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Description and Review

Netstat is an extremely powerful tool that can be used to view the network connection information on a machine. Netstat includes information such as listening/active ports and which protocols they are using. Established connections will also list the IP address (potentially an attacker's) that the machine is connected to. Many systems also allow you to specify a switch to view not only the PID of the programs connected to those ports, but also their names. Because many hacking attacks attempt to create backdoors on the victim machine in order to establish another means of connecting back into the machine later, being able to view open ports and the programs attached to them can be very beneficial. Netstat can help you quickly identify any suspicious network behavior on the machine and help point you in the right direction to files and connections that may require additional investigation.

Another great benefit to using the Netstat command is that it is one of the few commands that uses, for the most part, the same syntax on UNIX-like and Windows operating systems. Netstat also comes installed by default on nearly all systems.

Usage Instructions

To run Netstat, open the terminal/command shell and run the following command for Windows:

 netstat –anob

And the following command for UNIX systems:

 netstat –anp

This will output all listening and active connections on the machine, including the PID and name of the program attached to each connection. This also tells Netstat to return the IP address of connections, rather than trying to determine their host names. The -a parameter tells netstat to display all active connections and the TCP and UDP ports on which the computer is listening. The -n parameter tells netstat to show all the TCP connections but to not resolve the host-name, this will greatly increase the speed of the command. The -p parameter shows which processes are using which sockets. (see here for detailed explanations of Netstat switches on Windows systems, and here for detailed explanations of Netstat switches on UNIX systems).

Below is an example of an excerpt from the Netstat output (taken from a forensic investigation on a Windows machine for a class):

 Active Connections
 Proto  Local Address          Foreign Address        State           PID
 TCP                LISTENING       712
 TCP                LISTENING       4
 TCP               LISTENING       1256

As you can see, the bolded line shows that the srss.exe program is running with a PID of 1256 and is listening on TCP port 1337. Looking up port 1337 and srss.exe on the internet did not reveal any legitimate/standard programs by that name or port. Further review of the srss.exe file revealed that it was a disguised version of Netcat, running a backdoor on the system. Running Netstat helps us quickly determine which program to investigate first, allowing us to efficiently identify the intent of the attacker. If the attacker had been connected to that listening backdoor, his IP address would've also been shown in the "Foreign Address" column.

Netstat can also list out the routing table for the machine (which may reveal attempts by the attacker to create alternate routes in order to bypass firewalls and other restrictions) using the following command:

 netstat –rn

This is an example output from the internal route table:

   Kernel IP routing table
   Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface         UG        0 0          0 ens33     U         0 0          0 ens33   U         0 0          0 ens33