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.

-c n

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

-l n

Read ahead at most n lines. Default: 1000.

http://cr.yp.to/djbdns.html