SOCKS 4A: A Simple Extension to SOCKS 4 Protocol

本文链接：https://blog.youkuaiyun.com/vptr_grid/article/details/434765

本文介绍了SOCKS 4A，它是SOCKS 4协议的扩展，旨在让无法解析所有域名的主机使用SOCKS。文中说明了SOCKS 4客户端向服务器请求操作的数据包格式，以及4A版本中客户端无法解析域名时的处理方式，还提及服务器对请求包的检查和处理。

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	SOCKS 4A: A  Simple Extension to SOCKS 4 Protocol

			Ying-Da Lee
		yingda@best.com  or  yingda@esd.sgi.com

Please read SOCKS4.protocol first for an description of the version 4
protocol. This extension is intended to allow the use of SOCKS on hosts
which are not capable of resolving all domain names.

In version 4, the client sends the following packet to the SOCKS server
to request a CONNECT or a BIND operation:

		+----+----+----+----+----+----+----+----+----+----+....+----+
		| VN | CD | DSTPORT |      DSTIP        | USERID       |NULL|
		+----+----+----+----+----+----+----+----+----+----+....+----+
 # of bytes:	   1    1      2              4           variable       1

VN is the SOCKS protocol version number and should be 4. CD is the
SOCKS command code and should be 1 for CONNECT or 2 for BIND. NULL
is a byte of all zero bits.

For version 4A, if the client cannot resolve the destination host's
domain name to find its IP address, it should set the first three bytes
of DSTIP to NULL and the last byte to a non-zero value. (This corresponds
to IP address 0.0.0.x, with x nonzero. As decreed by IANA  -- The
Internet Assigned Numbers Authority -- such an address is inadmissible
as a destination IP address and thus should never occur if the client
can resolve the domain name.) Following the NULL byte terminating
USERID, the client must sends the destination domain name and termiantes
it with another NULL byte. This is used for both CONNECT and BIND requests.

A server using protocol 4A must check the DSTIP in the request packet.
If it represent address 0.0.0.x with nonzero x, the server must read
in the domain name that the client sends in the packet. The server
should resolve the domain name and make connection to the destination
host if it can. 

SOCKSified sockd may pass domain names that it cannot resolve to
the next-hop SOCKS server.