/etc/resolvconf/update-libc.d/sendmail is in sendmail-base 8.15.2-8.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | #!/bin/sh
# Make sendmail aware of changes to resolv.conf
# so that we can reset the statistics for unresolvable hostnames
# Bring in some script functions to handle dynamic configuration
. /usr/share/sendmail/dynamic;
# Note the interface that just changed - unfortunately, we know not how :(
update_resolv;
# If anything has been changed, update sendmail.cf and reload
update_sendmail;
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