The s6-dns-hosts-compile program

s6-dns-hosts-compile compiles the system's /etc/hosts file into a CDB file for more efficient later use by s6-dns programs that may use the data contained in the hosts file.

Interface

     s6-dns-hosts-compile [ -i ifile ] [ -o ofile ]

s6-dns-hosts-compile reads the /etc/hosts file, which must follow the traditional hosts format.
It outputs a CDB database in the /etc/hosts.cdb file.
It exits 0.

Options

-i ifile : read from ifile instead of /etc/hosts.
-o ofile : write to ofile instead of /etc/hosts.cdb.

Notes

The /etc/hosts file is an old way of specifying IP from/to domain name correspondences. It's convenient in a pinch when a user needs to locally override DNS data - for instance, to redirect common advertising sites to 127.0.0.1 - but it does not map exactly to the DNS view of domain names.
Due to this, and to its line-by-line text syntax, lookups using this file are terribly inefficient and rely on approximate qualification heuristics.
To avoid that, s6-dns tools prefer having the /etc/hosts data in a format that's both more DNS-like and more efficient; the file output by s6-dns-hosts-compile is the result.
When asked to take /etc/hosts data into account, s6-dns tools will first look for a /etc/hosts.cdb file and use the data from it. If absent, they will read the /etc/hosts file and compile its contents into a temporary cdb file under /tmp (which they immediately unlink), and use that cdb to query hosts data.
As a consequence, lookups are always efficient, but there's an initial compilation step. Using s6-dns-hosts-compile to produce a /etc/hosts.cdb file in advance saves the cost of that compilation step in subsequent invocations of s6-dns tools.
If s6-dns tools detect that /etc/hosts has been modified more recently than /etc/hosts.cdb, they will not use /etc/hosts.cdb and will perform the compilation step into a temporary file. So, remember to run s6-dns-hosts-compile after modifying your /etc/hosts if you want to keep the tiny performance improvement.
Some tools write link-local addresses in /etc/hosts i.e. IPv6 addresses followed by a % (percent) and a network interface name. This is nonstandard and unusable with the DNS protocol, which does not take interface names into account. s6-dns-hosts-compile will ignore such lines if they appear in your hosts file.