Having the extra entries doesn't hurt, because by default Nmap scans ports with the highest frequencies and low-frequency ports are simply skipped. And, though it may be unexpected, the excerpt shows that sometimes the UDP counterparts of popular TCP ports are found open.
Administrators sometimes change this file to reflect custom services running on their network. For example, an online services company I once consulted for had dozens of different custom daemons running on high-numbered ports. Doing this allows Nmap to display results for these ports using their proper names rather than unknown.
Remember that if you add entries without a port frequency figure, the frequency is taken to be zero, so the port will not be scanned by default. Use an option like -p [] to ensure that all named ports are scanned. Similarly, a certain registered port may be frequently wrong for a certain organization. In that case, I try to choose the most popular one for nmap-services.
Organizations which commonly use another service on such a port number may change the file accordingly. Services specific to a single organization should generally stay in their own nmap-services , but other port registrations can benefit everyone. Photo by Mateusz Dach from Pexels. I've used Nmap for years, and I continue to discover new things about it.
For example, recently I was teaching someone to scan a remote host for open TCP ports and then changed the default SSH port to one not detected during a default Nmap scan. To my surprise, after we changed the port, a typical, quick hacker-style scan readily revealed my attempt at security by obscurity. The problem is that I changed the default SSH port to , and the change was detected. A quick Nmap probe scans the 1, most popular ports, which I assumed were the ones between 1 and , with the 1, number being the computer 1, 1, The fact is that Nmap does indeed use 1, ports for a quick scan, but the operative word in the description above is that it uses the 1, most popular ports for scans.
The 1, most popular ports are not bound by the first 1, consecutive ports. The 1, ports are all over the place—ranging from one to These are only the top 1, TCP ports. If you want to see the corresponding 1, UDP ports, use this command:. Now, you might notice from your own command responses that these ports are in numerical order. That's fine for those who want to see all of the ports in numeric form and in numerical order , but unless you're a port number savant, you might want a better guide—one that's more human-readable, at least.
If so, try the following command:. I don't know how often the nmap-services file changes. I performed a diff on my old version against my updated version and they're the same, between 7. The point of showing you this information is so that you can place your configurable daemons outside of the standard scan range for Nmap.
Subexpressions to be captured such as version numbers are surrounded by parentheses as shown in most of the examples above. Next comes a delimiter character which the signature writer chooses. Next comes the field value, followed by the delimiter character. The following table describes the six fields:. Table 7. Any of the fields can be omitted. In fact, all of the fields can be omitted if no further information on the service is available.
In rare cases, a helper function can be applied to the replacement text before insertion. The following table describes the three helper functions available:. The softmatch directive is similar in format to the match directive discussed above. The main difference is that scanning continues after a softmatch, but it is limited to probes that are known to match the given service.
Also as with match , many softmatch statements can exist within a single Probe section. This line tells Nmap what ports the services identified by this probe are commonly found on. It should only be used once within each Probe section.
The syntax is a slightly simplified version of that taken by the Nmap -p option. See the examples above. This is the same as ' ports ' directive described above, except that these ports are often used to wrap a service in SSL. This optional directive cannot appear more than once per Probe. This rarely necessary directive specifies the amount of time Nmap should wait before giving up on the most recently defined Probe against a particular service. The Nmap default is usually fine.
This directive is only used for the Null probe. If a service closes the TCP connection before this timer runs out, then the service is labeled tcpwrapped. Otherwise, matching continues as usual. The rarity directive roughly corresponds to how infrequently this probe can be expected to return useful results. The higher the number, the more rare the probe is considered and the less likely it is to be tried against a service.
This optional directive specifies which probes should be used as fallbacks for if there are no matches in the current Probe section. If the fallback directive is present, Nmap first tries match lines from the probe itself, then those from the probes specified in the fallback directive from left to right. Here are some examples from nmap-service-probes which put this all together to save space many lines have been skipped. After reading this far into the section, the following should be understood.
Service and Application Version Detection. Exclude Directive.
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