mirror of
https://github.com/slackhq/nebula.git
synced 2026-05-16 04:47:38 +02:00
235 lines
7.0 KiB
Go
235 lines
7.0 KiB
Go
//go:build !android && !e2e_testing
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// +build !android,!e2e_testing
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package overlay
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import (
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"encoding/binary"
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"fmt"
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"golang.org/x/sys/unix"
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)
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// Size of the legacy struct virtio_net_hdr that the kernel prepends/expects on
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// a TUN opened with IFF_VNET_HDR (TUNSETVNETHDRSZ not set).
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const virtioNetHdrLen = 10
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// Maximum size we accept for a single read from a TUN with IFF_VNET_HDR. A
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// TSO superpacket can be up to 64KiB of payload plus a single L2/L3/L4 header
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// prefix plus the virtio header.
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const tunReadBufSize = 65535
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// Space for segmented output. Worst case is many small segments, each paying
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// an IP+TCP header. 128KiB comfortably covers the 64KiB payload ceiling.
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const tunSegBufSize = 131072
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type virtioNetHdr struct {
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Flags uint8
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GSOType uint8
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HdrLen uint16
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GSOSize uint16
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CsumStart uint16
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CsumOffset uint16
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}
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// decode reads a virtio_net_hdr in host byte order (TUN default; we never
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// call TUNSETVNETLE so the kernel matches our endianness).
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func (h *virtioNetHdr) decode(b []byte) {
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h.Flags = b[0]
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h.GSOType = b[1]
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h.HdrLen = binary.NativeEndian.Uint16(b[2:4])
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h.GSOSize = binary.NativeEndian.Uint16(b[4:6])
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h.CsumStart = binary.NativeEndian.Uint16(b[6:8])
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h.CsumOffset = binary.NativeEndian.Uint16(b[8:10])
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}
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// segmentInto splits a TUN-side packet described by hdr into one or more
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// IP packets, each appended to *out as a slice of scratch. scratch must be
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// sized to hold every segment (including replicated headers).
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func segmentInto(pkt []byte, hdr virtioNetHdr, out *[][]byte, scratch []byte) error {
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switch hdr.GSOType {
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case unix.VIRTIO_NET_HDR_GSO_NONE:
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if len(pkt) > len(scratch) {
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return fmt.Errorf("packet larger than segment buffer: %d > %d", len(pkt), len(scratch))
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}
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copy(scratch, pkt)
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seg := scratch[:len(pkt)]
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if hdr.Flags&unix.VIRTIO_NET_HDR_F_NEEDS_CSUM != 0 {
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if err := finishChecksum(seg, hdr); err != nil {
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return err
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}
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}
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*out = append(*out, seg)
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return nil
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case unix.VIRTIO_NET_HDR_GSO_TCPV4, unix.VIRTIO_NET_HDR_GSO_TCPV6:
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return segmentTCP(pkt, hdr, out, scratch)
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default:
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return fmt.Errorf("unsupported virtio gso type: %d", hdr.GSOType)
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}
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}
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// finishChecksum computes the L4 checksum for a non-GSO packet that the kernel
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// handed us with NEEDS_CSUM set. csum_start / csum_offset point at the 16-bit
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// checksum field; we zero it, fold a full sum (the field was pre-loaded with
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// the pseudo-header partial sum by the kernel), and store the result.
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func finishChecksum(seg []byte, hdr virtioNetHdr) error {
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cs := int(hdr.CsumStart)
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co := int(hdr.CsumOffset)
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if cs+co+2 > len(seg) {
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return fmt.Errorf("csum offsets out of range: start=%d offset=%d len=%d", cs, co, len(seg))
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}
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// The kernel stores a partial pseudo-header sum at [cs+co:]; sum over the
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// L4 region starting at cs, folding the prior partial in as the seed.
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partial := uint32(binary.BigEndian.Uint16(seg[cs+co : cs+co+2]))
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seg[cs+co] = 0
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seg[cs+co+1] = 0
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sum := checksumBytes(seg[cs:], partial)
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binary.BigEndian.PutUint16(seg[cs+co:cs+co+2], checksumFold(sum))
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return nil
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}
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// segmentTCP software-segments a TSO superpacket into one IP packet per MSS
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// chunk. The caller guarantees hdr.GSOType is TCPV4 or TCPV6.
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func segmentTCP(pkt []byte, hdr virtioNetHdr, out *[][]byte, scratch []byte) error {
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if hdr.GSOSize == 0 {
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return fmt.Errorf("gso_size is zero")
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}
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if int(hdr.HdrLen) > len(pkt) || hdr.HdrLen == 0 {
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return fmt.Errorf("hdr_len %d out of range (pkt %d)", hdr.HdrLen, len(pkt))
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}
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if hdr.CsumStart == 0 || hdr.CsumStart >= hdr.HdrLen {
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return fmt.Errorf("csum_start %d out of range (hdr_len %d)", hdr.CsumStart, hdr.HdrLen)
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}
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isV4 := hdr.GSOType == unix.VIRTIO_NET_HDR_GSO_TCPV4
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headerLen := int(hdr.HdrLen)
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csumStart := int(hdr.CsumStart)
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if isV4 && csumStart < 20 {
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return fmt.Errorf("csum_start %d too small for IPv4", csumStart)
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}
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if !isV4 && csumStart < 40 {
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return fmt.Errorf("csum_start %d too small for IPv6", csumStart)
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}
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if headerLen-csumStart < 20 {
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return fmt.Errorf("tcp header region too small: %d", headerLen-csumStart)
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}
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payload := pkt[headerLen:]
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payLen := len(payload)
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gso := int(hdr.GSOSize)
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numSeg := (payLen + gso - 1) / gso
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if numSeg == 0 {
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numSeg = 1
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}
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need := numSeg*headerLen + payLen
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if need > len(scratch) {
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return fmt.Errorf("scratch too small for %d segments: need %d have %d", numSeg, need, len(scratch))
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}
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origSeq := binary.BigEndian.Uint32(pkt[csumStart+4 : csumStart+8])
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origFlags := pkt[csumStart+13]
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const tcpFinPsh = 0x09 // FIN(0x01) | PSH(0x08)
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var origIPID uint16
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if isV4 {
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origIPID = binary.BigEndian.Uint16(pkt[4:6])
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}
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off := 0
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for i := 0; i < numSeg; i++ {
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segStart := i * gso
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segEnd := segStart + gso
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if segEnd > payLen {
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segEnd = payLen
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}
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segPayLen := segEnd - segStart
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// Materialise IP+TCP header and this segment's payload chunk.
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copy(scratch[off:], pkt[:headerLen])
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copy(scratch[off+headerLen:], payload[segStart:segEnd])
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seg := scratch[off : off+headerLen+segPayLen]
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off += headerLen + segPayLen
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// Fix IP header: total/payload length, v4 ID, v4 header csum.
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if isV4 {
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ihl := int(seg[0]&0x0f) * 4
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if ihl < 20 || ihl > csumStart {
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return fmt.Errorf("bad IPv4 IHL: %d", ihl)
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}
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binary.BigEndian.PutUint16(seg[2:4], uint16(headerLen+segPayLen))
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binary.BigEndian.PutUint16(seg[4:6], origIPID+uint16(i))
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seg[10] = 0
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seg[11] = 0
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binary.BigEndian.PutUint16(seg[10:12], checksumFold(checksumBytes(seg[:ihl], 0)))
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} else {
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// IPv6 payload length excludes the 40-byte fixed header but
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// includes any extension headers that sit between [40:csumStart].
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binary.BigEndian.PutUint16(seg[4:6], uint16(headerLen-40+segPayLen))
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}
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// Fix TCP header: seq, flags, checksum.
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segSeq := origSeq + uint32(segStart)
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binary.BigEndian.PutUint32(seg[csumStart+4:csumStart+8], segSeq)
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if i != numSeg-1 {
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seg[csumStart+13] = origFlags &^ tcpFinPsh
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} else {
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seg[csumStart+13] = origFlags
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}
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seg[csumStart+16] = 0
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seg[csumStart+17] = 0
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tcpLen := headerLen - csumStart + segPayLen
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var psum uint32
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if isV4 {
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psum = pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_TCP, tcpLen)
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} else {
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psum = pseudoHeaderIPv6(seg[8:24], seg[24:40], unix.IPPROTO_TCP, tcpLen)
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}
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binary.BigEndian.PutUint16(seg[csumStart+16:csumStart+18], checksumFold(checksumBytes(seg[csumStart:csumStart+tcpLen], psum)))
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*out = append(*out, seg)
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}
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return nil
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}
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func checksumBytes(b []byte, initial uint32) uint32 {
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sum := initial
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for len(b) >= 2 {
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sum += uint32(binary.BigEndian.Uint16(b[:2]))
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b = b[2:]
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}
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if len(b) == 1 {
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sum += uint32(b[0]) << 8
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}
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return sum
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}
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func checksumFold(sum uint32) uint16 {
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for sum>>16 != 0 {
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sum = (sum & 0xffff) + (sum >> 16)
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}
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return ^uint16(sum)
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}
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func pseudoHeaderIPv4(src, dst []byte, proto byte, tcpLen int) uint32 {
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sum := checksumBytes(src, 0)
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sum = checksumBytes(dst, sum)
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sum += uint32(proto)
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sum += uint32(tcpLen)
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return sum
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}
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func pseudoHeaderIPv6(src, dst []byte, proto byte, tcpLen int) uint32 {
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sum := checksumBytes(src, 0)
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sum = checksumBytes(dst, sum)
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sum += uint32(tcpLen >> 16)
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sum += uint32(tcpLen & 0xffff)
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sum += uint32(proto)
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return sum
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}
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