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44.11. Setting Up a Dialup PPP Session

Point-to-Point Protocol (PPP) is the way ISPs usually provide dialup access (largely because this is the default protocol Windows dialup uses). Unixes that can do dialup provide a PPP client, which you configure to call the ISP and set up a PPP connection. An established connection functions as a network connection -- you can use ifconfig (Section 44.8, Section 46.3) to examine it and packets will be routed to the PPP connection by default, and tools like traceroute (Section 46.4) can be used across it.

Unixes provide two ways to run PPP: kernel PPP, where the PPP code resides in the kernel and is therefore very fast but limited in features, and user PPP, where packets have to be copied back and forth between kernel space and user space, but a wide feature set can be provided. We'll give a quick overview of both.

Kernel PPP uses pppd and a fairly simple set of configuration commands. You provide pppd with the information needed to dial your modem appropriately and with whatever login information your ISP has provided you, and it connects. Generally you then have to set up /etc/resolv.conf to point to your ISP's DNS (Section 46.9) server. Some implementations of pppd don't even know how to dial the phone, and you'll have to use something like kermit to dial the phone first. pppd must also be run as root. Look at your platform's documentation for pppd for details on setting up kernel PPP on that platform.

Platforms that provide a user-space PPP client are a little easier to work with. User-space PPP clients can be run by users other than root (usually limited to a specific group); they tend to configure default routes, /etc/resolv.conf, and other details automatically; and they generally deal with PAP or CHAP authentication (which many ISPs use) a little more easily. Usually the user-space PPP client is just called ppp; look for its manpage to see what it requires to configure it.


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