PPP for Linux ------------- Paul Mackerras 17 April 2000 for ppp-2.4.0 1. Introduction --------------- The Linux PPP implementation includes both kernel and user-level parts. This package contains the user-level part, which consists of the PPP daemon (pppd) and associated utilities. In the past this package has contained updated kernel drivers. This is no longer necessary, as the current 2.2 and 2.3 kernel sources contain up-to-date drivers. The Linux PPP implementation is capable of being used both for initiating PPP connections (as a `client') or for handling incoming PPP connections (as a `server'). Note that this is an operational distinction, based on how the connection is created, rather than a distinction that is made in the PPP protocols themselves. Mostly this package is used for PPP connections over modems connected via asynchronous serial ports, so this guide concentrates on this situation. The PPP protocol consists of two parts. One is a scheme for framing and encoding packets, the other is a series of protocols called LCP, IPCP, PAP and CHAP, for negotiating link options and for authentication. This package similarly consists of two parts: a kernel module which handles PPP's low-level framing protocol, and a user-level program called pppd which implements PPP's negotiation protocols. The kernel module assembles/disassembles PPP frames, handles error detection, and forwards packets between the serial port and either the kernel network code or the user-level program pppd. IP packets go directly to the kernel network code. So once pppd has negotiated the link, it in practice lies completely dormant until you want to take the link down, when it negotiates a graceful disconnect. 2. Installation --------------- 2.1 Kernel driver Assuming you are running a recent 2.2 or 2.3 (or later) series kernel, the kernel source code will contain an up-to-date kernel PPP driver. If the PPP driver was included in your kernel configuration when your kernel was built, then you only need to install the user-level programs. Otherwise you will need to get the source tree for your kernel version, configure it with PPP included, and recompile. Most Linux distribution vendors ship kernels with PPP included in the configuration. The PPP driver can be either compiled into the kernel or compiled as a kernel module. If it is compiled into the kernel, the PPP driver is included in the kernel image which is loaded at boot time. If it is compiled as a module, the PPP driver is present in one or more files under /lib/modules and is loaded into the kernel when needed. The 2.2 series kernels contain an older version of the kernel PPP driver, one which doesn't support multilink. If you want multilink, you need to run the latest 2.3 series kernel. The kernel PPP driver was completely rewritten for the 2.3 series kernels to support multilink and to allow it to operate over diverse kinds of communication medium (the 2.2 driver only operates over serial ports and devices which look like serial ports, such as pseudo-ttys). Under the 2.2 kernels, if PPP is compiled as a module, the PPP driver modules should be present in the /lib/modules/`uname -r`/net directory (where `uname -r` represents the kernel version number). The PPP driver module itself is called ppp.o, and there will usually be compression modules there, ppp_deflate.o and bsd_comp.o, as well as slhc.o, which handles TCP/IP header compression. If the PPP driver is compiled into the kernel, the compression code will still be compiled as modules, for kernels before 2.2.17pre12. For 2.2.17pre12 and later, if the PPP driver is compiled in, the compression code will also. Under the 2.3 kernels, there are two PPP modules, ppp_generic.o and ppp_async.o, plus the compression modules (ppp_deflate.o, bsd_comp.o and slhc.o). If the PPP generic driver is compiled into the kernel, the other four can then be present either as modules or compiled into the kernel. There is a sixth module, ppp_synctty.o, which is used for synchronous tty devices such as high-speed WAN adaptors. 2.2 User-level programs If you obtained this package in .rpm or .deb format, you simply follow the usual procedure for installing the package. If you are using the .tar.gz form of this package, then cd into the ppp-2.4.0b2 directory you obtained by unpacking the archive and issue the following commands: $ ./configure $ make # make install The `make install' has to be done as root. This makes and installs four programs and their man pages: pppd, chat, pppstats and pppdump. If the /etc/ppp configuration directory doesn't exist, the `make install' step will create it and install some default configuration files. 2.3 System setup for 2.3 kernels Under the 2.3 series kernels, pppd needs to be able to open /dev/ppp, character device (108,0). If you are using devfs (the device filesystem), the /dev/ppp node will automagically appear when the ppp_generic module is loaded, or at startup if ppp_generic is compiled in. If you have ppp_generic as a module, and you are using devfsd (the devfs daemon), you will need to add a line like this to your /etc/devfsd.conf: LOOKUP ppp MODLOAD Otherwise you will need to create a /dev/ppp device node with the commands: # mknod /dev/ppp c 108 0 # chmod 600 /dev/ppp If you use module autoloading and have PPP as a module, you will need to add the following to your /etc/modules.conf or /etc/conf.modules: /dev/ppp ppp_generic alias char-major-108 ppp_generic alias tty-ldisc-3 ppp_async alias tty-ldisc-14 ppp_synctty alias ppp-compress-21 bsd_comp alias ppp-compress-24 ppp_deflate alias ppp-compress-26 ppp_deflate 2.4 System setup under 2.2 series kernels Under the 2.2 series kernels, you should add the following to your /etc/modules.conf or /etc/conf.modules: alias tty-ldisc-3 ppp alias ppp-compress-21 bsd_comp alias ppp-compress-24 ppp_deflate alias ppp-compress-26 ppp_deflate 3. Getting help with problems ----------------------------- If you have problems with your PPP setup, or you just want to ask some questions, or better yet if you can help others with their PPP questions, then you should join the linux-ppp mailing list. Send an email to majordomo@vger.rutgers.edu with a line in the body saying subscribe linux-ppp To leave the mailing list, send an email to majordomo@vger.rutgers.edu with a line in the body saying unsubscribe linux-ppp To send a message to the list, email it to linux-ppp@vger.rutgers.edu. You don't have to be subscribed to send messages to the list. You can also email me (paulus@linuxcare.com.au) but I am overloaded with email and I can't respond to most messages I get in a timely fashion. There are also several relevant news groups, such as comp.protocols.ppp, comp.os.linux.networking, or comp.os.linux.setup. 4. Configuring your dial-out PPP connections -------------------------------------------- Some Linux distribution makers include tools in their distributions for setting up PPP connections. For example, for Red Hat Linux and derivatives, you should probably use linuxconf or netcfg to set up your PPP connections. The two main windowing environments for Linux, KDE and Gnome, both come with GUI utilities for configuring and controlling PPP dial-out connections. They are convenient and relatively easy to configure. A third alternative is to use a PPP front-end package such as wvdial or ezppp. These also will handle most of the details of talking to the modem and setting up the PPP connection for you. Assuming that you don't want to use any of these tools, you want to set up the configuration manually yourself, then read on. This document gives a brief description and example. More details can be found by reading the pppd and chat man pages and the PPP-HOWTO. We assume that you have a modem that uses the Hayes-compatible AT command set connected to an async serial port (e.g. /dev/ttyS0) and that you are dialling out to an ISP. The trickiest and most variable part of setting up a dial-out PPP connection is the part which involves getting the modem to dial and then invoking PPP service at the far end. Generally, once both ends are talking PPP the rest is relatively straightforward. Now in fact pppd doesn't know anything about how to get modems to dial or what you have to say to the system at the far end to get it to talk PPP. That's handled by an external program such as chat, specified with the connect option to pppd. Chat takes a series of strings to expect from the modem interleaved with a series of strings to send to the modem. See the chat man page for more information. Here is a simple example for connecting to an ISP, assuming that the ISP's system starts talking PPP as soon as it answers the phone: pppd connect 'chat -v "" AT OK ATDT5551212 ~' \ /dev/ttyS0 57600 crtscts debug defaultroute Going through pppd's options in order: connect 'chat ...' This gives a command to run to contact the PPP server. Here the supplied 'chat' program is used to dial a remote computer. The whole command is enclosed in single quotes because pppd expects a one-word argument for the 'connect' option. The options to 'chat' itself are: -v verbose mode; log what we do to syslog "" don't wait for any prompt, but instead... AT send the string "AT" OK expect the response "OK", then ATDT5551212 dial the modem, then ~ wait for a ~ character, indicating the start of a PPP frame from the server /dev/ttyS0 specifies which serial port the modem is connected to 57600 specifies the baud rate to use crtscts use hardware flow control using the RTS & CTS signals debug log the PPP negotiation with syslog defaultroute add default network route via the PPP link Pppd will write error messages and debugging logs to the syslogd daemon using the facility name "daemon". These messages may already be logged to the console or to a file like /var/log/messages; consult your /etc/syslog.conf file to see. If you want to make all pppd messages go to a file such as /var/log/ppp-debug, add the line daemon.* /var/log/ppp-debug ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This is one or more tabs. Do not use spaces. to syslog.conf; make sure to put one or more TAB characters (not spaces!) between the two fields. Then you need to create an empty /var/log/ppp-debug file with a command such as touch /var/log/ppp-debug and then restart syslogd, usually by sending it a SIGHUP signal with a command like this: killall -HUP syslogd 4.1 Is the link up? The main way to tell if your PPP link is up and operational is the ifconfig ("interface configuration") command. Type /sbin/ifconfig at a shell prompt. It should print a list of interfaces including one like this example: ppp0 Link encap Point-to-Point Protocol inet addr 192.76.32.3 P-t-P 129.67.1.165 Mask 255.255.255.0 UP POINTOPOINT RUNNING MTU 1500 Metric 1 RX packets 33 errors 0 dropped 0 overrun 0 TX packets 42 errors 0 dropped 0 overrun 0 Assuming that ifconfig shows the ppp network interface, you can test the link using the ping command like this: /sbin/ping -c 3 129.67.1.165 where the address you give is the address shown as the P-t-P address in the ifconfig output. If the link is operating correctly, you should see output like this: PING 129.67.1.165 (129.67.1.165): 56 data bytes 64 bytes from 129.67.1.165: icmp_seq=0 ttl=255 time=268 ms 64 bytes from 129.67.1.165: icmp_seq=1 ttl=255 time=247 ms 64 bytes from 129.67.1.165: icmp_seq=2 ttl=255 time=266 ms --- 129.67.1.165 ping statistics --- 3 packets transmitted, 3 packets received, 0% packet loss round-trip min/avg/max = 247/260/268 ms