pppd(1M) System Administration Commands pppd(1M)NAMEpppd - point to point protocol daemon
SYNOPSISpppd [tty_name] [speed] [options]
DESCRIPTION
The point-to-point protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is composed of three
components: a facility for encapsulating datagrams over serial links,
an extensible link control protocol (LCP), and a family of network con‐
trol protocols (NCP) for establishing and configuring different net‐
work-layer protocols.
The encapsulation scheme is provided by driver code in the kernel. pppd
provides the basic LCP authentication support and several NCPs for
establishing and configuring the Internet Protocol (referred to as the
IP Control Protocol or "IPCP") and IPv6 (IPV6CP).
OPTIONS
The following sections discuss the pppd options:
Options Files
Options are taken from files and the command line. pppd reads options
from the files /etc/ppp/options, $HOME/.ppprc and /etc/ppp/options.tty‐
name (in that order) before processing the options on the command line.
(Command-line options are scanned for the terminal name before the
options.ttyname file is read.) To form the name of the options.ttyname
file, the initial /dev/ is removed from the terminal name, and any
remaining forward slash characters (/) are replaced with dots. For
example, with serial device /dev/cua/a, option file
/etc/ppp/options.cua.a is read.
An options file is parsed into a series of words that are delimited by
whitespace. Whitespace can be included in a word by enclosing the word
in double-quotes ("). A backslash (\) quotes the succeeding 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.
Frequently Used Options
<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
uses that terminal and does not fork to put
itself in the background. A value for this
option from a privileged source cannot be over‐
ridden by a non-privileged user.
<speed> Set the baud rate to <speed> (a decimal number).
The default is to leave the baud rate unchanged.
This option is normally needed for dial-out
only.
asyncmap <map> Set the async character map to <map>. The map
describes which control characters cannot be
successfully received over the serial line. pppd
asks 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 charac‐
ter 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, pppd attempts to
negotiate a value of 0. If the peer agrees, this
disables escaping of the standard control char‐
acters. Use the default-asyncmap option to dis‐
able negotiation and escape all control charac‐
ters.
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 the auth or the noauth option
is not specified, pppd allows the peer to use
only those 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,
including noauth, even if pppd is not being run
by root. The name string may not begin with a
slash ("/") or include consecutive periods
("..") as a pathname component.
callback number Request a callback to the given telephone number
using Microsoft CBCP.
connect script Use the executable or shell command specified by
script to set up the serial line. This script
would typically use the chat(1M) program to dial
the modem and start the remote PPP session. A
value for this option originating from a privi‐
leged source cannot be overridden by a non-priv‐
ileged user.
crtscts Use hardware flow control, that is, RTS/CTS, to
control the flow of data on the serial port. If
the crtscts, nocrtscts, cdtrcts or nocdtrcts
option is not provided, the hardware flow con‐
trol setting for the serial port is left
unchanged. Some serial ports lack a true RTS
output and use this mode to implement unidirec‐
tional flow control. The serial port suspends
transmission when requested by the modem by
means of CTS but cannot request the modem to
stop sending to the computer. This mode allows
the use of DTR as a modem control line.
defaultroute Add a default route to the system routing tables
when IPCP negotiation successfully completes,
using the peer as the gateway. This entry is
removed when the PPP connection is broken. This
option is privileged if the nodefaultroute
option is specified.
disconnect script Run the executable or shell command specified by
script after pppd terminates the link. Typi‐
cally, this script is used to command the modem
to hang up if hardware modem control signals are
not available. disconnect is not run if the
modem has already hung up. A value for this
option originating from a privileged source can‐
not be overridden by a non-privileged user.
escape xx,yy,... Specifies that certain characters be escaped on
transmission regardless of whether the peer
requests them to be escaped with its async con‐
trol 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 allows only control char‐
acters to be specified. Characters that cannot
be escaped are those containing hex values 0x20
through 0x3f and 0x5e.
file name Read options from file name. If this option is
used on the command line or in $HOME/.ppprc, the
file must be readable by the user invoking pppd.
See for a list of files that pppd always reads,
regardless of the use of this option.
init script Run the executable or shell command specified by
script to initialize the serial line. This
script would typically use the chat(1M) 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 Directs pppd to create a UUCP-style lock file
for the serial device to ensure exclusive access
to the device.
mru n Set the Maximum Receive Unit (MRU) value to n.
pppd asks the peer to send packets of no more
than n bytes. Minimum MRU value is 128. Default
MRU value is 1500. A value of 296 is recommended
for slow links (40 bytes for TCP/IP header + 256
bytes of data). For IPv6, MRU must be at least
1280.
mtu n Set the Maximum Transmit Unit (MTU) value to n.
Unless the peer requests a smaller value via MRU
negotiation, pppd requests the kernel networking
code to send data packets of no more than n
bytes through the PPP network interface. For
IPv6, MTU must be at least 1280.
passive Enables the "passive" option in the LCP. With
this option, pppd attempts to initiate a connec‐
tion; if no reply is received from the peer,
pppd waits passively for a valid LCP packet
instead of exiting, as it would without this
option.
Options
<local_IP_address>:<remote_IP_address>
Set the local and/or remote interface IP addresses. Either one may
be omitted, but the colon is required. The IP addresses are speci‐
fied with a host name or in decimal dot notation, for example:
:10.1.2.3. The default local address is the first IP address of the
system unless the noipdefault option is provided. The remote
address is 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.
allow-fcs fcs-type
Set allowable FCS type(s) for data sent to the peer. The fcs-type
is a comma-separated list of "crc16", "crc32", "null", or integers.
By default, all known types are allowed. If this option is speci‐
fied and the peer requests a type not listed, a LCP Configure-Nak
is sent to request only the listed types.
allow-ip address(es)
Allow peers to use the given IP address or subnet without authenti‐
cating themselves. The parameter is parsed in the same manner as
each element of the list of allowed IP addresses is parsed in the
secrets files. See the section more more details.
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 provide 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 nobsd‐
comp or bsdcomp 0 to disable BSD-Compress compression entirely. If
this option is read from a privileged source, a nonprivileged user
may not specify a code size larger than the value from the privi‐
leged source.
cdtrcts
Use a non-standard hardware flow control such as DTR/CTS to control
the flow of data on the serial port. If the crtscts, nocrtscts,
cdtrcts or nocdtrcts option is not specified, the hardware flow
control setting for the serial port is left unchanged. Some serial
ports lack a true RTS output. Such serial ports use this mode to
implement true bi-directional flow control. Note 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 chal‐
lenges) to n seconds. The default is 3.
connect-delay n
Wait for up to n milliseconds after the connect script finishes for
a valid PPP packet from the peer. When the wait period elapses or
when a valid PPP packet is received from the peer, pppd begins
negotiation by sending its first LCP packet. The default value is
1000 (1 second). A wait period applies only if the connect or pty
option is used.
datarate n
Set maximum data rate to n (in bytes per second) when using the
pty, notty, record, or socket options.
debug
Enables connection debugging facilities. If this option is given,
pppd logs 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 configuring /etc/syslog.conf appropriately.
default-asyncmap
Disable asyncmap negotiation, forcing all control characters to be
escaped for both the transmit and the receive direction.
default-fcs
Disable FCS Alternatives negotiation entirely. By default, no FCS
Alternatives option is sent to the peer, but the option is
accepted. If this option is specified by the peer, then LCP Config‐
ure-Reject is sent.
default-mru
Disable MRU [Maximum Receive Unit] negotiation. With this option,
pppd uses the default MRU value of 1500 bytes for the transmit and
receive directions.
deflate nr,nt,e
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 and effort level of e (1 to 9). If nt is not
specified, it defaults to the value given for nr. If e is not spec‐
ified, it defaults to 6. Values in the range 9 to 15 may be used
for nr and nt; larger values provide better compression but consume
more kernel memory for compression dictionaries. (Value 8 is not
permitted due to a zlib bug.) Alternatively, a value of 0 for nr or
nt disables compression in the corresponding direction. Use node‐
flate or deflate 0 to disable deflate compression entirely. (Note:
pppd requests deflate compression in preference to BSD-Compress if
the peer can do either.) If this option is read from a privileged
source, a nonprivileged user may not specify a code size larger
than the value from the privileged source.
demand
Initiate the link only on demand, that is, 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 ini‐
tially configures and enables the interface for IP traffic without
connecting to the peer. When traffic is available, pppd connects to
the peer and performs negotiation, authentication and other
actions. When completed, pppd passes data packets across the link.
The demand option implies the persist option. If this behavior is
not desired, use the nopersist option after the demand option. The
idle and holdoff options can be used in conjunction 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. With this configuration, pppd
uses the name porsche.Quotron.COM for accessing secrets in the
secrets file and as the default name when authenticating to the
peer. This option is privileged.
endpoint endpoint-value
Set the endpoint discriminator (normally used for RFC 1990 Multi‐
link PPP operation). The endpoint-value consists of a class identi‐
fier and a class-dependent value. The class identifier is one of
"null," "local," "IP," "MAC," "magic," "phone," or a decimal inte‐
ger. If present, the class-dependent value is separated from the
identifier by a colon (":") or period (".") . This value may be a
standard dotted-decimal IP address for class "IP," an optionally
colon-or-dot separated hex Ethernet address for class "MAC" (must
have 6 numbers), or an arbitrary string of bytes specified in hex
with optional colon or dot separators between bytes. Although this
option is available, this implementation does not support multi‐
link.
fcs fcs-type
Set FCS type(s) desired for data sent by the peer. The fcs-type is
a comma-separated list of crc16, crc32, null, or integers. By
default, an FCS Alternatives option is not specified, and the
medium-dependent FCS type is used. If this option is specified and
the peer sends an LCP Configure-Nak, only the listed types are
used. If none are in common, the FCS Alternatives option is omitted
from the next LCP Configure-Request to drop back to the default.
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 is effective only if the persist
or demand option is used. The holdoff period is not applied if the
link is terminated because it was idle.
ident string
Set the LCP Identification string. The default value is a version
string similar to that displayed by the --version option.
idle n
Specifies that pppd must disconnect if the link is idle for n sec‐
onds. The link is idle when no data packets (i.e. IP packets) are
being sent or received. Do not use this option with the persist
option but without the demand option.
ipcp-accept-local
With this option, pppd accepts the peer's idea of the local IP
address, even if the local IP address is specified in an option.
ipcp-accept-remote
With this option, pppd accepts the peer's idea of its remote IP
address, even if the remote IP address is 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 sent before sending
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. When
this option is given, the string supplied is given as the sixth
parameter to those scripts. See the section.
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 (for example: ::dead:beef). If the
ipv6cp-use-ipaddr option is given, the local and remote identifiers
are derived from the respective IPv4 addresses (see above). The
ipv6cp-use-persistent option can be used instead of the ipv6
<local>,<remote> option.
ipv6cp-accept-local
Accept peer's interface identifier for the local link identifier.
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 sent before sending
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 sec‐
onds (default 3).
ipv6cp-use-ipaddr
If either the local or remote IPv6 address is unspecified, use the
corresponding configured IPv4 address as a default interface iden‐
tifier. (This option uses the configured addresses, not the negoti‐
ated addresses. Do not use it with ipcp-accept-local if the local
IPv6 identifier is unspecified or with ipcp-accept-remote if the
remote IPv6 identifier is unspecified.)
ipv6cp-use-persistent
Use uniquely-available persistent value for link local address.
kdebug n
Enable debugging code in the kernel-level PPP driver. Argument n is
the sum of the following values: 1 to enable general debug mes‐
sages, 2 to request that contents of received packets be printed,
and 4 to request contents of transmitted packets be printed. Mes‐
sages printed by the kernel are logged by syslogd(1M) to a file
directed in the /etc/syslog.conf configuration file. Do not use the
kdebug option to debug failed links. Use the debug option instead.
lcp-echo-failure n
If this option is given, pppd presumes the peer to be dead if n LCP
Echo-Requests are sent without receiving a valid LCP Echo-Reply. If
this happens, pppd terminates the connection. This option requires
a non-zero value for the lcp-echo-interval parameter. This option
enables pppd to terminate after the physical connection is broken
(for example, if 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 sends an LCP Echo-Request frame to
the peer every n seconds. Normally the peer responds 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 con‐
nected.
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 sent 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 creates a file
named ppp-name.pid in /var/run containing its process ID. This is
useful in determining which instance of pppd is responsible for the
link to a given peer system. This is a privileged option.
local
Do not use modem control lines. With this option, pppd ignores the
state of the CD (Carrier Detect) signal from the modem and does not
change the state of the DTR (Data Terminal Ready) signal.
logfd n
Send log messages to file descriptor n. pppd sends 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. By default pppd sends log messages to stdout
(file descriptor 1) unless the serial port is open on stdout.
logfile filename
Append log messages to the file filename (and send the log messages
to syslog). The file is opened in append mode with the privileges
of the user who invoked pppd.
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 and the
system password database to be allowed access.
maxconnect n
Terminate the connection after it has been available for network
traffic for n seconds (that is, n seconds after the first network
control protocol starts). An LCP Time-Remaining message is sent
when the first NCP starts, and again when 5, 2, and 0.5 minutes are
remaining.
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 waits for the CD (Carrier Detect) signal from the
modem to be asserted when opening the serial device (unless a con‐
nect script is specified), and drops the DTR (Data Terminal Ready)
signal briefly when the connection is terminated and before execut‐
ing the connect script.
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 speci‐
fies the primary DNS address; the second instance (if given) speci‐
fies the secondary DNS address. If the first instance specifies a
name that resolves to multiple IP addresses, then the first two
addresses are used. (This option is present in some older versions
of pppd under the name dns-addr.)
ms-lanman
If pppd connects as a client to a Microsoft server and uses MS-
CHAPv1 for authentication, this option selects the LAN Manager
password style instead of Microsoft NT.
ms-wins <addr>
If pppd acts 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. As with
ms-dns, if the name specified resolves to multiple IP addresses,
then the first two will be taken as primary and secondary.
name name
Set the name of the local system for authentication purposes to
name. This is a privileged option. With this option, pppd uses
lines in the secrets files that have name as the second field to
look for a secret to use in authenticating the peer. In addition,
unless overridden with the user option, name is used as the name to
send to the peer when authenticating the local system. (Note that
pppd does not append the domain name to name.)
no-accm-test
Disable use of asyncmap (ACCM) checking using LCP Echo-Request mes‐
sages. If the lcp-echo-failure is used on an asynchronous line,
pppd includes all control characters in the first n LCP Echo-
Request messages. If the asyncmap is set incorrectly, the link
drops rather than continue operation with random failures. This
option disables that feature.
noaccomp
Disable HDLC 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. This option is
not necessary if noccp is specified.
noccp
Disable CCP (Compression Control Protocol) negotiation. This option
should only be required if the peer has bugs or becomes confused by
requests from pppd for CCP negotiation. If CCP is disabled, then
BSD and deflate compression do not need to be separately disabled.
nocrtscts
Disable hardware flow control (i.e. RTS/CTS) on the serial port. If
the crtscts, nocrtscts, cdtrcts or nocdtrcts options are not given,
the hardware flow control setting for the serial port is left
unchanged.
nocdtrcts
This option is a synonym for nocrtscts. Either option will disable
both forms of hardware flow control.
nodefaultroute
Disable the defaultroute option. You can prevent non-root users
from creating default routes with pppd 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. This option is not nec‐
essary if noccp is specified.
nodeflatedraft
Do not use Internet Draft (incorrectly assigned) algorithm number
for deflate compression. This option is not necessary if noccp is
specified.
nodetach
Do not detach from the controlling terminal. Without this option,
pppd forks to become a background process if a serial device other
than the terminal on the standard input is specified.
noendpoint
Do not send or accept the Multilink Endpoint Discriminator option.
noident
Disable use of LCP Identification. LCP Identification messages will
not be sent to the peer, but received messages will be logged.
(Specify this option twice to completely disable LCP Identifica‐
tion. In this case, pppd sends LCP Code-Reject in response to
received LCP Identification messages.)
noip
Disable IPCP negotiation and IP communication. Use this option only
if the peer has bugs or becomes confused by requests from pppd for
IPCP negotiation.
noipv6
Disable IPv6CP negotiation and IPv6 communication. IPv6 is not
enabled by default.
noipdefault
Disables the default behavior when no local IP address is speci‐
fied, which is to determine (if possible) the local IP address from
the hostname. With this option, the peer must supply the local IP
address during IPCP negotiation (unless it specified explicitly on
the command line or in an options file).
nolog
Do not send log messages to a file or file descriptor. This option
cancels the logfd and logfile options. nologfd acts as an alias for
this option.
nomagic
Disable magic number negotiation. With this option, pppd cannot
detect a looped-back line. Use this option only if the peer has
bugs. Do not use this option to work around the "Serial line is
looped back" error message.
nopam
This privileged option disables use of pluggable authentication
modules. If this option is specified, pppd reverts to standard
authentication mechanisms. The default is not to use PAM.
nopcomp
Disable protocol field compression negotiation in 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 is specified.
noplink
Cause pppd to use I_LINK instead of I_PLINK. This is the default.
When I_LINK is used, the system cleans up terminated interfaces
(even when SIGKILL is used) but does not allow ifconfig(1M) to
unplumb PPP streams or insert or remove modules dynamically. Use
the plink option if ifconfig(1M) modinsert, modremove or unplumb
support is needed.
nopredictor1
Do not accept or agree to Predictor-1 compression. (This option is
accepted for compatibility. The implementation does not support
Predictor-1 compression.)
noproxyarp
Disable the proxyarp option. If you want to prevent users from cre‐
ating proxy ARP entries with pppd, place this option in the
/etc/ppp/options file.
notty
Normally, pppd requires a terminal device. With this option, pppd
allocates itself a pseudo-tty master/slave pair and uses the slave
as its terminal device. pppd creates a child process to act as a
character shunt to transfer characters between the pseudo-tty mas‐
ter and its standard input and output. Thus, pppd transmits charac‐
ters on its standard output and receives 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 does not omit the
connection-ID byte from Van Jacobson compressed TCP/IP headers, nor
does it ask the peer to do so. This option is unnecessary if novj
is specified.
pam
This privileged option enables use of PAM. If this is specified,
pppd uses the pam(3PAM) framework for user authentication with a
service name of "ppp" if the login option and PAP authentication
are used. The default is not to use PAM.
papcrypt
Indicates that pppd should not accept a password which, before
encryption, is identical to the secret from the /etc/ppp/pap-
secrets file. Use this option if the secrets in the pap-secrets
file are in crypt(3C) format.
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 waits for the peer to authenticate
itself with PAP to n seconds (0= no limit). The default is 30 sec‐
onds.
password string
Password string for authentication to the peer.
persist
Do not exit after a connection is terminated; instead try to reopen
the connection.
plink
Cause pppd to use I_PLINK instead of I_LINK. The default is to use
I_LINK, which cleans up terminated interface (even if SIGKILL is
used), but does not allow ifconfig(1M) to unplumb PPP streams or
insert or remove modules dynamically. Use this option if ifcon‐
fig(1M) modinsert/modremove/unplumb support is needed. See also the
plumbed option.
plugin filename
Load the shared library object file filename as a plugin. This is a
privileged option. Unless the filename specifies an explicit path,
/etc/ppp/plugins and /usr/lib/inet/ppp will be searched for the
object to load in that order.
plumbed
This option indicates that pppd should find a plumbed interface and
use that for the session. If IPv4 addresses or IPv6 interface IDs
or link MTU are otherwise unspecified, they are copied from the
interface selected. This mode mimics some of the functionality of
the older aspppd implementation and may be helpful when pppd is
used with external applications that use ifconfig(1M).
pppmux timer
Enable PPP Multiplexing option negotiation and set transmit multi‐
plexing timeout to timer microseconds.
privgroup group-name
Allows members of group group-name to use privileged options. This
is a privileged option. Because there is no guarantee that members
of group-name cannot use pppd to become root themselves, you should
be careful using this option. Consider it equivalent to putting the
members of group-name in the root or sys group.
proxyarp
Add an entry to the system's Address Resolution Protocol (ARP) ta‐
ble with the IP address of the peer and the Ethernet address of
this system. When you use this option, the peer appears to other
systems to be on the local Ethernet. The remote address on the PPP
link must be in the same subnet as assigned to an Ethernet inter‐
face.
pty script
Specifies that the command script, and not a specific terminal
device is used for serial communication. pppd allocates itself a
pseudo-tty master/slave pair and uses the slave as its terminal
device. script runs 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 conjunction with the pty option, the child process will
have pipes on its standard input and output.)
receive-all
With this option, pppd accepts all control characters from the
peer, including those marked in the receive asyncmap. Without this
option, pppd discards those characters as specified in RFC 1662.
This option should be used only if the peer has bugs, as is often
found with dial-back implementations.
record filename
Directs pppd to record all characters sent and received to a file
named filename. filename is opened in append mode, using the user's
user-ID and permissions. Because this option uses a pseudo-tty and
a process to transfer characters between the pseudo-tty and the
real serial device, it increases the latency and CPU overhead of
transferring data over the PPP interface. Characters are stored in
a tagged format with timestamps that can be displayed in readable
form using the pppdump(1M) program. This option is generally used
when debugging the kernel portion of pppd (especially CCP compres‐
sion algorithms) and not for debugging link configuration problems.
See the debug option.
remotename name
Set the assumed name of the remote system for authentication pur‐
poses to name. Microsoft WindowsNT does not provide a system name
in its CHAP Challenge messages, and this option is often used to
work around this problem.
refuse-chap
With this option, pppd will not agree to authenticate itself to the
peer using standard Challenge Handshake Authentication Protocol
(CHAP). (MS-CHAP is not affected.)
refuse-mschap
Do not agree to authenticate to peer with MS-CHAPv1. If this option
is specified, requests for MS-CHAPv1 authentication from the peer
are declined with LCP Configure-Nak. That option does not disable
any other form of CHAP.
refuse-mschapv2
Do not agree to authenticate to peer with MS-CHAPv2. If specified,
this option requests that MS-CHAPv2 authentication from the peer be
declined with LCP Configure-Nak. That option does not disable any
other form of CHAP.
refuse-pap
With this option, pppd will not agree to authenticate itself to the
peer using Password Authentication Protocol (PAP).
require-chap
Require the peer to authenticate itself using standard CHAP authen‐
tication. MS-CHAP is not affected.
require-mschap
Require the peer to authenticate itself using MS-CHAPv1 authentica‐
tion.
require-mschapv2
Require the peer to authenticate itself using MS-CHAPv2 authentica‐
tion.
require-pap
Require the peer to authenticate itself using PAP authentication.
show-password
When logging 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. This
is like the "passive" option with older versions of pppd and is
retained for compatibility, but the current passive option is pre‐
ferred.
small-accm-test
When checking the asyncmap (ACCM) setting, pppd uses all 256 possi‐
ble values by default. See no-accm-test. This option restricts the
test so that only the 32 values affected by standard ACCM negotia‐
tion are tested. This option is useful on very slow links.
socket host:port
Connect to given host and port using TCP and run PPP over this con‐
nection.
sync
Use synchronous HDLC serial encoding instead of asynchronous. The
device used by pppd with this option must have sync support. Cur‐
rently supports zs, se, and hsi drivers.
unit n
Set PPP interface unit number to n, if possible.
updetach
With this option, pppd detaches from its controlling terminal after
establishing the PPP connection. When this is specified, messages
sent to stderr by the connect script, usually chat(1M), and debug‐
ging messages from the debug option are directed to pppd's standard
output.
usehostname
Enforce the use of the hostname with domain name appended, if
given, as the name of the local system for authentication purposes.
This overrides the name option. Because the name option is privi‐
leged, this option is normally not needed.
usepeerdns
Ask the peer for up to two DNS server addresses. 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 creates 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 ini‐
tiating PPP negotiation, after the connect script, if any, has com‐
pleted. A value for this option from a privileged source cannot be
overridden by a non-privileged user.
xonxoff
Use software flow control, that is, XON/XOFF, to control the flow
of data on the serial port.
Obsolete Options
The following options are obsolete:
+ua name Read a PAP user name and password from the file name.
This file must have two lines for name and password. Name
and password are sent to the peer when the peer requests
PAP authentication.
+ipv6 Enable IPv6 and IPv6CP without specifying interface iden‐
tifiers.
--version Show version number and exit.
--help Show brief help message and exit.
EXTENDED DESCRIPTION
The following sections discuss miscellaneous features of pppd:
Security
pppd allows system administrators to provide legitimate users with PPP
access to a server machine without fear of compromising the security of
the server or the network it runs on. Access control is provided by
restricting IP addresses the peer may use based on its authenticated
identity (if any), and through restrictions on options a non-privileged
user may use. Options that permit potentially insecure configurations
are privileged. Privileged options are accepted only in files that are
under the control of the system administrator or when pppd is being run
by root.
By default, pppd allows 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, meaning all peers must
authenticate themselves to set up a connection. On such a system, the
auth option is the default. Conversely, a system with a PPP link that
comprises the only connection to the Internet probably does not possess
a default route, so the peer can use virtually any IP address without
authenticating itself.
Security-sensitive options are privileged and cannot be accessed by a
non-privileged user running pppd, either on the command line, in the
user's $HOME/.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 run by the root
user, privileged options can be used without restriction. If the
/etc/ppp/options file does not exist, then only root may invoke pppd.
The /etc/ppp/options file must be created (but may be empty) to allow
ordinary non-root users to access pppd.
When opening the device, pppd uses the invoking user's user ID or the
root UID (that is, 0), depending if 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 that they would not normally have access to. Otherwise
pppd uses the invoking user's real UID when opening the device.
Authentication
During the authentication process, one peer convinces the other of its
identity by sending its name and some secret information to the other.
During authentication, the first peer becomes the "client" and the sec‐
ond becomes the "server." Authentication names can (but are not
required to) correspond to the peer's Internet hostnames.
pppd supports four authentication protocols: the Password Authentica‐
tion Protocol (PAP) and three forms of the Challenge Handshake Authen‐
tication Protocol (CHAP). With the PAP protocol, the client sends its
name and a cleartext password to the server to authenticate itself.
With CHAP, the server initiates the authentication exchange by sending
a challenge to the client who must respond with its name and a hash
value derived from the shared secret and the challenge.
The PPP protocol is symmetrical, meaning that each peer may be required
to authenticate itself to the other. Different authentication protocols
and names can be used for each exchange.
By default, pppd authenticates if requested and does not require
authentication from the peer. However, pppd does not authenticate
itself with a specific protocol if it has no secrets that can do so.
pppd stores authentication secrets in the /etc/ppp/pap-secrets (for
PAP), and /etc/ppp/chap-secrets (for CHAP) files. Both files use the
same format. pppd uses secrets files to authenticate itself to other
systems and to authenticate other systems to itself.
Secrets files contain one secret per line. Secrets are specific to a
particular combination of client and server and can only be used by
that client to authenticate itself to that server. Each line in a
secrets file has a minimum of three fields that contain the client and
server names followed by the secret. Often, these three fields are fol‐
lowed by IP addresses that are used by clients to connect to a server.
A secrets file is parsed into words, with client name, server name and
secrets fields allocated one word each. Embedded spaces or other spe‐
cial characters within a word must be quoted or escaped. Case is sig‐
nificant in all three fields.
A secret beginning with an at sign ("@") is followed by the name of a
file containing the secret. An asterisk (*) as the client or server
name matches any name. When choosing a match, pppd selects the one with
the fewest wildcards. Succeeding words on a line are interpreted by
pppd as acceptable IP addresses for that client. IP Addresses are dis‐
allowed if they appear in lines that contain only three words or lines
whose first word begins with a hyphen ("-"). To allow any address, use
"*". An address starting with an exclamation point ("!") indicates that
the specified address is not acceptable. An address may be followed by
"/" and a number n to indicate a whole subnet (all addresses that 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 inter‐
face unit number in use. In this case, the host part of the address is
set to the unit number, plus one.
When authenticating the peer, pppd chooses a secret with the peer's
name in the first field of the secrets file and the name of the local
system in the second field. The local system name defaults to the host‐
name, with the domain name appended if the domain option is used. The
default can be overridden with the name option unless the usehostname
option is used.
When authenticating to the peer, pppd first determines the name it will
use to identify itself to the peer. This name is specified with the
user option. If the user option is not used, the name defaults to the
host name of the local system. pppd then selects a secret from the
secrets file by searching for an entry with a local name in the first
field and the peer's name in the second field. pppd will know the name
of the peer if standard CHAP authentication is used because the peer
will have sent it in the Challenge packet. However, if MS-CHAP or PAP
is being used, pppd must 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. If that fails, pppd uses the null string as
the peer's name.
When authenticating the peer with PAP, the supplied password is
compared with data in the secrets file. If the password and secret do
not match, the password is encrypted using crypt() and checked against
the secret again. If the papcrypt option is given, the first unen‐
crypted comparison is omitted for better security, and entries must
thus be in encrypted crypt(3C) form.
If the login option is specified, the username and password are also
checked against the system password database. This allows you to set up
the pap-secrets file to enable PPP access only to certain users, and to
restrict the set of IP addresses available to users. Typically, when
using the login option, the secret in /etc/ppp/pap-secrets would be "",
which matches any password supplied by the peer. This makes having the
same secret in two places unnecessary. When login is used, the pam
option enables access control through pam(3PAM).
Authentication must be completed before IPCP (or other network proto‐
col) can be started. If the peer is required to authenticate itself and
fails, pppd closes LCP and terminates the link. If IPCP negotiates an
unacceptable IP address for the remote host, IPCP is closed. IP packets
are sent or received only when IPCP is open.
To allow hosts that cannot authenticate themselves to connect and
use one of a restricted set of IP addresses, add a line to the pap-
secrets file specifying the empty string for the client name and
secret.
Additional pppd options for a given peer may be specified by placing
them at the end of the secrets entry, separated by two dashes (--). For
example
peername servername secret ip-address -- novj
Routing
When IPCP negotiation is complete, pppd informs the kernel of the local
and remote IP addresses for the PPP interface and creates a host route
to the remote end of the link that enables peers to exchange IP pack‐
ets. Communication with other machines generally requires further modi‐
fication to routing tables and/or Address Resolution Protocol (ARP)
tables. In most cases the defaultroute and/or proxyarp options are suf‐
ficient for this, but further intervention may be necessary. If further
intervention is required, use the /etc/ppp/ip-up script or a routing
protocol daemon.
To add a default route through the remote host, use the defaultroute
option. This option is typically used for "client" systems; that is,
end-nodes that use the PPP link for access to the general Internet.
In some cases it is desirable to use proxy ARP, for example on a server
machine connected to a LAN, to allow other hosts to communicate with
the remote host. proxyarp instructs pppd to look for a network inter‐
face on the same subnet as the remote host. That is, an interface sup‐
porting broadcast and ARP that is not a point-to-point or loopback
interface and that is currently up. 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.
When the demand option is used, the interface IP addresses are already
set at the time when IPCP comes up. If pppd cannot negotiate the same
addresses it used to configure the interface, it changes the interface
IP addresses to the negotiated addresses. This may disrupt existing
connections. Using demand dialing with peers that perform dynamic IP
address assignment is not recommended.
Scripts
pppd invokes scripts at various stages during processing that are used
to perform site-specific ancillary processing. These scripts may be
shell scripts or executable programs. 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), enabling them to update routing tables,
run privileged daemons, or perform other tasks. Be sure 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 envi‐
ronment variables that give information about the link. The pppd envi‐
ronment variables are:
DEVICE Name of the serial tty device.
IFNAME Name of the network interface.
IPLOCAL IP address for the link's local end. This is set only
when IPCP has started.
IPREMOTE IP address for the link's remote end. This is set only
when IPCP has started.
PEERNAME Authenticated name of the peer. This is set only if the
peer authenticates itself.
SPEED Baud rate of the tty device.
ORIG_UID Real user-id of user who invoked pppd.
PPPLOGNAME Username of the real user-id who invoked pppd. This is
always set.
pppd also sets the following variables for the ip-down and auth-down
scripts:
CONNECT_TIME Number of seconds between the start of PPP negotiation
and connection termination.
BYTES_SENT Number of bytes sent at the level of the serial port
during the connection.
BYTES_RCVD Number of bytes received at the level of the serial
port during the connection.
LINKNAME Logical name of the link, set with the linkname
option.
If they exist, pppd invokes the following scripts. It is not an error
if they do not exist.
/etc/ppp/auth-up Program or script executed after the remote sys‐
tem successfully authenticates itself. It is
executed with five command-line arguments:
interface-name peer-name user-name tty-device
speed. Note that this script is not executed if
the peer does not authenticate itself, for exam‐
ple, when the noauth option is used.
/etc/ppp/auth-down Program or script executed when the link goes
down if /etc/ppp/auth-up was previously exe‐
cuted. It is executed in the same manner with
the same parameters as /etc/ppp/auth-up.
/etc/ppp/ip-up A program or script that is executed when the link
is available for sending and receiving IP packets
(that is, IPCP has come up). It is executed with
six command-line arguments: interface-name tty-
device speed local-IP-address remote-IP-address
ipparam.
/etc/ppp/ip-down 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.
/etc/ppp/ipv6-up Similar to /etc/ppp/ip-up, except that it is exe‐
cuted when the link is available for sending and
receiving IPv6 packets. Executed with six command-
line arguments: interface-name tty-device speed
local-link-local-address remote-link-local-address
ipparam.
/etc/ppp/ipv6-down Similar to /etc/ppp/ip-down, but executed when
IPv6 packets can no longer be transmitted on the
link. Executed with the same parameters as the
ipv6-up script.
EXAMPLES
Example 1 Using the auth Option
The following examples assume that the /etc/ppp/options file contains
the auth option.
pppd is commonly used to dial out to an ISP. You can do this using the
"pppd call isp" command where the /etc/ppp/peers/isp file is set up to
contain a line similar to the following:
cua/a 19200 crtscts connect '/usr/bin/chat -f /etc/ppp/chat-isp' noauth
For this example, chat(1M) is used to dial the ISP's modem and process
any login sequence required. The /etc/ppp/chat-isp file is used by chat
and could contain the following:
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "ERROR"
ABORT "NO ANSWER"
ABORT "BUSY"
ABORT "Username/Password Incorrect"
"" "at"
OK "at&f&d2&c1"
OK "atdt2468135"
"name:" "^Umyuserid"
"word:" "qmypassword"
"ispts" "q^Uppp"
"~-^Uppp-~"
See the chat(1M) man page for details of chat scripts.
Example 2 Using pppd with proxyarp
pppd can also 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 as shown in
the following example:
example% pppd proxyarp
Example 3 Providing a User with Access to PPP Facilities
To provide a user with access to the PPP facilities, allocate an IP
address for the user's machine, create an entry in /etc/ppp/pap-secrets
or /etc/ppp/chap-secrets. This enables the user's machine to authenti‐
cate itself. For example, to enable user "Joe" using machine "joespc"
to dial in to machine "server" and use the IP address "joespc.my.net,"
add the following entry to the /etc/ppp/pap-secrets or /etc/ppp/chap-
secrets files:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create another username, for example "ppp,"
whose login shell is /usr/bin/pppd and whose home directory is
/etc/ppp. If you run pppd this way, add the options to the
/etc/ppp/.ppprc file.
If your serial connection is complex, it may be useful to escape such
control characters as XON (^Q) and XOFF (^S), using asyncmap a0000. If
the path includes a telnet, escape ^] (asyncmap 200a0000). If the path
includes a rlogin command, add escape ff option to the options, because
rlogin removes the window-size-change sequence [0xff, 0xff, 0x73, 0x73,
followed by any 8 bytes] from the stream.
EXIT STATUS
The pppd exit status indicates errors or specifies why a link was ter‐
minated. Exit status values are:
0 pppd has detached or the connection was successfully established
and terminated at the peer's request.
1 An immediately fatal error occurred. For example, an essential
system call failed.
2 An error was detected in the options given. For example, two
mutually exclusive options were used, or /etc/ppp/options is
missing and the user is not root.
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 and 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 because no network protocols were
able to run.
11 The peer system failed or refused to authenticate itself.
12 The link was established successfully, but terminated because it
was idle.
13 The link was established successfully, but terminated because
the connect time limit was reached.
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 because a non-zero exit status was
returned.
19 Authentication to the peer failed.
FILES
/var/run/spppn.pid Process-ID for pppd process on PPP inter‐
face unit n.
/var/run/ppp-name.pid Process-ID for pppd process for logical
link name (see the linkname option).
/etc/ppp/pap-secrets Usernames, passwords and IP addresses for
PAP authentication. This file should be
owned by root and not readable or writable
by any other user, otherwise pppd will log
a warning.
/etc/ppp/chap-secrets Names, secrets and IP addresses for all
forms of CHAP authentication. The
/etc/ppp/pap-secrets file should be owned
by root should not readable or writable by
any other user, otherwise, pppd will log a
warning.
/etc/ppp/options System default options for pppd, read
before user default options or command-
line options.
$HOME/.ppprc User default options, read before
/etc/ppp/options.ttyname.
/etc/ppp/options.ttyname System default options for the serial port
in use; read after $HOME/.ppprc. The tty‐
name component of this filename is formed
when the initial /dev/ is stripped from
the port name (if present), and slashes
(if any) are converted to dots.
/etc/ppp/peers Directory with options files that may con‐
tain 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.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Availability │system/network/ppp │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Committed │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOchat(1M), ifconfig(1M), crypt(3C), pam(3PAM), attributes(5)
Haskin, D., Allen, E. RFC 2472 - IP Version 6 Over PPP. Network Working
Group. December 1998.
Jacobson, V. RFC 1144, Compressing TCP/IP Headers for Low-Speed Serial
Links. Network Working Group. February, 1990
Lloyd, B., Simpson, W. RFC 1334, PPP Authentication Protocols. Network
Working Group. October 1992.
McGregor, G. RFC 1332, The PPP Internet Protocol Control Protocol
(IPCP). Network Working Group. May 1992.
Rivest, R. RFC 1321, The MD5 Message-Digest Algorithm. Network Working
Group. April 1992
Simpson, W. RFC 1661, The Point-to-Point Protocol (PPP). Network Work‐
ing Group. July 1994.
Simpson, W. RFC 1662, HDLC-like Framing . Network Working Group. July
1994.
NOTES
These signals affect pppd behavior:
SIGINT, SIGTERM Terminate the link, restore the serial device set‐
tings and exit.
SIGHUP Terminate the link, restore the serial device set‐
tings and close the serial device. If the persist
or demand option is specified, pppd attempts to
reopen the serial device and start another connec‐
tion after the holdoff period. Otherwise pppd
exits. If received during the holdoff period,
SIGHUP causes pppd to end the holdoff period imme‐
diately.
SIGUSR1 Toggles the state of the debug option and prints
link status information to the log.
SIGUSR2 Causes pppd to renegotiate compression. This is
useful to re-enable compression after it has been
disabled as a result of a fatal decompression
error. (Fatal decompression errors generally indi‐
cate a bug in an implementation.)
DIAGNOSTICS
Messages are sent to the syslog daemon using facility LOG_DAEMON. To
see error and debug messages, edit the /etc/syslog.conf file to direct
the messages to the desired output device or file, or use the updetach
or logfile options.
The debug option causes the contents of all LCP, PAP, CHAP or IPCP con‐
trol packets sent or received to be logged. This is useful if PPP nego‐
tiation does not succeed or if authentication fails.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal,
which acts as a toggle to the pppd process.
SunOS 5.11 21 Nov 2001 pppd(1M)
