s6-dns: the s6-dnsip4-filter program
Keywords: s6-dns client s6-dnsip4-filter dnsip domain name ip address ipv4 filter

s6-dns
Software
skarnet.org

# 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.