When Nmap performs OS detection against a target and fails to find a perfect match, it usually repeats the attempt.
By default, Nmap tries five times if conditions are favorable for OS fingerprint submission, and twice when conditions aren't so good. Specifying a lower --max-os-tries value such as 1 speeds Nmap up, though you miss out on retries which could potentially identify the OS. Alternatively, a high value may be set to allow even more retries when conditions are favorable. This is rarely done, except to generate better fingerprints for submission and integration into the Nmap OS database.
OS Detection Chapter Nmap Reference Guide. OS Detection. OS detection is far more effective if at least one open and one closed TCP port are found. Set the --osscan-limit option and Nmap will not even try OS detection against hosts which do not meet this criteria. This can save substantial time, particularly on -Pn scans against many hosts. You still need to enable OS detection with -O or -A for the --osscan-limit option to have any effect.
Another OS detection option is --osscan-guess. When Nmap is unable to detect a perfect OS match, it sometimes offers up near-matches as possibilities. The match has to be very close for Nmap to do this by default.
If you specify this option or the equivalent --fuzzy option , Nmap will guess more aggressively. Nmap still tells you when an imperfect match is found and display its confidence level percentage for each guess.
When Nmap performs OS detection against a target and fails to find a perfect match, it usually repeats the attempt. By default, Nmap tries five times if conditions are favorable for OS fingerprint submission, and twice when conditions aren't so good.
The --max-os-tries option lets you change this maximum number of OS detection tries. Lowering it usually to 1 speeds Nmap up, though you miss out on retries which could potentially identify the OS. Alternatively, a high value may be set to allow even more retries when conditions are favorable.
This is rarely done, except to generate better fingerprints for submission and integration into the Nmap OS database. Like just about every other part of Nmap, results ultimately come from the target machine itself. While rare, systems are occasionally configured to confuse or mislead Nmap. Your best bet is to use numerous reconnaissance methods to explore a network, and don't trust any one of them. The most commonly useful results, such as TTL information, are printed to Nmap output whenever they are obtained.
Slightly less pertinent information, such as IP ID sequence generation and TCP sequence prediction difficulty, is only printed in verbose mode. But if you want all of the IP stack details that Nmap collected, you can find it in a compact form called a subject fingerprint.
Nmap sometimes prints this for user submission purposes when it doesn't recognize a host. You can also force Nmap to print it in normal, interactive, and XML formats by enabling debugging with -d. Usage and Examples Chapter 8. Remote OS Detection. The variable characteristics of each probe are described below:. The IP DF bit is not set. This probe is a UDP packet sent to a closed port.
The IP ID value is set to 0x for operating systems which allow us to set this. If the port is truly closed and there is no firewall in place, Nmap expects to receive an ICMP port unreachable message in return. The previous section describes probes sent by Nmap, and this one completes the puzzle by describing the barrage of tests performed on responses.
All numerical test values are given in hexadecimal notation, without leading zeros, unless noted otherwise. The tests are documented in roughly the order they appear in fingerprints. This test attempts to determine the smallest number by which the target host increments these values. For example, many hosts especially old ones always increment the ISN in multiples of 64, The first step in calculating this is creating an array of differences between probe responses.
The first element is the difference between the 1st and 2nd probe response ISNs. The second element is the difference between the 2nd and 3rd responses. There are five elements if Nmap receives responses to all six probes. Since the next couple of sections reference this array, we will call it diff1. If an ISN is lower than the previous one, Nmap looks at both the number of values it would have to subtract from the first value to obtain the second, and the number of values it would have to count up including wrapping the bit counter back to zero.
The smaller of those two values is stored in diff1. So the difference between 0x followed by 0x is 0xB This test value then records the greatest common divisor of all those elements. This value reports the average rate of increase for the returned TCP initial sequence number. Recall that a difference is taken between each two consecutive probe responses and stored in the previously discussed diff1 array. Those differences are each divided by the amount of time elapsed in seconds—will generally be about 0.
The array has one element for each diff1 value. An average is taken of the array values. If that average is less than one e.
Otherwise ISR is eight times the binary logarithm log base-2 of that average value, rounded to the nearest integer. While the ISR test measures the average rate of initial sequence number increments, this value measures the ISN variability. It roughly estimates how difficult it would be to predict the next ISN from the known sequence of six probe responses. This test is only performed if at least four responses were seen. We don't do the division for smaller GCD values because those are usually caused by chance.
A standard deviation of the array of the resultant values is then taken. If the result is one or less, SP is zero. Otherwise the binary logarithm of the result is computed, then it is multiplied by eight, rounded to the nearest integer, and stored as SP.
Please keep in mind that this test is only done for OS detection purposes and is not a full-blown audit of the target ISN generator. There are many algorithm weaknesses that lead to easy predictability even with a high SP value. There are three tests that examine the IP header ID field of responses. For each of these tests, the target's IP ID generation algorithm is classified based on the algorithm below.
Minor differences between tests are noted. Note that difference values assume that the counter can wrap. So the difference between an IP ID of 65, followed by a value of is 1, The difference between 2, followed by 1, is 64, Here are the calculation details:. This result isn't possible for II because there are not enough samples to support it. If any of the differences between two consecutive IDs exceeds 1,, and is not evenly divisible by , the test's value is RI random positive increments.
If the difference is evenly divisible by , it must be at least , to cause this RI result. If all of the differences are divisible by and no greater than 5,, the test is set to BI broken increment. This happens on systems like Microsoft Windows where the IP ID is sent in host byte order rather than network byte order. It works fine and isn't any sort of RFC violation, though it does give away host architecture details which can be useful to attackers.
If all of the differences are less than ten, the value is I incremental. We allow difference up to ten here rather than requiring sequential ordering because traffic from other hosts can cause sequence gaps.
If none of the previous steps identify the generation algorithm, the test is omitted from the fingerprint. If our six TCP IP ID values are , , , , , and , then our ICMP results are and , it is clear that not only are both sequences incremental, but they are both part of the same sequence.
If SS is included, the result is S if the sequence is shared and O other if it is not. That determination is made by the following algorithm:.
Otherwise it is O. TS is another test which attempts to determine target OS characteristics based on how it generates a series of numbers. It examines the TSval first four bytes of the option rather than the echoed TSecr last four bytes value. It takes the difference between each consecutive TSval and divides that by the amount of time elapsed between Nmap sending the two probes which generated those responses.
The resultant value gives a rate of timestamp increments per second. Nmap computes the average increments per second over all consecutive probes and then calculates the TS as follows:.
If any of the responses have no timestamp option, TS is set to U unsupported. The software for this infrastructure monitoring tool installs on Windows Server R2, , R2, and The first run of the utility will kick off the network discovery routines. These log all of the Layer 2 and Layer 3 devices switches and routers on your network and record them in a register.
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