ppp -- Point to Point Protocol (a.k.a. user-ppp)
ppp [-mode] [-nat] [-quiet] [-unitN] [system ...]
This is a user process PPP software package. Normally, PPP is implemented
as a part of the kernel (e.g., as managed by pppd(8)) and it's
thus somewhat hard to debug and/or modify its behaviour. However, in
this implementation PPP is done as a user process with the help of the
tunnel device driver (tun).
The -nat flag does the equivalent of a ``nat enable yes'', enabling ppp's
network address translation features. This allows ppp to act as a NAT or
masquerading engine for all machines on an internal LAN. Refer to
libalias(3) for details on the technical side of the NAT engine. Refer
to the NETWORK ADDRESS TRANSLATION (PACKET ALIASING) section of this manual
page for details on how to configure NAT in ppp.
The -quiet flag tells ppp to be silent at startup rather than displaying
the mode and interface to standard output.
The -unit flag tells ppp to only attempt to open /dev/tunN. Normally,
ppp will start with a value of 0 for N, and keep trying to open a tunnel
device by incrementing the value of N by one each time until it succeeds.
If it fails three times in a row because the device file is missing, it
The following modes are understood by ppp:
ppp opens the tun interface, configures it then goes into the
background. The link isn't brought up until outgoing data is
detected on the tun interface at which point ppp attempts to
bring up the link. Packets received (including the first one)
while ppp is trying to bring the link up will remain queued for a
default of 2 minutes. See the ``set choked'' command below.
In -auto mode, at least one ``system'' must be given on the command
line (see below) and a ``set ifaddr'' must be done in the
system profile that specifies a peer IP address to use when configuring
the interface. Something like ``10.0.0.1/0'' is usually
appropriate. See the ``pmdemand'' system in
/usr/share/examples/ppp/ppp.conf.sample for an example.
Here, ppp attempts to establish a connection with the peer immediately.
If it succeeds, ppp goes into the background and the
parent process returns an exit code of 0. If it fails, ppp exits
with a non-zero result.
In foreground mode, ppp attempts to establish a connection with
the peer immediately, but never becomes a daemon. The link is
created in background mode. This is useful if you wish to control
ppp's invocation from another process.
This is used for communicating over an already established connection,
usually when receiving incoming connections accepted by
getty(8). ppp ignores the ``set device'' line and uses descriptor
0 as the link. ppp will also ignore any configured chat
scripts unless the ``force-scripts'' option has been enabled.
If callback is configured, ppp will use the ``set device'' information
when dialing back.
This option is designed for machines connected with a dedicated
wire. ppp will always keep the device open and will ignore any
configured chat scripts unless the ``force-scripts'' option has
This mode is equivalent to -auto mode except that ppp will bring
the link back up any time it's dropped for any reason.
This is a no-op, and gives the same behaviour as if none of the
above modes have been specified. ppp loads any sections specified
on the command line then provides an interactive prompt.
One or more configuration entries or systems (as specified in
/etc/ppp/ppp.conf) may also be specified on the command line. ppp will
read the ``default'' system from /etc/ppp/ppp.conf at startup, followed
by each of the systems specified on the command line.
Provides an interactive user interface. Using its command mode, the user
can easily enter commands to establish the connection with the remote
end, check the status of connection and close the connection. All functions
can also be optionally password protected for security.
Supports both manual and automatic dialing. Interactive mode has a
``term'' command which enables you to talk to the device directly. When
you are connected to the remote peer and it starts to talk PPP, ppp
detects it and switches to packet mode automatically. Once you have
determined the proper sequence for connecting with the remote host, you
can write a chat script to define the necessary dialing and login procedure
for later convenience.
Supports on-demand dialup capability. By using -auto mode, ppp will act
as a daemon and wait for a packet to be sent over the PPP link. When
this happens, the daemon automatically dials and establishes the connection.
In almost the same manner -ddial mode (direct-dial mode) also
automatically dials and establishes the connection. However, it differs
in that it will dial the remote site any time it detects the link is
down, even if there are no packets to be sent. This mode is useful for
full-time connections where we worry less about line charges and more
about being connected full time. A third -dedicated mode is also available.
This mode is targeted at a dedicated link between two machines.
ppp will never voluntarily quit from dedicated mode - you must send it
the ``quit all'' command via its diagnostic socket. A SIGHUP will force
an LCP renegotiation, and a SIGTERM will force it to exit.
Supports client callback. ppp can use either the standard LCP callback
protocol or the Microsoft CallBack Control Protocol
Supports NAT or packet aliasing. Packet aliasing (a.k.a. IP masquerading)
allows computers on a private, unregistered network to access the
Internet. The PPP host acts as a masquerading gateway. IP addresses as
well as TCP and UDP port numbers are NAT'd for outgoing packets and deNAT'd
for returning packets.
Supports background PPP connections. In background mode, if ppp successfully
establishes the connection, it will become a daemon. Otherwise, it
will exit with an error. This allows the setup of scripts that wish to
execute certain commands only if the connection is successfully established.
Supports server-side PPP connections. In direct mode, ppp acts as server
which accepts incoming PPP connections on stdin/stdout.
Supports PAP and CHAP (rfc 1994, 2433 and 2759) authentication. With PAP
or CHAP, it is possible to skip the Unix style login(1) procedure, and
use the PPP protocol for authentication instead. If the peer requests
Microsoft CHAP authentication and ppp is compiled with DES support, an
appropriate MD4/DES response will be made.
Supports RADIUS (rfc 2138 & 2548) authentication. An extension to PAP
and CHAP, Remote Access Dial In User Service allows authentication information
to be stored in a central or distributed database along with various
per-user framed connection characteristics. If libradius(3) is
available at compile time, ppp will use it to make RADIUS requests when
configured to do so.
Supports Proxy Arp. ppp can be configured to make one or more proxy arp
entries on behalf of the peer. This allows routing from the peer to the
LAN without configuring each machine on that LAN.
Supports packet filtering. User can define four kinds of filters: the in
filter for incoming packets, the out filter for outgoing packets, the
dial filter to define a dialing trigger packet and the alive filter for
keeping a connection alive with the trigger packet.
Tunnel driver supports bpf. The user can use tcpdump(1) to check the
packet flow over the PPP link.
Supports PPP over TCP and PPP over UDP. If a device name is specified as
host:port[/tcp|udp], ppp will open a TCP or UDP connection for transporting
data rather than using a conventional serial device. UDP connections
force ppp into synchronous mode.
Supports PPP over ISDN. If ppp is given a raw B-channel i4b device to
open as a link, it's able to talk to the isdnd(8) daemon to establish an
Supports PPP over Ethernet (rfc 2516). If ppp is given a device specification
of the format PPPoE:iface[:provider] and if netgraph(4) is available,
ppp will attempt talk PPP over Ethernet to provider using the iface
On systems that do not support netgraph(4), an external program such as
pppoe(8) may be used.
Supports IETF draft Predictor-1 (rfc 1978) and DEFLATE (rfc 1979) [Toc] [Back]
compression. ppp supports not only VJ-compression but also Predictor-1
and DEFLATE compression. Normally, a modem has built-in compression
(e.g., v42.bis) and the system may receive higher data rates from it as a
result of such compression. While this is generally a good thing in most
other situations, this higher speed data imposes a penalty on the system
by increasing the number of serial interrupts the system has to process
in talking to the modem and also increases latency. Unlike VJ-compression,
Predictor-1 and DEFLATE compression pre-compresses all network
traffic flowing through the link, thus reducing overheads to a minimum.
Supports Microsoft's IPCP extensions (rfc 1877). Name Server Addresses
and NetBIOS Name Server Addresses can be negotiated with clients using
the Microsoft PPP stack (i.e., Win95, WinNT)
Supports Multi-link PPP (rfc 1990) It is possible to configure ppp to
open more than one physical connection to the peer, combining the bandwidth
of all links for better throughput.
Supports MPPE (draft-ietf-pppext-mppe) MPPE is Microsoft Point to Point
Encryption scheme. It is possible to configure ppp to participate in
Microsoft's Windows VPN. For now, ppp can only get encryption keys from
CHAP 81 authentication. ppp must be compiled with DES for MPPE to operate.
Supports IPV6CP (rfc 2023). An IPv6 connection can be made in addition
to or instead of the normal IPv4 connection.
ppp is installed as user root and group network, with permissions 04554.
By default, ppp will not run if the invoking user id is not zero. This
may be overridden by using the ``allow users'' command in
/etc/ppp/ppp.conf. When running as a normal user, ppp switches to user
id 0 in order to alter the system routing table, set up system lock files
and read the ppp configuration files. All external commands (executed
via the "shell" or "!bg" commands) are executed as the user id that
invoked ppp. Refer to the `ID0' logging facility if you're interested in
what exactly is done as user id zero.
When you first run ppp you may need to deal with some initial configuration
+o Your kernel must include a tunnel device (the GENERIC kernel includes
one by default). If it doesn't, or if you require more than one tun
interface, you'll need to rebuild your kernel with the following line
in your kernel configuration file:
pseudo-device tun N
where N is the maximum number of PPP connections you wish to support.
+o Check your /dev directory for the tunnel device entries /dev/tunN,
where `N' represents the number of the tun device, starting at zero.
If they don't exist, you can create them by running "sh ./MAKEDEV
tunN". This will create tun devices 0 through N.
+o Make sure that your system has a group named ``network'' in the
/etc/group file and that the group contains the names of all users
expected to use ppp. Refer to the group(5) manual page for details.
Each of these users must also be given access using the ``allow
users'' command in /etc/ppp/ppp.conf.
+o Create a log file. ppp uses syslog(3) to log information. A common
log file name is /var/log/ppp.log. To make output go to this file,
put the following lines in the /etc/syslog.conf file:
It is possible to have more than one PPP log file by creating a link
to the ppp executable:
# cd /usr/sbin
# ln ppp ppp0
in /etc/syslog.conf. Don't forget to send a HUP signal to syslogd(8)
after altering /etc/syslog.conf.
+o Although not strictly relevant to ppp's operation, you should configure
your resolver so that it works correctly. This can be done by
configuring a local DNS (using named(8)) or by adding the correct
`nameserver' lines to the file /etc/resolv.conf. Refer to the
resolv.conf(5) manual page for details.
Alternatively, if the peer supports it, ppp can be configured to ask
the peer for the nameserver address(es) and to update
/etc/resolv.conf automatically. Refer to the ``enable dns'' and
``resolv'' commands below for details.
In the following examples, we assume that your machine name is awfulhak.
when you invoke ppp (see PERMISSIONS above) with no arguments, you are
presented with a prompt:
ppp ON awfulhak>
The `ON' part of your prompt should always be in upper case. If it is in
lower case, it means that you must supply a password using the ``passwd''
command. This only ever happens if you connect to a running version of
ppp and have not authenticated yourself using the correct password.
You can start by specifying the device name and speed:
ppp ON awfulhak> set device /dev/cuaa0
ppp ON awfulhak> set speed 38400
Normally, hardware flow control (CTS/RTS) is used. However, under certain
circumstances (as may happen when you are connected directly to certain
PPP-capable terminal servers), this may result in ppp hanging as
soon as it tries to write data to your communications link as it is waiting
for the CTS (clear to send) signal - which will never come. Thus, if
you have a direct line and can't seem to make a connection, try turning
CTS/RTS off with ``set ctsrts off''. If you need to do this, check the
``set accmap'' description below too - you'll probably need to ``set
Usually, parity is set to ``none'', and this is ppp's default. Parity is
a rather archaic error checking mechanism that is no longer used because
modern modems do their own error checking, and most link-layer protocols
(that's what ppp is) use much more reliable checking mechanisms. Parity
has a relatively huge overhead (a 12.5% increase in traffic) and as a
result, it is always disabled (set to ``none'') when PPP is opened. However,
some ISPs (Internet Service Providers) may use specific parity settings
at connection time (before PPP is opened). Notably, Compuserve
insist on even parity when logging in:
ppp ON awfulhak> set parity even
You can now see what your current device settings look like:
ppp ON awfulhak> show physical
Link Type: interactive
Connect Count: 0
Queued Packets: 0
Phone Number: N/A
Device List: /dev/cuaa0
Characteristics: 38400bps, cs8, even parity, CTS/RTS on
Connect time: 0 secs
0 octets in, 0 octets out
Overall 0 bytes/sec
ppp ON awfulhak>
The term command can now be used to talk directly to the device:
ppp ON awfulhak> term
When the peer starts to talk in PPP, ppp detects this automatically and
returns to command mode.
ppp ON awfulhak> # No link has been established
Ppp ON awfulhak> # We've connected & finished LCP
PPp ON awfulhak> # We've authenticated
PPP ON awfulhak> # We've agreed IP numbers
If it does not, it's probable that the peer is waiting for your end to
start negotiating. To force ppp to start sending PPP configuration packets
to the peer, use the ``~p'' command to drop out of terminal mode and
enter packet mode.
If you never even receive a login prompt, it is quite likely that the
peer wants to use PAP or CHAP authentication instead of using Unix-style
login/password authentication. To set things up properly, drop back to
the prompt and set your authentication name and key, then reconnect:
ppp ON awfulhak> set authname myispusername
ppp ON awfulhak> set authkey myisppassword
ppp ON awfulhak> term
You may need to tell ppp to initiate negotiations with the peer here too:
ppp ON awfulhak> # No link has been established
Ppp ON awfulhak> # We've connected & finished LCP
PPp ON awfulhak> # We've authenticated
PPP ON awfulhak> # We've agreed IP numbers
You are now connected! Note that `PPP' in the prompt has changed to capital
letters to indicate that you have a peer connection. If only some
of the three Ps go uppercase, wait until either everything is uppercase
or lowercase. If they revert to lowercase, it means that ppp couldn't
successfully negotiate with the peer. A good first step for troubleshooting
at this point would be to
ppp ON awfulhak> set log local phase lcp ipcp
and try again. Refer to the ``set log'' command description below for
further details. If things fail at this point, it is quite important
that you turn logging on and try again. It is also important that you
note any prompt changes and report them to anyone trying to help you.
When the link is established, the show command can be used to see how
things are going:
PPP ON awfulhak> show physical
* Modem related information is shown here *
PPP ON awfulhak> show ccp
* CCP (compression) related information is shown here *
PPP ON awfulhak> show lcp
* LCP (line control) related information is shown here *
PPP ON awfulhak> show ipcp
* IPCP (IP) related information is shown here *
PPP ON awfulhak> show ipv6cp
* IPV6CP (IPv6) related information is shown here *
PPP ON awfulhak> show link
* Link (high level) related information is shown here *
PPP ON awfulhak> show bundle
* Logical (high level) connection related information is shown here *
At this point, your machine has a host route to the peer. This means
that you can only make a connection with the host on the other side of
the link. If you want to add a default route entry (telling your machine
to send all packets without another routing entry to the other side of
the PPP link), enter the following command:
PPP ON awfulhak> add default HISADDR
The string `HISADDR' represents the IP address of the connected peer. If
the ``add'' command fails due to an existing route, you can overwrite the
existing route using
PPP ON awfulhak> add! default HISADDR
This command can also be executed before actually making the connection.
If a new IP address is negotiated at connection time, ppp will update
your default route accordingly.
You can now use your network applications (ping, telnet, ftp etc.) in
other windows or terminals on your machine. If you wish to reuse the
current terminal, you can put ppp into the background using your standard
shell suspend and background commands (usually ``^Z'' followed by
Refer to the PPP COMMAND LIST section for details on all available commands.
To use automatic dialing, you must prepare some Dial and Login chat
scripts. See the example definitions in
/usr/share/examples/ppp/ppp.conf.sample (the format of /etc/ppp/ppp.conf
is pretty simple). Each line contains one comment, inclusion, label or
+o A line starting with a (``#'') character is treated as a comment
line. Leading whitespace are ignored when identifying comment lines.
+o An inclusion is a line beginning with the word `!include'. It must
have one argument - the file to include. You may wish to ``!include
~/.ppp.conf'' for compatibility with older versions of ppp.
+o A label name starts in the first column and is followed by a colon
+o A command line must contain a space or tab in the first column.
The /etc/ppp/ppp.conf file should consist of at least a ``default'' section.
This section is always executed. It should also contain one or
more sections, named according to their purpose, for example, ``MyISP''
would represent your ISP, and ``ppp-in'' would represent an incoming ppp
configuration. You can now specify the destination label name when you
invoke ppp. Commands associated with the ``default'' label are executed,
followed by those associated with the destination label provided. When
ppp is started with no arguments, the ``default'' section is still executed.
The load command can be used to manually load a section from the
ppp ON awfulhak> load MyISP
Note, no action is taken by ppp after a section is loaded, whether it's
the result of passing a label on the command line or using the ``load''
command. Only the commands specified for that label in the configuration
file are executed. However, when invoking ppp with the -background,
-ddial, or -dedicated switches, the link mode tells ppp to establish a
connection. Refer to the ``set mode'' command below for further details.
Once the connection is made, the `ppp' portion of the prompt will change
# ppp MyISP
ppp ON awfulhak> dial
Ppp ON awfulhak>
PPp ON awfulhak>
PPP ON awfulhak>
The Ppp prompt indicates that ppp has entered the authentication phase.
The PPp prompt indicates that ppp has entered the network phase. The PPP
prompt indicates that ppp has successfully negotiated a network layer
protocol and is in a usable state.
If the /etc/ppp/ppp.linkup file is available, its contents are executed
when the PPP connection is established. See the provided ``pmdemand''
example in /usr/share/examples/ppp/ppp.conf.sample which runs a script in
the background after the connection is established (refer to the
``shell'' and ``bg'' commands below for a description of possible substitution
strings). Similarly, when a connection is closed, the contents of
the /etc/ppp/ppp.linkdown file are executed. Both of these files have
the same format as /etc/ppp/ppp.conf.
In previous versions of ppp, it was necessary to re-add routes such as
the default route in the ppp.linkup file. ppp supports `sticky routes',
where all routes that contain the HISADDR, MYADDR, HISADDR6 or MYADDR6
literals will automatically be updated when the values of these variables
If you want to establish a connection using ppp non-interactively (such
as from a crontab(5) entry or an at(1) job) you should use the
-background option. When -background is specified, ppp attempts to
establish the connection immediately. If multiple phone numbers are
specified, each phone number will be tried once. If the attempt fails,
ppp exits immediately with a non-zero exit code. If it succeeds, then
ppp becomes a daemon, and returns an exit status of zero to its caller.
The daemon exits automatically if the connection is dropped by the remote
system, or it receives a TERM signal.
Demand dialing is enabled with the -auto or -ddial options. You must
also specify the destination label in /etc/ppp/ppp.conf to use. It must
contain the ``set ifaddr'' command to define the remote peers IP address.
(refer to /usr/share/examples/ppp/ppp.conf.sample)
# ppp -auto pmdemand
When -auto or -ddial is specified, ppp runs as a daemon but you can still
configure or examine its configuration by using the ``set server'' command
in /etc/ppp/ppp.conf, (for example, ``set server +3000 mypasswd'')
and connecting to the diagnostic port as follows:
# pppctl 3000 (assuming tun0)
PPP ON awfulhak> show who
tcp (127.0.0.1:1028) *
The ``show who'' command lists users that are currently connected to ppp
itself. If the diagnostic socket is closed or changed to a different
socket, all connections are immediately dropped.
In -auto mode, when an outgoing packet is detected, ppp will perform the
dialing action (chat script) and try to connect with the peer. In -ddial
mode, the dialing action is performed any time the line is found to be
down. If the connect fails, the default behaviour is to wait 30 seconds
and then attempt to connect when another outgoing packet is detected.
This behaviour can be changed using the ``set redial'' command:
set redial secs[+inc[-max]][.next] [attempts]
secs is the number of seconds to wait before attempting to connect
again. If the argument is the literal string `random', the
delay period is a random value between 1 and 30 seconds inclusive.
inc is the number of seconds that secs should be incremented each
time a new dial attempt is made. The timeout reverts to secs
only after a successful connection is established. The default
value for inc is zero.
max is the maximum number of times ppp should increment secs. The
default value for max is 10.
next is the number of seconds to wait before attempting to dial the
next number in a list of numbers (see the ``set phone'' command).
The default is 3 seconds. Again, if the argument is
the literal string `random', the delay period is a random value
between 1 and 30 seconds.
attempts is the maximum number of times to try to connect for each outgoing
packet that triggers a dial. The previous value is
unchanged if this parameter is omitted. If a value of zero is
specified for attempts, ppp will keep trying until a connection
So, for example:
set redial 10.3 4
will attempt to connect 4 times for each outgoing packet that causes a
dial attempt with a 3 second delay between each number and a 10 second
delay after all numbers have been tried. If multiple phone numbers are
specified, the total number of attempts is still 4 (it does not attempt
each number 4 times).
set redial 10+10-5.3 20
tells ppp to attempt to connect 20 times. After the first attempt, ppp
pauses for 10 seconds. After the next attempt it pauses for 20 seconds
and so on until after the sixth attempt it pauses for 1 minute. The next
14 pauses will also have a duration of one minute. If ppp connects, disconnects
and fails to connect again, the timeout starts again at 10 seconds.
Modifying the dial delay is very useful when running ppp in -auto mode on
both ends of the link. If each end has the same timeout, both ends wind
up calling each other at the same time if the link drops and both ends
have packets queued. At some locations, the serial link may not be reliable,
and carrier may be lost at inappropriate times. It is possible to
have ppp redial should carrier be unexpectedly lost during a session.
set reconnect timeout ntries
This command tells ppp to re-establish the connection ntries times on
loss of carrier with a pause of timeout seconds before each try. For
set reconnect 3 5
tells ppp that on an unexpected loss of carrier, it should wait 3 seconds
before attempting to reconnect. This may happen up to 5 times before ppp
gives up. The default value of ntries is zero (no reconnect). Care
should be taken with this option. If the local timeout is slightly
longer than the remote timeout, the reconnect feature will always be
triggered (up to the given number of times) after the remote side times
out and hangs up. NOTE: In this context, losing too many LQRs constitutes
a loss of carrier and will trigger a reconnect. If the -background
flag is specified, all phone numbers are dialed at most once until a connection
is made. The next number redial period specified with the ``set
redial'' command is honoured, as is the reconnect tries value. If your
redial value is less than the number of phone numbers specified, not all
the specified numbers will be tried. To terminate the program, type
PPP ON awfulhak> close
ppp ON awfulhak> quit all
A simple ``quit'' command will terminate the pppctl(8) or telnet(1) connection
but not the ppp program itself. You must use ``quit all'' to
terminate ppp as well.
RECEIVING INCOMING PPP CONNECTIONS (Method 1) [Toc] [Back]
To handle an incoming PPP connection request, follow these steps:
1. Make sure the modem and (optionally) /etc/rc.serial is configured
+o Use Hardware Handshake (CTS/RTS) for flow control.
+o Modem should be set to NO echo back (ATE0) and NO results string
2. Edit /etc/ttys to enable a getty(8) on the port where the modem is
attached. For example:
ttyd1 "/usr/libexec/getty std.38400" dialup on secure
Don't forget to send a HUP signal to the init(8) process to start
# kill -HUP 1
It is usually also necessary to train your modem to the same DTR
speed as the getty:
ppp ON awfulhak> set device /dev/cuaa1
ppp ON awfulhak> set speed 38400
ppp ON awfulhak> term
deflink: Entering terminal mode on /dev/cuaa1
Type `~?' for help
ppp ON awfulhak> quit
3. Create a /usr/local/bin/ppplogin file with the following contents:
exec /usr/sbin/ppp -direct incoming
Direct mode (-direct) lets ppp work with stdin and stdout. You can
also use pppctl(8) to connect to a configured diagnostic port, in
the same manner as with client-side ppp.
Here, the incoming section must be set up in /etc/ppp/ppp.conf.
Make sure that the incoming section contains the ``allow users''
command as appropriate.
4. Prepare an account for the incoming user.
ppp:xxxx:66:66:PPP Login User:/home/ppp:/usr/local/bin/ppplogin
Refer to the manual entries for adduser(8) and vipw(8) for details.
5. Support for IPCP Domain Name Server and NetBIOS Name Server negotiation
can be enabled using the ``accept dns'' and ``set nbns'' commands.
Refer to their descriptions below.
RECEIVING INCOMING PPP CONNECTIONS (Method 2) [Toc] [Back]
This method differs in that we use ppp to authenticate the connection
rather than login(1):
1. Configure your default section in /etc/gettytab with automatic ppp
recognition by specifying the ``pp'' capability:
2. Configure your serial device(s), enable a getty(8) and create
/usr/local/bin/ppplogin as in the first three steps for method 1
3. Add either ``enable chap'' or ``enable pap'' (or both) to
/etc/ppp/ppp.conf under the `incoming' label (or whatever label
4. Create an entry in /etc/ppp/ppp.secret for each incoming user:
Now, as soon as getty(8) detects a ppp connection (by recognising the
HDLC frame headers), it runs ``/usr/local/bin/ppplogin''.
It is VITAL that either PAP or CHAP are enabled as above. If they are
not, you are allowing anybody to establish a ppp session with your
machine without a password, opening yourself up to all sorts of potential
AUTHENTICATING INCOMING CONNECTIONS [Toc] [Back]
Normally, the receiver of a connection requires that the peer authenticates
itself. This may be done using login(1), but alternatively, you
can use PAP or CHAP. CHAP is the more secure of the two, but some
clients may not support it. Once you decide which you wish to use, add
the command `enable chap' or `enable pap' to the relevant section of
You must then configure the /etc/ppp/ppp.secret file. This file contains
one line per possible client, each line containing up to five fields:
name key [hisaddr [label [callback-number]]]
The name and key specify the client username and password. If key is
``*'' and PAP is being used, ppp will look up the password database
(passwd(5)) when authenticating. If the client does not offer a suitable
response based on any name/key combination in ppp.secret, authentication
If authentication is successful, hisaddr (if specified) is used when
negotiating IP numbers. See the ``set ifaddr'' command for details.
If authentication is successful and label is specified, the current system
label is changed to match the given label. This will change the subsequent
parsing of the ppp.linkup and ppp.linkdown files.
If authentication is successful and callback-number is specified and
``set callback'' has been used in ppp.conf, the client will be called
back on the given number. If CBCP is being used, callback-number may
also contain a list of numbers or a ``*'', as if passed to the ``set
cbcp'' command. The value will be used in ppp's subsequent CBCP phase.
PPP OVER TCP and UDP (a.k.a Tunnelling) [Toc] [Back]
Instead of running ppp over a serial link, it is possible to use a TCP
connection instead by specifying the host, port and protocol as the
set device ui-gate:6669/tcp
Instead of opening a serial device, ppp will open a TCP connection to the
given machine on the given socket. It should be noted however that ppp
doesn't use the telnet protocol and will be unable to negotiate with a
telnet server. You should set up a port for receiving this PPP connection
on the receiving machine (ui-gate). This is done by first updating
/etc/services to name the service:
ppp-in 6669/tcp # Incoming PPP connections over TCP
and updating /etc/inetd.conf to tell inetd(8) how to deal with incoming
connections on that port:
ppp-in stream tcp nowait root /usr/sbin/ppp ppp -direct ppp-in
Don't forget to send a HUP signal to inetd(8) after you've updated
/etc/inetd.conf. Here, we use a label named ``ppp-in''. The entry in
/etc/ppp/ppp.conf on ui-gate (the receiver) should contain the following:
set timeout 0
set ifaddr 10.0.4.1 10.0.4.2
and the entry in /etc/ppp/ppp.linkup should contain:
add 10.0.1.0/24 HISADDR
It is necessary to put the ``add'' command in ppp.linkup to ensure that
the route is only added after ppp has negotiated and assigned addresses
to its interface.
You may also want to enable PAP or CHAP for security. To enable PAP, add
the following line:
You'll also need to create the following entry in /etc/ppp/ppp.secret:
If MyAuthPasswd is a ``*'', the password is looked up in the passwd(5)
The entry in /etc/ppp/ppp.conf on awfulhak (the initiator) should contain
set escape 0xff
set device ui-gate:ppp-in/tcp
set timeout 30
set log Phase Chat Connect hdlc LCP IPCP IPV6CP CCP tun
set ifaddr 10.0.4.2 10.0.4.1
with the route setup in /etc/ppp/ppp.linkup:
add 10.0.2.0/24 HISADDR
Again, if you're enabling PAP, you'll also need this in the
set authname MyAuthName
set authkey MyAuthKey
We're assigning the address of 10.0.4.1 to ui-gate, and the address
10.0.4.2 to awfulhak. To open the connection, just type
awfulhak # ppp -background ui-gate
The result will be an additional "route" on awfulhak to the 10.0.2.0/24
network via the TCP connection, and an additional "route" on ui-gate to
the 10.0.1.0/24 network. The networks are effectively bridged - the
underlying TCP connection may be across a public network (such as the
Internet), and the PPP traffic is conceptually encapsulated (although not
packet by packet) inside the TCP stream between the two gateways.
The major disadvantage of this mechanism is that there are two "guaranteed
delivery" mechanisms in place - the underlying TCP stream and whatever
protocol is used over the PPP link - probably TCP again. If packets
are lost, both levels will get in each others way trying to negotiate
sending of the missing packet.
To avoid this overhead, it is also possible to do all this using UDP
instead of TCP as the transport by simply changing the protocol from
"tcp" to "udp". When using UDP as a transport, ppp will operate in synchronous
mode. This is another gain as the incoming data does not have
to be rearranged into packets.
Care should be taken when adding a default route through a tunneled setup
like this. It is quite common for the default route (added in
/etc/ppp/ppp.linkup) to end up routing the link's TCP connection through
the tunnel, effectively garrotting the connection. To avoid this, make
sure you add a static route for the benefit of the link:
set escape 0xff
set device ui-gate:ppp-in/tcp
add ui-gate x.x.x.x
where ``x.x.x.x'' is the IP number that your route to ``ui-gate'' would
When routing your connection accross a public network such as the Internet,
it is preferable to encrypt the data. This can be done with the
help of the MPPE protocol, although currently this means that you will
not be able to also compress the traffic as MPPE is implemented as a compression
layer (thank Microsoft for this). To enable MPPE encryption,
add the following lines to /etc/ppp/ppp.conf on the server:
disable deflate pred1
deny deflate pred1
ensuring that you've put the requisite entry in /etc/ppp/ppp.secret
(MSCHAPv2 is challenge based, so passwd(5) cannot be used)
MSCHAPv2 and MPPE are accepted by default, so the client end should work
without any additional changes (although ensure you have ``set authname''
and ``set authkey'' in your profile).
NETWORK ADDRESS TRANSLATION (PACKET ALIASING) [Toc] [Back]
The -nat command line option enables network address translation (a.k.a.
packet aliasing). This allows the ppp host to act as a masquerading
gateway for other computers over a local area network. Outgoing IP packets
are NAT'd so that they appear to come from the ppp host, and incoming
packets are de-NAT'd so that they are routed to the correct machine on
the local area network. NAT allows computers on private, unregistered
subnets to have Internet access, although they are invisible from the
outside world. In general, correct ppp operation should first be verified
with network address translation disabled. Then, the -nat option
should be switched on, and network applications (web browser, telnet(1),
ftp(1), ping(8), traceroute(8)) should be checked on the ppp host.
Finally, the same or similar applications should be checked on other computers
in the LAN. If network applications work correctly on the ppp
host, but not on other machines in the LAN, then the masquerading software
is working properly, but the host is either not forwarding or possibly
receiving IP packets. Check that IP forwarding is enabled in
/etc/rc.conf and that other machines have designated the ppp host as the
gateway for the LAN.
This implementation supports packet filtering. There are four kinds of
filters: the in filter, the out filter, the dial filter and the alive
filter. Here are the basics:
+o A filter definition has the following syntax:
set filter name rule-no action [!] [[host] src_addr[/width]
[dst_addr[/width]]] [proto [src cmp port] [dst cmp port] [estab]
[syn] [finrst] [timeout secs]]
1. Name should be one of `in', `out', `dial' or `alive'.
2. Rule-no is a numeric value between `0' and `39' specifying the
rule number. Rules are specified in numeric order according to
rule-no, but only if rule `0' is defined.
3. Action may be specified as `permit' or `deny', in which case, if
a given packet matches the rule, the associated action is taken
immediately. Action can also be specified as `clear' to clear
the action associated with that particular rule, or as a new
rule number greater than the current rule. In this case, if a
given packet matches the current rule, the packet will next be
matched against the new rule number (rather than the next rule
The action may optionally be followed with an exclamation mark
(``!''), telling ppp to reverse the sense of the following
4. [src_addr[/width]] and [dst_addr[/width]] are the source and
destination IP number specifications. If [/width] is specified,
it gives the number of relevant netmask bits, allowing the specification
of an address range.
Either src_addr or dst_addr may be given the values MYADDR,
HISADDR, MYADDR6 or HISADDR6 (refer to the description of the
``bg'' command for a description of these values). When these
values are used, the filters will be updated any time the values
change. This is similar to the behaviour of the ``add'' command
5. Proto may be any protocol from protocols(5).
6. Cmp is one of `lt', `eq' or `gt', meaning less-than, equal and
greater-than respectively. Port can be specified as a numeric
port or by service name from /etc/services.
7. The `estab', `syn', and `finrst' flags are only allowed when
proto is set to `tcp', and represent the TH_ACK, TH_SYN and
TH_FIN or TH_RST TCP flags respectively.
8. The timeout value adjusts the current idle timeout to at least
secs seconds. If a timeout is given in the alive filter as well
as in the in/out filter, the in/out value is used. If no timeout
is given, the default timeout (set using set timeout and
defaulting to 180 seconds) is used.
+o Each filter can hold up to 40 rules, starting from rule 0. The
entire rule set is not effective until rule 0 is defined, i.e., the
default is to allow everything through.
+o If no rule in a defined set of rules matches a packet, that packet
will be discarded (blocked). If there are no rules in a given filter,
the packet will be permitted.
+o It's possible to filter based on the payload of UDP frames where
those frames contain a PROTO_IP PPP frame header. See the
filter-decapsulation option below for further details.
+o Use ``set filter name -1'' to flush all rules.
SETTING THE IDLE TIMER [Toc] [Back]
To check/set the idle timer, use the ``show bundle'' and ``set timeout''
ppp ON awfulhak> set timeout 600
The timeout period is measured in seconds, the default value for which is
180 seconds (or 3 min). To disable the idle timer function, use the command
ppp ON awfulhak> set timeout 0
In -ddial and -dedicated modes, the idle timeout is ignored. In -auto
mode, when the idle timeout causes the PPP session to be closed, the ppp
program itself remains running. Another trigger packet will cause it to
attempt to re-establish the link.
PREDICTOR-1 and DEFLATE COMPRESSION [Toc] [Back]
ppp supports both Predictor type 1 and deflate compression. By default,
ppp will attempt to use (or be willing to accept) both compression protocols
when the peer agrees (or requests them). The deflate protocol is
preferred by ppp. Refer to the ``disable'' and ``deny'' commands if you
wish to disable this functionality.
It is possible to use a different compression algorithm in each direction
by using only one of ``disable deflate'' and ``deny deflate'' (assuming
that the peer supports both algorithms).
By default, when negotiating DEFLATE, ppp will use a window size of 15.
Refer to the ``set deflate'' command if you wish to change this behaviour.
A special algorithm called DEFLATE24 is also available, and is disabled
and denied by default. This is exactly the same as DEFLATE except that
it uses CCP ID 24 to negotiate. This allows ppp to successfully negotiate
DEFLATE with pppd version 2.3.*.
CONTROLLING IP ADDRESS [Toc] [Back]
For IPv4, ppp uses IPCP to negotiate IP addresses. Each side of the connection
specifies the IP address that it's willing to use, and if the
requested IP address is acceptable then ppp returns an ACK to the
requester. Otherwise, ppp returns NAK to suggest that the peer use a
different IP address. When both sides of the connection agree to accept
the received request (and send an ACK), IPCP is set to the open state and
a network level connection is established. To control this IPCP behaviour,
this implementation has the ``set ifaddr'' command for defining the
local and remote IP address:
set ifaddr [src_addr[/nn] [dst_addr[/nn] [netmask [trigger_addr]]]]
where, `src_addr' is the IP address that the local side is willing to
use, `dst_addr' is the IP address which the remote side should use and
`netmask' is the netmask that should be used. `Src_addr' defaults to the
current hostname(1), `dst_addr' defaults to 0.0.0.0, and `netmask'
defaults to whatever mask is appropriate for `src_addr'. It is only possible
to make `netmask' smaller than the default. The usual value is
255.255.255.255, as most kernels ignore the netmask of a POINTOPOINT
Some incorrect PPP implementations require that the peer negotiates a
specific IP address instead of `src_addr'. If this is the case,
`trigger_addr' may be used to specify this IP number. This will not
affect the routing table unless the other side agrees with this proposed
set ifaddr 18.104.22.168 22.214.171.124 255.255.255.255 0.0.0.0
The above specification means:
+o I will first suggest that my IP address should be 0.0.0.0, but I will
only accept an address of 126.96.36.199.
+o I strongly insist that the peer uses 188.8.131.52 as his own address
and won't permit the use of any IP address but 184.108.40.206. When
the peer requests another IP address, I will always suggest that it
+o The routing table entry will have a netmask of 0xffffffff.
This is all fine when each side has a pre-determined IP address, however
it is often the case that one side is acting as a server which controls
all IP addresses and the other side should go along with it. In order to
allow more flexible behaviour, the ``set ifaddr'' command allows the user
to specify IP addresses more loosely:
set ifaddr 220.127.116.11/24 18.104.22.168/20
A number followed by a slash (``/'') represents the number of bits significant
in the IP address. The above example means:
+o I'd like to use 22.214.171.124 as my address if it is possible, but
I'll also accept any IP address between 126.96.36.199 and
+o I'd like to make him use 188.8.131.52 as his own address, but I'll
also permit him to use any IP address between 184.108.40.206 and
+o As you may have already noticed, 220.127.116.11 is equivalent to saying
+o As an exception, 0 is equivalent to 0.0.0.0/0, meaning that I have no
preferred IP address and will obey the remote peers selection. When
using zero, no routing table entries will be made until a connection
+o 18.104.22.168/0 means that I'll accept/permit any IP address but I'll
suggest that 22.214.171.124 be used first.
When negotiating IPv6 addresses, no control is given to the user. IPV6CP
negotiation is fully automatic.
CONNECTING WITH YOUR INTERNET SERVICE PROVIDER [Toc] [Back]
The following steps should be taken when connecting to your ISP:
1. Describe your providers phone number(s) in the dial script using the
``set phone'' command. This command allows you to set multiple
phone numbers for dialing and redialing separated by either a pipe
(``|'') or a colon (``:''):
set phone telno[|backupnumber]...[:nextnumber]...
Numbers after the first in a pipe-separated list are only used if
the previous number was used in a failed dial or login script. Numbers
separated by a colon are used sequentially, irrespective of
what happened as a result of using the previous number. For example:
set phone "1234567|2345678:3456789|4567890"
Here, the 1234567 number is attempted. If the dial or login script
fails, the 2345678 number is used next time, but *only* if the dial
or login script fails. On the dial after this, the 3456789 number
is used. The 4567890 number is only used if the dial or login
script using the 3456789 fails. If the login script of the 2345678
number fails, the next number is still the 3456789 number. As many
pipes and colons can be used as are necessary (although a given site
would usually prefer to use either the pipe or the colon, but not
both). The next number redial timeout is used between all numbers.
When the end of the list is reached, the normal redial period is
used before starting at the beginning again. The selected phone
number is substituted for the \\T string in the ``set dial'' command
2. Set up your redial requirements using ``set redial''. For example,
if you have a bad telephone line or your provider is usually engaged
(not so common these days), you may want to specify the following:
set redial 10 4
This says that up to 4 phone calls should be attempted with a pause
of 10 seconds before dialing the first number again.
3. Describe your login procedure using the ``set dial'' and ``set
login'' commands. The ``set dial'' command is used to talk to your
modem and establish a link with your ISP, for example:
set dial "ABORT BUSY ABORT NO\\sCARRIER TIMEOUT 4 \"\" \
ATZ OK-ATZ-OK ATDT\\T TIMEOUT 60 CONNECT"
This modem "chat" string means:
+o Abort if the string "BUSY" or "NO CARRIER" are received.
+o Set the timeout to 4 seconds.
+o Expect nothing.
+o Send ATZ.
+o Expect OK. If that's not received within the 4 second timeout,
send ATZ and expect OK.
+o Send ATDTxxxxxxx where xxxxxxx is the next number in the phone
list from above.
+o Set the timeout to 60.
+o Wait for the CONNECT string.
Once the connection is established, the login script is executed.
This script is written in