path: root/man/man8/tc-bpf.8

.TH "BPF classifier and actions in tc" 8 "18 May 2015" "iproute2" "Linux"
.SH NAME
BPF \- BPF programmable classifier and actions for ingress/egress
queueing disciplines
.SH SYNOPSIS
.SS eBPF classifier (filter) or action:
.B tc filter ... bpf
[
.B object-file
OBJ_FILE ] [
.B section
CLS_NAME ] [
.B export
UDS_FILE ] [
.B verbose
] [
.B police
POLICE_SPEC ] [
.B action
ACTION_SPEC ] [
.B classid
CLASSID ]
.br
.B tc action ... bpf
[
.B object-file
OBJ_FILE ] [
.B section
CLS_NAME ] [
.B export
UDS_FILE ] [
.B verbose
]

.SS cBPF classifier (filter) or action:
.B tc filter ... bpf
[
.B bytecode-file
BPF_FILE |
.B bytecode
BPF_BYTECODE ] [
.B police
POLICE_SPEC ] [
.B action
ACTION_SPEC ] [
.B classid
CLASSID ]
.br
.B tc action ... bpf
[
.B bytecode-file
BPF_FILE |
.B bytecode
BPF_BYTECODE ]

.SH DESCRIPTION

Extended Berkeley Packet Filter (
.B eBPF
) and classic Berkeley Packet Filter
(originally known as BPF, for better distinction referred to as
.B cBPF
here) are both available as a fully programmable and highly efficient
classifier and actions. They both offer a minimal instruction set for
implementing small programs which can safely be loaded into the kernel
and thus executed in a tiny virtual machine from kernel space. An in-kernel
verifier guarantees that a specified program always terminates and neither
crashes nor leaks data from the kernel.

In Linux, it's generally considered that eBPF is the successor of cBPF.
The kernel internally transforms cBPF expressions into eBPF expressions and
executes the latter. Execution of them can be performed in an interpreter
or at setup time, they can be just-in-time compiled (JIT'ed) to run as
native machine code. Currently, x86_64, ARM64 and s390 architectures have
eBPF JIT support, whereas PPC, SPARC, ARM and MIPS have cBPF, but did not
(yet) switch to eBPF JIT support.

eBPF's instruction set has similar underlying principles as the cBPF
instruction set, it however is modelled closer to the underlying
architecture to better mimic native instruction sets with the aim to
achieve a better run-time performance. It is designed to be JIT'ed with
a one to one mapping, which can also open up the possibility for compilers
to generate optimized eBPF code through an eBPF backend that performs
almost as fast as natively compiled code. Given that LLVM provides such
an eBPF backend, eBPF programs can therefore easily be programmed in a
subset of the C language. Other than that, eBPF infrastructure also comes
with a construct called "maps". eBPF maps are key/value stores that are
shared between multiple eBPF programs, but also between eBPF programs and
user space applications.

For the traffic control subsystem, classifier and actions that can be
attached to ingress and egress qdiscs can be written in eBPF or cBPF. The
advantage over other classifier and actions is that eBPF/cBPF provides the
generic framework, while users can implement their highly specialized use
cases efficiently. This means that the classifier or action written that
way will not suffer from feature bloat, and can therefore execute its task
highly efficient. It allows for non-linear classification and even merging
the action part into the classification. Combined with efficient eBPF map
data structures, user space can push new policies like classids into the
kernel without reloading a classifier, or it can gather statistics that
are pushed into one map and use another one for dynamically load balancing
traffic based on the determined load, just to provide a few examples.

.SH PARAMETERS
.SS object-file
points to an object file that has an executable and linkable format (ELF)
and contains eBPF opcodes and eBPF map definitions. The LLVM compiler
infrastructure with
.B clang(1)
as a C language front end is one project that supports emitting eBPF object
files that can be passed to the eBPF classifier (more details in the
.B EXAMPLES
section). This option is mandatory when an eBPF classifier or action is
to be loaded.

.SS section
is the name of the ELF section from the object file, where the eBPF
classifier or action resides. By default the section name for the
classifier is called "classifier", and for the action "action". Given
that a single object file can contain multiple classifier and actions,
the corresponding section name needs to be specified, if it differs
from the defaults.

.SS export
points to a Unix domain socket file. In case the eBPF object file also
contains a section named "maps" with eBPF map specifications, then the
map file descriptors can be handed off via the Unix domain socket to
an eBPF "agent" herding all descriptors after tc lifetime. This can be
some third party application implementing the IPC counterpart for the
import, that uses them for calling into
.B bpf(2)
system call to read out or update eBPF map data from user space, for
example, for monitoring purposes or to push down new policies.

.SS verbose
if set, it will dump the eBPF verifier output, even if loading the eBPF
program was successful. By default, only on error, the verifier log is
being emitted to the user.

.SS police
is an optional parameter for an eBPF/cBPF classifier that specifies a
police in
.B tc(1)
which is attached to the classifier, for example, on an ingress qdisc.

.SS action
is an optional parameter for an eBPF/cBPF classifier that specifies a
subsequent action in
.B tc(1)
which is attached to a classifier.

.SS classid
.SS flowid
provides the default traffic control class identifier for this eBPF/cBPF
classifier. The default class identifier can also be overwritten by the
return code of the eBPF/cBPF program. A default return code of
.B -1
specifies the here provided default class identifier to be used. A return
code of the eBPF/cBPF program of 0 implies that no match took place, and
a return code other than these two will override the default classid. This
allows for efficient, non-linear classification with only a single eBPF/cBPF
program as opposed to having multiple individual programs for various class
identifiers which would need to reparse packet contents.

.SS bytecode
is being used for loading cBPF classifier and actions only. The cBPF bytecode
is directly passed as a text string in the form of
.B \'s,c t f k,c t f k,c t f k,...\'
, where
.B s
denotes the number of subsequent 4-tuples. One such 4-tuple consists of
.B c t f k
decimals, where
.B c
represents the cBPF opcode,
.B t
the jump true offset target,
.B f
the jump false offset target and
.B k
the immediate constant/literal. There are various tools that generate code
in this loadable format, for example,
.B bpf_asm
that ships with the Linux kernel source tree under
.B tools/net/
, so it is certainly not expected to hack this by hand. The
.B bytecode
or
.B bytecode-file
option is mandatory when a cBPF classifier or action is to be loaded.

.SS bytecode-file
also being used to load a cBPF classifier or action. It's effectively the
same as
.B bytecode
only that the cBPF bytecode is not passed directly via command line, but
rather resides in a text file.

.SH EXAMPLES
.SS eBPF TOOLING
A full blown example including eBPF agent code can be found inside the
iproute2 source package under:
.B examples/bpf/

As prerequisites, the kernel needs to have the eBPF system call namely
.B bpf(2)
enabled and ships with
.B cls_bpf
and
.B act_bpf
kernel modules for the traffic control subsystem. To enable eBPF/eBPF JIT
support, depending which of the two the given architecture supports:

.in +4n
.B echo 1 > /proc/sys/net/core/bpf_jit_enable
.in

A given restricted C file can be compiled via LLVM as:

.in +4n
.B clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj -o bpf.o
.in

The compiler invocation might still simplify in future, so for now,
it's quite handy to alias this construct in one way or another, for
example:
.in +4n
.nf
.sp
__bcc() {
        clang -O2 -emit-llvm -c $1 -o - | \\
        llc -march=bpf -filetype=obj -o "`basename $1 .c`.o"
}

alias bcc=__bcc
.fi
.in

A minimal, stand-alone unit, which matches on all traffic with the
default classid (return code of -1) looks like:

.in +4n
.nf
.sp
#include <linux/bpf.h>

#ifndef __section
# define __section(x)  __attribute__((section(x), used))
#endif

__section("classifier") int cls_main(struct __sk_buff *skb)
{
        return -1;
}

char __license[] __section("license") = "GPL";
.fi
.in

More examples can be found further below in subsection
.B eBPF PROGRAMMING
as focus here will be on tooling.

There can be various other sections, for example, also for actions.
Thus, an object file in eBPF can contain multiple entrance points.
Always a specific entrance point, however, must be specified when
configuring with tc. A license must be part of the restricted C code
and the license string syntax is the same as with Linux kernel modules.
The kernel reserves its right that some eBPF helper functions can be
restricted to GPL compatible licenses only, and thus may reject a program
from loading into the kernel when such a license mismatch occurs.

The resulting object file from the compilation can be inspected with
the usual set of tools that also operate on normal object files, for
example
.B objdump(1)
for inspecting ELF section headers:

.in +4n
.nf
.sp
objdump -h bpf.o
[...]
3 classifier    000007f8  0000000000000000  0000000000000000  00000040  2**3
                CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
4 action-mark   00000088  0000000000000000  0000000000000000  00000838  2**3
                CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
5 action-rand   00000098  0000000000000000  0000000000000000  000008c0  2**3
                CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
6 maps          00000030  0000000000000000  0000000000000000  00000958  2**2
                CONTENTS, ALLOC, LOAD, DATA
7 license       00000004  0000000000000000  0000000000000000  00000988  2**0
                CONTENTS, ALLOC, LOAD, DATA
[...]
.fi
.in

Adding an eBPF classifier from an object file that contains a classifier
in the default ELF section is trivial (note that instead of "object-file"
also shortcuts such as "obj" can be used):

.in +4n
.B bcc bpf.c
.br
.B tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1
.in

In case the classifier resides in ELF section "mycls", then that same
command needs to be invoked as:

.in +4n
.B tc filter add dev em1 parent 1: bpf obj bpf.o sec mycls flowid 1:1
.in

Dumping the classifier configuration will tell the location of the
classifier, in other words that it's from object file "bpf.o" under
section "mycls":

.in +4n
.B tc filter show dev em1
.br
.B filter parent 1: protocol all pref 49152 bpf
.br
.B filter parent 1: protocol all pref 49152 bpf handle 0x1 flowid 1:1 bpf.o:[mycls]
.in

The same program can also be installed on ingress qdisc side as opposed
to egress ...

.in +4n
.B tc qdisc add dev em1 handle ffff: ingress
.br
.B tc filter add dev em1 parent ffff: bpf obj bpf.o sec mycls flowid ffff:1
.in

\&... and again dumped from there:

.in +4n
.B tc filter show dev em1 parent ffff:
.br
.B filter protocol all pref 49152 bpf
.br
.B filter protocol all pref 49152 bpf handle 0x1 flowid ffff:1 bpf.o:[mycls]
.in

Attaching a classifier and action on ingress has the restriction that
it doesn't have an actual underlying queueing discipline. What ingress
can do is to classify, mangle, redirect or drop packets. When queueing
is required on ingress side, then ingress must redirect packets to the
.B ifb
device, otherwise policing can be used. Moreover, ingress can be used to
have an early drop point of unwanted packets before they hit upper layers
of the networking stack, perform network accounting with eBPF maps that
could be shared with egress, or have an early mangle and/or redirection
point to different networking devices.

Multiple eBPF actions and classifier can be placed into a single
object file within various sections. In that case, non-default section
names must be provided, which is the case for both actions in this
example:

.in +4n
.B tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1 \e
.br
.in +25n
.B                          action bpf obj bpf.o sec action-mark \e
.br
.B                          action bpf obj bpf.o sec action-rand ok
.in -25n
.in -4n

The advantage of this is that the classifier and the two actions can
then share eBPF maps with each other, if implemented in the programs.

In order to access eBPF maps from user space beyond
.B tc(8)
setup lifetime, the ownership can be transferred to an eBPF agent via
Unix domain sockets. There are two possibilities for implementing this:

.B 1)
implementation of an own eBPF agent that takes care of setting up
the Unix domain socket and implementing the protocol that
.B tc(8)
dictates. A code example of this can be found inside the iproute2
source package under:
.B examples/bpf/

.B 2)
use
.B tc exec
for transferring the eBPF map file descriptors through a Unix domain
socket, and spawning an application such as
.B sh(1)
\&. This approach's advantage is that tc will place the file descriptors
into the environment and thus make them available just like stdin, stdout,
stderr file descriptors, meaning, in case user applications run from within
this fd-owner shell, they can terminate and restart without losing eBPF
maps file descriptors. Example invocation with the previous classifier and
action mixture:

.in +4n
.B tc exec bpf imp /tmp/bpf
.br
.B tc filter add dev em1 parent 1: bpf obj bpf.o exp /tmp/bpf flowid 1:1 \e
.br
.in +25n
.B                          action bpf obj bpf.o sec action-mark \e
.br
.B                          action bpf obj bpf.o sec action-rand ok
.in -25n
.in -4n

Assuming that eBPF maps are shared with classifier and actions, it's
enough to export them once, for example, from within the classifier
or action command. tc will setup all eBPF map file descriptors at the
time when the object file is first parsed.

When a shell has been spawned, the environment will have a couple of
eBPF related variables. BPF_NUM_MAPS provides the total number of maps
that have been transferred over the Unix domain socket. BPF_MAP<X>'s
value is the file descriptor number that can be accessed in eBPF agent
applications, in other words, it can directly be used as the file
descriptor value for the
.B bpf(2)
system call to retrieve or alter eBPF map values. <X> denotes the
identifier of the eBPF map. It corresponds to the
.B id
member of
.B struct bpf_elf_map
\& from the tc eBPF map specification.

The environment in this example looks as follows:

.in +4n
.nf
.sp
sh# env | grep BPF
    BPF_NUM_MAPS=3
    BPF_MAP1=6
    BPF_MAP0=5
    BPF_MAP2=7
sh# ls -la /proc/self/fd
    [...]
    lrwx------. 1 root root 64 Apr 14 16:46 5 -> anon_inode:bpf-map
    lrwx------. 1 root root 64 Apr 14 16:46 6 -> anon_inode:bpf-map
    lrwx------. 1 root root 64 Apr 14 16:46 7 -> anon_inode:bpf-map
sh# my_bpf_agent
.fi
.in

eBPF agents are very useful in that they can prepopulate eBPF maps from
user space, monitor statistics via maps and based on that feedback, for
example, rewrite classids in eBPF map values during runtime. Given that eBPF
agents are implemented as normal applications, they can also dynamically
receive traffic control policies from external controllers and thus push
them down into eBPF maps to dynamically adapt to network conditions. Moreover,
eBPF maps can also be shared with other eBPF program types (e.g. tracing),
thus very powerful combination can therefore be implemented.

.SS eBPF PROGRAMMING

eBPF classifier and actions are being implemented in restricted C syntax
(in future, there could additionally be new language frontends supported).

The header file
.B linux/bpf.h
provides eBPF helper functions that can be called from an eBPF program.
This man page will only provide two minimal, stand-alone examples, have a
look at
.B examples/bpf
from the iproute2 source package for a fully fledged flow dissector
example to better demonstrate some of the possibilities with eBPF.

Supported 32 bit classifier return codes from the C program and their meanings:
.in +4n
.B 0
, denotes a mismatch
.br
.B -1
, denotes the default classid configured from the command line
.br
.B else
, everything else will override the default classid to provide a facility for
non-linear matching
.in

Supported 32 bit action return codes from the C program and their meanings (
.B linux/pkt_cls.h
):
.in +4n
.B TC_ACT_OK (0)
, will terminate the packet processing pipeline and allows the packet to
proceed
.br
.B TC_ACT_SHOT (2)
, will terminate the packet processing pipeline and drops the packet
.br
.B TC_ACT_UNSPEC (-1)
, will use the default action configured from tc (similarly as returning
.B -1
from a classifier)
.br
.B TC_ACT_PIPE (3)
, will iterate to the next action, if available
.br
.B TC_ACT_RECLASSIFY (1)
, will terminate the packet processing pipeline and start classification
from the beginning
.br
.B else
, everything else is an unspecified return code
.in

Both classifier and action return codes are supported in eBPF and cBPF
programs.

To demonstrate restricted C syntax, a minimal toy classifier example is
provided, which assumes that egress packets, for instance originating
from a container, have previously been marked in interval [0, 255]. The
program keeps statistics on different marks for user space and maps the
classid to the root qdisc with the marking itself as the minor handle:

.in +4n
.nf
.sp
#include <stdint.h>
#include <asm/types.h>

#include <linux/bpf.h>
#include <linux/pkt_sched.h>

#include "helpers.h"

struct tuple {
        long packets;
        long bytes;
};

#define BPF_MAP_ID_STATS        1 /* agent's map identifier */
#define BPF_MAX_MARK            256

struct bpf_elf_map __section("maps") map_stats = {
        .type           =       BPF_MAP_TYPE_ARRAY,
        .id             =       BPF_MAP_ID_STATS,
        .size_key       =       sizeof(uint32_t),
        .size_value     =       sizeof(struct tuple),
        .max_elem       =       BPF_MAX_MARK,
};

static inline void cls_update_stats(const struct __sk_buff *skb,
                                    uint32_t mark)
{
        struct tuple *tu;

        tu = bpf_map_lookup_elem(&map_stats, &mark);
        if (likely(tu)) {
                __sync_fetch_and_add(&tu->packets, 1);
                __sync_fetch_and_add(&tu->bytes, skb->len);
        }
}

__section("cls") int cls_main(struct __sk_buff *skb)
{
        uint32_t mark = skb->mark;

        if (unlikely(mark >= BPF_MAX_MARK))
                return 0;

        cls_update_stats(skb, mark);

        return TC_H_MAKE(TC_H_ROOT, mark);
}

char __license[] __section("license") = "GPL";
.fi
.in

Another small example is a port redirector which demuxes destination port
80 into the interval [8080, 8087] steered by RSS, that can then be attached
to ingress qdisc. The exercise of adding the egress counterpart and IPv6
support is left to the reader:

.in +4n
.nf
.sp
#include <asm/types.h>
#include <asm/byteorder.h>

#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/tcp.h>

#include "helpers.h"

static inline void set_tcp_dport(struct __sk_buff *skb, int nh_off,
                                 __u16 old_port, __u16 new_port)
{
        bpf_l4_csum_replace(skb, nh_off + offsetof(struct tcphdr, check),
                            old_port, new_port, sizeof(new_port));
        bpf_skb_store_bytes(skb, nh_off + offsetof(struct tcphdr, dest),
                            &new_port, sizeof(new_port), 0);
}

static inline int lb_do_ipv4(struct __sk_buff *skb, int nh_off)
{
        __u16 dport, dport_new = 8080, off;
        __u8 ip_proto, ip_vl;

        ip_proto = load_byte(skb, nh_off +
                             offsetof(struct iphdr, protocol));
        if (ip_proto != IPPROTO_TCP)
                return 0;

        ip_vl = load_byte(skb, nh_off);
        if (likely(ip_vl == 0x45))
                nh_off += sizeof(struct iphdr);
        else
                nh_off += (ip_vl & 0xF) << 2;

        dport = load_half(skb, nh_off + offsetof(struct tcphdr, dest));
        if (dport != 80)
                return 0;

        off = skb->queue_mapping & 7;
        set_tcp_dport(skb, nh_off - BPF_LL_OFF, __constant_htons(80),
                      __cpu_to_be16(dport_new + off));
        return -1;
}

__section("lb") int lb_main(struct __sk_buff *skb)
{
        int ret = 0, nh_off = BPF_LL_OFF + ETH_HLEN;

        if (likely(skb->protocol == __constant_htons(ETH_P_IP)))
                ret = lb_do_ipv4(skb, nh_off);

        return ret;
}

char __license[] __section("license") = "GPL";
.fi
.in

The related helper header file
.B helpers.h
in both examples was:

.in +4n
.nf
.sp
/* Misc helper macros. */
#define __section(x) __attribute__((section(x), used))
#define offsetof(x, y) __builtin_offsetof(x, y)
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

/* Used map structure */
struct bpf_elf_map {
    __u32 type;
    __u32 size_key;
    __u32 size_value;
    __u32 max_elem;
    __u32 id;
};

/* Some used BPF function calls. */
static int (*bpf_skb_store_bytes)(void *ctx, int off, void *from,
                                  int len, int flags) =
      (void *) BPF_FUNC_skb_store_bytes;
static int (*bpf_l4_csum_replace)(void *ctx, int off, int from,
                                  int to, int flags) =
      (void *) BPF_FUNC_l4_csum_replace;
static void *(*bpf_map_lookup_elem)(void *map, void *key) =
      (void *) BPF_FUNC_map_lookup_elem;

/* Some used BPF intrinsics. */
unsigned long long load_byte(void *skb, unsigned long long off)
    asm ("llvm.bpf.load.byte");
unsigned long long load_half(void *skb, unsigned long long off)
    asm ("llvm.bpf.load.half");
.fi
.in

Best practice, we recommend to only have a single eBPF classifier loaded
in tc and perform
.B all
necessary matching and mangling from there instead of a list of individual
classifier and separate actions. Just a single classifier tailored for a
given use-case will be most efficient to run.

.SS eBPF DEBUGGING

Both tc
.B filter
and
.B action
commands for
.B bpf
support an optional
.B verbose
parameter that can be used to inspect the eBPF verifier log. It is dumped
by default in case of an error.

In case the eBPF/cBPF JIT compiler has been enabled, it can also be
instructed to emit a debug output of the resulting opcode image into
the kernel log, which can be read via
.B dmesg(1)
:

.in +4n
.B echo 2 > /proc/sys/net/core/bpf_jit_enable
.in

The Linux kernel source tree ships additionally under
.B tools/net/
a small helper called
.B bpf_jit_disasm
that reads out the opcode image dump from the kernel log and dumps the
resulting disassembly:

.in +4n
.B bpf_jit_disasm -o
.in

Other than that, the Linux kernel also contains an extensive eBPF/cBPF
test suite module called
.B test_bpf
\&. Upon ...

.in +4n
.B modprobe test_bpf
.in

\&... it performs a diversity of test cases and dumps the results into
the kernel log that can be inspected with
.B dmesg(1)
\&. The results can differ depending on whether the JIT compiler is enabled
or not. In case of failed test cases, the module will fail to load. In
such cases, we urge you to file a bug report to the related JIT authors,
Linux kernel and networking mailing lists.

.SS cBPF

Although we generally recommend switching to implementing
.B eBPF
classifier and actions, for the sake of completeness, a few words on how to
program in cBPF will be lost here.

Likewise, the
.B bpf_jit_enable
switch can be enabled as mentioned already. Tooling such as
.B bpf_jit_disasm
is also independent whether eBPF or cBPF code is being loaded.

Unlike in eBPF, classifier and action are not implemented in restricted C,
but rather in a minimal assembler-like language or with the help of other
tooling.

The raw interface with tc takes opcodes directly. For example, the most
minimal classifier matching on every packet resulting in the default
classid of 1:1 looks like:

.in +4n
.B tc filter add dev em1 parent 1: bpf bytecode '1,6 0 0 4294967295,' flowid 1:1
.in

The first decimal of the bytecode sequence denotes the number of subsequent
4-tuples of cBPF opcodes. As mentioned, such a 4-tuple consists of
.B c t f k
decimals, where
.B c
represents the cBPF opcode,
.B t
the jump true offset target,
.B f
the jump false offset target and
.B k
the immediate constant/literal. Here, this denotes an unconditional return
from the program with immediate value of -1.

Thus, for egress classification, Willem de Bruijn implemented a minimal stand-alone
helper tool under the GNU General Public License version 2 for
.B iptables(8)
BPF extension, which abuses the
.B libpcap
internal classic BPF compiler, his code derived here for usage with
.B tc(8)
:

.in +4n
.nf
.sp
#include <pcap.h>
#include <stdio.h>

int main(int argc, char **argv)
{
        struct bpf_program prog;
        struct bpf_insn *ins;
        int i, ret, dlt = DLT_RAW;

        if (argc < 2 || argc > 3)
                return 1;
        if (argc == 3) {
                dlt = pcap_datalink_name_to_val(argv[1]);
                if (dlt == -1)
                        return 1;
        }

        ret = pcap_compile_nopcap(-1, dlt, &prog, argv[argc - 1],
                                  1, PCAP_NETMASK_UNKNOWN);
        if (ret)
                return 1;

        printf("%d,", prog.bf_len);
        ins = prog.bf_insns;

        for (i = 0; i < prog.bf_len - 1; ++ins, ++i)
                printf("%u %u %u %u,", ins->code,
                       ins->jt, ins->jf, ins->k);
        printf("%u %u %u %u",
               ins->code, ins->jt, ins->jf, ins->k);

        pcap_freecode(&prog);
        return 0;
}
.fi
.in

Given this small helper, any
.B tcpdump(8)
filter expression can be abused as a classifier where a match will
result in the default classid:

.in +4n
.B bpftool EN10MB 'tcp[tcpflags] & tcp-syn != 0' > /var/bpf/tcp-syn
.br
.B tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn flowid 1:1
.in

Basically, such a minimal generator is equivalent to:

.in +4n
.B tcpdump -iem1 -ddd 'tcp[tcpflags] & tcp-syn != 0' | tr '\\\\n' ',' > /var/bpf/tcp-syn
.in

Since
.B libpcap
does not support all Linux' specific cBPF extensions in its compiler, the
Linux kernel also ships under
.B tools/net/
a minimal BPF assembler called
.B bpf_asm
for providing full control. For detailed syntax and semantics on implementing
such programs by hand, see references under
.B FURTHER READING
\&.

Trivial toy example in
.B bpf_asm
for classifying IPv4/TCP packets, saved in a text file called
.B foobar
:

.in +4n
.nf
.sp
ldh [12]
jne #0x800, drop
ldb [23]
jneq #6, drop
ret #-1
drop: ret #0
.fi
.in

Similarly, such a classifier can be loaded as:

.in +4n
.B bpf_asm foobar > /var/bpf/tcp-syn
.br
.B tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn flowid 1:1
.in

For BPF classifiers, the Linux kernel provides additionally under
.B tools/net/
a small BPF debugger called
.B bpf_dbg
, which can be used to test a classifier against pcap files, single-step
or add various breakpoints into the classifier program and dump register
contents during runtime.

Implementing an action in classic BPF is rather limited in the sense that
packet mangling is not supported. Therefore, it's generally recommended to
make the switch to eBPF, whenever possible.

.SH FURTHER READING
Further and more technical details about the BPF architecture can be found
in the Linux kernel source tree under
.B Documentation/networking/filter.txt
\&.

Further details on eBPF
.B tc(8)
examples can be found in the iproute2 source
tree under
.B examples/bpf/
\&.

.SH SEE ALSO
.BR tc (8),
.BR tc-ematch (8)
.BR bpf (2)
.BR bpf (4)

.SH AUTHORS
Manpage written by Daniel Borkmann.

Please report corrections or improvements to the Linux kernel networking
mailing list:
.B <netdev@vger.kernel.org>