Almost Internet with SLiRP and PPP

Connecting to the Internet with Linux.
The Local Half: Installing PPP Under Linux

SLiRP is only half of your net connection; you also need to activate the other half of the connection on your local Linux machine. Linux, like SLiRP, “speaks” both SLIP and PPP—that is, support for TCP/IP networking over both SLIP connections and PPP connections is built into the Linux kernel. However, although both are available under Linux, I highly recommend using PPP instead of SLIP, for the following reasons:

  • PPP is an Internet Standard Protocol—this means that it has undergone a standardization process approved by the Internet Architecture Board (IAB) and is an official part of the Internet Protocol Suite. SLIP, by contrast, is an “Internet non-standard” and is not on the standard track.

  • PPP will work over some connections that are not 8-bit-transparent; SLIP will not.

  • PPP can support authentication, peer address negotiation, packet header compression, and point-to-point error correction; SLIP can support none of these (although Compressed SLIP, or CSLIP, does support packet header compression).

In short, it is my opinion that PPP is capable of providing a more robust and reliable connection than SLIP without significantly reducing the apparent speed of the connection.

PPP support under Linux comes in two halves. One half is part of the network system drivers, and comes built into the Linux kernel. The other half is a daemon process called pppd, which comes as a separate package.

Most distributions today come with PPP support built in, but if your Linux system is built from an older distribution, you may need to install PPP support. In order to properly install PPP networking under Linux, you need gcc installed, and either:

  • A Linux kernel version 1.1.13 or greater with PPP support compiled in, or

  • A Linux kernel version 1.0 or greater, plus either some experience compiling a Linux kernel or someone to help you do it.

If you don't know whether your kernel has PPP support compiled in, look for a message similar to the following when you boot up your machine:

PPP: version 0.2.7 (4 channels) [...]

You can also use the command dmesg | grep PPP to search the kernel boot messages for the above line. If no PPP messages exist, you will need to recompile your kernel to add PPP support (unless you are using the PPP kernel module; some distributions come with a PPP module ready to use). If you don't know what version your kernel is, use the uname -a command to display information about your system. If your kernel version is less than 1.1.13, you may need to recompile your kernel with a new PPP driver in order to use PPP. If you need to recompile your kernel, and especially if you need to install a newer PPP driver, see the documentation accompanying the pppd package and the Linux Documentation Project's PPP-HOWTO and Kernel-HOWTO for more information.

The steps involved in setting up PPP are as follows:

Getting pppd

The latest version of pppd at the writing of this article is ppp-2.1.2d.tar.gz. You may already have pppd; many Linux distributions include it. You can check for it with the following command: find / -name "pppd*" -print. If your distribution already has pppd installed, you can proceed to the configuration step. If you're going to be compiling pppd, untar the source package somewhere useful (such as the place you untarred SLiRP).

Compiling and installing pppd

The pppd package comes with thorough instructions, including some specifically for Linux users, located in pppd-2.1.2d/README and pppd-2.1.2d/README.linux. I recommend you read both those documents and any documentation located in pppd-2.1.2d/linux before compiling and installing pppd. Then, change to the pppd-2.1.2d/pppd directory and create a Makefile using cp Makefile.linux Makefile (I recommend copying instead of linking so that you can make changes to the makefile if necessary without affecting the original). Inspect the makefile to make sure BINDIR and MANDIR are set to the correct location to install the pppd binary and manual page respectively (to comply with the Linux Filesystem Standard, they should probably be set to /usr/sbin and /usr/man). Make any required changes to the makefile using your favorite text editor. Build pppd by running make. To install pppd, first become the superuser (either by logging in as root or using the su command) and type make install; this installs both the pppd binary and the manual page. If you are replacing an older version of pppd, you may wish to make a backup copy of the older pppd until you are sure the new one works correctly. [If you are using the latest development kernel, you will need the latest version of pppd as well; it will be the latest in the 2.2.0 series, kept in the same places as the older 2.1.2 series pppd sources—ED]