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One of the very first steps in any network reconnaissance
mission is to reduce a (sometimes huge) set of IP ranges into a
list of active or interesting hosts. Scanning every port of
every single IP address is slow and usually unnecessary. Of
course what makes a host interesting depends greatly on the
scan purposes. Network administrators may only be interested in
hosts running a certain service, while security auditors may
care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to
locate hosts on his internal network, while an external
penetration tester may use a diverse set of dozens of probes in
an attempt to evade firewall restrictions. Because host discovery needs are so diverse, Nmap offers a
wide variety of options for customizing the techniques used. Host
discovery is sometimes called ping scan, but it goes well beyond
the simple ICMP echo request packets associated with the
ubiquitous ping tool. Users can skip
the ping step entirely with a list scan (-sL ) or
by disabling ping (-PN ), or engage the network
with arbitrary combinations of multi-port TCP SYN/ACK, UDP, and
ICMP probes. The goal of these probes is to solicit responses
which demonstrate that an IP address is actually active (is being
used by a host or network device). On many networks, only a small
percentage of IP addresses are active at any given time. This is
particularly common with private address space
such as 10.0.0.0/8. That network has 16 million IPs, but I have
seen it used by companies with less than a thousand machines. Host
discovery can find those machines in a sparsely allocated sea of
IP addresses. If no host discovery options are given, Nmap
sends a TCP ACK
packet destined for port 80 and an ICMP echo request query
to each target machine. An exception to this is that an ARP scan is
used for any targets which are on a local ethernet network.
For unprivileged Unix shell users, a SYN packet is sent
instead of the ack using the connect()
system call. These defaults are equivalent to the
-PA -PE options. This host discovery is
often sufficient when scanning local networks, but a more
comprehensive set of discovery probes is recommended for
security auditing. The -P* options (which select
ping types) can be combined. You can increase your odds of
penetrating strict firewalls by sending many probe types using
different TCP ports/flags and ICMP codes. Also note that ARP
discovery (-PR ) is done by default against
targets on a local ethernet network even if you specify other
-P* options, because it is almost always faster
and more effective. By default, Nmap does host discovery and then performs a
port scan against each host it determines is online. This is true
even if you specify non-default host discovery types such as UDP
probes (-PU ). Read about the
-sP option to learn how to perform
only host discovery, or use
-PN to skip host discovery and port scan all
target hosts. The following options control host
discovery: -
-sL (List Scan)
The list scan is a degenerate form of host discovery
that simply lists each host of the network(s) specified,
without sending any packets to the target hosts. By
default, Nmap still does reverse-DNS resolution on the hosts
to learn their names. It is often surprising how much
useful information simple hostnames give out. For example,
fw.chi is the name of one company's Chicago firewall.
Nmap also reports the total number of
IP addresses at the end. The list scan is a good sanity
check to ensure that you have proper IP addresses for your
targets. If the hosts sport domain names you do not
recognize, it is worth investigating further to prevent
scanning the wrong company's network. Since the idea is to simply print a list of target
hosts, options for higher level functionality such as port
scanning, OS detection, or ping scanning cannot be combined
with this. If you wish to disable ping scanning while still
performing such higher level functionality, read up on the
-PN option. -
-sP (Ping Scan) This option tells Nmap to only
perform a ping scan (host discovery), then print out the available hosts
that responded to the scan. No further testing (such as
port scanning or OS detection) is performed. This is one
step more intrusive than the list scan, and can often be
used for the same purposes. It allows light reconnaissance
of a target network without attracting much attention.
Knowing how many hosts are up is more valuable to attackers
than the list provided by list scan of every single IP and host name. Systems administrators often find this option
valuable as well. It can easily be used to count available
machines on a network or monitor server availability. This
is often called a ping sweep, and is more reliable than
pinging the broadcast address because many hosts do not
reply to broadcast queries. The -sP option sends an ICMP echo
request and a TCP packet to port 80 by default. When
executed by an unprivileged user, only a SYN packet is sent
(using a connect() call) to port 80 on
the target. When a privileged user tries to scan targets
on a local ethernet network, ARP requests
(-PR ) are used unless
--send-ip was specified.
The -sP option can be combined with any of the
discovery probe types (the -P* options,
excluding -PN ) for greater flexibility.
If any of those probe type and port number options are
used, the default probes (ACK and echo request) are
overridden. When strict firewalls are in place between the
source host running Nmap and the target network, using
those advanced techniques is recommended. Otherwise hosts
could be missed when the firewall drops probes or their
responses. -
-PN (No ping)
This option skips the Nmap discovery stage altogether.
Normally, Nmap uses this stage to determine active machines
for heavier scanning. By default, Nmap only performs heavy
probing such as port scans, version detection, or OS
detection against hosts that are found to be up. Disabling
host discovery with -PN causes Nmap to
attempt the requested scanning functions against
every target IP address specified. So
if a class B sized target address space (/16) is specified
on the command line, all 65,536 IP addresses are scanned.
Proper host discovery is skipped as with the list scan, but
instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target
IP is active. For machines on a local ethernet network, ARP
scanning will still be performed (unless
--send-ip is specified) because Nmap needs
MAC addresses to further scan target hosts. This option flag for this used to be P0 (uses zero), but was renamed to avoid confusion with protocol ping's PO (uses the letter O) flag. -
-PS [portlist] (TCP SYN Ping)
This option sends an empty TCP packet with the SYN
flag set. The default destination port is 80 (configurable
at compile time by changing DEFAULT_TCP_PROBE_PORT_SPEC in
nmap.h ). Alternate ports can be
specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like
T: are not allowed. Examples are
-PS22 and
-PS22-25,80,113,1050,35000 . Note that there
can be no space between -PS and the port
list. If multiple probes are specified they will be sent in
parallel. The SYN flag suggests to the remote system that you
are attempting to establish a connection. Normally the
destination port will be closed, and a RST (reset) packet
sent back. If the port happens to be open, the target will
take the second step of a TCP 3-way-handshake by responding
with a SYN/ACK TCP packet. The machine running Nmap then
tears down the nascent connection by responding with a RST
rather than sending an ACK packet which would complete the
3-way-handshake and establish a full
connection. The RST packet is sent by the
kernel of the machine running Nmap in response to the
unexpected SYN/ACK, not by Nmap itself. Nmap does not care whether the port is open or closed.
Either the RST or SYN/ACK response discussed previously tell
Nmap that the host is available and responsive. On Unix boxes, only the privileged user
root is generally able to send and
receive raw TCP packets. For unprivileged users, a
workaround is automatically employed whereby the connect()
system call is initiated against each target port. This has
the effect of sending a SYN packet to the target host, in an
attempt to establish a connection. If connect() returns
with a quick success or an ECONNREFUSED failure, the
underlying TCP stack must have received a SYN/ACK or RST and
the host is marked available. If the connection attempt
is left hanging until a timeout is reached, the host is
marked as down. This workaround is also used for IPv6
connections, as raw IPv6 packet building support is not yet
available in Nmap. -
-PA [portlist] (TCP ACK Ping)
The TCP ACK ping is quite similar to the
just-discussed SYN ping. The difference, as you could
likely guess, is that the TCP ACK flag is set instead of the
SYN flag. Such an ACK packet purports to be acknowledging
data over an established TCP connection, but no such
connection exists. So remote hosts should always respond
with a RST packet, disclosing their existence in the
process. The -PA option uses the same default
port as the SYN probe (80) and can also take a list of
destination ports in the same format. If an unprivileged
user tries this, or an IPv6 target is specified, the
connect() workaround discussed previously is used. This
workaround is imperfect because connect() is actually
sending a SYN packet rather than an ACK. The reason for offering both SYN and ACK ping probes
is to maximize the chances of bypassing firewalls. Many
administrators configure routers and other simple firewalls
to block incoming SYN packets except for those destined for
public services like the company web site or mail server.
This prevents other incoming connections to the
organization, while allowing users to make unobstructed
outgoing connections to the Internet. This non-stateful
approach takes up few resources on the firewall/router and
is widely supported by hardware and software filters. The
Linux Netfilter/iptables firewall software offers the
--syn convenience option to implement this
stateless approach. When stateless firewall rules such as
this are in place, SYN ping probes (-PS )
are likely to be blocked when sent to closed target ports.
In such cases, the ACK probe shines as it cuts right through
these rules. Another common type of firewall uses stateful rules
that drop unexpected packets. This feature was initially
found mostly on high-end firewalls, though it has become
much more common over the years. The Linux
Netfilter/iptables system supports this through the
--state option, which categorizes packets
based on connection state. A SYN probe is more likely to
work against such a system, as unexpected ACK packets are
generally recognized as bogus and dropped. A solution to this quandary is
to send both SYN and ACK probes by specifying
-PS and -PA . -
-PU [portlist] (UDP Ping)
Another host discovery option is the UDP ping, which
sends an empty (unless --data-length is
specified) UDP packet to the given ports. The portlist
takes the same format as with the previously discussed
-PS and -PA options. If
no ports are specified, the default is 31338. This default
can be configured at compile-time by changing
DEFAULT_UDP_PROBE_PORT_SPEC in nmap.h .
A highly uncommon port is used by default because sending to
open ports is often undesirable for this particular scan
type. Upon hitting a closed port on the target machine, the
UDP probe should elicit an ICMP port unreachable packet in
return. This signifies to Nmap that the machine is up and
available. Many other types of ICMP errors, such as
host/network unreachables or TTL exceeded are indicative of
a down or unreachable host. A lack of response is also
interpreted this way. If an open port is reached, most
services simply ignore the empty packet and fail to return
any response. This is why the default probe port is 31338,
which is highly unlikely to be in use. A few services, such
as chargen, will respond to an empty UDP packet, and thus
disclose to Nmap that the machine is available. The primary advantage of this scan type is that it
bypasses firewalls and filters that only screen TCP. For
example, I once owned a Linksys BEFW11S4 wireless broadband
router. The external interface of this device filtered all
TCP ports by default, but UDP probes would still elicit port
unreachable messages and thus give away the device. -
-PE ;
-PP ;
-PM (ICMP Ping Types)
In addition to the unusual TCP and UDP host discovery
types discussed previously, Nmap can send the standard
packets sent by the ubiquitous
ping program. Nmap sends an ICMP
type 8 (echo request) packet to the target IP addresses,
expecting a type 0 (echo reply) in return from available
hosts. Unfortunately for network explorers, many hosts and
firewalls now block these packets, rather than responding as
required by
RFC 1122 For this reason, ICMP-only scans are rarely
reliable enough against unknown targets over the Internet.
But for system administrators monitoring an internal
network, they can be a practical and efficient approach.
Use the -PE option to enable this echo
request behavior. While echo request is the standard ICMP ping query,
Nmap does not stop there. The ICMP standard
(RFC 792) also specifies timestamp request, information
request, and address mask request packets as codes 13, 15,
and 17, respectively. While the ostensible purpose for
these queries is to learn information such as address masks
and current times, they can easily be used for host
discovery. A system that replies is up and available. Nmap
does not currently implement information request packets, as
they are not widely supported. RFC 1122 insists that
“a host SHOULD NOT implement these messages”.
Timestamp and address mask queries can be sent with the
-PP and -PM options,
respectively. A timestamp reply (ICMP code 14) or address
mask reply (code 18) discloses that the host is available.
These two queries can be valuable when administrators specifically
block echo request packets while forgetting that other ICMP
queries can be used for the same purpose. -
-PO [protolist] (IP Protocol Ping)
The newest host discovery option is the IP protocol ping,
which sends IP packets with the specified protocol number
set in their IP header. The protocol list
takes the same format as do port lists in the
previously discussed TCP and UDP host discovery options. If
no protocols are specified, the default is to send multiple
IP packets for ICMP (protocol 1), IGMP (protocol 2), and
IP-in-IP (protocol 4). The default protocols can be
configured at compile-time by changing
DEFAULT_PROTO_PROBE_PORT_SPEC in nmap.h .
Note that for the ICMP, IGMP, TCP (protocol 6), and UDP
(protocol 17), the packets are sent with the proper protocol
headers while other protocols are sent with no additional data
beyond the IP header (unless the
--data-length option is specified). This host discovery method looks for either responses
using the same protocol as a probe, or ICMP protocol
unreachable messages which signify that the given protocol
isn't supported on the destination host. Either type of
response signifies that the target host is alive. -
-PR (ARP Ping)
One of the most common Nmap usage scenarios is to scan
an ethernet LAN. On most LANs, especially those using
private address ranges specified by RFC 1918, the vast majority of
IP addresses are unused at any given time. When Nmap tries
to send a raw IP packet such as an ICMP echo request, the
operating system must determine the destination hardware
(ARP) address corresponding to the target IP so that it can
properly address the ethernet frame. This is often slow and
problematic, since operating systems weren't written with
the expectation that they would need to do millions of ARP
requests against unavailable hosts in a short time
period. ARP scan puts Nmap and its optimized algorithms in
charge of ARP requests. And if it gets a response back,
Nmap doesn't even need to worry about the IP-based ping
packets since it already knows the host is up. This makes
ARP scan much faster and more reliable than IP-based scans.
So it is done by default when scanning ethernet hosts that Nmap
detects are on a local ethernet network. Even if different
ping types (such as -PE or
-PS ) are specified, Nmap uses ARP instead
for any of the targets which are on the same LAN. If you
absolutely don't want to do an ARP scan, specify
--send-ip . -
--traceroute (Trace path to host)
Traceroutes are performed post-scan using information from the scan results to determine the port and protocol most likely to reach the target. It works with all scan types except connect scans (-sT) and idle scans (-sI). All traces use Nmap's dynamic timing model and are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live) in an attempt to elicit ICMP Time Exceeded messages from intermediate hops between the scanner and the target host. Standard traceroute implementation start with a TTL of 1 and increment the TTL until the destination host is reached. Nmap's traceroute starts with a high TTL and then decrements the TTL until it reaches 0. Doing it backwards lets nmap employ clever caching algorithms to speed up traces over multiple hosts. On average nmap sends 5-10 fewer packets per host, depending on network conditions. If a single subnet is being scanned (i.e. 192.168.0.0/24) nmap may only have to send a single packet to most hosts.
-
--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 -PT ) are very detailed, but the
TCP connect scan and ping (-sT ) are 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.
-
-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. -
-R (DNS resolution for all targets)
Tells Nmap to
always do reverse DNS
resolution on the target IP addresses. Normally reverse DNS is
only performed against responsive (online) hosts. -
--system-dns (Use system DNS resolver)
By default, Nmap resolves IP addresses by sending
queries directly to the name servers configured on your host
and then listening for responses. Many requests (often
dozens) are performed in parallel to improve performance.
Specify this option to use your system resolver instead (one
IP at a time via the getnameinfo() call). This is slower
and rarely useful unless you find a bug in the Nmap parallel
resolver (please let us know if you do). The system
resolver is always used for IPv6 scans. -
--dns-servers <server1[,server2],...>
(Servers to use for reverse DNS queries)
By default Nmap will try to determine your DNS servers
(for rDNS resolution) from your resolv.conf file (Unix) or
the Registry (Win32). Alternatively, you may use this
option to specify alternate servers. This option is not
honored if you are using --system-dns or an
IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your
target IP space. This option can also improve stealth, as
your requests can be bounced off just about any recursive
DNS server on the internet. This option also comes in handy when scanning private
networks. Sometimes only a few name servers provide
proper rDNS information, and you may not even know where
they are. You can scan the network for port 53 (perhaps
with version detection), then try Nmap list scans
(-sL ) specifying each name server one at a
time with --dns-servers until you find one
which works.
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