(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
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
These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (
Host discovery always works in large groups of hosts to improve speed
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
(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.
--max-parallelism option is sometimes set to one
to prevent Nmap from sending more than one probe at a time to hosts.
--scan-delay option, discussed later, is another
way to do this.
(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.
The exact formula is given in the section called “Idle Scan Implementation Algorithms”.
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
--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
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
--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 (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.
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,
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.
(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
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
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
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
technique is used in the section called “A practical example: bypassing default Snort 2.2.0 rules”
to defeat the default port scan detector in Snort IDS. Most other
intrusion detection systems can be defeated in the same way.
(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
--max-rate options are designed for these
--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.
--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
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
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
Many hosts have long used
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
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
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
filtered ports isn't
worth the extra time.
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
select, but not all will be present on any platform.
Use nmap -V to see which engines are supported.
(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. So 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
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
assumes that you are on an
extraordinarily fast network or are willing to sacrifice some accuracy
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,
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 will you specify will take
precedence over the timing template default for that parameter. I
-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
-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
really is. Their scan may take ten times longer than a
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
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
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.
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.
does the equivalent of
--initial-rtt-timeout 500ms --max-retries 6 and sets the maximum TCP scan delay
to 10 milliseconds.
does the equivalent of
--max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m as well as
setting the maximum TCP scan delay to 5 ms.