TCP/IP Tutorial and Technical Overview - IBM Redbooks

circuit-level gateway (see Figure 22-18 on page 805). However, in practice, there
are significant differences between them, such as:
► Circuit-level gateways can handle several TCP/IP applications, as well as
UDP applications, without any extra modifications on the client side for each
application. Therefore, this makes circuit-level gateways a good choice to
satisfy user requirements.
► Circuit-level gateways do not provide packet processing or filtering.
Therefore, a circuit-level gateway is generally referred to as a transparent
gateway.
► Application-level gateways have a lack of support for UDP.
► Circuit-level gateways are often used for outbound connections, while
application-level gateways (proxy) are used for both inbound and outbound
connections. Generally, when using both types combined, circuit-level
gateways can be used for outbound connections and application-level
gateways can be used for inbound connections to satisfy both security and
user requirements.
Circuit-level gateways can sometimes handle incoming UDP packets or TCP
connections. However, a client on the secure side must inform the gateway to
expect such packets. SOCKS v5 has this capability.
A well-known example of a circuit-level gateway is SOCKS (refer to 22.5,
“SOCKS” on page 846 for more information). Because the data that flows over
SOCKS is not monitored or filtered, a security problem can arise. To minimize
security problems, trusted services and resources need to be used on the
outside network (untrusted network).
804 TCP/IP Tutorial and Technical Overview