BPF classifier and actions in tc(8) Linux BPF classifier and actions in tc(8)
NAME
BPF - BPF programmable classifier and actions for ingress/egress queue-
ing disciplines
SYNOPSIS
eBPF classifier (filter) or action:
tc filter ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [ export
UDS_FILE ] [ verbose ] [ direct-action | da ] [ skip_hw | skip_sw ] [
police POLICE_SPEC ] [ action ACTION_SPEC ] [ classid CLASSID ]
tc action ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [ export
UDS_FILE ] [ verbose ]
cBPF classifier (filter) or action:
tc filter ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ] [
police POLICE_SPEC ] [ action ACTION_SPEC ] [ classid CLASSID ]
tc action ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ]
DESCRIPTION
Extended Berkeley Packet Filter ( eBPF ) and classic Berkeley Packet
Filter (originally known as BPF, for better distinction referred to as
cBPF here) are both available as a fully programmable and highly effi-
cient classifier and actions. They both offer a minimal instruction set
for implementing small programs which can safely be loaded into the ker-
nel 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 inter-
preter or at setup time, they can be just-in-time compiled (JIT'ed) to
run as native machine code.
Currently, the eBPF JIT compiler is available for the following archi-
tectures:
* x86_64 (since Linux 3.18)
* arm64 (since Linux 3.18)
* s390 (since Linux 4.1)
* ppc64 (since Linux 4.8)
* sparc64 (since Linux 4.12)
* mips64 (since Linux 4.13)
* arm32 (since Linux 4.14)
* x86_32 (since Linux 4.18)
Whereas the following architectures have cBPF, but did not (yet) switch
to eBPF JIT support:
* ppc32
* sparc32
* mips32
eBPF's instruction set has similar underlying principles as the cBPF in-
struction set, it however is modelled closer to the underlying architec-
ture 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 al-
most as fast as natively compiled code. Given that LLVM provides such an
eBPF backend, eBPF programs can therefore easily be programmed in a sub-
set 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 pro-
vides the generic framework, while users can implement their highly spe-
cialized 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 classifica-
tion and even merging the action part into the classification. Combined
with efficient eBPF map data structures, user space can push new poli-
cies 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.
PARAMETERS
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 com-
piler infrastructure with 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 EXAMPLES section). This option
is mandatory when an eBPF classifier or action is to be loaded.
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 clas-
sifier is called "classifier", and for the action "action". Given that a
single object file can contain multiple classifier and actions, the cor-
responding section name needs to be specified, if it differs from the
defaults.
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 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.
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.
direct-action | da
instructs eBPF classifier to not invoke external TC actions, instead use
the TC actions return codes (TC_ACT_OK, TC_ACT_SHOT etc.) for classi-
fiers.
skip_hw | skip_sw
hardware offload control flags. By default TC will try to offload fil-
ters to hardware if possible. skip_hw explicitly disables the attempt
to offload. skip_sw forces the offload and disables running the eBPF
program in the kernel. If hardware offload is not possible and this
flag was set kernel will report an error and filter will not be in-
stalled at all.
police
is an optional parameter for an eBPF/cBPF classifier that specifies a
police in tc(1) which is attached to the classifier, for example, on an
ingress qdisc.
action
is an optional parameter for an eBPF/cBPF classifier that specifies a
subsequent action in tc(1) which is attached to a classifier.
classid
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 -1 speci-
fies 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.
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 's,c t f k,c
t f k,c t f k,...' , where s denotes the number of subsequent 4-tuples.
One such 4-tuple consists of c t f k decimals, where c represents the
cBPF opcode, t the jump true offset target, f the jump false offset tar-
get and k the immediate constant/literal. There are various tools that
generate code in this loadable format, for example, bpf_asm that ships
with the Linux kernel source tree under tools/net/ , so it is certainly
not expected to hack this by hand. The bytecode or bytecode-file option
is mandatory when a cBPF classifier or action is to be loaded.
bytecode-file
also being used to load a cBPF classifier or action. It's effectively
the same as bytecode only that the cBPF bytecode is not passed directly
via command line, but rather resides in a text file.
EXAMPLES
eBPF TOOLING
A full blown example including eBPF agent code can be found inside the
iproute2 source package under: examples/bpf/
As prerequisites, the kernel needs to have the eBPF system call namely
bpf(2) enabled and ships with cls_bpf and act_bpf kernel modules for the
traffic control subsystem. To enable eBPF/eBPF JIT support, depending
which of the two the given architecture supports:
echo 1 > /proc/sys/net/core/bpf_jit_enable
A given restricted C file can be compiled via LLVM as:
clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj -o
bpf.o
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:
__bcc() {
clang -O2 -emit-llvm -c $1 -o - | \
llc -march=bpf -filetype=obj -o "`basename $1 .c`.o"
}
alias bcc=__bcc
A minimal, stand-alone unit, which matches on all traffic with the de-
fault classid (return code of -1) looks like:
#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";
More examples can be found further below in subsection 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. Al-
ways a specific entrance point, however, must be specified when config-
uring 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 re-
stricted 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
objdump(1) for inspecting ELF section headers:
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
[...]
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):
bcc bpf.c
tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1
In case the classifier resides in ELF section "mycls", then that same
command needs to be invoked as:
tc filter add dev em1 parent 1: bpf obj bpf.o sec mycls flowid 1:1
Dumping the classifier configuration will tell the location of the clas-
sifier, in other words that it's from object file "bpf.o" under section
"mycls":
tc filter show dev em1
filter parent 1: protocol all pref 49152 bpf
filter parent 1: protocol all pref 49152 bpf handle 0x1 flowid 1:1
bpf.o:[mycls]
The same program can also be installed on ingress qdisc side as opposed
to egress ...
tc qdisc add dev em1 handle ffff: ingress
tc filter add dev em1 parent ffff: bpf obj bpf.o sec mycls flowid
ffff:1
... and again dumped from there:
tc filter show dev em1 parent ffff:
filter protocol all pref 49152 bpf
filter protocol all pref 49152 bpf handle 0x1 flowid ffff:1
bpf.o:[mycls]
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 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 lay-
ers of the networking stack, perform network accounting with eBPF maps
that could be shared with egress, or have an early mangle and/or redi-
rection 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:
tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1 \
action bpf obj bpf.o sec action-mark \
action bpf obj bpf.o sec action-rand ok
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 tc(8) setup life-
time, the ownership can be transferred to an eBPF agent via Unix domain
sockets. There are two possibilities for implementing this:
1) implementation of an own eBPF agent that takes care of setting up the
Unix domain socket and implementing the protocol that tc(8) dictates. A
code example of this can be found inside the iproute2 source package un-
der: examples/bpf/
2) use tc exec for transferring the eBPF map file descriptors through a
Unix domain socket, and spawning an application such as sh(1) . This ap-
proach'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:
tc exec bpf imp /tmp/bpf
tc filter add dev em1 parent 1: bpf obj bpf.o exp /tmp/bpf flowid
1:1 \
action bpf obj bpf.o sec action-mark \
action bpf obj bpf.o sec action-rand ok
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 de-
scriptor value for the bpf(2) system call to retrieve or alter eBPF map
values. <X> denotes the identifier of the eBPF map. It corresponds to
the id member of struct bpf_elf_map from the tc eBPF map specification.
The environment in this example looks as follows:
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
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 dynam-
ically receive traffic control policies from external controllers and
thus push them down into eBPF maps to dynamically adapt to network con-
ditions. Moreover, eBPF maps can also be shared with other eBPF program
types (e.g. tracing), thus very powerful combination can therefore be
implemented.
eBPF PROGRAMMING
eBPF classifier and actions are being implemented in restricted C syntax
(in future, there could additionally be new language frontends sup-
ported).
The header file linux/bpf.h provides eBPF helper functions that can be
called from an eBPF program. This man page will only provide two mini-
mal, stand-alone examples, have a look at 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:
0 , denotes a mismatch
-1 , denotes the default classid configured from the command line
else , everything else will override the default classid to provide
a facility for non-linear matching
Supported 32 bit action return codes from the C program and their mean-
ings ( linux/pkt_cls.h ):
TC_ACT_OK (0) , will terminate the packet processing pipeline and
allows the packet to proceed
TC_ACT_SHOT (2) , will terminate the packet processing pipeline and
drops the packet
TC_ACT_UNSPEC (-1) , will use the default action configured from tc
(similarly as returning -1 from a classifier)
TC_ACT_PIPE (3) , will iterate to the next action, if available
TC_ACT_RECLASSIFY (1) , will terminate the packet processing
pipeline and start classification from the beginning
else , everything else is an unspecified return code
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:
#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,
.pinning = PIN_GLOBAL_NS,
};
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";
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:
#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";
The related helper header file helpers.h in both examples was:
/* 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)
/* Object pinning settings */
#define PIN_NONE 0
#define PIN_OBJECT_NS 1
#define PIN_GLOBAL_NS 2
/* ELF map definition */
struct bpf_elf_map {
__u32 type;
__u32 size_key;
__u32 size_value;
__u32 max_elem;
__u32 flags;
__u32 id;
__u32 pinning;
__u32 inner_id;
__u32 inner_idx;
};
/* 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");
Best practice, we recommend to only have a single eBPF classifier loaded
in tc and perform 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.
eBPF DEBUGGING
Both tc filter and action commands for bpf support an optional 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 in-
structed to emit a debug output of the resulting opcode image into the
kernel log, which can be read via dmesg(1) :
echo 2 > /proc/sys/net/core/bpf_jit_enable
The Linux kernel source tree ships additionally under tools/net/ a small
helper called bpf_jit_disasm that reads out the opcode image dump from
the kernel log and dumps the resulting disassembly:
bpf_jit_disasm -o
Other than that, the Linux kernel also contains an extensive eBPF/cBPF
test suite module called test_bpf . Upon ...
modprobe test_bpf
... it performs a diversity of test cases and dumps the results into the
kernel log that can be inspected with 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.
cBPF
Although we generally recommend switching to implementing eBPF classi-
fier and actions, for the sake of completeness, a few words on how to
program in cBPF will be lost here.
Likewise, the bpf_jit_enable switch can be enabled as mentioned already.
Tooling such as 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:
tc filter add dev em1 parent 1: bpf bytecode '1,6 0 0 4294967295,'
flowid 1:1
The first decimal of the bytecode sequence denotes the number of subse-
quent 4-tuples of cBPF opcodes. As mentioned, such a 4-tuple consists of
c t f k decimals, where c represents the cBPF opcode, t the jump true
offset target, f the jump false offset target and k the immediate con-
stant/literal. Here, this denotes an unconditional return from the pro-
gram 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 iptables(8) BPF extension, which abuses the libpcap internal classic
BPF compiler, his code derived here for usage with tc(8) :
#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;
}
Given this small helper, any tcpdump(8) filter expression can be abused
as a classifier where a match will result in the default classid:
bpftool EN10MB 'tcp[tcpflags] & tcp-syn != 0' > /var/bpf/tcp-syn
tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn
flowid 1:1
Basically, such a minimal generator is equivalent to:
tcpdump -iem1 -ddd 'tcp[tcpflags] & tcp-syn != 0' | tr '\n' ',' >
/var/bpf/tcp-syn
Since libpcap does not support all Linux' specific cBPF extensions in
its compiler, the Linux kernel also ships under tools/net/ a minimal BPF
assembler called bpf_asm for providing full control. For detailed syntax
and semantics on implementing such programs by hand, see references un-
der FURTHER READING .
Trivial toy example in bpf_asm for classifying IPv4/TCP packets, saved
in a text file called foobar :
ldh [12]
jne #0x800, drop
ldb [23]
jneq #6, drop
ret #-1
drop: ret #0
Similarly, such a classifier can be loaded as:
bpf_asm foobar > /var/bpf/tcp-syn
tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn
flowid 1:1
For BPF classifiers, the Linux kernel provides additionally under
tools/net/ a small BPF debugger called bpf_dbg , which can be used to
test a classifier against pcap files, single-step or add various break-
points 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 recom-
mended to make the switch to eBPF, whenever possible.
FURTHER READING
Further and more technical details about the BPF architecture can be
found in the Linux kernel source tree under Documentation/network-
ing/filter.txt .
Further details on eBPF tc(8) examples can be found in the iproute2
source tree under examples/bpf/ .
SEE ALSO
tc(8), tc-ematch(8) bpf(2) bpf(4)
AUTHORS
Manpage written by Daniel Borkmann.
Please report corrections or improvements to the Linux kernel networking
mailing list: <netdev@vger.kernel.org>
iproute2 18 May 201BPF classifier and actions in tc(8)
Generated by dwww version 1.16 on Tue Dec 16 04:13:50 CET 2025.