neulab/tldr huggingface dataset:
--script-updatedb This option updates the script database found in scripts/script.db which is used by Nmap to determine the available default scripts and categories. It is only necessary to update the database if you have added or removed NSE scripts from the default scripts directory or if you have changed the categories of any script. This option is generally used by itself: nmap --script-updatedb.
One of my highest Nmap development priorities has always been performance. A default scan (nmap hostname) of a host on my local network takes a fifth of a second. That is barely enough time to blink, but adds up when you are scanning hundreds or thousands of hosts. Moreover, certain scan options such as UDP scanning and version detection can increase scan times substantially. So can certain firewall configurations, particularly response rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate these scans, the user has ultimate control over how Nmap runs. Expert users carefully craft Nmap commands to obtain only the information they care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical tests, and upgrading to the latest version of Nmap (performance enhancements are made frequently). Optimizing timing parameters can also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in seconds by default, though you can append ‘ms’, ‘s’, ‘m’, or ‘h’ to the value to specify milliseconds, seconds, minutes, or hours. So the --host-timeout arguments 900000ms, 900, 900s, and 15m all do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel scan group sizes) Nmap has the ability to port scan or version scan multiple hosts in parallel. Nmap does this by dividing the target IP space into groups and then scanning one group at a time. In general, larger groups are more efficient. The downside is that host results can't be provided until the whole group is finished. So if Nmap started out with a group size of 50, the user would not receive any reports (except for the updates offered in verbose mode) until the first 50 hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It starts out with a group size as low as five so the first results come quickly and then increases the groupsize to as high as 1024. The exact default numbers depend on the options given. For efficiency reasons, Nmap uses larger group sizes for UDP or few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap will never exceed that size. Specify a minimum size with --min-hostgroup and Nmap will try to keep group sizes above that level. Nmap may have to use smaller groups than you specify if there are not enough target hosts left on a given interface to fulfill the specified minimum. Both may be set to keep the group size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery phase of a scan. This includes plain ping scans (-sn). Host discovery always works in large groups of hosts to improve speed and accuracy.
The primary use of these options is to specify a large minimum group size so that the full scan runs more quickly. A common choice is 256 to scan a network in /24 sized chunks. For a scan with many ports, exceeding that number is unlikely to help much. For scans of just a few port numbers, host group sizes of 2048 or more may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe parallelization) These options control the total number of probes that may be outstanding for a host group. They are used for port scanning and host discovery. By default, Nmap calculates an ever-changing ideal parallelism based on network performance. If packets are being dropped, Nmap slows down and allows fewer outstanding probes. The ideal probe number slowly rises as the network proves itself worthy. These options place minimum or maximum bounds on that variable. By default, the ideal parallelism can drop to one if the network proves unreliable and rise to several hundred in perfect conditions.
The most common usage is to set --min-parallelism to a number higher than one to speed up scans of poorly performing hosts or networks. This is a risky option to play with, as setting it too high may affect accuracy. Setting this also reduces Nmap's ability to control parallelism dynamically based on network conditions. A value of 10 might be reasonable, though I only adjust this value as a last resort.
The --max-parallelism option is sometimes set to one to prevent Nmap from sending more than one probe at a time to hosts. The --scan-delay option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout time (Adjust probe timeouts) Nmap maintains a running timeout value for determining how long it will wait for a probe response before giving up or retransmitting the probe. This is calculated based on the response times of previous probes.
If the network latency shows itself to be significant and variable, this timeout can grow to several seconds. It also starts at a conservative (high) level and may stay that way for a while when Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than the defaults can cut scan times significantly. This is particularly true for pingless (-Pn) scans, and those against heavily filtered networks. Don't get too aggressive though. The scan can end up taking longer if you specify such a low value that many probes are timing out and retransmitting while the response is in transit.
If all the hosts are on a local network, 100 milliseconds (--max-rtt-timeout 100ms) is a reasonable aggressive value. If routing is involved, ping a host on the network first with the ICMP ping utility, or with a custom packet crafter such as Nping that is more likely to get through a firewall. Look at the maximum round trip time out of ten packets or so. You might want to double that for the --initial-rtt-timeout and triple or quadruple it for the --max-rtt-timeout. I generally do not set the maximum RTT below 100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be useful when a network is so unreliable that even Nmap's default is too aggressive. Since Nmap only reduces the timeout down to the minimum when the network seems to be reliable, this need is unusual and should be reported as a bug to the nmap-dev mailing list.
--max-retries numtries (Specify the maximum number of port scan probe retransmissions) When Nmap receives no response to a port scan probe, it could mean the port is filtered. Or maybe the probe or response was simply lost on the network. It is also possible that the target host has rate limiting enabled that temporarily blocked the response. So Nmap tries again by retransmitting the initial probe. If Nmap detects poor network reliability, it may try many more times before giving up on a port. While this benefits accuracy, it also lengthens scan times. When performance is critical, scans may be sped up by limiting the number of retransmissions allowed. You can even specify --max-retries 0 to prevent any retransmissions, though that is only recommended for situations such as informal surveys where occasional missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten retransmissions. If a network seems reliable and the target hosts aren't rate limiting, Nmap usually only does one retransmission. So most target scans aren't even affected by dropping --max-retries to a low value such as three. Such values can substantially speed scans of slow (rate limited) hosts. You usually lose some information when Nmap gives up on ports early, though that may be preferable to letting the --host-timeout expire and losing all information about the target.
--host-timeout time (Give up on slow target hosts) Some hosts simply take a long time to scan. This may be due to poorly performing or unreliable networking hardware or software, packet rate limiting, or a restrictive firewall. The slowest few percent of the scanned hosts can eat up a majority of the scan time. Sometimes it is best to cut your losses and skip those hosts initially. Specify --host-timeout with the maximum amount of time you are willing to wait. For example, specify 30m to ensure that Nmap doesn't waste more than half an hour on a single host. Note that Nmap may be scanning other hosts at the same time during that half an hour, so it isn't a complete loss. A host that times out is skipped. No port table, OS detection, or version detection results are printed for that host.
The special value 0 can be used to mean “no timeout”, which can be used to override the T5 timing template, which sets the host timeout to 15 minutes.
--script-timeout time While some scripts complete in fractions of a second, others can take hours or more depending on the nature of the script, arguments passed in, network and application conditions, and more. The --script-timeout option sets a ceiling on script execution time. Any script instance which exceeds that time will be terminated and no output will be shown. If debugging (-d) is enabled, Nmap will report on each timeout. For host and service scripts, a script instance only scans a single target host or port and the timeout period will be reset for the next instance.
The special value 0 can be used to mean “no timeout”, which can be used to override the T5 timing template, which sets the script timeout to 10 minutes.
--scan-delay time; --max-scan-delay time (Adjust delay between probes) This option causes Nmap to wait at least the given amount of time between each probe it sends to a given host. This is particularly useful in the case of rate limiting.  Solaris machines (among many others) will usually respond to UDP scan probe packets with only one ICMP message per second. Any more than that sent by Nmap will be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate. Nmap tries to detect rate limiting and adjust the scan delay accordingly, but it doesn't hurt to specify it explicitly if you already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate limiting, the scan slows down dramatically. The --max-scan-delay option specifies the largest delay that Nmap will allow. A low --max-scan-delay can speed up Nmap, but it is risky. Setting this value too low can lead to wasteful packet retransmissions and possible missed ports when the target implements strict rate limiting.
Another use of --scan-delay is to evade threshold based intrusion detection and prevention systems (IDS/IPS).
--min-rate number; --max-rate number (Directly control the scanning rate) Nmap's dynamic timing does a good job of finding an appropriate speed at which to scan. Sometimes, however, you may happen to know an appropriate scanning rate for a network, or you may have to guarantee that a scan will be finished by a certain time. Or perhaps you must keep Nmap from scanning too quickly. The --min-rate and --max-rate options are designed for these situations.
When the --min-rate option is given Nmap will do its best to send packets as fast as or faster than the given rate. The argument is a positive real number representing a packet rate in packets per second. For example, specifying --min-rate 300 means that Nmap will try to keep the sending rate at or above 300 packets per second. Specifying a minimum rate does not keep Nmap from going faster if conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given maximum. Use --max-rate 100, for example, to limit sending to 100 packets per second on a fast network. Use --max-rate 0.1 for a slow scan of one packet every ten seconds. Use --min-rate and --max-rate together to keep the rate inside a certain range.
These two options are global, affecting an entire scan, not individual hosts. They only affect port scans and host discovery scans. Other features like OS detection implement their own timing.
There are two conditions when the actual scanning rate may fall below the requested minimum. The first is if the minimum is faster than the fastest rate at which Nmap can send, which is dependent on hardware. In this case Nmap will simply send packets as fast as possible, but be aware that such high rates are likely to cause a loss of accuracy. The second case is when Nmap has nothing to send, for example at the end of a scan when the last probes have been sent and Nmap is waiting for them to time out or be responded to. It's normal to see the scanning rate drop at the end of a scan or in between hostgroups. The sending rate may temporarily exceed the maximum to make up for unpredictable delays, but on average the rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning faster than a network can support may lead to a loss of accuracy. In some cases, using a faster rate can make a scan take longer than it would with a slower rate. This is because Nmap's adaptive retransmission algorithms will detect the network congestion caused by an excessive scanning rate and increase the number of retransmissions in order to improve accuracy. So even though packets are sent at a higher rate, more packets are sent overall. Cap the number of retransmissions with the --max-retries option if you need to set an upper limit on total scan time.
--defeat-rst-ratelimit Many hosts have long used rate limiting to reduce the number of ICMP error messages (such as port-unreachable errors) they send. Some systems now apply similar rate limits to the RST (reset) packets they generate. This can slow Nmap down dramatically as it adjusts its timing to reflect those rate limits. You can tell Nmap to ignore those rate limits (for port scans such as SYN scan which don't treat non-responsive ports as open) by specifying --defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear non-responsive because Nmap didn't wait long enough for a rate-limited RST response. With a SYN scan, the non-response results in the port being labeled filtered rather than the closed state we see when RST packets are received. This option is useful when you only care about open ports, and distinguishing between closed and filtered ports isn't worth the extra time.
--defeat-icmp-ratelimit Similar to --defeat-rst-ratelimit, the --defeat-icmp-ratelimit option trades accuracy for speed, increasing UDP scanning speed against hosts that rate-limit ICMP error messages. Because this option causes Nmap to not delay in order to receive the port unreachable messages, a non-responsive port will be labeled closed|filtered instead of the default open|filtered. This has the effect of only treating ports which actually respond via UDP as open. Since many UDP services do not respond in this way, the chance for inaccuracy is greater with this option than with --defeat-rst-ratelimit.
--nsock-engine iocp|epoll|kqueue|poll|select Enforce use of a given nsock IO multiplexing engine. Only the select(2)-based fallback engine is guaranteed to be available on your system. Engines are named after the name of the IO management facility they leverage. Engines currently implemented are epoll, kqueue, poll, and select, but not all will be present on any platform. By default, Nmap will use the "best" engine, i.e. the first one in this list that is supported. Use nmap -V to see which engines are supported on your platform.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing template) While the fine-grained timing controls discussed in the previous section are powerful and effective, some people find them confusing. Moreover, choosing the appropriate values can sometimes take more time than the scan you are trying to optimize. Fortunately, Nmap offers a simpler approach, with six timing templates. You can specify them with the -T option and their number (0–5) or their name. The template names are paranoid (0), sneaky (1), polite (2), normal (3), aggressive (4), and insane (5). The first two are for IDS evasion. Polite mode slows down the scan to use less bandwidth and target machine resources. Normal mode is the default and so -T3 does nothing. Aggressive mode speeds scans up by making the assumption that you are on a reasonably fast and reliable network. Finally insane mode assumes that you are on an extraordinarily fast network or are willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish to be, while leaving Nmap to pick the exact timing values. The templates also make some minor speed adjustments for which fine-grained control options do not currently exist. For example, -T4 prohibits the dynamic scan delay from exceeding 10 ms for TCP ports and -T5 caps that value at 5 ms. Templates can be used in combination with fine-grained controls, and the fine-grained controls that you specify will take precedence over the timing template default for that parameter. I recommend using -T4 when scanning reasonably modern and reliable networks. Keep that option even when you add fine-grained controls so that you benefit from those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would recommend always using -T4. Some people love -T5 though it is too aggressive for my taste. People sometimes specify -T2 because they think it is less likely to crash hosts or because they consider themselves to be polite in general. They often don't realize just how slow -T polite really is. Their scan may take ten times longer than a default scan. Machine crashes and bandwidth problems are rare with the default timing options (-T3) and so I normally recommend that for cautious scanners. Omitting version detection is far more effective than playing with timing values at reducing these problems.
While -T0 and -T1 may be useful for avoiding IDS alerts, they will take an extraordinarily long time to scan thousands of machines or ports. For such a long scan, you may prefer to set the exact timing values you need rather than rely on the canned -T0 and -T1 values.
The main effects of T0 are serializing the scan so only one port is scanned at a time, and waiting five minutes between sending each probe.  T1 and T2 are similar but they only wait 15 seconds and 0.4 seconds, respectively, between probes.  T3 is Nmap's default behavior, which includes parallelization.  -T4 does the equivalent of --max-rtt-timeout 1250ms --min-rtt-timeout 100ms --initial-rtt-timeout 500ms --max-retries 6 and sets the maximum TCP and SCTP scan delay to 10ms.  T5 does the equivalent of --max-rtt-timeout 300ms --min-rtt-timeout 50ms --initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m --script-timeout 10m --max-scan-delay as well as setting the maximum TCP and SCTP scan delay to 5ms. Maximum UDP scan delay is not set by T4 or T5, but it can be set with the --max-scan-delay option.
FIREWALL/IDS EVASION AND SPOOFING Many Internet pioneers envisioned a global open network with a universal IP address space allowing virtual connections between any two nodes. This allows hosts to act as true peers, serving and retrieving information from each other. People could access all of their home systems from work, changing the climate control settings or unlocking the doors for early guests. This vision of universal connectivity has been stifled by address space shortages and security concerns. In the early 1990s, organizations began deploying firewalls for the express purpose of reducing connectivity. Huge networks were cordoned off from the unfiltered Internet by application proxies, network address translation, and packet filters. The unrestricted flow of information gave way to tight regulation of approved communication channels and the content that passes over them.
Network obstructions such as firewalls can make mapping a network exceedingly difficult. It will not get any easier, as stifling casual reconnaissance is often a key goal of implementing the devices. Nevertheless, Nmap offers many features to help understand these complex networks, and to verify that filters are working as intended. It even supports mechanisms for bypassing poorly implemented defenses. One of the best methods of understanding your network security posture is to try to defeat it. Place yourself in the mind-set of an attacker, and deploy techniques from this section against your networks. Launch an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel through one of your own proxies.
In addition to restricting network activity, companies are increasingly monitoring traffic with intrusion detection systems (IDS). All of the major IDSs ship with rules designed to detect Nmap scans because scans are sometimes a precursor to attacks. Many of these products have recently morphed into intrusion prevention systems (IPS) that actively block traffic deemed malicious. Unfortunately for network administrators and IDS vendors, reliably detecting bad intentions by analyzing packet data is a tough problem. Attackers with patience, skill, and the help of certain Nmap options can usually pass by IDSs undetected. Meanwhile, administrators must cope with large numbers of false positive results where innocent activity is misdiagnosed and alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for evading firewall rules or sneaking past IDSs. They argue that these features are just as likely to be misused by attackers as used by administrators to enhance security. The problem with this logic is that these methods would still be used by attackers, who would just find other tools or patch the functionality into Nmap. Meanwhile, administrators would find it that much harder to do their jobs. Deploying only modern, patched FTP servers is a far more powerful defense than trying to prevent the distribution of tools implementing the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting firewalls and IDS systems. It takes skill and experience. A tutorial is beyond the scope of this reference guide, which only lists the relevant options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU) The -f option causes the requested scan (including host discovery scans) to use tiny fragmented IP packets. The idea is to split up the TCP header over several packets to make it harder for packet filters, intrusion detection systems, and other annoyances to detect what you are doing. Be careful with this! Some programs have trouble handling these tiny packets. The old-school sniffer named Sniffit segmentation faulted immediately upon receiving the first fragment. Specify this option once, and Nmap splits the packets into eight bytes or less after the IP header. So a 20-byte TCP header would be split into three packets. Two with eight bytes of the TCP header, and one with the final four. Of course each fragment also has an IP header. Specify -f again to use 16 bytes per fragment (reducing the number of fragments).  Or you can specify your own offset size with the --mtu option. Don't also specify -f if you use --mtu. The offset must be a multiple of eight. While fragmented packets won't get by packet filters and firewalls that queue all IP fragments, such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some networks can't afford the performance hit this causes and thus leave it disabled. Others can't enable this because fragments may take different routes into their networks. Some source systems defragment outgoing packets in the kernel. Linux with the iptables connection tracking module is one such example. Do a scan while a sniffer such as Wireshark is running to ensure that sent packets are fragmented. If your host OS is causing problems, try the --send-eth option to bypass the IP layer and send raw ethernet frames.
Fragmentation is only supported for Nmap's raw packet features, which includes TCP and UDP port scans (except connect scan and FTP bounce scan) and OS detection. Features such as version detection and the Nmap Scripting Engine generally don't support fragmentation because they rely on your host's TCP stack to communicate with target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) Causes a decoy scan to be performed, which makes it appear to the remote host that the host(s) you specify as decoys are scanning the target network too. Thus their IDS might report 5–10 port scans from unique IP addresses, but they won't know which IP was scanning them and which were innocent decoys. While this can be defeated through router path tracing, response-dropping, and other active mechanisms, it is generally an effective technique for hiding your IP address.
Separate each decoy host with commas, and you can optionally use ME as one of the decoys to represent the position for your real IP address. If you put ME in the sixth position or later, some common port scan detectors (such as Solar Designer's excellent Scanlogd) are unlikely to show your IP address at all. If you don't use ME, Nmap will put you in a random position. You can also use RND to generate a random, non-reserved IP address, or RND:number to generate number addresses.
Note that the hosts you use as decoys should be up or you might accidentally SYN flood your targets. Also it will be pretty easy to determine which host is scanning if only one is actually up on the network. You might want to use IP addresses instead of names (so the decoy networks don't see you in their nameserver logs). Right now random IP address generation is only supported with IPv4
Decoys are used both in the initial host discovery scan (using ICMP, SYN, ACK, or whatever) and during the actual port scanning phase. Decoys are also used during remote OS detection (-O). Decoys do not work with version detection or TCP connect scan. When a scan delay is in effect, the delay is enforced between each batch of spoofed probes, not between each individual probe. Because decoys are sent as a batch all at once, they may temporarily violate congestion control limits.
It is worth noting that using too many decoys may slow your scan and potentially even make it less accurate. Also, some ISPs will filter out your spoofed packets, but many do not restrict spoofed IP packets at all.
-S IP_Address (Spoof source address) In some circumstances, Nmap may not be able to determine your source address (Nmap will tell you if this is the case). In this situation, use -S with the IP address of the interface you wish to send packets through.
Another possible use of this flag is to spoof the scan to make the targets think that someone else is scanning them. Imagine a company being repeatedly port scanned by a competitor! The -e option and -Pn are generally required for this sort of usage. Note that you usually won't receive reply packets back (they will be addressed to the IP you are spoofing), so Nmap won't produce useful reports.
-e interface (Use specified interface) Tells Nmap what interface to send and receive packets on. Nmap should be able to detect this automatically, but it will tell you if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number) One surprisingly common misconfiguration is to trust traffic based only on the source port number. It is easy to understand how this comes about. An administrator will set up a shiny new firewall, only to be flooded with complaints from ungrateful users whose applications stopped working. In particular, DNS may be broken because the UDP DNS replies from external servers can no longer enter the network. FTP is another common example. In active FTP transfers, the remote server tries to establish a connection back to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of application-level proxies or protocol-parsing firewall modules. Unfortunately there are also easier, insecure solutions. Noting that DNS replies come from port 53 and active FTP from port 20, many administrators have fallen into the trap of simply allowing incoming traffic from those ports. They often assume that no attacker would notice and exploit such firewall holes. In other cases, administrators consider this a short-term stop-gap measure until they can implement a more secure solution. Then they forget the security upgrade.
Overworked network administrators are not the only ones to fall into this trap. Numerous products have shipped with these insecure rules. Even Microsoft has been guilty. The IPsec filters that shipped with Windows 2000 and Windows XP contain an implicit rule that allows all TCP or UDP traffic from port 88 (Kerberos). In another well-known case, versions of the Zone Alarm personal firewall up to 2.1.25 allowed any incoming UDP packets with the source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent) to exploit these weaknesses. Simply provide a port number and Nmap will send packets from that port where possible. Most scanning operations that use raw sockets, including SYN and UDP scans, support the option completely. The option notably doesn't have an effect for any operations that use normal operating system sockets, including DNS requests, TCP connect scan, version detection, and script scanning. Setting the source port also doesn't work for OS detection, because Nmap must use different port numbers for certain OS detection tests to work properly.
--data hex string (Append custom binary data to sent packets) This option lets you include binary data as payload in sent packets. hex string may be specified in any of the following formats: 0xAABBCCDDEEFF..., AABBCCDDEEFF...  or \xAA\xBB\xCC\xDD\xEE\xFF.... Examples of use are --data 0xdeadbeef and --data \xCA\xFE\x09. Note that if you specify a number like 0x00ff no byte-order conversion is performed. Make sure you specify the information in the byte order expected by the receiver.
--data-string string (Append custom string to sent packets) This option lets you include a regular string as payload in sent packets.  string can contain any string. However, note that some characters may depend on your system's locale and the receiver may not see the same information. Also, make sure you enclose the string in double quotes and escape any special characters from the shell. Examples: --data-string "Scan conducted by Security Ops, extension 7192" or --data-string "Ph34r my l33t skills". Keep in mind that nobody is likely to actually see any comments left by this option unless they are carefully monitoring the network with a sniffer or custom IDS rules.
--data-length number (Append random data to sent packets) Normally Nmap sends minimalist packets containing only a header. So its TCP packets are generally 40 bytes and ICMP echo requests are just 28. Some UDP ports and IP protocols get a custom payload by default. This option tells Nmap to append the given number of random bytes to most of the packets it sends, and not to use any protocol-specific payloads. (Use --data-length 0 for no random or protocol-specific payloads.  OS detection (-O) packets are not affected because accuracy there requires probe consistency, but most pinging and portscan packets support this. It slows things down a little, but can make a scan slightly less conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string (Send packets with specified ip options) The IP protocol[12] offers several options which may be placed in packet headers. Unlike the ubiquitous TCP options, IP options are rarely seen due to practicality and security concerns. In fact, many Internet routers block the most dangerous options such as source routing. Yet options can still be useful in some cases for determining and manipulating the network route to target machines. For example, you may be able to use the record route option to determine a path to a target even when more traditional traceroute-style approaches fail. Or if your packets are being dropped by a certain firewall, you may be able to specify a different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in values as the argument to --ip-options. Precede each hex number with \x then the two digits. You may repeat certain characters by following them with an asterisk and then the number of times you wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a hex string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options. Simply pass the letter R, T, or U to request record-route, record-timestamp, or both options together, respectively. Loose or strict source routing may be specified with an L or S followed by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received, specify --packet-trace. For more information and examples of using IP options with Nmap, see http://seclists.org/nmap-dev/2006/q3/52.
--ttl value (Set IP time-to-live field) Sets the IPv4 time-to-live field in sent packets to the given value.
--randomize-hosts (Randomize target host order) Tells Nmap to shuffle each group of up to 16384 hosts before it scans them. This can make the scans less obvious to various network monitoring systems, especially when you combine it with slow timing options. If you want to randomize over larger group sizes, increase PING_GROUP_SZ in nmap.h and recompile. An alternative solution is to generate the target IP list with a list scan (-sL -n -oN filename), randomize it with a Perl script, then provide the whole list to Nmap with -iL.
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) Asks Nmap to use the given MAC address
for all of the raw ethernet frames it sends. This option implies --send-eth to ensure that Nmap actually sends ethernet-level packets. The MAC given can take several formats. If it is simply the number 0, Nmap chooses a completely random MAC address for the session. If the given string is an even number of hex digits (with the pairs optionally separated by a colon), Nmap will use those as the MAC. If fewer than 12 hex digits are provided, Nmap fills in the remainder of the six bytes with random values. If the argument isn't a zero or hex string, Nmap looks through nmap-mac-prefixes to find a vendor name containing the given string (it is case insensitive). If a match is found, Nmap uses the vendor's OUI (three-byte prefix) and fills out the remaining three bytes randomly. Valid --spoof-mac argument examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco. This option only affects raw packet scans such as SYN scan or OS detection, not connection-oriented features such as version detection or the Nmap Scripting Engine.
--proxies Comma-separated list of proxy URLs (Relay TCP connections through a chain of proxies) Asks Nmap to establish TCP connections with a final target through supplied chain of one or more HTTP or SOCKS4 proxies. Proxies can help hide the true source of a scan or evade certain firewall restrictions, but they can hamper scan performance by increasing latency. Users may need to adjust Nmap timeouts and other scan parameters accordingly. In particular, a lower --max-parallelism may help because some proxies refuse to handle as many concurrent connections as Nmap opens by default.
This option takes a list of proxies as argument, expressed as URLs in the format proto://host:port. Use commas to separate node URLs in a chain. No authentication is supported yet. Valid protocols are HTTP and SOCKS4.
Warning: this feature is still under development and has limitations. It is implemented within the nsock library and thus has no effect on the ping, port scanning and OS discovery phases of a scan. Only NSE and version scan benefit from this option so far—other features may disclose your true address. SSL connections are not yet supported, nor is proxy-side DNS resolution (hostnames are always resolved by Nmap).
--badsum (Send packets with bogus TCP/UDP checksums) Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets sent to target hosts. Since virtually all host IP stacks properly drop these packets, any responses received are likely coming from a firewall or IDS that didn't bother to verify the checksum. For more details on this technique, see https://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums) Asks Nmap to use the deprecated Adler32 algorithm for calculating the SCTP checksum. If --adler32 is not given, CRC-32C (Castagnoli) is used.  RFC 2960[13] originally defined Adler32 as checksum algorithm for SCTP; RFC 4960[6] later redefined the SCTP checksums to use CRC-32C. Current SCTP implementations should be using CRC-32C, but in order to elicit responses from old, legacy SCTP implementations, it may be preferable to use Adler32.
Any security tool is only as useful as the output it generates. Complex tests and algorithms are of little value if they aren't presented in an organized and comprehensible fashion. Given the number of ways Nmap is used by people and other software, no single format can please everyone. So Nmap offers several formats, including the interactive mode for humans to read directly and XML for easy parsing by software.
In addition to offering different output formats, Nmap provides options for controlling the verbosity of output as well as debugging messages. Output types may be sent to standard output or to named files, which Nmap can append to or clobber. Output files may also be used to resume aborted scans.
Nmap makes output available in five different formats. The default is called interactive output, and it is sent to standard output (stdout). There is also normal output, which is similar to interactive except that it displays less runtime information and warnings since it is expected to be analyzed after the scan completes rather than interactively.
XML output is one of the most important output types, as it can be converted to HTML, easily parsed by programs such as Nmap graphical user interfaces, or imported into databases.
The two remaining output types are the simple grepable output which includes most information for a target host on a single line, and sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.
While interactive output is the default and has no associated command-line options, the other four format options use the same syntax. They take one argument, which is the filename that results should be stored in. Multiple formats may be specified, but each format may only be specified once. For example, you may wish to save normal output for your own review while saving XML of the same scan for programmatic analysis. You might do this with the options -oX myscan.xml -oN myscan.nmap. While this chapter uses the simple names like myscan.xml for brevity, more descriptive names are generally recommended. The names chosen are a matter of personal preference, though I use long ones that incorporate the scan date and a word or two describing the scan, placed in a directory named after the company I'm scanning.
While these options save results to files, Nmap still prints interactive output to stdout as usual. For example, the command nmap -oX myscan.xml target prints XML to myscan.xml and fills standard output with the same interactive results it would have printed if -oX wasn't specified at all. You can change this by passing a hyphen character as the argument to one of the format types. This causes Nmap to deactivate interactive output, and instead print results in the format you specified to the standard output stream. So the command nmap -oX - target will send only XML output to stdout.  Serious errors may still be printed to the normal error stream, stderr.
Unlike some Nmap arguments, the space between the logfile option flag (such as -oX) and the filename or hyphen is mandatory. If you omit the flags and give arguments such as -oG- or -oXscan.xml, a backwards compatibility feature of Nmap will cause the creation of normal format output files named G- and Xscan.xml respectively.
All of these arguments support strftime-like conversions in the filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D is the same as %m%d%y. A % followed by any other character just yields that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml' will use an XML file with a name in the form of scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to output files rather than clobbering them. All of these options are described below.
Nmap Output Formats
-oN filespec (normal output) Requests that normal output be directed to the given filename. As discussed above, this differs slightly from interactive output.
-oX filespec (XML output) Requests that XML output be directed to the given filename. Nmap includes a document type definition (DTD) which allows XML parsers to validate Nmap XML output. While it is primarily intended for programmatic use, it can also help humans interpret Nmap XML output. The DTD defines the legal elements of the format, and often enumerates the attributes and values they can take on. The latest version is always available from https://svn.nmap.org/nmap/docs/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free XML parsers are available for all major computer languages, including C/C++, Perl, Python, and Java. People have even written bindings for most of these languages to handle Nmap output and execution specifically. Examples are Nmap::Scanner[14] and Nmap::Parser[15] in Perl CPAN. In almost all cases that a non-trivial application interfaces with Nmap, XML is the preferred format.
The XML output references an XSL stylesheet which can be used to format the results as HTML. The easiest way to use this is simply to load the XML output in a web browser such as Firefox or IE. By default, this will only work on the machine you ran Nmap on (or a similarly configured one) due to the hard-coded nmap.xsl filesystem path. Use the --webxml or --stylesheet options to create portable XML files that render as HTML on any web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT) Script kiddie output is like interactive output, except that it is post-processed to better suit the l33t HaXXorZ who previously looked down on Nmap due to its consistent capitalization and spelling. Humor impaired people should note that this option is making fun of the script kiddies before flaming me for supposedly “helping them”.
-oG filespec (grepable output) This output format is covered last because it is deprecated. The XML output format is far more powerful, and is nearly as convenient for experienced users. XML is a standard for which dozens of excellent parsers are available, while grepable output is my own simple hack. XML is extensible to support new Nmap features as they are released, while I often must omit those features from grepable output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a simple format that lists each host on one line and can be trivially searched and parsed with standard Unix tools such as grep, awk, cut, sed, diff, and Perl. Even I usually use it for one-off tests done at the command line. Finding all the hosts with the SSH port open or that are running Solaris takes only a simple grep to identify the hosts, piped to an awk or cut command to print the desired fields.
Grepable output consists of comments (lines starting with a pound (#)) and target lines. A target line includes a combination of six labeled fields, separated by tabs and followed with a colon. The fields are Host, Ports, Protocols, Ignored State, OS, Seq Index, IP ID, and Status.
The most important of these fields is generally Ports, which gives details on each interesting port. It is a comma separated list of port entries. Each port entry represents one interesting port, and takes the form of seven slash (/) separated subfields. Those subfields are: Port number, State, Protocol, Owner, Service, SunRPC info, and Version info.
As with XML output, this man page does not allow for documenting the entire format. A more detailed look at the Nmap grepable output format is available from https://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats) As a convenience, you may specify -oA basename to store scan results in normal, XML, and grepable formats at once. They are stored in basename.nmap, basename.xml, and basename.gnmap, respectively. As with most programs, you can prefix the filenames with a directory path, such as ~/nmaplogs/foocorp/ on Unix or c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level), -vlevel (Set verbosity level) Increases the verbosity level, causing Nmap to print more information about the scan in progress. Open ports are shown as they are found and completion time estimates are provided when Nmap thinks a scan will take more than a few minutes. Use it twice or more for even greater verbosity: -vv, or give a verbosity level directly, for example -v3.
Most changes only affect interactive output, and some also affect normal and script kiddie output. The other output types are meant to be processed by machines, so Nmap can give substantial detail by default in those formats without fatiguing a human user. However, there are a few changes in other modes where output size can be reduced substantially by omitting some detail. For example, a comment line in the grepable output that provides a list of all ports scanned is only printed in verbose mode because it can be quite long.
-d (Increase debugging level), -dlevel (Set debugging level) When even verbose mode doesn't provide sufficient data for you, debugging is available to flood you with much more! As with the verbosity option (-v), debugging is enabled with a command-line flag (-d) and the debug level can be increased by specifying it multiple times, as in -dd, or by setting a level directly. For example, -d9 sets level nine. That is the highest effective level and will produce thousands of lines unless you run a very simple scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if you are simply confused as to what Nmap is doing and why. As this feature is mostly intended for developers, debug lines aren't always self-explanatory. You may get something like: Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar: 14987 to: 100000. If you don't understand a line, your only recourses are to ignore it, look it up in the source code, or request help from the development list (nmap-dev).  Some lines are self explanatory, but the messages become more obscure as the debug level is increased.
--reason (Host and port state reasons) Shows the reason each port is set to a specific state and the reason each host is up or down. This option displays the type of the packet that determined a port or hosts state. For example, A RST packet from a closed port or an echo reply from an alive host. The information Nmap can provide is determined by the type of scan or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed, but the TCP connect scan (-sT) is limited by the implementation of the connect system call. This feature is automatically enabled by the debug option (-d) and the results are stored in XML log files even if this option is not specified.
--stats-every time (Print periodic timing stats) Periodically prints a timing status message after each interval of time. The time is a specification of the kind described in the section called “TIMING AND PERFORMANCE”; so for example, use --stats-every 10s to get a status update every 10 seconds. Updates are printed to interactive output (the screen) and XML output.
--packet-trace (Trace packets and data sent and received) Causes Nmap to print a summary of every packet sent or received. This is often used for debugging, but is also a valuable way for new users to understand exactly what Nmap is doing under the covers. To avoid printing thousands of lines, you may want to specify a limited number of ports to scan, such as -p20-30. If you only care about the goings on of the version detection subsystem, use --version-trace instead. If you only care about script tracing, specify --script-trace. With --packet-trace, you get all of the above.