BPF(4)

💖 BPF(4) — Berkeley Packet Filter 💖

okay bestie, sit down, hydrate, because this is the Kernel Interfaces Manual page for bpf, section 4, and she is long. This is part 1 of 4, so don't go anywhere. 🫶

---

✨ NAME ✨

bpf — Berkeley Packet Filter

---

🌸 SYNOPSIS 🌸

device bpf

---

🦋 DESCRIPTION 🦋

The Berkeley Packet Filter is a raw interface to data link layers, and she does it in a protocol independent way, no cap. Every single packet on the network — even the ones that were literally addressed to other hosts — is accessible through this mechanism. nosy? yes. iconic? also yes. 💅

The packet filter shows up as a character special device, /dev/bpf. After you open the device, you must bind the file descriptor to a specific network interface using the BIOCSETIF ioctl. One interface can be shared by multiple listeners, and the filter underlying each descriptor will see an identical packet stream. besties sharing, we love to see it.

You need a separate device file for each minor device. If a file is already in use, the open will fail and errno will be set to EBUSY. 😭

Associated with each open instance of a bpf file is a user-settable packet filter. Whenever a packet is received by an interface, all file descriptors listening on that interface apply their filter. Each descriptor that accepts the packet receives its own copy. everyone gets their own, no sharing a plate.

The packet filter will support any link level protocol that has fixed length headers. Currently, only Ethernet, SLIP, and PPP drivers have been modified to interact with bpf.

Since packet data is in network byte order, applications should use the byteorder(3) macros to extract multi-byte values. do not just eyeball the bytes, I'm begging.

A packet can be sent out on the network by writing to a bpf file descriptor. The writes are unbuffered, meaning only one packet can be processed per write. Currently, only writes to Ethernets and SLIP links are supported.

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🎀 BUFFER MODES 🎀

bpf devices deliver packet data to the application via memory buffers provided by the application. The buffer mode is set using the BIOCSETBUFMODE ioctl, and read using the BIOCGETBUFMODE ioctl. two ioctls, one vibe.

⭐ Buffered read mode

By default, bpf devices operate in the BPF_BUFMODE_BUFFER mode, in which packet data is copied explicitly from kernel to user memory using the read(2) system call. The user process will declare a fixed buffer size that will be used both for sizing internal buffers and for all read(2) operations on the file. This size is queried using the BIOCGBLEN ioctl, and is set using the BIOCSBLEN ioctl. Note that an individual packet larger than the buffer size is necessarily truncated. 😬

🌈 Zero-copy buffer mode

bpf devices may also operate in the BPF_BUFMODE_ZEROCOPY mode, in which packet data is written directly into two user memory buffers by the kernel, avoiding both system call and copying overhead. Buffers are of fixed (and equal) size, page-aligned, and an even multiple of the page size. The maximum zero-copy buffer size is returned by the BIOCGETZMAX ioctl. Note that an individual packet larger than the buffer size is necessarily truncated. same warning, same energy.

The user process registers two memory buffers using the BIOCSETZBUF ioctl, which accepts a struct bpf_zbuf pointer as an argument:

struct bpf_zbuf {
        void *bz_bufa;
        void *bz_bufb;
        size_t bz_buflen;
};

bz_bufa is a pointer to the userspace address of the first buffer that will be filled, and bz_bufb is a pointer to the second buffer. bpf will then cycle between the two buffers as they fill and are acknowledged. it's giving relay race. 🏃‍♀️

Each buffer begins with a fixed-length header to hold synchronization and data length information for the buffer:

struct bpf_zbuf_header {
        volatile u_int  bzh_kernel_gen; /* Kernel generation number. */
        volatile u_int  bzh_kernel_len; /* Length of data in the buffer. */
        volatile u_int  bzh_user_gen;   /* User generation number. */
        /* ...padding for future use... */
};

The header structure of each buffer, including all padding, should be zeroed before it is configured using BIOCSETZBUF. Remaining space in the buffer will be used by the kernel to store packet data, laid out in the same format as with buffered read mode.

The kernel and the user process follow a simple acknowledgement protocol via the buffer header to synchronize access to the buffer: when the header generation numbers, bzh_kernel_gen and bzh_user_gen, hold the same value, the kernel owns the buffer, and when they differ, userspace owns the buffer.

While the kernel owns the buffer, the contents are unstable and may change asynchronously; while the user process owns the buffer, its contents are stable and will not be changed until the buffer has been acknowledged.

Initializing the buffer headers to all 0's before registering the buffer has the effect of assigning initial ownership of both buffers to the kernel. The kernel signals that a buffer has been assigned to userspace by modifying bzh_kernel_gen, and userspace acknowledges the buffer and returns it to the kernel by setting the value of bzh_user_gen to the value of bzh_kernel_gen.

In order to avoid caching and memory re-ordering effects, the user process must use atomic operations and memory barriers when checking for and acknowledging buffers:

#include <machine/atomic.h>

/*
 * Return ownership of a buffer to the kernel for reuse.
 */
static void
buffer_acknowledge(struct bpf_zbuf_header *bzh)
{

        atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);
}

/*
 * Check whether a buffer has been assigned to userspace by the kernel.
 * Return true if userspace owns the buffer, and false otherwise.
 */
static int
buffer_check(struct bpf_zbuf_header *bzh)
{

        return (bzh->bzh_user_gen !=
            atomic_load_acq_int(&bzh->bzh_kernel_gen));
}

The user process may force the assignment of the next buffer, if any data is pending, to userspace using the BIOCROTZBUF ioctl. This allows the user process to retrieve data in a partially filled buffer before the buffer is full, such as following a timeout; the process must recheck for buffer ownership using the header generation numbers, as the buffer will not be assigned to userspace if no data was present. 🙈

As in the buffered read mode, kqueue(2), poll(2), and select(2) may be used to sleep awaiting the availability of a completed buffer. They will return a readable file descriptor when ownership of the next buffer is assigned to user space.

In the current implementation, the kernel may assign zero, one, or both buffers to the user process; however, an earlier implementation maintained the invariant that at most one buffer could be assigned to the user process at a time. In order to both ensure progress and high performance, user processes should acknowledge a completely processed buffer as quickly as possible, returning it for reuse, and not block waiting on a second buffer while holding another buffer.

---

👑 IOCTLS 👑

The ioctl(2) command codes below are defined in <net/bpf.h>. All commands require these includes:

#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <net/bpf.h>

Additionally, BIOCGETIF and BIOCSETIF require <sys/socket.h> and <net/if.h>.

In addition to FIONREAD the following commands may be applied to any open bpf file. The (third) argument to ioctl(2) should be a pointer to the type indicated. pointer. not the value. pointer. 💅

• BIOCGBLEN — (u_int) Returns the required buffer length for reads on bpf files.
• BIOCSBLEN — (u_int) Sets the buffer length for reads on bpf files. The buffer must be set before the file is attached to an interface with BIOCSETIF. If the requested buffer size cannot be accommodated, the closest allowable size will be set and returned in the argument. A read call will result in EIO if it is passed a buffer that is not this size.
• BIOCGDLT — (u_int) Returns the type of the data link layer underlying the attached interface. EINVAL is returned if no interface has been specified. The device types, prefixed with “DLT_”, are defined in <net/bpf.h>.
• BIOCPROMISC — Forces the interface into promiscuous mode. All packets, not just those destined for the local host, are processed. Since more than one file can be listening on a given interface, a listener that opened its interface non-promiscuously may receive packets promiscuously. This problem can be remedied with an appropriate filter.
• BIOCFLUSH — Flushes the buffer of incoming packets, and resets the statistics that are returned by BIOCGSTATS.
• BIOCGETIF — (struct ifreq) Returns the name of the hardware interface that the file is listening on. The name is returned in the ifr_name field of the ifreq structure. All other fields are undefined. don't touch them.
• BIOCSETIF — (struct ifreq) Sets the hardware interface associate with the file. This command must be performed before any packets can be read. The device is indicated by name using the ifr_name field of the ifreq structure. Additionally, performs the actions of BIOCFLUSH.
• BIOCSRTIMEOUT
• BIOCGRTIMEOUT — (struct timeval) Set or get the read timeout parameter. The argument specifies the length of time to wait before timing out on a read request. This parameter is initialized to zero by open(2), indicating no timeout.
• BIOCGSTATS — (struct bpf_stat) Returns the following structure of packet statistics:

struct bpf_stat {
        u_int bs_recv;    /* number of packets received */
        u_int bs_drop;    /* number of packets dropped */
};

The fields are:

• bs_recv — the number of packets received by the descriptor since opened or reset (including any buffered since the last read call); and
• bs_drop — the number of packets which were accepted by the filter but dropped by the kernel because of buffer overflows (i.e., the application's reads are not keeping up with the packet traffic). 💀

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bestie this was only part 1, there's more coming, don't log off 💖

• BIOCIMMEDIATE (u_int) — flips "immediate mode" on or off based on whether the argument is truthy. 👀 With it on, your read comes back the literal moment a packet shows up. With it off, read just sits there blocking until either the kernel buffer fills up or a timeout happens. This is the moment for programs like rarpd(8) that genuinely have to answer messages in real time, no waiting around. For a brand-new file, the default is off. 💤
• BIOCSETF / BIOCSETFNR (struct bpf_program) — sets the read filter program the kernel uses to yeet away all the uninteresting packets. 🗑️ You hand over an array of instructions plus its length, using this lil guy:

struct bpf_program {
        int bf_len;
        struct bpf_insn *bf_insns;
};

The filter program itself is pointed to by the bf_insns field, and its length — counted in units of struct bpf_insn — goes in bf_len. 🫶 Peep the "FILTER MACHINE" section for the whole filter language lore. The only difference between the two: BIOCSETF also does everything BIOCFLUSH does, while BIOCSETFNR does not. That's it. That's the whole tea. ☕

• BIOCSETWF (struct bpf_program) — sets the write filter program, which is what the kernel uses to control what kind of packets you're allowed to write to the interface. ✍️ Same bpf filter program deal as BIOCSETF — go look at that one for the details.
• BIOCVERSION (struct bpf_version) — hands you back the major and minor version numbers of the filter language that the kernel currently recognizes. 🔢 Before you install any filter, you must check that the current version plays nice with the running kernel. They're compatible if the major numbers match and the application minor is less than or equal to the kernel minor. Kernel version lands in this struct:

struct bpf_version {
        u_short bv_major;
        u_short bv_minor;
};

The current version numbers are BPF_MAJOR_VERSION and BPF_MINOR_VERSION, both from <net/bpf.h>.

⚠️ An incompatible filter may cause undefined behavior — most likely an error out of ioctl(), or just haphazard, chaotic packet matching. 😭 Not the vibe. Check your versions.

• BIOCSHDRCMPLT / BIOCGHDRCMPLT (u_int) — set or get the status of the "header complete" flag. 🎀 Set it to zero if the link level source address should get filled in automatically by the interface output routine. Set it to one if the link level source address is gonna be written to the wire exactly as provided. This flag starts out at zero by default. ✨
• BIOCSSEESENT / BIOCGSEESENT (u_int) — okay so these are obsolete 👵 but they're still hanging around for compatibility. Use BIOCSDIRECTION and BIOCGDIRECTION instead, fr fr. They set or get the flag for whether locally generated packets on the interface should get returned by BPF. Set to zero to see only incoming packets on the interface. Set to one to see packets originating both locally and remotely on the interface. Initialized to one by default. 🦋
• BIOCSDIRECTION / BIOCGDIRECTION (u_int) — set or get the setting for whether incoming, outgoing, or all packets on the interface should get returned by BPF. 🧭 Set it to BPF_D_IN to see only incoming packets. Set it to BPF_D_INOUT to see packets originating locally and remotely on the interface. Set it to BPF_D_OUT to see only outgoing packets. This setting is initialized to BPF_D_INOUT by default. 👑
• BIOCSTSTAMP / BIOCGTSTAMP (u_int) — set or get the format and resolution of the time stamps BPF hands back. ⏰ Big menu, bestie, here we go:
  • Set to BPF_T_MICROTIME, BPF_T_MICROTIME_FAST, BPF_T_MICROTIME_MONOTONIC, or BPF_T_MICROTIME_MONOTONIC_FAST for time stamps in 64-bit struct timeval format.
  • Set to BPF_T_NANOTIME, BPF_T_NANOTIME_FAST, BPF_T_NANOTIME_MONOTONIC, or BPF_T_NANOTIME_MONOTONIC_FAST for time stamps in 64-bit struct timespec format.
  • Set to BPF_T_BINTIME, BPF_T_BINTIME_FAST, BPF_T_NANOTIME_MONOTONIC, or BPF_T_BINTIME_MONOTONIC_FAST for time stamps in 64-bit struct bintime format.
  • Set to BPF_T_NONE to just ignore the time stamp entirely. 🙈

All the 64-bit time stamp formats get wrapped in struct bpf_ts. 💅

The _FAST squad — that's BPF_T_MICROTIME_FAST, BPF_T_NANOTIME_FAST, BPF_T_BINTIME_FAST, BPF_T_MICROTIME_MONOTONIC_FAST, BPF_T_NANOTIME_MONOTONIC_FAST, and BPF_T_BINTIME_MONOTONIC_FAST — are analogs of the corresponding formats without the _FAST suffix, but they don't do a full time counter query, so their accuracy is one timer tick. ⚡

And BPF_T_MICROTIME_MONOTONIC, BPF_T_NANOTIME_MONOTONIC, BPF_T_BINTIME_MONOTONIC, BPF_T_MICROTIME_MONOTONIC_FAST, BPF_T_NANOTIME_MONOTONIC_FAST, and BPF_T_BINTIME_MONOTONIC_FAST store the time elapsed since kernel boot. 🌱

This setting is initialized to BPF_T_MICROTIME by default. ✨

• BIOCFEEDBACK (u_int) — sets packet feedback mode. 🔁 This lets injected packets get fed back as input to the interface when the output via the interface succeeds. When BPF_D_INOUT direction is set, injected outgoing packets are not returned by BPF, so you don't get duplicates. 🙅‍♀️ This flag is initialized to zero by default.
• BIOCLOCK — sets the locked flag on the bpf descriptor. 🔒 This prevents the execution of any ioctl commands that could change the underlying operating parameters of the device. Once it's locked, it's locked. periodt.
• BIOCGETBUFMODE / BIOCSETBUFMODE (u_int) — get or set the current bpf buffering mode. 🌈 Possible values are BPF_BUFMODE_BUFFER (buffered read mode) and BPF_BUFMODE_ZBUF (zero-copy buffer mode).
• BIOCSETZBUF (struct bpf_zbuf) — sets the current zero-copy buffer locations. 📍 You can only set buffer locations once zero-copy buffer mode has already been selected, and it has to happen before attaching to an interface. The buffers must be identical in size, page-aligned, and an integer multiple of pages in size. All three fields — bz_bufa, bz_bufb, and bz_buflen — must be filled out. 🫡 And if buffers have already been set for this device, the ioctl will fail. 💀
• BIOCGETZMAX (size_t) — gets the largest individual zero-copy buffer size that's allowed. 📏 Since zero-copy buffer mode uses two buffers, the real-world limit is actually twice the returned size. And because zero-copy buffers eat up kernel address space, conservative buffer size selection is suggested — especially when you've got multiple bpf descriptors in use on 32-bit systems. 😬 Be mindful.
• BIOCROTZBUF — forces ownership of the next buffer over to userspace, if there's any data present in the buffer. 📦 If there's no data in there, the buffer stays owned by the kernel. This is what lets consumers of zero-copy buffering implement timeouts and grab partially filled buffers. 🕰️ And to handle the case where there's no data in the buffer and therefore ownership doesn't get assigned, the user process has to check bzh_kernel_gen against bzh_user_gen. 💅

✨ BPF HEADER ✨

okay so bestie, before you read a single packet you need to know what's sitting in front of it 👀 Every packet that comes back from read(2) — or from a zero-copy buffer — has one of these structures glued to the front:

struct bpf_xhdr {
        struct bpf_ts   bh_tstamp;     /* time stamp */
        uint32_t        bh_caplen;     /* length of captured portion */
        uint32_t        bh_datalen;    /* original length of packet */
        u_short         bh_hdrlen;     /* length of bpf header (this struct
                                          plus alignment padding) */
};

struct bpf_hdr {
        struct timeval  bh_tstamp;     /* time stamp */
        uint32_t        bh_caplen;     /* length of captured portion */
        uint32_t        bh_datalen;    /* original length of packet */
        u_short         bh_hdrlen;     /* length of bpf header (this struct
                                          plus alignment padding) */
};

The fields, whose values are stored in host order, and are:

• bh_tstamp — the moment the packet filter got its hands on the packet 🕐
• bh_caplen — the length of the portion of the packet that actually got captured. It's the minimum of the truncation amount the filter specified and the length of the packet, fr fr.
• bh_datalen — the length of the packet off the wire. This value does not care about the truncation amount your filter specified, not even a little bit.
• bh_hdrlen — the length of the bpf header, which lowkey may not be equal to sizeof(struct bpf_xhdr) or sizeof(struct bpf_hdr).

The bh_hdrlen field exists to account for padding between the header and the link level protocol. The whole point here is to guarantee proper alignment of the packet data structures, which is required on alignment sensitive architectures and improves performance on many other architectures. The packet filter makes sure that the bpf_xhdr, bpf_hdr and the network layer header will be word aligned. Currently, bpf_hdr is used when the time stamp is set to BPF_T_MICROTIME, BPF_T_MICROTIME_FAST, BPF_T_MICROTIME_MONOTONIC, BPF_T_MICROTIME_MONOTONIC_FAST, or BPF_T_NONE for backward compatibility reasons. Otherwise, bpf_xhdr is used. However, bpf_hdr may be deprecated in the near future. Suitable precautions must be taken when accessing the link layer protocol fields on alignment restricted machines. (This is not a problem on an Ethernet, since the type field is a short falling on an even offset, and the addresses are probably accessed in a bytewise fashion). 💅

Additionally, individual packets are padded so that each one starts on a word boundary. This requires that an application has some knowledge of how to get from packet to packet. The macro BPF_WORDALIGN is defined in <net/bpf.h> to facilitate this process. It rounds up its argument to the nearest word aligned value (where a word is BPF_ALIGNMENT bytes wide).

For example, if ‘p’ points to the start of a packet, this expression will advance it to the next packet:

p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)

For the alignment mechanisms to work properly, the buffer passed to read(2) must itself be word aligned. The malloc(3) function will always return an aligned buffer. 🫶 no cap, malloc's got you.

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🌈 FILTER MACHINE 🌈

A filter program is an array of instructions, with all branches forwardly directed, terminated by a return instruction. Each instruction performs some action on the pseudo-machine state, which consists of an accumulator, index register, scratch memory store, and implicit program counter. it's giving tiny virtual machine and honestly? iconic ✨

The following structure defines the instruction format:

struct bpf_insn {
        u_short code;
        u_char  jt;
        u_char  jf;
        u_long k;
};

The k field is used in different ways by different instructions, and the jt and jf fields are used as offsets by the branch instructions. The opcodes are encoded in a semi-hierarchical fashion. There are eight classes of instructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU, BPF_JMP, BPF_RET, and BPF_MISC. Various other mode and operator bits are or'd into the class to give the actual instructions. The classes and modes are defined in <net/bpf.h>.

Below are the semantics for each defined bpf instruction. We use the convention that A is the accumulator, X is the index register, P[] packet data, and M[] scratch memory store. P[i:n] gives the data at byte offset “i” in the packet, interpreted as a word (n=4), unsigned halfword (n=2), or unsigned byte (n=1). M[i] gives the i'th word in the scratch memory store, which is only addressed in word units. The memory store is indexed from 0 to BPF_MEMWORDS - 1. k, jt, and jf are the corresponding fields in the instruction definition. “len” refers to the length of the packet.

🧚 BPF_LD

These instructions copy a value into the accumulator. The type of the source operand is specified by an “addressing mode” and can be a constant (BPF_IMM), packet data at a fixed offset (BPF_ABS), packet data at a variable offset (BPF_IND), the packet length (BPF_LEN), or a word in the scratch memory store (BPF_MEM). For BPF_IND and BPF_ABS, the data size must be specified as a word (BPF_W), halfword (BPF_H), or byte (BPF_B). The semantics of all the recognized BPF_LD instructions follow.

BPF_LD+BPF_W+BPF_ABS    A <- P[k:4]
BPF_LD+BPF_H+BPF_ABS    A <- P[k:2]
BPF_LD+BPF_B+BPF_ABS    A <- P[k:1]
BPF_LD+BPF_W+BPF_IND    A <- P[X+k:4]
BPF_LD+BPF_H+BPF_IND    A <- P[X+k:2]
BPF_LD+BPF_B+BPF_IND    A <- P[X+k:1]
BPF_LD+BPF_W+BPF_LEN    A <- len
BPF_LD+BPF_IMM          A <- k
BPF_LD+BPF_MEM          A <- M[k]

🧚 BPF_LDX

These instructions load a value into the index register. Note that the addressing modes are more restrictive than those of the accumulator loads, but they include BPF_MSH, a hack for efficiently loading the IP header length. (a hack!! we love a hack 💀)

BPF_LDX+BPF_W+BPF_IMM   X <- k
BPF_LDX+BPF_W+BPF_MEM   X <- M[k]
BPF_LDX+BPF_W+BPF_LEN   X <- len
BPF_LDX+BPF_B+BPF_MSH   X <- 4*(P[k:1]&0xf)

🧚 BPF_ST

This instruction stores the accumulator into the scratch memory. We do not need an addressing mode since there is only one possibility for the destination. one destination, one vibe 💅

BPF_ST                  M[k] <- A

🧚 BPF_STX

This instruction stores the index register in the scratch memory store.

BPF_STX                 M[k] <- X

🧚 BPF_ALU

The alu instructions perform operations between the accumulator and index register or constant, and store the result back in the accumulator. For binary operations, a source mode is required (BPF_K or BPF_X).

BPF_ALU+BPF_ADD+BPF_K   A <- A + k
BPF_ALU+BPF_SUB+BPF_K   A <- A - k
BPF_ALU+BPF_MUL+BPF_K   A <- A * k
BPF_ALU+BPF_DIV+BPF_K   A <- A / k
BPF_ALU+BPF_MOD+BPF_K   A <- A % k
BPF_ALU+BPF_AND+BPF_K   A <- A & k
BPF_ALU+BPF_OR+BPF_K    A <- A | k
BPF_ALU+BPF_XOR+BPF_K   A <- A ^ k
BPF_ALU+BPF_LSH+BPF_K   A <- A << k
BPF_ALU+BPF_RSH+BPF_K   A <- A >> k
BPF_ALU+BPF_ADD+BPF_X   A <- A + X
BPF_ALU+BPF_SUB+BPF_X   A <- A - X
BPF_ALU+BPF_MUL+BPF_X   A <- A * X
BPF_ALU+BPF_DIV+BPF_X   A <- A / X
BPF_ALU+BPF_MOD+BPF_X   A <- A % X
BPF_ALU+BPF_AND+BPF_X   A <- A & X
BPF_ALU+BPF_OR+BPF_X    A <- A | X
BPF_ALU+BPF_XOR+BPF_X   A <- A ^ X
BPF_ALU+BPF_LSH+BPF_X   A <- A << X
BPF_ALU+BPF_RSH+BPF_X   A <- A >> X
BPF_ALU+BPF_NEG         A <- -A

🧚 BPF_JMP

The jump instructions alter flow of control. Conditional jumps compare the accumulator against a constant (BPF_K) or the index register (BPF_X). If the result is true (or non-zero), the true branch is taken, otherwise the false branch is taken. Jump offsets are encoded in 8 bits so the longest jump is 256 instructions. However, the jump always (BPF_JA) opcode uses the 32 bit k field as the offset, allowing arbitrarily distant destinations. All conditionals use unsigned comparison conventions. (unsigned only, bestie — no negatives sneaking in here 🙈)

BPF_JMP+BPF_JA          pc += k
BPF_JMP+BPF_JGT+BPF_K   pc += (A > k) ? jt : jf
BPF_JMP+BPF_JGE+BPF_K   pc += (A >= k) ? jt : jf
BPF_JMP+BPF_JEQ+BPF_K   pc += (A == k) ? jt : jf
BPF_JMP+BPF_JSET+BPF_K  pc += (A & k) ? jt : jf
BPF_JMP+BPF_JGT+BPF_X   pc += (A > X) ? jt : jf
BPF_JMP+BPF_JGE+BPF_X   pc += (A >= X) ? jt : jf
BPF_JMP+BPF_JEQ+BPF_X   pc += (A == X) ? jt : jf
BPF_JMP+BPF_JSET+BPF_X  pc += (A & X) ? jt : jf

🧚 BPF_RET

The return instructions terminate the filter program and specify the amount of packet to accept (i.e., they return the truncation amount). A return value of zero indicates that the packet should be ignored. The return value is either a constant (BPF_K) or the accumulator (BPF_A).

BPF_RET+BPF_A           accept A bytes
BPF_RET+BPF_K           accept k bytes

🧚 BPF_MISC

The miscellaneous category was created for anything that does not fit into the above classes, and for any new instructions that might need to be added. Currently, these are the register transfer instructions that copy the index register to the accumulator or vice versa.

BPF_MISC+BPF_TAX        X <- A
BPF_MISC+BPF_TXA        A <- X

The bpf interface provides the following macros to facilitate array initializers: BPF_STMT(opcode, operand) and BPF_JUMP(opcode, operand, true_offset, false_offset). 🎀 use these, your array initializers will thank you.

✨ SYSCTL VARIABLES ✨

okay so there's a whole lil crew of sysctl(8) variables that control how the bpf subsystem behaves, and you do want to know about them bestie 💅

• net.bpf.optimize_writers: 0 — so like, a bunch of programs use BPF to send raw packets but never receive them (think cdpd, lldpd, dhcpd, dhcp relays, etc. — iconic send-only queens 👑). They genuinely do not need incoming packets delivered to them at all. Turning this option on makes new BPF users get attached to a write-only interface list until the program explicitly specifies a read filter via pcap_set_filter(). What does that get you? It removes any performance degradation for high-speed interfaces. slay.
• net.bpf.stats: — a binary interface for retrieving general statistics. yes it's giving mysterious. it's binary, bestie.
• net.bpf.zerocopy_enable: 0 — permits zero-copy to be used with net BPF readers.

🚨 Use with caution. for real. caution. i'm not kidding.

• net.bpf.maxinsns: 512 — the maximum number of instructions that a BPF program can contain. Wanna figure out roughly how many instructions any given filter needs? Use the tcpdump(1) -d option. handy fr fr.
• net.bpf.maxbufsize: 524288 — maximum buffer size to allocate for the packets buffer. big girl buffer energy.
• net.bpf.bufsize: 4096 — default buffer size to allocate for the packets buffer.

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🌸 EXAMPLES 🌸

bestie. bestie. it's example time and these are so cute, they're classic. let's go 🦋

🎀 Reverse ARP

The following filter is taken from the Reverse ARP Daemon. It accepts only Reverse ARP requests. no gatekeeping, just revarp requests, periodt.

struct bpf_insn insns[] = {
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
    BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
         sizeof(struct ether_header)),
    BPF_STMT(BPF_RET+BPF_K, 0),
};

🎀 IP between two hosts

This filter accepts only IP packets between host 128.3.112.15 and 128.3.112.35. very specific. very main character. 💜

struct bpf_insn insns[] = {
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
    BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
    BPF_STMT(BPF_RET+BPF_K, 0),
};

🎀 TCP finger packets

Finally, this filter returns only TCP finger packets. We must parse the IP header to reach the TCP header. The BPF_JSET instruction checks that the IP fragment offset is 0 so we are sure that we have a TCP header. detective work honestly 🔍

struct bpf_insn insns[] = {
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
    BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
    BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
    BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
    BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
    BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
    BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
    BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
    BPF_STMT(BPF_RET+BPF_K, 0),
};

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👀 SEE ALSO 👀

go read these next, bestie, they're the friend group:

• tcpdump(1)
• ioctl(2)
• kqueue(2)
• poll(2)
• select(2)
• byteorder(3)
• ng_bpf(4)
• bpf(9)

and also this absolute banger of a citation:

• McCanne, S. and Jacobson V., An efficient, extensible, and portable network monitor.

iykyk 📚

---

🌈 HISTORY 🌈

The Enet packet filter was created in 1980 by Mike Accetta and Rick Rashid at Carnegie-Mellon University. Jeffrey Mogul, at Stanford, ported the code to BSD and continued its development from 1983 on. Since then, it has evolved into the Ultrix Packet Filter at DEC, a STREAMS NIT module under SunOS 4.1, and BPF. the character development?? unmatched. 💖

---

👑 AUTHORS 👑

Steven McCanne, of Lawrence Berkeley Laboratory, implemented BPF in Summer 1990. Much of the design is due to Van Jacobson. legends only, no cap.

Support for zero-copy buffers was added by Robert N. M. Watson under contract to Seccuris Inc. ty for your service 🫶

---

💀 BUGS 💀

okay so here's the tea, and some of it is a lil cursed:

• The read buffer must be of a fixed size (returned by the BIOCGBLEN ioctl). fixed. size. don't fight it.
• A file that does not request promiscuous mode may receive promiscuously received packets as a side effect of another file requesting this mode on the same hardware interface. This could be fixed in the kernel with additional processing overhead. However, we favor the model where all files must assume that the interface is promiscuous, and if so desired, must utilize a filter to reject foreign packets. yeah. it's giving "we could fix it but we won't." 😭
• Data link protocols with variable length headers are not currently supported. nope. not today.
• The SEESENT, DIRECTION, and FEEDBACK settings have been observed to work incorrectly on some interface types, including those with hardware loopback rather than software loopback, and point-to-point interfaces. They appear to function correctly on a broad range of Ethernet-style interfaces.

⚠️ so if you're on one of those weird interface types, vibe check your results, bestie. don't trust it blindly.

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and that's the whole page, babe 🎀 this was the Debian build of BPF(4), dated October 21, 2016 — go forth and filter, you gorgeous packet-sniffing icon 💅✨🦋