--- name: "pfil_hook_get" section: "9freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/pfil/9freebsd" --- # pfil\_hook\_get(9freebsd) packet filter interface Part of [pfil(9freebsd)](/pfil/9freebsd.md). ## NAME **pfil**, **pfil\_head\_register**, **pfil\_head\_unregister**, **pfil\_head\_get**, **pfil\_add\_hook**, **pfil\_add\_hook\_flags**, **pfil\_remove\_hook**, **pfil\_remove\_hook\_flags**, **pfil\_run\_hooks**, **pfil\_rlock**, **pfil\_runlock**, **pfil\_wlock**, **pfil\_wunlock** – packet filter interface ## SYNOPSIS **#include \** **#include \** **#include \** **#include \** ``` typedef int (*pfil_func_t)(void *arg, struct mbuf **mp, struct ifnet *, int dir, struct inpcb); typedef int (*pfil_func_flags_t)(void *arg, struct mbuf **mp, struct ifnet *, int dir, int flags, struct inpcb); 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); int pfil_add_hook(pfil_func_t, void *arg, struct pfil_head *); int pfil_add_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *); int pfil_remove_hook(pfil_func_t, void *arg, struct pfil_head *); int pfil_remove_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *); int pfil_run_hooks(struct pfil_head *head, struct mbuf **mp, struct ifnet *, int dir, int flags, struct inpcb *); void pfil_rlock(struct pfil_head *, struct rm_priotracker *); void pfil_runlock(struct pfil_head *, struct rm_priotracker *); void pfil_wlock(struct pfil_head *); void pfil_wunlock(struct pfil_head *); ``` ## 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. Each filtering point uses common per-VNET rmlock by default. This can be changed by specifying *PFIL\_FLAG\_PRIVATE\_LOCK* as *flags* field in the *pfil\_head* structure. Note that specifying private lock can break filters sharing the same ruleset and/or state between different data link types. 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 *flags* argument will indicate if an outgoing packet is simply being forwarded with the value PFIL\_FWD. 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. Every filter hook is called with **pfil** read lock held. All heads uses the same lock within the same VNET instance. Packet filter can use this lock instead of own locking model to improve performance. Since **pfil** uses [rmlock(9)](/rmlock/9.md) **pfil\_rlock**() and **pfil\_runlock**() require *struct* *rm\_priotracker* to be passed as argument. Filter can acquire and release writer lock via **pfil\_wlock**() and **pfil\_wunlock**() functions. See [rmlock(9)](/rmlock/9.md) for more details. ## FILTERING POINTS Currently, filtering points are implemented for the following link types: - AF\_INET IPv4 packets. - AF\_INET6 IPv6 packets. - AF\_LINK Link-layer packets. ## 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), [if\_bridge(4)](/if_bridge/4.md), [rmlock(9)](/rmlock/9.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. **pfil** lock export was added in FreeBSD 10.0. ## BUGS 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.