services that can be compromised.
A potential target computer runs many 'services' that listen at 'well-known' 'ports'. By scanning which ports are available on the victim, the hacker finds potential vulnerabilities that can be exploited. Scan techniques can be differentiated broadly into Vanilla, Strobe, Stealth, FTP Bounce, Fragmented Packets, Sweep and UDP Scans. One of the primary activities that an attacker undertakes while attempting to penetrate the system is to compile an inventory of open ports using any of the port scanning techniques. On completion, this list helps the attacker identify various services that are running on the target system using a RFC compliant port list (discussed before under the services file). This allows further strategizing leading to system compromise. Port numbers are 16-bit unsigned numbers and can be broadly classified into three categories. Port 0-1023 is "well known ports", 1024 - 49151 are "registered ports" and 49152 - 65535 is "dynamic or private ports". Port scanning usually means scanning for TCP ports, which being a stateful protocol - based on acknowledgement, gives good feedback to the attacker. One problem with port scanning is that it is effortlessly logged by the services listening at the scanned ports.
This is because they detect an incoming connection, but do not receive any data, thereby generating an application error log. UDP, or connection-less (without acknowledgement) traffic, responds in a different manner. In order to scan for UDP ports, the attacker generally sends empty UDP datagram at the port. If the port is listening, the service will send back an error message or ignore the incoming datagram. If the port is closed, then the operating system sends back an "ICM P Port Unreachable" message. Here, by the method of exclusion, the attacker can find open ports.
Usually UDP ports are high end ports. Port scanning techniques can be broadly differentiated into open scan, half-open scan and stealth scan. There are other techniques such as ICMP echo and FTP bounce, and these are covered under sweeps and miscellaneous scans. How does an attacker decide on which scan to adopt? Well, this depends largely on the knowledge gained by the attacker during his reconnaissance regarding the type of network topology, IDS and other logging features present on the system. Predictably, an attacker would like to keep his actions undetected. One important aspect of information gathering is documentation. Most people don't like paperwork, but it's a requirement that can't be ignored. The best way to get off to a good start is to develop a systematic method to profile a target and record the results. Create a matrix with fields to record domain name, IP address, DNS servers, employee information, email addresses, IP address range, open ports, and banner details.

[Whois]

The primary tool to navigate these databases is Whois. Whois is a utility that interrogates the Internet domain name administration system and returns the domain ownership, address, location, phone number, and other details about a specified domain name. Whois is the primary tool used to query Domain Name Services (DNS). If you're performing this information gathering from a Linux computer, the good news is Whois is built in. From the Linux prompt, users can type in whois domainname.com or whois? to get a list of various options.

Windows users are not as fortunate as Linux users because Windows does not have a built-in Whois client. Windows users will have to use a third-party tool or website to obtain Whois information. One tool that a Windows user can use to perform Whois lookups is Sam Spade. There's also a variety of websites that you can use to obtain Whois information.


A Domain proxy is one way that organizations can protect their identity while still complying with laws that require domain ownership to be public information. Domain proxies work by applying anonymous contact information as well an anonymous email address. This information is displayed when someone performs a domain Whois.

The proxy then forwards any emails or contact information that might come to those addresses on to you. This information provides a contact person, address, phone number, and DNS servers. A hacker skilled in the art of social engineering might use this information to call the organization and pretend to be Kenneth, or he might use the phone number to war dial a range of phone numbers looking for modems.



[DNS Enumeration]

The attacker has also identified the names of the DNS servers. DNS servers might be targeted for zone transfers. A zone transfer is the mechanism used by DNS servers to update each other by transferring the contents of their database. DNS is structured as a hierarchy so that when you request DNS information, your request is passed up the hierarchy until a DNS server is found that can resolve the domain name request.
What's left at this step is to try and gather additional information from the organization's DNS servers. The primary tool to query DNS servers is nslookup. Nslookup provides machine name and address information. Both Linux and Windows have nslookup clients. Nslookup is used by typing nslookup from the command line followed by an IP address or a machine name. Doing so will cause nslookup to return the name, all known IP addresses, and all known CNAMES for the identified machine. Nslookup queries DNS servers for machine name and address information.
Using nslookup is rather straightforward. Let's look at an example in which nslookup is used to find out the IP addresses of Google's web servers. By entering nslookup www.google.com, the following response is obtained:

C:\>nslookup www.google.com
Server: dnsr1.sbcglobal.net
Address: 68.94.156.1
Non-authoritative answer:
Name: www.l.google.com
Addresses: 64.233.187.99, 64.233.187.104
Aliases: www.google.com


The first two lines of output say which DNS servers are being queried. In this case, it's dnsr1.sbcglobal.net in Texas. The non-authoritative answer lists two IP addresses for the Google web servers. Responses from non-authoritative servers do not contain copies of any domains. They have a cache file that is constructed from all the DNS lookups it has performed in the past for which it has gotten an authoritative response.
Nslookup can also be used in an interactive mode by just typing nslookup at the command prompt. In interactive mode, the user will be given a prompt of >; at which point, the user can enter a variety of options, including attempts to perform a zone transfer.
DNS normally moves information from one DNS server to another through the DNS zone transfer process. If a domain contains more than one name server, only one of these servers will be the primary. Any other servers in the domain will be secondary servers. Zone transfers are much like the DHCP process in that each is a four-step process. DNS zone transfers function as follows:

1. The secondary name server starts the process by requesting the SOA record from the primary name server.

2. The primary then checks the list of authorized servers, and if the secondary server's name is on that list, the SOA record is sent

3. The secondary must then check the SOA record to see if there is a match against the SOA it already maintains.

If the SOA is a match, the process stops here; however, if the SOA has a serial number that is higher, the secondary will need an update. The serial number indicates if changes were made since the last time the secondary server synchronized with the primary server. If an update is required, the secondary name server will send an All Zone Transfer (AXFR) request to the primary server.

4. Upon receipt of the AXFR, the primary server will send the entire zone file to the secondary name server.

A zone transfer is unlike a normal lookup in that the user is attempting to retrieve a copy of the entire zone file for a domain from a DNS server. This can provide a hacker or pen tester with a wealth of information. This is not something that the target organization should be allowing. Unlike lookups that primarily occur on UDP 53, unless the response is greater than 512 bytes, zone transfers use TCP 53. To attempt a zone transfer, you must be connected to a DNS server that is the authoritative server for that zone. Remember the nslookup information we previously gathered? It's shown here again for your convenience.

Registrant:
Pearson Technology Centre
Kenneth Simmons
200 Old Tappan Rd .
Old Tappan, NJ 07675 USA
Email: billing@superlibrary.comThis e-mail address is being protected from spambots. You need JavaScript enabled to view it
Phone: 001-201-7846187
Registrar Name....: REGISTER.COM, INC.
Registrar Whois...: whois.register.com
Registrar Homepage: www.register.com
DNS Servers:
usrxdns1.pearsontc.com
oldtxdns2.pearsontc.com


Review the last two entries. Both usrxdns1.pearsontc.com and oldtxdns2.pearsontc.com are the DNS authoritative servers for ExamCram.com. These are the addresses that an attacker will target to attempt a zone transfer. The steps to try and force a zone transfer are shown here:

1. nslookupEnter nslookup from the command line

2. server Enter the IP address of the authoritative server for that zone.

3. set type = anyTells nslookup to query for any record.

4. ls d Domain.com is the name of the targeted domain of the final step that performs the zone transfer.

One of two things will happen at this point; either you will receive an error message indicating that the transfer was unsuccessful, or you will be returned a wealth of information, as shown in the following:

C:\WINNT\system32>nslookup
Default Server: dnsr1.sbcglobal.net
Address: 128.112.3.12

server 172.6.1.114
set type=any
ls -d example.com

example.com. SOA hostmaster.sbc.net (950849 21600 3600 1728000 3600)
example.com. NS auth100.ns.sbc.net
example.com. NS auth110.ns.sbc.net
example.com. A 10.14.229.23
example.com. MX 10 dallassmtpr1.example.com
example.com. MX 20 dallassmtpr2.example.com
example.com. MX 30 lasmtpr1.example.com
lasmtpr1 A 192.172.243.240
dallassmtpr1 A 192.172.163.9
dallaslink2 A 192.172.161.4
spamassassin A 192.172.170.49
dallassmtpr2 A 192.172.163.7
dallasextra A 192.172.170.17
dallasgate A 192.172.163.22
lalink A 172.16.208.249
dallassmtp1 A 192.172.170.49
nygate A 192.172.3.250
www A 10.49.229.203
dallassmtp MX 10 dallassmtpr1.example.com
dallassmtp MX 20 dallassmtpr2.example.com
dallassmtp MX 30 lasmtpr1.example.com

Dig is another tool that can be used to provide this type of information. It's available for Linux and for Windows. Dig is a powerful tool that can be used to investigate the DNS system.
This type of information should not be made available to just anyone. Hackers can use this to find out what other servers are running on the network, and it can help them map the network and formulate what types of attacks to launch. Notice the first line that has example.com listed previously. Observe the final value of 3600 on that line. That is the TTL value discussed previously which would inform a hacker as to how long DNS poisoning would last. 3,600 seconds is 60 minutes. Zone transfers are intended for use by secondary DNS servers to synchronize with their primary DNS server. You should make sure that only specific IP addresses are allowed to request zone transfers. Although most Operating Systems restrict this by default, Windows 2000 did not. So, be aware of this if any 2000 servers are still in your network. All DNS servers should be tested. It is very often the case in which the primary has tight security, but the secondaries will allow zone transfers

[Google Hacking]

Most of us use Google or another search engine to locate information. What you might not know is that search engines, such as Google, have the capability to perform much more powerful searches than most people ever dream of. Not only can Google translate documents, perform news searches, do image searches, but it can also be used by hackers and attackers to do something that has been termed Google hacking. By using basic search techniques combined with advanced operators, Google can become a powerful vulnerability search tool.
Google can be used to uncover many pieces of sensitive information that shouldn't be revealed. To learn more about Google hacking, take a look at

http://johnny.ihackstuff.com

[Network Range]

Now that the pen test team has been able to locate name, phone numbers, addresses, some server names, and IP addresses, it's important to find out what range of IP addresses are available for scanning and further enumeration. If you take the IP address of a web server discovered earlier and enter it into the Whois lookup at www.arin.net, the network's range can be determined. As an example, 192.17.170.17 was entered into the ARIN Whois, and the following information was received:

OrgName: target network
OrgID: Target-2
Address: 1313 Mockingbird Road
City: Anytown
StateProv: Tx
PostalCode: 72341
Country: US
ReferralServer: rwhois://rwhois.exodus.net:4321/
NetRange: 192.17.12.0 - 192.17.12.255
CIDR: 192.17.0.0/24
NetName: SAVVIS
NetHandle: NET-192-17-12-0-1
Parent: NET-192-0-0-0-0

This means that the target network has 254 total addresses. The attacker can now focus his efforts on the range from 192.17.12.1 to 192.17.12.254 /24. If these results don't prove satisfactory, traceroute can be used for additional mapping.


[Traceroute]

The traceroute utility is used to determine the path to a target computer. Just as with nslookup, traceroute is available on Windows and UNIX platforms. In Windows, it is known as tracert because of 8.3 legacy filename constraints remaining from DOS. Traceroute was originally developed by Van Jacobson to view the path a packet follows from its source to its destination. Traceroute owes its functionality to the IP header time-to-live (TTL) field. You might remember from the discussion in Chapter 2, "The Technical Foundations of Hacking," that the TTL field is used to limit IP datagram's. Without a TTL, some IP datagram's might travel the Internet forever as there would be no means of timeout. TTL functions as a decrementing counter. Each hop that a datagram passes through reduces the TTL field by one. If the TTL value reaches 0, the datagram is discarded and a time exceeded in transit Internet Control Message Protocol (ICMP) message is created to inform the source of the failure. Linux tracer-oute is based on UDP, whereas Windows uses ICMP. To get a better idea of how this works, let's take a look at how Windows would process a tracer-oute. For this example, say that the target is three hops away. Windows would send out a packet with a TTL of 1. Upon reaching the first router, the packet TTL value would be decremented to 0, which would illicit a time exceeded in transit error message. This message would be sent back to the sender to indicate that the packet did not reach the remote host. Receipt of the message would inform Windows that it had yet to reach its destination, and the IP of the device in which the datagram timed out would be displayed. Next, Windows would increase the TTL to a value of 2. This datagram would make it through the first router, where the TTL value would be decremented to 1. Then it would make it through the second router; at which time, the TTL value would be decremented to 0 and the packet would expire. Therefore, the second router would create a time exceeded in transit error message and forward it to the original source. The IP address of this device would next be displayed on the user's computer. Finally, the TTL would be increased to 3. This datagram would easily make it past the first and second hop and arrive at the third hop. Because the third hop is the last hop before the target, the router would forward the packet to the destination and the target would issue a normal ICMP ping response. The output of this traceroute can be seen here:

C:\>tracert 192.168.1.200
Tracing route to 192.168.1.200:
1 10 ms <10 ms <10 ms
2 10 ms 10 ms 20 ms
3 20 ms 20 ms 20 ms 192.168.1.200
Trace complete.

Linux-based versions of traceroute work much the same way but use UDP. Traceroute sends these UDP packets targeted to high order port numbers that nothing should be listening on. Just as described previously, the TTL is increased until the target device is reached. Because traceroute is using a high order UDP port, typically 33434, the host should ignore the packets after generating port unreachable messages. These ICMP port unreachable messages are used by traceroute to notify the source that the destination has been reached.
It's advisable to check out more than one version of traceroute if you don't get the required results. Some techniques can also be used to try and slip traceroute passed a firewall or filtering device.
When UDP and ICMP are not allowed on the remote gateway, TCPTraceroute can be used. Another unique technique was developed by Michael Schiffman, who created a patch called traceroute.diff that allows you to specify the port that traceroute will use. With this handy tool, you could easily direct traceroute to use UDP port 53. Because that port is used for DNS queries, there's a good chance that it could be used to slip past the firewall.


[Identifying Active Machines]

Attackers will want to know if machines are alive before they attempt to attack. One of the most basic methods of identifying active machines is to perform a ping sweep. Although ping is found on just about every system running TCP/IP, it has been restricted by many organizations.
Ping uses ICMP and works by sending an echo request to a system and waiting for the target to send an echo reply back. If the target device is unreachable, a request time out is returned. Ping is a useful tool to identify active machines and to measure the speed at which packets are moved from one host to another or to get details like the TTL.

Ping does have a couple of drawbacks: First, only one system at a time is pinged and second, not all networks allow ping. To ping a large amount of hosts, a ping sweep is usually performed. Programs that perform ping sweeps typically sweep through a range of devices to determine which ones are active. Some of the programs that will perform ping sweeps include

- Angry IP Scanner
- Pinger
- WS_Ping_ProPack
- Network scan tools
- Super Scan
- Nmap

[Port Scanning]

Port scanning is the process of connecting to TCP and UDP ports for the purpose of finding what services and applications are running on the target device. After running applications, open ports and services are discovered, the hacker can then determine the best way to attack the system.
A good attacker takes time to build an attack plan and also phases his attack so that he is undetected. The primary step in mapping a target network will be to find the limits of the network and assess the perimeter defenses.
The attacker will seek to means of entry by building an inventory of the target network. This will give him an indication regarding any vulnerability that can be exploited and how well the network perimeters are guarded. An attacker might intrude with minimal footprint and lie low to assess what measures are being taken by the target network to detect the intrusion and defend it.


Common Ports and Protocols
Port Service Protocol
20/21 FTP TCP
22 SSH TCP
23 Telnet TCP
25 SMTP TCP
53 DNS TCP/UDP
69 TFTP UDP
80 HTTP TCP
110 POP3 TCP
135 RPC TCP
161/162 SNMP UDP
1433/1434 MSSQL TCP

As you have probably noticed, some of these applications run on TCP, whereas others run on UDP. Although it is certainly possible to scan for all 65,535 TCP and 65,535 UDP ports, many hackers will not. They will concentrate on the first 1,024 ports. These well-known ports are where we find most of the commonly used applications.
A list of well-known ports can be found at www.iana.org/assignments/port-numbers. Now, this is not to say that high order ports should be totally ignored because hackers might break into a system and open a high order port, such as 31337, to use as a backdoor. So, is one protocol easier to scan for than the other?
Well, the answer to that question is yes. TCP offers more opportunity for the hacker to manipulate than UDP. Let's take a look at why. TCP offers robust communication and is considered a connection protocol. TCP establishes a connection by using what is called a 3-way handshake. Those three steps proceed as follows:

The client sends the server a TCP packet with the sequence number flag (SYN Flag) set and an Initial Sequence Number (ISN).
The server replies by sending a packet with the SYN/ACK flag set to the client. The synchronize sequence number flag informs the client that it would like to communicate with it, whereas the acknowledgement flag informs the client that it received its initial packet. The acknowledgement number will be one digit higher than the client's ISN. The server will generate an ISN as well to keep track of every byte sent to the client. When the client receives the server's packet, it creates an ACK packet to acknowledge that the data has been received from the server. At this point, communication can begin.


[TCP Flag Types]

Flag Purpose

SYN Synchronize and Initial Sequence Number (ISN)
ACK Acknowledgement of packets received
FIN Final data flag used during the 4-step shutdown of a session
RST Reset bit used to close an abnormal connection
PSH Push data bit used to signal that data in the packet should be pushed to the beginning of the queue. Usually indicates an urgent message.
URG Urgent data bit used to signify that urgent control characters are present in this packet that should have priority.

At the conclusion of communication, TCP terminates the session by using a 4-step shutdown. Those four steps proceed as follows:

1. The client sends the server a packet with the FIN/ACK flags set.
2. The server sends a packet ACK flag set to acknowledge the clients packet.
3. The server then generates another packet with the FIN/ACK flags set to inform the client that it also is ready to conclude the session.
4. The client sends the server a packet with the ACK flag set to conclude the session.

The TCP system of communication makes for robust communication but also allows a hacker many ways to craft packets in an attempt to coax a server to respond or to try and avoid detection of an intrusion detection system (IDS). Many of these methods are built into Nmap and other port scanning tools, but before taking a look at those tools, some of the more popular port scanning techniques are listed here:

- TCP Connect scan This type of scan is the most reliable, although it is also the most detectable. It is easily logged and detected because a full connection is established. Open ports reply with a SYN/ACK, whereas closed ports respond with an RST/ACK.

- TCP SYN scan This type of scan is known as half open because a full TCP three-way connection is not established. This type of scan was originally developed to be stealthy and evade IDS systems although most now detect it. Open ports reply with a SYN/ACK, whereas closed ports respond with a RST/ACK.

- TCP FIN scan Forget trying to set up a connection; this technique jumps straight to the shutdown. This type of scan sends a FIN packet to the target port. Closed ports should send back an RST. This technique is usually effective only on UNIX devices.

- TCP NULL scan Sure, there should be some type of flag in the packet, but a NULL scan sends a packet with no flags set. If the OS has implemented TCP per RFC 793, closed ports will return an RST.

- TCP ACK scan This scan attempts to determine access control list (ACL) rule sets or identify if stateless inspection is being used. If an ICMP destination unreachable, communication administrative prohibited message is returned, the port is considered to be filtered.

- TCP XMAS scan Sorry, there are no Christmas presents here, just a port scan that has toggled on the FIN, URG, and PSH flags. Closed ports should return an RST.

Now let's look at UDP scans. UDP is unlike TCP. Although TCP is built on robust connections, UDP is based on speed. With TCP, the hacker has the ability to manipulate flags in an attempt to generate a TCP response or an error message from ICMP. UDP does not have flags, nor does UDP issue responses. It's a fire and forget protocol! The most you can hope for is a response from ICMP.
If the port is closed, ICMP will attempt to send an ICMP type 3 code 3 port unreachable message to the source of the UDP scan. But, if the network is blocking ICMP, no error message will be returned. Therefore, the response to the scans might simply be no response. If you are planning on doing UDP scans, plan for unreliable results.


[Nmap]


Nmap was developed by a hacker named Fyodor Yarochkin. This popular application is available for Windows and Linux as a GUI and command-line program. It is probably the most widely used port scanner ever developed. It can do many types of scans and OS identification. It also allows you to control the speed of the scan from slow to insane. Its popularity can be seen by the fact that it's incorporated into other products and was even used in the movie The Matrix. Nmap with the help option is shown here so that you can review some of its many switches. Nmap's documentation can be found at www.insecure.org

C:\nmap-3.93>nmap -h
Nmap 3.93 Usage: nmap [Scan Type(s)] [Options]
Some Common Scan Types ('*' options require root privileges)
* -sS TCP SYN stealth port scan (default if privileged (root))
-sT TCP connect() port scan (default for unprivileged users)
* -sU UDP port scan
-sP ping scan (Find any reachable machines)
* -sF,-sX,-sN Stealth FIN, Xmas, or Null scan (experts only)
-sV Version scan probes open ports determining service and app names/versions
-sR/-I RPC/Identd scan (use with other scan types)
Some Common Options (none are required, most can be combined):
* -O Use TCP/IP fingerprinting to guess remote operating system
-p ports to scan. Example range: '1-1024,1080,6666,31337'
-F Only scans ports listed in nmap-services
-v Verbose. Its use is recommended Use twice for greater effect.
-P0 Don't ping hosts (needed to scan www.microsoft.com and others)
* -Ddecoy_host1,decoy2[,...] Hide scan using many decoys
-6 scans via IPv6 rather than IPv4
-T General timing policy
-n/-R Never do DNS resolution/Always resolve [default: sometimes resolve]
-oN/-oX/-oG Output normal/XML/grepable scan logs to
-iL Get targets from file; Use '-' for stdin
* -S /-e Specify source address or network interface
--interactive Go into interactive mode (then press h for help)
--win_help Windows-specific features
Example: nmap -v -sS -O www.my.com 192.168.0.0/16 '192.88-90.*.*'

SEE THE MAN PAGE FOR MANY MORE OPTIONS. - http://insecure.org/nmap/man/


As can be seen from the output of the help menu in the previous listing, Nmap can run many types of scans. Nmap is considered a required tool for all ethical hackers. Nmap's output provides the open port's well-known service name, number, and protocol. They can either be open, closed, or filtered. If a port is open, it means that the target device will accept connections on that port. A closed port is not listening for connections, and a filtered port means that a firewall, filter, or other network device is guarding the port and preventing Nmap from fully probing it or determining its status. If a port is reported as unfiltered, it means that the port is closed and no firewall or router appears to be interfering with Nmap's attempts to determine its status. To run Nmap from the command line, type Nmap, followed by the switch, and then enter a single IP address or a range. For the example shown here, the sT option was used, which performs a TCP full 3-step connection.

C:\nmap-3.93>nmap -sT 192.168.1.108
Starting nmap 3.93 (http://www.insecure.org/nmap) at 2005-10-05 23:42 Central
Daylight Time
Interesting ports on Server (192.168.1.108):
(The 1653 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE
80/tcp open http
139/tcp open netbios-ssn
515/tcp open printer
548/tcp open afpovertcp
Nmap run completed -- 1 IP address (1 host up) scanned in 420.475 seconds

Several interesting ports were found on this computer, including 80 and 139. A UDP scan performed with the -sU switch returned the following results:

C:\nmap-3.93>nmap -sU 192.168.1.108
Starting nmap 3.93 (http://www.insecure.org/nmap) at 2005-10-05 23:47 Central
Daylight Time
Interesting ports on Server (192.168.1.108):
(The 1653 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE
69/udp open tftp
139/udp open netbios-ssn
Nmap run completed -- 1 IP address (1 host up) scanned in 843.713 seconds

Nmap also has a GUI version called NmapFE. Most of the options in NmapFe correspond directly to the command-line version. Some people call NmapFe the Nmap tutor because it displays the command-line syntax at the bottom of the GUI interface. It is no longer updated for Windows but is maintained for the Linux platform.


[FTP bounce]


A creative scan first detailed by 'Hobbit', takes advantage of the FTP servers with read/write access. The advantage of this scan can be both anonymity and accessibility. For instance suppose the target network allows FTP data transfer from only its recognized partners.
An attacker might discover a service business partner who has a FTP service running with a world-writeable directory that any anonymous user can drop files into and read them back from. It could even be the ISP hosting services on its FTP server.
The attacker, who has a FTP server and able to run in passive mode, logs in anonymously to the legitimate server and issues instructions for scanning or accessing the target server through a series of FTP commands. He may choose to make this into a batch file and execute it from the legitimate server to avoid detection.
If a connection is established as a means of active data transfer processing (DTP), the client knows a port is open, with a 150 and 226 response issued by the server. If the transfer fails a 425 error will be generated with a refused build data message. The PASV listener connection can be opened on any machine that grants a file write access to the attacker and used to bounce the scan attack for anonymity. Hobbit points out that "it does not even have to be an FTP server -- any utility that will listen on a known TCP port and read raw data from it into a file will do".


Often these scans are executed as batch files padded with junk so that the TCP windows are full and the connection stays alive long enough for the attacker to execute his commands. Fingerprinting the OS can help determine the TCP window size and allow the attacker to pad his commands for further access accordingly. Fingerprinting is discussed in detail later in this module. This scan is hard to trace, permits access to local networks and evades firewalls. However, most FTP servers have patched this vulnerability by adopting countermeasures such as preventing third party connections and disallowing listing of restricted ports. Another measure adopted has been to restrict write access.

[UDP Scan]


We have seen how private ports are assigned at the higher end and UDP scans try to detect the state of the port by transmitting a zero byte UDP packet to the target system and the concerned port. An open port does not respond, while a closed port will reply with an ICMP HOST UNREACHABLE response. Similar to inverse mapping, the absence of evidence is considered as the evidence of presence. The disadvantage to the attacker is that UDP is a connectionless protocol and unlike TCP does not retransmit packets if they are lost or dropped on the network. Moreover, it is easily detected and unreliable (false positives). Linux kernels limit ICMP error message rates, with destination unreachable set to 80 per 4 seconds, thereafter implementing a 1/4 second penalty if the count is exceeded. This makes the scan slow and moreover the scan requires root access. However, it avoids TCP based IDS and can scan non-TCP ports.

Sumber : http://www.hackerscenter.com