TCP/IP Tutorial and Technical Overview - IBM Redbooks

Client host SOCKS host
Client
IdentD
ep1 1080
113
Secure
network
ep2
epn: Ephemeral port number n
sss: Server port number sss
Figure 22-43 Socks TCP segment flow
Proxy Server
SOCKS
Non-secure
network
Server host
Real
server
The SOCKSv5 server listens for connections on a given port, usually 1080.
According to the connection type (TCP or UDP), the steps discussed in the
following sections establish a connection.
SOCKSv5 TCP connection
To establish a connection using TCP, the client first sends a TCP packet that
contains session request information through port 1080 to the server (see
Figure 22-43). If the access permissions allow this operation and the connection
request succeeds, the client enters an authentication negotiation. In this state,
the authentication type is determined, after which the client sends a relay
request. The SOCKSv5 server evaluates the request and either establishes the
connection or rejects it. The client sends the following message, which contains
a version identifier and method options (Figure 22-44).
Figure 22-44 SOCKSv5: Version identifier and method selection message format
ep3
1 byte 1 byte 1 to 255 bytes
ver nmethods methods
sss
Chapter 22. TCP/IP security 849