dnsfilter - reverse-resolves IP addresses, converting them to host names.

dnsfilter  reads  a series of lines from stdin, converts an IP address to a host name at the beginning of
       each line, and prints the results to stdout.

       If a line does not begin with an IP address, dnsfilter leaves the line alone.  If an IP address does  not
       have a host name listed in DNS, dnsfilter leaves the line alone.  If an IP address has a host name listed
       in DNS, dnsfilter inserts an equals sign and the host name before the first space or tab in the line.  If
       a  DNS  lookup fails temporarily, dnsfilter inserts a colon and a dash-separated error message before the
       first space or tab in the line.

       While dnsfilter is looking up an address in DNS, it reads ahead in the input and looks for more addresses
       to look up in parallel.

       dnsfilter - reverse-resolves IP addresses, converting them to host names.

opts is a series of getopt-style options:

       -cn   Do at most n DNS queries in parallel.  Default: 10.

       -ln   Read ahead at most n lines.  Default: 1000.

dnsip(1), dnsipq(1), dnsname(1), dnsmx(1), dnstxt(1), dnsqr(1), dnsq(1), dnstrace(1)

       http://cr.yp.to/djbdns.htmldnsfilter(1)

dnsfilter [ opts ]