The s6-dnsip4-filter program

s6-dnsip4-filter reads domain names on its standard input and prints the corresponding IPv4 addresses on its standard output.

Interface

     s6-dnsip4-filter [ -l maxlines ] [ -c maxconn ] [ -t timeout ] [ -f normalfmt ] [ -e errorfmt ]

s6-dnsip4-filter reads lines from its stdin, processes them, and prints the processed lines to its stdout, in the same order. It exits 0 when it reads EOF on stdin.
Every line starting with a domain name is processed. Lines not starting with a domain name are not, and the formatting rules treat them as a pure "remainder".
For every processed line, an A query is made, and the result is printed according to the formatting rules. The domain is not qualified before being resolved.
By default, s6-dnsip4-filter 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.

Options

-l maxlines : maximum lines. s6-dnsip4-filter will keep at most maxlines lines in memory at the same time. If line n is still waiting for resolution and cannot be printed, then s6-dnsip4-filter will stop reading from stdin before line n+maxlines, until the result arrives for line n. By default, maxlines is 256.
-c maxconn : maximum concurrent resolutions. s6-dnsip4-filter will perform at most maxconn resolutions in parallel at the same time. maxlines cannot be lesser than maxconn. By default, maxconn is 128.
-t timeout : if any resolution takes more than timeout milliseconds, then it is aborted and printed as a timeout error. By default, timeout is 0, which means no timeout.
-f normalfmt : print the positive results according to the normalfmt format string. By default, normalfmt is %s=%d%w%r, which means: print the original domain name, then an equal sign, then the corresponding IPv4 address, then the remainder of the line.
-e errorfmt : print the negative results according to the errorfmt format string. By default, errorfmt is %s=<%e%>%w%r, which means: print the original domain name, then an equal sign, then the error message between angle brackets, then the remainder of the line.

Formatting rules

normalfmt and errorfmt are format strings, i.e. they tell the program how a line must be printed. The following sequences are recognized:

%% prints a single % character
%s prints the original domain name
%d prints the IP address, if any
%e prints the error message, if any
%w%r prints the remainder of the input line

Example

     s6-dnsns google.com | s6-dnsip4-filter -f "%d"

prints all the nameserver addresses for the google.com domain. This is useful, for instance, to give the result as an argument to s6-dnsq.

Notes

s6-dnsip4-filter does not perform DNS resolutions itself. Instead, it forks a skadnsd child and sends it queries, getting the results asynchronously. The s6-dns filter programs have actually been written as example uses of the skadns library.