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Usage and Examples
| Chapter 8. Remote OS Detection |
## Usage and Examples The inner workings of OS detection are quite complex, but it is one of the easiest features to use. Simply add `-O` to your scan options. You may want to also increase the verbosity with `-v` for even more OS-related details. This is shown in Example 8.1, “OS Detection with Verbosity (`-O` `-v`)” Example 8.1. OS Detection with Verbosity (`-O` `-v`)
    # nmap -O -v scanme.nmap.org

    Starting Nmap ( http://nmap.org )
    Interesting ports on scanme.nmap.org (205.217.153.62):
    (The 1667 ports scanned but not shown below are in state: filtered)
    PORT    STATE  SERVICE
    22/tcp  open   ssh
    53/tcp  open   domain
    70/tcp  closed gopher
    80/tcp  open   http
    113/tcp closed auth
    Device type: general purpose
    Running: Linux 2.4.X|2.5.X|2.6.X
    OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
    Uptime 24.247 days (since Wed Feb 22 11:39:20 2006)
    Network Distance: 10 hops
    TCP Sequence Prediction: Class=random positive increments
                             Difficulty=2662996 (Good luck!)
    IP ID Sequence Generation: All zeros

    Nmap finished: 1 IP address (1 host up) scanned in 22.572 seconds
                   Raw packets sent: 3360 (148.612KB) | Rcvd: 25 (1264B)
Including the `-O -v` options caused Nmap to generate the following six extra line items: Device type All fingerprints are classified with one or more high-level device types, such as `router`, `printer`, `firewall`, or (as in this case) `general purpose`. These are further described in the section called “Device and OS classification (`Class` lines)” Several device types may be shown, in which case they will be separated with the pipe symbol as in “`Device Type: router|firewall`”. Running This field is also related to the OS classification scheme described in the section called “Device and OS classification (`Class` lines)” It shows the OS Family (`Linux` in this case) and OS generation if available. In this case, Nmap isn't sure whether the kernel is of the 2.4, 2.5, or 2.6 generation and so it writes all three separated by a pipe character (`2.4.X|2.5.X|2.6.X`). If there are multiple OS families, they are separated by commas (e.g. `OpenBSD 3.X, NetBSD 3.X|4.X`). If Nmap finds too many OS families to print concisely, it will omit this line. When there are no perfect matches, Nmap changes the field to “Running (JUST GUESSING)” and adds an accuracy percentage (100% is a perfect match) in parentheses after each candidate family name. If no fingerprints are close matches, the line is omitted. OS details This line gives the detailed description for each fingerprint that matches. While the `Device type` and `Running` lines are from predefined enumerated lists that are easy to parse by a computer, the OS details line contains free-form data which is useful to a human reading the report. This can include more exact version numbers, device models, and architectures specific to a given fingerprint. In this example, two fingerprints matched (“Linux 2.4.7 - 2.6.11” and “Linux 2.6.0 - 2.6.11”) and so they are comma separated. When there are no perfect matches but some close guesses, the field is renamed `Aggressive OS guesses` and fingerprints are shown followed by a percentage match in parentheses. Uptime As part OS detection, Nmap receives several SYN/ACK TCP packets in a row and checks the headers for a timestamp option. Many operating systems use a simple counter for this which starts at zero at boot time then increments at a constant rate such as twice per second. By looking at several responses, Nmap can determine the current values and rate of increase. Simple linear extrapolation determines boot time. The timestamp algorithm is used for OS detection too (see the section called “TCP timestamp option algorithm (`TS`)”) since the incrementation rate on different systems varies from 2Hz to 1,000Hz. While the uptime value itself isn't part of OS detection, it can be valuable and so is printed when available. Nmap omits the line if the target gives zeros or no timestamp options in its SYN/ACK packets, or if it does not reply at all. The line is also omitted if Nmap cannot discern the timestamp increment rate or it seems suspicious (like a 30-year uptime). Network distance A side effect of one of the OS detection tests allows Nmap to compute how many routers are between it and a target host. The distance is zero when you are scanning localhost, and one for a machine on the same network segment. Each additional router on the path adds one to the hop count. Nmap omits this line when it isn't able to compute this value (no reply to the relevant probe). TCP Sequence Prediction Systems with poor TCP initial sequence number generation are vulnerable to blind TCP spoofing attacks. In other words, you can make a full connection to those systems and send (but not receive) data while spoofing a different IP address. The target's logs will show the spoofed IP, and you can take advantage of any trust relationship between them. This attack was all the rage in the mid-nineties when people commonly used rlogin to allow logins to their account without any password from trusted IP addresses. Kevin Mitnick is alleged to have used this attack to break into Tsutomu Shimomura's computers in December 1994. The good news is that hardly anyone uses rlogin anymore, and many operating systems have been fixed to use unpredictable initial sequence numbers as proposed by RFC 1948 For these reasons, this line is only printed in verbose mode. Sadly, many vendors still ship vulnerable operating systems and devices Even the fixed ones often vary in implementation, which leaves them valuable for OS detection purposes. The class describes the ISN generation algorithm used by the target, and difficulty is a rough estimate of how hard the system makes blind IP spoofing (0 is the easiest). The parenthesized comment is based on the difficulty index and ranges from `trivial joke` to `easy`, `medium`, `formidable`, `worthy challenge`, and finally `good luck!` Further details about ISN tests are provided in the section called “TCP ISN greatest common denominator (`GCD`)” While the rlogin family is mostly a relic of the past, clever attackers can still find effective uses for blind TCP spoofing. For example, it allows for spoofed http requests. You don't see the results, but just the URL (POST or GET request) can have dramatic side effects. The spoofing allows attackers to hide their identity, frame someone else, or exploit IP address restrictions. IP ID sequence generation Many systems unwittingly give away sensitive information about their traffic levels based on how they generate the lowly 16-bit ID field in IP packets. This can be abused to spoof a port scan against other systems and for other mischievous purposes discussed in our idle scanning paper This field describes the ID generation algorithm that Nmap was able to discern. More information on how it classifies them is available in the section called “TCP IP ID sequence generation algorithm (`TI`)” Note that many systems use a different IP ID space for each host they communicate with. In that case, they may appear vulnerable (such as showing the `Incremental` class) while still being secure against attacks such as the idle scan. For this reason, and because the issue is rarely critical, the IP ID sequence generation line is only printed in verbose mode. If Nmap does not receive sufficient responses during OS detection, it will omit the whole line. The best way to test whether a host is vulnerable to being an idle scan zombie is to test it with `-sI`. While TCP fingerprinting is a powerful method for OS detection, interrogating open ports for clues is another effective approach. Some applications, such as Microsoft IIS, only run on a single platform (thus giving it away), while many other apps divulge their platform in overly verbose banner messages. Adding the `-sV` option enables Nmap version detection, which is trained to look for these clues (among others). In Example 8.2, “Using version scan to detect the OS”, Nmap catches the platform details from an FTP server. Example 8.2. Using version scan to detect the OS
    # nmap -sV -O -v 129.128.X.XX
    Starting Nmap ( http://nmap.org )
    Interesting ports on [hostname] (129.128.X.XX):
    (The 1225 ports scanned but not shown below are in state: closed)
    PORT      STATE    SERVICE      VERSION
    21/tcp    open     ftp          HP-UX 10.x ftpd 4.1
    22/tcp    open     ssh          OpenSSH 3.7.1p1 (protocol 1.99)
    111/tcp   open     rpc
    445/tcp   filtered microsoft-ds
    1526/tcp  open     oracle-tns   Oracle TNS Listener
    32775/tcp open     rpc
    No exact OS matches for host
    TCP Sequence Prediction: Class=truly random
                             Difficulty=9999999 (Good luck!)
    IP ID Sequence Generation: Incremental
    Service Info: OS: HP-UX
In this example, the line “`No exact OS matches for host`” means that TCP/IP fingerprinting failed to find an exact match. Fortunately, the `Service Info` field a few lines down discloses that the OS is HP-UX. If several operating systems were detected (which can happen with NAT gateway boxes that redirect ports to several different machines), the field would be `OSs` and the values would be comma separated. The `Service Info` line can also contain hostnames and device types found during the version scan. The focus of this article is on TCP/IP fingerprinting though, since version detection was covered in Chapter 7, Service and Application Version Detection With two effective OS detection methods available, which one should you use? The best answer is usually both. In some cases, such as a proxy firewall forwarding to an application on another host, the answers may legitimately differ. TCP/IP fingerprinting will identify the proxy while version scanning will generally detect the server running the proxied application. Even when no proxying or port forwarding is involved, using both techniques is beneficial. If they come out the same, that makes the results more credible. If they come out wildly different, investigate further to determine what is going on before relying on either. Since OS and version detection go together so well, Nmap has a special `-A` option to enable them both. 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 this 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 printed 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. Several programs have even been developed specifically to trick Nmap OS detection Your best bet is to use numerous reconnaissance methods to explore a network, and don't trust any one of them. TCP/IP fingerprinting requires collecting hundreds of details about the target's IP stack. The most commonly useful results, such as uptime and 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`). Then read the section called “Understanding an Nmap Fingerprint” to interpret it.
Chapter 8. Remote OS Detection |  | TCP/IP Fingerprinting Methods Supported by Nmap
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