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 2.1.2. 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
161 To leave the mail list, send 'unsubscribe linux-ppp' to the same
164 You can send to the list by mailing to
165 linux-ppp@vger.rutgers.edu. This is a majordomo mailing list and
166 is unlike the earlier version on hut.fi. There is no magic header
167 required for this list. In addition, it is mirrored to the usenet
168 group linux.act.ppp. You may choose to read the few messages posted
173 There are three applicable usenet news groups for the PPP code. Please
174 choose the group which applies the closest to the type of problem
175 which you are experiencing.
177 comp.os.linux.networking
178 - Trouble setting routes, running name services, using telnet, ftp,
180 - It will not compile.
183 - Trouble installing the package from BINARIES only. This does *NOT*
184 include problems with compiling the package.
187 - How do I use the package?
188 - How do I connect to .... services?
189 - Why does this not work?
190 - All other types of questions on how to use just the PPP code.
192 PLEASE don't assume that just because you are using PPP as your
193 network device driver that all problems with your networking are a
194 problem of PPP. PPP is *NOT* responsible for your modem disconnecting,
195 routing to other servers, running telnet, etc. Calling the problem
196 'ppp' on usenet may cause it to be ignored by the people who
197 actually work on the networking code.
199 kernel driver installation:
201 This depends on the kernel version you're using.
204 These versions are not supported.
206 Version 1.1.0 through 1.1.14
207 These versions are not supported.
209 Version 1.1.15 to 1.2.99
210 - Use the source to the ppp.c driver from the 'linux' directory and
211 replace the driver in the /usr/src/linux/drivers/net.
212 - Delete the file /usr/src/linux/drivers/net/ppp.h
213 - Add the following files to /usr/include/net:
219 - IF AND ONLY IF you are missing the following files then use the
220 copy provided in the 'linux' directory to supplement the files.
222 DO **NOT** REPLACE THE FILE IF IT CURRENTLY EXISTS.
227 Version 1.3.0 and later
228 The files have been properly updated.
230 Reboot with the new kernel. At startup, you should see
233 PPP: version 2.2.0 (dynamic channel allocation)
234 TCP compression code copyright 1989 Regents of the University of California
235 Dynamic channel allocation code copyright 1995 Caldera, Inc.
236 PPP line discipline registered.
240 First update /usr/include/net as described in the previous section.
242 Then, in this directory, issue the command:
246 Go to the 'pppd' directory and issue the commands:
251 This should build the program. If you have any errors then ensure
252 that you have the proper include files and haven't missed one.
254 If you are using shadow passwords *AND* have it installed, then you
255 should use the command:
259 rather than the non-shadow command listed earlier.
261 (Shadow library support will require the addition of some modules
262 to the shadow library. These were overlooked by the package author
263 and I will, or have already, notified him.)
265 This code has been built with the 4.5 and 4.6 subroutine libraries
266 and include files. If your include files are too old then you should
269 To install the package, issue the command:
273 This will install the binary in /usr/sbin and the man page into
276 pppd needs to be run as root. You can either make it setuid
277 root or just use it when you are root. 'make install' will try
278 to install it setuid root. Making pppd setuid root is
279 convenient for a single-user machine, but has security
280 implications which you should investigate carefully before
281 making it available on a multiuser machine.
283 The pppd process must have the following directories to work:
288 In addition, for the program to run, there must be a 'options' file
289 in the /etc/ppp directory. So, the following commands will accomplish
290 the required operations. They may have errors if the entries currently
293 Perform these commands as the 'root' user.
296 mkdir /var/run /etc/ppp
297 touch /etc/ppp/options
301 If you have not already done so, run `./configure' in this directory.
303 To compile the chat program, go to the 'chat' directory and issue
308 To install the package, issue the command:
312 This will install the binary in /usr/sbin and the man page into
315 pppstats installation:
317 If you have not already done so, run `./configure' in this directory.
319 To compile the pppstats program, go to the 'pppstats' directory
320 and issue the command:
324 To install the package, issue the command:
328 This will install the binary in /usr/sbin and the man page into
332 GENERAL NETWORK CONFIGURATION
334 Since many people don't use the Linux networking code at all until
335 they get a PPP link, this section describes generally what's needed to
336 get things running. In principle none of this is special to PPP. For
337 more details, you should consult the relevant Linux HOWTOs. If you
338 already understand network setup, you can skip this section.
340 The first file that requires attention is the rc script that does
341 network configuration at boot time, called /etc/rc.net or
342 /etc/rc.d/rc.net.{1,2} or something similar, depending on your Linux
343 distribution. This file should 'ifconfig' the loopback interface lo,
344 and should add an interface route for it. These lines might look
350 /sbin/ifconfig lo 127.0.0.1
351 /sbin/route add 127.0.0.1
353 However, it should *not* config an ethernet card or install any other
354 routes (unless you actually have an ethernet card, in which case I'll
355 assume you know what to do). Many distributions will provide scripts
356 that expect you to have an ethernet card.
358 You also need to decide whether you want to allow incoming
359 telnet/ftp/finger, etc. If so, you should have the rc startup script
360 run the 'inetd' daemon.
362 Next, you should set up /etc/hosts to have two lines. The first
363 should just give the loopback or localhost address and the second
364 should give your own host name and the IP address your PPP connection
365 will use. For example:
367 127.0.0.1 loopback localhost # useful aliases
368 192.1.1.17 billpc.president.whitehouse.gov bill # my hostname
369 192.1.1.23 chelseapc.president.whitehouse.gov chelseapc
371 where my IP address is 192.1.1.17 and my hostname is
372 billpc.president.whitehouse.gov. (Not really, you understand.) If
373 your PPP server does dynamic IP address assignment, give a guess as to
374 an address you might get (see also "Dynamic Address Assignment"
377 Finally, you need to configure the domain name system by putting
378 appropriate lines in /etc/resolv.conf . It should look something like
381 domain president.whitehouse.gov
382 search president.whitehouse.gov whitehouse.gov
384 nameserver 192.1.2.10
386 Assuming there are nameservers at 192.1.2.1 and 192.1.2.10, then when
387 you get connected with PPP, you can reach hosts whose full names are
388 'hillarypc.president.whitehouse.gov' and 'chelseapc.whitehouse.gov' by
389 the names 'hillarypc' and 'chelseapc'. You can probably find out the
390 right domain name to use and the IP numbers of nameservers from
391 whoever's providing your PPP link.
394 CONNECTING TO A PPP SERVER
396 To use PPP, you invoke the pppd program with appropriate options.
397 Everything you need to know is contained in the pppd(8) manual page.
398 However, it's useful to see some examples:
400 Example 1: A simple dial-up connection.
402 Here's a command for connecting to a PPP server by modem.
404 pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
405 /dev/cua1 38400 debug crtscts modem defaultroute 192.1.1.17
407 Going through pppd's options in order:
408 connect 'chat etc...' This gives a command to run to contact the
409 PPP server. Here the supplied 'chat' program is used to dial a
410 remote computer. The whole command is enclosed in single quotes
411 because pppd expects a one-word argument for the 'connect' option.
412 The options to 'chat' itself are:
414 -v verbose mode; log what we do to syslog
415 "" don't wait for any prompt, but instead...
416 ATDT5551212 dial the modem, then
417 CONNECT wait for answer
418 "" send a return (null text followed by usual return)
419 ogin: ppp word: whitewater log in.
421 /dev/cua1 specify the callout serial port cua1
422 38400 specify baud rate
423 debug log status in syslog
424 crtscts use hardware flow control between computer and modem
425 (at 38400 this is a must)
426 modem indicate that this is a modem device; pppd will hang up the
427 phone before and after making the call
428 defaultroute once the PPP link is established, make it the default
429 route; if you have a PPP link to the Internet this
430 is probably what you want
432 192.1.1.17 this is a degenerate case of a general option
433 of the form x.x.x.x:y.y.y.y . Here x.x.x.x is the local IP
434 address and y.y.y.y is the IP address of the remote end of the
435 PPP connection. If this option is not specified, or if just
436 one side is specified, then x.x.x.x defaults to the IP address
437 associated with the local machine's hostname (in /etc/hosts),
438 and y.y.y.y is determined by the remote machine. So if this
439 example had been taken from the fictional machine 'billpc',
440 this option would actually be redundant.
442 pppd will write error messages and debugging logs to the syslogd
443 daemon using the facility name "local2". (Verbose output from chat is
444 the same.) These messages may already be logged to the console or to
445 a file like /usr/adm/messages; consult your /etc/syslog.conf file to
446 see. If you want to make all pppd and chat messages go to the
447 console, add the line
449 local2.* /dev/console
450 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
451 This is one or more tabs. Do not use spaces.
453 to syslog.conf; make sure to put one or more TAB characters between
456 Example 2: Connecting to PPP server over hard-wired link.
458 This is a slightly more complicated example. This is the script I run
459 to make my own PPP link, which is over a hard-wired Gandalf link to an
460 Ultrix machine running Morningstar PPP.
462 pppd connect /etc/ppp/ppp-connect defaultroute noipdefault debug \
463 kdebug 0 /dev/cua0 9600
465 Here /etc/ppp/ppp-connect is the following script:
468 chat -v -t60 "" \; "service :" blackice ogin: callahan word: PASSWORD \
469 black% "stty -echo; ppp" "Starting PPP now" && sleep 5
471 This sends a break to wake up my terminal server, sends a semicolon
472 (which lets my terminal server do autobaud detection), then says we
473 want the service "blackice". It logs in, waits for a shell prompt
474 ("black%"), then starts PPP. The -t60 argument sets the timeout to a
475 minute, since things here are sometimes very slow.
477 The "&& sleep 5" causes the script to pause for 5 seconds, unless chat
478 fails in which case it exits immediately. This is just to give the
479 PPP server time to start (it's very slow). Also, the "stty -echo"
480 turned out to be very important for me; without it, my pppd would
481 sometimes start to send negotiation packets before the remote PPP
482 server had time to turn off echoing. The negotiation packets would
483 then get sent back to my local machine, be rejected (PPP is able to
484 detect loopback) and pppd would fail before the remote PPP server even
485 got going. The "stty -echo" command prevents this confusion. This
486 kind of problem should only ever affect a *very* few people who
487 connect to a PPP server that runs as a command on a slow Unix machine,
488 but I wanted to mention it because it took me several frustrating
491 The pppd options are mostly familiar. Two that are new are
492 "noipdefault" and "kdebug 1". "noipdefault" tells pppd to ask the
493 remote end for the IP address to use; this is necessary if the PPP
494 server implements dynamic IP address assignment as mine does (i.e., I
495 don't know what address I'll get ahead of time). "kdebug 1" sets the
496 kernel debugging level to 1, enabling slightly chattier messages from
499 Anyway, assuming your connection is working, you should see chat dial
500 the modem, then perhaps some messages from pppd (depending on your
501 syslog.conf setup), then some kernel messages like this:
503 ppp: channel ppp0 mtu changed to 1500
504 ppp: channel ppp0 open
505 ppp: channel ppp0 going up for IP packets!
507 (These messages will only appear if you gave the option "kdebug 1" and
508 have kern.info messages directed to the screen.) Simultaneously, pppd
509 is also writing interesting things to /usr/adm/messages (or other log
510 file, depending on syslog.conf).
515 If you think you've got a connection, there are a number of things you
522 (ifconfig may live elsewhere, depending on your distribution.)
523 This should show you all the network interfaces that are 'UP'. ppp0
524 should be one of them, and you should recognize the first IP address
525 as your own and the "P-t-P address" (or point-to-point address) the
526 address of your server. Here's what it looks like on my machine:
528 lo Link encap Local Loopback
529 inet addr 127.0.0.1 Bcast 127.255.255.255 Mask 255.0.0.0
530 UP LOOPBACK RUNNING MTU 2000 Metric 1
531 RX packets 0 errors 0 dropped 0 overrun 0
532 TX packets 0 errors 0 dropped 0 overrun 0
534 ppp0 Link encap Point-to-Point Protocol
535 inet addr 192.76.32.2 P-t-P 129.67.1.165 Mask 255.255.255.0
536 UP POINTOPOINT RUNNING MTU 1500 Metric 1
537 RX packets 33 errors 0 dropped 0 overrun 0
538 TX packets 42 errors 0 dropped 0 overrun 0
544 where z.z.z.z is the address of your name server. This should work.
545 Here's what it looks like for me:
547 waddington:~$ ping 129.67.1.165
548 PING 129.67.1.165 (129.67.1.165): 56 data bytes
549 64 bytes from 129.67.1.165: icmp_seq=0 ttl=255 time=268 ms
550 64 bytes from 129.67.1.165: icmp_seq=1 ttl=255 time=247 ms
551 64 bytes from 129.67.1.165: icmp_seq=2 ttl=255 time=266 ms
553 --- 129.67.1.165 ping statistics ---
554 3 packets transmitted, 3 packets received, 0% packet loss
555 round-trip min/avg/max = 247/260/268 ms
562 This should show three routes, something like this:
565 Destination Gateway Genmask Flags Metric Ref Use Iface
566 129.67.1.165 0.0.0.0 255.255.255.255 UH 0 0 6 ppp0
567 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
568 0.0.0.0 129.67.1.165 0.0.0.0 UG 0 0 6298 ppp0
570 If your output looks similar but doesn't have the destination 0.0.0.0
571 line (which refers to the default route used for connections), you may
572 have run pppd without the 'defaultroute' option.
574 At this point you can try telnetting/ftping/fingering whereever you
575 want, bearing in mind that you'll have to use numeric IP addresses
576 unless you've set up your /etc/resolv.conf correctly.
581 If you don't seem to get a connection, the thing to do is to collect
582 'debug' output from pppd. To do this, make sure you run pppd with the
583 'debug' option, and put the following two lines in your
584 /etc/syslog.conf file:
586 local2.* /dev/console
587 local2.* /usr/adm/ppplog
589 This will cause pppd's messages to be written to the current virtual
590 console and to the file /usr/adm/ppplog. Note that the left-hand
591 field and the right-hand field must be separated by at least one TAB
592 character. After modifying /etc/syslog.conf, you must execute the
593 command 'kill -HUP <pid>' where <pid> is the process ID of the
594 currently running syslogd process to cause it to re-read the
597 Some messages to look for:
598 - "pppd[NNN]: Connected..." means that the "connect" script has
599 completed successfully.
600 - "pppd[NNN]: sent [LCP ConfReq"... means that pppd has attempted to
601 begin negotiation with the remote end.
602 - "pppd[NNN]: recv [LCP ConfReq"... means that pppd has received a
603 negotiation frame from the remote end.
604 - "pppd[NNN]: ipcp up" means that pppd has reached the point where
605 it believes the link is ready for IP traffic to travel across it.
607 If you never see a "recv" message then there may be serious problems
608 with your link. (For example, the link may not be passing all 8
609 bits.) If that's the case, it would be useful to collect a debug log
610 which contains all the bytes being passed between your computer and
611 the remote PPP server. To do this, alter your syslog.conf lines to
614 local2.*,kern.* /dev/console
615 local2.*,kern.* /usr/adm/ppplog
617 and HUP the syslog daemon as before. Then, run pppd with the option
618 "kdebug 25". Whatever characters arrive over the PPP terminal line
619 will appear in the debugging output.
621 Occasionally you may see a message like
623 ppp_toss: tossing frame, reason = 4
625 The PPP code is throwing away a packet ("frame") from the remote
626 server because of a serial overrun. This means your CPU isn't able to
627 read characters from the serial port as quickly as they arrive; the
628 best solution is to get a 16550A serial chip, which gives the CPU some
629 grace period. Reasons other than 4 indicate other kinds of serial
630 errors, which should not occur.
632 During the initial connection sequence, you may see one or more
633 messages which indicate "bad fcs". This refers to a checksum error in
634 a received PPP frame, and usually occurs at the start of a session
635 when the peer system is sending some "text" messages, such as "hello
636 this is the XYZ company". Messages of "bad fcs" once the link is
637 established and the routes have been added are not normal and indicate
638 transmission errors or noise on the telephone line.
641 IF IT STILL DOESN'T WORK (OR, BUG REPORTS)
643 If you're still having difficulty, send the linux-ppp list a bug
644 report. It is extremely important to include as much information as
645 possible; for example:
647 - the version number of the kernel you are using
648 - the version number of Linux PPP you are using
649 - the exact command you use to start the PPP session
650 - log output from a session run with the 'debug' option, captured
651 using local2.*,kern.* in your syslog.conf file
652 - the type of PPP peer that you are connecting to (eg, Xyzzy Corp
653 terminal server, Morningstar PPP software, etc)
654 - the kind of connection you use (modem, hardwired, etc...)
657 DYNAMIC ADDRESS ASSIGNMENT
659 You can use Linux PPP with a PPP server which assigns a different IP
660 address every time you connect. This action is automatically performed
661 when you don't have a local IP address.
663 pppd connect 'chat -v "" ATDT5551212 CONNECT "" ogin: ppp word: whitewater' \
664 /dev/cua1 38400 noipdefault debug crtscts modem defaultroute
666 The noipdefault, added to the above example, suppresses the attempts
667 of pppd to deduce its own IP address by looking it up in the
668 /etc/hosts file. Since the process does not have an IP address, one
669 will be assigned to it from the configuration file on the remote
672 Sometimes you may get an error message like "Cannot assign requested
673 address" when you use a Linux client (for example, "talk"). This
674 happens when the IP address given in /etc/hosts for our hostname
675 differs from the IP address used by the PPP interface. The solution
676 is to use ifconfig ppp0 to get the interface address and then edit
677 /etc/hosts appropriately.
680 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS
682 Suppose you want to permit another machine to call yours up and start
683 a PPP session. This is possible using Linux PPP.
685 One way is to create an account named, say, 'ppp', with the login
686 shell being a short script that starts pppd. For example, the passwd
687 entry might look like this:
689 ppp:(encrypted password):102:50:PPP client login:/home/ppp:/usr/sbin/pppd
691 In addition, you would edit the file ~ppp/.ppprc to have the following
692 pieces of information:
700 Here we will insist that the remote machine use IP address 192.1.2.23,
701 while the local PPP interface will use the IP address associated with
702 this machine's hostname in /etc/hosts. The '-detach' option is required
703 for a server. It tells the pppd process not to terminate until the modem
704 is disconnected. Should it fork, the init process would restart the getty
705 process and the this would cause a severe conflict over the port.
707 The 'modem' option indicates that the connection is via a switched circuit
708 (using a modem) and that the pppd process should monitor the DCD signal
711 The 'crtscts' option tells the pppd process to use hardware RTS/CTS flow
712 control for the modem.
714 The 'lock' option tells pppd to lock the tty device. This will use the UUCP
715 style locking file in the lock directory.
717 This setup is sufficient if you just want to connect two machines so
718 that they can talk to one another. If you want to use Linux PPP to
719 connect a single machine to an entire network, or to connect two
720 networks together, then you need to arrange for packets to be routed
721 from the networks to the PPP link. Setting up a link between networks
722 is beyond the scope of this document; you should examine the routing
723 options in the manual page for pppd carefully and find out about
726 Let's consider just the first case. Suppose you have a Linux machine
727 attached to an Ethernet, and you want to allow its PPP peer to be able
728 to communicate with hosts on that Ethernet. To do this, you should
729 have the remote machine use an IP address that would normally appear
730 to be on the local Ethernet segment and you should give the 'proxyarp'
731 option to pppd on the server. Suppose, for example, we have this
734 192.1.2.23 192.1.2.17
735 +-----------+ PPP link +----------+
736 | chelseapc | ------------------- | billpc |
737 +-----------+ +----------+
739 ----------------------------------- 192.1.2.x
741 Here the PPP and Ethernet interfaces of billpc will have IP address
742 192.1.2.17. (It's OK for one or more PPP interfaces on a machine to
743 share an IP address with an Ethernet interface.) There is an
744 appropriate entry in /etc/passwd on billpc to allow chelseapc to call
745 in. It will run pppd when the user signs on to the system and pppd will
746 take the options from the user option file.
748 In addition, you would edit the file ~ppp/.ppprc to have the following
749 piece of information:
755 192.1.2.17:192.1.2.23
758 When the link comes up, pppd will enter a "proxy arp" entry for
759 chelseapc into the arp table on billpc. What this means effectively
760 is that billpc will pretend to the other machines on the 192.1.2.x
761 Ethernet that its Ethernet interface is ALSO the interface for
762 chelseapc (192.1.2.23) as well as billpc (192.1.2.17). In practice
763 this means that chelseapc can communicate just as if it was directly
764 connected to the Ethernet.
767 SETTING UP A MACHINE FOR INCOMING PPP CONNECTIONS WITH DYNAMIC IP
769 The use of dynamic IP assignments is not much different from that
770 using static IP addresses. Rather than putting the IP address into the
771 single file ~ppp/.ppprc, you would put the IP address for each of the
772 incoming terminals into the /etc/ppp/options.tty files. ('tty' is the
773 name of the tty device. For example /etc/ppp/options.ttyS0 is used for
774 the /dev/ttyS0 device.)
776 To each of the serial devices, you would attach a modem. To the
777 modems, attach the telephone lines. Place all of the telephone lines
778 into a hunt group so that the telephone system will select the
779 non-busy telephone and subsequently, the modem. By selecting the
780 modem, the user will select a tty device and the tty device will
781 select the IP address. Run a getty process against the tty device such
784 (The general consensus among the users is that you should *not* use
785 the agetty process to monitor a modem. Use either getty_ps' uugetty
786 process or mgetty from the mgetty+sendfax package.)
789 SECURITY CONCERNS ABOUT INCOMING PPP CONNECTIONS
791 The following security should be considered with the ppp connections.
793 1. Never put the pppd program file into the /etc/shells file. It is not
794 a legal shell for the general user. In addition, if the shell is missing
795 from the shells file, the ftpd process will not allow the user to access
796 the system via ftp. You would not want Joe Hacker using the ppp account
799 2. Ensure that the directory /etc/ppp is owned by 'root' and permits
800 only write access to the root user.
802 3. The files /etc/ppp/options must be owned by root and accessible only
803 from that user. Never permit any other user access to this file.
805 4. The files /etc/ppp/ip-up and /etc/ppp/ip-down will be executed by the
806 pppd process while it is root. Ensure that these files are writable only
809 5. If you use an incoming PPP connection, you should do the following as
812 a) Invalidate the files for rhosts and forward
813 rm -f ~ppp/.rhosts ~ppp/.forward
814 touch ~ppp/.rhosts ~ppp/.forward
815 chmod 444 ~ppp/.rhosts ~ppp/.forward
817 b) Prevent users from sending mail to the user 'ppp'.
819 This is best performed by creating a system alias 'ppp' and have it
820 point to the name "THIS_USER_CANNOT_RECEIVE_MAIL". It has no special
821 meaning other than the obvious one.
823 For sendmail, the sequence is fairly easy. Edit the /etc/aliases file
826 ppp:THIS_USER_CANNOT_RECEIVE_MAIL
828 Then run the sendmail program with the option '-bi' to rebuild the
831 c) Secure the ppp file properly.
832 chown root ~ppp/.ppprc
833 chmod 444 ~ppp/.ppprc
835 You may wish to extend the security by creating a group 'ppp' and putting
836 the ppp user into that group, along with the binaries for pppd and pppstats.
837 Then you may secure the binaries so that they are executable from the owner
838 (which should be root) and the group only. All other users would be denied
839 all access to the files and executables.
842 ADDITIONAL INFORMATION
844 Besides this document, additional information may be found in:
846 - The file README in the source package
847 - The PPP-HOWTO on sunsite.unc.edu
848 - The Net-2-HOWTO on sunsite.unc.edu
849 - The Network Administration Guide published by O'Rielly and Associates
851 Please consult these sources of information should you have questions
852 about the program. If you still can not find your answer then ask either
853 the usenet news groups or the mail list.
859 The dip program used by Linux is not directly supported by the PPP
860 package as such. Please don't ask the PPP porting group questions
861 about dip. It does work in two areas.
863 1. If you use it as a parameter to 'connect' then you can use the scripting
864 language and establish the connection. You would use the standard set of
867 2. dip-3.3.7m-uri and later versions support a 'mode ppp' function
868 which will invoke the pppd program. That is all that it does. It will
869 not pass any parameters to pppd other than its required '-detach' to
870 allow dip to detect the normal termination of pppd.
872 The following information comes from John Phillips in an article which he
873 posted to comp.os.linux.setup.
875 Assuming that you already know how dip supports SLIP, these points
876 are relative to a working SLIP set-up.
878 1. You need dip-3.3.7m-uri, and, of course, PPP compiled into the
881 2. Make sure pppd is where dip thinks it is: /usr/lib/ppp/pppd, or
882 make a link from there to where pppd really is. (Or re-compile dip
883 to tell it where pppd is on your system - see pathnames.h).
885 3. The key differences between the dip script for PPP, compared to one
888 a. Use "mode PPP" instead of "mode SLIP"
890 b. Don't set certain options such as mtu and default - these are set
891 by pppd from the file /etc/ppp/options. Mine looks like this:
900 The actual parameters and values may depend on your IP supplier
903 c. Tell your IP supplier's start-up code to use ppp, not slip: I
904 use "send nolqm,idle=240\n" instead of "send slip,idle=240,mru=576\n"
905 at the "protocol: " prompt. ("nolqm" asks for ppp without the line
906 quality monitoring protocol, which is not - I think - supported in
907 Linux PPP.) This prompt may be different (or absent) with another
910 d. You don't need "get $local <name>", since the ppp protocol
911 negotiates this at start-up. You still need "get $remote <name>".
912 (This may also vary with IP supplier - you may need to set some
913 more parameters in /etc/ppp/options to work with yours - see "man
914 pppd" for details of the options supported by pppd.)
916 4. The dip script will exit after dialling and starting up pppd. When
917 ppp negotiation is completed and IP comes up, pppd runs /etc/ppp/ip-up.
918 This file can contain things you want to run when the network comes up
919 (e.g. running the mail queue).
921 5. When IP goes down (e.g. after you close down the link with "dip -k"),
922 pppd runs /etc/ppp/ip-down, which can contain things you want to do on