Vulnerability analysis part 2: Host discovery using nmap

By david on September 7, 2015

I know IP address on my attacking machine is 192.168.238.137, and I know the subnet mask is 255.255.255.0. In CIDR notation, this means the subnet is /24. This tells me which part of my IP address is the network prefix (192.168.238), and which part is the host number (.137).

We can list out all the IP addresses in this range by using a list scan (-sP). This scan sends no packets to listed IP addresses, but it will attempt a reverse DNS resolution on each host.

# nmap -sL 192.168.238.0/24

Since there are 256 addresses outputted by this command, I will not paste them here.

Let’s explore other machines with ping sweep. This scan does not scan any ports, but only reports on which hosts are up by pinging them. Older versions of nmap us -sP. Newer versions will use -sn

# nmap -sn 192.168.238.0/24
Starting Nmap 6.49BETA4 ( https://nmap.org ) at 2015-09-06 18:49 EDT
Nmap scan report for 192.168.238.1
Host is up (0.00039s latency).
MAC Address: 00:50:56:C0:00:08 (VMware)
Nmap scan report for 192.168.238.2
Host is up (0.00012s latency).
MAC Address: 00:50:56:EA:8D:7F (VMware)
Nmap scan report for 192.168.238.137
Host is up (-0.10s latency).
MAC Address: 00:0C:29:54:6E:48 (VMware)
Nmap scan report for 192.168.238.254
Host is up (0.00011s latency).
MAC Address: 00:50:56:EF:B0:9E (VMware)
Nmap scan report for 192.168.238.136
Host is up.
Nmap done: 256 IP addresses (5 hosts up) scanned in 2.28 seconds

I know the 192.168.238.2 addresses is my default gateway, and 192.168.238.136 is my local machine. What are those other three addresses? One of them is the Metasploitable machine. I’m not sure what the other two are.

A little digging, and I find out what these addresses are.

192.168.238.1: Host machines
192.168.238.2: Default gateway
192.168.238.254: VMware DHCP server
192.168.238.136: Kali machine
192.168.238.137 must be our Metasploitable VM.

The ping scan will ping the host, send a TCP SYN, a TCP ACK, and request an ICMP timestamp. If any of these requests receive a response, we know the host is alive. If there was a firewall or other device blocking these requests, the ping scan might show that no host is up at an IP address, and you will need to use a different method of host discovery. The -Pn scan will attempt a port scan on every IP address in the given range. We’ll speed it up with the -F flag to only scan 100 ports.

# nmap -F -Pn 192.168.238.0/24
Starting Nmap 6.49BETA4 ( https://nmap.org ) at 2015-09-07 17:12 EDT
Nmap scan report for 192.168.238.1
Host is up (0.00032s latency).
Not shown: 97 filtered ports
PORT STATE SERVICE
135/tcp open msrpc
139/tcp open netbios-ssn
445/tcp open microsoft-ds
MAC Address: 00:50:56:C0:00:08 (VMware)
Nmap scan report for 192.168.238.2
Host is up (0.00011s latency).
Not shown: 99 closed ports
PORT STATE SERVICE
53/tcp open domain
MAC Address: 00:50:56:EA:8D:7F (VMware)
Nmap scan report for 192.168.238.137
Host is up (0.00018s latency).
Not shown: 82 closed ports
PORT STATE SERVICE
21/tcp open ftp
22/tcp open ssh
23/tcp open telnet
25/tcp open smtp
53/tcp open domain
80/tcp open http
111/tcp open rpcbind
139/tcp open netbios-ssn
445/tcp open microsoft-ds
513/tcp open login
514/tcp open shell
2049/tcp open nfs
2121/tcp open ccproxy-ftp
3306/tcp open mysql
5432/tcp open postgresql
5900/tcp open vnc
6000/tcp open X11
8009/tcp open ajp13
MAC Address: 00:0C:29:54:6E:48 (VMware)
Nmap scan report for 192.168.238.254
Host is up (-0.10s latency).
All 100 scanned ports on 192.168.238.254 are filtered
MAC Address: 00:50:56:F7:A6:AC (VMware)
Nmap scan report for 192.168.238.136
Host is up (0.0000020s latency).
All 100 scanned ports on 192.168.238.136 are closed
Nmap done: 256 IP addresses (5 hosts up) scanned in 16.43 seconds

Some additional scan types:

TCP SYN pings initiate the first step in the TCP handshake on a given port. If the host responds with TCP ACK, we know the host is alive, and we close the handshake with a RST packet.

TCP ACK pings send the ACK to a host, making the host think a handshake was initiated. Since the host knows it didn’t send a SYN, it will respond with a RST packet to stop the handshake, letting us know the host is alive.

UDP pings send a UDP packet to a certain port. It receives an ICMP port unreachable packet if the port is closed, but that lets us know the host is alive.

Up next: Port scanning