This paper concerns the problem of the absence of ingress filtering at the network edge, one of the main causes of important network security issues. Numerous network operators do not deploy the best current practice-Source Address Validation (SAV) that aims at mitigating these issues. We perform the first Internet-wide active measurement study to enumerate networks not filtering incoming packets by their source address. The measurement method consists of identifying closed and open DNS resolvers handling requests coming from the outside of the network with the source address from the range assigned inside the network under the test. The proposed method provides the most complete picture of the inbound SAV deployment state at network providers. We reveal that 32 673 Autonomous Systems (ASes) and 197 641 Border Gateway Protocol (BGP) prefixes are vulnerable to spoofing of inbound traffic. Finally, using the data from the Spoofer project and performing an open resolver scan, we compare the filtering policies in both directions.

This paper reports on the first Internet-wide active measurement study to enumerate networks not filtering incoming packets based on their source address. Our method identifies closed and open DNS resolvers handling requests from the outside of the network with the source address in the prefix of the tested network. The study gives the most complete picture of the inbound Source Address Validation deployment at network providers: 32,673 IPv4 ASes and 197,641 IPv4 BGP prefixes are vulnerable to spoofing of inbound traffic. CCS CONCEPTS • Networks → Network measurement; Security.

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DDoS attacks are one of the biggest threats to the modern Internet as their magnitude is constantly increasing. They are highly effective because of the amplification and reflection potential of different Internet protocols. In this paper, we show how a single DNS query triggers a response packet flood to the query source, possibly because of middleboxes located in networks with routing loops. We send DNS A requests to 3 billion routable IPv4 hosts and find 15,909 query destinations from 1,742 autonomous systems that trigger up to 46.7 million repeating responses. We perform traceroute measurements towards destination hosts that resulted in the highest amplification, locate 115 routing loops on the way, and notify corresponding network operators. Finally, we analyze two years of historical scan data and find that such "mega amplifiers" are prevalent. In the worst case, a single DNS A request triggered 655 million responses, all returned to a single host.