s6-dns: the s6-dnsname-filter program
Keywords: s6-dns client s6-dnsname-filter dnsname domain name ptr ip address filter

s6-dns
Software
skarnet.org

# The s6-dnsname-filter program

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

## Interface

         s6-dnsname-filter [ -4 ] [ -6 ] [ -l maxlines ] [ -c maxconn ] [ -t timeout ] [ -f normalfmt ] [ -e errorfmt ]

- s6-dnsname-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 an IP address is processed. Lines not starting with an IP address are not, and the formatting rules treat them as a pure "remainder".
- For every processed line, the read IP address is converted to an `in-addr.arpa.` or `ip6.arpa.` domain on which a PTR query is made, and the result is printed according to the formatting rules.
- By default, s6-dnsname-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

- `-4` : scan for IPv4 addresses.
- `-6` : scan for IPv6 addresses. Both the `-4` and `-6` options may be given. If none is given, by default s6-dnsname-filter will only scan for IPv4 addresses.
- `-l` maxlines : maximum lines. s6-dnsname-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-dnsname-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-dnsname-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 IP address, then an equal sign, then the corresponding domain name, 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 IP address, 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 IP address
- `%d` prints the domain name, if any
- `%e` prints the error message, if any
- `%w%r` prints the remainder of the input line

## Notes

s6-dnsname-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.