Socks5协议

+----+----------+----------+
|VER | NMETHODS | METHODS  |
+----+----------+----------+
| 1  |    1     | 1 to 255 |
+----+----------+----------+

- VER       protocol version: X'05'
- NMETHODS  the length of methods
- METHODS
    - X'00' NO AUTHENTICATION REQUIRED
    - X'01' GSSAPI
    - X'02' USERNAME/PASSWORD
    - X'03' to X'7F' IANA ASSIGNED
    - X'80' to X'FE' RESERVED FOR PRIVATE METHODS
    - X'FF' NO ACCEPTABLE METHODS

+----+--------+
|VER | METHOD |
+----+--------+
| 1  |   1    |
+----+--------+

- VER       X'05' SOCKS5
- METHOD    one of the methods given in METHODS

If the selected METHOD is X'FF', none of the methods listed by the
client are acceptable, and the client MUST close the connection.

The client and server then enter a method-specific sub-negotiation.

+----+-----+-------+------+----------+----------+
|VER | CMD |  RSV  | ATYP | DST.ADDR | DST.PORT |
+----+-----+-------+------+----------+----------+
| 1  |  1  | X'00' |  1   | Variable |    2     |
+----+-----+-------+------+----------+----------+

- VER protocol version: X'05'
- CMD
    - CONNECT X'01'
    - BIND X'02'
    - UDP ASSOCIATE X'03'
- RSV    RESERVED
- ATYP   address type of following address
    - IP V4 address: X'01'
    - DOMAINNAME: X'03'
    - IP V6 address: X'04'
- DST.ADDR desired destination address
- DST.PORT desired destination port in network octet order

- ATYP and DST.ADDR
    - X'01' IPv4
        DST.ADDR 4字节
    - X'03' Host
        +---------------+----------+
        |length of host |   host   |
        +---------------+----------+
        |       1       | Variable |
        +---------------+----------+
    - X'04' IPv6
        DST.ADDR 16字节

    +----+-----+-------+------+----------+----------+
    |VER | REP |  RSV  | ATYP | BND.ADDR | BND.PORT |
    +----+-----+-------+------+----------+----------+
    | 1  |  1  | X'00' |  1   | Variable |    2     |
    +----+-----+-------+------+----------+----------+

    - VER    protocol version: X'05'
    - REP    Reply field:
       - X'00' succeeded
       - X'01' general SOCKS server failure
       - X'02' connection not allowed by ruleset
       - X'03' Network unreachable
       - X'04' Host unreachable
       - X'05' Connection refused
       - X'06' TTL expired
       - X'07' Command not supported
       - X'08' Address type not supported
       - X'09' to X'FF' unassigned
    - RSV    RESERVED, must be set to X'00'
    - ATYP   address type of following address
        - IP V4 address: X'01'
        - DOMAINNAME: X'03'
        - IP V6 address: X'04'
     - BND.ADDR server bound address
     - BND.PORT server bound port in network octet order

In the reply to a CONNECT, BND.PORT contains the port number that the
server assigned to connect to the target host, while BND.ADDR
contains the associated IP address. 

The BIND request is used in protocols which require the client to
accept connections from the server.

Two replies are sent from the SOCKS server to the client during a
BIND operation.  

The first is sent after the server creates and binds
a new socket.  The BND.PORT field contains the port number that the
SOCKS server assigned to listen for an incoming connection.  The
BND.ADDR field contains the associated IP address.  

The second reply occurs only after the anticipated incoming
connection succeeds or fails.
In the second reply, the BND.PORT and BND.ADDR fields contain the
address and port number of the connecting host.

The UDP ASSOCIATE request is used to establish an association within
the UDP relay process to handle UDP datagrams.  The DST.ADDR and
DST.PORT fields contain the address and port that the client expects
to use to send UDP datagrams on for the association.  

In the reply to a UDP ASSOCIATE request, the BND.PORT and BND.ADDR
fields indicate the port number/address where the client MUST send
UDP request messages to be relayed.

When a reply (REP value other than X'00') indicates a failure, the
SOCKS server MUST terminate the TCP connection shortly after sending
the reply.  This must be no more than 10 seconds after detecting the
condition that caused a failure.

If the reply code (REP value of X'00') indicates a success, and the
request was either a BIND or a CONNECT, the client may now start
passing data.

A UDP-based client MUST send its datagrams to the UDP relay server at
the UDP port indicated by BND.PORT in the reply to the UDP ASSOCIATE
request. Each UDP datagram carries a UDP request.

+----+------+------+----------+----------+----------+
|RSV | FRAG | ATYP | DST.ADDR | DST.PORT |   DATA   |
+----+------+------+----------+----------+----------+
| 2  |  1   |  1   | Variable |    2     | Variable |
+----+------+------+----------+----------+----------+

- RSV  Reserved X'0000'
- FRAG    Current fragment number
- ATYP    address type of following addresses:
   - IP V4 address: X'01'
   - DOMAINNAME: X'03'
   - IP V6 address: X'04'
- DST.ADDR       desired destination address
- DST.PORT       desired destination port
- DATA     user data

When a UDP relay server decides to relay a UDP datagram, it does so
silently, without any notification to the requesting client.
Similarly, it will drop datagrams it cannot or will not relay.

The UDP relay server MUST acquire from the SOCKS server the expected
IP address of the client that will send datagrams to the BND.PORT
given in the reply to UDP ASSOCIATE.  It MUST drop any datagrams
arriving from any source IP address other than the one recorded for
the particular association.

The FRAG field indicates whether or not this datagram is one of a
number of fragments.  If implemented, the high-order bit indicates
end-of-fragment sequence, while a value of X'00' indicates that this
datagram is standalone.  Values between 1 and 127 indicate the
fragment position within a fragment sequence.  Each receiver will
have a REASSEMBLY QUEUE and a REASSEMBLY TIMER associated with these
fragments.  The reassembly queue must be reinitialized and the
associated fragments abandoned whenever the REASSEMBLY TIMER expires,
or a new datagram arrives carrying a FRAG field whose value is less
than the highest FRAG value processed for this fragment sequence.
The reassembly timer MUST be no less than 5 seconds.  It is
recommended that fragmentation be avoided by applications wherever
possible.

Implementation of fragmentation is optional; an implementation that
does not support fragmentation MUST drop any datagram whose FRAG
field is other than X'00'.


The programming interface for a SOCKS-aware UDP MUST report an
available buffer space for UDP datagrams that is smaller than the
actual space provided by the operating system:

- if ATYP is X'01' - 10+method_dependent octets smaller
- if ATYP is X'03' - 262+method_dependent octets smaller
- if ATYP is X'04' - 20+method_dependent octets smaller