This is a list of Frequently Asked Questions about using ppp-2.x and some answers.

Q: Can you give me an example of how I might set up my machine to dial out to an ISP?

A: Here's an example for dialling out to an ISP via a modem on /dev/tty02. The modem uses hardware (CTS/RTS) flow control, and the serial port is run at 38400 baud. The ISP assigns our IP address.

To configure pppd for this connection, create a file under /etc/ppp/peers called (say) my-isp containing the following:

tty02 crtscts 38400
connect 'chat -v -f /etc/ppp/chat/my-isp'
The ppp connection is then initiated using the following command:
pppd call my-isp
Of course, if the directory containing pppd is not in your path, you will need to give the full pathname for pppd, for example, /usr/sbin/pppd.

When you run this, pppd will use the chat program to dial the ISP and invoke its ppp service. Chat will read the file specified with -f, namely /etc/ppp/chat/my-isp, to find a list of strings to expect to receive, and strings to send. This file would contain something like this:

ABORT "Username/Password Incorrect"
"" "at"
OK "at&d2&c1"
OK "atdt2479381"
"name:" "^Uusername"
"word:" "\qpassword"
"annex" "\q^Uppp"
"Switching to PPP-ppp-Switching to PPP"
You will need to change the details here. The first string on each line is a string to expect to receive; the second is the string to send. You can add or delete lines according to the dialog required to access your ISP's system. This example is for a modem with a standard AT command set, dialling out to an Annex terminal server. The \q toggles "quiet" mode; when quiet mode is on, the strings to be sent are replaced by ?????? in the log. You may need to go through the dialog manually using kermit or tip first to determine what should go in the script.

To terminate the link, run the following script, called (say) kill-ppp:

if [ -f $piddir/$unit.pid ]; then
  kill -1 `cat $piddir/$unit.pid`

On some systems (SunOS, Solaris, Ultrix), you will need to change /var/run to /etc/ppp.

Q: Can you give me an example of how I could set up my office machine so I can dial in to it from home?

A: Let's assume that the office machine is called "office" and is on a local ethernet subnet. Call the home machine "home" and give it an IP address on the same subnet as "office". We'll require both machines to authenticate themselves to each other.

Set up the files on "office" as follows:

/etc/ppp/options contains:

auth		# require the peer to authenticate itself
# other options can go here if desired
/etc/ppp/chap-secrets contains:
home	office	"beware the frub-jub"	home
office	home	"bird, my son!%&*"	-
Set up a modem on a serial port so that users can dial in to the modem and get a login prompt.

On "home", set up the files as follows:

  • /etc/ppp/options contains the same as on "office".
  • /etc/ppp/chap-secrets contains:
    home	office	"beware the frub-jub"	-
    office	home	"bird, my son!%&*"	office
  • Create a file called /etc/ppp/peers/office containing the following:
    tty02 crtscts 38400
    connect 'chat -v -f /etc/ppp/chat/office'
    (You may need to change some of the details here.)
  • Create the /etc/ppp/chat/office file containing the following:
    ABORT "ogin incorrect"
    "" "at"
    OK "at&d2&c1"
    OK "atdt2479381"
    "name:" "^Uusername"
    "word:" "\qpassword"
    "$" "\q^U/usr/sbin/pppd proxyarp"
    You will need to change the details. Note that the "$" in the second-last line is expecting the shell prompt after a successful login - you may need to change it to "%" or something else.
You then initiate the connection (from home) with the command:
pppd call office

Q: When I try to establish a connection, the modem successfully dials the remote system, but then hangs up a few seconds later. How do I find out what's going wrong?

A: There are a number of possible problems here. The first thing to do is to ensure that pppd's messages are visible. Pppd uses the syslog facility to log messages which help to identify specific problems. Messages from pppd have facility "daemon" and levels ranging from "debug" to "error".

Usually it is useful to see messages of level "notice" or higher on the console. To see these, find the line in /etc/syslog.conf which has /dev/console on the right-hand side, and add "daemon.notice" in the list on the left. The line will end up looking something like this:

*.err;kern.debug;auth.notice;mail.crit;daemon.notice	/dev/console
Note that the whitespace is tabs, *not* spaces.

If you are having problems, it may be useful to see messages of level "info" as well, in which case you would change "daemon.notice" to "daemon.info".

In addition, it is useful to collect pppd's debugging output in a file - the debug option to pppd causes it to log the contents of all control packets sent and received in human-readable form. To do this, add a line like this to /etc/syslog.conf:

daemon,local2.debug		/etc/ppp/log
and create an empty /etc/ppp/log file.

When you change syslog.conf, you will need to send a HUP signal to syslogd to causes it to re-read syslog.conf. You can do this with a command like this (as root):

	kill -HUP `cat /etc/syslogd.pid`
(On some systems, you need to use /var/run/syslog.pid instead of /etc/syslogd.pid.)

After setting up syslog like this, you can use the -v flag to chat and the `debug' option to pppd to get more information. Try initiating the connection again; when it fails, inspect /etc/ppp/log to see what happened and where the connection failed.

Q: When I try to establish a connection, I get an error message saying "Serial link is not 8-bit clean". Why?

A: The most common cause is that your connection script hasn't successfully dialled out to the remote system and invoked ppp service there. Instead, pppd is talking to something (a shell or login process on the remote machine, or maybe just the modem) which is only outputting 7-bit characters.

This can also arise with a modem which uses an AT command set if the dial command is issued before pppd is invoked, rather than within a connect script started by pppd. If the serial port is set to 7 bits/character plus parity when the last AT command is issued, the modem serial port will be set to the same setting.

Note that pppd *always* sets the local serial port to 8 bits per character, with no parity and 1 stop bit. So you shouldn't need to issue an stty command before invoking pppd.

Q: When I try to establish a connection, I get an error message saying "Serial line is looped back". Why?

A: Probably your connection script hasn't successfully dialled out to the remote system and invoked ppp service there. Instead, pppd is talking to something which is just echoing back the characters it receives. The -v option to chat can help you find out what's going on. It can be useful to include "~" as the last expect string to chat, so chat won't return until it's seen the start of the first PPP frame from the remote system.

Another possibility is that your phone connection has dropped for some obscure reason and the modem is echoing the characters it receives from your system.

Q: I installed pppd successfully, but when I try to run it, I get a message saying something like "peer authentication required but no authentication files accessible".

A: When pppd is used on a machine which already has a connection to the Internet (or to be more precise, one which has a default route in its routing table), it will require all peers to authenticate themselves. The reason for this is that if you don't require authentication, you have a security hole, because the peer can basically choose any IP address it wants, even the IP address of some trusted host (for example, a host mentioned in some .rhosts file).

On machines which don't have a default route, pppd does not require the peer to authenticate itself. The reason is that such machines would mostly be using pppd to dial out to an ISP which will refuse to authenticate itself. In that case the peer can use any IP address as long as the system does not already have a route to that address. For example, if you have a local ethernet network, the peer can't use an address on that network. (In fact it could if it authenticated itself and it was permitted to use that address by the pap-secrets or chap-secrets file.)

There are 3 ways around the problem:

  1. If possible, arrange for the peer to authenticate itself, and create the necessary secrets files (/etc/ppp/pap-secrets and/or /etc/ppp/chap-secrets).
  2. If the peer refuses to authenticate itself, and will always be using the same IP address, or one of a small set of IP addresses, you can create an entry in the /etc/ppp/pap-secrets file like this:
      ""	  *	  ""	  allowed.addr.domain
    (that is, using the empty string for the client name and password fields). Of couse, you replace the 4th and following fields in the example above with the IP address(es) that the peer may use. You can use either hostnames or numeric IP addresses.
  3. You can add the `noauth' option to the /etc/ppp/options file. Pppd will then not ask the peer to authenticate itself. If you do this, I *strongly* recommend that you remove the set-uid bit from the permissions on the pppd executable, with a command like this:
    	chmod u-s /usr/sbin/pppd
    Then, an intruder could only use pppd maliciously if they had already become root, in which case they couldn't do any more damage using pppd than they could anyway.

Q: What do I need to put in the secrets files?

A: Three things:

  • secrets (i.e. passwords) to use for authenticating this host to other hosts (i.e., for proving our identity to others);
  • secrets which other hosts can use for authenticating themselves to us (i.e., so that they can prove their identity to us); and
  • information about which IP addresses other hosts may use, once they have authenticated themselves.
There are two authentication files: /etc/ppp/pap-secrets, which contains secrets for use with PAP (the Password Authentication Protocol), and /etc/ppp/chap-secrets, which contains secrets for use with CHAP (the Challenge Handshake Authentication Protocol). Both files have the same simple format, which is as follows:
  • The file contains a series of entries, each of which contains a secret for authenticating one machine to another.
  • Each entry is contained on a single logical line. A logical line may be continued across several lines by placing a backslash (\) at the end of each line except the last.
  • Each entry has 3 or more fields, separated by whitespace (spaces and/or tabs). These fields are, in order:
    1. The name of the machine that is authenticating itself (the "client").
    2. The name of the machine that is authenticating the client (the "server").
    3. The secret to be used for authenticating that client to that server. If this field begins with the at-sign `@', the rest of the field is taken as the name of a file containing the actual secret.
    4. The 4th and any following fields list the IP address(es) that the client may use.
  • The file may contain comments, which begin with a `#' and continue to the end of the line.
  • Double quotes `"' should be used around a field if it contains characters with special significance, such as space, tab, `#', etc.
  • The backslash `\' may be used before characters with special significance (space, tab, `#', `\', etc.) to remove that significance.
Some important points to note:
  • A machine can be *both* a "client" and a "server" for the purposes of authentication - this happens when both peers require the other to authenticate itself. So A would authenticate itself to B, and B would also authenticate itself to A (possibly using a different authentication protocol).
  • If both the "client" and the "server" are running ppp-2.x, they need to have a similar entry in the appropriate secrets file; the first two fields are *not* swapped on the client, compared to the server. So the client might have an entry like this:
    	ay	bee	"our little secret"	-
    and the corresponding entry on the server could look like this:
    	ay	bee	"our little secret"

Q: Explain about PAP and CHAP?

A: PAP stands for the Password Authentication Protocol. With this protocol, the "client" (the machine that needs to authenticate itself) sends its name and a password, in clear text, to the "server". The server returns a message indicating whether the name and password are valid.

CHAP stands for the Challenge Handshake Authentication Protocol. It is designed to address some of the deficiencies and vulnerabilities of PAP. Like PAP, it is based on the client and server having a shared secret, but the secret is never passed in clear text over the link. Instead, the server sends a "challenge" - an arbitrary string of bytes, and the client must prove it knows the shared secret by generating a hash value from the challenge combined with the shared secret, and sending the hash value back to the server. The server also generates the hash value and compares it with the value received from the client.

At a practical level, CHAP can be slightly easier to configure than PAP because the server sends its name with the challenge. Thus, when finding the appropriate secret in the secrets file, the client knows the server's name. In contrast, with PAP, the client has to find its password (i.e. the shared secret) before it has received anything from the server. Thus, it may be necessary to use the `remotename' option to pppd when using PAP authentication so that it can select the appropriate secret from /etc/ppp/pap-secrets.

Microsoft also has a variant of CHAP which uses a different hashing arrangement from normal CHAP. There is a client-side implementation of Microsoft's CHAP in ppp-2.3; see README.MSCHAP80.

Q: When the modem hangs up, without the remote system having terminated the connection properly, pppd does not notice the hangup, but just keeps running. How do I get pppd to notice the hangup and exit?

A: Pppd detects modem hangup by looking for an end-of-file indication from the serial driver, which should be generated when the CD (carrier detect) signal on the serial port is deasserted. For this to work:

  • The modem has to be set to assert CD when the connection is made and deassert it when the phone line hangs up. Usually the AT&C1 modem command sets this mode.
  • The cable from the modem to the serial port must connect the CD signal (on pin 8).
  • Some serial drivers have a "software carrier detect" mode, which must be *disabled*. The method of doing this varies between systems. Under SunOS, use the ttysoftcar command. Under NetBSD, edit /etc/ttys to remove the "softcar" flag from the line for the serial port, and run ttyflags.

Q: Why should I use PPP compression (BSD-Compress or Deflate) when my modem already does V.42 compression? Won't it slow the CPU down a lot?

A: Using PPP compression is preferable, especially when using modems over phone lines, for the following reasons:

  • The V.42 compression in the modem isn't very strong - it's an LZW technique (same as BSD-Compress) with a 10, 11 or 12 bit code size. With BSD-Compress you can use a code size of up to 15 bits and get much better compression, or you can use Deflate and get even better compression ratios.
  • I have found that enabling V.42 compression in my 14.4k modem increases the round-trip time for a character to be sent, echoed and returned by around 40ms, from 160ms to 200ms (with error correction enabled). This is enough to make it feel less responsive on rlogin or telnet sessions. Using PPP compression adds less than 5ms (small enough that I couldn't measure it reliably). I admit my modem is a cheapie and other modems may well perform better.
  • While compression and decompression do require some CPU time, they reduce the amount of time spent in the serial driver to transmit a given amount of data. Many machines require an interrupt for each character sent or received, and the interrupt handler can take a significant amount of CPU time. So the increase in CPU load isn't as great as you might think. My measurements indicate that a system with a 33MHz 486 CPU should be able to do Deflate compression for serial link speeds of up to 100kb/s or more. It depends somewhat on the type of data, of course; for example, when compressing a string of nulls with Deflate, it's hard to get a high output data rate from the compressor, simply because it compresses strings of nulls so well that it has to eat a very large amount of input data to get each byte of output.

Q: I get messages saying "Unsupported protocol (...) received". What do these mean?

A: If you only get one or two when pppd starts negotiating with the peer, they mean that the peer wanted to negotiate some PPP protocol that pppd doesn't understand. This doesn't represent a problem, it simply means that there is some functionality that the peer supports that pppd doesn't, so that functionality can't be used.

If you get them sporadically while the link is operating, or if the protocol numbers (in parentheses) don't correspond to any valid PPP protocol that the peer might be using, then the problem is probably that characters are getting corrupted on the receive side, or that extra characters are being inserted into the receive stream somehow. If this is happening, most packets that get corrupted should get discarded by the FCS (Frame Check Sequence, a 16-bit CRC) check, but a small number may get through.

One possibility may be that you are receiving broadcast messages on the remote system which are being sent over your serial link. Another possibility is that your modem is set for XON/XOFF (software) flow control and is inserting ^Q and ^S characters into the receive data stream.

Q: I get messages saying "Protocol-Reject for unsupported protocol ...". What do these mean?

A: This is the other side of the previous question. If characters are getting corrupted on the way to the peer, or if your system is inserting extra bogus characters into the transmit data stream, the peer may send protocol-reject messages to you, resulting in the above message (since your pppd doesn't recognize the protocol number either.)

Q: I get a message saying something like "ioctl(TIOCSETD): Operation not permitted". How do I fix this?

A: This is because pppd is not running as root. If you have not installed pppd setuid-root, you will have to be root to run it. If you have installed pppd setuid-root and you still get this message, it is probably because your shell is using some other copy of pppd than the installed one - for example, if you are in the pppd directory where you've just built pppd and your $PATH has . before /usr/sbin (or wherever pppd gets installed).

Q: Has your package been ported to HP/UX or IRIX or AIX?

A: No. I don't have access to systems running HP/UX or AIX. No-one has volunteered to port it to HP/UX. I had someone who did a port for AIX 4.x, but who is no longer able to maintain it. And apparently AIX 3.x is quite different, so it would need a separate port.

IRIX includes a good PPP implementation in the standard distribution, as far as I know.

Q: Under SunOS 4, when I try to modload the ppp modules, I get the message "can't open /dev/vd: No such device".

A: First check in /dev that there is an entry like this:

crw-r--r--  1  root         57,   0 Oct 2  1991 vd
If not, make one (mknod /dev/vd c 57 0). If the problem still exists, probably your kernel has been configured without the vd driver included. The vd driver is needed for loadable module support.

First, identify the config file that was used. When you boot your machine, or if you run /etc/dmesg, you'll see a line that looks something like this:

SunOS Release 4.1.3_U1 (CAP_XBOX) #7: Thu Mar 21 15:31:56 EST 1996
			this is the config file name
The config file will be in the /sys/`arch -k`/conf directory (arch -k should return sun4m for a SparcStation 10, sun3x for a Sun 3/80, etc.). Look in there for a line saying "options VDDRV". If that line isn't present (or is commented out), add it (or uncomment it).

You then need to rebuild the kernel as described in the SunOS manuals. Basically you need to run config and make like this:

	/usr/etc/config CAP_XBOX
	cd ../CAP_XBOX
(replacing the string CAP_XBOX by the name of the config file for your kernel, of course).

Then copy the new kernel to /:

	mv /vmunix /vmunix.working
	cp vmunix /
and reboot. Modload should then work.

Q: I'm running Linux (or NetBSD or FreeBSD), and my system comes with PPP already. Should I consider installing this package? Why?

A: The PPP that is already installed in your system is (or is derived from) some version of this PPP package. You can find out what version of this package is already installed with the command "pppd --help". If this is older than the latest version, you may wish to install the latest version so that you can take advantage of the new features or bug fixes.

Q: I'm running pppd in demand mode, and I find that pppd often dials out unnecessarily when I try to make a connection within my local machine or with a machine on my local LAN. What can I do about this?

A: Very often the cause of this is that a program is trying to contact a nameserver to resolve a hostname, and the nameserver (specified in /etc/resolv.conf, usually) is on the far side of the ppp link. You can try executing a command such as `ping myhost' (where myhost is the name of the local machine, or some other machine on a local LAN), to see whether that starts the ppp link. If it does, check the setup of your /etc/hosts file to make sure you have the local machine and any hosts on your local LAN listed, and /etc/resolv.conf and/or /etc/nsswitch.conf files to make sure you resolve hostnames from /etc/hosts if possible before trying to contact a nameserver.

Q: Since I installed ppp-2.3.6, dialin users to my server have been getting this message when they run pppd:
peer authentication required but no suitable secret(s) found for 
authenticating any peer to us (ispserver)
A: In 2.3.6, the default is to let an unauthenticated peer only use IP addresses to which the machine doesn't already have a route. So on a machine with a default route, everyone has to authenticate. If you really don't want that, you can put `noauth' in the /etc/ppp/options file. Note that there is then no check on who is using which IP address. IMHO, this is undesirably insecure, but I guess it may be tolerable as long as you don't use any .rhosts files or anything like that. I recommend that you require dialin users to authenticate, even if just with PAP using their login password (using the `login' option to pppd). If you do use `noauth', you should at least have a pppusers group and set the permissions on pppd to allow only user and group to execute it.