When an UDP packet is sent to a target port, there might be three scenarios : However, the lack of a feedback mechanism makes it difficult to identify open ports. This gives more emphasis on speed than quality. UDP works on the principle “Fire and Forget” which means that it sends packets directed to targets at certain ports and hopes that they would make it. This means that there’s no “feedback mechanism” like TCP. UDP scans are much less reliable than the previous two as UDP connections are stateless by nature. Infact, when run with sudo privileges, nmap defaults to SYN Scans, otherwise it defaults to TCP scan. It is also to be noted that SYN scans also require sudo privileges because it needs to craft raw packets. However, it is not advisable to run SYN Scans on production environments as it might break certain unstable applications. Often, SYN Scans are not logged by applications running on the ports as most applications start logging a connection only after it has been fully established which is not the case with SYN Scans.They might be able to bypass some primitive firewalls.This method is an improvement on the previous ones because: This prevents the server from repeatedly trying to make the requests and massively reduces scan times. In the previous method where we were sending back a TCP packet with the ACK flag set after receiving an SYN/ACK packet, now we would be sending an RST packet. SYN scans, also known as “Half-Open” or “Stealth Scan” are an improvement over the previous method. Also this method is extremely slow as it waits for the entire TCP 3 way handshake. This is not a very reliable scan technique as it is easy to configure a firewall rule to respond back with RST packets or drop all incoming packets. The target responds back with a TCP packet with the SYN/ACK flags set which would signify that the port is open and then Nmap would respond with a TCP packet with the ACK flag set and hence would complete the TCP 3-way handshake.Target doesn’t respond at all, probably due to a firewall dropping all incoming packets in which case the port will be considered filtered.The target responds with an RST packet that signifies that the port is closed.In this type of scan, Nmap sends a TCP packet to a port with the SYN flag set. Nmap supports a lot of different scan types. nmap -p- 127.0.0.1: This scans all the ports on the localhost.nmap -p 1-100 127.0.0.1: This scans ports from 1 to 100 on localhost.nmap -p 80 127.0.0.0.1: This scans port 80 on localhost.It can be a single port as well as a range of ports. Presently this enables OS detection (-O), version scanning (-sV), script scanning (-sC) and traceroute (–traceroute) Deprecated format as users are now moving towards XML outputs. -oG: Produce “grepable” output and store it to a file.-oX: Produce output in a clean, XML format and store it in a given file.-oN: Redirect normal output to a given filename.However you can specify the format of your choice with : -oA: Same Nmap output in “normal”, XML and grepable formats.You can always specify the verbosity level by specifying a number like this. The minimum level of verbosity advised for use. You can even set the verbosity level as such : -sV: Probe open ports to determine service/version info.Nmap is strong and powerful networking scanning to tool which allows for customizing our scans with the help of flags passed via the command line. Note that you may also need to run it with sudo privileges at times to perform some particular types of scans, Nmap Switches It also comes in handy during network auditing! It is essentially a port scanner that helps you scan networks and identify various ports and services available in the network, besides also providing further information on targets, including reverse DNS names, operating system guesses, device types, and MAC addresses. Nmap is probably the most famous reconnaissance tool among Pentesters and Hacker.
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