27.2. Configuring PPP

27.2. Configuring PPP
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FreeBSD provides built-in support for managing dial-up PPP connections using ppp(8). The default FreeBSD kernel provides support for tun which is used to interact with a modem hardware. Configuration is performed by editing at least one configuration file, and configuration files containing examples are provided. Finally, ppp is used to start and manage connections.

In order to use a PPP connection, the following items are needed:

A dial-up account with an Internet Service Provider (ISP).
A dial-up modem.
The dial-up number for the ISP.
The login name and password assigned by the ISP.
The IP address of one or more DNS servers. Normally, the ISP provides these addresses. If it did not, FreeBSD can be configured to use DNS negotiation.

If any of the required information is missing, contact the ISP.

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

The IP address of the default gateway. If this information is unknown, the ISP will automatically provide the correct value during connection setup. When configuring PPP on FreeBSD, this address is referred to as HISADDR.
The subnet mask. If the ISP has not provided one, 255.255.255.255 will be used in the ppp(8) configuration file.
If the ISP has assigned a static IP address and hostname, it should be input into the configuration file. Otherwise, this information will be automatically provided during connection setup.

The rest of this section demonstrates how to configure FreeBSD for common PPP connection scenarios. The required configuration file is /etc/ppp/ppp.conf and additional files and examples are available in /usr/share/examples/ppp/.

Note:

Throughout this section, many of the file examples display line numbers. These line numbers have been added to make it easier to follow the discussion and are not meant to be placed in the actual file.

When editing a configuration file, proper indentation is important. Lines that end in a : start in the first column (beginning of the line) while all other lines should be indented as shown using spaces or tabs.

27.2.1. Basic Configuration

In order to configure a PPP connection, first edit /etc/ppp/ppp.conf with the dial-in information for the ISP. This file is described as follows:

1     default:
2       set log Phase Chat LCP IPCP CCP tun command
3       ident user-ppp VERSION
4       set device /dev/cuau0
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
10
11    provider:
12      set phone "(123) 456 7890"
13      set authname foo
14      set authkey bar
15      set timeout 300
16      set ifaddr x.x.x.x/0 y.y.y.y/0 255.255.255.255 0.0.0.0
17      add default HISADDR

Line 1:

Identifies the default entry. Commands in this entry (lines 2 through 9) are executed automatically when ppp is run.

Line 2:

Enables verbose logging parameters for testing the connection. Once the configuration is working satisfactorily, this line should be reduced to:

set log phase tun

Line 3:

Displays the version of ppp(8) to the PPP software running on the other side of the connection.

Line 4:

Identifies the device to which the modem is connected, where COM1 is /dev/cuau0 and COM2 is /dev/cuau1.

Line 5:

Sets the connection speed. If 115200 does not work on an older modem, try 38400 instead.

Lines 6 & 7:

The dial string written as an expect-send syntax. Refer to chat(8) for more information.

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 \.

Line 8:

Sets the idle timeout for the link in seconds.

Line 9:

Instructs the peer to confirm the DNS settings. If the local network is running its own DNS server, this line should be commented out, by adding a # at the beginning of the line, or removed.

Line 10:

A blank line for readability. Blank lines are ignored by ppp(8).

Line 11:

Identifies an entry called provider. This could be changed to the name of the ISP so that load ISP can be used to start the connection.

Line 12:

Use the phone number for the ISP. Multiple phone numbers may be specified using the colon (:) or pipe character (|) as a separator. To rotate through the numbers, use a colon. 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 enclose the entire set of phone numbers between quotation marks (") to prevent dialing failures.

Lines 13 & 14:

Use the user name and password for the ISP.

Line 15:

Sets the default idle timeout in seconds for the connection. In this example, the connection will be closed automatically after 300 seconds of inactivity. To prevent a timeout, set this value to zero.

Line 16:

Sets the interface addresses. The values used depend upon whether a static IP address has been obtained from the ISP or if it instead negotiates a dynamic IP address during connection.

If the ISP has allocated a static IP address and default gateway, replace x.x.x.x with the static IP address and replace y.y.y.y with the IP address of the default gateway. If the ISP has only provided a static IP address without a gateway address, replace y.y.y.y with 10.0.0.2/0.

If the IP address changes whenever a connection is made, change this line to the following value. This tells ppp(8) to use the IP Configuration Protocol (IPCP) to negotiate a dynamic IP address:

set ifaddr 10.0.0.1/0 10.0.0.2/0 255.255.255.255 0.0.0.0

Line 17:

Keep this line as-is as it adds a default route to the gateway. The HISADDR will automatically be replaced with the gateway address specified on line 16. It is important that this line appears after line 16.

Depending upon whether ppp(8) is started manually or automatically, a /etc/ppp/ppp.linkup may also need to be created which contains the following lines. This file is required when running ppp in -auto mode. This file is used after the connection has been established. At this point, the IP address will have been assigned and it is now be possible to add the routing table entries. When creating this file, make sure that provider matches the value demonstrated in line 11 of ppp.conf.

provider:
      add default HISADDR

This file is also needed when the default gateway address is “guessed” in a static IP address configuration. In this case, remove line 17 from ppp.conf and create /etc/ppp/ppp.linkup with the above two lines. More examples for this file can be found in /usr/share/examples/ppp/.

By default, the ppp command must be run as the root user. To change this default, add the account of the user who should run ppp to the network group in /etc/group.

Then, give the user access to one or more entries in /etc/ppp/ppp.conf using the allow command. For example, to give fred and mary permission to only the provider: entry, add this line to the provider: section:

allow users fred mary

To give the specified users access to all entries, put that line in the default section instead.

27.2.2. Advanced Configuration

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 203.14.100.1 203.14.100.2
set nbns 203.14.100.5

And for PPP version 2 and above:

accept dns
set dns 203.14.100.1 203.14.100.2
set nbns 203.14.100.5

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.

27.2.2.1. PAP and CHAP Authentication

Some ISPs set their system up so that the authentication part of the connection is done using either of the PAP or CHAP authentication mechanisms. If this is the case, the ISP will not give a login: prompt at connection, 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”.

The following alterations must be made:

13      set authname MyUserName
14      set authkey MyPassword
15      set login

Line 13:

This line specifies the PAP/CHAP user name. Insert the correct value for MyUserName.

Line 14:

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

16      accept PAP

16      accept CHAP

to make it obvious that this is the intention, but PAP and CHAP are both accepted by default.

Line 15:

The ISP will not normally require a login to the server when using PAP or CHAP. Therefore, disable the “set login” string.

27.2.2.2. Using PPP Network Address Translation Capability

PPP has ability to use internal NAT without kernel diverting capabilities. This functionality may be enabled by the following line in /etc/ppp/ppp.conf:

nat enable yes

Alternatively, NAT may be enabled by command-line option -nat. There is also /etc/rc.conf knob named ppp_nat, which is enabled by default.

When using this feature, it may be useful to include the following /etc/ppp/ppp.conf options to enable incoming connections forwarding:

nat port tcp 10.0.0.2:ftp ftp
nat port tcp 10.0.0.2:http http

or do not trust the outside at all

nat deny_incoming yes

27.2.3. Final System Configuration

While ppp is now configured, some edits still need to be made to /etc/rc.conf.

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

hostname="foo.example.com"

If the ISP has supplied a static IP address and name, use this name as the host name.

Look for the network_interfaces variable. To configure the system to dial the 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 the ppp daemon in automatic mode. If this machine acts as a gateway, consider including -alias. Refer to the manual page for further details.

Make sure that the router program is set to NO with the following line in /etc/rc.conf:

router_enable="NO"

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

It is probably a good idea to ensure 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:

sendmail_flags="-bd"

The downside is that sendmail is forced to re-examine the mail queue whenever the ppp link. To automate this, include !bg in ppp.linkup:

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

An alternative is to set up a “dfilter” to block SMTP traffic. Refer to the sample files for further details.

27.2.4. Using `ppp`

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

# ppp

and then dial provider to start the PPP session, or, to configure ppp to establish sessions automatically when there is outbound traffic and start_if.tun0 does not exist, type:

# ppp -auto provider

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 the 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.

27.2.5. Configuring Dial-in Services

Section 26.4, “Dial-in Service” provides a good description on enabling dial-up services using getty(8).

An alternative to getty is comms/mgetty+sendfax port), 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 the modem will not answer the phone.

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

Refer to http://mgetty.greenie.net/doc/mgetty_toc.html for more information on mgetty.

By default the comms/mgetty+sendfax port comes with the AUTO_PPP option enabled allowing 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 compiled, and installed the comms/mgetty+sendfax port on his system.

Ensure that /usr/local/etc/mgetty+sendfax/login.config has the following:

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

This tells mgetty to run ppp-pap-dialup for detected PPP connections.

Create an executable file called /etc/ppp/ppp-pap-dialup containing the following:

#!/bin/sh
exec /usr/sbin/ppp -direct pap$IDENT

For each dial-up 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.

pap:
  enable pap
  set ifaddr 203.14.100.1 203.14.100.20-203.14.100.40
  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 the /etc/passwd file.

enable passwdauth

To assign some users a static IP number, specify the number as the third argument in /etc/ppp/ppp.secret. See /usr/share/examples/ppp/ppp.secret.sample for examples.