Protocol specification¶
Basic protocol design¶
fastd uses UDP as the transport protocol for its packets. UDP has been chosen instead of raw IP packets (as they are used by IPIP and 6in4 tunnels or IPsec) to simplify the deployment of multiple fastd instances on the same host using different UDP ports and allow passing through common NAT routers without explicit configuration.
The first byte of the UDP payload is used to discern the different packet types
used by fastd. For now only two values for the first byte have been defined:
0x01
indicates a handshake packet, and 0x02
a data packet. All other
values are reserved for future use and must be ignored by current implementations.
Handshake format¶
The initial 0x01
byte together with the next three bytes form the 4-byte handshake header; the rest of
the packet after the header consists of a list of TLV records. The second header byte is reserved and must
always be 0x00
; the following two header bytes contain the length of the following TLV records in bytes
encoded as Big Endian.
The following TLV records start with a 2-byte type field, followed by a 2-byte length field and the arbitrary-length value. There is no special alignment defined for the TLV records. All integers that are part of the TLV format (in particular, the type and length fields) are encoded in little-endian byte order.
TLV record types¶
Record ID | Value description | Format | Values |
---|---|---|---|
0x0000 |
Handshake type | 1-byte unsigned integer | {1, 2, 3} |
0x0001 |
Reply code | 1-byte unsigned integer | {0 (success), 1 (mandatory record missing), 2 (unacceptable value)} |
0x0002 |
Error detail | 1/2-byte unsigned integer | Record type which caused an error |
0x0003 |
Flags (currently unused) | variable-length bit field | So far, no values are defined |
0x0004 |
Mode | 1-byte unsigned integer | {0 (TAP mode), 1 (TUN mode)} |
0x0005 |
Protocol name | variable-length string | “ec25519-fhmqvc” |
0x0006 |
Sender key | 32-byte public key | |
0x0007 |
Recipient key | 32-byte public key | |
0x0008 |
Sender handshake key | 32-byte public key | |
0x0009 |
Recipient handshake key | 32-byte public key | |
0x000a |
Authentication tag (obsolete) | 32-byte opaque value | Not used anymore |
0x000b |
MTU | 2-byte unsigned integer | |
0x000c |
Method name | variable-length string | |
0x000d |
Version name | variable-length string | |
0x000e |
Method list | zero-separated string list | |
0x000f |
TLV authentication tag | 32-byte opaque value |
Handshake protocol¶
The following specification describes the current handshake as it is performed by fastd versions since v11.
The handshake protocol consists of three packets. See also: ec25519, FHMQV-C
The following fields are sent in all three packets as different fastd versions expect them in different parts of the handshake:
- Mode (TUN/TAP)
- MTU
- fastd version (e.g.
v15
) - Protocol name (
ec25519-fhmqvc
)
Handshake request¶
The first packet of a handshake contains the following additional fields:
- Handshake type (0x01)
- FHMQV-C values:
- Sender key \(\hat{A}\)
- Recipient key \(\hat{B}\)
- Sender handshake key \(X\)
The recipient key may be omitted if the recipient identity is unknown because the handshake was triggered by an unexpected data packet.
Handshake reply¶
The second packet of a handshake contains the following additional fields:
- Handshake type (0x02)
- Reply code (0x00)
- Method list (list of all supported methods)
- FHMQV-C values:
- Sender key \(\hat{B}\)
- Recipient key \(\hat{A}\)
- Sender handshake key \(Y\)
- Recipient handshake key \(X\)
- TLV authentication tag \(\text{MAC}_B\)
Handshake finish¶
The second packet of a handshake contains the following additional fields:
- Handshake type (0x03)
- Reply code (0x00)
- Method (the chosen encryption/authentication scheme)
- FHMQV-C values:
- Sender key \(\hat{A}\)
- Recipient key \(\hat{B}\)
- Sender handshake key \(X\)
- Recipient handshake key \(Y\)
- TLV authentication tag \(\text{MAC}_A\)
Handshake error¶
When an unacceptable handshake is received, fastd will respond with an error packet. The error packet contains the following fields:
- Handshake type (the type of the packet that is answered plus 1)
- Reply code (0x01 when a record is missing from the handshake, 0x02 when a value is unacceptable)
- Error detail (the record type ID which caused the error)
Payload packets¶
The payload packet structure is defined by the methods; at the moment most methods use the same format, starting with a 24 byte header, followed by the actual payload:
- Byte 1: Packet type (0x02)
- Byte 2: Flags (method-specific; unused, always 0x00)
- Bytes 3-8: Packet sequence number/nonce (big endian; incremented by 2 for each packet; one side of a connection uses the even sequence numbers and the other side the odd ones)
- Bytes 9-24: Authentication tag (method-specific)
The null
method uses only a 1 byte header: The packet type is directly followed by the payload data.