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Remote Physical Device Fingerprinting. Authors Tadayoshi Kohno, Andre Broido, KC Claffy Appears in IEEE Symposium on Security and Privacy, 2005 Presented by Peter Matthews. Introduction. There are a number of reliable techniques for remote operating system fingerprinting nmap XProbe

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Remote Physical Device Fingerprinting

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Remote Physical Device Fingerprinting


Tadayoshi Kohno, Andre Broido, KC Claffy

Appears in

IEEE Symposium on Security and Privacy, 2005

Presented by

Peter Matthews


  • There are a number of reliable techniques for remote operating system fingerprinting

    • nmap

    • XProbe

  • Paper proposes the next step:

    • Remotely fingerprint a physical devicewithout that device's knowledge or cooperation

Clock Drift

  • A standard clock circuit in a computer system uses a quartz crystal oscillator as its time base, similar to any modern wristwatch

    • Some amount of imprecision in the oscillatory frequency

  • Clocks using these thus exhibit drift over time when compared to the actual time

Clock Skew

  • A clock will usually have some offset

    • Offset(t) = time_reported(t) – true_time(t)

  • The clock skew S is the change in this offset over time

    • S = d Offset(t) / dt

  • Measured in ppm (μs/s)

  • How much skew?

    • +/- 4 seconds a day common

    • (25 minutes a year)

  • Importantly, paper argues skew of a device is (generally) consistent and distinctive to that device

    • Thus can use as a fingerprint for this device

24 hours later

Network Time Protocol (NTP)

  • Hierarchical server system

    • Top-level servers synchronized via atomic clocks, GPS

  • Used to synchronizes system clock periodically

Techniques for Determining Clock Skew of a Remote Device

  • 3 approaches given

TCP Timestamps Option (TSopt)

  • 32-bit timestamp contained in each packet, taken from a virtual clock that is “at least approximately proportional to real time”

  • Frequency (Hz) between 1 and 1000

    • Windows 2000, XP – 10 Hz (100 ms resolution)

    • Redhat 9.0 – 100 Hz (10 ms resolution)

  • Usually reset to zero upon reboot

  • Usually not affected by changes to the device's system clock (NTP synchronization does not affect)

Exploiting the TCP Timestamps Option (Passive Approach)

  • The measurer – any entity capable of observing TCP packets from the fingerprintee

  • Create a trace of TCP packets from fingerprintee

  • For each packet plot a point

    • X value: Amount of actual time passed between reception of first packet in trace and the current packet

    • Y value: The offset observed for this packet, based on timestamp

  • Use linear programming to determine the equation of the line y = αx + β that best upper-bounds this set of points

    • α is the estimate of the clock skew

    • β is an initial observed offset

TSopt clock skew estimates for two sources from a OC-48 link of a US Tier 1 ISP over a two hour period.

Exploiting the TCP Timestamps Option (Semi-Passive Approach)

  • Windows 2000 and XP machines do not set timestamp flag in their initial SYN packets

  • RFC 1323 mandates that noneof the following TCP packets in the connection can include timestamp

  • Thus, previous approach will not work if a Windows machine is behind NAT, firewall

  • Paper’s trick: The measurer includes timestamp in the responding SYN/ACK packet

  • Windows machines then include timestamp in all subsequent packets of this connection

SYN, TSopt=0

SYN, TSopt=1

SYN-ACK, TSopt=1


data, timestamp

data, timestamp

ICMP Timestamps

  • Reports value of system clock (milliseconds past midnight)

  • RFC 792 requires frequency is 1000 Hz (1 ms resolution)

  • If system clock is updated via NTP regularly, will be relatively accurate

    • However, most hosts do so infrequently

Exloiting ICMP Timestamp Requests (Active Approach)

  • The measurer: entity capable of sending ICMP Timestamp Request and storing the fingerprintee's subsequent ICMP Timestamp Reply messages

  • Limitation: Fingerprintee must not be behind a NAT or firewall that filters ICMP

  • Estimation of clock skew is similar to that in TSopt methods.

Questions Concerning Clock Skew

  • What is the distribution of clock skews among devices?

  • How stable are these clock skews over time?

  • Can these clock skews be measured accurately, independent of network topology and access technology?

Distribution of Clock Skews

Figure 1: Histogram of TSopt clock skew estimates for sources in a 2 hour network trace from a OC-48 link of a US Tier 1 ISP. (Considered only sources that sent packet over a period of at least 50 minutes per hour, and sent at least 2000 packets per hour.)

  • Could this skew simply reflect different operating system and hardware configurations?

  • To answer this, TSopt clock offsets were measured for 69 Micron 448 MHz Pentium II machines running Windows XP SP1 over 38 days

  • ~48 TCP packets with timestamp per hour

  • TSopt clock offsets measured for 69 homogenous machines over 96 hours.

  • Clock skew can be used to distinguish some, but not all machines.

Stability of Clock Skews

  • The network traces from this experiment were then divided into 12 and 24 hour periods

    • All periods of the same length for each machine were then compared

  • Range of maximum difference for a single device:

    • 12-hour period: 1.29 – 7.33 ppm, 2.28 ppm mean

    • 24-hour period: 0.01 – 5.32 ppm, 0.71 ppm mean

  • Supports authors’ claim that modern processors have relatively stable clock skews

Independence of Access Technology

  • Fingerprinting of a laptop connected via various access technologies and locations

  • Skew estimates all within a 1 ppm range

Independence of Network Topology

  • PlanetLab machines located globally used as measurer, same laptop as previous experiment

  • Excepting measurement from IIT, skew estimates within .5 ppm range

  • Generally speaking, estimates are largely independent of topology and distance between fingerprinter and fingerprintee

Other OS Factors

  • For tested operating systems, system clock and TSopt clock effectively have the same clock skew


  • Detecting virtual honeynets and virtual hosts

  • Counting number of devices behind a NAT

  • Tracking individual devices

    • With some probability

  • Arguing that a given device was not involved in a recorded event

  • Could use un-anonymized traces on a link to assign hosts in an anonymized trace


  • Shows that it is possible to extract relevant security information from data considered noise

  • Approach could be used with any other protocols that leak information about a device’s clock


  • Further experimentation required

    • Laptop running Windows XP SP2 has a noticeably different TSopt clock skew after switching to battery power

    • Newer processors throttle their speeds based on temperature and load, affects voltage from power supply

  • Easy to circumvent particular methods

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

    • Randomize TSopt timestamp

    • Filter ICMP timestamp

Possible Extensions of Paper’s Approach

  • Utilization of approach with other protocols that leak information about a device’s clock

  • Use of profiling in combination with skew data

    • Skew is within a certain range and machine visits certain websites frequently

    • OS profiling techniques


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