---
name: "pfil_add_hook"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/pfil_add_hook"
---

# pfil\_add\_hook(9)

packet filter interface

## NAME

**pfil**, **pfil\_head\_register**, **pfil\_head\_unregister**, **pfil\_head\_get**, **pfil\_hook\_get**, **pfil\_add\_hook**, **pfil\_remove\_hook**, **pfil\_run\_hooks** – packet filter interface

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/mbuf.h\>**

**#include \<net/if.h\>**

**#include \<net/pfil.h\>**

*int*\

**pfil\_head\_register**(*struct pfil\_head \*head*);

*int*\

**pfil\_head\_unregister**(*struct pfil\_head \*head*);

*struct pfil\_head \**\

**pfil\_head\_get**(*int af*, *u\_long dlt*);

*struct packet\_filter\_hook \**\

**pfil\_hook\_get**(*int dir*, *struct pfil\_head \*head*);

*void*\

**pfil\_add\_hook**(*int (\*func)()*, *void \*arg*, *int flags*, *struct pfil\_head \**);

*void*\

**pfil\_remove\_hook**(*int (\*func)()*, *void \*arg*, *int flags*, *struct pfil\_head \**);

*int*\

**(\*func)**(*void \*arg*, *struct mbuf \*\*mp*, *struct ifnet \**, *int dir*, *struct inpcb \**);

*int*\

**pfil\_run\_hooks**(*struct pfil\_head \*head*, *struct mbuf \*\*mp*, *struct ifnet \**, *int dir*, *struct inpcb \**);

## DESCRIPTION

The **pfil** framework allows for a specified function to be invoked for every incoming or outgoing packet for a particular network I/O stream. These hooks may be used to implement a firewall or perform packet transformations.

Packet filtering points are registered with **pfil\_head\_register**(). Filtering points are identified by a key (*void \**) and a data link type (*int*) in the *pfil\_head* structure. Packet filters use the key and data link type to look up the filtering point with which they register themselves. The key is unique to the filtering point. The data link type is a [bpf(4)](/bpf/4.md) DLT constant indicating what kind of header is present on the packet at the filtering point. Filtering points may be unregistered with the **pfil\_head\_unregister**() function.

Packet filters register/unregister themselves with a filtering point with the **pfil\_add\_hook**() and **pfil\_remove\_hook**() functions, respectively. The head is looked up using the **pfil\_head\_get**() function, which takes the key and data link type that the packet filter expects. Filters may provide an argument to be passed to the filter when invoked on a packet.

When a filter is invoked, the packet appears just as if it “came off the wire”. That is, all protocol fields are in network byte order. The filter is called with its specified argument, the pointer to the pointer to the *mbuf* containing the packet, the pointer to the network interface that the packet is traversing, and the direction (PFIL\_IN or PFIL\_OUT) that the packet is traveling. The filter may change which mbuf the *mbuf \*\** argument references. The filter returns an error (errno) if the packet processing is to stop, or 0 if the processing is to continue. If the packet processing is to stop, it is the responsibility of the filter to free the packet.

## RETURN VALUES

If successful, **pfil\_head\_get**() returns the *pfil\_head* structure for the given key/dlt. The **pfil\_add\_hook**() and **pfil\_remove\_hook**() functions return 0 if successful. If called with flag PFIL\_WAITOK, **pfil\_remove\_hook**() is expected to always succeed.

The **pfil\_head\_unregister**() function might sleep!

## SEE ALSO

[bpf(4)](/bpf/4.md), [bridge(4)](/bridge/4.md)

## HISTORY

The **pfil** interface first appeared in NetBSD 1.3. The **pfil** input and output lists were originally implemented as ⟨sys/queue.h⟩ LIST structures; however this was changed in NetBSD 1.4 to TAILQ structures. This change was to allow the input and output filters to be processed in reverse order, to allow the same path to be taken, in or out of the kernel.

The **pfil** interface was changed in 1.4T to accept a 3rd parameter to both **pfil\_add\_hook**() and **pfil\_remove\_hook**(), introducing the capability of per-protocol filtering. This was done primarily in order to support filtering of IPv6.

In 1.5K, the **pfil** framework was changed to work with an arbitrary number of filtering points, as well as be less IP-centric.

Fine-grained locking was added in FreeBSD 5.2.

## BUGS

The **pfil\_hook\_get**() function is only safe for internal use.

FreeBSD implements only hooks for AF\_INET and AF\_INET6. Packets diverted through these hooks have data in host byte order contrary to the above statements.

The [bridge(4)](/bridge/4.md) diverts inbound AF\_INET traffic, but contrary to the above statements, the data is provided in host byte order.

When a *pfil\_head* is being modified, no traffic is diverted (to avoid deadlock). This means that traffic may be dropped unconditionally for a short period of time. **pfil\_run\_hooks**() will return ENOBUFS to indicate this.