The s6-dnsname program

s6-dnsname finds the name associated to an IPv4 or IPv6 address.

Interface

     s6-dnsname [ -4 | -6 ] [ -H | -h ] [ -r ] [ -t timeout ] ip

s6-dnsname converts the IP address ip to a name ending in in-addr.arpa. or ip6.arpa. then makes a PTR query for this name. It waits for the result and prints the obtained names, one per line, then exits 0.
If the domain exists but no relevant field has been found, it exits 1.
If the DNS answered but no answer is available, it prints a relevant error message and exits 2.
By default, s6-dnsname looks for DNS cache addresses in the /etc/resolv.conf file. If the DNSCACHEIP environment variable is set and contains a list of IP (v4 or v6) addresses, separated by commas, semicolons, spaces, tabs, newlines or carriage returns, then this list is used instead.

-4 : interpret ip as an IPv4 address.
-6 : interpret ip as an IPv6 address. If neither of the -4 and -6 is given, or if both are given, then ip will be interpreted as v4 or v6 depending on its syntax.
-H : do not use data from /etc/hosts. This is the default.
-h : use data from /etc/hosts, if available. If there's a compiled /etc/hosts.cdb file that is newer than /etc/hosts, it will be used instead. (See s6-dns-hosts-compile for details.) If the lookup in the hosts database returns at least one result, then no DNS lookup is performed.
-r : random. By default, the program does not sort the result, but prints them in the order received from the DNS. With this option, it performs a random permutation on the results before printing them.
-t timeout : if the resolution takes more than timeout milliseconds, then it exits 99 right away with an error message. By default, timeout is 0, which means no timeout.

If the underlying skalibs has been compiled with IPv6 support disabled, s6-dnsname will not be able to use IPv6 transport for its resolution, but it will still accept and resolve IPv6 addresses.