Network exploration tool and security / port scanner

-sS (TCP SYN scan) .
    SYN scan is the default and most popular scan option for good reasons. It can be performed quickly,
    scanning thousands of ports per second on a fast network not hampered by restrictive firewalls. SYN
    scan is relatively unobtrusive and stealthy, since it never completes TCP connections. It also works
    against any compliant TCP stack rather than depending on idiosyncrasies of specific platforms as
    Nmap´s FIN/NULL/Xmas, Maimon and idle scans do. It also allows clear, reliable differentiation
    between the open, closed, and filtered states.

    This technique is often referred to as half-open scanning, because you don´t open a full TCP
    connection. You send a SYN packet, as if you are going to open a real connection and then wait for a
    response. A SYN/ACK indicates the port is listening (open), while a RST (reset) is indicative of a
    non-listener. If no response is received after several retransmissions, the port is marked as
    filtered. The port is also marked filtered if an ICMP unreachable error (type 3, code 1, 2, 3, 9, 10,
    or 13) is received.

-sV (Version detection) .
    Enables version detection, as discussed above. Alternatively, you can use -A, which enables version
    detection among other things.

-sC .
    Performs a script scan using the default set of scripts. It is equivalent to --script=default. Some
    of the scripts in this category are considered intrusive and should not be run against a target
    network without permission.

-n (No DNS resolution) .
    Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds. Since DNS can be
    slow even with Nmap´s built-in parallel stub resolver, this option can slash scanning times.

--open (Show only open (or possibly open) ports) .
    Sometimes you only care about ports you can actually connect to (open ones), and don´t want results
    cluttered with closed, filtered, and closed|filtered ports. Output customization is normally done
    after the scan using tools such as grep, awk, and Perl, but this feature was added due to
    overwhelming requests. Specify --open to only see open, open|filtered, and unfiltered ports. These
    three ports are treated just as they normally are, which means that open|filtered and unfiltered may
    be condensed into counts if there are an overwhelming number of them.

--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.

--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 lengthen 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.

nmap [Scan Type...] [Options] {target specification}

-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.

-v (Increase 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.

    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.

-p port ranges (Only scan specified ports) .
    This option specifies which ports you want to scan and overrides the default. Individual port numbers
    are OK, as are ranges separated by a hyphen (e.g.  1-1023). The beginning and/or end values of a
    range may be omitted, causing Nmap to use 1 and 65535, respectively. So you can specify -p- to scan
    ports from 1 through 65535. Scanning port zero.  is allowed if you specify it explicitly. For IP
    protocol scanning (-sO), this option specifies the protocol numbers you wish to scan for (0–255).

    When scanning both TCP and UDP ports, you can specify a particular protocol by preceding the port
    numbers by T: or U:. The qualifier lasts until you specify another qualifier. For example, the
    argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports 53, 111,and 137, as well as the
    listed TCP ports. Note that to scan both UDP and TCP, you have to specify -sU and at least one TCP
    scan type (such as -sS, -sF, or -sT). If no protocol qualifier is given, the port numbers are added
    to all protocol lists.  Ports can also be specified by name according to what the port is referred to
    in the nmap-services. You can even use the wildcards * and ? with the names. For example, to scan FTP
    and all ports whose names begin with “http”, use -p ftp,http*. Be careful about shell expansions and
    quote the argument to -p if unsure.

    Ranges of ports can be surrounded by square brackets to indicate ports inside that range that appear
    in nmap-services. For example, the following will scan all ports in nmap-services equal to or below
    1024: -p [-1024]. Be careful with shell expansions and quote the argument to -p if unsure.

--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 (-sP). 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 Class C 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.