A series of six TCP probes is sent to generate these four test response lines. The probes are sent exactly 100 milliseconds apart so the total time taken is 500 ms. Exact timing is important as some of the sequence algorithms we detect (initial sequence numbers, IP IDs, and TCP timestamps) are time dependent. This timing value was chosen to take 500 ms so that we can reliably detect the common 2 Hz TCP timestamp sequences.

Each probe is a TCP SYN packet to a detected open port on the remote machine. The sequence and acknowledgment numbers are random (but saved so Nmap can differentiate responses). Detection accuracy requires probe consistency, so there is no data payload even if the user requested one with --data-length.

These packets vary in the TCP options they use and the TCP window field value. The following list provides the options and values for all six packets. The listed window field values do not reflect window scaling. EOL is the end-of-options-list option, which many sniffing tools don't show by default.

The results of these tests include four result category lines. The first, SEQ, contains results based on sequence analysis of the probe packets. These test results are GCD, SP, ISR, TI, II, TS, and SS. The next line, OPS contains the TCP options received for each of the probes (the test names are O1 through 06). Similarly, the WIN line contains window sizes for the probe responses (named W1 through W6). The final line related to these probes, T1, contains various test values for packet #1. Those results are for the R, DF, T, TG, W, S, A, F, O, RD, and Q tests. These tests are only reported for the first probe since they are almost always the same for each probe.

The IE test involves sending two ICMP echo request packets to the target. The first one has the IP DF bit set, a type-of-service (TOS) byte value of zero, a code of nine (even though it should be zero), the sequence number 295, a random IP ID and ICMP request identifier, and 120 bytes of 0x00 for the data payload.

The second ping query is similar, except a TOS of four (IP_TOS_RELIABILITY) is used, the code is zero, 150 bytes of data is sent, and the ICMP request ID and sequence numbers are incremented by one from the previous query values.

The results of both of these probes are combined into a IE line containing the R, DFI, T, TG, and CD tests. The R value is only true (Y) if both probes elicit responses. The T, and CD values are for the response to the first probe only, since they are highly unlikely to differ. DFI is a custom test for this special dual-probe ICMP case.

These ICMP probes follow immediately after the TCP sequence probes to ensure valid results of the shared IP ID sequence number test (see the section called “Shared IP ID sequence Boolean (SS)”).

This probe tests for explicit congestion notification (ECN) support in the target TCP stack. ECN is a method for improving Internet performance by allowing routers to signal congestion problems before they start having to drop packets. It is documented in RFC 3168. Nmap tests this by sending a SYN packet which also has the ECN CWR and ECE congestion control flags set. For an unrelated (to ECN) test, the urgent field value of 0xF7F5 is used even though the urgent flag is not set. The acknowledgment number is zero, sequence number is random, window size field is three, and the reserved bit which immediately precedes the CWR bit is set. TCP options are WScale (10), NOP, MSS (1460), SACK permitted, NOP, NOP. The probe is sent to an open port.

If a response is received, the R, DF, T, TG, W, O, CC, and Q tests are performed and recorded.

The six T2 through T7 tests each send one TCP probe packet. With one exception, the TCP options data in each case is (in hex) 03030A0102040109080AFFFFFFFF000000000402. Those 20 bytes correspond to window scale (10), NOP, MSS (265), Timestamp (TSval: 0xFFFFFFFF; TSecr: 0), then SACK permitted. The exception is that T7 uses a Window scale value of 15 rather than 10. The variable characteristics of each probe are described below:

In each of these cases, a line is added to the fingerprint with results for the R, DF, T, TG, W, S, A, F, O, RD, and Q tests.

This probe is a UDP packet sent to a closed port. The character ‘C’ (0x43) is repeated 300 times for the data field. The IP ID value is set to 0x1042 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. That response is then subjected to the R, DF, T, TG, IPL, UN, RIPL, RID, RIPCK, RUCK, and RUD tests.

The SEQ test sends six TCP SYN packets to an open port of the target machine and collects SYN/ACK packets back. Each of these SYN/ACK packets contains a 32-bit initial sequence number (ISN). 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 64,000.

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 32-bit counter back to zero). The smaller of those two values is stored in diff1. So the difference between 0x20000 followed by 0x15000 is 0xB000. The difference between 0xFFFFFF00 and 0xC000 is 0xC0FF. This test value then records the greatest common divisor of all those elements. This GCD is also used for calculating the SP result.

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 0.1) between sending the two probes which generated them. The result is an array, which we'll call seq_rates containing the rates of ISN counter increases per second. 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.g. a constant ISN is used), ISR is zero. 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. The calculation uses the difference array (seq_rates) and GCD values discussed in the previous section.

This test is only performed if at least four responses were seen. If the previously computed GCD value is greater than nine, the elements of the previously computed seq_rates array are divided by that value. 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.

This Boolean value records whether the target shares its IP ID sequence between the TCP and ICMP protocols. If our six TCP IP ID values are 117, 118, 119, 120, 121, and 122, then our ICMP results are 123 and 124, it is clear that not only are both sequences incremental, but they are both part of the same sequence. If, on the other hand, the TCP IP ID values are 117–122 but the ICMP values are 32,917 and 32,918, two different sequences are being used.

This test is only included if II is RI, BI, or I and TI is the same. 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:

Let avg be the final TCP sequence response IP ID minus the first TCP sequence response IP ID, divided by the difference in probe numbers. If probe #1 returns an IP ID of 10,000 and probe #6 returns 20,000, avg would be (20,000 − 10,000) / (6 − 1), which equals 2,000.

If the first ICMP echo response IP ID is less than the final TCP sequence response IP ID plus three times avg, the SS result is S. Otherwise it is O.

TS is another test which attempts to determine target OS characteristics based on how it generates a series of numbers. This one looks at the TCP timestamp option (if any) in responses to the SEQ probes. 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:

This test records the TCP header options in a packet. It preserves the original ordering and also provides some information about option values. Because RFC 793 doesn't require any particular ordering, implementations often come up with unique orderings. Some platforms don't implement all options (they are, of course, optional). When you combine all of those permutations with the number of different option values that implementations use, this test provides a veritable trove of information. The value for this test is a string of characters representing the options being used. Several options take arguments that come immediately after the character. Supported options and arguments are all shown in Table 8.1.

As an example, the string M5B4NW3NNT11 means the packet includes the MSS option (value 0x5B4) followed by a NOP. Next comes a window scale option with a value of three, then two more NOPs. The final option is a timestamp, and neither of its two fields were zero. If there are no TCP options in a response, the test will exist but the value string will be empty. If no probe was returned, the test is omitted.

While this test is generally named O, the six probes sent for sequence generation purposes are a special case. Those are inserted into the special OPS test line and take the names O1 through O6 to distinguish which probe packet they relate to. The “O” stands for “options”. Despite the different names, each test O1 through O6 is processed exactly the same way as the other O tests.

This test simply records the 16-bit TCP window size of the received packet. It is quite effective, since there are more than 80 values that at least one OS is known to send. A down side is that some operating systems have more than a dozen possible values by themselves. This leads to false negative results until we collect all of the possible window sizes used by an operating system.

While this test is generally named W, the six probes sent for sequence generation purposes are a special case. Those are inserted into a special WIN test line and take the names W1 through W6. The window size is recorded for all of the sequence number probes because they differ in TCP MSS option values, which causes some operating systems to advertise a different window size. Despite the different names, each test is processed exactly the same way.

This test simply records whether the target responded to a given probe. Possible values are Y and N. If there is no reply, remaining fields for the test are omitted.

A risk with this test involves probes that are dropped by a firewall. This leads to R=N in the subject fingerprint. Yet the reference fingerprint in nmap-os-db may have R=Y if the target OS usually replies. Thus the firewall could prevent proper OS detection. To reduce this problem, reference fingerprints generally omit the R=Y test from the IE and U1 probes, which are the ones most likely to be dropped. In addition, if Nmap is missing a closed TCP port for a target, it will not set R=N for the T5, T6, or T7 tests even if the port it tries is non-responsive. After all, the lack of a closed port may be because they are all filtered.

The IP header contains a single bit which forbids routers from fragmenting a packet. If the packet is too large for routers to handle, they will just have to drop it (and ideally return a “destination unreachable, fragmentation needed” response). This test records Y if the bit is set, and N if it isn't.

This is simply a modified version of the DF test that is used for the special IE probes. It compares results of the don't fragment bit for the two ICMP echo request probes sent. It has four possible values, which are enumerated in Table 8.2.

IP packets contain a field named time-to-live (TTL) which is decremented every time they traverse a router. If the field reaches zero, the packet must be discarded. This prevents packets from looping endlessly. Because operating systems differ on which TTL they start with, it can be used for OS detection. Nmap determines how many hops away it is from the target by examining the ICMP port unreachable response to the U1 probe. That response includes the original IP packet, including the already-decremented TTL field, received by the target. By subtracting that value from our as-sent TTL, we learn how many hops away the machine is. Nmap then adds that hop distance to the probe response TTL to determine what the initial TTL was when that ICMP probe response packet was sent. That initial TTL value is stored in the fingerprint as the T result.

Even though an eight-bit field like TTL can never hold values greater than 0xFF, this test occasionally results in values of 0x100 or higher. This occurs when a system (could be the source, a target, or a system in between) corrupts or otherwise fails to correctly decrement the TTL. It can also occur due to asymmetric routes.

Nmap can also learn from the system interface and routing tables when the hop distance is zero (localhost scan) or one (on the same network segment). This value is used when Nmap prints the hop distance for the user, but it is not used for T result computation.

It is not uncommon for Nmap to receive no response to the U1 probe, which prevents Nmap from learning how many hops away a target is. Firewalls and NAT devices love to block unsolicited UDP packets. But since common TTL values are spread well apart and targets are rarely more than 20 hops away, Nmap can make a pretty good guess anyway. Most systems send packets with an initial TTL of 32, 60, 64, 128, or 255. So the TTL value received in the response is rounded up to the next value out of 32, 64, 128, or 255. 60 is not in that list because it cannot be reliably distinguished from 64. It is rarely seen anyway. The resulting guess is stored in the TG field. This TTL guess field is not printed in a subject fingerprint if the actual TTL (T) value was discovered.

This test is only used for the ECN probe. That probe is a SYN packet which includes the CWR and ECE congestion control flags. When the response SYN/ACK is received, those flags are examined to set the CC (congestion control) test value as described in Table 8.3.

This tests for two quirks that a few implementations have in their TCP stack. The first is that the reserved field in the TCP header (right after the header length) is nonzero. This is particularly likely to happen in response to the ECN test as that one sets a reserved bit in the probe. If this is seen in a packet, an “R” is recorded in the Q string.

The other quirk Nmap tests for is a nonzero urgent pointer field value when the URG flag is not set. This is also particularly likely to be seen in response to the ECN probe, which sets a non-zero urgent field. A “U” is appended to the Q string when this is seen.

The Q string must always be generated in alphabetical order. If no quirks are present, the Q test is empty but still shown.

This test examines the 32-bit sequence number field in the TCP header. Rather than record the field value as some other tests do, this one examines how it compares to the TCP acknowledgment number from the probe that elicited the response. It then records the appropriate value as shown in Table 8.4.

This test is the same as S except that it tests how the acknowledgment number in the response compares to the sequence number in the respective probe. The four possible values are given in Table 8.5.

This field records the TCP flags in the response. Each letter represents one flag, and they occur in the same order as in a TCP packet (from high-bit on the left, to the low ones). So the value AS represents the ACK and SYN bits set, while the value SA is illegal (wrong order). The possible flags are shown in Table 8.6.

Some operating systems return ASCII data such as error messages in reset packets. This is explicitly allowed by section 4.2.2.12 of RFC 1122. When Nmap encounters such data, it performs a CRC32 checksum and reports the results. When there is no data, RD is set to zero. Some of the few operating systems that may return data in their reset packets are HP-UX and versions of Mac OS prior to Mac OS X.

This test records the total length (in octets) of an IP packet. It is only used for the port unreachable response elicited by the U1 test. That length varies by implementation because they are allowed to choose how much data from the original probe to include, as long as they meet the minimum RFC 1122 requirement. That requirement is to include the original IP header and at least eight bytes of data.

An ICMP port unreachable message header is eight bytes long, but only the first four are used. RFC 792 states that the last four bytes must be zero. A few implementations (mostly ethernet switches and some specialized embedded devices) set it anyway. The value of those last four bytes is recorded in this field.

ICMP port unreachable messages (as are sent in response to the U1 probe) are required to include the IP header which generated them. This header should be returned just as they received it, but some implementations send back a corrupted version due to changes they made during IP processing. This test simply records the returned IP total length value. If the correct value of 0x148 (328) is returned, the value G (for good) is stored instead of the actual value.

The U1 probe has a static IP ID value of 0x1042. If that value is returned in the port unreachable message, the value G is stored for this test. Otherwise the exact value returned is stored. Some systems, such as Solaris, manipulate IP ID values for raw IP packets that Nmap sends. In such cases, this test is skipped. We have found that some systems, particularly HP and Xerox printers, flip the bytes and return 0x4210 instead.

The IP checksum is one value that we don't expect to remain the same when returned in a port unreachable message. After all, each network hop during transit changes the checksum as the TTL is decremented. However, the checksum we receive should match the enclosing IP packet. If it does, the value G (good) is stored for this test. If the returned value is zero, then Z is stored. Otherwise the result is I (invalid).

The UDP header checksum value should be returned exactly as it was sent. If it is, G is recorded for this test. Otherwise the value actually returned is recorded.

This test checks the integrity of the (possibly truncated) returned UDP payload. If all the payload bytes are the expected ‘C’ (0x43), or if the payload was truncated to zero length, G is recorded; otherwise, I (invalid) is recorded.

The code value of an ICMP echo reply (type zero) packet is supposed to be zero. But some implementations wrongly send other values, particularly if the echo request has a nonzero code (as one of the IE tests does). The response code values for the two probes are combined into a CD value as described in Table 8.7.