  Internet-wide scans are a common active measurement approach to study the
Internet, e.g., studying security properties or protocol adoption. They involve
probing large address ranges (IPv4 or parts of IPv6) for specific ports or
protocols. Besides their primary use for probing (e.g., studying protocol
adoption), we show that - at the same time - they provide valuable insights
into the Internet control plane informed by ICMP responses to these probes - a
currently unexplored secondary use. We collect one week of ICMP responses
(637.50M messages) to several Internet-wide ZMap scans covering multiple TCP
and UDP ports as well as DNS-based scans covering > 50% of the domain name
space. This perspective enables us to study the Internet's control plane as a
by-product of Internet measurements. We receive ICMP messages from ~171M
different IPs in roughly 53K different autonomous systems. Additionally, we
uncover multiple control plane problems, e.g., we detect a plethora of outdated
and misconfigured routers and uncover the presence of large-scale persistent
routing loops in IPv4.
