DragonFly On-Line Manual Pages
PPPD(8) DragonFly System Manager's Manual PPPD(8)
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 exten-
sible Link Control Protocol (LCP), and a family of Network Control Proto-
cols (NCP) for establishing and configuring different network-layer pro-
The encapsulation scheme is provided by driver code in the kernel. pppd
provides the basic LCP, authentication support, and an NCP for establish-
ing and configuring the Internet Protocol (IP) (called the IP Control
FREQUENTLY USED OPTIONS
Communicate over the named device. The string ``/dev/'' is
prepended if necessary. If no device name is given, or if the name
of the terminal connected to the standard input is given, pppd will
use that terminal, and will not fork to put itself in the back-
ground. This option is privileged if the noauth option is used.
Set the baud rate to speed (a decimal number). On systems such as
4.4BSD and NetBSD, any speed can be specified, providing that it is
supported by the serial device driver. Other systems (e.g. SunOS,
Linux) allow only a limited set of speeds.
Specifies a packet filter to be applied to data packets to deter-
mine which packets are to be regarded as link activity, and there-
fore 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 regu-
larly 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 expres-
sion should be enclosed in single-quotes to prevent whitespace in
the expression from being interpreted by the shell. This option is
only available if both the kernel and pppd were compiled with
Set the async character map to map. This map describes which con-
trol 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.
Require the peer to authenticate itself before allowing network
packets to be sent or received.
Read options from the file /etc/ppp/peers/name. This file may con-
tain 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
Use the executable or shell command specified by script to set up
the serial line. This script would typically use the chat(8) pro-
gram to dial the modem and start the remote ppp session. This
option is privileged if the noauth option is used.
Attempt dial-out connection to remote system no more than specified
number of times. The default is 1. If the connection is not made,
pppd will exit. Requires that persist has been specified.
Use hardware flow control (i.e. RTS/CTS) to control the flow of
data on the serial port. If neither the crtscts nor the nocrtscts
option is given, the hardware flow control setting for the serial
port is left unchanged.
Add a default route to the system routing tables, using the peer as
the gateway, when IPCP negotiation is successfully completed. This
entry is removed when the PPP connection is broken. This option is
privileged if the nodefaultroute option has been specified.
Run the executable or shell command specified by script after pppd
has terminated the link. This script could, for example, issue
commands to the modem to cause it to hang up if hardware modem con-
trol signals were not available. The disconnect script is not run
if the modem has already hung up. This option is privileged if the
noauth option is used.
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 to 0x3f or 0x5e.
Read options from file name (the format is described below). The
file must be readable by the user who has invoked pppd.
Specifies that pppd should create a UUCP-style lock file for the
serial device to ensure exclusive access to the device.
Set the Maximum Receive Unit (MRU) value to size. pppd will ask
the peer to send packets of no more than size 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).
Set the Maximum Transmit Unit (MTU) value to size. 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
size bytes through the PPP network interface.
Enables the ``passive'' option in the LCP. With this option, pppd
will attempt to initiate a connection; if no reply is received from
the peer, pppd will then just wait passively for a valid LCP packet
from the peer, instead of exiting, as it would without this option.
Set the local and/or remote interface IP addresses. Either one may
be omitted. The IP addresses can be specified with a host name or
in decimal dot notation (e.g. 126.96.36.199). 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 dif-
ferent value from the peer in the IPCP negotiation, unless the
ipcp-accept-local and/or ipcp-accept-remote options are given,
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 BSD-Compress compression entirely.
If this option is given, pppd will rechallenge the peer every
Set the maximum number of CHAP challenge transmissions to number.
The default is 10.
Set the CHAP restart interval (retransmission timeout for chal-
lenges) to number seconds. The default is 3.
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
Disable asyncmap negotiation, forcing all control characters to be
escaped for both the transmit and the receive direction.
Disable Maximum Receive Unit (MRU) negotiation. With this option,
pppd will use the default MRU value of 1500 bytes for both the
transmit and receive direction.
Request that the peer compress packets that it sends, using the
Deflate scheme, with a maximum window size of 2**nr bytes, and
agree to compress packets sent to the peer with a maximum window
size of 2**nt bytes. If nt is not specified, it defaults to the
value given for nr. Values in the range 8 to 15 may be used for nr
and nt; larger values give better compression but consume more ker-
nel 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-Com-
press if the peer can do either.
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 pack-
ets (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 conjunction with the demand option.
Append the domain name name to the local host name for authentica-
tion 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 privi-
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.
Specifies that pppd should disconnect if the link is idle for
number seconds. The link is idle when no data packets (i.e. IP
packets) are being sent or received. If the active-filter option
is given, data packets which are rejected by the specified activity
filter also count as the link being idle. Note: it is not advis-
able to use this option with the persist option without the demand
With this option, pppd will accept the peer's idea of our local IP
address, even if the local IP address was specified in an option.
With this option, pppd will accept the peer's idea of its (remote)
IP address, even if the remote IP address was specified in an
Set the maximum number of IPCP configure-request transmissions to
number. The default is 10.
Set the maximum number of IPCP configure-NAKs returned before
starting to send configure-Rejects instead to number. The default
Set the maximum number of IPCP terminate-request transmissions to
number. The default is 3.
Set the IPCP restart interval (retransmission timeout) to number
seconds. The default is 3.
Provides an extra parameter to the ip-up and ip-down scripts. If
this option is given, the string supplied is given as the 6th
parameter to those scripts.
Enable the IPXCP and IPX protocols. This option is presently only
supported under Linux, and only if your kernel has been configured
to include IPX support.
Set the IPX network number in the IPXCP configure request frame to
number, a hexadecimal number (without a leading 0x). There is no
valid default. If this option is not specified, the network number
is obtained from the peer. If the peer does not have the network
number, the IPX protocol will not be started.
Set the IPX node numbers. The two node numbers are separated from
each other with a colon character. The first number local is the
local node number. The second number remote is the peer's node
number. Each node number is a hexadecimal number, at most 10 dig-
its 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.
Set the name of the router. This is a string and is sent to the
peer as information data.
Set the routing protocol to be received by this option. More than
one instance of ipx-routing may be specified. The values may be 0
for NONE, 2 for IRIP/SAP, and 4 for NLSP. The NONE option may be
specified as the only instance of ipx-routing.
Accept the peer's NAK for the node number specified in the ipx-node
option. If a node number was specified, and non-zero, the default
is to insist that the value be used. If you include this option
then you will permit the peer to override the entry of the node
Accept the peer's NAK for the network number specified in the ipx-
network option. If a network number was specified, and non-zero,
the default is to insist that the value be used. If you include
this option then you will permit the peer to override the entry of
the node number.
Use the peer's network number specified in the configure request
frame. If a node number was specified for the peer and this option
was not specified, the peer will be forced to use the value which
you have specified.
Set the maximum number of IPXCP configure request frames which the
system will send. The default is 10.
Set the maximum number of IPXCP NAK frames which the local system
will send before it rejects the options. The default is 3.
Set the maximum number of IPXCP terminate request frames before the
local system considers that the peer is not listening to them. The
default is 3.
Enable debugging code in the kernel-level PPP driver. The argument
level is a number which is the sum of the following values: 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 syslogd(8), to a file as directed in
the /etc/syslog.conf configuration file.
If this option is given, pppd will presume the peer to be dead if
number 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.
If this option is given, pppd will send an LCP echo-request frame
to the peer every number 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
Set the maximum number of LCP configure-request transmissions to
number. The default is 10.
Set the maximum number of LCP configure-NAKs returned before start-
ing to send configure-Rejects instead to number. The default is
Set the maximum number of LCP terminate-request transmissions to
number. The default is 3.
Set the LCP restart interval (retransmission timeout) to number
seconds. The default is 3.
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) sig-
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.
Terminate the connection when it has been available for network
traffic for number seconds (i.e. number seconds after the first
network control protocol comes up).
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 Termi-
nal 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.
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. Note: this option was present in
some older versions of pppd under the name dns-addr.
If pppd is acting as a server for Microsoft Windows or Samba
clients, this option allows pppd to supply one or two WINS (Windows
Internet Name Services) server addresses to the clients. The first
instance of this option specifies the primary WINS address; the
second instance (if given) specifies the secondary WINS address.
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 sys-
tem to the peer. Note: pppd does not append the domain name to
Set the interface netmask to mask, 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 appro-
priate network mask for the class of the remote IP address, ORed
with the netmasks for any non point-to-point network interfaces in
the system which are on the same network.
Disable Address/Control compression in both directions (send and
Do not require the peer to authenticate itself. This option is
privileged if the auth option is specified in /etc/ppp/options.
Disables BSD-Compress compression; pppd will not request or agree
to compress packets using the BSD-Compress scheme.
Disable Compression Control Protocol (CCP) negotiation. This
option should only be required if the peer is buggy and gets con-
fused by requests from pppd for CCP negotiation.
Disable hardware flow control (i.e. RTS/CTS) on the serial port.
If neither the crtscts nor the nocrtscts option is given, the hard-
ware flow control setting for the serial port is left unchanged.
Disable the defaultroute option. The system administrator who
wishes to prevent users from creating default routes with pppd can
do so by placing this option in the /etc/ppp/options file.
Disables Deflate compression; pppd will not request or agree to
compress packets using the Deflate scheme.
Don't detach from the controlling terminal. Without this option,
if a serial device other than the terminal on the standard input is
specified, pppd will fork to become a background process.
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.
Disables the default behaviour 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 will have to supply the
local IP address during IPCP negotiation (unless it specified
explicitly on the command line or in an options file).
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.
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.
Disable protocol field compression negotiation in both the receive
and the transmit direction.
Exit once a connection has been made and terminated. This is the
default unless the persist or demand option has been specified.
Do not accept or agree to Predictor-1 compression.
Disable the proxyarp option. The system administrator who wishes
to prevent users from creating proxy ARP entries with pppd can do
so by placing this option in the /etc/ppp/options file.
Disable Van Jacobson style TCP/IP header compression in both the
transmit and the receive direction.
Disable the connection-ID compression option in Van Jacobson style
TCP/IP header compression. With this option, pppd will not omit
the connection-ID byte from Van Jacobson compressed TCP/IP headers,
nor ask the peer to do so.
Indicates that all secrets in the /etc/ppp/pap-secrets file which
are used for checking the identity of the peer are encrypted, and
thus pppd should not accept a password which, before encryption, is
identical to the secret from the /etc/ppp/pap-secrets file.
Set the maximum number of PAP authenticate-request transmissions to
number. The default is 10.
Set the PAP restart interval (retransmission timeout) to number
seconds. The default is 3.
Set the maximum time that pppd will wait for the peer to authenti-
cate itself with PAP to number seconds (0 means no limit).
Specifies a packet filter to applied to data packets being sent or
received to determine which packets should be allowed to pass.
Packets which are rejected by the filter are silently discarded.
This option can be used to prevent specific network daemons (such
as routed) using up link bandwidth, or to provide a basic firewall
capability. The filter-expression syntax is as described for
tcpdump(1), except that qualifiers which are inappropriate for a
PPP link, such as ether and arp, are not permitted. Generally the
filter expression should be enclosed in 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.
Do not exit after a connection is terminated; instead try to reopen
Request that the peer compress frames that it sends using Predic-
tor-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.
Add an entry to this system's Address Resolution Protocol (ARP) ta-
ble 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.
Set the assumed name of the remote system for authentication pur-
poses to name.
With this option, pppd will not agree to authenticate itself to the
peer using CHAP.
With this option, pppd will not agree to authenticate itself to the
peer using PAP.
Require the peer to authenticate itself using Challenge Handshake
Authentication Protocol (CHAP) authentication.
Require the peer to authenticate itself using Password Authentica-
tion Protocol (PAP) authentication.
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).
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).
Sets the name used for authenticating the local system to the peer
Sets the number of connection slots to be used by the Van Jacobson
TCP/IP header compression and decompression code to number, which
must be between 2 and 16 inclusively.
Run the executable or shell command specified by script before ini-
tiating PPP negotiation, after the connect script (if any) has com-
pleted. This option is privileged if the noauth option is used.
Use software flow control (i.e. XON/XOFF) to control the flow of
data on the serial port.
The following signals have the specified effect when sent to pppd:
SIGINT, SIGTERM These signals cause pppd to terminate the link (by
closing LCP), restore the serial device settings,
SIGHUP This signal causes pppd to terminate the link,
restore the serial device settings, and close the
serial device. If the persist or demand option
has been specified, pppd will try to reopen the
serial device and start another connection (after
the holdoff period). Otherwise pppd will exit.
If this signal is received during the holdoff
period, it causes pppd to end the holdoff period
SIGUSR1 This signal toggles the state of the debug option.
SIGUSR2 This signal causes pppd to renegotiate compres-
sion. This can be useful to re-enable compression
after it has been disabled as a result of a fatal
decompression error. (Fatal decompression errors
generally indicate a bug in one or other implemen-
pppd provides system administrators with sufficient access control that
PPP access to a server machine can be provided to legitimate users with-
out fear of compromising the security of the server or the network it's
on. In part this is provided by the /etc/ppp/options file, where the
administrator can place options to restrict the ways in which pppd can be
used, and in part by the PAP and CHAP secrets files, where the adminis-
trator can restrict the set of IP addresses which individual users may
The normal way that pppd should be set up is to have the auth option in
the /etc/ppp/options file. (This may become the default in later
releases.) If users wish to use pppd to dial out to a peer which will
refuse to authenticate itself (such as an internet service provider), the
system administrator should create an options file under /etc/ppp/peers
containing the noauth option, the name of the serial port to use, and the
connect option (if required), plus any other appropriate options. In
this way, pppd can be set up to allow non-privileged users to make unau-
thenticated connections only to trusted peers.
As indicated above, some security-sensitive options are privileged, which
means that they may not be used by an ordinary non-privileged user run-
ning 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. Privi-
leged 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, priv-
ileged options can be used without restriction.
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 cleart-
ext 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 differ-
ent 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
Each line in a secrets file contains one secret. A given secret is spe-
cific 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 connect-
ing to the specified server.
A secrets file is parsed into words as for an 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. Any fol-
lowing words on the same line are taken to be a list of acceptable IP
addresses for that client, or an override for local:remote addresses (the
same format used on the command line or in the options file) when on a
line that contains a specific client name (not a wildcard nor empty). 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 number, to
indicate a whole subnet, i.e. all addresses which have the same value in
the most significant number bits. 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.
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 remotename option.
Otherwise, if the remote IP address was specified by a name (rather than
in numeric form), that name will be used as the peer's name. Failing
that, pppd will use the null string as the 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(3) 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 sup-
plied by the peer. This avoids the need to have the same secret in two
Additional checks are performed when the login option is used. If the
file /etc/ppp/ppp.deny exists, and the user is listed in it, the authen-
tication fails. If the file /etc/ppp/ppp.shells exists and the user's
normal login shell is not listed, the authentication fails.
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 authenti-
cate 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 pap-secrets
file which specifies the empty string for the client and password, it is
possible to allow restricted access to hosts which refuse to authenticate
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 fur-
ther intervention is required. The /etc/ppp/ip-up script can be used for
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
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 net-
work interface on the same subnet as the remote host (an interface sup-
porting broadcast and ARP, which is up and not a point-to-point or loop-
back interface). If found, pppd creates a permanent, published ARP entry
with the IP address of the remote host and the hardware address of the
network interface found.
When the demand option is used, the interface IP addresses have already
been set at the point when IPCP comes up. If pppd has not been able to
negotiate the same addresses that it used to configure the interface (for
example when the peer is an ISP that uses dynamic IP address assignment),
pppd has to change the interface IP addresses to the negotiated
addresses. This may disrupt existing connections, and the use of demand
dialling with peers that do dynamic IP address assignment is not recom-
The environment variables that pppd sets are:
DEVICE The name of the serial tty device being used.
IFNAME The name of the network interface being used.
IPLOCAL The IP address for the local end of the link. This is only set
when IPCP has come up.
IPREMOTE The IP address for the remote end of the link. This is only
set when IPCP has come up.
PEERNAME The authenticated name of the peer. This is only set if the
peer authenticates itself.
SPEED The baud rate of the tty device.
UID The real user-id of the user who invoked pppd.
pppd reads options from the files /etc/ppp/options, ~/.ppprc and
/etc/ppp/options.tty_name (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.tty_name file is read.)
An options file is parsed into a series of words, delimited by white-
space. 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
/var/run/pppunit.pid Process-ID for pppd process on ppp interface
/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. pppd will log a warning if this is not
/etc/ppp/chap-secrets Names, secrets and IP addresses for CHAP
authentication. As for /etc/ppp/pap-secrets,
this file should be owned by root and not
readable or writable by any other user. pppd
will log a warning if this is not the case.
/etc/ppp/options System default options for pppd, read before
user default options or command-line options.
~/.ppprc User default options, read before
/etc/ppp/options.tty_name System default options for the serial port
being used, read after ~/.ppprc. In forming
the tty_name part of this filename, an initial
``/dev/'' is stripped from the port name (if
present), and any slashes in the remaining
part are converted to dots.
/etc/ppp/peers A directory containing options files which may
contain privileged options, even if pppd was
invoked by a user other than root. The system
administrator can create options files in this
directory to permit non-privileged users to
dial out without requiring the peer to authen-
ticate, but only to certain trusted peers.
/etc/ppp/ppp.deny Lists users who may not use the system pass-
word PAP authentication.
/etc/ppp/ppp.shells Lists user shells which are approved for sys-
tem password PAP authentication logins.
/usr/share/examples/pppd/ Sample pppd configuration files.
pppd also invokes scripts at various stages in its processing which can
be used to perform site-specific ancillary processing. These scripts are
usually shell scripts, but could be executable code files instead. pppd
does not wait for the scripts to finish. The scripts are executed as
root (with the real and effective user-id set to 0), so that they can do
things such as update routing tables or run privileged daemons. Be care-
ful that the contents of these scripts do not compromise your system's
security. pppd runs the scripts with standard input, output and error
redirected to /dev/null, and with an environment that is empty except for
some environment variables that give information about the link. pppd
invokes the following scripts, if they exist. It is not an error if they
/etc/ppp/auth-up A program or script which is executed after the
remote system successfully authenticates itself. It
is executed with the following parameters:
iface-name peer-name user-name tty-dev speed
Note that this script is not executed if the peer
doesn't authenticate itself, for example when the
noauth option is used.
/etc/ppp/auth-down A program or script which is 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 which is executed when the link
is available for sending and receiving IP packets
(that is, IPCP has come up). It is executed with the
iface-name tty-dev speed local-IP remote-IP 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.
/etc/ppp/ipx-up A program or script which is executed when the link
is available for sending and receiving IPX packets
(that is, IPXCP has come up). It is executed with
the following parameters:
iface-name tty-dev speed network-number
local-IPX-router-name remote-IPX-router-name ipparam
The local-IPX-routing-protocol and remote-IPX-rout-
ing-protocol field may be one of the following:
``NONE'' to indicate that there is no routing proto-
``RIP'' to indicate that RIP/SAP should be used
``NLSP'' to indicate that Novell NLSP should be used
``RIP NLSP'' to indicate that both RIP/SAP and NLSP
should be used
/etc/ppp/ipx-down A program or script which is executed when the link
is no longer available for sending and receiving IPX
packets. This script can be used for undoing the
effects of the /etc/ppp/ipx-up script. It is invoked
in the same manner and with the same parameters.
The following examples assume that the /etc/ppp/options file contains the
auth option (as in the default /etc/ppp/options file in the ppp distribu-
Probably the most common use of pppd is to dial out to an ISP. This can
be done with a command such as
# pppd call isp
where the /etc/ppp/peers/isp file is set up by the system administrator
to contain something like this:
ttyS0 19200 crtscts
connect '/usr/sbin/chat -v -f /etc/ppp/chat-isp'
In this example, we are using chat to dial the ISP's modem and go through
any logon sequence required. The /etc/ppp/chat-isp file contains the
script used by chat; it could for example contain something like this:
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "NO ANSWER"
ABORT "Username/Password Incorrect"
See the chat(8) man page for details of chat scripts.
pppd can also be used to provide a dial-in ppp service for users. If the
users already have login accounts, the simplest way to set up the ppp
service is to let the users log in to their accounts and run pppd
(installed setuid-root) with a command such as
# pppd proxyarp
To allow a user to use the PPP facilities, you need to allocate an IP
address for that user's machine and create an entry in
/etc/ppp/pap-secrets or /etc/ppp/chap-secrets (depending on which authen-
tication method the PPP implementation on the user's machine supports),
so that the user's machine can authenticate itself. For example, if Joe
has a machine called joespc which is to be allowed to dial in to the
machine called server and use the IP address joespc.my.net, you would add
an entry like this to /etc/ppp/pap-secrets or /etc/ppp/chap-secrets:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create a username called (for example) ppp, whose
login shell is pppd and whose home directory is /etc/ppp. Options to be
used when pppd is run this way can be put in /etc/ppp/.ppprc.
If your serial connection is any more complicated than a piece of wire,
you may need to arrange for some control characters to be escaped. In
particular, it is often useful to escape XON (^Q) and XOFF (^S), using
asyncmap a0000. If the path includes a telnet, you probably should
escape ^] as well (asyncmap 200a0000). If the path includes an rlogin,
you will need to use the escape ff option on the end which is running the
rlogin client, since many rlogin implementations are not transparent;
they will remove the sequence [0xff, 0xff, 0x73, 0x73, followed by any 8
bytes] from the stream.
Messages are sent to the syslog daemon using facility LOG_DAEMON. (This
can be overridden by recompiling pppd with the macro LOG_PPP defined as
the desired facility.) In order to see the error and debug messages, you
will need to edit your /etc/syslog.conf file to direct the messages to
the desired output device or file.
The debug option causes the contents of all control packets sent or
received to be logged, that is, all LCP, PAP, CHAP or IPCP packets. This
can be useful if the PPP negotiation does not succeed or if authentica-
tion fails. If debugging is enabled at compile time, the debug option
also causes other debugging messages to be logged.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal to
the pppd process. This signal acts as a toggle.
tcpdump(1), crypt(3), syslog.conf(5), chat(8), ppp(8), syslogd(8)
V. Jacobson, Compressing TCP/IP headers for low-speed serial links, RFC
1144, February 1990.
R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321, April 1992.
G. McGregor, PPP Internet Protocol Control Protocol (IPCP), RFC 1332, May
B. Lloyd and W.A. Simpson, PPP authentication protocols, RFC 1334,
W.A. Simpson, The Point-to-Point Protocol (PPP), RFC 1661, July 1994.
W.A. Simpson, PPP in HDLC-like Framing, RFC 1662, July 1994.
pppd was written by Paul Mackerras <Paul.Mackerras@cs.anu.edu.au>, based
on earlier work by Drew Perkins, Brad Clements, Karl Fox, Greg Christy,
and Brad Parker.
DragonFly 4.1 August 31, 2013 DragonFly 4.1