PPPD(8) PPPD(8)
NAME
pppd - Point to Point Protocol daemon
SYNOPSIS
pppd [ 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 parts: a method for encapsulating
datagrams over serial links, an extensible Link Control
Protocol (LCP), and a family of Network Control Protocols
(NCP) for establishing and configuring different network-
layer protocols.
The encapsulation scheme is provided by driver code in the
kernel. Pppd provides the basic LCP, authentication sup-
port, and an NCP for establishing and configuring the
Internet Protocol (IP) (called the IP Control Protocol,
IPCP).
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 con-
nected to the standard input is given, pppd will
use that terminal, and will not fork to put itself
in the background. This option is privileged if
the noauth option is used.
<speed>
Set the baud rate to <speed> (a decimal number).
On systems such as 4.4BSD and OpenBSD, any speed
can be specified. Other systems (e.g. SunOS) allow
only a limited set of speeds.
active-filter filter-expression
Specifies a packet filter to be applied to data
packets to determine which packets are to be
regarded as link activity, and therefore reset the
idle timer, or cause the link to be brought up in
demand-dialling mode. This option is useful in
conjunction with the idle option if there are pack-
ets being sent or received regularly over the link
(for example, routing information packets) which
would otherwise prevent the link from ever appear-
ing to be idle. The filter-expression syntax is as
described for tcpdump(1), except that qualifiers
which are inappropriate for a PPP link, such as
ether and arp, are not permitted. Generally the
filter expression should be enclosed in single-
quotes to prevent whitespace in the expression from
being interpreted by the shell. This option is
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PPPD(8) PPPD(8)
currently only available under OpenBSD, and then
only if both the kernel and pppd were compiled with
PPP_FILTER defined.
asyncmap <map>
Set the async character map to <map>. This map
describes which control characters cannot be suc-
cessfully 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 num-
ber with each bit representing a character to
escape. Bit 0 (00000001) represents the character
0x00; bit 31 (80000000) represents the character
0x1f or ^_. If multiple asyncmap options are
given, the values are ORed together. If no
asyncmap option is given, no async character map
will be negotiated for the receive direction; the
peer should then escape all control characters. To
escape transmitted characters, use the escape
option.
auth Require the peer to authenticate itself before
allowing network packets to be sent or received.
call name
Read options from the file /etc/ppp/peers/name.
This file may contain privileged options, such as
noauth, even if pppd is not being run by root. The
name string may not begin with / or include .. as a
pathname component. The format of the options file
is described below.
connect script
Use the executable or shell command specified by
script to set up the serial line. This script
would typically use the chat(8) program to dial the
modem and start the remote ppp session. This
option is privileged if the noauth option is used.
crtscts
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.
defaultroute
Add a default route to the system routing tables,
using the peer as the gateway, when IPCP negotia-
tion is successfully completed. This entry is
removed when the PPP connection is broken. This
option is privileged if the nodefaultroute option
has been specified.
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PPPD(8) PPPD(8)
disconnect script
Run the executable or shell command specified by
script after pppd has terminated the link. This
script could, for example, issue commands to the
modem to cause it to hang up if hardware modem 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.
escape xx,yy,...
Specifies that certain characters should be escaped
on transmission (regardless of whether the peer
requests them to be escaped with its async control
character map). The characters to be escaped are
specified as a list of hex numbers separated by
commas. Note that almost any character can be
specified for the escape option, unlike the
asyncmap option which only allows control charac-
ters to be specified. The characters which may not
be escaped are those with hex values 0x20 - 0x3f or
0x5e.
file name
Read options from file name (the format is
described below). The file must be readable by the
user who has invoked pppd.
lock Specifies that pppd should create a UUCP-style lock
file for the serial device to ensure exclusive
access to the device.
mru n Set the MRU [Maximum Receive Unit] value to n. Pppd
will ask the peer to send packets of no more than n
bytes. The minimum MRU value is 128. The default
MRU value is 1500. A value of 296 is recommended
for slow links (40 bytes for TCP/IP header + 256
bytes of data).
mtu n Set the MTU [Maximum Transmit Unit] value to n.
Unless the peer requests a smaller value via MRU
negotiation, pppd will request that the kernel net-
working code send data packets of no more than n
bytes through the PPP network interface.
passive
Enables the "passive" option in the LCP. With this
option, pppd will attempt to initiate a connection;
if no reply is received from the peer, pppd will
then just wait passively for a valid LCP packet
from the peer, instead of exiting, as it would
without this option.
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PPPD(8) PPPD(8)
OPTIONS
<local_IP_address>:<remote_IP_address>
Set the local and/or remote interface IP addresses.
Either one may be omitted. The IP addresses can be
specified with a host name or in decimal dot nota-
tion (e.g. 150.234.56.78). The default local
address is the (first) IP address of the system
(unless the noipdefault option is given). The
remote address will be obtained from the peer if
not specified in any option. Thus, in simple
cases, this option is not required. If a local
and/or remote IP address is specified with this
option, pppd will not accept a different value from
the peer in the IPCP negotiation, unless the ipcp-
accept-local and/or ipcp-accept-remote options are
given, respectively.
bsdcomp nr,nt
Request that the peer compress packets that it
sends, using the BSD-Compress scheme, with a maxi-
mum 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.
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 transmis-
sions to n (default 10).
chap-restart n
Set the CHAP restart interval (retransmission time-
out for challenges) to n seconds (default 3).
debug Enables connection debugging facilities. If this
option is given, pppd will log the contents of all
control packets sent or received in a readable
form. The packets are logged through syslog with
facility daemon and level debug. This information
can be directed to a file by setting up /etc/sys-
log.conf appropriately (see syslog.conf(5)).
default-asyncmap
Disable asyncmap negotiation, forcing all control
characters to be escaped for both the transmit and
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PPPD(8) PPPD(8)
the receive direction.
default-mru
Disable MRU [Maximum Receive Unit] negotiation.
With this option, pppd will use the default MRU
value of 1500 bytes for both the transmit and
receive direction.
deflate nr,nt
Request that the peer compress packets that it
sends, using the Deflate scheme, with a maximum
window size of 2**nr bytes, and agree to compress
packets sent to the peer with a maximum window size
of 2**nt bytes. If nt is not specified, it
defaults to the value given for nr. Values in the
range 8 to 15 may be used for nr and nt; larger
values give better compression but consume more
kernel memory for compression dictionaries. Alter-
natively, a value of 0 for nr or nt disables com-
pression 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.)
demand Initiate the link only on demand, i.e. when data
traffic is present. With this option, the remote
IP address must be specified by the user on the
command line or in an options file. Pppd will ini-
tially configure the interface and enable it for IP
traffic without connecting to the peer. When traf-
fic is available, pppd will connect to the peer and
perform negotiation, authentication, etc. When
this is completed, pppd will commence passing data
packets (i.e., IP packets) across the link.
The demand option implies the persist option. If
this behaviour is not desired, use the nopersist
option after the demand option. The idle and hold-
off options are also useful in conjuction with the
demand option.
domain d
Append the domain name d to the local host name for
authentication purposes. For example, if gethost-
name() returns the name porsche, but the fully
qualified domain name is porsche.Quotron.COM, you
could specify domain Quotron.COM. Pppd would then
use the name porsche.Quotron.COM for looking up
secrets in the secrets file, and as the default
name to send to the peer when authenticating itself
to the peer. This option is privileged.
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PPPD(8) PPPD(8)
holdoff n
Specifies how many seconds to wait before re-initi-
ating the link after it terminates. This option
only has any effect if the persist or demand option
is used. The holdoff period is not applied if the
link was terminated because it was idle.
idle n Specifies that pppd should disconnect if the link
is idle for n seconds. The link is idle when no
data packets (i.e. IP packets) are being sent or
received. Note: it is not advisable to use this
option with the persist option without the demand
option. If the active-filter option is given, data
packets which are rejected by the specified activ-
ity filter also count as the link being idle.
ipcp-accept-local
With this option, pppd will accept the peer's idea
of our local IP address, even if the local IP
address was specified in an option.
ipcp-accept-remote
With this option, pppd will accept the peer's idea
of its (remote) IP address, even if the remote IP
address was specified in an option.
ipcp-max-configure n
Set the maximum number of IPCP configure-request
transmissions to n (default 10).
ipcp-max-failure n
Set the maximum number of IPCP configure-NAKs
returned before starting to send configure-Rejects
instead to n (default 10).
ipcp-max-terminate n
Set the maximum number of IPCP terminate-request
transmissions to n (default 3).
ipcp-restart n
Set the IPCP restart interval (retransmission time-
out) to n seconds (default 3).
ipparam string
Provides an extra parameter to the ip-up and ip-
down scripts. If this option is given, the string
supplied is given as the 6th parameter to those
scripts.
ipx Enable the IPXCP and IPX protocols. This option is
presently only supported under Linux, and only if
your kernel has been configured to include IPX sup-
port.
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PPPD(8) PPPD(8)
ipx-network n
Set the IPX network number in the IPXCP configure
request frame to n, a hexadecimal number (without a
leading 0x). There is no valid default. If this
option is not specified, the network number is
obtained from the peer. If the peer does not have
the network number, the IPX protocol will not be
started.
ipx-node n:m
Set the IPX node numbers. The two node numbers are
separated from each other with a colon character.
The first number n is the local node number. The
second number m is the peer's node number. Each
node number is a hexadecimal number, at most 10
digits long. The node numbers on the ipx-network
must be unique. There is no valid default. If this
option is not specified then the node numbers are
obtained from the peer.
ipx-router-name <string>
Set the name of the router. This is a string and is
sent to the peer as information data.
ipx-routing n
Set the routing protocol to be received by this
option. More than one instance of ipx-routing may
be specified. The 'none' option (0) may be speci-
fied as the only instance of ipx-routing. The val-
ues may be 0 for NONE, 2 for RIP/SAP, and 4 for
NLSP.
ipxcp-accept-local
Accept the peer's NAK for the node number specified
in the ipx-node option. If a node number was speci-
fied, and non-zero, the default is to insist that
the value be used. If you include this option then
you will permit the peer to override the entry of
the node number.
ipxcp-accept-network
Accept the peer's NAK for the network number speci-
fied 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.
ipxcp-accept-remote
Use the peer's network number specified in the con-
figure request frame. If a node number was speci-
fied for the peer and this option was not speci-
fied, the peer will be forced to use the value
which you have specified.
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PPPD(8) PPPD(8)
ipxcp-max-configure n
Set the maximum number of IPXCP configure request
frames which the system will send to n. The default
is 10.
ipxcp-max-failure n
Set the maximum number of IPXCP NAK frames which
the local system will send before it rejects the
options. The default value is 3.
ipxcp-max-terminate n
Set the maximum nuber of IPXCP terminate request
frames before the local system considers that the
peer is not listening to them. The default value is
3.
kdebug n
Enable debugging code in the kernel-level PPP
driver. The argument n 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 syslog(1) to a file as directed in the
/etc/syslog.conf configuration file.
lcp-echo-failure n
If this option is given, pppd will presume the peer
to be dead if n LCP echo-requests are sent without
receiving a valid LCP echo-reply. If this happens,
pppd will terminate the connection. Use of this
option requires a non-zero value for the lcp-echo-
interval parameter. This option can be used to
enable pppd to terminate after the physical connec-
tion has been broken (e.g., the modem has hung up)
in situations where no hardware modem control lines
are available.
lcp-echo-interval n
If this option is given, pppd will send an LCP
echo-request frame to the peer every n seconds.
Normally the peer should respond to the echo-
request by sending an echo-reply. This option can
be used with the lcp-echo-failure option to detect
that the peer is no longer connected.
lcp-max-configure n
Set the maximum number of LCP configure-request
transmissions to n (default 10).
lcp-max-failure n
Set the maximum number of LCP configure-NAKs
returned before starting to send configure-Rejects
8
PPPD(8) PPPD(8)
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 time-
out) to n seconds (default 3).
local Don't use the modem control lines. With this
option, pppd will ignore the state of the CD (Car-
rier Detect) signal from the modem and will not
change the state of the DTR (Data Terminal Ready)
signal.
login Use the system password database for authenticating
the peer using PAP, and record the user in the sys-
tem wtmp file. Note that the peer must have an
entry in the /etc/ppp/pap-secrets file as well as
the system password database to be allowed access.
maxconnect n
Terminate the connection when it has been available
for network traffic for n seconds (i.e. n seconds
after the first network control protocol comes up).
modem Use the modem control lines. This option is the
default. With this option, pppd will wait for the
CD (Carrier Detect) signal from the modem to be
asserted when opening the serial device (unless a
connect script is specified), and it will drop the
DTR (Data Terminal Ready) signal briefly when the
connection is terminated and before executing the
connect script. On Ultrix, this option implies
hardware flow control, as for the crtscts option.
modem_chat
Use the modem control lines during the chat script.
The default is to ignore the state of the CD (Car-
rier Detect) signal from the modem during the chat
script. If you are using a cua device (as opposed
to a tty device) you should set this option.
ms-dns <addr>
If pppd is acting as a server for Microsoft Windows
clients, this option allows pppd to supply one or
two DNS (Domain Name Server) addresses to the
clients. The first instance of this option speci-
fies the primary DNS address; the second instance
(if given) specifies the secondary DNS address.
(This option was present in some older versions of
pppd under the name dns-addr.)
9
PPPD(8) PPPD(8)
ms-wins <addr>
If pppd is acting as a server for Microsoft Windows
or "Samba" clients, this option allows pppd to sup-
ply one or two WINS (Windows Internet Name Ser-
vices) 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.
name name
Set the name of the local system for authentication
purposes to name. This is a privileged option.
With this option, pppd will use lines in the
secrets files which have name as the second field
when looking for a secret to use in authenticating
the peer. In addition, unless overridden with the
user option, name will be used as the name to send
to the peer when authenticating the local system to
the peer. (Note that pppd does not append the
domain name to name.)
netmask n
Set the interface netmask to n, a 32 bit netmask in
"decimal dot" notation (e.g. 255.255.255.0). If
this option is given, the value specified is ORed
with the default netmask. The default netmask is
chosen based on the negotiated remote IP address;
it is the appropriate network mask for the class of
the remote IP address, ORed with the netmasks for
any non point-to-point network interfaces in the
system which are on the same network.
noaccomp
Disable Address/Control compression in both direc-
tions (send and receive).
noauth Do not require the peer to authenticate itself.
This option is privileged if the auth option is
specified in /etc/ppp/options.
nobsdcomp
Disables BSD-Compress compression; pppd will not
request or agree to compress packets using the BSD-
Compress scheme.
noccp Disable CCP (Compression Control Protocol) negotia-
tion. This option should only be required if the
peer is buggy and gets confused by requests from
pppd for CCP negotiation.
nocrtscts
Disable hardware flow control (i.e. RTS/CTS) on the
serial port. If neither the crtscts nor the
nocrtscts option is given, the hardware flow
10
PPPD(8) PPPD(8)
control setting for the serial port is left
unchanged.
nodefaultroute
Disable the defaultroute option. The system admin-
istrator who wishes to prevent users from creating
default routes with pppd can do so by placing this
option in the /etc/ppp/options file.
nodeflate
Disables Deflate compression; pppd will not request
or agree to compress packets using the Deflate
scheme.
nodetach
Don't detach from the controlling terminal. With-
out this option, if a serial device other than the
terminal on the standard input is specified, pppd
will fork to become a background process.
noip Disable IPCP negotiation and IP communication.
This option should only be required if the peer is
buggy and gets confused by requests from pppd for
IPCP negotiation.
noipdefault
Disables the default behaviour when no local IP
address is specified, which is to determine (if
possible) the local IP address from the hostname.
With this option, the peer will have to supply the
local IP address during IPCP negotiation (unless it
specified explicitly on the command line or in an
options file).
noipx Disable the IPXCP and IPX protocols. This option
should only be required if the peer is buggy and
gets confused by requests from pppd for IPXCP nego-
tiation.
nomagic
Disable magic number negotiation. With this
option, pppd cannot detect a looped-back line.
This option should only be needed if the peer is
buggy.
nopcomp
Disable protocol field compression negotiation in
both the receive and the transmit direction.
nopersist
Exit once a connection has been made and termi-
nated. This is the default unless the persist or
demand option has been specified.
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PPPD(8) PPPD(8)
nopredictor1
Do not accept or agree to Predictor-1 comprssion.
noproxyarp
Disable the proxyarp option. The system adminis-
trator 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.
novj Disable Van Jacobson style TCP/IP header compres-
sion in both the transmit and the receive direc-
tion.
novjccomp
Disable the connection-ID compression option in Van
Jacobson style TCP/IP header compression. With
this option, pppd will not omit the connection-ID
byte from Van Jacobson compressed TCP/IP headers,
nor ask the peer to do so.
papcrypt
Indicates that all secrets in the /etc/ppp/pap-
secrets file which are used for checking the iden-
tity of the peer are encrypted, and thus pppd
should not accept a password which, before encryp-
tion, is identical to the secret from the
/etc/ppp/pap-secrets file.
pap-max-authreq n
Set the maximum number of PAP authenticate-request
transmissions to n (default 10).
pap-restart n
Set the PAP restart interval (retransmission time-
out) to n seconds (default 3).
pap-timeout n
Set the maximum time that pppd will wait for the
peer to authenticate itself with PAP to n seconds
(0 means no limit).
pass-filter filter-expression
Specifies a packet filter to applied to data pack-
ets being sent or received to determine which pack-
ets 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 tcp-
dump(1), except that qualifiers which are inappro-
priate for a PPP link, such as ether and arp, are
not permitted. Generally the filter expression
should be enclosed in single-quotes to prevent
12
PPPD(8) PPPD(8)
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
OpenBSD, and then only if both the kernel and pppd
were compiled with PPP_FILTER defined.
persist
Do not exit after a connection is terminated;
instead try to reopen the connection.
predictor1
Request that the peer compress frames that it sends
using Predictor-1 compression, and agree to com-
press transmitted frames with Predictor-1 if
requested. This option has no effect unless the
kernel driver supports Predictor-1 compression.
proxyarp
Add an entry to this system's ARP [Address Resolu-
tion Protocol] table with the IP address of the
peer and the Ethernet address of this system. This
will have the effect of making the peer appear to
other systems to be on the local ethernet.
remotename name
Set the assumed name of the remote system for
authentication purposes to name.
refuse-chap
With this option, pppd will not agree to authenti-
cate itself to the peer using CHAP.
refuse-pap
With this option, pppd will not agree to authenti-
cate itself to the peer using PAP.
require-chap
Require the peer to authenticate itself using CHAP
[Challenge Handshake Authentication Protocol]
authentication.
require-pap
Require the peer to authenticate itself using PAP
[Password Authentication Protocol] authentication.
silent With this option, pppd will not transmit LCP pack-
ets to initiate a connection until a valid LCP
packet is received from the peer (as for the `pas-
sive' option with ancient versions of pppd).
usehostname
Enforce the use of the hostname (with domain name
13
PPPD(8) PPPD(8)
appended, if given) as the name of the local system
for authentication purposes (overrides the name
option).
user name
Sets the name used for authenticating the local
system to the peer to name.
vj-max-slots n
Sets the number of connection slots to be used by
the Van Jacobson TCP/IP header compression and
decompression code to n, which must be between 2
and 16 (inclusive).
welcome script
Run the executable or shell command specified by
script before initiating PPP negotiation, after the
connect script (if any) has completed. This option
is privileged if the noauth option is used.
xonxoff
Use software flow control (i.e. XON/XOFF) to con-
trol the flow of data on the serial port.
OPTIONS FILES
Options can be taken from files as well as the command
line. Pppd reads options from the files /etc/ppp/options,
~/.ppprc and /etc/ppp/options.ttyname (in that order)
before processing the options on the command line. (In
fact, the command-line options are scanned to find the
terminal name before the options.ttyname file is read.)
In forming the name of the options.ttyname file, the ini-
tial /dev/ is removed from the terminal name, and any
remaining / characters are replaced with dots.
An options file is parsed into a series of words, delim-
ited by whitespace. Whitespace can be included in a word
by enclosing the word in double-quotes ("). A backslash
(\) quotes the following character. A hash (#) starts a
comment, which continues until the end of the line. There
is no restriction on using the file or call options within
an options file.
SECURITY
pppd provides system administrators with sufficient access
control that PPP access to a server machine can be pro-
vided to legitimate users without 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 administrator can restrict the
set of IP addresses which individual users may use.
14
PPPD(8) PPPD(8)
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 con-
nect option (if required), plus any other appropriate
options. In this way, pppd can be set up to allow non-
privileged users to make unauthenticated 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 running a setuid-root pppd,
either on the command line, in the user's ~/.ppprc file,
or in an options file read using the file option. 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, privileged options can be used
without restriction.
AUTHENTICATION
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 authentica-
tion 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 Chal-
lenge Handshake Authentication Protocol (CHAP). PAP
involves the client sending its name and a cleartext pass-
word 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
15
PPPD(8) PPPD(8)
occur. The two exchanges could use different authentica-
tion protocols, and in principle, different names could be
used in the two exchanges.
The default behaviour of pppd is to agree to authenticate
if requested, and to not require authentication from the
peer. However, pppd will not agree to authenticate itself
with a particular protocol if it has no secrets which
could be used to do so.
Pppd stores secrets for use in authentication in secrets
files (/etc/ppp/pap-secrets for PAP, /etc/ppp/chap-secrets
for CHAP). Both secrets files have the same format. The
secrets files can contain secrets for pppd to use in
authenticating itself to other systems, as well as secrets
for pppd to use when authenticating other systems to
itself.
Each line in a secrets file contains one secret. A given
secret is specific to a particular combination of client
and server - it can only be used by that client to authen-
ticate itself to that server. Thus each line in a secrets
file has at least 3 fields: the name of the client, the
name of the server, and the secret. These fields may be
followed by a list of the IP addresses that the specified
client may use when connecting to the specified server.
A secrets file is parsed into words as for a options file,
so the client name, server name and secrets fields must
each be one word, with any embedded spaces or other spe-
cial characters quoted or escaped. Any following words on
the same line are taken to be a list of acceptable IP
addresses for that client. If there are only 3 words on
the line, or if the first word is "-", then all IP
addresses are disallowed. To allow any address, use "*".
A word starting with "!" indicates that the specified
address is not acceptable. An address may be followed by
"/" and a number n, to indicate a whole subnet, i.e. all
addresses which have the same value in the most signifi-
cant n 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 authen-
ticating 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
16
PPPD(8) PPPD(8)
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 pass-
word is first compared with the secret from the secrets
file. If the password doesn't match the secret, the pass-
word is encrypted using crypt() and checked against the
secret again. Thus secrets for authenticating the peer
can be stored in encrypted form if desired. If the
papcrypt option is given, the first (unencrypted) compari-
son is omitted, for better security.
Furthermore, if the login option was specified, the user-
name 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 cer-
tain users, and to restrict the set of IP addresses that
each user can use. Typically, when using the login
option, the secret in /etc/ppp/pap-secrets would be "",
which will match any password supplied by the peer. This
avoids the need to have the same secret in two places.
Authentication must be satisfactorily completed before
IPCP (or any other Network Control Protocol) can be
started. If the peer is required to authenticate itself,
and fails to do so, pppd will terminated the link (by
closing LCP). If IPCP negotiates an unacceptable IP
address for the remote host, IPCP will be closed. IP
packets can only be sent or received when IPCP is open.
In some cases it is desirable to allow some hosts which
can't authenticate themselves to connect and use one of a
restricted set of IP addresses, even when the local host
17
PPPD(8) PPPD(8)
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 authenti-
cate themselves.
ROUTING
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 rout-
ing tables and/or ARP (Address Resolution Protocol)
tables. In most cases the defaultroute and/or proxyarp
options are sufficient for this, but in some cases further
intervention is required. The /etc/ppp/ip-up script can
be used for this.
Sometimes it is desirable to add a default route through
the remote host, as in the case of a machine whose only
connection to the Internet is through the ppp interface.
The defaultroute option causes pppd to create such a
default route when IPCP comes up, and delete it when the
link is terminated.
In some cases it is desirable to use proxy ARP, for exam-
ple on a server machine connected to a LAN, in order to
allow other hosts to communicate with the remote host.
The proxyarp option causes pppd to look for a network
interface on the same subnet as the remote host (an inter-
face supporting broadcast and ARP, which is up and not a
point-to-point or loopback interface). If found, pppd
creates a permanent, published ARP entry with the IP
address of the remote host and the hardware address of the
network interface found.
When the demand option is used, the interface IP addresses
have already been set at the point when IPCP comes up. If
pppd has not been able to negotiate the same addresses
that it used to configure the interface (for example when
the peer is an ISP that uses dynamic IP address assign-
ment), pppd has to change the interface IP addresses to
the negotiated addresses. This may disrupt existing con-
nections, and the use of demand dialling with peers that
do dynamic IP address assignment is not recommended.
EXAMPLES
The following examples assume that the /etc/ppp/options
file contains the auth option (as in the default
/etc/ppp/options file in the ppp distribution).
18
PPPD(8) PPPD(8)
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'
noauth
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 "ERROR"
ABORT "NO ANSWER"
ABORT "BUSY"
ABORT "Username/Password Incorrect"
"" "at"
OK "at&d0&c1"
OK "atdt2468135"
"name:" "^Umyuserid"
"word:" "\qmypassword"
"ispts" "\q^Uppp"
"~-^Uppp-~"
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 sim-
plest 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 authentication method the PPP imple-
mentation 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
19
PPPD(8) PPPD(8)
Alternatively, you can create a username called (for exam-
ple) "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 use-
ful 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.
DIAGNOSTICS
Messages are sent to the syslog daemon using facility
LOG_DAEMON. (This can be overriden by recompiling pppd
with the macro LOG_PPP defined as the desired facility.)
In order to see the error and debug messages, you will
need to edit your /etc/syslog.conf file to direct the mes-
sages to the desired output device or file.
The debug option causes the contents of all control pack-
ets sent or received to be logged, that is, all LCP, PAP,
CHAP or IPCP packets. This can be useful if the PPP nego-
tiation does not succeed or if authentication fails. If
debugging is enabled at compile time, the debug option
also causes other debugging messages to be logged.
Debugging can also be enabled or disabled by sending a
SIGUSR1 signal to the pppd process. This signal acts as a
toggle.
SCRIPTS
Pppd invokes scripts at various stages in its processing
which can be used to perform site-specific ancillary pro-
cessing. These scripts are usually shell scripts, but
could be executable code files instead. Pppd does not
wait for the scripts to finish. The scripts are executed
as root (with the real and effective user-id set to 0), so
that they can do things such as update routing tables or
run privileged daemons. Be careful that the 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. The environment vari-
ables that pppd sets are:
DEVICE The name of the serial tty device being used.
20
PPPD(8) PPPD(8)
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 invokes the following scripts, if they exist. It is
not an error if they don't exist.
/etc/ppp/auth-up
A program or script which is executed after the
remote system successfully authenticates itself.
It is executed with the parameters
interface-name peer-name user-name tty-device speed
Note that this script is not executed if the peer
doesn't authenticate itself, for example when the
noauth option is used.
/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 parameters
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 param-
eters as the ip-up script.
21
PPPD(8) PPPD(8)
/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 parameters
interface-name tty-device speed network-number
local-IPX-node-address remote-IPX-node-address
local-IPX-routing-protocol remote-IPX-routing-pro-
tocol local-IPX-router-name remote-IPX-router-name
ipparam pppd-pid
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 pro-
tocol
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 param-
eters as the ipx-up script.
FILES
/var/run/pppn.pid (BSD or Linux), /etc/ppp/pppn.pid (oth-
ers)
Process-ID for pppd process on ppp interface unit
n.
/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 the case.
/etc/ppp/chap-secrets
Names, secrets and IP addresses for CHAP authenti-
cation. 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 warn-
ing if this is not the case.
/etc/ppp/options
System default options for pppd, read before user
default options or command-line options.
22
PPPD(8) PPPD(8)
~/.ppprc
User default options, read before
/etc/ppp/options.ttyname.
/etc/ppp/options.ttyname
System default options for the serial port being
used, read after ~/.ppprc. In forming the ttyname
part of this filename, an initial /dev/ is stripped
from the port name (if present), and any slashes in
the remaining part are converted to dots.
/etc/ppp/peers
A directory containing options files which may con-
tain privileged options, even if pppd was invoked
by a user other than root. The system administra-
tor 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.
SEE ALSO
RFC1144
Jacobson, V. Compressing TCP/IP headers for low-
speed serial links. February 1990.
RFC1321
Rivest, R. The MD5 Message-Digest Algorithm.
April 1992.
RFC1332
McGregor, G. PPP Internet Protocol Control Proto-
col (IPCP). May 1992.
RFC1334
Lloyd, B.; Simpson, W.A. PPP authentication proto-
cols. October 1992.
RFC1661
Simpson, W.A. The Point-to-Point Protocol (PPP).
July 1994.
RFC1662
Simpson, W.A. PPP in HDLC-like Framing. July
1994.
NOTES
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,
and exit.
23
PPPD(8) PPPD(8)
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 immedi-
ately.
SIGUSR1
This signal toggles the state of the debug option.
SIGUSR2
This signal causes pppd to renegotiate compression.
This can be useful to re-enable compression after
it has been disabled as a result of a fatal decom-
pression error. (Fatal decompression errors gener-
ally indicate a bug in one or other implementa-
tion.)
AUTHORS
Paul Mackerras (Paul.Mackerras@cs.anu.edu.au), based on
earlier work by Drew Perkins, Brad Clements, Karl Fox,
Greg Christy, and Brad Parker.
24
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. |
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