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 ] [ ex-
port 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 ] [ ex-
port 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
kernel and thus executed in a tiny virtual machine from kernel space.
An in-kernel verifier guarantees that a specified program always termi-
nates 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 in-
terpreter 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
instruction set, it however is modelled closer to the underlying archi-
tecture 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
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 pro-
vides 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 classifica-
tion. 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.
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
corresponding 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 clas-
sifiers.
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 over-
written by the return code of the eBPF/cBPF program. A default return
code of -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 in-
dividual 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 repre-
sents the cBPF opcode, t the jump true offset target, f the jump false
offset target 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 byte-
code-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, depend-
ing 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 ex-
ample:
__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 classi-
fier in the default ELF section is trivial (note that instead of "ob-
ject-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
classifier, 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 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:
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) dic-
tates. A code example of this can be found inside the iproute2 source
package under: 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
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 los-
ing 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 specifica-
tion.
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 dy-
namically 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 pro-
gram types (e.g. tracing), thus very powerful combination can therefore
be implemented.
eBPF PROGRAMMING
eBPF classifier and actions are being implemented in restricted C syn-
tax (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 counter-
part 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 al-
ready. 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
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 iptables(8) BPF extension, which abuses the libpcap internal clas-
sic 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 refer-
ences under 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 network-
ing mailing list: <netdev@vger.kernel.org>
iproute2 18 May 20BPF classifier and actions in tc(8)
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