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 datagrams over serial links, an
extensible Link Control Protocol (LCP), and a family of Network Control
Protocols (NCP) for establishing and configuring different networklayer
protocols.
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 Internet Protocol (IP) (called the IP
Control Protocol, IPCP).
FREQUENTLY USED OPTIONS [Toc] [Back] <tty_name>
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 standard 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.
<speed>
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 limited set of speeds.
asyncmap <map>
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) represents 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 transmitted 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.
call name
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.
connect script
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.
crtscts
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.
defaultroute
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.
disconnect script
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 nonprivileged
user.
escape xx,yy,...
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 0x5e.
file name
Read options from file name (the format is described below).
The file must be readable by the user who has invoked pppd.
init script
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 packets 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)
passive
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.
<local_IP_address>:<remote_IP_address>
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. 150.234.56.78). 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.
ipv6 <local_interface_identifier>,<remote_interface_identifier>
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 is
randomized.
active-filter filter-expression
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 conjunction
with the idle option if there are packets being sent or
received regularly over the link (for example, routing information
packets) which would otherwise prevent 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.
allow-ip address(es)
Allow peers to use the given IP address or subnet without
authenticating themselves. The parameter is parsed as for each
element of the list of allowed IP addresses in the secrets files
(see the AUTHENTICATION section below).
bsdcomp nr,nt
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 compression dictionaries. Alternatively,
a value of 0 for nr or nt disables compression in the corresponding
direction. Use nobsdcomp or bsdcomp 0 to disable BSDCompress
compression entirely.
cdtrcts
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 control. The sacrifice is
that this flow control mode does not permit using DTR as a modem
control line.
chap-interval n
If this option is given, pppd will rechallenge the peer every n
seconds.
chap-max-challenge n
Set the maximum number of CHAP challenge transmissions to n
(default 10).
chap-restart n
Set the CHAP restart interval (retransmission timeout for challenges)
to n seconds (default 3).
connect-delay n
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 packets are logged through
syslog with facility daemon and level debug. This information
can be directed to a file by setting up /etc/syslog.conf appropriately
(see syslog.conf(5)).
default-asyncmap
Disable asyncmap negotiation, forcing all control characters to
be escaped for both the transmit and the receive direction.
default-mru
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.
deflate nr,nt
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 compression in the corresponding
direction. Use nodeflate or deflate 0 to disable
Deflate compression entirely. (Note: pppd requests Deflate compression
in preference 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 link.
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.
domain d
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 output 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.
endpoint <epdisc>
Sets the endpoint discriminator sent by the local machine to the
peer during multilink 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-decimal 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.
hide-password
When logging the contents of PAP packets, this option causes
pppd to exclude the password string from the log. This is the
default.
holdoff n
Specifies how many seconds to wait before re-initiating the link
after it terminates. 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.
ipcp-accept-local
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.
ipcp-accept-remote
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.
ipcp-max-configure n
Set the maximum number of IPCP configure-request transmissions
to n (default 10).
ipcp-max-failure n
Set the maximum number of IPCP configure-NAKs returned before
starting to send configure-Rejects instead to n (default 10).
ipcp-max-terminate n
Set the maximum number of IPCP terminate-request transmissions
to n (default 3).
ipcp-restart n
Set the IPCP restart interval (retransmission timeout) to n seconds
(default 3).
ipparam string
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.
ipv6cp-max-configure n
Set the maximum number of IPv6CP configure-request transmissions
to n (default 10).
ipv6cp-max-failure n
Set the maximum number of IPv6CP configure-NAKs returned before
starting to send configure-Rejects instead to n (default 10).
ipv6cp-max-terminate n
Set the maximum number of IPv6CP terminate-request transmissions
to n (default 3).
ipv6cp-restart n
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.
ipx-network n
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.
ipx-node n:m
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 the peer.
ipx-router-name <string>
Set the name of the router. This is a string and is sent to the
peer as information data.
ipx-routing n
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 NLSP.
ipxcp-accept-local
Accept the peer's NAK for the node number specified in the ipxnode
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.
ipxcp-accept-network
Accept the peer's NAK for the network number specified in the
ipx-network option. If a network number was specified, and nonzero,
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.
ipxcp-accept-remote
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.
ipxcp-max-configure n
Set the maximum number of IPXCP configure request frames which
the system will send to n. The default is 10.
ipxcp-max-failure n
Set the maximum number of IPXCP NAK frames which the local system
will send before it rejects the options. The default value
is 3.
ipxcp-max-terminate n
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.
kdebug n
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 kernel debug messages. (Note
that these messages are usually only useful for debugging 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.
lcp-echo-failure n
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 terminate after the physical connection has been broken
(e.g., the modem has hung up) in situations where no hardware
modem control lines are available.
lcp-echo-interval n
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.
lcp-max-configure n
Set the maximum number of LCP configure-request transmissions to
n (default 10).
lcp-max-failure n
Set the maximum number of LCP configure-NAKs returned before
starting to send configure-Rejects instead to n (default 10).
lcp-max-terminate n
Set the maximum number of LCP terminate-request transmissions to
n (default 3).
lcp-restart n
Set the LCP restart interval (retransmission timeout) to n seconds
(default 3).
linkname name
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.
logfd n
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.
logfile filename
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 append mode.
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
access.
maxconnect n
Terminate the connection when it has been available for network
traffic for n seconds (i.e. n seconds after the first network
control protocol comes up).
maxfail n
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.
mpshortseq
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 individual links. This
option is currently only available under Linux, and only has any
effect if multilink is enabled (see the multilink option).
ms-dns <addr>
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.)
ms-wins <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.
multilink
Enables the use of the PPP multilink protocol. If the peer also
supports multilink, 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.
name name
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 authenticating 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.)
netmask n
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-topoint
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-to-point interface.)
noaccomp
Disable Address/Control compression in both directions (send and
receive).
noauth Do not require the peer to authenticate itself. This option is
privileged.
nobsdcomp
Disables BSD-Compress compression; pppd will not request or
agree to compress packets 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 negotiation.
nocrtscts
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.
nocdtrcts
This option is a synonym for nocrtscts. Either of these options
will disable both forms of hardware flow control.
nodefaultroute
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.
nodeflate
Disables Deflate compression; pppd will not request or agree to
compress packets using the Deflate scheme.
nodetach
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.
noendpoint
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 negotiation.
noipdefault
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.
noktune
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.
nomagic
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 Linux.
nompshortseq
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 available under Linux, and only
has any effect if multilink is enabled.
nomultilink
Disables the use of PPP multilink. This option is currently
only available under Linux.
nopcomp
Disable protocol field compression negotiation in both the
receive and the transmit direction.
nopersist
Exit once a connection has been made and terminated. This is
the default unless the persist or demand option has been specified.
nopredictor1
Do not accept or agree to Predictor-1 compression.
noproxyarp
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 /etc/ppp/options
file.
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 characters
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 standard 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 receive direction.
novjccomp
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.
papcrypt
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
/etc/ppp/pap-secrets file.
pap-max-authreq n
Set the maximum number of PAP authenticate-request transmissions
to n (default 10).
pap-restart n
Set the PAP restart interval (retransmission timeout) to n seconds
(default 3).
pap-timeout n
Set the maximum time that pppd will wait for the peer to authenticate
itself with PAP to n seconds (0 means no limit).
pass-filter filter-expression
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 single-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 currently only
available under NetBSD, and then only if both the kernel and
pppd were compiled with PPP_FILTER defined.
persist
Do not exit after a connection is terminated; instead try to
reopen the connection. The maxfail option still has an effect on
persistent connections.
plugin filename
Load the shared library object file filename as a plugin. This
is a privileged option.
predictor1
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.
privgroup group-name
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.
proxyarp
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.
pty script
Specifies that the command script is to be used to communicate
rather than a specific 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.)
receive-all
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 is
buggy.
record filename
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 permissions. 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.
remotename name
Set the assumed name of the remote system for authentication
purposes to name.
refuse-chap
With this option, pppd will not agree to authenticate itself to
the peer using CHAP.
refuse-pap
With this option, pppd will not agree to authenticate itself to
the peer using PAP.
require-chap
Require the peer to authenticate itself using CHAP [Challenge
Handshake Authentication Protocol] authentication.
require-pap
Require the peer to authenticate itself using PAP [Password
Authentication Protocol] authentication.
show-password
When logging the contents of PAP packets, this option causes
pppd to show the password 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 SyncLink adapters under Linux and
FreeBSD 2.2.8 and later.
updetach
With this option, pppd will detach from its controlling terminal
once it has successfully established the ppp connection (to the
point where the first network control protocol, usually the IP
control protocol, has come up).
usehostname
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.
usepeerdns
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.
user name
Sets the name used for authenticating the local system to the
peer to name.
vj-max-slots n
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 (inclusive).
welcome script
Run the executable or shell command specified by script before
initiating PPP negotiation, 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.
xonxoff
Use software flow control (i.e. XON/XOFF) to control the flow of
data on the serial port.
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 processing 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 file.
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 security 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 configurations; 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 connection.
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 itself.
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 privileges 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 internet hostnames of the peers, but this is not essential.
At present, pppd supports two authentication protocols: the Password
Authentication Protocol (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 authenticate 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 systems, 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 particular 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 specified server.
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 characters 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 second 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 remote-
name 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 peer's
name.
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 authenticating 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 generally 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 papsecrets
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. Communication
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
loopback 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 under Linux.
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
|
