PPPD(8)PPPD(8)NAMEpppd - Point to Point Protocol daemon
SYNOPSISpppd [ tty_name ] [ speed ] [ options ]
DESCRIPTION
The Point-to-Point Protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is composed of three
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 support, 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 connected 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 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.
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 conjunc‐
tion with the idle option if there are packets being sent or
received regularly over the link (for example, routing informa‐
tion 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 inap‐
propriate for a PPP link, such as ether and arp, are not permit‐
ted. Generally the filter expression should be enclosed in sin‐
gle-quotes to prevent whitespace in the expression from being
interpreted by the shell. This option only available 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 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) rep‐
resents 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 char‐
acters. 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.
connect-max-attempts <n>
Attempt dial-out connection to remote system no more than speci‐
fied number of times (default = 1). If the connection is not
made, pppd will exit. Requires that persist has been specified.
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 negotiation is successfully completed.
This entry is removed when the PPP connection is broken. This
option is privileged if the nodefaultroute option has been spec‐
ified.
disconnect script
Run the executable or shell command specified by script after
pppd has terminated the link. This script could, for example,
issue commands to the modem to cause it to hang up if hardware
modem control signals were not available. The disconnect script
is not run if the modem has already hung up. This option is
privileged if the noauth option is used.
escape xx,yy,...
Specifies that certain characters should be escaped on transmis‐
sion (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 com‐
mas. Note that almost any character can be specified for the
escape option, unlike the asyncmap option which only allows con‐
trol characters to be specified. The characters which may not
be escaped are those with hex values 0x20 - 0x3f or 0x5e.
file name
Read options from file name (the format is described below).
The file must be readable by the user who has invoked pppd.
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 networking 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 with‐
out this option.
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 notation (e.g. 150.234.56.78). The
default local address is the (first) IP address of the system
(unless the noipdefault option is given). The remote address
will be obtained from the peer if not specified in any option.
Thus, in simple cases, this option is not required. If a local
and/or remote IP address is specified with this option, pppd
will not accept a different value from the peer in the IPCP
negotiation, unless the ipcp-accept-local and/or ipcp-accept-
remote options are given, respectively.
bsdcomp nr,nt
Request that the peer compress packets that it sends, using the
BSD-Compress scheme, with a maximum code size of nr bits, and
agree to compress packets sent to the peer with a maximum code
size of nt bits. If nt is not specified, it defaults to the
value given for nr. Values in the range 9 to 15 may be used for
nr and nt; larger values give better compression but consume
more kernel memory for compression dictionaries. Alternatively,
a value of 0 for nr or nt disables compression in the corre‐
sponding 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 transmissions to n
(default 10).
chap-restart n
Set the CHAP restart interval (retransmission timeout for chal‐
lenges) 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/syslog.conf appro‐
priately (see syslog.conf(5)).
default-asyncmap
Disable asyncmap negotiation, forcing all control characters to
be escaped for both the transmit and the receive direction.
default-mru
Disable MRU [Maximum Receive Unit] negotiation. With this
option, pppd will use the default MRU value of 1500 bytes for
both the transmit and receive direction.
deflate nr,nt
Request that the peer compress packets that it sends, using the
Deflate scheme, with a maximum window size of 2**nr bytes, and
agree to compress packets sent to the peer with a maximum window
size of 2**nt bytes. If nt is not specified, it defaults to the
value given for nr. Values in the range 8 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 corre‐
sponding direction. Use nodeflate or deflate 0 to disable
Deflate compression entirely. (Note: pppd requests Deflate com‐
pression 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 speci‐
fied 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 avail‐
able, pppd will connect to the peer and perform negotiation,
authentication, etc. When this is completed, pppd will commence
passing data packets (i.e., IP packets) across the link.
The demand option implies the persist option. If this behaviour
is not desired, use the nopersist option after the demand
option. The idle and holdoff options are also useful in con‐
junction with the demand option.
domain d
Append the domain name d 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
privileged.
holdoff n
Specifies how many seconds to wait before re-initiating the link
after it terminates. This option only has any effect if the
persist or demand option is used. The holdoff period is not
applied if the link was terminated because it was idle.
idle n Specifies that pppd should disconnect if the link is idle for n
seconds. The link is idle when no data packets (i.e. IP pack‐
ets) are being sent or received. Note: it is not advisable to
use this option with the persist option without the demand
option. If the active-filter option is given, data packets
which are rejected by the specified activity filter also count
as the link being idle.
ipcp-accept-local
With this option, pppd will accept the peer's idea of our local
IP address, even if the local IP address was specified in an
option.
ipcp-accept-remote
With this option, pppd will accept the peer's idea of its
(remote) IP address, even if the remote IP address was specified
in an option.
ipcp-max-configure n
Set the maximum number of IPCP configure-request transmissions
to n (default 10).
ipcp-max-failure n
Set the maximum number of IPCP configure-NAKs returned before
starting to send configure-Rejects instead to n (default 10).
ipcp-max-terminate n
Set the maximum number of IPCP terminate-request transmissions
to n (default 3).
ipcp-restart n
Set the IPCP restart interval (retransmission timeout) to n sec‐
onds (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 support.
ipx-network n
Set the IPX network number in the IPXCP configure request frame
to n, a hexadecimal number (without a leading 0x). There is no
valid default. If this option is not specified, the network
number is obtained from the peer. If the peer does not have the
network number, the IPX protocol will not be started.
ipx-node n:m
Set the IPX node numbers. The two node numbers are separated
from each other with a colon character. The first number n is
the local node number. The second number m is the peer's node
number. Each node number is a hexadecimal number, at most 10
digits long. The node numbers on the ipx-network must be
unique. There is no valid default. If this option is not spec‐
ified then the node numbers are obtained from the peer.
ipx-router-name <string>
Set the name of the router. This is a string and is sent to the
peer as information data.
ipx-routing n
Set the routing protocol to be received by this option. More
than one instance of ipx-routing may be specified. The 'none'
option (0) may be specified as the only instance of ipx-routing.
The values may be 0 for NONE, 2 for RIP/SAP, and 4 for NLSP.
ipxcp-accept-local
Accept the peer's NAK for the node number specified in the ipx-
node option. If a node number was specified, and non-zero, the
default is to insist that the value be used. If you include
this option then you will permit the peer to override the entry
of the node number.
ipxcp-accept-network
Accept the peer's NAK for the network number specified in the
ipx-network option. If a network number was specified, and 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 configure request
frame. If a node number was specified for the peer and this
option was not specified, the peer will be forced to use the
value which you have specified.
ipxcp-max-configure n
Set the maximum number of IPXCP configure request frames which
the system will send to n. The default is 10.
ipxcp-max-failure n
Set the maximum number of IPXCP NAK frames which the local sys‐
tem will send before it rejects the options. The default value
is 3.
ipxcp-max-terminate n
Set the maximum number 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 argu‐
ment 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 con‐
tents of transmitted packets be printed. On most systems, mes‐
sages 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 connec‐
tion. Use of this option requires a non-zero value for the lcp-
echo-interval parameter. This option can be used to enable pppd
to terminate after the physical connection has been broken
(e.g., the modem has hung up) in situations where no hardware
modem control lines are available.
lcp-echo-interval n
If this option is given, pppd will send an LCP echo-request
frame to the peer every n seconds. Normally the peer should
respond to the echo-request by sending an echo-reply. This
option can be used with the lcp-echo-failure option to detect
that the peer is no longer connected.
lcp-max-configure n
Set the maximum number of LCP configure-request transmissions to
n (default 10).
lcp-max-failure n
Set the maximum number of LCP configure-NAKs returned before
starting to send configure-Rejects instead to n (default 10).
lcp-max-terminate n
Set the maximum number of LCP terminate-request transmissions to
n (default 3).
lcp-restart n
Set the LCP restart interval (retransmission timeout) to n sec‐
onds (default 3).
local Don't use the modem control lines. With this option, pppd will
ignore the state of the CD (Carrier Detect) signal from the
modem and will not change the state of the DTR (Data Terminal
Ready) signal.
login Use the system password database for authenticating the peer
using PAP, and record the user in the system wtmp file. Note
that the peer must have an entry in the /etc/ppp/pap-secrets
file as well as the system password database to be allowed
access.
maxconnect n
Terminate the connection when it has been available for network
traffic for n seconds (i.e. n seconds after the first network
control protocol comes up).
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 ter‐
minated and before executing the connect script. On Ultrix,
this option implies hardware flow control, as for the crtscts
option.
ms-dns <addr>
If pppd is acting as a server for Microsoft Windows clients,
this option allows pppd to supply one or two DNS (Domain Name
Server) addresses to the clients. The first instance of this
option specifies the primary DNS address; the second instance
(if given) specifies the secondary DNS address. (This option
was present in some older versions of pppd under the name dns-
addr.)
ms-wins <addr>
If pppd is acting as a server for Microsoft Windows or "Samba"
clients, this option allows pppd to supply one or two WINS (Win‐
dows 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.
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 directions (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) negotiation. This
option should only be required if the peer is buggy and gets
confused by requests from pppd for CCP negotiation.
nocrtscts
Disable hardware flow control (i.e. RTS/CTS) on the serial port.
If neither the crtscts nor the nocrtscts option is given, the
hardware flow control setting for the serial port is left
unchanged.
nodefaultroute
Disable the defaultroute option. The system administrator who
wishes to prevent users from creating default routes with pppd
can do so by placing this option in the /etc/ppp/options file.
nodeflate
Disables Deflate compression; pppd will not request or agree to
compress packets using the Deflate scheme.
nodetach
Don't detach from the controlling terminal. Without this
option, if a serial device other than the terminal on the stan‐
dard 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 spec‐
ified, which is to determine (if possible) the local IP address
from the hostname. With this option, the peer will have to sup‐
ply the local IP address during IPCP negotiation (unless it
specified explicitly on the command line or in an options file).
noipx Disable the IPXCP and IPX protocols. This option should only be
required if the peer is buggy and gets confused by requests from
pppd for IPXCP negotiation.
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 terminated. This is
the default unless the persist or demand option has been speci‐
fied.
nopredictor1
Do not accept or agree to Predictor-1 compression.
noproxyarp
Disable the proxyarp option. The system administrator who
wishes to prevent users from creating proxy ARP entries with
pppd can do so by placing this option in the /etc/ppp/options
file.
novj Disable Van Jacobson style TCP/IP header compression in both the
transmit and the receive direction.
novjccomp
Disable the connection-ID compression option in Van Jacobson
style TCP/IP header compression. With this option, pppd will
not omit the connection-ID byte from Van Jacobson compressed
TCP/IP headers, nor ask the peer to do so.
papcrypt
Indicates that all secrets in the /etc/ppp/pap-secrets file
which are used for checking the identity of the peer are
encrypted, and thus pppd should not accept a password which,
before encryption, is identical to the secret from the
/etc/ppp/pap-secrets file.
pap-max-authreq n
Set the maximum number of PAP authenticate-request transmissions
to n (default 10).
pap-restart n
Set the PAP restart interval (retransmission timeout) to n sec‐
onds (default 3).
pap-timeout n
Set the maximum time that pppd will wait for the peer to authen‐
ticate itself with PAP to n seconds (0 means no limit).
pass-filter filter-expression
Specifies a packet filter to applied to data packets being sent
or received to determine which packets should be allowed to
pass. Packets which are rejected by the filter are silently
discarded. This option can be used to prevent specific network
daemons (such as routed) using up link bandwidth, or to provide
a basic firewall capability. The filter-expression syntax is as
described for tcpdump(1), except that qualifiers which are inap‐
propriate for a PPP link, such as ether and arp, are not permit‐
ted. Generally the filter expression should be enclosed in sin‐
gle-quotes to prevent whitespace in the expression from being
interpreted by the shell. Note that it is possible to apply
different constraints to incoming and outgoing packets using the
inbound and outbound qualifiers. This option is currently only
available under NetBSD, and then only if both the kernel and
pppd were compiled with PPP_FILTER defined.
persist
Do not exit after a connection is terminated; instead try to
reopen the connection.
predictor1
Request that the peer compress frames that it sends using Pre‐
dictor-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.
proxyarp
Add an entry to this system's ARP [Address Resolution Protocol]
table with the IP address of the peer and the Ethernet address
of this system. This will have the effect of making the peer
appear to other systems to be on the local ethernet.
remotename name
Set the assumed name of the remote system for authentication
purposes to name.
refuse-chap
With this option, pppd will not agree to authenticate itself to
the peer using CHAP.
refuse-pap
With this option, pppd will not agree to authenticate itself to
the peer using PAP.
require-chap
Require the peer to authenticate itself using CHAP [Challenge
Handshake Authentication Protocol] authentication.
require-pap
Require the peer to authenticate itself using PAP [Password
Authentication Protocol] authentication.
silent With this option, pppd will not transmit LCP packets to initiate
a connection until a valid LCP packet is received from the peer
(as for the `passive' option with ancient versions of pppd).
usehostname
Enforce the use of the hostname (with domain name appended, if
given) as the name of the local system for authentication pur‐
poses (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 Jacob‐
son 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 control 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 initial /dev/ is
removed from the terminal name, and any remaining / characters are
replaced with dots.
An options file is parsed into a series of words, delimited by 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
options file.
SECURITYpppd provides system administrators with sufficient access control that
PPP access to a server machine can be provided to legitimate users
without fear of compromising the security of the server or the network
it's on. 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.
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 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.
Privileged options may be used in /etc/ppp/options file or in an
options file read using the call option. If pppd is being run by the
root user, privileged options can be used without restriction.
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 pass‐
word) with the server, and authenticates itself by proving that it
knows that secret. Very often, the names used for authentication cor‐
respond to the internet hostnames of the peers, but this is not essen‐
tial.
At present, pppd supports two authentication protocols: the Password
Authentication Protocol (PAP) and the Challenge Handshake Authentica‐
tion Protocol (CHAP). PAP involves the client sending its name and a
cleartext password to the server to authenticate itself. In contrast,
the server initiates the CHAP authentication exchange by sending a
challenge to the client (the challenge packet includes the server's
name). The client must respond with a response which includes its name
plus a hash value derived from the shared secret and the challenge, in
order to prove that it knows the secret.
The PPP protocol, being symmetrical, allows both peers to require the
other to authenticate itself. In that case, two separate and indepen‐
dent authentication exchanges will occur. The two exchanges could use
different authentication protocols, and in principle, different names
could be used in the two exchanges.
The default behaviour of pppd is to agree to authenticate if requested,
and to not require authentication from the peer. However, pppd will
not agree to authenticate itself with a particular protocol if it has
no secrets which could be used to do so.
Pppd stores secrets for use in authentication in secrets files
(/etc/ppp/pap-secrets for PAP, /etc/ppp/chap-secrets for CHAP). Both
secrets files have the same format. The secrets files can contain
secrets for pppd to use in authenticating itself to other systems, as
well as secrets for pppd to use when authenticating other systems to
itself.
Each line in a secrets file contains one secret. A given secret is
specific to a particular combination of client and server - it can only
be used by that client to authenticate itself to that server. Thus
each line in a secrets file has at least 3 fields: the name of the
client, the name of the server, and the secret. These fields may be
followed by a list of the IP addresses that the specified client may
use when connecting to the specified server.
A secrets file is parsed into words as for 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
following 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 wild‐
card 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 speci‐
fied address is not acceptable. An address may be followed by "/" and
a number n, to indicate a whole subnet, i.e. all addresses which have
the same value in the most significant n bits. Note that case is sig‐
nificant 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 remote‐
name option. Otherwise, if the remote IP address was specified by a
name (rather than in numeric form), that name will be used as the
peer's name. Failing that, pppd will use the null string as the peer's
name.
When authenticating the peer with PAP, the supplied password is first
compared with the secret from the secrets file. If the password
doesn't match the secret, the password is encrypted using crypt() and
checked against the secret again. Thus secrets for authenticating the
peer can be stored in encrypted form if desired. If the papcrypt
option is given, the first (unencrypted) comparison is omitted, for
better security.
Furthermore, if the login option was specified, the username and pass‐
word 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 pass‐
word supplied by the peer. This avoids the need to have the same
secret in two places.
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
authentication 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 termi‐
nated 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 pass‐
word, it is possible to allow restricted access to hosts which refuse
to authenticate 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. Communica‐
tion with other machines generally requires further modification to
routing tables and/or ARP (Address Resolution Protocol) tables. In
most cases the defaultroute and/or proxyarp options are sufficient for
this, but in some cases further intervention is required. The
/etc/ppp/ip-up script can be used for this.
Sometimes it is desirable to add a default route through the remote
host, as in the case of a machine whose only connection to the Internet
is through the ppp interface. The defaultroute option causes pppd to
create such a default route when IPCP comes up, and delete it when the
link is terminated.
In some cases it is desirable to use proxy ARP, for example on a server
machine connected to a LAN, in order to allow other hosts to communi‐
cate with the remote host. The proxyarp option causes pppd to look for
a network interface on the same subnet as the remote host (an interface
supporting broadcast and ARP, which is up and not a point-to-point or
loopback interface). If found, pppd creates a permanent, published ARP
entry with the IP address of the remote host and the hardware address
of the network interface found.
When the demand option is used, the interface IP addresses have already
been set at the point when IPCP comes up. If pppd has not been able to
negotiate the same addresses that it used to configure the interface
(for example when the peer is an ISP that uses dynamic IP address
assignment), pppd has to change the interface IP addresses to the nego‐
tiated addresses. This may disrupt existing connections, 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).
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 con‐
tains 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 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 authentication
method the PPP implementation on the user's machine supports), so that
the user's machine can authenticate itself. For example, if Joe has a
machine called "joespc" which is to be allowed to dial in to the
machine called "server" and use the IP address joespc.my.net, you would
add an entry like this to /etc/ppp/pap-secrets or /etc/ppp/chap-
secrets:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create a username called (for example) "ppp",
whose login shell is pppd and whose home directory is /etc/ppp.
Options to be used when pppd is run this way can be put in
/etc/ppp/.ppprc.
If your serial connection is any more complicated than a piece of wire,
you may need to arrange for some control characters to be escaped. In
particular, it is often useful to escape XON (^Q) and XOFF (^S), using
asyncmap a0000. If the path includes a telnet, you probably should
escape ^] as well (asyncmap 200a0000). If the path includes an rlogin,
you will need to use the escape ff option on the end which is running
the rlogin client, since many rlogin implementations are not transpar‐
ent; 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 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
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 processing. These scripts are
usually shell scripts, but could be executable code files instead.
Pppd does not wait for the scripts to finish. The scripts are executed
as root (with the real and effective user-id set to 0), so that they
can do things such as update routing tables or run privileged daemons.
Be careful that the 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 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 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 executed. 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 parameters as
the ip-up script.
/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-protocol local-IPX-router-name remote-IPX-
router-name ipparam pppd-pid
The local-IPX-routing-protocol and remote-IPX-routing-protocol
field may be one of the following:
NONE to indicate that there is no routing protocol
RIP to indicate that RIP/SAP should be used
NLSP to indicate that Novell NLSP should be used
RIP NLSP to indicate that both RIP/SAP and NLSP should be used
/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 as the ipx-up script.
FILES
/var/run/pppn.pid (BSD or Linux), /etc/ppp/pppn.pid (others)
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 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.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 contain privi‐
leged options, even if pppd was invoked by a user other than
root. The system administrator can create options files in this
directory to permit non-privileged users to dial out without
requiring the peer to authenticate, but only to certain trusted
peers.
/etc/ppp/ppp.deny
Lists users who may not use the system password PAP authentica‐
tion.
/etc/ppp/ppp.shells
Lists user shells which are approved for system password PAP
authentication logins.
/usr/share/examples/pppd/
Sample pppd configuration files.
SEE ALSOchat(8), ppp(8)
RFC 1144
Jacobson, V. Compressing TCP/IP headers for low-speed serial
links. February 1990.
RFC 1321
Rivest, R. The MD5 Message-Digest Algorithm. April 1992.
RFC 1332
McGregor, G. PPP Internet Protocol Control Protocol (IPCP).
May 1992.
RFC 1334
Lloyd, B.; Simpson, W.A. PPP authentication protocols. October
1992.
RFC 1661
Simpson, W.A. The Point-to-Point Protocol (PPP). July 1994.
RFC 1662
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.
SIGHUP This signal causes pppd to terminate the link, restore the
serial device settings, and close the serial device. If the
persist or demand option has been specified, pppd will try to
reopen the serial device and start another connection (after the
holdoff period). Otherwise pppd will exit. If this signal is
received during the holdoff period, it causes pppd to end the
holdoff period immediately.
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 decompression error. (Fatal decompression
errors generally indicate a bug in one or other implementation.)
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.
PPPD(8)
