Censys
Certificate | DNS | Dataset | IP | NetworkCensys performs regular scans for common protocols (e.g., DNS, HTTP(S), SSH). Provides a search for TLS certificates.
Access is free, but requires registration. The website no longer provides free bulk access. Bulk access requires a commercial or a research license. The free access is limited to 1000 API calls per day.
@InProceedings{censys15,
author = {Zakir Durumeric and David Adrian and Ariana Mirian and Michael Bailey and J. Alex Halderman},
title = {A Search Engine Backed by {I}nternet-Wide Scanning},
booktitle = {Proceedings of the 22nd {ACM} Conference on Computer and Communications Security},
month = oct,
year = 2015
}