---
name: "dnsfilter"
section: "1"
platform: "linux"
package: "djbdns-utils"
version: "1:1.05-22"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://carta.tech/man/dnsfilter"
---

# dnsfilter(1)

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

## NAME

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

## SYNOPSIS

**dnsfilter** \[ *opts* \]

## DESCRIPTION

**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.

## OPTIONS

*opts* is a series of getopt-style options:

- **-c *n***

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

- **-l *n***

  Read ahead at most  *n* lines. Default: 1000.

## SEE ALSO

[dnsip(1)](/man/dnsip/1.md), [dnsipq(1)](/man/dnsipq/1.md), [dnsname(1)](/man/dnsname/1.md), [dnsmx(1)](/man/dnsmx/1.md), [dnstxt(1)](/man/dnstxt/1.md), [dnsqr(1)](/man/dnsqr/1.md), [dnsq(1)](/man/dnsq/1.md), [dnstrace(1)](/man/dnstrace/1.md)

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