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
as a part of the kernel (e.g., as managed by pppd(8)) and
somewhat hard to debug and/or modify its behaviour. However, in this implementation
PPP is done as a user process with the help of
device driver, tun(4).
The -nat flag does the equivalent of a ``nat enable yes'',
network address translation features. This allows ppp to
act as a NAT or
masquerading engine for all machines on an internal LAN.
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
the mode and interface to standard output.
The -unit flag tells ppp to only attempt to open /dev/tunN.
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
If it fails three times in a row because the device file is
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
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
line (see below) and a set ifaddr must be done in
the system profile
that specifies a peer IP address to use when
interface. Something like ``10.0.0.1/0'' is usually
See the ``pmdemand'' system in
/etc/ppp/ppp.conf.sample for an
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 receiving incoming connections.
ppp ignores the
set device line and uses descriptor 0 as the link.
If callback is configured, ppp will use the set
when dialing back.
This option is designed for machines connected with
wire. ppp will always keep the device open and will
any configured chat scripts.
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
/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
end, check the status of the 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
and switches to packet mode automatically. Once you have
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
this happens, the daemon automatically dials and establishes
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 fulltime
connections where we worry less about line charges and
being connected full time. A third -dedicated mode is also
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
protocol or the Microsoft CallBack Control Protocol
Supports NAT or packet aliasing. Packet aliasing (a.k.a. IP
allows computers on a private, unregistered network to
Internet. The PPP host acts as a masquerading gateway. IP
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
establishes the connection, it will become a daemon.
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)
PAP or CHAP, it is possible to skip the Unix style login(1)
and use the PPP protocol for authentication instead. If the
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.
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
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(8) to
packet flow over the PPP link.
Supports PPP over TCP and PPP over UDP. If a device name is
host:port[/tcp|udp], ppp will open a TCP or UDP connection
data rather than using a conventional serial device.
force ppp into synchronous mode.
Supports PPP over Ethernet (RFC 2516). PPP over Ethernet is
with the external program pppoe(8).
Supports IETF draft Predictor-1 (RFC 1978) and DEFLATE (RFC
compression. ppp supports not only VJ-compression but also
and DEFLATE compression. Normally, a modem has built-in
(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
in talking to the modem and also increases latency. Unlike
Predictor-1 and DEFLATE compression pre-compresses all
traffic flowing through the link, thus reducing overheads to
Supports Microsoft's IPCP extensions (RFC 1877). Name Server Addresses
and NetBIOS Name Server Addresses can be negotiated with
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
CHAP 81 authentication. ppp must be compiled with DES for
MPPE to operate.
Supports IPV6CP (RFC 2023). An IPv6 connection can be made
to or instead of the normal IPv4 connection.
ppp is installed as user ``root'' and group ``network'',
04550. 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
via the shell or !bg commands) are executed as the user ID
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
one by default). If it doesn't, you'll need to rebuild
with the following line in your kernel configuration
Tun interfaces can be created at runtime using the
create command or by opening the character special device /dev/tunN.
See ifconfig(8) and tun(4) for more information.
+o Check your /dev directory for the tunnel device entries
where `N' represents the number of the tun device,
starting at zero.
If they don't exist, you can create them by running "sh
tunN". This will create tun devices 0 through N.
+o Make sure that your system has a group named ``network''
/etc/group file and that the group contains the names of
expected to use ppp. Refer to the group(5) manual page
Each of these users must also be given access using the
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
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
after altering /etc/syslog.conf.
+o Although not strictly relevant to ppp's operation, you
your resolver so that it works correctly. This can
be done by
configuring a local DNS (using named(8)) or by adding
``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
commands below for details.
In the following examples, we assume that your machine name
``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
command. This only ever happens if you connect to a running
ppp and have not authenticated yourself using the correct
You can start by specifying the device name and speed:
ppp ON awfulhak> set device /dev/cua00
ppp ON awfulhak> set speed 38400
Normally, hardware flow control (CTS/RTS) is used. However,
circumstances (as may happen when you are connected directly to certain
PPP-capable terminal servers), this may result in ppp
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,
CTS/RTS off with ``set ctsrts off''. If you need to do
this, check the
``set accmap'' description below too - you'll probably need
Usually, parity is set to ``none'', and this is ppp's default. Parity is
a rather archaic error checking mechanism that is no longer
modern modems do their own error checking, and most linklayer 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
some ISPs (Internet Service Providers) may use specific
at connection time (before PPP is opened). Notably,
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/cua00
Characteristics: 38400bps, cs8, even parity, CTS/RTS
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 &
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
If you never even receive a login prompt, it is quite likely
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
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 &
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 upper case, wait until either everything
is upper case
or lower case. If they revert to lower case, it means that
successfully negotiate with the peer. A good first step for
at this point would be:
ppp ON awfulhak> set log local phase lcp ipcp
...and try again. Refer to the set log command description
further details. If things fail at this point, it is quite
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
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.
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
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
existing route using add!:
PPP ON awfulhak> add! default HISADDR
This command can also be executed before actually making the
If a new IP address is negotiated at connection time, ppp
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
current terminal, you can put ppp into the background using
shell suspend and background commands (usually `^Z' followed
Refer to the PPP COMMAND LIST section for details on all
To use automatic dialing, you must prepare some Dial and Login chat
scripts. See the example definitions in
format of /etc/ppp/ppp.conf is pretty simple). Each line
comment, inclusion, label, or command:
+o A line starting with a `#' character is treated as a
Leading whitespace is ignored when identifying comment
+o An inclusion is a line beginning with the word ``!include''. It must
have one argument - the file to include. You may wish
~/.ppp.conf'' for compatibility with older versions of
+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
The /etc/ppp/ppp.conf file should consist of at least a
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
configuration. You can now specify the destination label
name when you
invoke ppp. Commands associated with the ``default'' label
followed by those associated with the destination label provided. When
ppp is started with no arguments, the ``default'' section is
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,
the result of passing a label on the command line or using
the load command.
Only the commands specified for that label in the
file are executed. However, when invoking ppp with the
-ddial, or -dedicated switches, the link mode tells ppp to
connection. Refer to the set mode command below for further
Once the connection is made, the ``ppp'' portion of the
change to ``PPP'':
# 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
when the PPP connection is established. See the provided
example in /etc/ppp/ppp.conf.sample which runs a script in
after the connection is established (refer to the shell and
below for a description of possible substitution strings).
when a connection is closed, the contents of the
file are executed. Both of these files have the same format
In previous versions of ppp, it was necessary to re-add
routes such as
the default route in the ppp.linkup file. ppp supports
where all routes that contain the HISADDR, MYADDR, HISADDR6,
literals will automatically be updated when the values of
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
-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
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
daemon exits automatically if the connection is dropped by
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 peer's
(refer to /etc/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
all connections are immediately dropped.
In -auto mode, when an outgoing packet is detected, ppp will
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
This behaviour can be changed using the set redial command:
set redial secs[+inc[-max]][.next] [attempts]
secs The number of seconds to wait before attempting to
again. If the argument is the literal string
`random', the delay
period is a random value between 1 and 30 seconds inclusive.
inc 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
for inc is zero.
max The maximum number of times ppp should increment
secs. The default
value for max is 10.
next The number of seconds to wait before attempting to
next number in a list of numbers (see the set
The default is 3 seconds. Again, if the argument
is the literal
string `random', the delay period is a random
1 and 30 seconds.
attempts The maximum number of times to try to connect for
packet that triggers a dial. The previous value
if this parameter is omitted. If a value of zero
for attempts, ppp will keep trying until a connection is made.
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
delay after all numbers have been tried. If multiple phone
specified, the total number of attempts is still 4 (it does
each number 4 times).
set redial 10+10-5.3 20
...tells ppp to attempt to connect 20 times. After the
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
next 14 pauses will also have a duration of one minute. If
disconnects, and fails to connect again, the timeout starts
again at 10
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
have packets queued. At some locations, the serial link may
not be reliable,
and carrier may be lost at inappropriate times. It is
have ppp redial should carrier be unexpectedly lost during a
set reconnect timeout ntries
This command tells ppp to re-establish the connection ntries
loss of carrier with a pause of timeout seconds before each
try. For example,
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
Care should be taken with this option. If the local timeout
longer than the remote timeout, the reconnect feature will
triggered (up to the given number of times) after the remote
out and hangs up. NOTE: In this context, losing too many
a loss of carrier and will trigger a reconnect. If
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
ppp as well.
RECEIVING INCOMING PPP CONNECTIONS (Method 1) [Toc] [Back]
To handle an incoming PPP connection request, follow these
1. Make sure the modem is configured correctly:
+o Use Hardware Handshake (CTS/RTS) for flow control.
+o Modem should be set to NO echo back (ATE0) and NO
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
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
speed as the getty:
ppp ON awfulhak> set device /dev/cua01
ppp ON awfulhak> set speed 38400
ppp ON awfulhak> term
deflink: Entering terminal mode on /dev/cua01
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
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)
5. Support for IPCP Domain Name Server and NetBIOS Name
can be enabled using the accept dns and set nbns
to their descriptions below.
RECEIVING INCOMING PPP CONNECTIONS (Method 2) [Toc] [Back]
This method differs in that we use ppp to authenticate the
rather than login(1):
1. Configure your default section in /etc/gettytab with
recognition by specifying the `pp' capability:
default: :pp=/usr/local/bin/ppplogin: .....
2. Configure your serial device(s), enable a getty(8), and
/usr/local/bin/ppplogin as in the first three steps for
3. Add either ``enable chap'' or ``enable pap'' (or both)
/etc/ppp/ppp.conf under the ``incoming'' label (or
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
AUTHENTICATING INCOMING CONNECTIONS [Toc] [Back]
Normally, the receiver of a connection requires that the
itself. This may be done using login(1), but alternatively, you
can use PAP or CHAP. CHAP is the more secure of the two,
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
one line per possible client, each line containing up to
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
when authenticating. If the client does not offer a suitable response
based on any name/key combination in ppp.secret, authentication fails.
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
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
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
command. The value will be used in ppp's subsequent CBCP
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 device:
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
on the receiving machine (ui-gate). This is done by
/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
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
/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
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
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
If MyAuthPasswd is a `*', the password is looked up in the
The entry in /etc/ppp/ppp.conf on awfulhak (the initiator)
set escape 0xff
set device ui-gate:ppp-in/tcp
set timeout 30
set log Phase Chat Connect hdlc LCP IPCP IPV6CP CCP
set ifaddr 10.0.4.2 10.0.4.1
...with the route set up 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
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
network via the TCP connection, and an additional "route" on
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
packet by packet) inside the TCP stream between the two
The major disadvantage of this mechanism is that there are
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
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
mode. This is another gain as the incoming data does not
have to be rearranged
Care should be taken when adding a default route through a
like this. It is quite common for the default route
/etc/ppp/ppp.linkup) to end up routing the link's TCP connection through
the tunnel, effectively garrotting the connection. To avoid
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 ``uigate'' would
When routing your connection across a public network such as
it is preferable to encrypt the data. This can be done
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
add the following lines to /etc/ppp/ppp.conf on the server:
disable deflate pred1
deny deflate pred1
Ensure that you've put the requisite entry in
(MSCHAPv2 is challenge based, so passwd(5) cannot be used).
MSCHAPv2 and MPPE are accepted by default, so the client end
without any additional changes (although ensure you have
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,
packets are de-NAT'd so that they are routed to the correct
the local area network. NAT allows computers on private,
subnets to have Internet access, although they are invisible
outside world. In general, correct ppp operation should
first be verified
with network address translation disabled. Then, the
should be switched on, and network applications (web browser, telnet(1),
ftp(1), ping(8), traceroute(8), etc.) 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/sysctl.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
filter. Here are the basics:
+o A filter definition has the following syntax:
set filter name rule-no action [!] [[host]
[dst_addr[/width]]] [proto [src cmp port] [dst cmp port]
[syn] [finrst] [timeout secs]]
1. Name should be one of ``in'', ``out'', ``dial'', or
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
taken immediately. Action can also be specified as
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 match.
4. [src_addr[/width]] and [dst_addr[/width]] are the
destination IP number specifications. If [/width]
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
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
change. This is similar to the behaviour of the
add command below.
5. Proto may be any protocol from protocols(5).
6. Cmp is one of `lt', `eq', or `gt', meaning lessthan, equal, and
greater-than, respectively. Port can be specified
as a numeric
port or by a 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_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,
will be discarded (blocked). If there are no rules in a
the packet will be permitted.
+o It's possible to filter based on the payload of UDP
those frames contain a PROTO_IP PPP frame header. See
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
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 following
ppp ON awfulhak> set timeout 0
In -ddial and -dedicated modes, the idle timeout is ignored.
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.
ppp will attempt to use (or be willing to accept) both compression protocols
when the peer agrees (or requests them). The deflate
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
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
A special algorithm called DEFLATE24 is also available, and
and denied by default. This is exactly the same as DEFLATE
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
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
this implementation has the set ifaddr command for defining
the local and
remote IP address:
set ifaddr [src_addr[/nn] [dst_addr[/nn] [netmask
src_addr is the IP address that the local side is willing to
dst_addr is the IP address which the remote side should use,
is the netmask that should be used. src_addr defaults to
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
netmask smaller than the default. The usual value is
most kernels ignore the netmask of a POINTOPOINT interface.
Some incorrect PPP implementations require that the peer negotiates a
specific IP address instead of src_addr. If this is the
trigger_addr may be used to specify this IP number. This
will not affect
the routing table unless the other side agrees with this
set ifaddr 18.104.22.168 22.214.171.124
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
the peer requests another IP address, I will always suggest that it
+o The routing table entry will have a netmask of
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
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 184.108.40.206/24 220.127.116.11/20
A number followed by a slash (`/') represents the number of
in the IP address. The above example means:
+o I'd like to use 18.104.22.168 as my address if it is
I'll also accept any IP address between 22.214.171.124
+o I'd like to make him use 126.96.36.199 as his own address, but I'll
also permit him to use any IP address between
+o As you may have already noticed, 188.8.131.52 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 peer's selection. When
using zero, no routing table entries will be made until
+o 184.108.40.206/0 means that I'll accept/permit any IP address but I'll
suggest that 220.127.116.11 be used first.
When negotiating IPv6 addresses, no control is given to the
negotiation is fully automatic.
CONNECTING WITH YOUR INTERNET SERVICE PROVIDER [Toc] [Back]
The following steps should be taken when connecting to your
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
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
separated by a colon are used sequentially, irrespective of
what happened as a result of using the previous number.
set phone "1234567|2345678:3456789|4567890"
Here, the 1234567 number is attempted. If the dial or
fails, the 2345678 number is used next time, but *only*
if the dial
or login script fails. On the dial after this, the
is used. The 4567890 number is only used if the dial
script using the 3456789 fails. Irrespective of
whether the login
script of the 2345678 number succeeds or fails, the
next number is
still the 3456789 number.
As many pipes and colons can be used as are necessary
given site would usually prefer to use either the pipe
or the colon,
but not both). The next number redial timeout is used
numbers. When the end of the list is reached, the normal redial period
is used before starting at the beginning again.
phone number is substituted for the \T string in the
set dial command
2. Set up your redial requirements using set redial. For
you have a bad telephone line or your provider is usually engaged
(not so common these days), you may want to specify the
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" is 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
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
This script is written in the same style as the dial
care should be taken to avoid having your password