Simple Proxy Protocol Header
The client source IP and port is encoded in a fixed-length, 38-octet long header and prepended to the payload of each proxied UDP datagram in the format described below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Magic Number | |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| |+ +| |+ Client Address +| |+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| | |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| |+ +| |+ Proxy Address +| |+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| | Client Port |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Proxy Port | Payload... |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+The contents of the header are below.
16-bit fixed value set to 0x56EC for SPP. This field should be used to identify the SPP protocol and its SPP 38-byte header.
128-bit address of the originator of the proxied UDP datagram, that is, the client. An IPv6 address if the client used IPv6 addressing, or an IPv4-mapped IPv6 address (refer to RFC 4291 ↗) in case of an IPv4 client.
128-bit address of the recipient of the proxied UDP datagram, that is the proxy. Contents should be interpreted in the same way as the Client Address.
16-bit source port number of the proxied UDP datagram. In other words, the UDP port number from which the client sent the datagram.
16-bit destination port number of the proxied UDP datagram. In other words, the UDP port number on which the proxy received the datagram.
Data following the header carried by the datagram. Magic number, addresses, and port numbers are encoded in network byte order.
A corresponding C structure describing the header is:
struct { uint16_t magic; uint8_t client_addr[16]; uint8_t proxy_addr[16]; uint16_t client_port; uint16_t proxy_port;};