Tool introduction: masscan

When performing a port scan against an IT infrastructure, we must assume that services can be accessed via any IP address. As a result, we must scan every IT system, even if we have received a server list from the client. Documentation can be incomplete, especially if the network has grown historically and was managed by multiple administrators. However, since we usually have limited time for a pentest, the choice of port scanner depends on the size of the network. While Nmap can be used to efficiently identify all services on an IT system, the tool is not optimized for a wide scan of several thousand IP addresses. However, this size is often used for internal IT infrastructures because the three private IPv4 address ranges can be used:

• 10.0.0.0/8 (16.777.216 IPs)
• 172.16.0.0/12 (1.048.576 IPs)
• 192.168.0.0/16 (65.536 IPs)

For such cases, tools like masscan have been developed, which, according to their own statement, can scan the entire Internet for the accessibility of one port in only 3 minutes (see man page). This statement can be quite realistic if one would have an exceptionally good routing and network connection. As an example, web servers on the 192.168.0.0/16 network can be identified within a few minutes using the following command, since 2000 SYN packets are sent per second:

sudo masscan -p80,443 --rate=2000 -e tun0 192.168.0.0/16

-p: specifies the port(s) to be scanned
--rate: specifies the desired rate for transmitting packets per second.
-e: use the named raw network interface, such as "eth0" or "dna1"

However, care must be taken when configuring the rate limit, as we may negatively affect the availability of firewalls or other network devices. With masscan we are basically able to identify common services such as DNS servers, web servers, SSH servers, etc. in large network areas in a foreseeable time, while we can then use Nmap to carry out a complete port scan for the active IP addresses.