Idle scan on an open port

The idle scan is a TCPport scan method that consists of sending spoofed packets to a computer^[1] to find out what services are available. This is accomplished by impersonating another computer called a 'zombie' (that is not transmitting or receiving information) and observing the behavior of the 'zombie' system.

Information[edit]

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This action can be done through common software network utilities such as nmap and hping. The attack involves sending forged packets to a specific machine target in an effort to find distinct characteristics of another zombie machine.The attack is sophisticated because there is no interaction between the attacker computer and the target: the attacker interacts only with the 'zombie' computer.

This exploit functions with two purposes, as a port scanner and a mapper of trusted IP relationships between machines. The target system interacts with the 'zombie' computer and difference in behaviour can be observed using different 'zombies' with evidence of different privileges granted by the target to different computers.

Discovered by Salvatore Sanfilippo (also known by his handle 'Antirez') in 1998,^[2] the idle scan has been used by many black hat 'hackers' to covertly identify open ports on a target computer in preparation for attacking it.Although it was originally named dumb scan, the term idle scan was coined in 1999, after the publication of a proof of concept 16-bit identification field (IPID) scanner named idlescan, by Filipe Almeida (aka LiquidK).^[3] This type of scan can also be referenced as zombie scan; all the nomenclatures are due to the nature of one of the computers involved in the attack.

TCP/IP basics[edit]

The design and operation of the Internet is based on the Internet Protocol Suite, commonly also called TCP/IP.IP is the primary protocol in the Internet Layer of the Internet Protocol Suite and has the task of delivering datagrams from the source host to the destination host solely based on their addresses. For this purpose, IP defines addressing methods and structures for datagram encapsulation. It is a connectionless protocol and relies on the transmission of packets. Every IP packet from a given source has an ID that uniquely identifies IP datagram.^{[clarification needed]}

TCP provides reliable, ordered delivery of a stream of bytes from a program on one computer to another program on another computer. TCP is the protocol that major Internet applications rely on, such as the World Wide Web, e-mail, and file transfer. Each of these applications (web server, email server, FTP server) is called a network service. In this system, network services are identified using two components: a host address and a port number. There are 65536 distinct and usable port numbers per host. Most services use a limited range of numbers by default, and the default port number for a service is almost always used.

Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host. See: List of TCP and UDP port numbers.

The result of a scan on a port is usually generalized into one of three categories:

Open or Accepted: The host sent a reply indicating that a service is listening on the port.
Closed or Denied or Not Listening: The host sent a reply indicating that connections will be denied to the port.
Filtered, Dropped or Blocked: There was no reply from the host.

Open ports present two vulnerabilities of which administrators must be wary:

Security and stability concerns associated with the program responsible for delivering the service - Open ports.
Security and stability concerns associated with the operating system that is running on the host - Open or Closed ports.

Filtered ports do not tend to present vulnerabilities.The host in a local network can be protected by a firewall that filters, according with rules that its administrator set up, packets. This is done to deny services to hosts not known and prevent intrusion in the inside network.The IP protocol is network layer transmission protocol.

Basic mechanics[edit]

Idle scans take advantage of predictable Identification field value from IP header: every IP packet from a given source has an ID that uniquely identifies fragments of an original IP datagram; the protocol implementation assigns values to this mandatory field generally by a fixed value (1) increment. Because transmitted packets are numbered in a sequence you can say how many packets are transmitted between two packets that you receive.

An attacker would first scan for a host with a sequential and predictable sequence number (IPID). The latest versions of Linux, Solaris, OpenBSD, and Windows Vista are not suitable as zombie, since the IPID has been implemented with patches^[4] that randomized the IPID.^[1] Computers chosen to be used in this stage are known as 'zombies'.

Once a suitable zombie is found the next step would be to try to establish a TCP connection with a given service (port) of the target system, impersonating the zombie. It is done by sending a SYN packet to the target computer, spoofing the IP address from the zombie, i.e. with the source address equal to zombie IP address.

If the port of the target computer is open it will accept the connection for the service, responding with a SYN/ACK packet back to the zombie.

The zombie computer will then send a RST packet to the target computer (to reset the connection) because it did not actually send the SYN packet in the first place.

Since the zombie had to send the RST packet it will increment its IPID. This is how an attacker would find out if the targets port is open. The attacker will send another packet to the zombie. If the IPID is incremented only by a step then the attacker would know that the particular port is closed.

The method assumes that zombie has no other interactions: if there is any message sent for other reasons between the first interaction of the attacker with the zombie and the second interaction other than RST message, there will be a false positive.

The first stage of an idle scan

The second stage of an idle scan

Finding a zombie host[edit]

The first step in executing an idle scan is to find an appropriate zombie. It needs to assign IP ID packets incrementally on a global (rather than per-host it communicates with) basis. It should be idle (hence the scan name), as extraneous traffic will bump up its IP ID sequence, confusing the scan logic. The lower the latency between the attacker and the zombie, and between the zombie and the target, the faster the scan will proceed.^[5]
Note that when a port is open, IPIDs increment by 2. Following is the sequence:
1. Attacker to target -> SYN, target to zombie ->SYN/ACK, Zombie to target -> RST (IPID increment by 1)
2. Now attacker tries to probe zombie for result. Attacker to Zombie ->SYN/ACK, Zombie to Attacker -> RST (IPID increment by 1)
So, in this process IPID increments by 2 finally.

When an idle scan is attempted, tools (for example nmap) tests the proposed zombie and reports any problems with it. If one doesn't work, try another. Enough Internet hosts are vulnerable that zombie candidates aren't hard to find.A common approach is to simply execute a ping sweep of some network. Choosing a network near your source address, or near the target, produces better results. You can try an idle scan using each available host from the ping sweep results until you find one that works. As usual, it is best to ask permission before using someone's machines for unexpected purposes such as idle

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