  In recent years, DNS over Encrypted (DoE) methods have been regarded as a
novel trend within the realm of the DNS ecosystem. In these DoE methods, DNS
over HTTPS (DoH) provides encryption to protect data confidentiality while
providing better obfuscation to avoid censorship by multiplexing port 443 with
web services. This development introduced certain inconveniences in discovering
publicly available DoH services. In this paper, we propose the E-DoH method for
elegant and efficient DoH service detection. First, we optimized the probing
mechanism to enable a single DoH connection to accomplish multiple tasks
including service discovery, correctness validation and dependency
construction. Second, we propose an efficient DoH detection tool. This tool can
enhance probing efficiency while significantly reduce the required traffic
volume. Third, based on the above optimization methods, we conducted an
exploration of the IPv4 space and performed an in-depth analysis of DoH based
on the collected information. Through experiments, our approach demonstrates a
remarkable 80% improvement in time efficiency, and only requires 4%-20% traffic
volume to complete the detection task. In wild detection, our approach
discovered 46k DoH services, which nearly doubles the number discovered by the
state-of-the-art. Based on the collected data, we present several intriguing
conclusions about the current DoH service ecosystem.
