Configuring the Point-to-Point Protocol (PPP)

Complex PPP connection scenarios

Many networking scenarios use PPP to connect two hosts via modems or a dedicated serial line. While these scenarios may require some effort to achieve a successful connection, they are simple in that they only involve configuration of one connection. This section provides a description of two more complex scenarios that require multiple connections, including one or more PPP links, to resolve the scenario's networking needs.

PPP gateway scenario

The first complex connection scenario is illustrated by ``PPP gateway scenario''.

PPP gateway scenario

The requirements of this scenario are that host A needs to access the Internet but has no direct access. To gain Internet access, host A must use PPP to connect to host B, which does have Internet access. Host B must act as a gateway to the Internet for host A. This scenario requires that:

a successful PPP link be established between host A and host B.
host B be configured as a PPP gateway (see ``PPP gateway'').
host B may need to have proxy ARP entries for other hosts to be able to send packets to host A (see ``Proxy ARP'').
host A have a default route in its IP routing table that points to the PPP interface associated with the PPP link to host B. To add such a default route, you can use the command:

route add default remote_IP_address
remote_IP_address must be the remote IP address for the PPP link to host B.

PPP server scenario

The second complex connection scenario is illustrated by ``PPP server scenario''.

PPP server scenario

The requirements of this scenario are for a single host to act as a PPP server. Multiple hosts connect to this server using PPP, and the server provides services (such as Internet access) to these hosts over the PPP connections. There are two special configuration issues for a PPP server:

the PPP server must be configured as a PPP gateway (see ``PPP gateway'').
the PPP server must be represented by a unique IP address to each client.

A configuration decision is whether this address (and the unique IP address by which the server will know the client) will be assigned by the server or by each client.

NOTE: The following configuration requirements apply only to the server configuration. Since the clients may be running a non-SCO PPP implementation, no assumptions are made about the clients' configurations.

If the server assigns addresses, this requires the following server configuration:

The server must have an incoming remote access configuration in its /etc/ppphosts file for each client from which it will accept a PPP link request.
NOTE: For servers that assign IP addresses from an IP address pool, it is possible to have more than one client log in using the same login accounts. This makes configuration simpler by requiring fewer PPP client accounts on the server.
The server must specify unique local and remote IP addresses in each client entry. These addresses may be specified at configuration time for each entry, or a pool of IP addresses may be created from which PPP chooses when the link is brought up. For more information on how to implement this choice, see ``Configuring an endpoint for remote access''.
Each client host must agree to use the remote and local IP addresses that the PPP server specifies for that client.

If the clients assign addresses, this requires the following configuration:

The remote access endpoints configured on the server that will be used by the clients must permit IP address negotiation. That is, they must not specify the noipaddr configuration parameter.
The remote access endpoints configured on the server that will be used by the clients must not specify local or remote IP addresses.
Each client must specify unique remote and local IP addresses when negotiating for a PPP link with the server.
Each client must support IP address negotiation with the server.

NOTE: It is possible for two or more clients to log in using the same login account. This makes configuration simpler by requiring fewer PPP login accounts.