bpf(2)
NAME
bpf - perform a command on an extended BPF map or program
SYNOPSIS
#include <linux/bpf.h> int bpf(int cmd, union bpf_attr *attr, unsigned int size);
DESCRIPTION
The bpf() system call performs a range of operations related to
extended Berkeley Packet Filters. Extended BPF (or eBPF) is similar to
the original ("classic") BPF (cBPF) used to filter network packets.
For both cBPF and eBPF programs, the kernel statically analyzes the
programs before loading them, in order to ensure that they cannot harm
the running system.
eBPF extends cBPF in multiple ways, including the ability to call a
fixed set of in-kernel helper functions (via the BPF_CALL opcode
extension provided by eBPF) and access shared data structures such as
eBPF maps.
Extended BPF Design/Architecture
eBPF maps are a generic data structure for storage of different data
types. Data types are generally treated as binary blobs, so a user
just specifies the size of the key and the size of the value at map-
creation time. In other words, a key/value for a given map can have an
arbitrary structure.
A user process can create multiple maps (with key/value-pairs being
opaque bytes of data) and access them via file descriptors. Different
eBPF programs can access the same maps in parallel. It's up to the
user process and eBPF program to decide what they store inside maps.
There's one special map type, called a program array. This type of map
stores file descriptors referring to other eBPF programs. When a
lookup in the map is performed, the program flow is redirected in-place
to the beginning of another eBPF program and does not return back to
the calling program. The level of nesting has a fixed limit of 32, so
that infinite loops cannot be crafted. At runtime, the program file
descriptors stored in the map can be modified, so program functionality
can be altered based on specific requirements. All programs referred
to in a program-array map must have been previously loaded into the
kernel via bpf(). If a map lookup fails, the current program continues
its execution. See BPF_MAP_TYPE_PROG_ARRAY below for further details.
Generally, eBPF programs are loaded by the user process and
automatically unloaded when the process exits. In some cases, for
example, tc-bpf(8), the program will continue to stay alive inside the
kernel even after the process that loaded the program exits. In that
case, the tc subsystem holds a reference to the eBPF program after the
file descriptor has been closed by the user-space program. Thus,
whether a specific program continues to live inside the kernel depends
on how it is further attached to a given kernel subsystem after it was
loaded via bpf().
Each eBPF program is a set of instructions that is safe to run until
its completion. An in-kernel verifier statically determines that the
eBPF program terminates and is safe to execute. During verification,
the kernel increments reference counts for each of the maps that the
eBPF program uses, so that the attached maps can't be removed until the
program is unloaded.
eBPF programs can be attached to different events. These events can be
the arrival of network packets, tracing events, classification events
by network queueing disciplines (for eBPF programs attached to a tc(8)
classifier), and other types that may be added in the future. A new
event triggers execution of the eBPF program, which may store
information about the event in eBPF maps. Beyond storing data, eBPF
programs may call a fixed set of in-kernel helper functions.
The same eBPF program can be attached to multiple events and different
eBPF programs can access the same map:
tracing tracing tracing packet packet packet
event A event B event C on eth0 on eth1 on eth2
| | | | | ^
| | | | v |
--> tracing <-- tracing socket tc ingress tc egress
prog_1 prog_2 prog_3 classifier action
| | | | prog_4 prog_5
|--- -----| |------| map_3 | |
map_1 map_2 --| map_4 |--
Arguments
The operation to be performed by the bpf() system call is determined by
the cmd argument. Each operation takes an accompanying argument,
provided via attr, which is a pointer to a union of type bpf_attr (see
below). The size argument is the size of the union pointed to by attr.
The value provided in cmd is one of the following:
BPF_MAP_CREATE
Create a map and return a file descriptor that refers to the
map. The close-on-exec file descriptor flag (see fcntl(2)) is
automatically enabled for the new file descriptor.
BPF_MAP_LOOKUP_ELEM
Look up an element by key in a specified map and return its
value.
BPF_MAP_UPDATE_ELEM
Create or update an element (key/value pair) in a specified map.
BPF_MAP_DELETE_ELEM
Look up and delete an element by key in a specified map.
BPF_MAP_GET_NEXT_KEY
Look up an element by key in a specified map and return the key
of the next element.
BPF_PROG_LOAD
Verify and load an eBPF program, returning a new file descriptor
associated with the program. The close-on-exec file descriptor
flag (see fcntl(2)) is automatically enabled for the new file
descriptor.
The bpf_attr union consists of various anonymous structures that are
used by different bpf() commands:
union bpf_attr {
struct { /* Used by BPF_MAP_CREATE */
__u32 map_type;
__u32 key_size; /* size of key in bytes */
__u32 value_size; /* size of value in bytes */
__u32 max_entries; /* maximum number of entries
in a map */
};
struct { /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY
commands */
__u32 map_fd;
__aligned_u64 key;
union {
__aligned_u64 value;
__aligned_u64 next_key;
};
__u64 flags;
};
struct { /* Used by BPF_PROG_LOAD */
__u32 prog_type;
__u32 insn_cnt;
__aligned_u64 insns; /* 'const struct bpf_insn *' */
__aligned_u64 license; /* 'const char *' */
__u32 log_level; /* verbosity level of verifier */
__u32 log_size; /* size of user buffer */
__aligned_u64 log_buf; /* user supplied 'char *'
buffer */
__u32 kern_version;
/* checked when prog_type=kprobe
(since Linux 4.1) */
};
} __attribute__((aligned(8)));
eBPF maps
Maps are a generic data structure for storage of different types of
data. They allow sharing of data between eBPF kernel programs, and
also between kernel and user-space applications.
Each map type has the following attributes:
* type
* maximum number of elements
* key size in bytes
* value size in bytes
The following wrapper functions demonstrate how various bpf() commands
can be used to access the maps. The functions use the cmd argument to
invoke different operations.
BPF_MAP_CREATE
The BPF_MAP_CREATE command creates a new map, returning a new
file descriptor that refers to the map.
int
bpf_create_map(enum bpf_map_type map_type,
unsigned int key_size,
unsigned int value_size,
unsigned int max_entries)
{
union bpf_attr attr = {
.map_type = map_type,
.key_size = key_size,
.value_size = value_size,
.max_entries = max_entries
};
return bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
}
The new map has the type specified by map_type, and attributes
as specified in key_size, value_size, and max_entries. On
success, this operation returns a file descriptor. On error, -1
is returned and errno is set to EINVAL, EPERM, or ENOMEM.
The key_size and value_size attributes will be used by the
verifier during program loading to check that the program is
calling bpf_map_*_elem() helper functions with a correctly
initialized key and to check that the program doesn't access the
map element value beyond the specified value_size. For example,
when a map is created with a key_size of 8 and the eBPF program
calls
bpf_map_lookup_elem(map_fd, fp - 4)
the program will be rejected, since the in-kernel helper
function
bpf_map_lookup_elem(map_fd, void *key)
expects to read 8 bytes from the location pointed to by key, but
the fp - 4 (where fp is the top of the stack) starting address
will cause out-of-bounds stack access.
Similarly, when a map is created with a value_size of 1 and the
eBPF program contains
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
the program will be rejected, since it accesses the value
pointer beyond the specified 1 byte value_size limit.
Currently, the following values are supported for map_type:
enum bpf_map_type {
BPF_MAP_TYPE_UNSPEC, /* Reserve 0 as invalid map type */
BPF_MAP_TYPE_HASH,
BPF_MAP_TYPE_ARRAY,
BPF_MAP_TYPE_PROG_ARRAY,
};
map_type selects one of the available map implementations in the
kernel. For all map types, eBPF programs access maps with the
same bpf_map_lookup_elem() and bpf_map_update_elem() helper
functions. Further details of the various map types are given
below.
BPF_MAP_LOOKUP_ELEM
The BPF_MAP_LOOKUP_ELEM command looks up an element with a given
key in the map referred to by the file descriptor fd.
int
bpf_lookup_elem(int fd, const void *key, void *value)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
};
return bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}
If an element is found, the operation returns zero and stores
the element's value into value, which must point to a buffer of
value_size bytes.
If no element is found, the operation returns -1 and sets errno
to ENOENT.
BPF_MAP_UPDATE_ELEM
The BPF_MAP_UPDATE_ELEM command creates or updates an element
with a given key/value in the map referred to by the file
descriptor fd.
int
bpf_update_elem(int fd, const void *key, const void *value,
uint64_t flags)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
.flags = flags,
};
return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}
The flags argument should be specified as one of the following:
BPF_ANY
Create a new element or update an existing element.
BPF_NOEXIST
Create a new element only if it did not exist.
BPF_EXIST
Update an existing element.
On success, the operation returns zero. On error, -1 is
returned and errno is set to EINVAL, EPERM, ENOMEM, or E2BIG.
E2BIG indicates that the number of elements in the map reached
the max_entries limit specified at map creation time. EEXIST
will be returned if flags specifies BPF_NOEXIST and the element
with key already exists in the map. ENOENT will be returned if
flags specifies BPF_EXIST and the element with key doesn't exist
in the map.
BPF_MAP_DELETE_ELEM
The BPF_MAP_DELETE_ELEM command deleted the element whose key is
key from the map referred to by the file descriptor fd.
int
bpf_delete_elem(int fd, const void *key)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
};
return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
}
On success, zero is returned. If the element is not found, -1
is returned and errno is set to ENOENT.
BPF_MAP_GET_NEXT_KEY
The BPF_MAP_GET_NEXT_KEY command looks up an element by key in
the map referred to by the file descriptor fd and sets the
next_key pointer to the key of the next element.
int
bpf_get_next_key(int fd, const void *key, void *next_key)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.next_key = ptr_to_u64(next_key),
};
return bpf(BPF_MAP_GET_NEXT_KEY, &attr, sizeof(attr));
}
If key is found, the operation returns zero and sets the
next_key pointer to the key of the next element. If key is not
found, the operation returns zero and sets the next_key pointer
to the key of the first element. If key is the last element, -1
is returned and errno is set to ENOENT. Other possible errno
values are ENOMEM, EFAULT, EPERM, and EINVAL. This method can
be used to iterate over all elements in the map.
close(map_fd)
Delete the map referred to by the file descriptor map_fd. When
the user-space program that created a map exits, all maps will
be deleted automatically (but see NOTES).
eBPF map types
The following map types are supported:
BPF_MAP_TYPE_HASH
Hash-table maps have the following characteristics:
* Maps are created and destroyed by user-space programs. Both
user-space and eBPF programs can perform lookup, update, and
delete operations.
* The kernel takes care of allocating and freeing key/value
pairs.
* The map_update_elem() helper will fail to insert new element
when the max_entries limit is reached. (This ensures that
eBPF programs cannot exhaust memory.)
* map_update_elem() replaces existing elements atomically.
Hash-table maps are optimized for speed of lookup.
BPF_MAP_TYPE_ARRAY
Array maps have the following characteristics:
* Optimized for fastest possible lookup. In the future the
verifier/JIT compiler may recognize lookup() operations that
employ a constant key and optimize it into constant pointer.
It is possible to optimize a non-constant key into direct
pointer arithmetic as well, since pointers and value_size are
constant for the life of the eBPF program. In other words,
array_map_lookup_elem() may be 'inlined' by the verifier/JIT
compiler while preserving concurrent access to this map from
user space.
* All array elements pre-allocated and zero initialized at init
time
* The key is an array index, and must be exactly four bytes.
* map_delete_elem() fails with the error EINVAL, since elements
cannot be deleted.
* map_update_elem() replaces elements in a nonatomic fashion;
for atomic updates, a hash-table map should be used instead.
There is however one special case that can also be used with
arrays: the atomic built-in __sync_fetch_and_add() can be
used on 32 and 64 bit atomic counters. For example, it can
be applied on the whole value itself if it represents a
single counter, or in case of a structure containing multiple
counters, it could be used on individual counters. This is
quite often useful for aggregation and accounting of events.
Among the uses for array maps are the following:
* As "global" eBPF variables: an array of 1 element whose key
is (index) 0 and where the value is a collection of 'global'
variables which eBPF programs can use to keep state between
events.
* Aggregation of tracing events into a fixed set of buckets.
* Accounting of networking events, for example, number of
packets and packet sizes.
BPF_MAP_TYPE_PROG_ARRAY (since Linux 4.2)
A program array map is a special kind of array map whose map
values contain only file descriptors referring to other eBPF
programs. Thus, both the key_size and value_size must be
exactly four bytes. This map is used in conjunction with the
bpf_tail_call() helper.
This means that an eBPF program with a program array map
attached to it can call from kernel side into
void bpf_tail_call(void *context, void *prog_map, unsigned int index);
and therefore replace its own program flow with the one from the
program at the given program array slot, if present. This can
be regarded as kind of a jump table to a different eBPF program.
The invoked program will then reuse the same stack. When a jump
into the new program has been performed, it won't return to the
old program anymore.
If no eBPF program is found at the given index of the program
array (because the map slot doesn't contain a valid program file
descriptor, the specified lookup index/key is out of bounds, or
the limit of 32 nested calls has been exceed), execution
continues with the current eBPF program. This can be used as a
fall-through for default cases.
A program array map is useful, for example, in tracing or
networking, to handle individual system calls or protocols in
their own subprograms and use their identifiers as an individual
map index. This approach may result in performance benefits,
and also makes it possible to overcome the maximum instruction
limit of a single eBPF program. In dynamic environments, a
user-space daemon might atomically replace individual
subprograms at run-time with newer versions to alter overall
program behavior, for instance, if global policies change.
eBPF programs
The BPF_PROG_LOAD command is used to load an eBPF program into the
kernel. The return value for this command is a new file descriptor
associated with this eBPF program.
char bpf_log_buf[LOG_BUF_SIZE];
int
bpf_prog_load(enum bpf_prog_type type,
const struct bpf_insn *insns, int insn_cnt,
const char *license)
{
union bpf_attr attr = {
.prog_type = type,
.insns = ptr_to_u64(insns),
.insn_cnt = insn_cnt,
.license = ptr_to_u64(license),
.log_buf = ptr_to_u64(bpf_log_buf),
.log_size = LOG_BUF_SIZE,
.log_level = 1,
};
return bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
}
prog_type is one of the available program types:
enum bpf_prog_type {
BPF_PROG_TYPE_UNSPEC, /* Reserve 0 as invalid
program type */
BPF_PROG_TYPE_SOCKET_FILTER,
BPF_PROG_TYPE_KPROBE,
BPF_PROG_TYPE_SCHED_CLS,
BPF_PROG_TYPE_SCHED_ACT,
};
For further details of eBPF program types, see below.
The remaining fields of bpf_attr are set as follows:
* insns is an array of struct bpf_insn instructions.
* insn_cnt is the number of instructions in the program referred to by
insns.
* license is a license string, which must be GPL compatible to call
helper functions marked gpl_only. (The licensing rules are the same
as for kernel modules, so that also dual licenses, such as "Dual
BSD/GPL", may be used.)
* log_buf is a pointer to a caller-allocated buffer in which the in-
kernel verifier can store the verification log. This log is a
multi-line string that can be checked by the program author in order
to understand how the verifier came to the conclusion that the eBPF
program is unsafe. The format of the output can change at any time
as the verifier evolves.
* log_size size of the buffer pointed to by log_bug. If the size of
the buffer is not large enough to store all verifier messages, -1 is
returned and errno is set to ENOSPC.
* log_level verbosity level of the verifier. A value of zero means
that the verifier will not provide a log; in this case, log_buf must
be a NULL pointer, and log_size must be zero.
Applying close(2) to the file descriptor returned by BPF_PROG_LOAD will
unload the eBPF program (but see NOTES).
Maps are accessible from eBPF programs and are used to exchange data
between eBPF programs and between eBPF programs and user-space
programs. For example, eBPF programs can process various events (like
kprobe, packets) and store their data into a map, and user-space
programs can then fetch data from the map. Conversely, user-space
programs can use a map as a configuration mechanism, populating the map
with values checked by the eBPF program, which then modifies its
behavior on the fly according to those values.
eBPF program types
The eBPF program type (prog_type) determines the subset of kernel
helper functions that the program may call. The program type also
determines the program input (context)---the format of struct bpf_context
(which is the data blob passed into the eBPF program as the first
argument).
For example, a tracing program does not have the exact same subset of
helper functions as a socket filter program (though they may have some
helpers in common). Similarly, the input (context) for a tracing
program is a set of register values, while for a socket filter it is a
network packet.
The set of functions available to eBPF programs of a given type may
increase in the future.
The following program types are supported:
BPF_PROG_TYPE_SOCKET_FILTER (since Linux 3.19)
Currently, the set of functions for BPF_PROG_TYPE_SOCKET_FILTER
is:
bpf_map_lookup_elem(map_fd, void *key)
/* look up key in a map_fd */
bpf_map_update_elem(map_fd, void *key, void *value)
/* update key/value */
bpf_map_delete_elem(map_fd, void *key)
/* delete key in a map_fd */
The bpf_context argument is a pointer to a struct __sk_buff.
BPF_PROG_TYPE_KPROBE (since Linux 4.1)
[To be documented]
BPF_PROG_TYPE_SCHED_CLS (since Linux 4.1)
[To be documented]
BPF_PROG_TYPE_SCHED_ACT (since Linux 4.1)
[To be documented]
Events
Once a program is loaded, it can be attached to an event. Various
kernel subsystems have different ways to do so.
Since Linux 3.19, the following call will attach the program prog_fd to
the socket sockfd, which was created by an earlier call to socket(2):
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
&prog_fd, sizeof(prog_fd));
Since Linux 4.1, the following call may be used to attach the eBPF
program referred to by the file descriptor prog_fd to a perf event file
descriptor, event_fd, that was created by a previous call to
perf_event_open(2):
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
EXAMPLES
/* bpf+sockets example:
* 1. create array map of 256 elements
* 2. load program that counts number of packets received
* r0 = skb->data[ETH_HLEN + offsetof(struct iphdr, protocol)]
* map[r0]++
* 3. attach prog_fd to raw socket via setsockopt()
* 4. print number of received TCP/UDP packets every second
*/
int
main(int argc, char **argv)
{
int sock, map_fd, prog_fd, key;
long long value = 0, tcp_cnt, udp_cnt;
map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key),
sizeof(value), 256);
if (map_fd < 0) {
printf("failed to create map '%s'\n", strerror(errno));
/* likely not run as root */
return 1;
}
struct bpf_insn prog[] = {
BPF_MOV64_REG(BPF_REG_6, BPF_REG_1), /* r6 = r1 */
BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol)),
/* r0 = ip->proto */
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
/* *(u32 *)(fp - 4) = r0 */
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), /* r2 = fp */
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), /* r2 = r2 - 4 */
BPF_LD_MAP_FD(BPF_REG_1, map_fd), /* r1 = map_fd */
BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
/* r0 = map_lookup(r1, r2) */
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
/* if (r0 == 0) goto pc+2 */
BPF_MOV64_IMM(BPF_REG_1, 1), /* r1 = 1 */
BPF_XADD(BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0),
/* lock *(u64 *) r0 += r1 */
BPF_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
BPF_EXIT_INSN(), /* return r0 */
};
prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
sizeof(prog), "GPL");
sock = open_raw_sock("lo");
assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
sizeof(prog_fd)) == 0);
for (;;) {
key = IPPROTO_TCP;
assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
key = IPPROTO_UDP
assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
printf("TCP %lld UDP %lld packets0, tcp_cnt, udp_cnt);
sleep(1);
}
return 0;
}
Some complete working code can be found in the samples/bpf directory in
the kernel source tree.
RETURN VALUE
For a successful call, the return value depends on the operation:
BPF_MAP_CREATE
The new file descriptor associated with the eBPF map.
BPF_PROG_LOAD
The new file descriptor associated with the eBPF program.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
ERRORS
E2BIG The eBPF program is too large or a map reached the max_entries
limit (maximum number of elements).
EACCES For BPF_PROG_LOAD, even though all program instructions are
valid, the program has been rejected because it was deemed
unsafe. This may be because it may have accessed a disallowed
memory region or an uninitialized stack/register or because the
function constraints don't match the actual types or because
there was a misaligned memory access. In this case, it is
recommended to call bpf() again with log_level = 1 and examine
log_buf for the specific reason provided by the verifier.
EBADF fd is not an open file descriptor.
EFAULT One of the pointers (key or value or log_buf or insns) is
outside the accessible address space.
EINVAL The value specified in cmd is not recognized by this kernel.
EINVAL For BPF_MAP_CREATE, either map_type or attributes are invalid.
EINVAL For BPF_MAP_*_ELEM commands, some of the fields of union
bpf_attr that are not used by this command are not set to zero.
EINVAL For BPF_PROG_LOAD, indicates an attempt to load an invalid
program. eBPF programs can be deemed invalid due to
unrecognized instructions, the use of reserved fields, jumps out
of range, infinite loops or calls of unknown functions.
ENOENT For BPF_MAP_LOOKUP_ELEM or BPF_MAP_DELETE_ELEM, indicates that
the element with the given key was not found.
ENOMEM Cannot allocate sufficient memory.
EPERM The call was made without sufficient privilege (without the
CAP_SYS_ADMIN capability).
VERSIONS
The bpf() system call first appeared in Linux 3.18.
CONFORMING TO
The bpf() system call is Linux-specific.
NOTES
In the current implementation, all bpf() commands require the caller to have the CAP_SYS_ADMIN capability. eBPF objects (maps and programs) can be shared between processes. For example, after fork(2), the child inherits file descriptors referring to the same eBPF objects. In addition, file descriptors referring to eBPF objects can be transferred over UNIX domain sockets. File descriptors referring to eBPF objects can be duplicated in the usual way, using dup(2) and similar calls. An eBPF object is deallocated only after all file descriptors referring to the object have been closed. eBPF programs can be written in a restricted C that is compiled (using the clang compiler) into eBPF bytecode. Various features are omitted from this restricted C, such as loops, global variables, variadic functions, floating-point numbers, and passing structures as function arguments. Some examples can be found in the samples/bpf/*_kern.c files in the kernel source tree. The kernel contains a just-in-time (JIT) compiler that translates eBPF bytecode into native machine code for better performance. The JIT compiler is disabled by default, but its operation can be controlled by writing one of the following integer strings to the file /proc/sys/net/core/bpf_jit_enable: 0 Disable JIT compilation (default). 1 Normal compilation. 2 Debugging mode. The generated opcodes are dumped in hexadecimal into the kernel log. These opcodes can then be disassembled using the program tools/net/bpf_jit_disasm.c provided in the kernel source tree. JIT compiler for eBPF is currently available for the x86-64, arm64, and s390 architectures.
SEE ALSO
seccomp(2), socket(7), tc(8), tc-bpf(8) Both classic and extended BPF are explained in the kernel source file Documentation/networking/filter.txt.
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
