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Sometimes the IP address that we are scanning may be a firewall or a network device which is using NAT and behind this device there may be multiple systems.

It can be important to understand that we are in this scenario, to understand this, we may first sweep all the ports if possible, and then try with subsets of the open port to check what kind of OS guessing is made by nmap, if the guessing is different depending on the chosen ports, then we most probably are dealing with a NATed network.

In this cases we can use TCP timestamps to infer what is the configuration:

  • if timestamps are significantly different(> 1s): It is very likely that timestamps are coming from different systems
  • if timestamps are euql (<1s): It is likely that timestamps are coming from the same system
  • If only one port responds without set TCP timestamp options, it is safe to assume that two different systems are responding. If TSopts are not included in the answer or TSval = 0 on both ports, then no knowledge can be gained, because it could be the same system having timestamps disabled or timestamps got disabled on all systems.

This can also be understood by using a simpler tool like hping.

Known problems are that some firewalls or NAT gateways terminate the tcp connection and build a new one -- in this way the timestamp is the one of the firewall/reverse-proxy and in this scenario it is impossible to find out more about the network behind the firewall/proxy/NAT gateway using timestamps. Another known problem comes out if multiple servers are run virtually (so as virtual machines or containers or any other virtualization technique) on the same machine, in this case they can have very similar timestamps.

Another cool way to use TCP timestamps is to understand if we are dealing with load-balancers. While there are many ways of figuring this out, TCP timestamps offer us an alternative way to access this information. In practice a load balancer can be detected if there are different timestamps associated to a single port, in this case the service is likely to be load-balanced.

Currently the best way to defend against all possible information gathering done using TCP timestamps is disabling them, which comes with the cost of lost Protection Against Wrapped Sequence numbers (PAWS) and worse Round-Trip Time Measurement (RTTM).

Disabling timestamps can be done on GNU/Linux using:

echo 0 > /proc/sys/net/ipv4/tcp_timestamps

while on Windows can be done by executing:

Tcp1323Opts = 0