1.\" $OpenBSD: bpf.4,v 1.38 2016/04/28 19:07:19 natano Exp $ 2.\" $NetBSD: bpf.4,v 1.7 1995/09/27 18:31:50 thorpej Exp $ 3.\" 4.\" Copyright (c) 1990 The Regents of the University of California. 5.\" All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that: (1) source code distributions 9.\" retain the above copyright notice and this paragraph in its entirety, (2) 10.\" distributions including binary code include the above copyright notice and 11.\" this paragraph in its entirety in the documentation or other materials 12.\" provided with the distribution, and (3) all advertising materials mentioning 13.\" features or use of this software display the following acknowledgement: 14.\" ``This product includes software developed by the University of California, 15.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of 16.\" the University nor the names of its contributors may be used to endorse 17.\" or promote products derived from this software without specific prior 18.\" written permission. 19.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED 20.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF 21.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. 22.\" 23.\" This document is derived in part from the enet man page (enet.4) 24.\" distributed with 4.3BSD Unix. 25.\" 26.Dd $Mdocdate: April 28 2016 $ 27.Dt BPF 4 28.Os 29.Sh NAME 30.Nm bpf 31.Nd Berkeley Packet Filter 32.Sh SYNOPSIS 33.Cd "pseudo-device bpfilter" 34.Sh DESCRIPTION 35The Berkeley Packet Filter provides a raw interface to data link layers in 36a protocol-independent fashion. 37All packets on the network, even those destined for other hosts, are 38accessible through this mechanism. 39.Pp 40The packet filter appears as a character special device, 41.Pa /dev/bpf . 42After opening the device, the file descriptor must be bound to a specific 43network interface with the 44.Dv BIOCSETIF 45.Xr ioctl 2 . 46A given interface can be shared between multiple listeners, and the filter 47underlying each descriptor will see an identical packet stream. 48.Pp 49Associated with each open instance of a 50.Nm 51file is a user-settable 52packet filter. 53Whenever a packet is received by an interface, all file descriptors 54listening on that interface apply their filter. 55Each descriptor that accepts the packet receives its own copy. 56.Pp 57Reads from these files return the next group of packets that have matched 58the filter. 59To improve performance, the buffer passed to read must be the same size as 60the buffers used internally by 61.Nm bpf . 62This size is returned by the 63.Dv BIOCGBLEN 64.Xr ioctl 2 65and can be set with 66.Dv BIOCSBLEN . 67Note that an individual packet larger than this size is necessarily truncated. 68.Pp 69A packet can be sent out on the network by writing to a 70.Nm 71file descriptor. 72Each descriptor can also have a user-settable filter 73for controlling the writes. 74Only packets matching the filter are sent out of the interface. 75The writes are unbuffered, meaning only one packet can be processed per write. 76.Pp 77Once a descriptor is configured, further changes to the configuration 78can be prevented using the 79.Dv BIOCLOCK 80.Xr ioctl 2 . 81.Sh IOCTL INTERFACE 82The 83.Xr ioctl 2 84command codes below are defined in 85.In net/bpf.h . 86All commands require these includes: 87.Pp 88.nr nS 1 89.In sys/types.h 90.In sys/time.h 91.In sys/ioctl.h 92.In net/bpf.h 93.nr nS 0 94.Pp 95Additionally, 96.Dv BIOCGETIF 97and 98.Dv BIOCSETIF 99require 100.In sys/socket.h 101and 102.In net/if.h . 103.Pp 104The (third) argument to the 105.Xr ioctl 2 106call should be a pointer to the type indicated. 107.Pp 108.Bl -tag -width Ds -compact 109.It Dv BIOCGBLEN Fa "u_int *" 110Returns the required buffer length for reads on 111.Nm 112files. 113.Pp 114.It Dv BIOCSBLEN Fa "u_int *" 115Sets the buffer length for reads on 116.Nm 117files. 118The buffer must be set before the file is attached to an interface with 119.Dv BIOCSETIF . 120If the requested buffer size cannot be accommodated, the closest allowable 121size will be set and returned in the argument. 122A read call will result in 123.Er EINVAL 124if it is passed a buffer that is not this size. 125.Pp 126.It Dv BIOCGDLT Fa "u_int *" 127Returns the type of the data link layer underlying the attached interface. 128.Er EINVAL 129is returned if no interface has been specified. 130The device types, prefixed with 131.Dq DLT_ , 132are defined in 133.In net/bpf.h . 134.Pp 135.It Dv BIOCGDLTLIST Fa "struct bpf_dltlist *" 136Returns an array of the available types of the data link layer 137underlying the attached interface: 138.Bd -literal -offset indent 139struct bpf_dltlist { 140 u_int bfl_len; 141 u_int *bfl_list; 142}; 143.Ed 144.Pp 145The available types are returned in the array pointed to by the 146.Va bfl_list 147field while their length in 148.Vt u_int 149is supplied to the 150.Va bfl_len 151field. 152.Er ENOMEM 153is returned if there is not enough buffer space and 154.Er EFAULT 155is returned if a bad address is encountered. 156The 157.Va bfl_len 158field is modified on return to indicate the actual length in 159.Vt u_int 160of the array returned. 161If 162.Va bfl_list 163is 164.Dv NULL , 165the 166.Va bfl_len 167field is set to indicate the required length of the array in 168.Vt u_int . 169.Pp 170.It Dv BIOCSDLT Fa "u_int *" 171Changes the type of the data link layer underlying the attached interface. 172.Er EINVAL 173is returned if no interface has been specified or the specified 174type is not available for the interface. 175.Pp 176.It Dv BIOCPROMISC 177Forces the interface into promiscuous mode. 178All packets, not just those destined for the local host, are processed. 179Since more than one file can be listening on a given interface, a listener 180that opened its interface non-promiscuously may receive packets promiscuously. 181This problem can be remedied with an appropriate filter. 182.Pp 183The interface remains in promiscuous mode until all files listening 184promiscuously are closed. 185.Pp 186.It Dv BIOCFLUSH 187Flushes the buffer of incoming packets and resets the statistics that are 188returned by 189.Dv BIOCGSTATS . 190.Pp 191.It Dv BIOCLOCK 192This ioctl is designed to prevent the security issues associated 193with an open 194.Nm 195descriptor in unprivileged programs. 196Even with dropped privileges, an open 197.Nm 198descriptor can be abused by a rogue program to listen on any interface 199on the system, send packets on these interfaces if the descriptor was 200opened read-write and send signals to arbitrary processes using the 201signaling mechanism of 202.Nm bpf . 203By allowing only 204.Dq known safe 205ioctls, the 206.Dv BIOCLOCK 207ioctl prevents this abuse. 208The allowable ioctls are 209.Dv BIOCFLUSH , 210.Dv BIOCGBLEN , 211.Dv BIOCGDIRFILT , 212.Dv BIOCGDLT , 213.Dv BIOCGDIRFILT , 214.Dv BIOCGDLTLIST , 215.Dv BIOCGETIF , 216.Dv BIOCGHDRCMPLT , 217.Dv BIOCGRSIG , 218.Dv BIOCGRTIMEOUT , 219.Dv BIOCGSTATS , 220.Dv BIOCIMMEDIATE , 221.Dv BIOCLOCK , 222.Dv BIOCSRTIMEOUT , 223.Dv BIOCVERSION , 224.Dv TIOCGPGRP , 225and 226.Dv FIONREAD . 227Use of any other ioctl is denied with error 228.Er EPERM . 229Once a descriptor is locked, it is not possible to unlock it. 230A process with root privileges is not affected by the lock. 231.Pp 232A privileged program can open a 233.Nm 234device, drop privileges, set the interface, filters and modes on the 235descriptor, and lock it. 236Once the descriptor is locked, the system is safe 237from further abuse through the descriptor. 238Locking a descriptor does not prevent writes. 239If the application does not need to send packets through 240.Nm bpf , 241it can open the device read-only to prevent writing. 242If sending packets is necessary, a write-filter can be set before locking the 243descriptor to prevent arbitrary packets from being sent out. 244.Pp 245.It Dv BIOCGETIF Fa "struct ifreq *" 246Returns the name of the hardware interface that the file is listening on. 247The name is returned in the 248.Fa ifr_name 249field of the 250.Li struct ifreq . 251All other fields are undefined. 252.Pp 253.It Dv BIOCSETIF Fa "struct ifreq *" 254Sets the hardware interface associated with the file. 255This command must be performed before any packets can be read. 256The device is indicated by name using the 257.Fa ifr_name 258field of the 259.Li struct ifreq . 260Additionally, performs the actions of 261.Dv BIOCFLUSH . 262.Pp 263.It Dv BIOCSRTIMEOUT Fa "struct timeval *" 264.It Dv BIOCGRTIMEOUT Fa "struct timeval *" 265Sets or gets the read timeout parameter. 266The 267.Ar timeval 268specifies the length of time to wait before timing out on a read request. 269This parameter is initialized to zero by 270.Xr open 2 , 271indicating no timeout. 272.Pp 273.It Dv BIOCGSTATS Fa "struct bpf_stat *" 274Returns the following structure of packet statistics: 275.Bd -literal -offset indent 276struct bpf_stat { 277 u_int bs_recv; 278 u_int bs_drop; 279}; 280.Ed 281.Pp 282The fields are: 283.Bl -tag -width bs_recv 284.It Fa bs_recv 285Number of packets received by the descriptor since opened or reset (including 286any buffered since the last read call). 287.It Fa bs_drop 288Number of packets which were accepted by the filter but dropped by the kernel 289because of buffer overflows (i.e., the application's reads aren't keeping up 290with the packet traffic). 291.El 292.Pp 293.It Dv BIOCIMMEDIATE Fa "u_int *" 294Enables or disables 295.Dq immediate mode , 296based on the truth value of the argument. 297When immediate mode is enabled, reads return immediately upon packet reception. 298Otherwise, a read will block until either the kernel buffer becomes full or a 299timeout occurs. 300This is useful for programs like 301.Xr rarpd 8 , 302which must respond to messages in real time. 303The default for a new file is off. 304.Pp 305.It Dv BIOCSETF Fa "struct bpf_program *" 306Sets the filter program used by the kernel to discard uninteresting packets. 307An array of instructions and its length are passed in using the following 308structure: 309.Bd -literal -offset indent 310struct bpf_program { 311 u_int bf_len; 312 struct bpf_insn *bf_insns; 313}; 314.Ed 315.Pp 316The filter program is pointed to by the 317.Fa bf_insns 318field, while its length in units of 319.Li struct bpf_insn 320is given by the 321.Fa bf_len 322field. 323Also, the actions of 324.Dv BIOCFLUSH 325are performed. 326.Pp 327See section 328.Sx FILTER MACHINE 329for an explanation of the filter language. 330.Pp 331.It Dv BIOCSETWF Fa "struct bpf_program *" 332Sets the filter program used by the kernel to filter the packets 333written to the descriptor before the packets are sent out on the 334network. 335See 336.Dv BIOCSETF 337for a description of the filter program. 338This ioctl also acts as 339.Dv BIOCFLUSH . 340.Pp 341Note that the filter operates on the packet data written to the descriptor. 342If the 343.Dq header complete 344flag is not set, the kernel sets the link-layer source address 345of the packet after filtering. 346.Pp 347.It Dv BIOCVERSION Fa "struct bpf_version *" 348Returns the major and minor version numbers of the filter language currently 349recognized by the kernel. 350Before installing a filter, applications must check that the current version 351is compatible with the running kernel. 352Version numbers are compatible if the major numbers match and the application 353minor is less than or equal to the kernel minor. 354The kernel version number is returned in the following structure: 355.Bd -literal -offset indent 356struct bpf_version { 357 u_short bv_major; 358 u_short bv_minor; 359}; 360.Ed 361.Pp 362The current version numbers are given by 363.Dv BPF_MAJOR_VERSION 364and 365.Dv BPF_MINOR_VERSION 366from 367.In net/bpf.h . 368An incompatible filter may result in undefined behavior (most likely, an 369error returned by 370.Xr ioctl 2 371or haphazard packet matching). 372.Pp 373.It Dv BIOCSRSIG Fa "u_int *" 374.It Dv BIOCGRSIG Fa "u_int *" 375Sets or gets the receive signal. 376This signal will be sent to the process or process group specified by 377.Dv FIOSETOWN . 378It defaults to 379.Dv SIGIO . 380.Pp 381.It Dv BIOCSHDRCMPLT Fa "u_int *" 382.It Dv BIOCGHDRCMPLT Fa "u_int *" 383Sets or gets the status of the 384.Dq header complete 385flag. 386Set to zero if the link level source address should be filled in 387automatically by the interface output routine. 388Set to one if the link level source address will be written, 389as provided, to the wire. 390This flag is initialized to zero by default. 391.Pp 392.It Dv BIOCSFILDROP Fa "u_int *" 393.It Dv BIOCGFILDROP Fa "u_int *" 394Sets or gets the status of the 395.Dq filter drop 396flag. 397If non-zero, packets matching any filters will be reported to the 398associated interface so that they can be dropped. 399.Pp 400.It Dv BIOCSDIRFILT Fa "u_int *" 401.It Dv BIOCGDIRFILT Fa "u_int *" 402Sets or gets the status of the 403.Dq direction filter 404flag. 405If non-zero, packets matching the specified direction (either 406.Dv BPF_DIRECTION_IN 407or 408.Dv BPF_DIRECTION_OUT ) 409will be ignored. 410.El 411.Ss Standard ioctls 412.Nm 413now supports several standard ioctls which allow the user to do asynchronous 414and/or non-blocking I/O to an open 415.Nm 416file descriptor. 417.Pp 418.Bl -tag -width Ds -compact 419.It Dv FIONREAD Fa "int *" 420Returns the number of bytes that are immediately available for reading. 421.Pp 422.It Dv FIONBIO Fa "int *" 423Sets or clears non-blocking I/O. 424If the argument is non-zero, enable non-blocking I/O. 425If the argument is zero, disable non-blocking I/O. 426If non-blocking I/O is enabled, the return value of a read while no data 427is available will be 0. 428The non-blocking read behavior is different from performing non-blocking 429reads on other file descriptors, which will return \-1 and set 430.Va errno 431to 432.Er EAGAIN 433if no data is available. 434Note: setting this overrides the timeout set by 435.Dv BIOCSRTIMEOUT . 436.Pp 437.It Dv FIOASYNC Fa "int *" 438Enables or disables asynchronous I/O. 439When enabled (argument is non-zero), the process or process group specified 440by 441.Dv FIOSETOWN 442will start receiving 443.Dv SIGIO 444signals when packets arrive. 445Note that you must perform an 446.Dv FIOSETOWN 447command in order for this to take effect, as the system will not do it by 448default. 449The signal may be changed via 450.Dv BIOCSRSIG . 451.Pp 452.It Dv FIOSETOWN Fa "int *" 453.It Dv FIOGETOWN Fa "int *" 454Sets or gets the process or process group (if negative) that should receive 455.Dv SIGIO 456when packets are available. 457The signal may be changed using 458.Dv BIOCSRSIG 459(see above). 460.El 461.Ss BPF header 462The following structure is prepended to each packet returned by 463.Xr read 2 : 464.Bd -literal -offset indent 465struct bpf_hdr { 466 struct bpf_timeval bh_tstamp; 467 u_int32_t bh_caplen; 468 u_int32_t bh_datalen; 469 u_int16_t bh_hdrlen; 470}; 471.Ed 472.Pp 473The fields, stored in host order, are as follows: 474.Bl -tag -width Ds 475.It Fa bh_tstamp 476Time at which the packet was processed by the packet filter. 477.It Fa bh_caplen 478Length of the captured portion of the packet. 479This is the minimum of the truncation amount specified by the filter and the 480length of the packet. 481.It Fa bh_datalen 482Length of the packet off the wire. 483This value is independent of the truncation amount specified by the filter. 484.It Fa bh_hdrlen 485Length of the BPF header, which may not be equal to 486.Li sizeof(struct bpf_hdr) . 487.El 488.Pp 489The 490.Fa bh_hdrlen 491field exists to account for padding between the header and the link level 492protocol. 493The purpose here is to guarantee proper alignment of the packet data 494structures, which is required on alignment-sensitive architectures and 495improves performance on many other architectures. 496The packet filter ensures that the 497.Fa bpf_hdr 498and the network layer header will be word aligned. 499Suitable precautions must be taken when accessing the link layer protocol 500fields on alignment restricted machines. 501(This isn't a problem on an Ethernet, since the type field is a 502.Li short 503falling on an even offset, and the addresses are probably accessed in a 504bytewise fashion). 505.Pp 506Additionally, individual packets are padded so that each starts on a 507word boundary. 508This requires that an application has some knowledge of how to get from packet 509to packet. 510The macro 511.Dv BPF_WORDALIGN 512is defined in 513.In net/bpf.h 514to facilitate this process. 515It rounds up its argument to the nearest word aligned value (where a word is 516.Dv BPF_ALIGNMENT 517bytes wide). 518For example, if 519.Va p 520points to the start of a packet, this expression will advance it to the 521next packet: 522.Pp 523.Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen); 524.Pp 525For the alignment mechanisms to work properly, the buffer passed to 526.Xr read 2 527must itself be word aligned. 528.Xr malloc 3 529will always return an aligned buffer. 530.Ss Filter machine 531A filter program is an array of instructions with all branches forwardly 532directed, terminated by a 533.Dq return 534instruction. 535Each instruction performs some action on the pseudo-machine state, which 536consists of an accumulator, index register, scratch memory store, and 537implicit program counter. 538.Pp 539The following structure defines the instruction format: 540.Bd -literal -offset indent 541struct bpf_insn { 542 u_int16_t code; 543 u_char jt; 544 u_char jf; 545 u_int32_t k; 546}; 547.Ed 548.Pp 549The 550.Fa k 551field is used in different ways by different instructions, and the 552.Fa jt 553and 554.Fa jf 555fields are used as offsets by the branch instructions. 556The opcodes are encoded in a semi-hierarchical fashion. 557There are eight classes of instructions: 558.Dv BPF_LD , 559.Dv BPF_LDX , 560.Dv BPF_ST , 561.Dv BPF_STX , 562.Dv BPF_ALU , 563.Dv BPF_JMP , 564.Dv BPF_RET , 565and 566.Dv BPF_MISC . 567Various other mode and operator bits are logically OR'd into the class to 568give the actual instructions. 569The classes and modes are defined in 570.In net/bpf.h . 571Below are the semantics for each defined 572.Nm 573instruction. 574We use the convention that A is the accumulator, X is the index register, 575P[] packet data, and M[] scratch memory store. 576P[i:n] gives the data at byte offset 577.Dq i 578in the packet, interpreted as a word (n=4), unsigned halfword (n=2), or 579unsigned byte (n=1). 580M[i] gives the i'th word in the scratch memory store, which is only addressed 581in word units. 582The memory store is indexed from 0 to 583.Dv BPF_MEMWORDS Ns \-1 . 584.Fa k , 585.Fa jt , 586and 587.Fa jf 588are the corresponding fields in the instruction definition. 589.Dq len 590refers to the length of the packet. 591.Bl -tag -width Ds 592.It Dv BPF_LD 593These instructions copy a value into the accumulator. 594The type of the source operand is specified by an 595.Dq addressing mode 596and can be a constant 597.Pf ( Dv BPF_IMM ) , 598packet data at a fixed offset 599.Pf ( Dv BPF_ABS ) , 600packet data at a variable offset 601.Pf ( Dv BPF_IND ) , 602the packet length 603.Pf ( Dv BPF_LEN ) , 604or a word in the scratch memory store 605.Pf ( Dv BPF_MEM ) . 606For 607.Dv BPF_IND 608and 609.Dv BPF_ABS , 610the data size must be specified as a word 611.Pf ( Dv BPF_W ) , 612halfword 613.Pf ( Dv BPF_H ) , 614or byte 615.Pf ( Dv BPF_B ) . 616The semantics of all recognized 617.Dv BPF_LD 618instructions follow. 619.Pp 620.Bl -tag -width 32n -compact 621.Sm off 622.It Xo Dv BPF_LD No + Dv BPF_W No + 623.Dv BPF_ABS 624.Xc 625.Sm on 626A <- P[k:4] 627.Sm off 628.It Xo Dv BPF_LD No + Dv BPF_H No + 629.Dv BPF_ABS 630.Xc 631.Sm on 632A <- P[k:2] 633.Sm off 634.It Xo Dv BPF_LD No + Dv BPF_B No + 635.Dv BPF_ABS 636.Xc 637.Sm on 638A <- P[k:1] 639.Sm off 640.It Xo Dv BPF_LD No + Dv BPF_W No + 641.Dv BPF_IND 642.Xc 643.Sm on 644A <- P[X+k:4] 645.Sm off 646.It Xo Dv BPF_LD No + Dv BPF_H No + 647.Dv BPF_IND 648.Xc 649.Sm on 650A <- P[X+k:2] 651.Sm off 652.It Xo Dv BPF_LD No + Dv BPF_B No + 653.Dv BPF_IND 654.Xc 655.Sm on 656A <- P[X+k:1] 657.Sm off 658.It Xo Dv BPF_LD No + Dv BPF_W No + 659.Dv BPF_LEN 660.Xc 661.Sm on 662A <- len 663.Sm off 664.It Dv BPF_LD No + Dv BPF_IMM 665.Sm on 666A <- k 667.Sm off 668.It Dv BPF_LD No + Dv BPF_MEM 669.Sm on 670A <- M[k] 671.El 672.It Dv BPF_LDX 673These instructions load a value into the index register. 674Note that the addressing modes are more restricted than those of the 675accumulator loads, but they include 676.Dv BPF_MSH , 677a hack for efficiently loading the IP header length. 678.Pp 679.Bl -tag -width 32n -compact 680.Sm off 681.It Xo Dv BPF_LDX No + Dv BPF_W No + 682.Dv BPF_IMM 683.Xc 684.Sm on 685X <- k 686.Sm off 687.It Xo Dv BPF_LDX No + Dv BPF_W No + 688.Dv BPF_MEM 689.Xc 690.Sm on 691X <- M[k] 692.Sm off 693.It Xo Dv BPF_LDX No + Dv BPF_W No + 694.Dv BPF_LEN 695.Xc 696.Sm on 697X <- len 698.Sm off 699.It Xo Dv BPF_LDX No + Dv BPF_B No + 700.Dv BPF_MSH 701.Xc 702.Sm on 703X <- 4*(P[k:1]&0xf) 704.El 705.It Dv BPF_ST 706This instruction stores the accumulator into the scratch memory. 707We do not need an addressing mode since there is only one possibility for 708the destination. 709.Pp 710.Bl -tag -width 32n -compact 711.It Dv BPF_ST 712M[k] <- A 713.El 714.It Dv BPF_STX 715This instruction stores the index register in the scratch memory store. 716.Pp 717.Bl -tag -width 32n -compact 718.It Dv BPF_STX 719M[k] <- X 720.El 721.It Dv BPF_ALU 722The ALU instructions perform operations between the accumulator and index 723register or constant, and store the result back in the accumulator. 724For binary operations, a source mode is required 725.Pf ( Dv BPF_K 726or 727.Dv BPF_X ) . 728.Pp 729.Bl -tag -width 32n -compact 730.Sm off 731.It Xo Dv BPF_ALU No + BPF_ADD No + 732.Dv BPF_K 733.Xc 734.Sm on 735A <- A + k 736.Sm off 737.It Xo Dv BPF_ALU No + BPF_SUB No + 738.Dv BPF_K 739.Xc 740.Sm on 741A <- A - k 742.Sm off 743.It Xo Dv BPF_ALU No + BPF_MUL No + 744.Dv BPF_K 745.Xc 746.Sm on 747A <- A * k 748.Sm off 749.It Xo Dv BPF_ALU No + BPF_DIV No + 750.Dv BPF_K 751.Xc 752.Sm on 753A <- A / k 754.Sm off 755.It Xo Dv BPF_ALU No + BPF_AND No + 756.Dv BPF_K 757.Xc 758.Sm on 759A <- A & k 760.Sm off 761.It Xo Dv BPF_ALU No + BPF_OR No + 762.Dv BPF_K 763.Xc 764.Sm on 765A <- A | k 766.Sm off 767.It Xo Dv BPF_ALU No + BPF_LSH No + 768.Dv BPF_K 769.Xc 770.Sm on 771A <- A << k 772.Sm off 773.It Xo Dv BPF_ALU No + BPF_RSH No + 774.Dv BPF_K 775.Xc 776.Sm on 777A <- A >> k 778.Sm off 779.It Xo Dv BPF_ALU No + BPF_ADD No + 780.Dv BPF_X 781.Xc 782.Sm on 783A <- A + X 784.Sm off 785.It Xo Dv BPF_ALU No + BPF_SUB No + 786.Dv BPF_X 787.Xc 788.Sm on 789A <- A - X 790.Sm off 791.It Xo Dv BPF_ALU No + BPF_MUL No + 792.Dv BPF_X 793.Xc 794.Sm on 795A <- A * X 796.Sm off 797.It Xo Dv BPF_ALU No + BPF_DIV No + 798.Dv BPF_X 799.Xc 800.Sm on 801A <- A / X 802.Sm off 803.It Xo Dv BPF_ALU No + BPF_AND No + 804.Dv BPF_X 805.Xc 806.Sm on 807A <- A & X 808.Sm off 809.It Xo Dv BPF_ALU No + BPF_OR No + 810.Dv BPF_X 811.Xc 812.Sm on 813A <- A | X 814.Sm off 815.It Xo Dv BPF_ALU No + BPF_LSH No + 816.Dv BPF_X 817.Xc 818.Sm on 819A <- A << X 820.Sm off 821.It Xo Dv BPF_ALU No + BPF_RSH No + 822.Dv BPF_X 823.Xc 824.Sm on 825A <- A >> X 826.Sm off 827.It Dv BPF_ALU No + BPF_NEG 828.Sm on 829A <- -A 830.El 831.It Dv BPF_JMP 832The jump instructions alter flow of control. 833Conditional jumps compare the accumulator against a constant 834.Pf ( Dv BPF_K ) 835or the index register 836.Pf ( Dv BPF_X ) . 837If the result is true (or non-zero), the true branch is taken, otherwise the 838false branch is taken. 839Jump offsets are encoded in 8 bits so the longest jump is 256 instructions. 840However, the jump always 841.Pf ( Dv BPF_JA ) 842opcode uses the 32-bit 843.Fa k 844field as the offset, allowing arbitrarily distant destinations. 845All conditionals use unsigned comparison conventions. 846.Pp 847.Bl -tag -width 32n -compact 848.Sm off 849.It Dv BPF_JMP No + BPF_JA 850pc += k 851.Sm on 852.Sm off 853.It Xo Dv BPF_JMP No + BPF_JGT No + 854.Dv BPF_K 855.Xc 856.Sm on 857pc += (A > k) ? jt : jf 858.Sm off 859.It Xo Dv BPF_JMP No + BPF_JGE No + 860.Dv BPF_K 861.Xc 862.Sm on 863pc += (A >= k) ? jt : jf 864.Sm off 865.It Xo Dv BPF_JMP No + BPF_JEQ No + 866.Dv BPF_K 867.Xc 868.Sm on 869pc += (A == k) ? jt : jf 870.Sm off 871.It Xo Dv BPF_JMP No + BPF_JSET No + 872.Dv BPF_K 873.Xc 874.Sm on 875pc += (A & k) ? jt : jf 876.Sm off 877.It Xo Dv BPF_JMP No + BPF_JGT No + 878.Dv BPF_X 879.Xc 880.Sm on 881pc += (A > X) ? jt : jf 882.Sm off 883.It Xo Dv BPF_JMP No + BPF_JGE No + 884.Dv BPF_X 885.Xc 886.Sm on 887pc += (A >= X) ? jt : jf 888.Sm off 889.It Xo Dv BPF_JMP No + BPF_JEQ No + 890.Dv BPF_X 891.Xc 892.Sm on 893pc += (A == X) ? jt : jf 894.Sm off 895.It Xo Dv BPF_JMP No + BPF_JSET No + 896.Dv BPF_X 897.Xc 898.Sm on 899pc += (A & X) ? jt : jf 900.El 901.It Dv BPF_RET 902The return instructions terminate the filter program and specify the 903amount of packet to accept (i.e., they return the truncation amount) 904or, for the write filter, the maximum acceptable size for the packet 905(i.e., the packet is dropped if it is larger than the returned 906amount). 907A return value of zero indicates that the packet should be ignored/dropped. 908The return value is either a constant 909.Pf ( Dv BPF_K ) 910or the accumulator 911.Pf ( Dv BPF_A ) . 912.Pp 913.Bl -tag -width 32n -compact 914.It Dv BPF_RET No + Dv BPF_A 915Accept A bytes. 916.It Dv BPF_RET No + Dv BPF_K 917Accept k bytes. 918.El 919.It Dv BPF_MISC 920The miscellaneous category was created for anything that doesn't fit into 921the above classes, and for any new instructions that might need to be added. 922Currently, these are the register transfer instructions that copy the index 923register to the accumulator or vice versa. 924.Pp 925.Bl -tag -width 32n -compact 926.Sm off 927.It Dv BPF_MISC No + Dv BPF_TAX 928.Sm on 929X <- A 930.Sm off 931.It Dv BPF_MISC No + Dv BPF_TXA 932.Sm on 933A <- X 934.El 935.El 936.Pp 937The 938.Nm 939interface provides the following macros to facilitate array initializers: 940.Bd -filled -offset indent 941.Dv BPF_STMT ( Ns Ar opcode , 942.Ar operand ) 943.Pp 944.Dv BPF_JUMP ( Ns Ar opcode , 945.Ar operand , 946.Ar true_offset , 947.Ar false_offset ) 948.Ed 949.Sh FILES 950.Bl -tag -width /dev/bpf -compact 951.It Pa /dev/bpf 952.Nm 953device 954.El 955.Sh EXAMPLES 956The following filter is taken from the Reverse ARP daemon. 957It accepts only Reverse ARP requests. 958.Bd -literal -offset indent 959struct bpf_insn insns[] = { 960 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12), 961 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3), 962 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20), 963 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1), 964 BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) + 965 sizeof(struct ether_header)), 966 BPF_STMT(BPF_RET+BPF_K, 0), 967}; 968.Ed 969.Pp 970This filter accepts only IP packets between host 128.3.112.15 and 971128.3.112.35. 972.Bd -literal -offset indent 973struct bpf_insn insns[] = { 974 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12), 975 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8), 976 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26), 977 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2), 978 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30), 979 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4), 980 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3), 981 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30), 982 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1), 983 BPF_STMT(BPF_RET+BPF_K, (u_int)-1), 984 BPF_STMT(BPF_RET+BPF_K, 0), 985}; 986.Ed 987.Pp 988Finally, this filter returns only TCP finger packets. 989We must parse the IP header to reach the TCP header. 990The 991.Dv BPF_JSET 992instruction checks that the IP fragment offset is 0 so we are sure that we 993have a TCP header. 994.Bd -literal -offset indent 995struct bpf_insn insns[] = { 996 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12), 997 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10), 998 BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23), 999 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8), 1000 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20), 1001 BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0), 1002 BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14), 1003 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14), 1004 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0), 1005 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16), 1006 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1), 1007 BPF_STMT(BPF_RET+BPF_K, (u_int)-1), 1008 BPF_STMT(BPF_RET+BPF_K, 0), 1009}; 1010.Ed 1011.Sh SEE ALSO 1012.Xr ioctl 2 , 1013.Xr read 2 , 1014.Xr select 2 , 1015.Xr signal 3 , 1016.Xr MAKEDEV 8 , 1017.Xr tcpdump 8 1018.Rs 1019.%A McCanne, S. 1020.%A Jacobson, V. 1021.%D January 1993 1022.%J 1993 Winter USENIX Conference 1023.%T The BSD Packet Filter: A New Architecture for User-level Packet Capture 1024.Re 1025.Sh HISTORY 1026The Enet packet filter was created in 1980 by Mike Accetta and Rick Rashid 1027at Carnegie-Mellon University. 1028Jeffrey Mogul, at Stanford, ported the code to 1029.Bx 1030and continued its 1031development from 1983 on. 1032Since then, it has evolved into the Ultrix Packet Filter at DEC, a STREAMS 1033NIT module under SunOS 4.1, and BPF. 1034.Sh AUTHORS 1035.An -nosplit 1036.An Steve McCanne 1037of Lawrence Berkeley Laboratory implemented BPF in Summer 1990. 1038Much of the design is due to 1039.An Van Jacobson . 1040.Sh BUGS 1041The read buffer must be of a fixed size (returned by the 1042.Dv BIOCGBLEN 1043ioctl). 1044.Pp 1045A file that does not request promiscuous mode may receive promiscuously 1046received packets as a side effect of another file requesting this mode on 1047the same hardware interface. 1048This could be fixed in the kernel with additional processing overhead. 1049However, we favor the model where all files must assume that the interface 1050is promiscuous, and if so desired, must utilize a filter to reject foreign 1051packets. 1052