It is long known attackers may use forged source IP address to conceal their real locations. To capture the spoofers, a number of IP traceback mechanisms have been proposed. However, due to the challenges of deployment, there has been not a widely adopted IP traceback solution, at least at the Internet level. As a result, the mist on the locations of spoofers has never been dissipated till now.
This paper proposes passive IP traceback (PIT) that bypasses the deployment difficulties of IP traceback techniques. PIT investigates Internet Control Message Protocol error messages (named path backscatter) triggered by spoofing traffic, and tracks the spoofers based on public available information (e.g., topology). In this way, PIT can find the spoofers without any deployment requirement.
This paper illustrates the causes, collection, and the statistical results on path backscatter, demonstrates the processes and effectiveness of PIT, and shows the captured locations of spoofers through applying PIT on the path backscatter data set.
These results can help further reveal IP spoofing, which has been studied for long but never well understood. Though PIT cannot work in all the spoofing attacks, it may be the most useful mechanism to trace spoofers before an Internet-level traceback system has been deployed in real. 
IP spoofing, which means attackers launching attacks with forged source IP addresses, has been recognized as a serious security problem on the Internet for long. By using addresses that are assigned to others or not assigned at all, attackers can avoid exposing their real locations, or enhance the effect of attacking, or launch reflection based attacks. A number of notorious attacks rely on IP spoofing, including SYN flooding, SMURF, DNS amplification etc. A DNS amplification attack which severely degraded the service of a Top Level Domain (TLD) name server is reported in though there has been a popular conventional wisdom that DoS attacks are launched from botnets and spoofing is no longer critical, the report of ARBOR on NANOG 50th meeting shows spoofing is still significant in observed DoS attacks. Indeed, based on the captured backscatter messages from UCSD Network Telescopes, spoofing activities are still frequently observed.
To capture the origins of IP spoofing traffic is of great importance. As long as the real locations of spoofers are not disclosed, they cannot be deterred from launching further attacks. Even just approaching the spoofers, for example, determining the ASes or networks they reside in, attackers can be located in a smaller area, and filters can be placed closer to the attacker before attacking traffic get aggregated. The last but not the least, identifying the origins of spoofing traffic can help build a reputation system for ASes, which would be helpful to push the corresponding ISPs to verify IP source address.
Instead of proposing another IP traceback mechanism with improved tracking capability, we propose a novel solution, named Passive IP Traceback (PIT), to bypass the challenges in deployment. Routers may fail to forward an IP spoofing packet due to various reasons, e.g., TTL exceeding. In such cases, the routers may generate an ICMP error message (named path backscatter) and send the message to the spoofed source address. Because the routers can be close to the spoofers, the path backscatter messages may potentially disclose the locations of the spoofers. PIT exploits these path backscatter messages to find the location of the spoofers. With the locations of the spoofers known, the victim can seek help from the corresponding ISP to filter out the attacking packets, or take other counterattacks. PIT is especially useful for the victims in reflection based spoofing attacks, e.g., DNS amplification attacks. The victims can find the locations of the spoofers directly from the attacking traffic.
In this article, at first we illustrate the generation, types, collection, and the security issues of path backscatter messages in section III. Then in section IV, we present PIT, which tracks the location of the spoofers based on path backscatter messages together with the topology and routing information. We discuss how to apply PIT when both topology and routing are known, or only topology is known, or neither are known respectively. We also present two effective algorithms to apply PIT in large scale networks. In the following section, at first we show the statistical results on path backscatter messages. Then we evaluate the two key mechanisms of PIT which work without routing information. At last, we give the tracking result when applying PIT on the path backscatter message dataset: a number of ASes in which spoofers are found.
Our work has the following contributions:
1) This is the first article known which deeply investigates path backscatter messages. These messages are valuable to help understand spoofing activities. Though Moore et al. [8] has exploited backscatter messages, which are generated by the targets of spoofing messages, to study Denial of Services (DoS), path backscatter messages, which are sent by intermediate devices rather than the targets, have not been used in traceback. 2) A practical and effective IP traceback solution based on path backscatter messages, i.e., PIT, is proposed. PIT bypasses the deployment difficulties of existing IP traceback mechanisms and actually is already in force. Though given the limitation that path backscatter messages are not generated with stable possibility, PIT cannot work in all the attacks, but it does work in a number of spoofing activities. At least it may be the most useful traceback mechanism before an AS-level traceback system has been deployed in real. 3) Through applying PIT on the path backscatter dataset, a number of locations of spoofers are captured and presented. Though this is not a complete list, it is the first known list disclosing the locations of spoofers.