diff options
Diffstat (limited to 'pkg/tcpip/header/tcp.go')
| -rw-r--r-- | pkg/tcpip/header/tcp.go | 518 |
1 files changed, 518 insertions, 0 deletions
diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go new file mode 100644 index 000000000..995df4076 --- /dev/null +++ b/pkg/tcpip/header/tcp.go @@ -0,0 +1,518 @@ +// Copyright 2016 The Netstack Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package header + +import ( + "encoding/binary" + + "gvisor.googlesource.com/gvisor/pkg/tcpip" + "gvisor.googlesource.com/gvisor/pkg/tcpip/seqnum" +) + +const ( + srcPort = 0 + dstPort = 2 + seqNum = 4 + ackNum = 8 + dataOffset = 12 + tcpFlags = 13 + winSize = 14 + tcpChecksum = 16 + urgentPtr = 18 +) + +const ( + // MaxWndScale is maximum allowed window scaling, as described in + // RFC 1323, section 2.3, page 11. + MaxWndScale = 14 + + // TCPMaxSACKBlocks is the maximum number of SACK blocks that can + // be encoded in a TCP option field. + TCPMaxSACKBlocks = 4 +) + +// Flags that may be set in a TCP segment. +const ( + TCPFlagFin = 1 << iota + TCPFlagSyn + TCPFlagRst + TCPFlagPsh + TCPFlagAck + TCPFlagUrg +) + +// Options that may be present in a TCP segment. +const ( + TCPOptionEOL = 0 + TCPOptionNOP = 1 + TCPOptionMSS = 2 + TCPOptionWS = 3 + TCPOptionTS = 8 + TCPOptionSACKPermitted = 4 + TCPOptionSACK = 5 +) + +// TCPFields contains the fields of a TCP packet. It is used to describe the +// fields of a packet that needs to be encoded. +type TCPFields struct { + // SrcPort is the "source port" field of a TCP packet. + SrcPort uint16 + + // DstPort is the "destination port" field of a TCP packet. + DstPort uint16 + + // SeqNum is the "sequence number" field of a TCP packet. + SeqNum uint32 + + // AckNum is the "acknowledgement number" field of a TCP packet. + AckNum uint32 + + // DataOffset is the "data offset" field of a TCP packet. + DataOffset uint8 + + // Flags is the "flags" field of a TCP packet. + Flags uint8 + + // WindowSize is the "window size" field of a TCP packet. + WindowSize uint16 + + // Checksum is the "checksum" field of a TCP packet. + Checksum uint16 + + // UrgentPointer is the "urgent pointer" field of a TCP packet. + UrgentPointer uint16 +} + +// TCPSynOptions is used to return the parsed TCP Options in a syn +// segment. +type TCPSynOptions struct { + // MSS is the maximum segment size provided by the peer in the SYN. + MSS uint16 + + // WS is the window scale option provided by the peer in the SYN. + // + // Set to -1 if no window scale option was provided. + WS int + + // TS is true if the timestamp option was provided in the syn/syn-ack. + TS bool + + // TSVal is the value of the TSVal field in the timestamp option. + TSVal uint32 + + // TSEcr is the value of the TSEcr field in the timestamp option. + TSEcr uint32 + + // SACKPermitted is true if the SACK option was provided in the SYN/SYN-ACK. + SACKPermitted bool +} + +// SACKBlock represents a single contiguous SACK block. +type SACKBlock struct { + // Start indicates the lowest sequence number in the block. + Start seqnum.Value + + // End indicates the sequence number immediately following the last + // sequence number of this block. + End seqnum.Value +} + +// TCPOptions are used to parse and cache the TCP segment options for a non +// syn/syn-ack segment. +type TCPOptions struct { + // TS is true if the TimeStamp option is enabled. + TS bool + + // TSVal is the value in the TSVal field of the segment. + TSVal uint32 + + // TSEcr is the value in the TSEcr field of the segment. + TSEcr uint32 + + // SACKBlocks are the SACK blocks specified in the segment. + SACKBlocks []SACKBlock +} + +// TCP represents a TCP header stored in a byte array. +type TCP []byte + +const ( + // TCPMinimumSize is the minimum size of a valid TCP packet. + TCPMinimumSize = 20 + + // TCPProtocolNumber is TCP's transport protocol number. + TCPProtocolNumber tcpip.TransportProtocolNumber = 6 +) + +// SourcePort returns the "source port" field of the tcp header. +func (b TCP) SourcePort() uint16 { + return binary.BigEndian.Uint16(b[srcPort:]) +} + +// DestinationPort returns the "destination port" field of the tcp header. +func (b TCP) DestinationPort() uint16 { + return binary.BigEndian.Uint16(b[dstPort:]) +} + +// SequenceNumber returns the "sequence number" field of the tcp header. +func (b TCP) SequenceNumber() uint32 { + return binary.BigEndian.Uint32(b[seqNum:]) +} + +// AckNumber returns the "ack number" field of the tcp header. +func (b TCP) AckNumber() uint32 { + return binary.BigEndian.Uint32(b[ackNum:]) +} + +// DataOffset returns the "data offset" field of the tcp header. +func (b TCP) DataOffset() uint8 { + return (b[dataOffset] >> 4) * 4 +} + +// Payload returns the data in the tcp packet. +func (b TCP) Payload() []byte { + return b[b.DataOffset():] +} + +// Flags returns the flags field of the tcp header. +func (b TCP) Flags() uint8 { + return b[tcpFlags] +} + +// WindowSize returns the "window size" field of the tcp header. +func (b TCP) WindowSize() uint16 { + return binary.BigEndian.Uint16(b[winSize:]) +} + +// Checksum returns the "checksum" field of the tcp header. +func (b TCP) Checksum() uint16 { + return binary.BigEndian.Uint16(b[tcpChecksum:]) +} + +// SetSourcePort sets the "source port" field of the tcp header. +func (b TCP) SetSourcePort(port uint16) { + binary.BigEndian.PutUint16(b[srcPort:], port) +} + +// SetDestinationPort sets the "destination port" field of the tcp header. +func (b TCP) SetDestinationPort(port uint16) { + binary.BigEndian.PutUint16(b[dstPort:], port) +} + +// SetChecksum sets the checksum field of the tcp header. +func (b TCP) SetChecksum(checksum uint16) { + binary.BigEndian.PutUint16(b[tcpChecksum:], checksum) +} + +// CalculateChecksum calculates the checksum of the tcp segment given +// the totalLen and partialChecksum(descriptions below) +// totalLen is the total length of the segment +// partialChecksum is the checksum of the network-layer pseudo-header +// (excluding the total length) and the checksum of the segment data. +func (b TCP) CalculateChecksum(partialChecksum uint16, totalLen uint16) uint16 { + // Add the length portion of the checksum to the pseudo-checksum. + tmp := make([]byte, 2) + binary.BigEndian.PutUint16(tmp, totalLen) + checksum := Checksum(tmp, partialChecksum) + + // Calculate the rest of the checksum. + return Checksum(b[:b.DataOffset()], checksum) +} + +// Options returns a slice that holds the unparsed TCP options in the segment. +func (b TCP) Options() []byte { + return b[TCPMinimumSize:b.DataOffset()] +} + +// ParsedOptions returns a TCPOptions structure which parses and caches the TCP +// option values in the TCP segment. NOTE: Invoking this function repeatedly is +// expensive as it reparses the options on each invocation. +func (b TCP) ParsedOptions() TCPOptions { + return ParseTCPOptions(b.Options()) +} + +func (b TCP) encodeSubset(seq, ack uint32, flags uint8, rcvwnd uint16) { + binary.BigEndian.PutUint32(b[seqNum:], seq) + binary.BigEndian.PutUint32(b[ackNum:], ack) + b[tcpFlags] = flags + binary.BigEndian.PutUint16(b[winSize:], rcvwnd) +} + +// Encode encodes all the fields of the tcp header. +func (b TCP) Encode(t *TCPFields) { + b.encodeSubset(t.SeqNum, t.AckNum, t.Flags, t.WindowSize) + binary.BigEndian.PutUint16(b[srcPort:], t.SrcPort) + binary.BigEndian.PutUint16(b[dstPort:], t.DstPort) + b[dataOffset] = (t.DataOffset / 4) << 4 + binary.BigEndian.PutUint16(b[tcpChecksum:], t.Checksum) + binary.BigEndian.PutUint16(b[urgentPtr:], t.UrgentPointer) +} + +// EncodePartial updates a subset of the fields of the tcp header. It is useful +// in cases when similar segments are produced. +func (b TCP) EncodePartial(partialChecksum, length uint16, seqnum, acknum uint32, flags byte, rcvwnd uint16) { + // Add the total length and "flags" field contributions to the checksum. + // We don't use the flags field directly from the header because it's a + // one-byte field with an odd offset, so it would be accounted for + // incorrectly by the Checksum routine. + tmp := make([]byte, 4) + binary.BigEndian.PutUint16(tmp, length) + binary.BigEndian.PutUint16(tmp[2:], uint16(flags)) + checksum := Checksum(tmp, partialChecksum) + + // Encode the passed-in fields. + b.encodeSubset(seqnum, acknum, flags, rcvwnd) + + // Add the contributions of the passed-in fields to the checksum. + checksum = Checksum(b[seqNum:seqNum+8], checksum) + checksum = Checksum(b[winSize:winSize+2], checksum) + + // Encode the checksum. + b.SetChecksum(^checksum) +} + +// ParseSynOptions parses the options received in a SYN segment and returns the +// relevant ones. opts should point to the option part of the TCP Header. +func ParseSynOptions(opts []byte, isAck bool) TCPSynOptions { + limit := len(opts) + + synOpts := TCPSynOptions{ + // Per RFC 1122, page 85: "If an MSS option is not received at + // connection setup, TCP MUST assume a default send MSS of 536." + MSS: 536, + // If no window scale option is specified, WS in options is + // returned as -1; this is because the absence of the option + // indicates that the we cannot use window scaling on the + // receive end either. + WS: -1, + } + + for i := 0; i < limit; { + switch opts[i] { + case TCPOptionEOL: + i = limit + case TCPOptionNOP: + i++ + case TCPOptionMSS: + if i+4 > limit || opts[i+1] != 4 { + return synOpts + } + mss := uint16(opts[i+2])<<8 | uint16(opts[i+3]) + if mss == 0 { + return synOpts + } + synOpts.MSS = mss + i += 4 + + case TCPOptionWS: + if i+3 > limit || opts[i+1] != 3 { + return synOpts + } + ws := int(opts[i+2]) + if ws > MaxWndScale { + ws = MaxWndScale + } + synOpts.WS = ws + i += 3 + + case TCPOptionTS: + if i+10 > limit || opts[i+1] != 10 { + return synOpts + } + synOpts.TSVal = binary.BigEndian.Uint32(opts[i+2:]) + if isAck { + // If the segment is a SYN-ACK then store the Timestamp Echo Reply + // in the segment. + synOpts.TSEcr = binary.BigEndian.Uint32(opts[i+6:]) + } + synOpts.TS = true + i += 10 + case TCPOptionSACKPermitted: + if i+2 > limit || opts[i+1] != 2 { + return synOpts + } + synOpts.SACKPermitted = true + i += 2 + + default: + // We don't recognize this option, just skip over it. + if i+2 > limit { + return synOpts + } + l := int(opts[i+1]) + // If the length is incorrect or if l+i overflows the + // total options length then return false. + if l < 2 || i+l > limit { + return synOpts + } + i += l + } + } + + return synOpts +} + +// ParseTCPOptions extracts and stores all known options in the provided byte +// slice in a TCPOptions structure. +func ParseTCPOptions(b []byte) TCPOptions { + opts := TCPOptions{} + limit := len(b) + for i := 0; i < limit; { + switch b[i] { + case TCPOptionEOL: + i = limit + case TCPOptionNOP: + i++ + case TCPOptionTS: + if i+10 > limit || (b[i+1] != 10) { + return opts + } + opts.TS = true + opts.TSVal = binary.BigEndian.Uint32(b[i+2:]) + opts.TSEcr = binary.BigEndian.Uint32(b[i+6:]) + i += 10 + case TCPOptionSACK: + if i+2 > limit { + // Malformed SACK block, just return and stop parsing. + return opts + } + sackOptionLen := int(b[i+1]) + if i+sackOptionLen > limit || (sackOptionLen-2)%8 != 0 { + // Malformed SACK block, just return and stop parsing. + return opts + } + numBlocks := (sackOptionLen - 2) / 8 + opts.SACKBlocks = []SACKBlock{} + for j := 0; j < numBlocks; j++ { + start := binary.BigEndian.Uint32(b[i+2+j*8:]) + end := binary.BigEndian.Uint32(b[i+2+j*8+4:]) + opts.SACKBlocks = append(opts.SACKBlocks, SACKBlock{ + Start: seqnum.Value(start), + End: seqnum.Value(end), + }) + } + i += sackOptionLen + default: + // We don't recognize this option, just skip over it. + if i+2 > limit { + return opts + } + l := int(b[i+1]) + // If the length is incorrect or if l+i overflows the + // total options length then return false. + if l < 2 || i+l > limit { + return opts + } + i += l + } + } + return opts +} + +// EncodeMSSOption encodes the MSS TCP option with the provided MSS values in +// the supplied buffer. If the provided buffer is not large enough then it just +// returns without encoding anything. It returns the number of bytes written to +// the provided buffer. +func EncodeMSSOption(mss uint32, b []byte) int { + // mssOptionSize is the number of bytes in a valid MSS option. + const mssOptionSize = 4 + + if len(b) < mssOptionSize { + return 0 + } + b[0], b[1], b[2], b[3] = TCPOptionMSS, mssOptionSize, byte(mss>>8), byte(mss) + return mssOptionSize +} + +// EncodeWSOption encodes the WS TCP option with the WS value in the +// provided buffer. If the provided buffer is not large enough then it just +// returns without encoding anything. It returns the number of bytes written to +// the provided buffer. +func EncodeWSOption(ws int, b []byte) int { + if len(b) < 3 { + return 0 + } + b[0], b[1], b[2] = TCPOptionWS, 3, uint8(ws) + return int(b[1]) +} + +// EncodeTSOption encodes the provided tsVal and tsEcr values as a TCP timestamp +// option into the provided buffer. If the buffer is smaller than expected it +// just returns without encoding anything. It returns the number of bytes +// written to the provided buffer. +func EncodeTSOption(tsVal, tsEcr uint32, b []byte) int { + if len(b) < 10 { + return 0 + } + b[0], b[1] = TCPOptionTS, 10 + binary.BigEndian.PutUint32(b[2:], tsVal) + binary.BigEndian.PutUint32(b[6:], tsEcr) + return int(b[1]) +} + +// EncodeSACKPermittedOption encodes a SACKPermitted option into the provided +// buffer. If the buffer is smaller than required it just returns without +// encoding anything. It returns the number of bytes written to the provided +// buffer. +func EncodeSACKPermittedOption(b []byte) int { + if len(b) < 2 { + return 0 + } + + b[0], b[1] = TCPOptionSACKPermitted, 2 + return int(b[1]) +} + +// EncodeSACKBlocks encodes the provided SACK blocks as a TCP SACK option block +// in the provided slice. It tries to fit in as many blocks as possible based on +// number of bytes available in the provided buffer. It returns the number of +// bytes written to the provided buffer. +func EncodeSACKBlocks(sackBlocks []SACKBlock, b []byte) int { + if len(sackBlocks) == 0 { + return 0 + } + l := len(sackBlocks) + if l > TCPMaxSACKBlocks { + l = TCPMaxSACKBlocks + } + if ll := (len(b) - 2) / 8; ll < l { + l = ll + } + if l == 0 { + // There is not enough space in the provided buffer to add + // any SACK blocks. + return 0 + } + b[0] = TCPOptionSACK + b[1] = byte(l*8 + 2) + for i := 0; i < l; i++ { + binary.BigEndian.PutUint32(b[i*8+2:], uint32(sackBlocks[i].Start)) + binary.BigEndian.PutUint32(b[i*8+6:], uint32(sackBlocks[i].End)) + } + return int(b[1]) +} + +// EncodeNOP adds an explicit NOP to the option list. +func EncodeNOP(b []byte) int { + if len(b) == 0 { + return 0 + } + b[0] = TCPOptionNOP + return 1 +} + +// AddTCPOptionPadding adds the required number of TCPOptionNOP to quad align +// the option buffer. It adds padding bytes after the offset specified and +// returns the number of padding bytes added. The passed in options slice +// must have space for the padding bytes. +func AddTCPOptionPadding(options []byte, offset int) int { + paddingToAdd := -offset & 3 + // Now add any padding bytes that might be required to quad align the + // options. + for i := offset; i < offset+paddingToAdd; i++ { + options[i] = TCPOptionNOP + } + return paddingToAdd +} |