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Shadowsocks is a secure split proxy loosely based on SOCKS5.
client <---> ss-local <--[encrypted]--> ss-remote <---> target
The Shadowsocks local component (ss-local) acts like a traditional SOCKS5 server and provides proxy service to clients. It encrypts and forwards data streams and packets from the client to the Shadowsocks remote component (ss-remote), which decrypts and forwards to the target. Replies from target are similarly encrypted and relayed by ss-remote back to ss-local, which decrypts and eventually returns to the original client.
Addresses used in Shadowsocks follow the SOCKS5 address format:
[1-byte type][variable-length host][2-byte port]
The following address types are defined:
0x01: host is a 4-byte IPv4 address.
0x03: host is a variable length string, starting with a 1-byte length, followed by up to 255-byte domain name.
0x04: host is a 16-byte IPv6 address.
The port number is a 2-byte big-endian unsigned integer.
ss-local initiates a TCP connection to ss-remote by sending an encrypted data stream starting with the target address followed by payload data. The exact encryption scheme differs depending on the cipher used.
ss-remote receives the encrypted data stream, decrypts and parses the leading target address. It then establishes a new TCP connection to the target and forwards payload data to it. ss-remote receives reply from the target, encrypts and forwards it back to the ss-local, until ss-local disconnects.
ss-local sends an encrypted data packet containing the target address and payload to ss-remote.
Upon receiving the encrypted packet, ss-remote decrypts and parses the target address. It then sends a new data packet containing only the payload to the target. ss-remote receives data packets back from target and prepends the target address to the payload in each packet, then sends encrypted copies back to ss-local.
Essentially, ss-remote is performing Network Address Translation for ss-local.