16.2. Using User PPP

Originally contributed by Brian Somers. With input from Nik Clayton, Dirk Fr闣berg, and Peter Childs.

16.2.1. User PPP Assumptions

This document assumes you have the following:

  • An account with an Internet Service Provider (ISP) which you connect to using PPP.

  • Further, you have a modem or other device connected to your system and configured correctly, which allows you to connect to your ISP.

  • The dial-up number(s) of your ISP.

  • Your login name and password. (Either a regular Unix-style login and password pair, or a PAP or CHAP login and password pair.)

  • The IP address of one or more name servers. Normally, you will be given two IP addresses by your ISP to use for this. If they have not given you at least one, then you can use the enable dns command in your ppp.conf file to tell ppp to set the name servers for you. This feature depends on your ISPs PPP implementation supporting DNS negotiation.

The following information may be supplied by your ISP, but is not completely necessary:

  • The IP address of your ISP's gateway. The gateway is the machine to which you will connect and will be set up as your default route. If you do not have this information, we can make one up and your ISP's PPP server will tell us the correct value when we connect.

    This IP number is referred to as HISADDR by ppp.

  • The netmask you should use. If your ISP has not provided you with one, you can safely use

  • If your ISP provides you with a static IP address and hostname, you can enter it. Otherwise, we simply let the peer assign whatever IP address it sees fit.

If you do not have any of the required information, contact your ISP.

Note: Throughout this section, many of the examples showing the contents of configuration files are numbered by line. These numbers serve to aid in the presentation and discussion only and are not meant to be placed in the actual file. Proper indentation with tab and space characters is also important. Preparing the Kernel

As previously mentioned, ppp uses the tun device. If this device has not been compiled into your kernel, ppp will load it on demand as a module. The tunnel driver is dynamic, so any number of devices may be created (you are not limited by any kernel configuration values).

Note: It should be noted that the tunnel driver creates devices on demand, so ifconfig -a will not necessarily show any tun devices. Check the tun Device

Under normal circumstances, most users will only use one tun device (/dev/tun0). References to tun0 below may be changed to tunN where N is any unit number corresponding to your system.

For FreeBSD installations that do not have DEVFS enabled, the existence of the tun0 device should be verified (this is not necessary if DEVFS is enabled as device nodes will be created on demand).

The easiest way to make sure that the tun0 device is configured correctly is to remake the device. To remake the device, do the following:

    # cd /dev
    # sh MAKEDEV tun0

If you need 16 tunnel devices in your kernel, you will need to create them. This can be done by executing the following commands:

    # cd /dev
    # sh MAKEDEV tun15 Name Resolution Configuration

The resolver is the part of the system that looks up IP addresses into hostnames and vice versa. It can be configured to look for maps that describe IP to hostname mappings in one of two places. The first is a file called /etc/hosts. Read hosts(5) for more information. The second is the Internet Domain Name Service (DNS), a distributed data base. For more information on DNS and DNS services, refer to Section 17.9.

The resolver is a set of system calls that perform the name mappings, but you have to tell it where to find the information. For versions of FreeBSD prior to 5.0. This is done by editing the file /etc/host.conf. FreeBSD 5.0 uses the /etc/nsswitch.conf file. Edit /etc/host.conf

For versions of FreeBSD prior to 5.0, this file should contain the following two lines (in this order):


This instructs the resolver to first look in the file /etc/hosts, and to then consult the DNS if the name was not found. Edit /etc/nsswitch.conf

For FreeBSD version 5.0 or above, this file should contain at least the following line:

    hosts: files, dns

This instructs the resolver to first look in the file /etc/hosts, and to then consult DNS if the name was not found. Edit /etc/hosts

This file may contain the IP addresses and names of machines on your local network. At a bare minimum it should contain entries for the machine which will be running ppp. Assuming that your machine is called foo.example.com with the IP address, /etc/hosts should contain:	localhost.example.com	localhost
    ::1	localhost.example.com	localhost	foo.example.com		foo

The first two lines define the alias localhost as a synonym for the current machine. Regardless of your own IP address, the IP addresses for these lines should always be and ::1. The last line maps the name foo.example.com (and the shorthand foo) to the IP address

Note: and localhost are known as loopback addresses, which loopback to the local machine.

If your provider allocates you a static IP address and name, and you are not using that as your host name, add this to the /etc/hosts too. Edit /etc/resolv.conf

The /etc/resolv.conf file tells the resolver how to behave. Normally, you will need to enter the following line(s):

    domain example.com
    nameserver x.x.x.x
    nameserver y.y.y.y

The x.x.x.x and y.y.y.y addresses are those given to you by your ISP. Add as many nameserver lines as your ISP provides. The domain line is set to your hosts domain name. Refer to the resolv.conf(5) manual page for details of other possible entries in this file.

If you are running a local name server, replace the above nameserver lines with:


The enable dns command (entered in the /etc/ppp/ppp.conf file - see below) will tell PPP to request that your ISP confirms the nameserver values. If your ISP supplies different addresses (or if there are no nameserver lines in /etc/resolv.conf), PPP will rewrite the file with the ISP-supplied values. PPP Configuration

Both ppp and pppd (the kernel level implementation of PPP) use the configuration files located in the /etc/ppp directory. Examples for user ppp can be found in /usr/share/examples/ppp/.

Configuring ppp requires that you edit a number of files, depending on your requirements. What you put in them depends to some extent on whether your ISP allocates IP addresses statically (i.e., you get given one IP address, and always use that one) or dynamically (i.e., your IP address changes each time you connect to your ISP). PPP and Static IP Addresses

You will need to edit the /etc/ppp/ppp.conf configuration file. It should look similar to the example below.

Note: Lines that end in a : start in the first column (beginning of the line)-- all other lines should be indented as shown using spaces or tabs.

    1     default:
    2       set log Phase Chat LCP IPCP CCP tun command
    3       ident user-ppp VERSION (built COMPILATIONDATE)
    4       set device /dev/cuaa0
    5       set speed 115200
    6       set dial "ABORT BUSY ABORT NO\\sCARRIER TIMEOUT 5 \
    7                 \"\" AT OK-AT-OK ATE1Q0 OK \\dATDT\\T TIMEOUT 40 CONNECT"
    8       set timeout 180
    9       enable dns
    11    provider:
    12      set phone "(123) 456 7890"
    13      set authname foo
    14      set authkey bar
    15      set login "TIMEOUT 10 \"\" \"\" gin:--gin: \\U word: \\P col: ppp"
    16      set timeout 300
    17      set ifaddr x.x.x.x y.y.y.y
    18      add default HISADDR

Line 1:

Identifies the default entry. Commands in this entry are executed automatically when ppp is run.

Line 2:

Enables logging parameters. When the configuration is working satisfactorily, this line should be reduced to saying

    set log phase tun
in order to avoid excessive log file sizes.

Line 3:

Tells PPP how to identify itself to the peer. PPP identifies itself to the peer if it has any trouble negotiating and setting up the link, providing information that the peers administrator may find useful when investigating such problems.

Line 4:

Identifies the device to which the modem is connected. COM1 is /dev/cuaa0 and COM2 is /dev/cuaa1.

Line 5:

Sets the speed you want to connect at. If 115200 does not work (it should with any reasonably new modem), try 38400 instead.

Line 6 & 7:

The dial string. User PPP uses an expect-send syntax similar to the chat(8) program. Refer to the manual page for information on the features of this language.

Note that this command continues onto the next line for readability. Any command in ppp.conf may do this if the last character on the line is a ``\'' character.

Line 8:

Sets the idle timeout for the link. 180 seconds is the default, so this line is purely cosmetic.

Line 9:

Tells PPP to ask the peer to confirm the local resolver settings. If you run a local name server, this line should be commented out or removed.

Line 10:

A blank line for readability. Blank lines are ignored by PPP.

Line 11:

Identifies an entry for a provider called "provider".

Line 12:

Sets the phone number for this provider. Multiple phone numbers may be specified using the colon (:) or pipe character (|)as a separator. The difference between the two separators is described in ppp(8). To summarize, if you want to rotate through the numbers, use a colon. If you want to always attempt to dial the first number first and only use the other numbers if the first number fails, use the pipe character. Always quote the entire set of phone numbers as shown.

Line 13 & 14:

Identifies the user name and password. When connecting using a Unix-style login prompt, these values are referred to by the set login command using the \U and \P variables. When connecting using PAP or CHAP, these values are used at authentication time.

Line 15:

If you are using PAP or CHAP, there will be no login at this point, and this line should be commented out or removed. See PAP and CHAP authentication for further details.

The login string is of the same chat-like syntax as the dial string. In this example, the string works for a service whose login session looks like this:

    J. Random Provider
    login: foo
    password: bar
    protocol: ppp

You will need to alter this script to suit your own needs. When you write this script for the first time, you should ensure that you have enabled "chat" logging so you can determine if the conversation is going as expected.

Line 16:

Sets the default idle timeout (in seconds) for the connection. Here, the connection will be closed automatically after 300 seconds of inactivity. If you never want to timeout, set this value to zero or use the -ddial command line switch.

Line 17:

Sets the interface addresses. The string x.x.x.x should be replaced by the IP address that your provider has allocated to you. The string y.y.y.y should be replaced by the IP address that your ISP indicated for their gateway (the machine to which you connect). If your ISP has not given you a gateway address, use If you need to use a "guessed" address, make sure that you create an entry in /etc/ppp/ppp.linkup as per the instructions for PPP and Dynamic IP addresses. If this line is omitted, ppp cannot run in -auto mode.

Line 18:

Adds a default route to your ISP's gateway. The special word HISADDR is replaced with the gateway address specified on line 9. It is important that this line appears after line 9, otherwise HISADDR will not yet be initialized.

If you do not wish to run ppp in -auto, this line should be moved to the ppp.linkup file.

It is not necessary to add an entry to ppp.linkup when you have a static IP address and are running ppp in -auto mode as your routing table entries are already correct before you connect. You may however wish to create an entry to invoke programs after connection. This is explained later with the sendmail example.

Example configuration files can be found in the /usr/share/examples/ppp/ directory. PPP and Dynamic IP Addresses

If your service provider does not assign static IP addresses, ppp can be configured to negotiate the local and remote addresses. This is done by "guessing" an IP address and allowing ppp to set it up correctly using the IP Configuration Protocol (IPCP) after connecting. The ppp.conf configuration is the same as PPP and Static IP Addresses, with the following change:

    17      set ifaddr

Again, do not include the line number, it is just for reference. Indentation of at least one space is required.

Line 17:

The number after the / character is the number of bits of the address that ppp will insist on. You may wish to use IP numbers more appropriate to your circumstances, but the above example will always work.

The last argument ( tells PPP to start negotiations using address rather than and is necessary for some ISPs. Do not use as the first argument to set ifaddr as it prevents PPP from setting up an initial route in -auto mode.

If you are not running in -auto mode, you will need to create an entry in /etc/ppp/ppp.linkup. ppp.linkup is used after a connection has been established. At this point, ppp will have assigned the interface addresses and it will now be possible to add the routing table entries:

    1     provider:
    2      add default HISADDR

Line 1:

On establishing a connection, ppp will look for an entry in ppp.linkup according to the following rules: First, try to match the same label as we used in ppp.conf. If that fails, look for an entry for the IP address of our gateway. This entry is a four-octet IP style label. If we still have not found an entry, look for the MYADDR entry.

Line 2:

This line tells ppp to add a default route that points to HISADDR. HISADDR will be replaced with the IP number of the gateway as negotiated by the IPCP.

See the pmdemand entry in the files /usr/share/examples/ppp/ppp.conf.sample and /usr/share/examples/ppp/ppp.linkup.sample for a detailed example. Receiving Incoming Calls

When you configure ppp to receive incoming calls on a machine connected to a LAN, you must decide if you wish to forward packets to the LAN. If you do, you should allocate the peer an IP number from your LAN's subnet, and use the command enable proxy in your /etc/ppp/ppp.conf file. You should also confirm that the /etc/rc.conf file contains the following:

    gateway_enable="YES" Which getty?

Configuring FreeBSD for Dial-up Services provides a good description on enabling dial-up services using getty(8).

An alternative to getty is mgetty, a smarter version of getty designed with dial-up lines in mind.

The advantages of using mgetty is that it actively talks to modems, meaning if port is turned off in /etc/ttys then your modem will not answer the phone.

Later versions of mgetty (from 0.99beta onwards) also support the automatic detection of PPP streams, allowing your clients script-less access to your server.

Refer to Mgetty and AutoPPP for more information on mgetty. PPP Permissions

The ppp command must normally be run as the root user. If however, you wish to allow ppp to run in server mode as a normal user by executing ppp as described below, that user must be given permission to run ppp by adding them to the network group in /etc/group.

You will also need to give them access to one or more sections of the configuration file using the allow command:

    allow users fred mary

If this command is used in the default section, it gives the specified users access to everything. PPP Shells for Dynamic-IP Users

Create a file called /etc/ppp/ppp-shell containing the following:

    IDENT=`echo $0 | sed -e 's/^.*-\(.*\)$/\1/'`
    if [ x$IDENT = xdialup ]; then
            IDENT=`basename $TTY`
    echo "PPP for $CALLEDAS on $TTY"
    echo "Starting PPP for $IDENT"
    exec /usr/sbin/ppp -direct $IDENT

This script should be executable. Now make a symbolic link called ppp-dialup to this script using the following commands:

    # ln -s ppp-shell /etc/ppp/ppp-dialup

You should use this script as the shell for all of your dialup users. This is an example from /etc/password for a dialup PPP user with username pchilds (remember do not directly edit the password file, use vipw).

    pchilds:*:1011:300:Peter Childs PPP:/home/ppp:/etc/ppp/ppp-dialup

Create a /home/ppp directory that is world readable containing the following 0 byte files:

    -r--r--r--   1 root     wheel           0 May 27 02:23 .hushlogin
    -r--r--r--   1 root     wheel           0 May 27 02:22 .rhosts

which prevents /etc/motd from being displayed. PPP Shells for Static-IP Users

Create the ppp-shell file as above, and for each account with statically assigned IPs create a symbolic link to ppp-shell.

For example, if you have three dialup customers, fred, sam, and mary, that you route class C networks for, you would type the following:

    # ln -s /etc/ppp/ppp-shell /etc/ppp/ppp-fred
    # ln -s /etc/ppp/ppp-shell /etc/ppp/ppp-sam
    # ln -s /etc/ppp/ppp-shell /etc/ppp/ppp-mary

Each of these users dialup accounts should have their shell set to the symbolic link created above (for example, mary's shell should be /etc/ppp/ppp-mary). Setting up ppp.conf for Dynamic-IP Users

The /etc/ppp/ppp.conf file should contain something along the lines of:

      set debug phase lcp chat
      set timeout 0
      set ifaddr
      enable proxy
      set ifaddr
      enable proxy

Note: The indenting is important.

The default: section is loaded for each session. For each dialup line enabled in /etc/ttys create an entry similar to the one for ttyd0: above. Each line should get a unique IP address from your pool of IP addresses for dynamic users. Setting up ppp.conf for Static-IP Users

Along with the contents of the sample /usr/share/examples/ppp/ppp.conf above you should add a section for each of the statically assigned dialup users. We will continue with our fred, sam, and mary example.

      set ifaddr
      set ifaddr
      set ifaddr

The file /etc/ppp/ppp.linkup should also contain routing information for each static IP user if required. The line below would add a route for the class C via the client's ppp link.

      add netmask HISADDR
      add netmask HISADDR
      add netmask HISADDR More on mgetty, AutoPPP, and MS Extensions mgetty and AutoPPP

Configuring and compiling mgetty with the AUTO_PPP option enabled allows mgetty to detect the LCP phase of PPP connections and automatically spawn off a ppp shell. However, since the default login/password sequence does not occur it is necessary to authenticate users using either PAP or CHAP.

This section assumes the user has successfully configured, compiled, and installed a version of mgetty with the AUTO_PPP option (v0.99beta or later).

Make sure your /usr/local/etc/mgetty+sendfax/login.config file has the following in it:

    /AutoPPP/ -     -		      /etc/ppp/ppp-pap-dialup

This will tell mgetty to run the ppp-pap-dialup script for detected PPP connections.

Create a file called /etc/ppp/ppp-pap-dialup containing the following (the file should be executable):

    exec /usr/sbin/ppp -direct pap$IDENT

For each dialup line enabled in /etc/ttys, create a corresponding entry in /etc/ppp/ppp.conf. This will happily co-exist with the definitions we created above.

      enable pap	      
      set ifaddr
      enable proxy

Each user logging in with this method will need to have a username/password in /etc/ppp/ppp.secret file, or alternatively add the following option to authenticate users via PAP from /etc/password file.

    enable passwdauth

If you wish to assign some users a static IP number, you can specify the number as the third argument in /etc/ppp/ppp.secret. See /usr/share/examples/ppp/ppp.secret.sample for examples. MS Extensions

It is possible to configure PPP to supply DNS and NetBIOS nameserver addresses on demand.

To enable these extensions with PPP version 1.x, the following lines might be added to the relevant section of /etc/ppp/ppp.conf.

    enable msext
    set ns
    set nbns

And for PPP version 2 and above:

    accept dns
    set dns
    set nbns

This will tell the clients the primary and secondary name server addresses, and a NetBIOS nameserver host.

In version 2 and above, if the set dns line is omitted, PPP will use the values found in /etc/resolv.conf. PAP and CHAP Authentication

Some ISPs set their system up so that the authentication part of your connection is done using either of the PAP or CHAP authentication mechanisms. If this is the case, your ISP will not give a login: prompt when you connect, but will start talking PPP immediately.

PAP is less secure than CHAP, but security is not normally an issue here as passwords, although being sent as plain text with PAP, are being transmitted down a serial line only. There is not much room for crackers to "eavesdrop".

Referring back to the PPP and Static IP addresses or PPP and Dynamic IP addresses sections, the following alterations must be made:

    7       set login
    12      set authname MyUserName
    13      set authkey MyPassword

Line 7:

Your ISP will not normally require that you log into the server if you are using PAP or CHAP. You must therefore disable your "set login" string.

Line 12:

This line specifies your PAP/CHAP user name. You will need to insert the correct value for MyUserName.

Line 13:

This line specifies your PAP/CHAP password. You will need to insert the correct value for MyPassword. You may want to add an additional line, such as:

    15      accept PAP


    15      accept CHAP

to make it obvious that this is the intention, but PAP and CHAP are both accepted by default. Changing Your ppp Configuration on the Fly

It is possible to talk to the ppp program while it is running in the background, but only if a suitable diagnostic port has been set up. To do this, add the following line to your configuration:

    set server /var/run/ppp-tun%d DiagnosticPassword 0177

This will tell PPP to listen to the specified Unix-domain socket, asking clients for the specified password before allowing access. The %d in the name is replaced with the tun device number that is in use.

Once a socket has been set up, the pppctl(8) program may be used in scripts that wish to manipulate the running program. Final System Configuration

You now have ppp configured, but there are a few more things to do before it is ready to work. They all involve editing the /etc/rc.conf file.

Working from the top down in this file, make sure the hostname= line is set, e.g.:


If your ISP has supplied you with a static IP address and name, it is probably best that you use this name as your host name.

Look for the network_interfaces variable. If you want to configure your system to dial your ISP on demand, make sure the tun0 device is added to the list, otherwise remove it.

    network_interfaces="lo0 tun0" ifconfig_tun0=

Note: The ifconfig_tun0 variable should be empty, and a file called /etc/start_if.tun0 should be created. This file should contain the line:

    ppp -auto mysystem

This script is executed at network configuration time, starting your ppp daemon in automatic mode. If you have a LAN for which this machine is a gateway, you may also wish to use the -alias switch. Refer to the manual page for further details.

Set the router program to NO with following line in your /etc/rc.conf:


It is important that the routed daemon is not started (it is started by default), as routed tends to delete the default routing table entries created by ppp.

It is probably worth your while ensuring that the sendmail_flags line does not include the -q option, otherwise sendmail will attempt to do a network lookup every now and then, possibly causing your machine to dial out. You may try:


The downside of this is that you must force sendmail to re-examine the mail queue whenever the ppp link is up by typing:

    # /usr/sbin/sendmail -q

You may wish to use the !bg command in ppp.linkup to do this automatically:

    1     provider:
    2       delete ALL
    3       add 0 0 HISADDR
    4       !bg sendmail -bd -q30m

If you do not like this, it is possible to set up a "dfilter" to block SMTP traffic. Refer to the sample files for further details.

Now the only thing left to do is reboot the machine.

All that is left is to reboot the machine. After rebooting, you can now either type:

    # ppp

and then dial provider to start the PPP session, or, if you want ppp to establish sessions automatically when there is outbound traffic (and you have not created the start_if.tun0 script), type:

    # ppp -auto provider Summary

To recap, the following steps are necessary when setting up ppp for the first time:

Client side:

  1. Ensure that the tun device is built into your kernel.

  2. Ensure that the tunX device file is available in the /dev directory.

  3. Create an entry in /etc/ppp/ppp.conf. The pmdemand example should suffice for most ISPs.

  4. If you have a dynamic IP address, create an entry in /etc/ppp/ppp.linkup.

  5. Update your /etc/rc.conf file.

  6. Create a start_if.tun0 script if you require demand dialing.

Server side:

  1. Ensure that the tun device is built into your kernel.

  2. Ensure that the tunX device file is available in the /dev directory.

  3. Create an entry in /etc/passwd (using the vipw(8) program).

  4. Create a profile in this users home directory that runs ppp -direct direct-server or similar.

  5. Create an entry in /etc/ppp/ppp.conf. The direct-server example should suffice.

  6. Create an entry in /etc/ppp/ppp.linkup.

  7. Update your /etc/rc.conf file.