![]() ![]() However, for non-Windows servers protected by a firewall, this can be a way to get through. Windows operating systems are in this group, and scanning them with NULL scans will produce unreliable results. This is not normally a valid packet and some hosts will not know what to do with this. NULL Scan Another very stealthy scan that sets all the TCP header flags to off or null. Most but not all computers will send a RST packet back if they get this input, so the FIN scan can show false positives and negatives, but it may get under the radar of some IDS programs and other countermeasures. Machines running Windows OS, however, do not implement this slowdown feature, so you should be able to use UDP to scan Windows hosts normally.į IN Scan This is a stealthy scan, like the SYN scan, but sends a TCP FIN packet instead. It may be quite slow, since some machines intentionally slow down responses to this kind of traffic to avoid being overwhelmed. However, it can also reveal Trojan horses running on high UDP ports and hidden RPC services. Since UDP does not respond with a positive acknowledgment like TCP and only responds to an incoming UDP packet when the port is closed, this type of scan can sometimes show false positives. UDP Scan This scan checks to see if there are any UDP ports listening. However, some machines may be configured not to respond to a ping (for example, machines running the new XP firewall) but still have services running on them, so a ping sweep is not as accurate as a full port scan. If you don’t really care about what services are running and you just want to know which IP addresses are up, this is a lot faster than a full port scan. P ing Sweep This does a simple ping of all the addresses to see which ones are answering to ICMP. Also, if you don’t have administrator or root privileges on the Nmap machine, you won’t be able to run anything other than a Connect scan because the specially crafted packets for other scans require low-level OS access. However, if stealth or bandwidth is not an issue, a Connect scan is sometimes more accurate than the SYN scan. This scan is not only noisy but also puts more load on the machines being scanned and the network. T CP Connect This works much like the SYN scan, except it completes the full TCP handshake and makes a full connection. However, the remote system will hold this “half socket” open until it times out from not receiving a response. The SYN scan does not complete the TCP handshake by sending an ACK back to the machine as far as the scanee is concerned, it never sees a valid connection. ![]() If it doesn’t get a response, it assumes the port is closed. If it gets a SYN ACK packet back, then Nmap knows there is a service running there. It works by sending a single TCP SYN packet to each possible port. It is quieter than a TCP Connect scan, that is, it won’t show up on most simple logs. S YN SCAN This is the default scan and is good for most purposes. Now the client and server can start data transfer. 3 The client again sends an ACK, followed by Y+1, thus completing the handshake. 2 The server acknowledges with a SYN-ACK (Acknowledgement), X+1 and a randomly generated number, Y. ![]() In the TCP three-way handshake:ġ A “client” initiates communication with a SYN (Synchronise) packet with a randomly generated number, X. The TCP three-way handshake is used to establish and reset connections, and this concept is key to understanding various NMap scan types. ICMP packets are used to convey error messages, if any. TCP is a connection-oriented protocol, which assures delivery of packets. This practice has its own uses, like with live audio/video transfers, where real-time delivery is a must. However, that does not mean it is an unreliable protocol higher-level applications must take care to verify that data has been received at the other end. ![]() UDP is a connection-less protocol that does not assure the delivery of packets at the other end. Application layer : FTP, HTTP, SNMP, BOOTP, DHCP.NMAP (Image Source Google) Various TCP/IP protocols ![]()
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