PFIL(9) | Kernel Developer's Manual | PFIL(9)

# NAME

`pfil`, `pfil_head_register`, `pfil_head_unregister`, `pfil_head_get`, `pfil_hook_get`, `pfil_add_hook`, `pfil_remove_hook`, `pfil_run_hooks`, `pfil_add_ihook`, `pfil_remove_ihook`, `pfil_run_addrhooks`, `pfil_run_ifhooks` —
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 *ph);

int
`pfil_head_unregister`(struct pfil_head *ph);

struct pfil_head *
`pfil_head_get`(int af, u_long dlt);

struct packet_filter_hook *
`pfil_hook_get`(int dir, struct pfil_head *ph);

int
`pfil_add_hook`(int (*func)(), void *arg, int flags, struct pfil_head *ph);

int
`pfil_remove_hook`(int (*func)(), void *arg, int flags, struct pfil_head *ph);

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

int
`pfil_run_hooks`(struct pfil_head *ph, struct mbuf **mp, struct ifnet *ifp, int dir);

int
`pfil_add_ihook`(void (*ifunc)(), void *arg, int flags, struct pfil_head *ph);

int
`pfil_remove_ihook`(void (*ifunc)(), void *arg, int flags, struct pfil_head *ph);

void
`(*ifunc)`(void *arg, unsigned long cmd, void *ptr);

void
`pfil_run_addrhooks`(struct pfil_head *ph, unsigned long, struct ifaddr *ifa);

void
`pfil_run_ifhooks`(struct pfil_head *ph, unsigned long, struct ifnet *ifp);

# 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) `DLT_`type 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 (either `PFIL_IN` or `PFIL_OUT`, see also below) that the packet is traveling. The filter may change which mbuf the mbuf ** argument references. The filter returns an 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.

The flags parameter, used in the `pfil_add_hook`() and `pfil_remove_hook`() functions, indicates when the filter should be called. The flags are:

`PFIL_IN`
  call me on incoming packets
`PFIL_OUT`
  call me on outgoing packets
`PFIL_ALL`
  call me on all of the above

By the same token, event handlers register/unregister themselves with the `pfil_add_ihook`() and `pfil_remove_ihook`() functions, respectively. The event handler is called with its specified argument, the event id (either `PFIL_IFNET_ATTACH` or `PFIL_IFNET_DETACH`, see also below) or ioctl number, and the pointer to the network interface or the pointer to the ifaddr.

The flags parameter, used in the `pfil_add_ihook`() and `pfil_remove_ihook`() functions, indicates when the filter should be called. The flags are:

`PFIL_IFADDR`
  call me on interface reconfig (cmd is ioctl #)
`PFIL_IFNET`
  call me on interface attach/detach (cmd is either `PFIL_IFNET_ATTACH` or `PFIL_IFNET_DETACH`)

# SEE ALSO

bpf(4)

# 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.

`pfil_add_ihook`() and `pfil_remove_ihook`() were added in NetBSD 8.0.

# AUTHORS

The `pfil` interface was designed and implemented by Matthew R. Green, with help from Darren Reed, Jason R. Thorpe, and Charles M. Hannum. Darren Reed added support for IPv6 in addition to IPv4. Jason R. Thorpe added support for multiple hooks and other clean up.

# BUGS

The current `pfil` implementation will need changes to suit a threaded kernel model.

January 17, 2018 | NetBSD 9.2