17.4.3.3. Binding and Redirection Options

The service configuration files for xinetd support binding the service to an IP address and redirecting incoming requests for that service to another IP address, hostname, or port.
Binding is controlled with the bind option in the service-specific configuration files and links the service to one IP address on the system. Once configured, the bind option only allows requests for the proper IP address to access the service. In this way, different services can be bound to different network interfaces based on need.
This is particularly useful for systems with multiple network adapters or with multiple IP addresses configured. On such a system, insecure services, like Telnet, can be configured to listen only on the interface connected to a private network and not to the interface connected with the Internet.
The redirect option accepts an IP address or hostname followed by a port number. It configures the service to redirect any requests for this service to the specified host and port number. This feature can be used to point to another port number on the same system, redirect the request to different IP address on the same machine, shift the request to a totally different system and port number, or any combination of these options. In this way, a user connecting to certain service on a system may be rerouted to another system with no disruption.
The xinetd daemon is able to accomplish this redirection by spawning a process that stays alive for the duration of the connection between the requesting client machine and the host actually providing the service, transferring data between the two systems.
But the advantages of the bind and redirect options are most clearly evident when they are used together. By binding a service to a particular IP address on a system and then redirecting requests for this service to a second machine that only the first machine can see, an internal system can be used to provide services for a totally different network. Alternatively, these options can be used to limit the exposure of a particular service on a multi-homed machine to a known IP address, as well as redirect any requests for that service to another machine specially configured for that purpose.
For example, consider a system that is used as a firewall with this setting for its Telnet service:
service telnet
{
        socket_type		= stream
        wait			= no
        server			= /usr/sbin/in.telnetd
        log_on_success		+= DURATION USERID
        log_on_failure		+= USERID
        bind                    = 123.123.123.123
        redirect                = 10.0.1.13 23
}
The bind and redirect options in this file ensures that the Telnet service on the machine is bound to the external IP address (123.123.123.123), the one facing the Internet. In addition, any requests for Telnet service sent to 123.123.123.123 are redirected via a second network adapter to an internal IP address (10.0.1.13) that only the firewall and internal systems can access. The firewall then send the communication between the two systems, and the connecting system thinks it is connected to 123.123.123.123 when it is actually connected to a different machine.
This feature is particularly useful for users with broadband connections and only one fixed IP address. When using Network Address Translation (NAT), the systems behind the gateway machine, which are using internal-only IP addresses, are not available from outside the gateway system. However, when certain services controlled by xinetd are configured with the bind and redirect options, the gateway machine can act as a proxy between outside systems and a particular internal machine configured to provide the service. In addition, the various xinetd access control and logging options are also available for additional protection.