Category: Nmap filtered ports scanning

This is a mistake, as exploitable UDP services are quite common and attackers certainly don't ignore the whole protocol. Fortunately, Nmap can help inventory UDP ports. UDP scan is activated with the -sU option. For most ports, this packet will be empty no payloadbut for a few of the more common ports a protocol-specific payload will be sent.

The most curious element of this table may be the open filtered state. It is a symptom of the biggest challenges with UDP scanning: open ports rarely respond to empty probes.

If ports in all other states would respond, then open ports could all be deduced by elimination. Unfortunately, firewalls and filtering devices are also known to drop packets without responding. So when Nmap receives no response after several attempts, it cannot determine whether the port is open or filtered. When Nmap was released, filtering devices were rare enough that Nmap could and did simply assume that the port was open. The Internet is better guarded now, so Nmap changed in version 3.

This scan of Felix demonstrates the open filtered ambiguity issue as well as another problem: UDP scanning can be slow. Scanning a thousand ports took almost 17 minutes in this case due to ICMP response rate limiting performed by Felix and most other Linux systems. Nmap provides ways to work around both problems, as described by the following two sections. In the case of the Felix scan, all but the three open filtered ports were closed.

So the scan was still successful in narrowing down potentially open ports to a handful. That is not always the case. In this case, the scan didn't narrow down the open ports at all. All are open filtered. A new strategy is called for. Yet it also shows that, on rare occasions, the UDP service listening on a port will respond in kind, proving that the port is open.As a novice performing automotive repair, I can struggle for hours trying to fit my rudimentary tools hammer, duct tape, wrench, etc.

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 Npcap has already been loaded into the OS.

Requiring root privileges was a serious limitation when Nmap was released inas 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.

Basic guide to NMAP (Kali Linux 2.0)

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. 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.

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.

Of the scans listed in this section, unprivileged users can only execute connect and FTP bounce scans. SYN scan is the default and most popular scan option for good reasons. It can be performed quickly, scanning thousands of ports per second on a fast network not hampered by restrictive firewalls. It is also relatively unobtrusive and stealthy since it never completes TCP connections.

It also allows clear, reliable differentiation between the openclosedand filtered states. This technique is often referred to as half-open scanning, because you don't open a full TCP connection.

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You send a SYN packet, as if you are going to open a real connection and then wait for a response. 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 0, 1, 2, 3, 9, 10, or 13 is received.

nmap filtered ports scanning

This is the case when a user does not have raw packet privileges. Instead of writing raw packets as most other scan types do, Nmap asks the underlying operating system to establish a connection with the target machine and port by issuing the connect system call. This is the same high-level system call that web browsers, P2P clients, and most other network-enabled applications use to establish a connection.

Rather than read raw packet responses off the wire, Nmap uses this API to obtain status information on each connection attempt.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. It only takes a minute to sign up. The result of nmap on the first ports gives 22 and 80 as open, as I expect. However a few ports appear as "filtered". My question is: why do port 21, 25 and appear as "filtered" and the other ports do not appear as filtered?

If it's normal to see 21,25 and as "filtered", then why aren't all the other ports appearing as "filtered" too!? The excellent " Nmap Network Discovery " book, written by its creator Fyodor explains this very well. I quote. The filtering could be from a dedicated firewall device, router rules, or host-based firewall software.

These ports frustrate attackers because they provide so little information. Sometimes they respond with ICMP error messages such as type 3 code 13 destination unreachable: communication administratively prohibitedbut filters that simply drop probes without responding are far more common.

This forces Nmap to retry several times just in case the probe was dropped due to network congestion rather than filtering. This sort of filtering slows scans down dramatically.

This occurs for scan types in which open ports give no response. The lack of response could also mean that a packet filter dropped the probe or any response it elicited. So Nmap does not know for sure whether the port is open or being filtered.

Because in your ISP, router, your network administrator, anything between them, or yourself are filtering them. These ports have a pretty bad history, the is the port used by the Microsoft instant messaging protocol aka MSN and friends which I believe you may or may not have set an specific rule. By default, Nmap scans only the most common 1, ports for each protocol tcp,udp. If your port is outside that then it won't scan it and hence won't report it.

However, you can specify ports you want to scan with -p option. Sign up to join this community.

The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Why are some ports reported by nmap filtered and not the others?

Ask Question. Asked 5 years, 10 months ago. Active 5 months ago.One of my goals in developing Nmap is to keep the most common usage simple, while retaining the flexibility for custom and advanced scans. This is accomplished with the command-line interface by offering dozens of options, but choosing sane defaults when they are not specified.

nmap filtered ports scanning

Meanwhile, advanced users sometimes specify so many options that their terminal line wraps around. A similar balance must be struck with command output. The most important results should stick out even to the occasional user who hasn't even read the man page.

Yet the output should be comprehensive and concise enough to suit professional penetration testers who run Nmap against thousands of machines daily.

Users smart enough to read this book or the Nmap source code benefit from greater control of the scanner and insights into what Nmap output really means. This tutorial demonstrates some common Nmap port scanning scenarios and explains the output. Rather than attempt to be comprehensive, the goal is simply to acquaint new users well enough to understand the rest of this chapter. The simplest Nmap command is just nmap by itself. This prints a cheat sheet of common Nmap options and syntax.

If an IP address is specified instead of a hostname this lookup is skipped. If not, Nmap reports that fact and exits.

I could have specified -Pn to skip this test. This query can be skipped with the -n option to improve speed and stealthiness. Launches a TCP port scan of the most popular 1, ports listed in nmap-services.

A SYN stealth scan is usually used, but connect scan is substituted instead for non-root Unix users who lack the privileges necessary to send raw packets. Prints the results to standard output in normal human-readable format, and exits.

The time Nmap started and version number are normally provided as well, though these were generally removed from this book for consistency and to avoid line wrapping. The ports considered most interesting because they are open or in a rarely-seen state for that host are itemized individually. When many ports are in a single non-open state, they are considered a default state, and aggregated onto a single line to avoid diluting the results with thousands of uninteresting entries.

In this case, Nmap notes that ports are filtered. The interesting ports table comes next, and provides the key scan results. The columns vary depending on options used, but in this case provide the port number and protocol, state, and service protocol for each port. The service here is just a guess made by looking up the port in nmap-services.

The service would be listed as unknown if any of the ports had no name registered in that file. Three of these ports are open and three are closed.

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Finally, Nmap reports some basic timing stats before it exits. These stats are the number of targets specified, the number of those that the ping scan found to be up, and the total time taken. While this simple command is often all that is needed, advanced users often go much further. Finally, -T4 enables a more aggressive timing policy to speed up the scan.

More complex: nmap -p0- -v -A -T4 scanme. Fortunately the extra output is easy to understand. The first 13 new lines are runtime information letting the user know what is happening as she stares expectantly at the terminal, hoping for good news.

What constitutes good news depends on whether she is a systems administrator who has to fix problems, a pen-tester who needs some issues to report on, or a black-hat cracker trying to exploit them.

About a dozen similar lines were removed for brevity.Please ensure you are using the latest version before reporting that a feature doesn't work as described. It was designed to rapidly scan large networks, although it works fine against single hosts.

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. That table lists the port number and protocol, service name, and state.

The state is either openfilteredclosedor unfiltered.

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 -sONmap 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.

The only Nmap arguments used in this example are -Ato enable OS and version detection, script scanning, and traceroute; -T4 for faster execution; and then the hostname. Nmap Reference Guide. A representative Nmap scan nmap -A -T4 scanme.While mapping out firewall rules can be valuable, bypassing rules is often the primary goal.

Nmap implements many techniques for doing this, though most are only effective against poorly configured networks.

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Unfortunately, those are common. Individual techniques each have a low probability of success, so try as many different methods as possible. The attacker need only find one misconfiguration to succeed, while the network defenders must close every hole.

The previous section discussed using an ACK scan to map out which target network ports are filtered. However, it could not determine which of the accessible ports were open or closed. Nmap offers several scan methods that are good at sneaking past firewalls while still providing the desired port state information.

FIN scan is one such technique. The SYN scan showed only two open ports, perhaps due to firewall restrictions. Meanwhile, the ACK scan is unable to recognize open ports from closed ones. Many other scan types are worth trying, since the target firewall rules and target host type determine which techniques will work. One surprisingly common misconfiguration is to trust traffic based only on the source port number. It is easy to understand how this comes about.

An administrator will set up a shiny new firewall, only to be flooded with complains from ungrateful users whose applications stopped working.

FTP is another common example. In active FTP transfers, the remote server tries to establish a connection back to the client to transfer the requested file.

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Secure solutions to these problems exist, often in the form of application-level proxies or protocol-parsing firewall modules. Unfortunately there are also easier, insecure solutions. Noting that DNS replies come from port 53 and active FTP from port 20, many administrators have fallen into the trap of simply allowing incoming traffic from those ports.

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They often assume that no attacker would notice and exploit such firewall holes. In other cases, administrators consider this a short-term stop-gap measure until they can implement a more secure solution. Then they forget the security upgrade. Overworked network administrators are not the only ones to fall into this trap. Numerous products have shipped with these insecure rules. Even Microsoft has been guilty. Apple fans shouldn't get too smug about this because the firewall which shipped with Mac OS X Tiger is just as bad.

Yet another pathetic example of this configuration is that Zone Alarm personal firewall versions up to 2. 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.

Some output has been removed for brevity and clarity. Note that the closed port 88 was the hint that lead JJ to try using it as a source port. For further information on this vulnerability, see Microsoft Knowledge Base Article While IPv6 has not exactly taken the world by storm, it is reasonably popular in Japan and certain other regions.While Nmap has grown in functionality over the years, it began as an efficient port scanner, and that remains its core function. 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: openclosedfilteredunfilteredopen filteredor closed filtered. These states are not intrinsic properties of the port itself, but describe how Nmap sees them.

Finding these is often the primary goal of port scanning. Security-minded people know that each open port is an avenue for attack. Attackers and pen-testers want to exploit the open ports, while administrators try to close or protect them with firewalls without thwarting legitimate users.

Open ports are also interesting for non-security scans because they show services available for use on the network. A closed port is accessible it receives and responds to Nmap probe packetsbut there is no application listening on it.

They can be helpful in showing that a host is up on an IP address host discovery, or ping scanningand as part of OS detection. Because closed ports are reachable, it may be worth scanning later in case some open up. Administrators may want to consider blocking such ports with a firewall. Then they would appear in the filtered state, discussed next. Nmap cannot determine whether the port is open because packet filtering prevents its probes from reaching the port.

The filtering could be from a dedicated firewall device, router rules, or host-based firewall software.

nmap filtered ports scanning

These ports frustrate attackers because they provide so little information. Sometimes they respond with ICMP error messages such as type 3 code 13 destination unreachable: communication administratively prohibitedbut filters that simply drop probes without responding are far more common. This forces Nmap to retry several times just in case the probe was dropped due to network congestion rather than filtering.

This slows down the scan dramatically. The unfiltered state means that a port is accessible, but Nmap is unable to determine whether it is open or closed. Only the ACK scan, which is used to map firewall rulesets, classifies ports into this state.


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