Auditing network services with nmap

To remotely aud* the network to see what services are running on each host, without having to log in to each one, you need a tool like nmap. * is available for all the major operating systems, including Windows. The version that’s in the Linux repositories is usually quite old. So, if you’re using something other than Kali, your can download nmap.

sudo nmap -sS IP_ADDRESS
  • -sS: The lowercase s denotes the type of scan that we want to do. The uppercase S denotes that we’re doing a SYN packet scan.

  • IP_ADDRESS: scanning a single machine or a group of machines or an entire network.

  • Something like Not shown: 996 closed ports: The fact that it’s showing all of these closed ports instead of filtered ports tells me that there’s no firewall on this machine.

Port states

An Nmap scan will show the target machine’s ports in one of three states:

  • filtered: This means that the port is blocked by a firewall.

  • open: This means that the port is not blocked by a firewall and that the service that’s associated with that port is running.

  • closed: This means that the port is not blocked by a firewall, and that the service that’s associated with that port is not running.

Scan types

There are lots of different scanning options, each with its own purpose. The SYN packet scan that we’re using here is considered a stealthy type of scan because * generates less network traffic and fewer system log entries than certain other types of scans. With this type of scan, Nmap sends a SYN packet to a port on the target machine, as if * were trying to create a TCP connection to that machine. If the target machine responds with a SYN/ACK packet, * means that the port is in an open state and is ready to create the TCP connection. If the target machine responds with an RST packet, * means that the port is in a closed state. If there’s no response at all, * means that the port is filtered, blocked by a firewall. As a normal Linux administrator, this is one of the types of scans that you would do most of the time.

A discovery scan is useful for when you need to just see what devices are on the network:

sudo nmap -sn IP_ADDRESS/24

With the -sn option, nmap will detect whether you’re scanning the local subnet or a remote subnet. If the subnet is local, nmap will send out an Address Resolution Protocol (ARP) broadcast that requests the IPv4 addresses of every device on the subnet. That is a reliable way of discovering devices because ARP isn’t something that will ever be blocked by a firewall.

sudo nmap -A IP_ADDRESS

This scan:

  • identifies open, closed, and filtered TCP ports.

  • identifies the version of the running services.

  • runs a set of vulnerability scanning scripts that come with nmap.

  • attempts to identify the operating system of the target host.

5900/tcp open vnc Apple remote desktop vnc
| vnc-info:
| Protocol version: 3.889
| Security types:
|_ Mac OS X security type (30)
1 service unrecognized despite returning data. If you know the
service/version, please submit the following fingerprint at :
MAC Address: 00:0A:95:8B:E0:C0 (Apple)
Device type: general purpose

VNC can be handy at times. It’s like Microsoft’s Remote Desktop service for Windows, except that it’s free, open source software. But it’s also a security problem because it’s an unencrypted protocol. So, all your information goes across the network in plain text. If you must use VNC, run it through an SSH tunnel.