dawei
http://nmap.org/bennieston-tutorial/
Nmap is a free,open-source port scanner available for both UNIX and Windows. It has an optional graphical front-end,NmapFE,and supports a wide variety of scan types,each one with different benefits and drawbacks.
This article describes some of these scan types,explaining their relative benefits and just how they actually work. It also offers tips about which types of scan would be best against which types of host.
The article assumes you have Nmap installed (or that you know how to install it. Instructions are available on the Nmap website,http://www.insecure.org/nmap/install/inst-source.html),and that you have the required privileges to run the scans detailed (many scans require root or Administrator privileges).
A frequently asked questions section has been added since the first version of this article,and this is included as the last section in this version. This is a fully revised and updated version of this tutorial,re-typed and converted to a TeX format,allowing more output formats to be utilised. At the time of writing,the latest Nmap version was 4.11.
This information is provided to assist users of Nmap in scanning their own networks,or networks for which they have been given permission to scan,in order to determine the security of such networks. it is not intended to assist with scanning remote sites with the intention of breaking into or exploiting services on those sites,or for imformation gathering purposes beyond those allowed by law. I hereby disclaim any responsibility for actions taken based upon the information in this article,and urge all who seek information towards a destructive end to reconsider their life,and do something constructive instead.
The two basic scan types used most in Nmap are TCP connect() scanning [-sT] and SYN scanning (also known as half-open,or stealth scanning) [-sS].
These two types are explained in detail below.
These scans are so called because UNIX sockets programming uses a system call namedconnect()to begin a TCP connection to a remote site. Ifconnect()succeeds,a connection was made. If it fails,the connection could not be made (the remote system is offline,the port is closed,or some other error occurred along the way). This allows a basic type of port scan,which attempts to connect to every port in turn,and notes whether or not the connection succeeded. Once the scan is completed,ports to which a connection could be established are listed asopen,the rest are said to be closed.
This method of scanning is very effective,and provides a clear picture of the ports you can and cannot access. If a connect() scan lists a port as open,you can definitely connect to it – that is what the scanning computer just did! There is,however,a major drawback to this kind of scan; it is very easy to detect on the system being scanned. If a firewall or intrusion detection system is running on the victim,attempts to connect() to every port on the system will almost always trigger a warning. Indeed,with modern firewalls,an attempt to connect to a single port which has been blocked or has not been specifically “opened” will usually result in the connection attempt being logged. Additionally,most servers will log connections and their source IP,so it would be easy to detect the source of a TCP connect() scan.
For this reason,the TCP Stealth Scan was developed.
I’ll begin this section with an overview of the TCP connection process. Those familiar with TCP/IP can skip the first few paragraphs.
When a TCP connection is made between two systems,a process known as a “three way handshake” occurs. This involves the exchange of three packets,and synchronises the systems with each other (necessary for the error correction built into TCP. Refer to a good TCP/IP book for more details.
The system initiating the connection sends a packet to the system it wants to connect to. TCP packets have a header section with aflagsfield. Flags tell the receiving end something about the type of packet,and thus what the correct response is.
Here,I will talk about only four of the possible flags. These are SYN (Synchronise),ACK (Acknowledge),FIN (Finished) and RST (Reset). SYN packets include a TCP sequence number,which lets the remote system know what sequence numbers to expect in subsequent communication. ACK acknowledges receipt of a packet or set of packets,FIN is sent when a communication is finished,requesting that the connection be closed,and RST is sent when the connection is to be reset (closed immediately).
To initiate a TCP connection,the initiating system sends a SYN packet to the destination,which will respond with a SYN of its own,and an ACK,acknowledging the receipt of the first packet (these are combined into a single SYN/ACK packet). The first system then sends an ACK packet to acknowledge receipt of the SYN/ACK,and data transfer can then begin.
SYN or Stealth scanning makes use of this procedure by sending a SYN packet and looking at the response. If SYN/ACK is sent back,the port is open and the remote end is trying to open a TCP connection. The scanner then sends an RST to tear down the connection before it can be established fully; often preventing the connection attempt appearing in application logs. If the port is closed,an RST will be sent. If it is filtered,the SYN packet will have been dropped and no response will be sent. In this way,Nmap can detect three port states – open,closed and filtered. Filtered ports may require further probing since they could be subject to firewall rules which render them open to some IPs or conditions,and closed to others.
Modern firewalls and Intrusion Detection Systems can detect SYN scans,but in combination with other features of Nmap,it is possible to create a virtually undetectable SYN scan by altering timing and other options (explained later).
With the multitude of modern firewalls and IDS’ now looking out for SYN scans,these three scan types may be useful to varying degrees. Each scan type refers to the flags set in the TCP header. The idea behind these type of scans is that a closed port should respond with an RST upon receiving packets,whereas an open port should just drop them (it’s listening for packets with SYN set). This way,you never make even part of a connection,and never send a SYN packet; which is what most IDS’ look out for.
The FIN scan sends a packet with only the FIN flag set,the Xmas Tree scan sets the FIN,URG and PUSH flags (see a good TCP/IP book for more details) and the Null scan sends a packet with no flags switched on.
These scan types will work against any system where the TCP/IP implementation follows RFC 793. Microsoft Windows does not follow the RFC,and will ignore these packets even on closed ports. This technicality allows you to detect an MS Windows system by running SYN along with one of these scans. If the SYN scan shows open ports,and the FIN/NUL/XMAS does not,chances are you’re looking at a Windows box (though OS Fingerprinting is a much more reliable way of determining the OS running on a target!)
The sample below shows a SYN scan and a FIN scan,performed against a Linux system. The results are,predictably,the same,but the FIN scan is less likely to show up in a logging system.
1 [chaos]# nmap -sS 127.0.0.1 2 3 Starting Nmap 4.01 at 2006-07-06 17:23 BST 4 Interesting ports on chaos (127.0.0.1): 5 (The 1668 ports scanned but not shown below are in state: 6 closed) 7 PORT STATE SERVICE 8 21/tcp open ftp 9 22/tcp open ssh 10 631/tcp open ipp 11 6000/tcp open X11 12 13 Nmap finished: 1 IP address (1 host up) scanned in 0.207 14 seconds 15 [chaos]# nmap -sF 127.0.0.1 16 17 Starting Nmap 4.01 at 2006-07-06 17:23 BST 18 Interesting ports on chaos (127.0.0.1): 19 (The 1668 ports scanned but not shown below are in state: 20 closed) 21 PORT STATE SERVICE 22 21/tcp open|filtered ftp 23 22/tcp open|filtered ssh 24 631/tcp open|filtered ipp 25 6000/tcp open|filtered X11 26 27 Nmap finished: 1 IP address (1 host up) scanned in 1.284 28 seconds
