1 PPP for Linux Version 2.2.0
6 Michael Callahan callahan@maths.ox.ac.uk
7 Al Longyear longyear@netcom.com
12 CHANGES FROM THE PREVIOUS VERSION
15 GENERAL NETWORK CONFIGURATION
16 CONNECTING TO A PPP SERVER
19 IF IT STILL DOESN'T WORK (OR, BUG REPORTS)
20 DYNAMIC ADDRESS ASSIGNMENT
21 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS
22 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS WITH DYNAMIC IP
23 ADDITIONAL INFORMATION
30 This file is substantially derived from the previous version for
31 the pppd process 2.2.0. Michael Callahan wrote that version. This
32 particular version was written, modified, hacked, changed, whatever,
33 by Al Longyear. If you find errors in this document, they are probably
34 mine and not Michael's.
36 This is a PPP driver for Linux. It has been used by many people and
37 seems to be quite stable. It is capable of being used either as a
38 'client'--for connecting a Linux machine to a local Internet provider,
39 for example--or as a 'server'--allowing a Linux machine with a modem
40 and an Ethernet connection to the Internet to provide dial-in PPP
41 links. (In fact, the PPP protocol does not make the distinction
42 between client and server, but this is the way people often think
45 The PPP protocol consists of two parts. One is a scheme for framing
46 and encoding packets, the other is a series of protocols called LCP,
47 IPCP, UPAP and CHAP, for negotiating link options and for
48 authentication. This package similarly consists of two parts: a
49 kernel module which handles PPP's low-level framing protocol, and a
50 user-level program called pppd which implements PPP's negotiation
53 The kernel module assembles/disassembles PPP frames, handles error
54 detection, and forwards packets between the serial port and either the
55 kernel network code or the user-level program pppd. IP packets go
56 directly to the kernel network code. So once pppd has negotiated the
57 link, it in practice lies completely dormant until you want to take
58 the link down, when it negotiates a graceful disconnect.
63 I (MJC) wrote the original kernel driver from scratch. Laurence
64 Culhane and Fred van Kempen's slip.c was priceless as a model (a
65 perusal of the files will reveal that I often mimicked what slip.c
66 did). Otherwise I just implemented what pppd needs, using RFC1331 as
67 a guide. For the most part, the Linux driver provides the same
68 interface as the free 386BSD and SunOS drivers. The exception is that
69 Linux has no support for asynchronous I/O, so I hacked an ioctl into
70 the PPP kernel module that provides a signal when packets appear and
71 made pppd use this instead.
73 Al Longyear ported version 2.2 of pppd (from the free package
74 ppp-2.2.0) to Linux. He also provided several enhancements to both
75 the kernel driver and the OS-independent part of pppd. His
76 contributions to Linux PPP have been immense, and so this release
77 is being distributed over both our names.
79 The pppd program comes from the free distribution of PPP for Suns and
80 386BSD machines, maintained by Paul Mackerras. This package lists
81 "thanks to" Brad Parker, Greg Christy, Drew D. Perkins, Rick Adams and
84 Jim Freeman added the code to support a ppp module and to dynamically
85 extend the number of ppp devices. The Space.c module should not have
86 any devices defined for this logic to work.
89 CHANGES FROM THE PREVIOUS VERSION
91 - The number of devices for the PPP device has been made dynamic. It was
92 previously configured with the default value of four devices.
94 - The problems dealing with other systems such as Windows NT and their
95 authenticiation has been corrected. It will now generate the proper
96 responses to allow the system to choose a valid authentication protocol.
98 - The kernel debug value has changed. Previously it was a level. It is now
99 a bit map with various bits meaning certain types of debug information.
101 0 - No debug information is generated
103 2 - Log incoming packets
104 4 - Log outgoing packets
105 8 - Log tty output buffers
106 16 - Log tty input buffers
108 If you wish to use any combination, add the values together. For example,
109 '7' will log debug messages and incoming packages and outgoing packets.
111 The default setting is 0.
113 The simple IP trace which ppp.c performed when 'kdebug' was greater than
114 127 has been removed. You should use tcpdump for this type of trace
117 - Support is added for compression control protocol. At the present time
118 only the BSD-Compress compression protocol is supported.
120 - There are two queues for output frames. This avoids some problems which
121 occured with the previous version and some PPP packages which exchanged
122 echo frames with Linux.
124 - The echo frames are now proper. Previously, it would generate extra
125 characters and this caused some providers to not respond to the 'junk'.
127 - The max-echo-failure option will now properly disconnect the line.
129 - There are other changes which are listed in the general README file. Please
130 read that file as well for changes.
132 - There is no limit to the number of ppp devices which you may use. Jim Freeman
133 has added code to create them upon demand and to re-use the ones which have
134 been closed. There is no code, nor plans to write code, to remove (delete)
135 the un-used devices. So, if your system goes to a spurt and uses 3000 ppp
136 devices, it will remain at that level until you next reload the kernel.
141 The IPX support is still minimal. There is code which will only work with
142 the 1.3 version of the networking software. The pppd process will still
143 require changes to support the IPXCP and a change to the driver to properly
144 enable/disable the IPX frames. Jim Freeman is reportily working on the IPX
150 This version of PPP has been tested on 1.1.x (x>=14) It will probably
151 not work on kernels much earlier than this due to a change in the
152 routing code. If you have an earlier kernel, please upgrade.
154 joining the PPP channel of linux-activists:
156 This isn't really part of installation, but if you DO use
157 Linux PPP you should do this. Send a message with the line
159 contained in the body to majordomo@vger.rutgers.edu
163 subscribe linux-ppp myname@mail.address
165 if you wish the linux-ppp information sent to a different mail
168 To leave the mail list, send 'unsubscribe linux-ppp' to the same
171 You can send to the list by mailing to
172 linux-ppp@vger.rutgers.edu. This is a majordomo mailing list and
173 is unlike the earlier version on hut.fi. There is no magic header
174 required for this list. In addition, it is mirrored to the usenet
175 group linux.act.ppp. You may choose to read the few messages posted
180 There are three applicable usenet news groups for the PPP code. Please
181 choose the group which applies the closest to the type of problem
182 which you are experiencing.
184 comp.os.linux.networking
185 - Trouble setting routes, running name services, using telnet, ftp,
187 - It will not compile.
190 - Trouble installing the package from BINARIES only. This does *NOT*
191 include problems with compiling the package.
194 - How do I use the package?
195 - How do I connect to .... services?
196 - Why does this not work?
197 - All other types of questions on how to use just the PPP code.
199 PLEASE don't assume that just because you are using PPP as your
200 network device driver that all problems with your networking are a
201 problem of PPP. PPP is *NOT* responsible for your modem disconnecting,
202 routing to other servers, running telnet, etc. Calling the problem
203 'ppp' on usenet may cause it to be ignored by the people who
204 actually work on the networking code.
206 Installation procedure:
208 The installation procedure has been totally revised for this
209 version. Due to feedback from other users, it was felt that a more
210 automated installation procedure be performed.
212 Use the following procedure for all kernel versions. There are six steps
213 numbered one through six. Please do them in order and not skip one.
215 1. Issue the command:
219 from the top level directory of pppd. This is the directory which
220 contains this README.linux file. The result of this will be to build a
221 set of symbolic links to the makefiles. They should link 'Makefile' to
222 'Makefile.linux' in each of the directories.
224 2. Issue the command:
228 from the top level directory. This will install the various include
229 files and source files into the proper directory for the linux
230 kernel. If you don't have the kernel installed in the /usr/src/kernel
231 directory then it will not work. Instead it will print a message to
232 the effect that you need to specify the kernel location on the
235 The actual message will say:
237 There appears to be no kernel source distribution in /usr/src/linux.
238 Give the top-level kernel source directory as the argument to
240 usage: kinstall.sh [linux-source-directory]
242 If, and only if, you receive this message, do the following:
244 a. Change to the 'linux' directory with the command:
248 b. Issue the command:
250 ./kinstall.sh /usr/src/linux
252 or use the proper location for the kernel rather than
253 /usr/src/linux. For example, if you have the kernel installed in
254 /usr1/kernel then the command would be:
256 ./kinstall.sh /usr1/kernel
258 The script will validate that the kernel is properly installed into
259 that directory and check the level of the kernel. The installation
260 will not be accepted if your kernel is too early.
262 The installation procedure will copy only the files which are
263 needed. It will not replace any file which should not be
264 replaced. Please don't second-guess the installation script and
265 attempt to do the procedure on your own. There are some very subtle
266 dependencies and if you are not careful, the installation will not
269 You are free to run the installation script as many times as you
270 wish. The additional executions will only change the files which have
275 You must rebuild the kernel with this package. The driver is totally
276 new and will not work with the older daemon and the newer daemon will
277 not work with the older kernel driver. If you don't know how to build
278 a kernel, then you should read the README file in the kernel source
281 4. Build the programs.
283 The programs are built next. The command to build the programs is fairly
284 simple. Just issue the command:
288 from the top level directory where this README.linux file is located.
290 5. Install the programs.
292 You may use the command
296 to install the various programs. They will be installed into the
297 /usr/lib/ppp directory. You may not like this directory for the
298 executables. The directory name is called BINDIR and is set in the
299 file 'linux/Makefile.linux'.
301 6. Reboot to the new kernel.
303 After building the new kernel, you will need to actually use it. Reboot
304 the Linux system and you may then use the new pppd program.
307 GENERAL NETWORK CONFIGURATION
309 Since many people don't use the Linux networking code at all until
310 they get a PPP link, this section describes generally what's needed to
311 get things running. In principle none of this is special to PPP. For
312 more details, you should consult the relevant Linux HOWTOs. If you
313 already understand network setup, you can skip this section.
315 The first file that requires attention is the rc script that does
316 network configuration at boot time, called /etc/rc.net or
317 /etc/rc.d/rc.net.{1,2} or something similar, depending on your Linux
318 distribution. This file should 'ifconfig' the loopback interface lo,
319 and should add an interface route for it. These lines might look
325 /sbin/ifconfig lo 127.0.0.1
326 /sbin/route add 127.0.0.1
328 However, it should *not* config an ethernet card or install any other
329 routes (unless you actually have an ethernet card, in which case I'll
330 assume you know what to do). Many distributions will provide scripts
331 that expect you to have an ethernet card.
333 You also need to decide whether you want to allow incoming
334 telnet/ftp/finger, etc. If so, you should have the rc startup script
335 run the 'inetd' daemon.
337 Next, you should set up /etc/hosts to have two lines. The first
338 should just give the loopback or localhost address and the second
339 should give your own host name and the IP address your PPP connection
340 will use. For example:
342 127.0.0.1 loopback localhost # useful aliases
343 192.1.1.17 billpc.president.whitehouse.gov bill # my hostname
344 192.1.1.23 chelseapc.president.whitehouse.gov chelseapc
346 where my IP address is 192.1.1.17 and my hostname is
347 billpc.president.whitehouse.gov. (Not really, you understand.) If
348 your PPP server does dynamic IP address assignment, give a guess as to
349 an address you might get (see also "Dynamic Address Assignment"
352 Finally, you need to configure the domain name system by putting
353 appropriate lines in /etc/resolv.conf . It should look something like
356 domain president.whitehouse.gov
357 search president.whitehouse.gov whitehouse.gov
359 nameserver 192.1.2.10
361 Assuming there are nameservers at 192.1.2.1 and 192.1.2.10, then when
362 you get connected with PPP, you can reach hosts whose full names are
363 'hillarypc.president.whitehouse.gov' and 'chelseapc.whitehouse.gov' by
364 the names 'hillarypc' and 'chelseapc'. You can probably find out the
365 right domain name to use and the IP numbers of nameservers from
366 whoever's providing your PPP link.
369 CONNECTING TO A PPP SERVER
371 To use PPP, you invoke the pppd program with appropriate options.
372 Everything you need to know is contained in the pppd(8) manual page.
373 However, it's useful to see some examples:
375 Example 1: A simple dial-up connection.
377 Here's a command for connecting to a PPP server by modem.
379 pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
380 /dev/cua1 38400 debug crtscts modem defaultroute 192.1.1.17
382 Going through pppd's options in order:
383 connect 'chat etc...' This gives a command to run to contact the
384 PPP server. Here the supplied 'chat' program is used to dial a
385 remote computer. The whole command is enclosed in single quotes
386 because pppd expects a one-word argument for the 'connect' option.
387 The options to 'chat' itself are:
389 -v verbose mode; log what we do to syslog
390 "" don't wait for any prompt, but instead...
391 ATDT5551212 dial the modem, then
392 CONNECT wait for answer
393 "" send a return (null text followed by usual return)
394 ogin: ppp word: whitewater log in.
396 /dev/cua1 specify the callout serial port cua1
397 38400 specify baud rate
398 debug log status in syslog
399 crtscts use hardware flow control between computer and modem
400 (at 38400 this is a must)
401 modem indicate that this is a modem device; pppd will hang up the
402 phone before and after making the call
403 defaultroute once the PPP link is established, make it the default
404 route; if you have a PPP link to the Internet this
405 is probably what you want
407 192.1.1.17 this is a degenerate case of a general option
408 of the form x.x.x.x:y.y.y.y . Here x.x.x.x is the local IP
409 address and y.y.y.y is the IP address of the remote end of the
410 PPP connection. If this option is not specified, or if just
411 one side is specified, then x.x.x.x defaults to the IP address
412 associated with the local machine's hostname (in /etc/hosts),
413 and y.y.y.y is determined by the remote machine. So if this
414 example had been taken from the fictional machine 'billpc',
415 this option would actually be redundant.
417 pppd will write error messages and debugging logs to the syslogd
418 daemon using the facility name "daemon". These messages may already be
419 logged to the console or to a file like /usr/adm/messages; consult
420 your /etc/syslog.conf file to see. If you want to make all pppd
421 messages go to the console, add the line
423 daemon.* /dev/console
424 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
425 This is one or more tabs. Do not use spaces.
427 to syslog.conf; make sure to put one or more TAB characters between
430 Example 2: Connecting to PPP server over hard-wired link.
432 This is a slightly more complicated example. This is the script I run
433 to make my own PPP link, which is over a hard-wired Gandalf link to an
434 Ultrix machine running Morningstar PPP.
436 pppd connect /etc/ppp/ppp-connect defaultroute noipdefault debug \
437 kdebug 0 /dev/cua0 9600
439 Here /etc/ppp/ppp-connect is the following script:
442 chat -v -t60 "" \; "service :" blackice ogin: callahan word: PASSWORD \
443 black% "stty -echo; ppp" "Starting PPP now" && sleep 5
445 This sends a break to wake up my terminal server, sends a semicolon
446 (which lets my terminal server do autobaud detection), then says we
447 want the service "blackice". It logs in, waits for a shell prompt
448 ("black%"), then starts PPP. The -t60 argument sets the timeout to a
449 minute, since things here are sometimes very slow.
451 The "&& sleep 5" causes the script to pause for 5 seconds, unless chat
452 fails in which case it exits immediately. This is just to give the
453 PPP server time to start (it's very slow). Also, the "stty -echo"
454 turned out to be very important for me; without it, my pppd would
455 sometimes start to send negotiation packets before the remote PPP
456 server had time to turn off echoing. The negotiation packets would
457 then get sent back to my local machine, be rejected (PPP is able to
458 detect loopback) and pppd would fail before the remote PPP server even
459 got going. The "stty -echo" command prevents this confusion. This
460 kind of problem should only ever affect a *very* few people who
461 connect to a PPP server that runs as a command on a slow Unix machine,
462 but I wanted to mention it because it took me several frustrating
465 The pppd options are mostly familiar. Two that are new are
466 "noipdefault" and "kdebug 1". "noipdefault" tells pppd to ask the
467 remote end for the IP address to use; this is necessary if the PPP
468 server implements dynamic IP address assignment as mine does (i.e., I
469 don't know what address I'll get ahead of time). "kdebug 1" sets the
470 kernel debugging level to 1, enabling slightly chattier messages from
473 Anyway, assuming your connection is working, you should see chat dial
474 the modem, then perhaps some messages from pppd (depending on your
475 syslog.conf setup), then some kernel messages like this:
477 ppp: channel ppp0 mtu changed to 1500
478 ppp: channel ppp0 open
479 ppp: channel ppp0 going up for IP packets!
481 (These messages will only appear if you gave the option "kdebug 1" and
482 have kern.info messages directed to the screen.) Simultaneously, pppd
483 is also writing interesting things to /usr/adm/messages (or other log
484 file, depending on syslog.conf).
489 If you think you've got a connection, there are a number of things you
496 (ifconfig may live elsewhere, depending on your distribution.)
497 This should show you all the network interfaces that are 'UP'. ppp0
498 should be one of them, and you should recognize the first IP address
499 as your own and the "P-t-P address" (or point-to-point address) the
500 address of your server. Here's what it looks like on my machine:
502 lo Link encap Local Loopback
503 inet addr 127.0.0.1 Bcast 127.255.255.255 Mask 255.0.0.0
504 UP LOOPBACK RUNNING MTU 2000 Metric 1
505 RX packets 0 errors 0 dropped 0 overrun 0
506 TX packets 0 errors 0 dropped 0 overrun 0
508 ppp0 Link encap Point-to-Point Protocol
509 inet addr 192.76.32.2 P-t-P 129.67.1.165 Mask 255.255.255.0
510 UP POINTOPOINT RUNNING MTU 1500 Metric 1
511 RX packets 33 errors 0 dropped 0 overrun 0
512 TX packets 42 errors 0 dropped 0 overrun 0
518 where z.z.z.z is the address of your name server. This should work.
519 Here's what it looks like for me:
521 waddington:~$ ping 129.67.1.165
522 PING 129.67.1.165 (129.67.1.165): 56 data bytes
523 64 bytes from 129.67.1.165: icmp_seq=0 ttl=255 time=268 ms
524 64 bytes from 129.67.1.165: icmp_seq=1 ttl=255 time=247 ms
525 64 bytes from 129.67.1.165: icmp_seq=2 ttl=255 time=266 ms
527 --- 129.67.1.165 ping statistics ---
528 3 packets transmitted, 3 packets received, 0% packet loss
529 round-trip min/avg/max = 247/260/268 ms
536 This should show three routes, something like this:
539 Destination Gateway Genmask Flags Metric Ref Use Iface
540 129.67.1.165 0.0.0.0 255.255.255.255 UH 0 0 6 ppp0
541 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
542 0.0.0.0 129.67.1.165 0.0.0.0 UG 0 0 6298 ppp0
544 If your output looks similar but doesn't have the destination 0.0.0.0
545 line (which refers to the default route used for connections), you may
546 have run pppd without the 'defaultroute' option.
548 At this point you can try telnetting/ftping/fingering whereever you
549 want, bearing in mind that you'll have to use numeric IP addresses
550 unless you've set up your /etc/resolv.conf correctly.
555 If you don't seem to get a connection, the thing to do is to collect
556 'debug' output from pppd. To do this, make sure you run pppd with the
557 'debug' option, and put the following two lines in your
558 /etc/syslog.conf file:
560 daemon.* /dev/console
561 daemon.* /usr/adm/ppplog
563 This will cause pppd's messages to be written to the current virtual
564 console and to the file /usr/adm/ppplog. Note that the left-hand
565 field and the right-hand field must be separated by at least one TAB
566 character. After modifying /etc/syslog.conf, you must execute the
567 command 'kill -HUP <pid>' where <pid> is the process ID of the
568 currently running syslogd process to cause it to re-read the
571 Some messages to look for:
572 - "pppd[NNN]: Connected..." means that the "connect" script has
573 completed successfully.
574 - "pppd[NNN]: sent [LCP ConfReq"... means that pppd has attempted to
575 begin negotiation with the remote end.
576 - "pppd[NNN]: recv [LCP ConfReq"... means that pppd has received a
577 negotiation frame from the remote end.
578 - "pppd[NNN]: ipcp up" means that pppd has reached the point where
579 it believes the link is ready for IP traffic to travel across it.
581 If you never see a "recv" message then there may be serious problems
582 with your link. (For example, the link may not be passing all 8
583 bits.) If that's the case, it would be useful to collect a debug log
584 which contains all the bytes being passed between your computer and
585 the remote PPP server. To do this, alter your syslog.conf lines to
588 daemon.*,kern.* /dev/console
589 daemon.*,kern.* /usr/adm/ppplog
591 and HUP the syslog daemon as before. Then, run pppd with the option
592 "kdebug 25". Whatever characters arrive over the PPP terminal line
593 will appear in the debugging output.
595 Occasionally you may see a message like
597 ppp_toss: tossing frame, reason = 4
599 The PPP code is throwing away a packet ("frame") from the remote
600 server because of a serial overrun. This means your CPU isn't able to
601 read characters from the serial port as quickly as they arrive; the
602 best solution is to get a 16550A serial chip, which gives the CPU some
603 grace period. Reasons other than 4 indicate other kinds of serial
604 errors, which should not occur.
606 During the initial connection sequence, you may see one or more
607 messages which indicate "bad fcs". This refers to a checksum error in
608 a received PPP frame, and usually occurs at the start of a session
609 when the peer system is sending some "text" messages, such as "hello
610 this is the XYZ company". Messages of "bad fcs" once the link is
611 established and the routes have been added are not normal and indicate
612 transmission errors or noise on the telephone line.
615 IF IT STILL DOESN'T WORK (OR, BUG REPORTS)
617 If you're still having difficulty, send the linux-ppp list a bug
618 report. It is extremely important to include as much information as
619 possible; for example:
621 - the version number of the kernel you are using
622 - the version number of Linux PPP you are using
623 - the exact command you use to start the PPP session
624 - log output from a session run with the 'debug' option, captured
625 using daemon.*,kern.* in your syslog.conf file
626 - the type of PPP peer that you are connecting to (eg, Xyzzy Corp
627 terminal server, Morningstar PPP software, etc)
628 - the kind of connection you use (modem, hardwired, etc...)
631 DYNAMIC ADDRESS ASSIGNMENT
633 You can use Linux PPP with a PPP server which assigns a different IP
634 address every time you connect. This action is automatically performed
635 when you don't have a local IP address.
637 pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
638 /dev/cua1 38400 noipdefault debug crtscts modem defaultroute
640 The noipdefault, added to the above example, suppresses the attempts
641 of pppd to deduce its own IP address by looking it up in the
642 /etc/hosts file. Since the process does not have an IP address, one
643 will be assigned to it from the configuration file on the remote
646 Sometimes you may get an error message like "Cannot assign requested
647 address" when you use a Linux client (for example, "talk"). This
648 happens when the IP address given in /etc/hosts for our hostname
649 differs from the IP address used by the PPP interface. The solution
650 is to use ifconfig ppp0 to get the interface address and then edit
651 /etc/hosts appropriately.
654 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS
656 Suppose you want to permit another machine to call yours up and start
657 a PPP session. This is possible using Linux PPP.
659 One way is to create an account named, say, 'ppp', with the login
660 shell being a short script that starts pppd. For example, the passwd
661 entry might look like this:
663 ppp:(encrypted password):102:50:PPP client login:/home/ppp:/usr/sbin/pppd
665 In addition, you would edit the file ~ppp/.ppprc to have the following
666 pieces of information:
674 Here we will insist that the remote machine use IP address 192.1.2.23,
675 while the local PPP interface will use the IP address associated with
676 this machine's hostname in /etc/hosts. The '-detach' option is required
677 for a server. It tells the pppd process not to terminate until the modem
678 is disconnected. Should it fork, the init process would restart the getty
679 process and the this would cause a severe conflict over the port.
681 The 'modem' option indicates that the connection is via a switched circuit
682 (using a modem) and that the pppd process should monitor the DCD signal
685 The 'crtscts' option tells the pppd process to use hardware RTS/CTS flow
686 control for the modem.
688 The 'lock' option tells pppd to lock the tty device. This will use the UUCP
689 style locking file in the lock directory.
691 This setup is sufficient if you just want to connect two machines so
692 that they can talk to one another. If you want to use Linux PPP to
693 connect a single machine to an entire network, or to connect two
694 networks together, then you need to arrange for packets to be routed
695 from the networks to the PPP link. Setting up a link between networks
696 is beyond the scope of this document; you should examine the routing
697 options in the manual page for pppd carefully and find out about
700 Let's consider just the first case. Suppose you have a Linux machine
701 attached to an Ethernet, and you want to allow its PPP peer to be able
702 to communicate with hosts on that Ethernet. To do this, you should
703 have the remote machine use an IP address that would normally appear
704 to be on the local Ethernet segment and you should give the 'proxyarp'
705 option to pppd on the server. Suppose, for example, we have this
708 192.1.2.23 192.1.2.17
709 +-----------+ PPP link +----------+
710 | chelseapc | ------------------- | billpc |
711 +-----------+ +----------+
713 ----------------------------------- 192.1.2.x
715 Here the PPP and Ethernet interfaces of billpc will have IP address
716 192.1.2.17. (It's OK for one or more PPP interfaces on a machine to
717 share an IP address with an Ethernet interface.) There is an
718 appropriate entry in /etc/passwd on billpc to allow chelseapc to call
719 in. It will run pppd when the user signs on to the system and pppd will
720 take the options from the user option file.
722 In addition, you would edit the file ~ppp/.ppprc to have the following
723 piece of information:
729 192.1.2.17:192.1.2.23
732 When the link comes up, pppd will enter a "proxy arp" entry for
733 chelseapc into the arp table on billpc. What this means effectively
734 is that billpc will pretend to the other machines on the 192.1.2.x
735 Ethernet that its Ethernet interface is ALSO the interface for
736 chelseapc (192.1.2.23) as well as billpc (192.1.2.17). In practice
737 this means that chelseapc can communicate just as if it was directly
738 connected to the Ethernet.
741 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS WITH DYNAMIC IP
743 The use of dynamic IP assignments is not much different from that
744 using static IP addresses. Rather than putting the IP address into the
745 single file ~ppp/.ppprc, you would put the IP address for each of the
746 incoming terminals into the /etc/ppp/options.tty files. ('tty' is the
747 name of the tty device. For example /etc/ppp/options.ttyS0 is used for
748 the /dev/ttyS0 device.)
750 To each of the serial devices, you would attach a modem. To the
751 modems, attach the telephone lines. Place all of the telephone lines
752 into a hunt group so that the telephone system will select the
753 non-busy telephone and subsequently, the modem. By selecting the
754 modem, the user will select a tty device and the tty device will
755 select the IP address. Run a getty process against the tty device such
758 (The general consensus among the users is that you should *not* use
759 the agetty process to monitor a modem. Use either getty_ps' uugetty
760 process or mgetty from the mgetty+sendfax package.)
763 SECURITY CONCERNS ABOUT INCOMING PPP CONNECTIONS
765 The following security should be considered with the ppp connections.
767 1. Never put the pppd program file into the /etc/shells file. It is not
768 a legal shell for the general user. In addition, if the shell is missing
769 from the shells file, the ftpd process will not allow the user to access
770 the system via ftp. You would not want Joe Hacker using the ppp account
773 2. Ensure that the directory /etc/ppp is owned by 'root' and permits
774 only write access to the root user.
776 3. The files /etc/ppp/options must be owned by root and accessible only
777 from that user. Never permit any other user access to this file.
779 4. The files /etc/ppp/ip-up and /etc/ppp/ip-down will be executed by the
780 pppd process while it is root. Ensure that these files are writable only
783 5. If you use an incoming PPP connection, you should do the following as
786 a) Invalidate the files for rhosts and forward
787 rm -f ~ppp/.rhosts ~ppp/.forward
788 touch ~ppp/.rhosts ~ppp/.forward
789 chmod 444 ~ppp/.rhosts ~ppp/.forward
791 b) Prevent users from sending mail to the user 'ppp'.
793 This is best performed by creating a system alias 'ppp' and have it
794 point to the name "THIS_USER_CANNOT_RECEIVE_MAIL". It has no special
795 meaning other than the obvious one.
797 For sendmail, the sequence is fairly easy. Edit the /etc/aliases file
800 ppp:THIS_USER_CANNOT_RECEIVE_MAIL
802 Then run the sendmail program with the option '-bi' to rebuild the
805 c) Secure the ppp file properly.
806 chown root ~ppp/.ppprc
807 chmod 444 ~ppp/.ppprc
809 You may wish to extend the security by creating a group 'ppp' and putting
810 the ppp user into that group, along with the binaries for pppd and pppstats.
811 Then you may secure the binaries so that they are executable from the owner
812 (which should be root) and the group only. All other users would be denied
813 all access to the files and executables.
816 ADDITIONAL INFORMATION
818 Besides this document, additional information may be found in:
820 - The file README in the source package
821 - The PPP-HOWTO on sunsite.unc.edu
822 - The Net-2-HOWTO on sunsite.unc.edu
823 - The Network Administration Guide published by O'Rielly and Associates
825 Please consult these sources of information should you have questions
826 about the program. If you still can not find your answer then ask either
827 the usenet news groups or the mail list.
833 The dip program used by Linux is not directly supported by the PPP
834 package as such. Please don't ask the PPP porting group questions
835 about dip. It does work in two areas.
837 1. If you use it as a parameter to 'connect' then you can use the scripting
838 language and establish the connection. You would use the standard set of
841 2. dip-3.3.7m-uri and later versions support a 'mode ppp' function
842 which will invoke the pppd program. That is all that it does. It will
843 not pass any parameters to pppd other than its required '-detach' to
844 allow dip to detect the normal termination of pppd.
846 The following information comes from John Phillips in an article which he
847 posted to comp.os.linux.setup.
849 Assuming that you already know how dip supports SLIP, these points
850 are relative to a working SLIP set-up.
852 1. You need dip-3.3.7m-uri, and, of course, PPP compiled into the
855 2. Make sure pppd is where dip thinks it is: /usr/lib/ppp/pppd, or
856 make a link from there to where pppd really is. (Or re-compile dip
857 to tell it where pppd is on your system - see pathnames.h).
859 3. The key differences between the dip script for PPP, compared to one
862 a. Use "mode PPP" instead of "mode SLIP"
864 b. Don't set certain options such as mtu and default - these are set
865 by pppd from the file /etc/ppp/options. Mine looks like this:
874 The actual parameters and values may depend on your IP supplier
877 c. Tell your IP supplier's start-up code to use ppp, not slip: I
878 use "send nolqm,idle=240\n" instead of "send slip,idle=240,mru=576\n"
879 at the "protocol: " prompt. ("nolqm" asks for ppp without the line
880 quality monitoring protocol, which is not - I think - supported in
881 Linux PPP.) This prompt may be different (or absent) with another
884 d. You don't need "get $local <name>", since the ppp protocol
885 negotiates this at start-up. You still need "get $remote <name>".
886 (This may also vary with IP supplier - you may need to set some
887 more parameters in /etc/ppp/options to work with yours - see "man
888 pppd" for details of the options supported by pppd.)
890 4. The dip script will exit after dialling and starting up pppd. When
891 ppp negotiation is completed and IP comes up, pppd runs /etc/ppp/ip-up.
892 This file can contain things you want to run when the network comes up
893 (e.g. running the mail queue).
895 5. When IP goes down (e.g. after you close down the link with "dip -k"),
896 pppd runs /etc/ppp/ip-down, which can contain things you want to do on