NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SYNTAX | SOURCE EXAMPLES | USAGE EXAMPLE | LEGAL | HISTORY | SEE ALSO | AUTHOR | COLOPHON | COLOPHON

BPFC(8)                      netsniff-ng toolkit                     BPFC(8)

NAME         top

       bpfc - a Berkeley Packet Filter assembler and compiler

SYNOPSIS         top

       bpfc { [options] | [source-file] }

DESCRIPTION         top

       bpfc is a small Berkeley Packet Filter assembler and compiler which
       is able to translate BPF assembler-like mnemonics into a numerical or
       C-like format, that can be read by tools such as netsniff-ng,
       iptables (xt_bpf) and many others. BPF is the one and only upstream
       filtering construct that is used in combination with packet(7)
       sockets, but also seccomp-BPF for system call sandboxing.

       The Linux kernel and also BSD kernels implement "virtual machine"
       like constructs and JIT compilers that mimic a small register-based
       machine in BPF architecture and execute filter code that is, for
       example, composed by bpfc on a data buffer that is given by network
       packets. The purpose of this is to shift computation in time, so that
       the kernel can drop or truncate incoming packets as early as possible
       without having to push them to user space for further analysis first.
       Meanwhile, BPF constructs also find application in other areas such
       as in the communication between user and kernel space like system
       call sand-boxing.

       At the time of writing this man page, the only other available BPF
       compiler is part of the pcap(3) library and accessible through a
       high-level filter language that might be familiar to many people as
       tcpdump-like filters.

       However, it is quite often useful to bypass that compiler and write
       optimized code that cannot be produced by the pcap(3) compiler, or is
       wrongly optimized, or is defective on purpose in order to debug test
       kernel code. Also, a reason to use bpfc could be to try out some new
       BPF extensions that are not supported by other compilers.
       Furthermore, bpfc can be useful to verify JIT compiler behavior or to
       find possible bugs that need to be fixed.

       bpfc is implemented with the help of flex(1) and bison(1), tokenizes
       the source file in the first stage and parses its content into an
       AST. In two code generation stages it emits target opcodes. bpfc
       furthermore supports Linux kernel BPF extensions. More about that can
       be found in the syntax section.

       The Linux kernel BPF JIT compiler is automatically turned on if
       detected by netsniff-ng. However, it can also be manually turned on
       through the command ''echo "1" > /proc/sys/net/core/bpf_jit_enable''
       (normal working mode) or ''echo "2" >
       /proc/sys/net/core/bpf_jit_enable'' (debug mode where emitted opcodes
       of the image are printed to the kernel log). An architecture agnostic
       BPF JIT image disassembler can be found in the kernel source tree
       under ''tools/net/bpf_jit_disasm.c'' or within the netsniff-ng Git
       repository.

OPTIONS         top

   -i <source-file/->, --input <source-file/->
       Read BPF assembly instruction from an input file or from stdin.

   -p, --cpp
       Pass the bpf program through the C preprocessor before reading it in
       bpfc. This allows #define and #include directives (e.g. to include
       definitions from system headers) to be used in the bpf program.

   -D <name>=<definition>, --define <name>=<definition>
       Add macro definition for the C preprocessor to use it within bpf
       file. This option is used in combination with the -p,--cpp option.

   -f <format>, --format <format>
       Specify a different output format than the default that is netsniff-
       ng compatible. The <format> specifier can be: C, netsniff-ng, xt_bpf,
       tcpdump.

   -b, --bypass
       Bypass basic filter validation when emitting opcodes. This can be
       useful for explicitly creating malformed BPF expressions for
       injecting into the kernel, for example, for bug testing.

   -V, --verbose
       Be more verbose and display some bpfc debugging information.

   -d, --dump
       Dump all supported instructions to stdout.

   -v, --version
       Show version information and exit.

   -h, --help
       Show user help and exit.

SYNTAX         top

       The BPF architecture resp. register machine consists of the following
       elements:

           Element          Description

           A                32 bit wide accumulator
           X                32 bit wide X register
           M[]              16 x 32 bit wide misc registers aka “scratch
       memory store”, addressable from 0 to 15

       A program, that is translated by bpfc into ''opcodes'' is an array
       that consists of the following elements:

           o:16, jt:8, jf:8, k:32

       The element o is a 16 bit wide opcode that has a particular
       instruction encoded, jt and jf are two 8 bit wide jump targets, one
       for condition
        ''true'', one for condition ''false''. Last but not least the 32 bit
       wide element k contains a miscellaneous argument that can be
       interpreted in different ways depending on the given instruction
       resp. opcode.

       The instruction set consists of load, store, branch, alu,
       miscellaneous and return instructions that are also represented in
       bpfc syntax. This table also includes bpfc's own extensions. All
       operations are based on unsigned data structures:

          Instruction      Addressing mode      Description

          ld               1, 2, 3, 4, 10       Load word into A
          ldi              4                    Load word into A
          ldh              1, 2                 Load half-word into A
          ldb              1, 2                 Load byte into A
          ldx              3, 4, 5, 10          Load word into X
          ldxi             4                    Load word into X
          ldxb             5                    Load byte into X

          st               3                    Copy A into M[]
          stx              3                    Copy X into M[]

          jmp              6                    Jump to label
          ja               6                    Jump to label
          jeq              7, 8                 Jump on k == A
          jneq             8                    Jump on k != A
          jne              8                    Jump on k != A
          jlt              8                    Jump on k < A
          jle              8                    Jump on k <= A
          jgt              7, 8                 Jump on k > A
          jge              7, 8                 Jump on k >= A
          jset             7, 8                 Jump on k & A

          add              0, 4                 A + <x>
          sub              0, 4                 A - <x>
          mul              0, 4                 A * <x>
          div              0, 4                 A / <x>
          mod              0, 4                 A % <x>
          neg              0, 4                 !A
          and              0, 4                 A & <x>
          or               0, 4                 A | <x>
          xor              0, 4                 A ^ <x>
          lsh              0, 4                 A << <x>
          rsh              0, 4                 A >> <x>

          tax                                   Copy A into X
          txa                                   Copy X into A

          ret              4, 9                 Return

          Addressing mode  Syntax               Description

           0               x/%x                 Register X
           1               [k]                  BHW at byte offset k in the
       packet
           2               [x + k]              BHW at the offset X + k in
       the packet
           3               M[k]                 Word at offset k in M[]
           4               #k                   Literal value stored in k
           5               4*([k]&0xf)          Lower nibble * 4 at byte
       offset k in the packet
           6               L                    Jump label L
           7               #k,Lt,Lf             Jump to Lt if true,
       otherwise jump to Lf
           8               #k,Lt                Jump to Lt if predicate is
       true
           9               a/%a                 Accumulator A
          10               extension            BPF extension (see next
       table)

          Extension (and alias)                 Description

          #len, len, #pktlen, pktlen            Length of packet (skb->len)
          #pto, pto, #proto, proto              Ethernet type field
       (skb->protocol)
          #type, type                           Packet type (**)
       (skb->pkt_type)
          #poff, poff                           Detected payload start
       offset
          #ifx, ifx, #ifidx, ifidx              Interface index
       (skb->dev->ifindex)
          #nla, nla                             Netlink attribute of type X
       with offset A
          #nlan, nlan                           Nested Netlink attribute of
       type X with offset A
          #mark, mark                           Packet mark (skb->mark)
          #que, que, #queue, queue, #Q, Q       NIC queue index
       (skb->queue_mapping)
          #hat, hat, #hatype, hatype            NIC hardware type (**)
       (skb->dev->type)
          #rxh, rxh, #rxhash, rxhash            Receive hash (skb->rxhash)
          #cpu, cpu                             Current CPU
       (raw_smp_processor_id())
          #vlant, vlant, #vlan_tci, vlan_tci    VLAN TCI value
       (vlan_tx_tag_get(skb))
          #vlanp, vlanp                         VLAN present
       (vlan_tx_tag_present(skb))

          Further extension details (**)        Value

          #type, type                           0 - to us / host
                                                1 - to all / broadcast
                                                2 - to group / multicast
                                                3 - to others (promiscuous
       mode)
                                                4 - outgoing of any type

          #hat, hat, #hatype, hatype            1 - Ethernet 10Mbps
                                                8 - APPLEtalk
                                               19 - ATM
                                               24 - IEEE 1394 IPv4 - RFC
       2734
                                               32 - InfiniBand
                                              768 - IPIP tunnel
                                              769 - IP6IP6 tunnel
                                              772 - Loopback device
                                              778 - GRE over IP
                                              783 - Linux-IrDA
                                              801 - IEEE 802.11
                                              802 - IEEE 802.11 + Prism2
       header
                                              803 - IEEE 802.11 + radiotap
       header
                                              823 - GRE over IP6
                                              824 - Netlink
                                              [...] See
       include/uapi/linux/if_arp.h

       Note that the majority of BPF extensions are available on Linux only.

       There are two types of comments in bpfc source-files:

         1. Multi-line C-style comments:        /* put comment here */
         2. Single-line ASM-style comments:     ;  put comment here

       Used Abbreviations:

         BHW: byte, half-word, or word

SOURCE EXAMPLES         top

       In this section, we give a couple of examples of bpfc source files,
       in other words, some small example filter programs:

   Only return packet headers (truncate packets):
         ld poff
         ret a

   Only allow ARP packets:
         ldh [12]
         jne #0x806, drop
         ret #-1
         drop: ret #0

   Only allow IPv4 TCP packets:
         ldh [12]
         jne #0x800, drop
         ldb [23]
         jneq #6, drop
         ret #-1
         drop: ret #0

   Only allow IPv4 TCP SSH traffic:
         ldh [12]
         jne #0x800, drop
         ldb [23]
         jneq #6, drop
         ldh [20]
         jset #0x1fff, drop
         ldxb 4 * ([14] & 0xf)
         ldh [x + 14]
         jeq #0x16, pass
         ldh [x + 16]
         jne #0x16, drop
         pass: ret #-1
         drop: ret #0

   A loadable x86_64 seccomp-BPF filter to allow a given set of syscalls:
         ld [4]                  /* offsetof(struct seccomp_data, arch) */
         jne #0xc000003e, bad    /* AUDIT_ARCH_X86_64 */
         ld [0]                  /* offsetof(struct seccomp_data, nr) */
         jeq #15, good           /* __NR_rt_sigreturn */
         jeq #231, good          /* __NR_exit_group */
         jeq #60, good           /* __NR_exit */
         jeq #0, good            /* __NR_read */
         jeq #1, good            /* __NR_write */
         jeq #5, good            /* __NR_fstat */
         jeq #9, good            /* __NR_mmap */
         jeq #14, good           /* __NR_rt_sigprocmask */
         jeq #13, good           /* __NR_rt_sigaction */
         jeq #35, good           /* __NR_nanosleep */
         bad: ret #0             /* SECCOMP_RET_KILL */
         good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */

   Allow any (hardware accelerated) VLAN:
         ld vlanp
         jeq #0, drop
         ret #-1
         drop: ret #0

   Only allow traffic for (hardware accelerated) VLAN 10:
         ld vlant
         jneq #10, drop
         ret #-1
         drop: ret #0

   More pedantic check for the above VLAN example:
         ld vlanp
         jeq #0, drop
         ld vlant
         jneq #10, drop
         ret #-1
         drop: ret #0

   Filter rtnetlink messages
         ldh #proto       /* A = skb->protocol */

         jneq #0, skip    /* check for NETLINK_ROUTE */
         ldb [4]          /* A = nlmsg_type */

         jneq #0x10, skip /* check type == RTNL_NEWLINK */
         ldx #16          /* X = offset(ifinfomsg) */

         ldb [x + 4]      /* offset(ifi_index) */
         jneq #0x3, skip  /* check ifindex == 3 */

         ld #32           /* A = len(nlmsghdr) + len(ifinfomsg), payload
       offset */
         ldx #16          /* X = IFLA_OPERSTATE */
         ld #nla          /* A = offset(IFLA_OPERSTATE) */
         jeq #0, skip
         tax
         ldb [x + 4]      /* A = value(IFLA_OPERSTATE) */
         jneq #0x6, skip  /* check oper state is UP */

         ret #-1
         skip: ret #0

USAGE EXAMPLE         top

   bpfc fubar
       Compile the source file ''fubar'' into BPF opcodes. Opcodes will be
       directed to stdout.

   bpfc -f xt_bpf -b -p -i fubar, resp. iptables -A INPUT -m bpf --bytecode
       `bpfc -f xt_bpf -i fubar` -j LOG
       Compile the source file ''fubar'' into BPF opcodes, bypass basic
       filter validation and emit opcodes in netfilter's xt_bpf readable
       format. Note that the source file ''fubar'' is first passed to the C
       preprocessor for textual replacements before handing over to the bpfc
       compiler.

   bpfc -
       Read bpfc instruction from stdin and emit opcodes to stdout.

   bpfc foo > bar, resp. netsniff-ng -f bar ...
       Compile filter instructions from file foo and redirect bpfc's output
       into the file bar, that can then be read by netsniff-ng(8) through
       option -f.

   bpfc -f tcpdump -i fubar
       Output opcodes from source file fubar in the same behavior as
       ''tcpdump -ddd''.

LEGAL         top

       bpfc is licensed under the GNU GPL version 2.0.

HISTORY         top

       bpfc was originally written for the netsniff-ng toolkit by Daniel
       Borkmann. It is currently maintained by Tobias Klauser
       <tklauser@distanz.ch> and Daniel Borkmann <dborkma@tik.ee.ethz.ch>.

SEE ALSO         top

       netsniff-ng(8), trafgen(8), mausezahn(8), ifpps(8), flowtop(8),
       astraceroute(8), curvetun(8)

AUTHOR         top

       Manpage was written by Daniel Borkmann.

COLOPHON         top

       This page is part of the Linux netsniff-ng toolkit project. A
       description of the project, and information about reporting bugs, can
       be found at http://netsniff-ng.org/.

COLOPHON         top

       This page is part of the netsniff-ng (a free Linux networking
       toolkit) project.  Information about the project can be found at 
       ⟨http://netsniff-ng.org/⟩.  If you have a bug report for this manual
       page, send it to netsniff-ng@googlegroups.com.  This page was
       obtained from the project's upstream Git repository 
       ⟨git://github.com/netsniff-ng/netsniff-ng.git⟩ on 2017-09-15.  If you
       discover any rendering problems in this HTML version of the page, or
       you believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in this
       COLOPHON (which is not part of the original manual page), send a mail
       to man-pages@man7.org

Linux                           03 March 2013                        BPFC(8)

Pages that refer to this page: astraceroute(8)curvetun(8)flowtop(8)ifpps(8)mausezahn(8)netsniff-ng(8)trafgen(8)