Documentation
¶
Overview ¶
Package gre decodes Generic Routing Encapsulation packets per RFC 2784 (base) + RFC 2890 (Key + Sequence Number) + RFC 2637 (PPTP Enhanced GRE, Version=1).
Wrap-vs-native judgement
Native. All three RFCs are fully public; GRE is a tight bit-packed header with optional fields gated by flag bits. No crypto, no compression, no varints. Operators paste IP-payload bytes (protocol number 47 in the outer IP header) from a `tcpdump -X proto 47` line, a Wireshark Follow-IP-Stream view, or any GRE-emitting tool and get the documented header plus encapsulated protocol identification for routing into a downstream decoder.
What this package covers
**4-byte mandatory header** (RFC 2784 §2):
byte 0: C (Checksum present, bit 7), R (Routing present — deprecated, bit 6), K (Key present, bit 5), S (Sequence Number present, bit 4), s (Strict Source Route — deprecated, bit 3), Recur (Recursion Control — deprecated, bits 0-2).
byte 1: Flags (5 bits) + Version (3 bits). Version 0 = standard GRE; Version 1 = PPTP Enhanced GRE.
bytes 2-3: Protocol Type (EtherType of the encapsulated payload). **8-entry name table**:
0x0800 IPv4
0x86DD IPv6
0x6558 Transparent Ethernet Bridging (L2 tunnel, EoGRE)
0x880B PPP (PPP-in-GRE)
0x8847 MPLS unicast
0x8848 MPLS multicast
0x6559 Raw Frame Relay
0x0806 ARP
**Optional fields** (gated by flag bits, in this order):
If C or R set: 4 bytes = 16-bit Checksum + 16-bit Offset (Offset is only meaningful when R is set; R is deprecated).
If K set (RFC 2890): 4 bytes Key (used to demultiplex multiple GRE tunnels between the same endpoints).
If S set (RFC 2890): 4 bytes Sequence Number (for in-order delivery; rarely used in practice).
**PPTP Enhanced GRE** (RFC 2637, Version=1) — Microsoft PPTP overloads the K bit + Key field: the 4 bytes are interpreted as PayloadLength (uint16 BE) + Call ID (uint16 BE). PPTP additionally adds an Acknowledgement Number (4 bytes) when the A bit (bit 7 of byte 1) is set. PPTP always has K=1, S is optional, A is optional.
**Recognised variant**: surfaces 'standard GRE (RFC 2784)' for V=0 or 'PPTP Enhanced GRE (RFC 2637)' for V=1.
**Encapsulated payload bytes** are surfaced as hex with a header-offset hint for the operator to feed into the appropriate downstream decoder.
What this package does NOT cover (deliberately out of scope)
IP framing — feed the IP-payload bytes after the outer IPv4 / IPv6 header strip (IP protocol number 47 for GRE).
Inner payload decoding — operators pipe the post-GRE bytes to `ip_packet_decode` (for IPv4/IPv6 payloads), to a future Ethernet decoder (for TEB payloads), to `arp_decode` (for ARP), etc.
Routing field (R bit) body — the RFC 1701 routing entries are deprecated and we only surface the Checksum + Offset bytes.
PPP frame dissection inside PPTP — the post-Ack PPP frame is a separate Spec.
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
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Types ¶
type Result ¶
type Result struct {
Variant string `json:"variant"`
Version int `json:"version"`
ChecksumPresent bool `json:"checksum_present"`
RoutingPresent bool `json:"routing_present"`
KeyPresent bool `json:"key_present"`
SequencePresent bool `json:"sequence_present"`
StrictSource bool `json:"strict_source_route"`
Recur int `json:"recursion_control"`
AckPresent bool `json:"ack_present,omitempty"`
FlagsByte0Hex string `json:"flags_byte0_hex"`
FlagsByte1Hex string `json:"flags_byte1_hex"`
ProtocolType int `json:"protocol_type"`
ProtocolTypeHex string `json:"protocol_type_hex"`
ProtocolName string `json:"protocol_name"`
Checksum *uint16 `json:"checksum,omitempty"`
Offset *uint16 `json:"offset,omitempty"`
Key *uint32 `json:"key,omitempty"`
KeyHex string `json:"key_hex,omitempty"`
SequenceNumber *uint32 `json:"sequence_number,omitempty"`
AckNumber *uint32 `json:"ack_number,omitempty"`
PPTPPayloadLen *uint16 `json:"pptp_payload_length,omitempty"`
PPTPCallID *uint16 `json:"pptp_call_id,omitempty"`
HeaderBytes int `json:"header_bytes"`
PayloadLength int `json:"payload_length"`
PayloadHex string `json:"payload_hex,omitempty"`
TotalBytes int `json:"total_bytes"`
Notes []string `json:"notes,omitempty"`
}
Result is the top-level decoded view.
Source Files