![]() If the port is closed the machine sends RST If the port is open, the machine replies with SYN/ACK Its stealth comes from not performing a 3-way handshake to complete the connection and the packet exchange is as follows: Stealth Scan, also known as SYN scan or half-open scan, is the default and most popular technique. There are multiple techniques you can use for port scanning: Open Port: Nmap receives “syn-ack” as the probe responseĬlosed Port: Nmap receives an “RST” as the probe responseįiltered: Nmap marks the port as open | filtered when it does not receive any response, which could be due to firewall filtering Nmap identifies the status of ports based on the response it receives for an SYN request. If a list of live IP addresses already exists, host discovery is not necessary and you can move to the next step, finding open ports. ![]() If the host is down, there will be no response. If the host is up, it will answer with an RST packet since the connection doesn’t exist. Lack of a response for a certain period leads to marking the host as down.ĭuring a TCP ACK scan, Nmap sends an empty TCP packet with the ACK flag set to port 80. ![]() If the machine replies with an SYN/ACK or RST packet for the specified port, Nmap knows the host is up. With a TCP SYN scan, Nmap sends an SYN packet to a given port on the target. TCP scans represent another way to discover hosts, using commands to send out TCP SYN or TCP ACK ping messages: Keep in mind that ICMP messages may be blocked by some firewalls, so this technique may not always work. Once again, if a type 18 packet is received, the host is alive. The -PM option sends ICMP address mask (netmask) requests (type 17), expecting an ICMP address mask reply (type 18) in return. If a type 14 ICMP packet is received, then Nmap assumes the host is alive. Using the -PP option, Nmap will send ICMP timestamp requests (type 13), expecting ICMP timestamp replies (type 14) in return. A live host will send back a reply, signaling its presence on the network. ICMP scan can also identify live hosts by sending an ICMP Echo request. Nmap -n -sn -PR -send-eth 192.168.100.1-20Ībove, you can see an ARP request and reply captured by Wireshark. Getting an ARP reply means that the hosts exist and since this ARP is needed for routing packets, a firewall won’t interfere in the exchange. Scanning the ports at this stage would generate too much traffic, take time and resources, and is likely to trigger security alerts.īelow are some methods to identify live IPs:ĪRP scanning can be used to stealthily discover the hosts in the local LAN. If the target is unknown and large, the recommendation is to identify hosts first. To discover available hosts, the following packets are sent (as seen in the below screen capture below from Wireshark packet analyzer): In the older version of the tool, the option for ping sweep was -sP in the newer version, it is -sn. This way, the user gets a complete list of open ports and the services running on them.īy default, Nmap uses requests to identify a live IP. It uses unicornscan to scan all 65535 ports, and then feeds the results to Nmap for service fingerprinting. Onetwopunch is a powerful script that combines the features of unicornscan and Nmap tools for faster and more accurate results. It has both a command line and a graphical interface, and the default transmission rate is 100 packets per second. Masscan is widely known as the fastest port scanner. Based on the live IPs detected, it can scan for ports and services, reveal MAC addresses, as well as resolve hostnames. Unicornscan is useful for collecting network and OS information, and it comes with features like asynchronous TCP and UDP scanning, port scanning, and service and OS fingerprinting.Īngry IP Scanner is a GUI-based tool for high-speed scanning, allowing users to run ping sweeps of the network. Nmap is not the only port scanner available, and other tools in this category are suitable for particular needs. In this article, we will describe how Nmap can help you to: There is also a graphical version known as Zenmap, which offers easy access to scanning options and network mapping diagrams. Some of its features include Host Discovery, Port Scan, Service and OS fingerprinting, and Basic Vulnerability detection. Our focus is on Nmap (Network Mapper), by far the most popular tool for network discovery and port scanning. Multiple tools can produce good results, but some port scanners are better for a particular task than others. For an attacker, this is the first step to get info about the target’s network and identify a potential way in, since services running on an open port could be vulnerable to attacks. Network administrators and penetration testers use port scanning to discover open communication channels on computer systems. This article is a deep dive into how Nmap works, to understand its internal structure, and master its functionality.
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