---
name: "accf_dns"
section: "9"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/accf_dns"
---

# accf\_dns(9)

buffer incoming DNS requests until the whole first request is present

## NAME

**accf\_dns** – buffer incoming DNS requests until the whole first request is present

## SYNOPSIS

**options** **INET** **options** **ACCEPT\_FILTER\_DNS**

In [rc.conf(5)](/rc.conf/5.md): **kld\_list="accf\_dns"**

## DESCRIPTION

This is a filter to be placed on a socket that will be using **accept**() to receive incoming connections.

It prevents the application from receiving the connected descriptor via **accept**() until a whole DNS request is available on the socket. It does this by reading the first two bytes of the request, to determine its size, and waiting until the required amount of data is available to be read.

The *ACCEPT\_FILTER\_DNS* kernel option is also a module that can be enabled at runtime via [kldload(8)](/kldload/8.md) if the INET option has been compiled into the kernel.

## EXAMPLES

If the **accf\_dns** module is available in the kernel, the following code will enable the DNS accept filter on a socket *sok*.

```
	struct accept_filter_arg afa;
	bzero(&afa, sizeof(afa));
	strcpy(afa.af_name, "dnsready");
	setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));
```

## SEE ALSO

[setsockopt(2)](/setsockopt/2.md), [accept\_filter(9)](/accept_filter/9.md), [accf\_data(9)](/accf_data/9.md), [accf\_http(9)](/accf_http/9.md)

## HISTORY

The accept filter mechanism was introduced in FreeBSD 4.0.

## AUTHORS

This manual page and the filter were written by David Malone.