Il CD di internet/CD.iso/SOURCE/N/PPP/_PPP.TAR/usr/lib/ppp/SETUP

Text File | 1994-10-17 | 7.6 KB | 188 lines

        Setting up a PPP link.

Setting up a PPP link between two machines involves several steps:

1. Prepare both of the machines which are to be connected:
   1A. Make and install the pppd, pppstats and chat programs.
   1B. Install the ppp driver in the kernel.
The README.* files give details on this step.

2. Decide on the IP addresses to be used and the level of
authentication required by each machine, and set up the /etc/ppp
directories accordingly.

3. Set up the serial link between the two machines and run pppd on
each machine.  The two pppd's then negotiate and set up the link.

Step 1 is described in README.bsd and README.sun.  The remaining steps
are described below.  Steps 1 and 2 need only be done once; step 3 is
done each time the link is to be established.


Choosing IP addresses.

If a host is already connected to the Internet via a LAN such as
Ethernet, then it will already have at least one IP address assigned,
which will usually be the IP address of the LAN interface.  In such
cases, it is usually most convenient to use that address as the local
IP address of the PPP interface(s) on that host.  This is OK because
the PPP interface(s) are point-to-point interfaces.

If a host is not connected to the Internet, then an IP address needs
to be assigned for it.  If PPP is to be used to link it to another
host which is connected to the Internet, is is usually most convenient
to assign it an address on the same subnet as the remote host.  If the
other host is not connected to the Internet either, then the choice of
IP addresses is quite arbitrary.

Authentication.

The level of authentication required depends on the situation, but
generally hosts which are connected to the Internet via a LAN should
be set up to (a) require the remote host to authenticate itself, and
(b) restrict the remote host's choice of IP address, based on its
identity.  Otherwise the possibility exists for a remote host to
impersonate another host on the local subnet.  (However, when you are
first installing PPP, it is probably easier to leave authentication
disabled until you get to the point where you can successfully
establish a link.)

Setting up /etc/ppp.

The /etc/ppp directory contains various files used by pppd; it should
be created by the system administrator when installing PPP.  It would
typically contain the following files:

    chap-secrets    Secrets used for authenticating with CHAP
    pap-secrets        Secrets used for authenticating with PAP
    options        Options that the system administrator wants to
            apply whenever pppd is run
    ppp<n>.pid        File containing the process ID of the pppd on
            ppp unit <n>; created by pppd

Since this directory contains files of secrets used for
authentication, it should not be in a partition which is accessible
from other hosts (e.g., exported by NFS).

The `options' file contains any options which the system administrator
wants pppd to use whenever it is run.  If authentication is to be
required, this should contain the `auth' and `usehostname' options.
If a netmask option is required, it is usually convenient to put it
here also.  If pppd has been compiled with the -DREQ_SYSOPTIONS=1 flag
on the command line, then the `options' file must be readable by pppd,
otherwise pppd will refuse to run.

Secrets for PAP (Password Authentication Protocol) authentication are
stored in /etc/ppp/pap-secrets; secrets for CHAP (Cryptographic
Authentication Protocol) are stored in /etc/ppp/chap-secrets.  These
files have the same format, and store secrets both for authenticating
other hosts, and for authenticating this host to others.  The format
is that there are 3 or more words per line, which are:

    client - name of the machine to be authenticated
    server - name of the machine requiring the authentication
    secret - password or CHAP secret known by both client and server
    IP addresses - zero or more IP addresses which the client may
        use (if we are the server).

For example, if a LAN-connected host called "worksun" is to require
authentication, and a host "bsdbox" is to connect to it and
authenticate itself using CHAP, then both machines should have a
/etc/ppp/chap-secrets file, which should contain a line something
like:

    bsdbox    worksun    "an unguessable secret"    bsdbox.my.domain

Setting up syslog.

pppd issues messages using syslog facility daemon (or local2 if it has
been compiled with debugging enabled); chat uses facility local2.  It
is useful to see messages of priority 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' on the left.  It is
also useful to add a line like this:

    daemon,local2.debug        /etc/ppp/ppp-log

If you do this, you will need to create an empty /etc/ppp/ppp-log
file.

After modifying syslog.conf, you will then need to send a HUP signal
to syslogd (or reboot).


Setting up a PPP link.

Establishing a PPP connection between two machines basically involves
setting up a serial link and running pppd on both ends of the link.
How this is done depends on the nature of the serial link, which may
be as simple as a null modem cable between two machines, or it may
involve modems, terminal servers, telnet sessions, etc.  The usual
scenario is that you establish the serial link (e.g., by dialling out
from a modem) and use it to log in to a remote machine.

The next step is to run pppd on the remote machine.  Do not supply a
serial device name here; pppd will use the session's controlling
terminal.  (This may be a pty, e.g., if the serial link contains a
telnet session, except under Ultrix.)  A typical command would be

    pppd passive

If the remote machine is connected to the Internet via a LAN, it is
often useful to add the `proxyarp' option.  The `asyncmap' option is
also useful if the serial line is not completely transparent;
`asyncmap 200a0000' is appropriate if the serial link includes a
telnet.

Some people find it convenient to set up a `ppp' username on the
remote machine, with no password and a shell script which runs pppd as
its login shell.

The next step is to run pppd on the local machine, using a command
like:

    pppd /dev/ttya 19200

When a device is given, as in this command line, pppd will put itself
in the background.  The two pppd's should then negotiate and bring up
the link.  If you have edited /etc/syslog.conf as described above, you
will see messages from pppd giving the local and remote IP addresses
of the link when it is successfully established.

If the local machine has no other connection to the Internet, you can
ask pppd to add a default route via the remote host by adding the
`defaultroute' option to the pppd command.

Other random points about running pppd:
    - If you want the local address of the PPP link to be
      different from the (first) IP address of the host, you need
      to put the desired address on the pppd command line with a
      colon appended.
    - The performance will probably be better if you reduce the
      MRU (maximum receive unit) on both ends; 296 is a good
      value.  To do this, use the option `mru 296'.


Terminating the PPP link.

When you wish terminate the PPP link, you should send a TERM or INTR
signal to one of the pppd's, e.g., with a command like:

    kill `cat /etc/ppp/ppp0.pid`

on SunOS or Ultrix, or

    kill `cat /var/run/ppp0.pid`

on {386,Net,Free}BSD.

That pppd will inform the other pppd to terminate, and they will both
clean up and exit.

If pppd is attached to a hardware serial port connected to a modem,
then it should get a HUP signal when the modem hangs up, which will
cause it to clean up and exit.  Whether it does or not depends on the
driver, and on Suns, on the setting of the `tty soft carrier' flag,
which is manipulated by the /usr/etc/ttysoftcar program (see
ttysoftcar(8)).