| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Search Irongeek.com:  
Help Irongeek.com pay for bandwidth and research equipment: |
Nmap (Network Mapper) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental information on each depending on the options used. Key among that information is the
interesting ports table. That table lists the port number and protocol, service name, and state. The state is either
open,
filtered,
closed, or
unfiltered. Open means that an application on the target machine is listening for connections/packets on that port.
Filtered
means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is
open
or
closed.
Closed
ports have no application listening on them, though they could open up at any time. Ports are classified as
unfiltered
when they are responsive to Nmap’s probes, but Nmap cannot determine whether they are open or closed. Nmap reports the state combinations
open|filtered
and
closed|filtered
when it cannot determine which of the two states describe a port. The port table may also include software version details when version detection has been requested. When an IP protocol scan is requested (-sO), Nmap provides information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further information on targets, including reverse DNS names, operating system guesses, device types, and MAC addresses.
A typical Nmap scan is shown in
Example 14.1, A representative Nmap scan. The only Nmap arguments used in this example are
-A, to enable OS and version detection,
-T4
for faster execution, and then the two target hostnames.
Example 14.1. A representative Nmap scan.sp
Starting nmap ( http://insecure.org/nmap/ )
Interesting ports on scanme.nmap.org (205.217.153.62):
(The 1663 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 3.9p1 (protocol 1.99)
53/tcp open domain
70/tcp closed gopher
80/tcp open http Apache httpd 2.0.52 ((Fedora))
113/tcp closed auth
Device type: general purpose
Running: Linux 2.4.X|2.5.X|2.6.X
OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)
Interesting ports on playground.nmap.org (192.168.0.40):
(The 1659 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE VERSION
135/tcp open msrpc Microsoft Windows RPC
139/tcp open netbios-ssn
389/tcp open ldap?
445/tcp open microsoft-ds Microsoft Windows XP microsoft-ds
1002/tcp open windows-icfw?
1025/tcp open msrpc Microsoft Windows RPC
1720/tcp open H.323/Q.931 CompTek AquaGateKeeper
5800/tcp open vnc-http RealVNC 4.0 (Resolution 400x250; VNC TCP port: 5900)
5900/tcp open vnc VNC (protocol 3.8)
MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
Device type: general purpose
Running: Microsoft Windows NT/2K/XP
OS details: Microsoft Windows XP Pro RC1+ through final release
Service Info: OSs: Windows, Windows XP
Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds
The newest version of Nmap can be obtained from
http://insecure.org/nmap/. The newest version of the man page is available from
http://insecure.org/nmap/man/.
This options summary is printed when Nmap is run with no arguments, and the latest version is always available at
http://insecure.org/nmap/data/nmap.usage.txt. It helps people remember the most common options, but is no substitute for the in-depth documentation in the rest of this manual. Some obscure options aren’t even included here.
Everything on the Nmap command-line that isn’t an option (or option argument) is treated as a target host specification. The simplest case is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For this, Nmap supports CIDR-style addressing. You can append
/numbits
to an IP address or hostname and Nmap will scan every IP address for which the first
numbits
are the same as for the reference IP or hostname given. For example, 192.168.10.0/24 would scan the 256 hosts between 192.168.10.0 (binary:
11000000 10101000 00001010 00000000) and 192.168.10.255 (binary:
11000000 10101000 00001010 11111111), inclusive. 192.168.10.40/24 would do exactly the same thing. Given that the host scanme.nmap.org is at the IP address 205.217.153.62, the specification scanme.nmap.org/16 would scan the 65,536 IP addresses between 205.217.0.0 and 205.217.255.255. The smallest allowed value is /1, which scans half the Internet. The largest value is 32, which scans just the named host or IP address because all address bits are fixed.
CIDR notation is short but not always flexible enough. For example, you might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or .255 because they are commonly broadcast addresses. Nmap supports this through octet range addressing. Rather than specify a normal IP address, you can specify a comma separated list of numbers or ranges for each octet. For example, 192.168.0-255.1-254 will skip all addresses in the range that end in .0 and or .255. Ranges need not be limited to the final octets: the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for all IP addresses ending in 13.37. This sort of broad sampling can be useful for Internet surveys and research.
IPv6 addresses can only be specified by their fully qualified IPv6 address or hostname. CIDR and octet ranges aren’t supported for IPv6 because they are rarely useful.
Nmap accepts multiple host specifications on the command line, and they don’t need to be the same type. The command
nmap scanme.nmap.org 192.168.0.0/16 10.0.0,1,3-7.0-255
does what you would expect.
While targets are usually specified on the command lines, the following options are also available to control target selection:
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 (-P0), 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 RFC1918-blessed 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
-P0
to skip host discovery and port scan all target hosts. The following options control host discovery:
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
-P0
option.
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, 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
-P0) 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.
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.
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.
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.
While echo request is the standard ICMP ping query, Nmap does not stop there. The ICMP standard ([2]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 admins specifically block echo request packets while forgetting that other ICMP queries can be used for the same purpose.
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.
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.
While Nmap has grown in functionality over the years, it began as an efficient port scanner, and that remains its core function. The simple command
nmap target
scans more than 1660 TCP ports on the host
target. While many port scanners have traditionally lumped all ports into the open or closed states, Nmap is much more granular. It divides ports into six states:
open,
closed,
filtered,
unfiltered,
open|filtered, or
closed|filtered.
These states are not intrinsic properties of the port itself, but describe how Nmap sees them. For example, an Nmap scan from the same network as the target may show port 135/tcp as open, while a scan at the same time with the same options from across the Internet might show that port as
filtered.
The six port states recognized by Nmap
As a novice performing automotive repair, I can struggle for hours trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to the task at hand. When I fail miserably and tow my jalopy to a real mechanic, he invariably fishes around in a huge tool chest until pulling out the perfect gizmo which makes the job seem effortless. The art of port scanning is similar. Experts understand the dozens of scan techniques and choose the appropriate one (or combination) for a given task. Inexperienced users and script kiddies, on the other hand, try to solve every problem with the default SYN scan. Since Nmap is free, the only barrier to port scanning mastery is knowledge. That certainly beats the automotive world, where it may take great skill to determine that you need a strut spring compressor, then you still have to pay thousands of dollars for it.
Most of the scan types are only available to privileged users. This is because they send and receive raw packets, which requires root access on UNIX systems. Using an administrator account on Windows is recommended, though Nmap sometimes works for unprivileged users on that platform when WinPcap has already been loaded into the OS. Requiring root privileges was a serious limitation when Nmap was released in 1997, as many users only had access to shared shell accounts. Now, the world is different. Computers are cheaper, far more people have always-on direct Internet access, and desktop UNIX systems (including Linux and MAC OS X) are prevalent. A Windows version of Nmap is now available, allowing it to run on even more desktops. For all these reasons, users have less need to run Nmap from limited shared shell accounts. This is fortunate, as the privileged options make Nmap far more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all of its insights are based on packets returned by the target machines (or firewalls in front of them). Such hosts may be untrustworthy and send responses intended to confuse or mislead Nmap. Much more common are non-RFC-compliant hosts that do not respond as they should to Nmap probes. FIN, Null, and Xmas scans are particularly susceptible to this problem. Such issues are specific to certain scan types and so are discussed in the individual scan type entries.
This section documents the dozen or so port scan techniques supported by Nmap. Only one method may be used at a time, except that UDP scan (-sU) may be combined with any one of the TCP scan types. As a memory aid, port scan type options are of the form
-sC, where
C
is a prominent character in the scan name, usually the first. The one exception to this is the deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan, though it substitutes a connect scan if the user does not have proper privileges to send raw packets (requires root access on UNIX) or if IPv6 targets were specified. Of the scans listed in this section, unprivileged users can only execute connect and ftp bounce scans.
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.
When SYN scan is available, it is usually a better choice. Nmap has less control over the high level
connect()
call than with raw packets, making it less efficient. The system call completes connections to open target ports rather than performing the half-open reset that SYN scan does. Not only does this take longer and require more packets to obtain the same information, but target machines are more likely to log the connection. A decent IDS will catch either, but most machines have no such alarm system. Many services on your average UNIX system will add a note to syslog, and sometimes a cryptic error message, when Nmap connects and then closes the connection without sending data. Truly pathetic services crash when this happens, though that is uncommon. An administrator who sees a bunch of connection attempts in her logs from a single system should know that she has been connect scanned.
UDP scan is activated with the
-sU
option. It can be combined with a TCP scan type such as SYN scan (-sS) to check both protocols during the same run.
UDP scan works by sending an empty (no data) UDP header to every targeted port. If an ICMP port unreachable error (type 3, code 3) is returned, the port is
closed. Other ICMP unreachable errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as
filtered. Occasionally, a service will respond with a UDP packet, proving that it is
open. If no response is received after retransmissions, the port is classified as
open|filtered. This means that the port could be open, or perhaps packet filters are blocking the communication. Versions scan (-sV) can be used to help differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and filtered ports rarely send any response, leaving Nmap to time out and then conduct retransmissions just in case the probe or response were lost. Closed ports are often an even bigger problem. They usually send back an ICMP port unreachable error. But unlike the RST packets sent by closed TCP ports in response to a SYN or connect scan, many hosts rate limit ICMP port unreachable messages by default. Linux and Solaris are particularly strict about this. For example, the Linux 2.4.20 kernel limits destination unreachable messages to one per second (in
net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid flooding the network with useless packets that the target machine will drop. Unfortunately, a Linux-style limit of one packet per second makes a 65,536-port scan take more than 18 hours. Ideas for speeding your UDP scans up include scanning more hosts in parallel, doing a quick scan of just the popular ports first, scanning from behind the firewall, and using
--host-timeout
to skip slow hosts.
When scanning systems compliant with this RFC text, any packet not containing SYN, RST, or ACK bits will result in a returned RST if the port is closed and no response at all if the port is open. As long as none of those three bits are included, any combination of the other three (FIN, PSH, and URG) are OK. Nmap exploits this with three scan types:
The key advantage to these scan types is that they can sneak through certain non-stateful firewalls and packet filtering routers. Another advantage is that these scan types are a little more stealthy than even a SYN scan. Don’t count on this though -- most modern IDS products can be configured to detect them. The big downside is that not all systems follow RFC 793 to the letter. A number of systems send RST responses to the probes regardless of whether the port is open or not. This causes all of the ports to be labeled
closed. Major operating systems that do this are Microsoft Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does work against most UNIX-based systems though. Another downside of these scans is that they can’t distinguish
open
ports from certain
filtered
ones, leaving you with the response
open|filtered.
The ACK scan probe packet has only the ACK flag set (unless you use
--scanflags). When scanning unfiltered systems,
open
and
closed
ports will both return a RST packet. Nmap then labels them as
unfiltered, meaning that they are reachable by the ACK packet, but whether they are
open
or
closed
is undetermined. Ports that don’t respond, or send certain ICMP error messages back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled
filtered.
This scan relies on an implementation detail of a minority of systems out on the Internet, so you can’t always trust it. Systems that don’t support it will usually return all ports
closed. Of course, it is possible that the machine really has no open ports. If most scanned ports are
closed
but a few common port numbers (such as 22, 25, 53) are
filtered, the system is most likely susceptible. Occasionally, systems will even show the exact opposite behavior. If your scan shows 1000 open ports and 3 closed or filtered ports, then those three may very well be the truly open ones.
The
--scanflags
argument can be a numerical flag value such as 9 (PSH and FIN), but using symbolic names is easier. Just mash together any combination of
URG,
ACK,
PSH,
RST,
SYN, and
FIN. For example,
--scanflags URGACKPSHRSTSYNFIN
sets everything, though it’s not very useful for scanning. The order these are specified in is irrelevant.
In addition to specifying the desired flags, you can specify a TCP scan type (such as
-sA
or
-sF). That base type tells Nmap how to interpret responses. For example, a SYN scan considers no-response to indicate a
filtered
port, while a FIN scan treats the same as
open|filtered. Nmap will behave the same way it does for the base scan type, except that it will use the TCP flags you specify instead. If you don’t specify a base type, SYN scan is used.
Besides being extraordinarily stealthy (due to its blind nature), this scan type permits mapping out IP-based trust relationships between machines. The port listing shows open ports
from the perspective of the zombie host.
So you can try scanning a target using various zombies that you think might be trusted (via router/packet filter rules).
You can add a colon followed by a port number to the zombie host if you wish to probe a particular port on the zombie for IPID changes. Otherwise Nmap will use the port it uses by default for tcp pings (80).
Besides being useful in its own right, protocol scan demonstrates the power of open source software. While the fundamental idea is pretty simple, I had not thought to add it nor received any requests for such functionality. Then in the summer of 2000, Gerhard Rieger conceived the idea, wrote an excellent patch implementing it, and sent it to the nmap-hackers mailing list. I incorporated that patch into the Nmap tree and released a new version the next day. Few pieces of commercial software have users enthusiastic enough to design and contribute their own improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of iterating through the port number field of a UDP packet, it sends IP packet headers and iterates through the 8-bit IP protocol field. The headers are usually empty, containing no data and not even the proper header for the claimed protocol. The three exceptions are TCP, UDP, and ICMP. A proper protocol header for those is included since some systems won’t send them otherwise and because Nmap already has functions to create them. Instead of watching for ICMP port unreachable messages, protocol scan is on the lookout for ICMP
protocol
unreachable messages. If Nmap receives any response in any protocol from the target host, Nmap marks that protocol as
open. An ICMP protocol unreachable error (type 3, code 2) causes the protocol to be marked as
closed
Other ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the protocol to be marked
filtered
(though they prove that ICMP is
open
at the same time). If no response is received after retransmissions, the protocol is marked
open|filtered
This vulnerability was widespread in 1997 when Nmap was released, but has largely been fixed. Vulnerable servers are still around, so it is worth trying when all else fails. If bypassing a firewall is your goal, scan the target network for open port 21 (or even for any ftp services if you scan all ports with version detection), then try a bounce scan using each. Nmap will tell you whether the host is vulnerable or not. If you are just trying to cover your tracks, you don’t need to (and, in fact, shouldn’t) limit yourself to hosts on the target network. Before you go scanning random Internet addresses for vulnerable FTP servers, consider that sysadmins may not appreciate you abusing their servers in this way.
In addition to all of the scan methods discussed previously, Nmap offers options for specifying which ports are scanned and whether the scan order is randomized or sequential. By default, Nmap scans all ports up to and including 1024 as well as higher numbered ports listed in the
nmap-services
file for the protocol(s) being scanned.
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 & 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.
Point Nmap at a remote machine and it might tell you that ports 25/tcp, 80/tcp, and 53/udp are open. Using its
nmap-services
database of about 2,200 well-known services, Nmap would report that those ports probably correspond to a mail server (SMTP), web server (HTTP), and name server (DNS) respectively. This lookup is usually accurate -- the vast majority of daemons listening on TCP port 25 are, in fact, mail servers. However, you should not bet your security on this! People can and do run services on strange ports.
Even if Nmap is right, and the hypothetical server above is running SMTP, HTTP, and DNS servers, that is not a lot of information. When doing vulnerability assessments (or even simple network inventories) of your companies or clients, you really want to know which mail and DNS servers and versions are running. Having an accurate version number helps dramatically in determining which exploits a server is vulnerable to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan methods, version detection interrogates those ports to determine more about what is actually running. The
nmap-service-probes
database contains probes for querying various services and match expressions to recognize and parse responses. Nmap tries to determine the service protocol (e.g. ftp, ssh, telnet, http), the application name (e.g. ISC Bind, Apache httpd, Solaris telnetd), the version number, hostname, device type (e.g. printer, router), the OS family (e.g. Windows, Linux) and sometimes miscellaneous details like whether an X server is open to connections, the SSH protocol version, or the KaZaA user name). Of course, most services don’t provide all of this information. If Nmap was compiled with OpenSSL support, it will connect to SSL servers to deduce the service listening behind that encryption layer. When RPC services are discovered, the Nmap RPC grinder (-sR) is automatically used to determine the RPC program and version numbers. Some UDP ports are left in the
open|filtered
state after a UDP port scan is unable to determine whether the port is open or filtered. Version detection will try to elicit a response from these ports (just as it does with open ports), and change the state to open if it succeeds.
open|filtered
TCP ports are treated the same way. Note that the Nmap
-A
option enables version detection among other things. A paper documenting the workings, usage, and customization of version detection is available at
http://insecure.org/nmap/vscan/.
When Nmap receives responses from a service but cannot match them to its database, it prints out a special fingerprint and a URL for you to submit if to if you know for sure what is running on the port. Please take a couple minutes to make the submission so that your find can benefit everyone. Thanks to these submissions, Nmap has about 3,000 pattern matches for more than 350 protocols such as smtp, ftp, http, etc.
Version detection is enabled and controlled with the following options:
One of Nmap’s best-known features is remote OS detection using TCP/IP stack fingerprinting. Nmap sends a series of TCP and UDP packets to the remote host and examines practically every bit in the responses. After performing dozens of tests such as TCP ISN sampling, TCP options support and ordering, IPID sampling, and the initial window size check, Nmap compares the results to its
nmap-os-fingerprints
database of more than 1500 known OS fingerprints and prints out the OS details if there is a match. Each fingerprint includes a freeform textual description of the OS, and a classification which provides the vendor name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type (general purpose, router, switch, game console, etc).
If Nmap is unable to guess the OS of a machine, and conditions are good (e.g. at least one open port and one closed port were found), Nmap will provide a URL you can use to submit the fingerprint if you know (for sure) the OS running on the machine. By doing this you contribute to the pool of operating systems known to Nmap and thus it will be more accurate for everyone.
OS detection enables several other tests which make use of information that is gathered during the process anyway. One of these is uptime measurement, which uses the TCP timestamp option (RFC 1323) to guess when a machine was last rebooted. This is only reported for machines which provide this information. Another is TCP Sequence Predictability Classification. This measures approximately how hard it is to establish a forged TCP connection against the remote host. It is useful for exploiting source-IP based trust relationships (rlogin, firewall filters, etc) or for hiding the source of an attack. This sort of spoofing is rarely performed any more, but many machines are still vulnerable to it. The actual difficulty number is based on statistical sampling and may fluctuate. It is generally better to use the English classification such as
worthy challenge
or
trivial joke. This is only reported in normal output in verbose (-v) mode. When verbose mode is enabled along with
-O, IPID Sequence Generation is also reported. Most machines are in the
incremental
class, which means that they increment the ID field in the IP header for each packet they send. This makes them vulnerable to several advanced information gathering and spoofing attacks.
A paper documenting the workings, usage, and customization of OS detection is available at
http://insecure.org/nmap/osdetect/.
OS detection is enabled and controlled with the following options:
This option, and all other vestiges of the old OS detection system, will likely be removed in late 2006 or in 2007.
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 tens or hundreds of 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 milliseconds by default, though you can append s, m, or h to the value to specify seconds, minutes, or hours. So the
--host-timeout
arguments
900000,
900s, and
15m
all do the same thing.
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.
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.
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 ten 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. This can be useful in combination with
--scan-delay
(discussed later), although the latter usually serves the purpose well enough by itself.
Specifying a lower
--max-rtt-timeout
and
--initial-rtt-timeout
than the defaults can cut scan times significantly. This is particularly true for pingless (-P0) 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 is a reasonable aggressive
--max-rtt-timeout
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 hping2 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 100ms, no matter what the ping times are. Nor do I exceed 1000ms.
--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.
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.
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. 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).
Using this option can reduce accuracy, as some ports will appear nonresponse 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 optional is useful when you only care about open ports, and distinguishing between
closed
and
filtered
ports isn’t worth the extra time.
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 10ms for TCP ports and
-T5
caps that value at 5 milliseconds. 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 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 1250 --initial-rtt-timeout 500 --max-retries 6
and sets the maximum TCP scan delay to 10 milliseconds.
T5
does the equivalent of
--max-rtt-timeout 300 --min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2 --host-timeout 15m
as well as setting the maximum TCP scan delay to 5ms.
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 mindset 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.
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 6th 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.
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).
Decoys are used both in the initial ping 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.
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.
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
-P0
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.
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 admins 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, admins 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. Nmap must use different port numbers for certain OS detection tests to work properly, and DNS requests ignore the
--source-port
flag because Nmap relies on system libraries to handle those. Most TCP scans, including SYN scan, support the option completely, as does UDP scan.
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/q3/0052.html.
Any security tools 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.
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
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
[7]Nmap::Scanner
and
[8]Nmap::Parser
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.
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 6 labeled fields, separated by tabs and followed with a colon. The fields are
Host,
Ports,
Protocols,
Ignored State,
OS,
Seq Index,
IPID, 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
http://www.unspecific.com/nmap-oG-output.
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.
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.
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.
This section describes some important (and not-so-important) options that don’t really fit anywhere else.
While IPv6 hasn’t exactly taken the world by storm, it gets significant use in some (usually Asian) countries and most modern operating systems support it. To use Nmap with IPv6, both the source and target of your scan must be configured for IPv6. If your ISP (like most of them) does not allocate IPv6 addresses to you, free tunnel brokers are widely available and work fine with Nmap. One of the better ones is run by BT Exact at
https://tb.ipv6.btexact.com/. I have also used one that Hurricane Electric provides at
http://ipv6tb.he.net/. 6to4 tunnels are another popular, free approach.
During the execution of nmap, all key presses are captured. This allows you to interact with the program without aborting and restarting it. Certain special keys will change options, while any other keys will print out a status message telling you about the scan. The convention is that
lowercase letters increase
the amount of printing, and
uppercase letters decrease
the printing. You may also press ? for help.
Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing Service Scan
Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15 remaining)
Here are some Nmap usage examples, from the simple and routine to a little more complex and esoteric. Some actual IP addresses and domain names are used to make things more concrete. In their place you should substitute addresses/names from
your own network.. While I don’t think port scanning other networks is or should be illegal, some network administrators don’t appreciate unsolicited scanning of their networks and may complain. Getting permission first is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap and not testing exploits or denial of service attacks. To conserve bandwidth, please do not initiate more than a dozen scans against that host per day. If this free scanning target service is abused, it will be taken down and Nmap will report
Failed to resolve given hostname/IP: scanme.nmap.org. These permissions also apply to the hosts
scanme2.nmap.org,
scanme3.nmap.org, and so on, though those hosts do not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine
scanme.nmap.org
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out of the 255 machines on
class C
network where Scanme resides. It also tries to determine what operating system is running on each host that is up and running. This requires root privileges because of the SYN scan and OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of each of the 255 possible 8 bit subnets in the 198.116 class B address space. This tests whether the systems run sshd, DNS, pop3d, imapd, or port 4564. For any of these ports found open, version detection is used to determine what application is running.
nmap -v -iR 100000 -P0 -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web servers (port 80). Host enumeration is disabled with
-P0
since first sending a couple probes to determine whether a host is up is wasteful when you are only probing one port on each target host anyway.
nmap -P0 -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap 216.163.128.20/20
This scans 4096 IPs for any webservers (without pinging them) and saves the output in grepable and XML formats.
Like its author, Nmap isn’t perfect. But you can help make it better by sending bug reports or even writing patches. If Nmap doesn’t behave the way you expect, first upgrade to the latest version available from
http://insecure.org/nmap/. If the problem persists, do some research to determine whether it has already been discovered and addressed. Try Googling the error message or browsing the Nmap-dev archives at
http://seclists.org/. Read this full munual page as well. If nothing comes of this, mail a bug report to
<nmap-dev@insecure.org>. Please include everything you have learned about the problem, as well as what version of Nmap you are running and what operating system version it is running on. Problem reports and Nmap usage questions sent to nmap-dev@insecure.org are far more likely to be answered than those sent to Fyodor directly.
Code patches to fix bugs are even better than bug reports. Basic instructions for creating patch files with your changes are available at
http://insecure.org/nmap/data/HACKING. Patches may be sent to nmap-dev (recommended) or to Fyodor directly.
Fyodor
<fyodor@insecure.org>
( http://insecure.org)
Hundreds of people have made valuable contributions to Nmap over the years. These are detailed in the
CHANGELOG
file which is distributed with Nmap and also available from
http://insecure.org/nmap/changelog.html.
The Nmap Security Scanner is (C) 1996-2005 Insecure.Com LLC. Nmap is also a registered trademark of Insecure.Com LLC. This program is free software; you may redistribute and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; Version 2. This guarantees your right to use, modify, and redistribute this software under certain conditions. If you wish to embed Nmap technology into proprietary software, we may be willing to sell alternative licenses (contact
<sales@insecure.com>). Many security scanner vendors already license Nmap technology such as host discovery, port scanning, OS detection, and service/version detection.
Note that the GPL places important restrictions on
derived works, yet it does not provide a detailed definition of that term. To avoid misunderstandings, we consider an application to constitute a
derivative work
for the purpose of this license if it does any of the following:
We don’t consider these to be added restrictions on top of the GPL, but just a clarification of how we interpret
derived works
as it applies to our GPL-licensed Nmap product. This is similar to the way Linus Torvalds has announced his interpretation of how
derived works
applies to Linux kernel modules. Our interpretation refers only to Nmap - we don’t speak for any other GPL products.
If you have any questions about the GPL licensing restrictions on using Nmap in non-GPL works, we would be happy to help. As mentioned above, we also offer alternative license to integrate Nmap into proprietary applications and appliances. These contracts have been sold to many security vendors, and generally include a perpetual license as well as providing for priority support and updates as well as helping to fund the continued development of Nmap technology. Please email
<sales@insecure.com>
for further information.
As a special exception to the GPL terms, Insecure.Com LLC grants permission to link the code of this program with any version of the OpenSSL library which is distributed under a license identical to that listed in the included Copying.OpenSSL file, and distribute linked combinations including the two. You must obey the GNU GPL in all respects for all of the code used other than OpenSSL. If you modify this file, you may extend this exception to your version of the file, but you are not obligated to do so.
If you received these files with a written license agreement or contract stating terms other than the terms above, then that alternative license agreement takes precedence over these comments.
This Nmap Reference Guide is (C) 2005 Insecure.Com LLC. It is hereby placed under version 2.5 of the
[9]Creative Commons Attribution License. This allows you redistribute and modify the work as you desire, as long as you credit the original source. Alternatively, you may choose to treat this document as falling under the same license as Nmap itself (discussed previously).
Source is provided to this software because we believe users have a right to know exactly what a program is going to do before they run it. This also allows you to audit the software for security holes (none have been found so far).
Source code also allows you to port Nmap to new platforms, fix bugs, and add new features. You are highly encouraged to send your changes to
<fyodor@insecure.org>
for possible incorporation into the main distribution. By sending these changes to Fyodor or one of the Insecure.Org development mailing lists, it is assumed that you are offering Fyodor and Insecure.Com LLC the unlimited, non-exclusive right to reuse, modify, and relicense the code. Nmap will always be available Open Source, but this is important because the inability to relicense code has caused devastating problems for other Free Software projects (such as KDE and NASM). We also occasionally relicense the code to third parties as discussed above. If you wish to specify special license conditions of your contributions, just say so when you send them.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details at
http://www.gnu.org/copyleft/gpl.html, or in the COPYING file included with Nmap.
It should also be noted that Nmap has occasionally been known to crash poorly written applications, TCP/IP stacks, and even operating systems. While this is extremely rare, it is important to keep in mind.
Nmap should never be run against mission critical systems
unless you are prepared to suffer downtime. We acknowledge here that Nmap may crash your systems or networks and we disclaim all liability for any damage or problems Nmap could cause.
Because of the slight risk of crashes and because a few black hats like to use Nmap for reconnaissance prior to attacking systems, there are administrators who become upset and may complain when their system is scanned. Thus, it is often advisable to request permission before doing even a light scan of a network.
Nmap should never be installed with special privileges (e.g. suid root) for security reasons.
This product includes software developed by the
[10]Apache Software Foundation. A modified version of the
[11]Libpcap portable packet capture library
is distributed along with nmap. The Windows version of Nmap utilized the libpcap-derived
[12]WinPcap library
instead. Regular expression support is provided by the
[13]PCRE library, which is open source software, written by Philip Hazel. Certain raw networking functions use the
[14]Libdnet
networking library, which was written by Dug Song. A modified version is distributed with Nmap. Nmap can optionally link with the
[15]OpenSSL cryptography toolkit
for SSL version detection support. All of the third-party software described in this paragraph is freely redistributable under BSD-style software licenses.
US Export Control: Insecure.Com LLC believes that Nmap falls under US ECCN (export control classification number) 5D992. This category is called
Information Security software not controlled by 5D002. The only restriction of this classification is AT (anti-terrorism), which applies to almost all goods and denies export to a handful of rogue nations such as Iran and North Korea. Thus exporting Nmap does not require any special license, permit, or other governmental authorization.
15 most recent posts on Irongeek.com:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Printable version of this article
If you would like to republish one of the articles from this site on your
webpage or print journal please contact IronGeek.
Copyright 2020, IronGeek
Louisville / Kentuckiana Information Security Enthusiast