NMAP, also known as Network Mapper, is a versatile tool for network mapping and security auditing. One of its essential features includes the ability to scan the User Datagram Protocol (UDP) for network diagnostics and security audits. UDP is a lower-level IP communication protocol, mostly used where speed is important, and error checking and correction is not as necessary, like streaming and gaming.
To perform a UDP scan with NMAP, you can use the “-sU” command-line option. Below is an example command:
`nmap -sU -p 123
This command tells NMAP to execute a UDP scan (-sU) on port 123 (-p 123) of the target system (
However, it is necessary to understand that UDP scanning can be slower than TCP scanning, mainly because of differences in the protocol specifications. Unlike TCP, UDP does not have a mechanism for verifying whether a message was received, so the absence of a response can be interpreted in two ways: the port may be open, or the packet may have been dropped.
UDP scans could also result in false positives due to “port unreachable” ICMP error messages. For instance, suppose a “port unreachable” message is generated by an application layer above the ICMP layer. In that case, NMAP can mistakenly interpret this as a closed port when the port is actually open but guarded by a tool producing the error.
To mitigate these issues, NMAP employs several methods. First, it sends a payload with the UDP packet that will likely return a response if the port is open. If an ICMP “port unreachable” error returns, the port is classified as closed. If there’s no response, the port is classified as open|filtered, denoting that the packet was either received, or it was dropped/filtered somewhere along the route. If some other response is received, the port is classified as open.
However, the classification open|filtered can be quite unclear. To resolve, the “—reason” option can be used to include reasoning information in output. An example of a command to perform this is:
`nmap -sU —reason -p 123
Furthermore, slower speed of UDP scanning can be improved by increasing the intensity of the scan. The “-T4” option is often a good balance between speed and accuracy:
`nmap -sU -T4 -p 123
A clear understanding of NMAP and UDP will yield more benefits in applying NMAP to perform UDP scanning. Reading NMAP’s official documentation and understanding UDP specification (RFC 768) would be helpful.
Sources:
- Gordon Fyodor Lyon (2009). “NMAP Network Scanning”. NMAP.Org
- J. Postel (1980). “RFC 768 – User Datagram Protocol”. IETF.org.
NOTE: Remember to obtain proper authorizations before performing any scan to prevent legal issues. It is illegal to scan networks without explicit permission.
