pppd - Point to Point Protocol daemon
pppd [ tty_name ] [ speed ] [ options ]
The Point-to-Point Protocol (PPP) provides a method for transmitting datagrams over serial
point-to-point links. PPP is composed of three parts: a method for encapsulating data-
grams over serial links, an extensible Link Control Protocol (LCP), and a family of Net-
work Control Protocols (NCP) for establishing and configuring different network-layer pro-
The encapsulation scheme is provided by driver code in the kernel. Pppd provides the
basic LCP, authentication support, and an NCP for establishing and configuring the Inter-
net Protocol (IP) (called the IP Control Protocol, IPCP).
FREQUENTLY USED OPTIONS
Communicate over the named device. The string "/dev/" is prepended if necessary.
If no device name is given, or if the name of the terminal connected to the stan-
dard input is given, pppd will use that terminal, and will not fork to put itself
in the background. A value for this option from a privileged source cannot be
overridden by a non-privileged user.
Set the baud rate to <speed> (a decimal number). On systems such as 4.4BSD and
NetBSD, any speed can be specified. Other systems (e.g. SunOS) allow only a lim-
ited set of speeds.
Set the async character map to <map>. This map describes which control characters
cannot be successfully received over the serial line. Pppd will ask the peer to
send these characters as a 2-byte escape sequence. The argument is a 32 bit hex
number with each bit representing a character to escape. Bit 0 (00000001) repre-
sents the character 0x00; bit 31 (80000000) represents the character 0x1f or ^_.
If multiple asyncmap options are given, the values are ORed together. If no
asyncmap option is given, no async character map will be negotiated for the receive
direction; the peer should then escape all control characters. To escape transmit-
ted characters, use the escape option.
auth Require the peer to authenticate itself before allowing network packets to be sent
or received. This option is the default if the system has a default route. If
neither this option nor the noauth option is specified, pppd will only allow the
peer to use IP addresses to which the system does not already have a route.
Read options from the file /etc/ppp/peers/name. This file may contain privileged
options, such as noauth, even if pppd is not being run by root. The name string
may not begin with / or include .. as a pathname component. The format of the
options file is described below.
Use the executable or shell command specified by script to set up the serial line.
This script would typically use the chat(8) program to dial the modem and start the
remote ppp session. A value for this option from a privileged source cannot be
overridden by a non-privileged user.
Use hardware flow control (i.e. RTS/CTS) to control the flow of data on the serial
port. If neither the crtscts, the nocrtscts, the cdtrcts nor the nocdtrcts option
is given, the hardware flow control setting for the serial port is left unchanged.
Some serial ports (such as Macintosh serial ports) lack a true RTS output. Such
serial ports use this mode to implement unidirectional flow control. The serial
port will suspend transmission when requested by the modem (via CTS) but will be
unable to request the modem stop sending to the computer. This mode retains the
ability to use DTR as a modem control line.
Add a default route to the system routing tables, using the peer as the gateway,
when IPCP negotiation is successfully completed. This entry is removed when the
PPP connection is broken. This option is privileged if the nodefaultroute option
has been specified.
Run the executable or shell command specified by script after pppd has terminated
the link. This script could, for example, issue commands to the modem to cause it
to hang up if hardware modem control signals were not available. The disconnect
script is not run if the modem has already hung up. A value for this option from a
privileged source cannot be overridden by a non-privileged user.
Specifies that certain characters should be escaped on transmission (regardless of
whether the peer requests them to be escaped with its async control character map).
The characters to be escaped are specified as a list of hex numbers separated by
commas. Note that almost any character can be specified for the escape option,
unlike the asyncmap option which only allows control characters to be specified.
The characters which may not be escaped are those with hex values 0x20 - 0x3f or
Read options from file name (the format is described below). The file must be
readable by the user who has invoked pppd.
Run the executable or shell command specified by script to initialize the serial
line. This script would typically use the chat(8) program to configure the modem
to enable auto answer. A value for this option from a privileged source cannot be
overridden by a non-privileged user.
lock Specifies that pppd should create a UUCP-style lock file for the serial device to
ensure exclusive access to the device.
mru n Set the MRU [Maximum Receive Unit] value to n. Pppd will ask the peer to send pack-
ets of no more than n bytes. The minimum MRU value is 128. The default MRU value
is 1500. A value of 296 is recommended for slow links (40 bytes for TCP/IP header
+ 256 bytes of data). (Note that for IPv6 MRU must be at least 1280)
mtu n Set the MTU [Maximum Transmit Unit] value to n. Unless the peer requests a smaller
value via MRU negotiation, pppd will request that the kernel networking code send
data packets of no more than n bytes through the PPP network interface. (Note that
for IPv6 MTU must be at least 1280)
Enables the "passive" option in the LCP. With this option, pppd will attempt to
initiate a connection; if no reply is received from the peer, pppd will then just
wait passively for a valid LCP packet from the peer, instead of exiting, as it
would without this option.
Set the local and/or remote interface IP addresses. Either one may be omitted.
The IP addresses can be specified with a host name or in decimal dot notation (e.g.
188.8.131.52). The default local address is the (first) IP address of the system
(unless the noipdefault option is given). The remote address will be obtained from
the peer if not specified in any option. Thus, in simple cases, this option is not
required. If a local and/or remote IP address is specified with this option, pppd
will not accept a different value from the peer in the IPCP negotiation, unless the
ipcp-accept-local and/or ipcp-accept-remote options are given, respectively.
Set the local and/or remote 64-bit interface identifier. Either one may be omitted.
The identifier must be specified in standard ascii notation of IPv6 addresses (e.g.
::dead:beef). If the ipv6cp-use-ipaddr option is given, the local identifier is the
local IPv4 address (see above). On systems which supports a unique persistent id,
such as EUI-48 derived from the Ethernet MAC address, ipv6cp-use-persistent option
can be used to replace the ipv6 <local>,<remote> option. Otherwise the identifier
Specifies a packet filter to be applied to data packets to determine which packets
are to be regarded as link activity, and therefore reset the idle timer, or cause
the link to be brought up in demand-dialling mode. This option is useful in con-
junction with the idle option if there are packets being sent or received regularly
over the link (for example, routing information packets) which would otherwise pre-
vent the link from ever appearing to be idle. The filter-expression syntax is as
described for tcpdump(1), except that qualifiers which are inappropriate for a PPP
link, such as ether and arp, are not permitted. Generally the filter expression
should be enclosed in single-quotes to prevent whitespace in the expression from
being interpreted by the shell. This option is currently only available under Net-
BSD, and then only if both the kernel and pppd were compiled with PPP_FILTER
Allow peers to use the given IP address or subnet without authenticating them-
selves. The parameter is parsed as for each element of the list of allowed IP
addresses in the secrets files (see the AUTHENTICATION section below).
Request that the peer compress packets that it sends, using the BSD-Compress
scheme, with a maximum code size of nr bits, and agree to compress packets sent to
the peer with a maximum code size of nt bits. If nt is not specified, it defaults
to the value given for nr. Values in the range 9 to 15 may be used for nr and nt;
larger values give better compression but consume more kernel memory for compres-
sion dictionaries. Alternatively, a value of 0 for nr or nt disables compression
in the corresponding direction. Use nobsdcomp or bsdcomp 0 to disable BSD-Compress
Use a non-standard hardware flow control (i.e. DTR/CTS) to control the flow of data
on the serial port. If neither the crtscts, the nocrtscts, the cdtrcts nor the
nocdtrcts option is given, the hardware flow control setting for the serial port is
left unchanged. Some serial ports (such as Macintosh serial ports) lack a true RTS
output. Such serial ports use this mode to implement true bi-directional flow con-
trol. The sacrifice is that this flow control mode does not permit using DTR as a
modem control line.
If this option is given, pppd will rechallenge the peer every n seconds.
Set the maximum number of CHAP challenge transmissions to n (default 10).
Set the CHAP restart interval (retransmission timeout for challenges) to n seconds
Wait for up n milliseconds after the connect script finishes for a valid PPP packet
from the peer. At the end of this time, or when a valid PPP packet is received
from the peer, pppd will commence negotiation by sending its first LCP packet. The
default value is 1000 (1 second). This wait period only applies if the connect or
pty option is used.
debug Enables connection debugging facilities. If this option is given, pppd will log
the contents of all control packets sent or received in a readable form. The pack-
ets are logged through syslog with facility daemon and level debug. This informa-
tion can be directed to a file by setting up /etc/syslog.conf appropriately (see
Disable asyncmap negotiation, forcing all control characters to be escaped for both
the transmit and the receive direction.
Disable MRU [Maximum Receive Unit] negotiation. With this option, pppd will use
the default MRU value of 1500 bytes for both the transmit and receive direction.
Request that the peer compress packets that it sends, using the Deflate scheme,
with a maximum window size of 2**nr bytes, and agree to compress packets sent to
the peer with a maximum window size of 2**nt bytes. If nt is not specified, it
defaults to the value given for nr. Values in the range 9 to 15 may be used for nr
and nt; larger values give better compression but consume more kernel memory for
compression dictionaries. Alternatively, a value of 0 for nr or nt disables com-
pression in the corresponding direction. Use nodeflate or deflate 0 to disable
Deflate compression entirely. (Note: pppd requests Deflate compression in prefer-
ence to BSD-Compress if the peer can do either.)
demand Initiate the link only on demand, i.e. when data traffic is present. With this
option, the remote IP address must be specified by the user on the command line or
in an options file. Pppd will initially configure the interface and enable it for
IP traffic without connecting to the peer. When traffic is available, pppd will
connect to the peer and perform negotiation, authentication, etc. When this is
completed, pppd will commence passing data packets (i.e., IP packets) across the
The demand option implies the persist option. If this behaviour is not desired,
use the nopersist option after the demand option. The idle and holdoff options are
also useful in conjuction with the demand option.
Append the domain name d to the local host name for authentication purposes. For
example, if gethostname() returns the name porsche, but the fully qualified domain
name is porsche.Quotron.COM, you could specify domain Quotron.COM. Pppd would then
use the name porsche.Quotron.COM for looking up secrets in the secrets file, and as
the default name to send to the peer when authenticating itself to the peer. This
option is privileged.
dryrun With the dryrun option, pppd will print out all the option values which have been
set and then exit, after parsing the command line and options files and checking
the option values, but before initiating the link. The option values are logged at
level info, and also printed to standard output unless the device on standard out-
put is the device that pppd would be using to communicate with the peer.
dump With the dump option, pppd will print out all the option values which have been
set. This option is like the dryrun option except that pppd proceeds as normal
rather than exiting.
Sets the endpoint discriminator sent by the local machine to the peer during multi-
link negotiation to <epdisc>. The default is to use the MAC address of the first
ethernet interface on the system, if any, otherwise the IPv4 address corresponding
to the hostname, if any, provided it is not in the multicast or locally-assigned IP
address ranges, or the localhost address. The endpoint discriminator can be the
string null or of the form type:value, where type is a decimal number or one of the
strings local, IP, MAC, magic, or phone. The value is an IP address in dotted-dec-
imal notation for the IP type, or a string of bytes in hexadecimal, separated by
periods or colons for the other types. For the MAC type, the value may also be the
name of an ethernet or similar network interface. This option is currently only
available under Linux.
When logging the contents of PAP packets, this option causes pppd to exclude the
password string from the log. This is the default.
Specifies how many seconds to wait before re-initiating the link after it termi-
nates. This option only has any effect if the persist or demand option is used.
The holdoff period is not applied if the link was terminated because it was idle.
idle n Specifies that pppd should disconnect if the link is idle for n seconds. The link
is idle when no data packets (i.e. IP packets) are being sent or received. Note:
it is not advisable to use this option with the persist option without the demand
option. If the active-filter option is given, data packets which are rejected by
the specified activity filter also count as the link being idle.
With this option, pppd will accept the peer's idea of our local IP address, even if
the local IP address was specified in an option.
With this option, pppd will accept the peer's idea of its (remote) IP address, even
if the remote IP address was specified in an option.
Set the maximum number of IPCP configure-request transmissions to n (default 10).
Set the maximum number of IPCP configure-NAKs returned before starting to send con-
figure-Rejects instead to n (default 10).
Set the maximum number of IPCP terminate-request transmissions to n (default 3).
Set the IPCP restart interval (retransmission timeout) to n seconds (default 3).
Provides an extra parameter to the ip-up and ip-down scripts. If this option is
given, the string supplied is given as the 6th parameter to those scripts.
Set the maximum number of IPv6CP configure-request transmissions to n (default 10).
Set the maximum number of IPv6CP configure-NAKs returned before starting to send
configure-Rejects instead to n (default 10).
Set the maximum number of IPv6CP terminate-request transmissions to n (default 3).
Set the IPv6CP restart interval (retransmission timeout) to n seconds (default 3).
ipx Enable the IPXCP and IPX protocols. This option is presently only supported under
Linux, and only if your kernel has been configured to include IPX support.
Set the IPX network number in the IPXCP configure request frame to n, a hexadecimal
number (without a leading 0x). There is no valid default. If this option is not
specified, the network number is obtained from the peer. If the peer does not have
the network number, the IPX protocol will not be started.
Set the IPX node numbers. The two node numbers are separated from each other with a
colon character. The first number n is the local node number. The second number m
is the peer's node number. Each node number is a hexadecimal number, at most 10
digits long. The node numbers on the ipx-network must be unique. There is no valid
default. If this option is not specified then the node numbers are obtained from
Set the name of the router. This is a string and is sent to the peer as information
Set the routing protocol to be received by this option. More than one instance of
ipx-routing may be specified. The 'none' option (0) may be specified as the only
instance of ipx-routing. The values may be 0 for NONE, 2 for RIP/SAP, and 4 for
Accept the peer's NAK for the node number specified in the ipx-node option. If a
node number was specified, and non-zero, the default is to insist that the value be
used. If you include this option then you will permit the peer to override the
entry of the node number.
Accept the peer's NAK for the network number specified in the ipx-network option.
If a network number was specified, and non-zero, the default is to insist that the
value be used. If you include this option then you will permit the peer to override
the entry of the node number.
Use the peer's network number specified in the configure request frame. If a node
number was specified for the peer and this option was not specified, the peer will
be forced to use the value which you have specified.
Set the maximum number of IPXCP configure request frames which the system will send
to n. The default is 10.
Set the maximum number of IPXCP NAK frames which the local system will send before
it rejects the options. The default value is 3.
Set the maximum nuber of IPXCP terminate request frames before the local system
considers that the peer is not listening to them. The default value is 3.
Enable debugging code in the kernel-level PPP driver. The argument values depend
on the specific kernel driver, but in general a value of 1 will enable general ker-
nel debug messages. (Note that these messages are usually only useful for debug-
ging the kernel driver itself.) For the Linux 2.2.x kernel driver, the value is a
sum of bits: 1 to enable general debug messages, 2 to request that the contents of
received packets be printed, and 4 to request that the contents of transmitted
packets be printed. On most systems, messages printed by the kernel are logged by
syslog(1) to a file as directed in the /etc/syslog.conf configuration file.
ktune Enables pppd to alter kernel settings as appropriate. Under Linux, pppd will
enable IP forwarding (i.e. set /proc/sys/net/ipv4/ip_forward to 1) if the proxyarp
option is used, and will enable the dynamic IP address option (i.e. set
/proc/sys/net/ipv4/ip_dynaddr to 1) in demand mode if the local address changes.
If this option is given, pppd will presume the peer to be dead if n LCP echo-
requests are sent without receiving a valid LCP echo-reply. If this happens, pppd
will terminate the connection. Use of this option requires a non-zero value for
the lcp-echo-interval parameter. This option can be used to enable pppd to termi-
nate after the physical connection has been broken (e.g., the modem has hung up) in
situations where no hardware modem control lines are available.
If this option is given, pppd will send an LCP echo-request frame to the peer every
n seconds. Normally the peer should respond to the echo-request by sending an
echo-reply. This option can be used with the lcp-echo-failure option to detect
that the peer is no longer connected.
Set the maximum number of LCP configure-request transmissions to n (default 10).
Set the maximum number of LCP configure-NAKs returned before starting to send con-
figure-Rejects instead to n (default 10).
Set the maximum number of LCP terminate-request transmissions to n (default 3).
Set the LCP restart interval (retransmission timeout) to n seconds (default 3).
Sets the logical name of the link to name. Pppd will create a file named ppp-
name.pid in /var/run (or /etc/ppp on some systems) containing its process ID. This
can be useful in determining which instance of pppd is responsible for the link to
a given peer system. This is a privileged option.
local Don't use the modem control lines. With this option, pppd will ignore the state of
the CD (Carrier Detect) signal from the modem and will not change the state of the
DTR (Data Terminal Ready) signal.
Send log messages to file descriptor n. Pppd will send log messages to at most one
file or file descriptor (as well as sending the log messages to syslog), so this
option and the logfile option are mutually exclusive. The default is for pppd to
send log messages to stdout (file descriptor 1), unless the serial port is already
open on stdout.
Append log messages to the file filename (as well as sending the log messages to
syslog). The file is opened with the privileges of the user who invoked pppd, in
login Use the system password database for authenticating the peer using PAP, and record
the user in the system wtmp file. Note that the peer must have an entry in the
/etc/ppp/pap-secrets file as well as the system password database to be allowed
Terminate the connection when it has been available for network traffic for n sec-
onds (i.e. n seconds after the first network control protocol comes up).
Terminate after n consecutive failed connection attempts. A value of 0 means no
limit. The default value is 10.
modem Use the modem control lines. This option is the default. With this option, pppd
will wait for the CD (Carrier Detect) signal from the modem to be asserted when
opening the serial device (unless a connect script is specified), and it will drop
the DTR (Data Terminal Ready) signal briefly when the connection is terminated and
before executing the connect script. On Ultrix, this option implies hardware flow
control, as for the crtscts option.
mp Enables the use of PPP multilink; this is an alias for the `multilink' option.
This option is currently only available under Linux.
Enables the use of short (12-bit) sequence numbers in multilink headers, as opposed
to 24-bit sequence numbers. This option is only available under Linux, and only
has any effect if multilink is enabled (see the multilink option).
mrru n Sets the Maximum Reconstructed Receive Unit to n. The MRRU is the maximum size for
a received packet on a multilink bundle, and is analogous to the MRU for the indi-
vidual links. This option is currently only available under Linux, and only has
any effect if multilink is enabled (see the multilink option).
If pppd is acting as a server for Microsoft Windows clients, this option allows
pppd to supply one or two DNS (Domain Name Server) addresses to the clients. The
first instance of this option specifies the primary DNS address; the second
instance (if given) specifies the secondary DNS address. (This option was present
in some older versions of pppd under the name dns-addr.)
If pppd is acting as a server for Microsoft Windows or "Samba" clients, this option
allows pppd to supply one or two WINS (Windows Internet Name Services) server
addresses to the clients. The first instance of this option specifies the primary
WINS address; the second instance (if given) specifies the secondary WINS address.
Enables the use of the PPP multilink protocol. If the peer also supports multi-
link, then this link can become part of a bundle between the local system and the
peer. If there is an existing bundle to the peer, pppd will join this link to that
bundle, otherwise pppd will create a new bundle. See the MULTILINK section below.
This option is currently only available under Linux.
Set the name of the local system for authentication purposes to name. This is a
privileged option. With this option, pppd will use lines in the secrets files
which have name as the second field when looking for a secret to use in authenti-
cating the peer. In addition, unless overridden with the user option, name will be
used as the name to send to the peer when authenticating the local system to the
peer. (Note that pppd does not append the domain name to name.)
Set the interface netmask to n, a 32 bit netmask in "decimal dot" notation (e.g.
255.255.255.0). If this option is given, the value specified is ORed with the
default netmask. The default netmask is chosen based on the negotiated remote IP
address; it is the appropriate network mask for the class of the remote IP address,
ORed with the netmasks for any non point-to-point network interfaces in the system
which are on the same network. (Note: on some platforms, pppd will always use
255.255.255.255 for the netmask, if that is the only appropriate value for a point-
Disable Address/Control compression in both directions (send and receive).
noauth Do not require the peer to authenticate itself. This option is privileged.
Disables BSD-Compress compression; pppd will not request or agree to compress pack-
ets using the BSD-Compress scheme.
noccp Disable CCP (Compression Control Protocol) negotiation. This option should only be
required if the peer is buggy and gets confused by requests from pppd for CCP nego-
Disable hardware flow control (i.e. RTS/CTS) on the serial port. If neither the
crtscts nor the nocrtscts nor the cdtrcts nor the nocdtrcts option is given, the
hardware flow control setting for the serial port is left unchanged.
This option is a synonym for nocrtscts. Either of these options will disable both
forms of hardware flow control.
Disable the defaultroute option. The system administrator who wishes to prevent
users from creating default routes with pppd can do so by placing this option in
the /etc/ppp/options file.
Disables Deflate compression; pppd will not request or agree to compress packets
using the Deflate scheme.
Don't detach from the controlling terminal. Without this option, if a serial
device other than the terminal on the standard input is specified, pppd will fork
to become a background process.
Disables pppd from sending an endpoint discriminator to the peer or accepting one
from the peer (see the MULTILINK section below). This option should only be
required if the peer is buggy.
noip Disable IPCP negotiation and IP communication. This option should only be required
if the peer is buggy and gets confused by requests from pppd for IPCP negotiation.
noipv6 Disable IPv6CP negotiation and IPv6 communication. This option should only be
required if the peer is buggy and gets confused by requests from pppd for IPv6CP
Disables the default behaviour when no local IP address is specified, which is to
determine (if possible) the local IP address from the hostname. With this option,
the peer will have to supply the local IP address during IPCP negotiation (unless
it specified explicitly on the command line or in an options file).
noipx Disable the IPXCP and IPX protocols. This option should only be required if the
peer is buggy and gets confused by requests from pppd for IPXCP negotiation.
Opposite of the ktune option; disables pppd from changing system settings.
nolog Do not send log messages to a file or file descriptor. This option cancels the
logfd and logfile options.
Disable magic number negotiation. With this option, pppd cannot detect a looped-
back line. This option should only be needed if the peer is buggy.
nomp Disables the use of PPP multilink. This option is currently only available under
Disables the use of short (12-bit) sequence numbers in the PPP multilink protocol,
forcing the use of 24-bit sequence numbers. This option is currently only avail-
able under Linux, and only has any effect if multilink is enabled.
Disables the use of PPP multilink. This option is currently only available under
Disable protocol field compression negotiation in both the receive and the transmit
Exit once a connection has been made and terminated. This is the default unless
the persist or demand option has been specified.
Do not accept or agree to Predictor-1 compression.
Disable the proxyarp option. The system administrator who wishes to prevent users
from creating proxy ARP entries with pppd can do so by placing this option in the
notty Normally, pppd requires a terminal device. With this option, pppd will allocate
itself a pseudo-tty master/slave pair and use the slave as its terminal device.
Pppd will create a child process to act as a `character shunt' to transfer charac-
ters between the pseudo-tty master and its standard input and output. Thus pppd
will transmit characters on its standard output and receive characters on its stan-
dard input even if they are not terminal devices. This option increases the
latency and CPU overhead of transferring data over the ppp interface as all of the
characters sent and received must flow through the character shunt process. An
explicit device name may not be given if this option is used.
novj Disable Van Jacobson style TCP/IP header compression in both the transmit and the
Disable the connection-ID compression option in Van Jacobson style TCP/IP header
compression. With this option, pppd will not omit the connection-ID byte from Van
Jacobson compressed TCP/IP headers, nor ask the peer to do so.
Indicates that all secrets in the /etc/ppp/pap-secrets file which are used for
checking the identity of the peer are encrypted, and thus pppd should not accept a
password which, before encryption, is identical to the secret from the
Set the maximum number of PAP authenticate-request transmissions to n (default 10).
Set the PAP restart interval (retransmission timeout) to n seconds (default 3).
Set the maximum time that pppd will wait for the peer to authenticate itself with
PAP to n seconds (0 means no limit).
Specifies a packet filter to applied to data packets being sent or received to
determine which packets should be allowed to pass. Packets which are rejected by
the filter are silently discarded. This option can be used to prevent specific
network daemons (such as routed) using up link bandwidth, or to provide a basic
firewall capability. The filter-expression syntax is as described for tcpdump(1),
except that qualifiers which are inappropriate for a PPP link, such as ether and
arp, are not permitted. Generally the filter expression should be enclosed in sin-
gle-quotes to prevent whitespace in the expression from being interpreted by the
shell. Note that it is possible to apply different constraints to incoming and
outgoing packets using the inbound and outbound qualifiers. This option is cur-
rently only available under NetBSD, and then only if both the kernel and pppd were
compiled with PPP_FILTER defined.
Do not exit after a connection is terminated; instead try to reopen the connection.
Load the shared library object file filename as a plugin. This is a privileged
Request that the peer compress frames that it sends using Predictor-1 compression,
and agree to compress transmitted frames with Predictor-1 if requested. This
option has no effect unless the kernel driver supports Predictor-1 compression.
Allows members of group group-name to use privileged options. This is a privileged
option. Use of this option requires care as there is no guarantee that members of
group-name cannot use pppd to become root themselves. Consider it equivalent to
putting the members of group-name in the kmem or disk group.
Add an entry to this system's ARP [Address Resolution Protocol] table with the IP
address of the peer and the Ethernet address of this system. This will have the
effect of making the peer appear to other systems to be on the local ethernet.
Specifies that the command script is to be used to communicate rather than a spe-
cific terminal device. Pppd will allocate itself a pseudo-tty master/slave pair
and use the slave as its terminal device. The script will be run in a child
process with the pseudo-tty master as its standard input and output. An explicit
device name may not be given if this option is used. (Note: if the record option
is used in conjuction with the pty option, the child process will have pipes on its
standard input and output.)
With this option, pppd will accept all control characters from the peer, including
those marked in the receive asyncmap. Without this option, pppd will discard those
characters as specified in RFC1662. This option should only be needed if the peer
Specifies that pppd should record all characters sent and received to a file named
filename. This file is opened in append mode, using the user's user-ID and permis-
sions. This option is implemented using a pseudo-tty and a process to transfer
characters between the pseudo-tty and the real serial device, so it will increase
the latency and CPU overhead of transferring data over the ppp interface. The
characters are stored in a tagged format with timestamps, which can be displayed in
readable form using the pppdump(8) program.
Set the assumed name of the remote system for authentication purposes to name.
With this option, pppd will not agree to authenticate itself to the peer using
With this option, pppd will not agree to authenticate itself to the peer using PAP.
Require the peer to authenticate itself using CHAP [Challenge Handshake Authentica-
tion Protocol] authentication.
Require the peer to authenticate itself using PAP [Password Authentication Proto-
When logging the contents of PAP packets, this option causes pppd to show the pass-
word string in the log message.
silent With this option, pppd will not transmit LCP packets to initiate a connection until
a valid LCP packet is received from the peer (as for the `passive' option with
ancient versions of pppd).
sync Use synchronous HDLC serial encoding instead of asynchronous. The device used by
pppd with this option must have sync support. Currently supports Microgate Syn-
cLink adapters under Linux and FreeBSD 2.2.8 and later.
With this option, pppd will detach from its controlling terminal once it has suc-
cessfully established the ppp connection (to the point where the first network con-
trol protocol, usually the IP control protocol, has come up).
Enforce the use of the hostname (with domain name appended, if given) as the name
of the local system for authentication purposes (overrides the name option). This
option is not normally needed since the name option is privileged.
Ask the peer for up to 2 DNS server addresses. The addresses supplied by the peer
(if any) are passed to the /etc/ppp/ip-up script in the environment variables DNS1
and DNS2. In addition, pppd will create an /etc/ppp/resolv.conf file containing
one or two nameserver lines with the address(es) supplied by the peer.
Sets the name used for authenticating the local system to the peer to name.
Sets the number of connection slots to be used by the Van Jacobson TCP/IP header
compression and decompression code to n, which must be between 2 and 16 (inclu-
Run the executable or shell command specified by script before initiating PPP nego-
tiation, after the connect script (if any) has completed. A value for this option
from a privileged source cannot be overridden by a non-privileged user.
Use software flow control (i.e. XON/XOFF) to control the flow of data on the serial
Options can be taken from files as well as the command line. Pppd reads options from the
files /etc/ppp/options, ~/.ppprc and /etc/ppp/options.ttyname (in that order) before pro-
cessing the options on the command line. (In fact, the command-line options are scanned
to find the terminal name before the options.ttyname file is read.) In forming the name
of the options.ttyname file, the initial /dev/ is removed from the terminal name, and any
remaining / characters are replaced with dots.
An options file is parsed into a series of words, delimited by whitespace. Whitespace can
be included in a word by enclosing the word in double-quotes ("). A backslash (\) quotes
the following character. A hash (#) starts a comment, which continues until the end of
the line. There is no restriction on using the file or call options within an options
pppd provides system administrators with sufficient access control that PPP access to a
server machine can be provided to legitimate users without fear of compromising the secu-
rity of the server or the network it's on. This control is provided through restrictions
on which IP addresses the peer may use, based on its authenticated identity (if any), and
through restrictions on which options a non-privileged user may use. Several of pppd's
options are privileged, in particular those which permit potentially insecure configura-
tions; these options are only accepted in files which are under the control of the system
administrator, or if pppd is being run by root.
The default behaviour of pppd is to allow an unauthenticated peer to use a given IP
address only if the system does not already have a route to that IP address. For example,
a system with a permanent connection to the wider internet will normally have a default
route, and thus all peers will have to authenticate themselves in order to set up a con-
nection. On such a system, the auth option is the default. On the other hand, a system
where the PPP link is the only connection to the internet will not normally have a default
route, so the peer will be able to use almost any IP address without authenticating
As indicated above, some security-sensitive options are privileged, which means that they
may not be used by an ordinary non-privileged user running a setuid-root pppd, either on
the command line, in the user's ~/.ppprc file, or in an options file read using the file
option. Privileged options may be used in /etc/ppp/options file or in an options file
read using the call option. If pppd is being run by the root user, privileged options can
be used without restriction.
When opening the device, pppd uses either the invoking user's user ID or the root UID
(that is, 0), depending on whether the device name was specified by the user or the system
administrator. If the device name comes from a privileged source, that is,
/etc/ppp/options or an options file read using the call option, pppd uses full root privi-
leges when opening the device. Thus, by creating an appropriate file under
/etc/ppp/peers, the system administrator can allow users to establish a ppp connection via
a device which they would not normally have permission to access. Otherwise pppd uses the
invoking user's real UID when opening the device.
Authentication is the process whereby one peer convinces the other of its identity. This
involves the first peer sending its name to the other, together with some kind of secret
information which could only come from the genuine authorized user of that name. In such
an exchange, we will call the first peer the "client" and the other the "server". The
client has a name by which it identifies itself to the server, and the server also has a
name by which it identifies itself to the client. Generally the genuine client shares
some secret (or password) with the server, and authenticates itself by proving that it
knows that secret. Very often, the names used for authentication correspond to the inter-
net hostnames of the peers, but this is not essential.
At present, pppd supports two authentication protocols: the Password Authentication Proto-
col (PAP) and the Challenge Handshake Authentication Protocol (CHAP). PAP involves the
client sending its name and a cleartext password to the server to authenticate itself. In
contrast, the server initiates the CHAP authentication exchange by sending a challenge to
the client (the challenge packet includes the server's name). The client must respond
with a response which includes its name plus a hash value derived from the shared secret
and the challenge, in order to prove that it knows the secret.
The PPP protocol, being symmetrical, allows both peers to require the other to authenti-
cate itself. In that case, two separate and independent authentication exchanges will
occur. The two exchanges could use different authentication protocols, and in principle,
different names could be used in the two exchanges.
The default behaviour of pppd is to agree to authenticate if requested, and to not require
authentication from the peer. However, pppd will not agree to authenticate itself with a
particular protocol if it has no secrets which could be used to do so.
Pppd stores secrets for use in authentication in secrets files (/etc/ppp/pap-secrets for
PAP, /etc/ppp/chap-secrets for CHAP). Both secrets files have the same format. The
secrets files can contain secrets for pppd to use in authenticating itself to other sys-
tems, as well as secrets for pppd to use when authenticating other systems to itself.
Each line in a secrets file contains one secret. A given secret is specific to a particu-
lar combination of client and server - it can only be used by that client to authenticate
itself to that server. Thus each line in a secrets file has at least 3 fields: the name
of the client, the name of the server, and the secret. These fields may be followed by a
list of the IP addresses that the specified client may use when connecting to the speci-
A secrets file is parsed into words as for a options file, so the client name, server name
and secrets fields must each be one word, with any embedded spaces or other special char-
acters quoted or escaped. Note that case is significant in the client and server names
and in the secret.
If the secret starts with an `@', what follows is assumed to be the name of a file from
which to read the secret. A "*" as the client or server name matches any name. When
selecting a secret, pppd takes the best match, i.e. the match with the fewest wildcards.
Any following words on the same line are taken to be a list of acceptable IP addresses for
that client. If there are only 3 words on the line, or if the first word is "-", then all
IP addresses are disallowed. To allow any address, use "*". A word starting with "!"
indicates that the specified address is not acceptable. An address may be followed by "/"
and a number n, to indicate a whole subnet, i.e. all addresses which have the same value
in the most significant n bits. In this form, the address may be followed by a plus sign
("+") to indicate that one address from the subnet is authorized, based on the ppp network
interface unit number in use. In this case, the host part of the address will be set to
the unit number plus one.
Thus a secrets file contains both secrets for use in authenticating other hosts, plus
secrets which we use for authenticating ourselves to others. When pppd is authenticating
the peer (checking the peer's identity), it chooses a secret with the peer's name in the
first field and the name of the local system in the second field. The name of the local
system defaults to the hostname, with the domain name appended if the domain option is
used. This default can be overridden with the name option, except when the usehostname
option is used.
When pppd is choosing a secret to use in authenticating itself to the peer, it first
determines what name it is going to use to identify itself to the peer. This name can be
specified by the user with the user option. If this option is not used, the name defaults
to the name of the local system, determined as described in the previous paragraph. Then
pppd looks for a secret with this name in the first field and the peer's name in the sec-
ond field. Pppd will know the name of the peer if CHAP authentication is being used,
because the peer will have sent it in the challenge packet. However, if PAP is being
used, pppd will have to determine the peer's name from the options specified by the user.
The user can specify the peer's name directly with the remotename option. Otherwise, if
the remote IP address was specified by a name (rather than in numeric form), that name
will be used as the peer's name. Failing that, pppd will use the null string as the
When authenticating the peer with PAP, the supplied password is first compared with the
secret from the secrets file. If the password doesn't match the secret, the password is
encrypted using crypt() and checked against the secret again. Thus secrets for authenti-
cating the peer can be stored in encrypted form if desired. If the papcrypt option is
given, the first (unencrypted) comparison is omitted, for better security.
Furthermore, if the login option was specified, the username and password are also checked
against the system password database. Thus, the system administrator can set up the pap-
secrets file to allow PPP access only to certain users, and to restrict the set of IP
addresses that each user can use. Typically, when using the login option, the secret in
/etc/ppp/pap-secrets would be "", which will match any password supplied by the peer.
This avoids the need to have the same secret in two places.
Authentication must be satisfactorily completed before IPCP (or any other Network Control
Protocol) can be started. If the peer is required to authenticate itself, and fails to do
so, pppd will terminated the link (by closing LCP). If IPCP negotiates an unacceptable IP
address for the remote host, IPCP will be closed. IP packets can only be sent or received
when IPCP is open.
In some cases it is desirable to allow some hosts which can't authenticate themselves to
connect and use one of a restricted set of IP addresses, even when the local host gener-
ally requires authentication. If the peer refuses to authenticate itself when requested,
pppd takes that as equivalent to authenticating with PAP using the empty string for the
username and password. Thus, by adding a line to the pap-secrets file which specifies the
empty string for the client and password, it is possible to allow restricted access to
hosts which refuse to authenticate themselves.
When IPCP negotiation is completed successfully, pppd will inform the kernel of the local
and remote IP addresses for the ppp interface. This is sufficient to create a host route
to the remote end of the link, which will enable the peers to exchange IP packets. Commu-
nication with other machines generally requires further modification to routing tables
and/or ARP (Address Resolution Protocol) tables. In most cases the defaultroute and/or
proxyarp options are sufficient for this, but in some cases further intervention is
required. The /etc/ppp/ip-up script can be used for this.
Sometimes it is desirable to add a default route through the remote host, as in the case
of a machine whose only connection to the Internet is through the ppp interface. The
defaultroute option causes pppd to create such a default route when IPCP comes up, and
delete it when the link is terminated.
In some cases it is desirable to use proxy ARP, for example on a server machine connected
to a LAN, in order to allow other hosts to communicate with the remote host. The proxyarp
option causes pppd to look for a network interface on the same subnet as the remote host
(an interface supporting broadcast and ARP, which is up and not a point-to-point or loop-
back interface). If found, pppd creates a permanent, published ARP entry with the IP
address of the remote host and the hardware address of the network interface found.
When the demand option is used, the interface IP addresses have already been set at the
point when IPCP comes up. If pppd has not been able to negotiate the same addresses that
it used to configure the interface (for example when the peer is an ISP that uses dynamic
IP address assignment), pppd has to change the interface IP addresses to the negotiated
addresses. This may disrupt existing connections, and the use of demand dialling with
peers that do dynamic IP address assignment is not recommended.
Multilink PPP provides the capability to combine two or more PPP links between a pair of
machines into a single `bundle', which appears as a single virtual PPP link which has the
combined bandwidth of the individual links. Currently, multilink PPP is only supported
Pppd detects that the link it is controlling is connected to the same peer as another link
using the peer's endpoint discriminator and the authenticated identity of the peer (if it
authenticates itself). The endpoint discriminator is a block of data which is hopefully
unique for each peer. Several types of data can be used, including locally-assigned
strings of bytes, IP addresses, MAC addresses, randomly strings of bytes, or E-164 phone
numbers. The endpoint discriminator sent to the peer by pppd can be set using the end-
In circumstances the peer may send no endpoint discriminator or a non-unique value. The
optional bundle option adds an extra string which is added to the peer's endpoint discrim-
inator and authenticated identity when matching up links to be joined together in a bun-
dle. The bundle option can also be used to allow the establishment of multiple bundles
between the local system and the peer. Pppd uses a TDB database in /var/run/pppd.tdb to
match up links.
Assuming that multilink is enabled and the peer is willing to negotiate multilink, then
when pppd is invoked to bring up the first link to the peer, it will detect that no other
link is connected to the peer and create a new bundle, that is, another ppp network inter-
face unit. When another pppd is invoked to bring up another link to the peer, it will
detect the existing bundle and join its link to it. Currently, if the first pppd termi-
nates (for example, because of a hangup or a received signal) the bundle is destroyed.
The following examples assume that the /etc/ppp/options file contains the auth option (as
in the default /etc/ppp/options file in the ppp distribution).
Probably the most common use of pppd is to dial out to an ISP. This can be done with a
command such as
pppd call isp
where the /etc/ppp/peers/isp file is set up by the system administrator to contain some-
thing like this:
ttyS0 19200 crtscts
connect '/usr/sbin/chat -v -f /etc/ppp/chat-isp'
In this example, we are using chat to dial the ISP's modem and go through any logon
sequence required. The /etc/ppp/chat-isp file contains the script used by chat; it could
for example contain something like this:
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "NO ANSWER"
ABORT "Username/Password Incorrect"
See the chat(8) man page for details of chat scripts.
Pppd can also be used to provide a dial-in ppp service for users. If the users already
have login accounts, the simplest way to set up the ppp service is to let the users log in
to their accounts and run pppd (installed setuid-root) with a command such as
To allow a user to use the PPP facilities, you need to allocate an IP address for that
user's machine and create an entry in /etc/ppp/pap-secrets or /etc/ppp/chap-secrets
(depending on which authentication method the PPP implementation on the user's machine
supports), so that the user's machine can authenticate itself. For example, if Joe has a
machine called "joespc" which is to be allowed to dial in to the machine called "server"
and use the IP address joespc.my.net, you would add an entry like this to /etc/ppp/pap-
secrets or /etc/ppp/chap-secrets:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create a username called (for example) "ppp", whose login shell is
pppd and whose home directory is /etc/ppp. Options to be used when pppd is run this way
can be put in /etc/ppp/.ppprc.
If your serial connection is any more complicated than a piece of wire, you may need to
arrange for some control characters to be escaped. In particular, it is often useful to
escape XON (^Q) and XOFF (^S), using asyncmap a0000. If the path includes a telnet, you
probably should escape ^] as well (asyncmap 200a0000). If the path includes an rlogin,
you will need to use the escape ff option on the end which is running the rlogin client,
since many rlogin implementations are not transparent; they will remove the sequence
[0xff, 0xff, 0x73, 0x73, followed by any 8 bytes] from the stream.
Messages are sent to the syslog daemon using facility LOG_DAEMON. (This can be overriden
by recompiling pppd with the macro LOG_PPP defined as the desired facility.) In order to
see the error and debug messages, you will need to edit your /etc/syslog.conf file to
direct the messages to the desired output device or file.
The debug option causes the contents of all control packets sent or received to be logged,
that is, all LCP, PAP, CHAP or IPCP packets. This can be useful if the PPP negotiation
does not succeed or if authentication fails. If debugging is enabled at compile time, the
debug option also causes other debugging messages to be logged.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal to the pppd process.
This signal acts as a toggle.
The exit status of pppd is set to indicate whether any error was detected, or the reason
for the link being terminated. The values used are:
0 Pppd has detached, or otherwise the connection was successfully established and
terminated at the peer's request.
1 An immediately fatal error of some kind occurred, such as an essential system call
failing, or running out of virtual memory.
2 An error was detected in processing the options given, such as two mutually exclu-
sive options being used.
3 Pppd is not setuid-root and the invoking user is not root.
4 The kernel does not support PPP, for example, the PPP kernel driver is not included
or cannot be loaded.
5 Pppd terminated because it was sent a SIGINT, SIGTERM or SIGHUP signal.
6 The serial port could not be locked.
7 The serial port could not be opened.
8 The connect script failed (returned a non-zero exit status).
9 The command specified as the argument to the pty option could not be run.
10 The PPP negotiation failed, that is, it didn't reach the point where at least one
network protocol (e.g. IP) was running.
11 The peer system failed (or refused) to authenticate itself.
12 The link was established successfully and terminated because it was idle.
13 The link was established successfully and terminated because the connect time limit
14 Callback was negotiated and an incoming call should arrive shortly.
15 The link was terminated because the peer is not responding to echo requests.
16 The link was terminated by the modem hanging up.
17 The PPP negotiation failed because serial loopback was detected.
18 The init script failed (returned a non-zero exit status).
19 We failed to authenticate ourselves to the peer.
Pppd invokes scripts at various stages in its processing which can be used to perform
site-specific ancillary processing. These scripts are usually shell scripts, but could be
executable code files instead. Pppd does not wait for the scripts to finish. The scripts
are executed as root (with the real and effective user-id set to 0), so that they can do
things such as update routing tables or run privileged daemons. Be careful that the con-
tents of these scripts do not compromise your system's security. Pppd runs the scripts
with standard input, output and error redirected to /dev/null, and with an environment
that is empty except for some environment variables that give information about the link.
The environment variables that pppd sets are:
DEVICE The name of the serial tty device being used.
IFNAME The name of the network interface being used.
The IP address for the local end of the link. This is only set when IPCP has come
The IP address for the remote end of the link. This is only set when IPCP has come
The authenticated name of the peer. This is only set if the peer authenticates
SPEED The baud rate of the tty device.
The real user-id of the user who invoked pppd.
The username of the real user-id that invoked pppd. This is always set.
For the ip-down and auth-down scripts, pppd also sets the following variables giving sta-
tistics for the connection:
The number of seconds from when the PPP negotiation started until the connection
The number of bytes sent (at the level of the serial port) during the connection.
The number of bytes received (at the level of the serial port) during the connec-
The logical name of the link, set with the linkname option.
Pppd invokes the following scripts, if they exist. It is not an error if they don't
A program or script which is executed after the remote system successfully authen-
ticates itself. It is executed with the parameters
interface-name peer-name user-name tty-device speed
Note that this script is not executed if the peer doesn't authenticate itself, for
example when the noauth option is used.
A program or script which is executed when the link goes down, if /etc/ppp/auth-up
was previously executed. It is executed in the same manner with the same parame-
ters as /etc/ppp/auth-up.
A program or script which is executed when the link is available for sending and
receiving IP packets (that is, IPCP has come up). It is executed with the parame-
interface-name tty-device speed local-IP-address remote-IP-address ipparam
A program or script which is executed when the link is no longer available for
sending and receiving IP packets. This script can be used for undoing the effects
of the /etc/ppp/ip-up script. It is invoked in the same manner and with the same
parameters as the ip-up script.
Like /etc/ppp/ip-up, except that it is executed when the link is available for
sending and receiving IPv6 packets. It is executed with the parameters
interface-name tty-device speed local-link-local-address remote-link-local-address
Similar to /etc/ppp/ip-down, but it is executed when IPv6 packets can no longer be
transmitted on the link. It is executed with the same parameters as the ipv6-up
A program or script which is executed when the link is available for sending and
receiving IPX packets (that is, IPXCP has come up). It is executed with the param-
interface-name tty-device speed network-number local-IPX-node-address remote-IPX-
node-address local-IPX-routing-protocol remote-IPX-routing-protocol local-IPX-
router-name remote-IPX-router-name ipparam pppd-pid
The local-IPX-routing-protocol and remote-IPX-routing-protocol field may be one of
NONE to indicate that there is no routing protocol
RIP to indicate that RIP/SAP should be used
NLSP to indicate that Novell NLSP should be used
RIP NLSP to indicate that both RIP/SAP and NLSP should be used
A program or script which is executed when the link is no longer available for
sending and receiving IPX packets. This script can be used for undoing the effects
of the /etc/ppp/ipx-up script. It is invoked in the same manner and with the same
parameters as the ipx-up script.
/var/run/pppn.pid (BSD or Linux), /etc/ppp/pppn.pid (others)
Process-ID for pppd process on ppp interface unit n.
/var/run/ppp-name.pid (BSD or Linux), /etc/ppp/ppp-name.pid (others)
Process-ID for pppd process for logical link name (see the linkname option).
Usernames, passwords and IP addresses for PAP authentication. This file should be
owned by root and not readable or writable by any other user. Pppd will log a
warning if this is not the case.
Names, secrets and IP addresses for CHAP authentication. As for /etc/ppp/pap-
secrets, this file should be owned by root and not readable or writable by any
other user. Pppd will log a warning if this is not the case.
System default options for pppd, read before user default options or command-line
User default options, read before /etc/ppp/options.ttyname.
System default options for the serial port being used, read after ~/.ppprc. In
forming the ttyname part of this filename, an initial /dev/ is stripped from the
port name (if present), and any slashes in the remaining part are converted to
A directory containing options files which may contain privileged options, even if
pppd was invoked by a user other than root. The system administrator can create
options files in this directory to permit non-privileged users to dial out without
requiring the peer to authenticate, but only to certain trusted peers.
Jacobson, V. Compressing TCP/IP headers for low-speed serial links. February
Rivest, R. The MD5 Message-Digest Algorithm. April 1992.
McGregor, G. PPP Internet Protocol Control Protocol (IPCP). May 1992.
Lloyd, B.; Simpson, W.A. PPP authentication protocols. October 1992.
Simpson, W.A. The Point-to-Point Protocol (PPP). July 1994.
Simpson, W.A. PPP in HDLC-like Framing. July 1994.
Haskin, D. IP Version 6 over PPP December 1998.
The following signals have the specified effect when sent to pppd.
These signals cause pppd to terminate the link (by closing LCP), restore the serial
device settings, and exit.
SIGHUP This signal causes pppd to terminate the link, restore the serial device settings,
and close the serial device. If the persist or demand option has been specified,
pppd will try to reopen the serial device and start another connection (after the
holdoff period). Otherwise pppd will exit. If this signal is received during the
holdoff period, it causes pppd to end the holdoff period immediately.
This signal toggles the state of the debug option.
This signal causes pppd to renegotiate compression. This can be useful to re-
enable compression after it has been disabled as a result of a fatal decompression
error. (Fatal decompression errors generally indicate a bug in one or other imple-
Paul Mackerras (Paul.Mackerras@cs.anu.edu.au), based on earlier work by Drew Perkins, Brad
Clements, Karl Fox, Greg Christy, and Brad Parker.