PPPD(8) PPPD(8)
NAME
pppd - Point to Point Protocol daemon
SYNOPSIS
pppd [ options ] [ tty_name ] [ speed ]
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, pppd will use the controlling
terminal, and will not fork to put itself in the
background.
<speed>
Set the baud rate to <speed>. On systems such as
4.4BSD and NetBSD, any speed can be specified.
Other systems (e.g. SunOS) allow only a limited set
of speeds.
asyncmap <map>
Set the async character map to <map>. This map
describes which control characters cannot be
successfully received over the serial line. pppd
will ask the peer to send these characters as a
2-byte escape sequence. The argument is a 32 bit
hex number with each bit representing a character
to escape. Bit 0 (00000001) represents the
character 0x00; bit 31 (80000000) represents the
character 0x1f or ^_. If multiple asyncmap options
are given, the values are ORed together. If no
asyncmap option is given, no async character map
will be negotiated for the receive direction; the
peer will then escape all control characters.
auth Require the peer to authenticate itself before
allowing network packets to be sent or received.
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PPPD(8) PPPD(8)
connect <p>
Use the executable or shell command specified by
<p> to set up the serial line. This script would
typically use the "chat" program to dial the modem
and start the remote ppp session.
crtscts
Use hardware flow control (i.e. RTS/CTS) to control
the flow of data on the serial port.
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.
disconnect <p>
Run the executable or shell command specified by
<p> after pppd has terminated the link. This
script could, for example, issue commands to the
modem to cause it to hang up.
escape xx,yy,...
Specifies that certain characters should be escaped
on transmission (regardless of whether the peer
requests them to be escaped with its async control
character map). The characters to be escaped are
specified as a list of hex numbers separated by
commas. Note that almost any character can be
specified for the escape option, unlike the
asyncmap option which only allows control
characters to be specified. The characters which
may not be escaped are those with hex values 0x20 -
0x3f or 0x5e.
file <f>
Read options from file <f> (the format is described
below).
lock Specifies that pppd should use a UUCP-style lock on
the serial device to ensure exclusive access to the
device.
mru <n>
Set the MRU [Maximum Receive Unit] value to <n> for
negotiation. 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).
netmask <n>
Set the interface netmask to <n>, a 32 bit netmask
in "decimal dot" notation (e.g. 255.255.255.0).
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PPPD(8) PPPD(8)
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 does
without this option).
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 old versions of pppd).
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.
-all Don't request or allow negotiation of any options
for LCP and IPCP (use default values).
-ac Disable Address/Control compression negotiation
(use default, i.e. address/control field
disabled).
-am Disable asyncmap negotiation (use the default
asyncmap, i.e. escape all control characters).
-as <n>
Same as asyncmap <n>
-d Increase debugging level (same as the debug
option).
-detach
Don't fork to become a background process
(otherwise pppd will do so if a serial device is
specified).
-ip Disable IP address negotiation (with this option,
the remote IP address must be specified with an
option on the command line or in an options file).
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PPPD(8) PPPD(8)
-mn Disable magic number negotiation. With this
option, pppd cannot detect a looped-back line.
-mru Disable MRU [Maximum Receive Unit] negotiation (use
default, i.e. 1500).
-p Same as the passive option.
-pc Disable protocol field compression negotiation (use
default, i.e. protocol field compression
disabled).
+ua <p>
Agree to authenticate using PAP [Password
Authentication Protocol] if requested by the peer,
and use the data in file <p> for the user and
password to send to the peer. The file contains the
remote user name, followed by a newline, followed
by the remote password, followed by a newline.
This option is obsolescent.
+pap Require the peer to authenticate itself using PAP.
-pap Don't agree to authenticate using PAP.
+chap Require the peer to authenticate itself using CHAP
[Cryptographic Handshake Authentication Protocol]
authentication.
-chap Don't agree to authenticate using CHAP.
-vj Disable negotiation of Van Jacobson style IP header
compression (use default, i.e. no compression).
debug Increase debugging level (same as -d). If this
option is given, pppd will log the contents of all
control packets sent or received in a readable
form. The packets are logged through syslog with
facility daemon and level debug. This information
can be directed to a file by setting up
/etc/syslog.conf appropriately (see
syslog.conf(5)).
domain <d>
Append the domain name <d> to the local host name
for authentication purposes. For example, if
gethostname() returns the name porsche, but the
fully qualified domain name is porsche.Quotron.COM,
you would use the domain option to set the domain
name to Quotron.COM.
modem Use the modem control lines. (This option is not
fully implemented.)
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PPPD(8) PPPD(8)
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.
local Don't use the modem control lines.
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.
name <n>
Set the name of the local system for authentication
purposes to <n>.
user <u>
Set the user name to use for authenticating this
machine with the peer using PAP to <u>.
usehostname
Enforce the use of the hostname as the name of the
local system for authentication purposes (overrides
the name option).
remotename <n>
Set the assumed name of the remote system for
authentication purposes to <n>.
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.
login Use the system password database for authenticating
the peer using PAP.
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).
lcp-restart <n>
Set the LCP restart interval (retransmission
timeout) to <n> seconds (default 3).
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PPPD(8) PPPD(8)
lcp-max-terminate <n>
Set the maximum number of LCP terminate-request
transmissions to <n> (default 3).
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).
ipcp-restart <n>
Set the IPCP restart interval (retransmission
timeout) to <n> seconds (default 3).
ipcp-max-terminate <n>
Set the maximum number of IPCP terminate-request
transmissions to <n> (default 3).
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).
pap-restart <n>
Set the PAP restart interval (retransmission
timeout) to <n> seconds (default 3).
pap-max-authreq <n>
Set the maximum number of PAP authenticate-request
transmissions to <n> (default 10).
chap-restart <n>
Set the CHAP restart interval (retransmission
timeout for challenges) to <n> seconds (default 3).
chap-max-challenge <n>
Set the maximum number of CHAP challenge
transmissions to <n> (default 10).
chap-interval <n>
If this option is given, pppd will rechallenge the
peer every <n> seconds.
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.
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PPPD(8) PPPD(8)
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.
OPTIONS FILES
Options can be taken from files as well as the command
line. pppd reads options from the files /etc/ppp/options
and $HOME/.ppprc before looking at the command line. An
options file is parsed into a series of words, delimited
by whitespace. Whitespace can be included in a word by
enclosing the word in quotes ("). A backslash (\) quotes
the following character. A hash (#) starts a comment,
which continues until the end of the line.
AUTHENTICATION
pppd provides system administrators with sufficient access
control that PPP access to a server machine can be
provided to legitimate users without fear of compromising
the security of the server or the network it's on. In
part this is provided by the /etc/ppp/options file, where
the administrator can place options to require
authentication whenever pppd is run, 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 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.
Authentication is based on secrets, which are selected
from secrets files (/etc/ppp/pap-secrets for PAP,
/etc/ppp/chap-secrets for CHAP). Both secrets files have
the same format, and both can store secrets for several
combinations of server (authenticating peer) and client
(peer being authenticated). Note that pppd can be both a
server and client, and that different protocols can be
used in the two directions if desired.
A secrets file is parsed into words as for a options file.
A secret is specified by a line containing at least 3
words, in the order client, server, secret. 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, it is assumed that any IP
address is OK; to disallow all IP addresses, use "-". 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.
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PPPD(8) PPPD(8)
Thus a secrets file contains both secrets for use in
authenticating other hosts, plus secrets which we use for
authenticating ourselves to others. Which secret to use
is chosen based on the names of the host (the `local
name') and its peer (the `remote name'). The local name
is set as follows:
if the usehostname option is given,
then the local name is the hostname of this machine
(with the domain appended, if given)
else if the name option is given,
then use the argument of the first name option seen
else if the local IP address is specified with a hostname,
then use that name
else use the hostname of this machine (with the domain
appended, if given)
When authenticating ourselves using PAP, there is also a
`username' which is the local name by default, but can be
set with the user option or the +ua option.
The remote name is set as follows:
if the remotename option is given,
then use the argument of the last remotename option
seen
else if the remote IP address is specified with a
hostname,
then use that host name
else the remote name is the null string "".
Secrets are selected from the PAP secrets file as follows:
* For authenticating the peer, look for a secret with
client == username specified in the PAP authenticate-
request, and server == local name.
* For authenticating ourselves to the peer, look for a
secret with client == our username, server == remote
name.
When authenticating the peer with PAP, a secret of ""
matches any password supplied by the peer. 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 the login option was
specified, the username and password are also checked
against the system password database. Thus, the system
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PPPD(8) PPPD(8)
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.
Secrets are selected from the CHAP secrets file as
follows:
* For authenticating the peer, look for a secret with
client == name specified in the CHAP-Response message,
and server == local name.
* For authenticating ourselves to the peer, look for a
secret with client == local name, and server == name
specified in the CHAP-Challenge message.
Authentication must be satisfactorily completed before
IPCP (or any other Network Control Protocol) can be
started. If authentication fails, pppd will terminated
the link (by closing LCP). If IPCP negotiates an
unacceptable IP address for the remote host, IPCP will be
closed. IP packets can only be sent or received when IPCP
is open.
In some cases it is desirable to allow some hosts which
can't authenticate themselves to connect and use one of a
restricted set of IP addresses, even when the local host
generally requires authentication. If the peer refuses to
authenticate itself when requested, pppd takes that as
equivalent to authenticating with PAP using the empty
string for the username and password. Thus, by adding a
line to the pap-secrets file which specifies the empty
string for the client and password, it is possible to
allow restricted access to hosts which refuse to
authenticate 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
routing tables and/or ARP (Address Resolution Protocol)
tables. In some cases this will be done automatically
through the actions of the routed or gated daemons, but in
most cases some further intervention is required.
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.
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PPPD(8) PPPD(8)
In some cases it is desirable to use proxy ARP, for
example on a server machine connected to a LAN, in order
to allow other hosts to communicate with the remote host.
The proxyarp option causes pppd to look for a network
interface on the same subnet as the remote host (an
interface supporting broadcast and ARP, which is up and
not a point-to-point or loopback interface). If found,
pppd creates a permanent, published ARP entry with the IP
address of the remote host and the hardware address of the
network interface found.
EXAMPLES
In the simplest case, you can connect the serial ports of
two machines and issue a command like
pppd /dev/ttya 9600 passive
to each machine, assuming there is no getty running on the
serial ports. If one machine has a getty running, you can
use kermit or tip on the other machine to log in to the
first machine and issue a command like
pppd passive
Then exit from the communications program (making sure the
connection isn't dropped), and issue a command like
pppd /dev/ttya 9600
The process of logging in to the other machine and
starting pppd can be automated by using the connect option
to run chat, for example:
pppd /dev/ttya 38400 connect 'chat "" "" "login:"
"username" "Password:" "password" "% " "exec pppd
passive"'
(Note however that running chat like this will leave the
password visible in the parameter list of pppd and chat.)
If your serial connection is any more complicated than a
piece of wire, you may need to arrange for some control
characters to be escaped. In particular, it is often
useful to escape XON (^Q) and XOFF (^S), using asyncmap
a0000. If the path includes a telnet, you probably should
escape ^] as well (asyncmap 200a0000). If the path
includes an rlogin, you will need to use the escape ff
option on the end which is running the rlogin client,
since many rlogin implementations are not transparent;
they will remove the sequence [0xff, 0xff, 0x73, 0x73,
followed by any 8 bytes] from the stream.
DIAGNOSTICS
Messages are sent to the syslog daemon using facility
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PPPD(8) PPPD(8)
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
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. If debugging is enabled at
compile time, the debug option also causes other debugging
messages to be logged.
Debugging can also be enabled by sending a SIGUSR1 to the
pppd process. Debugging may be disabled by sending a
SIGUSR2 to the pppd process.
FILES
/var/run/pppn.pid (BSD), /etc/ppp/pppn.pid (SunOS)
Process-ID for pppd process on ppp interface unit
n.
/etc/ppp/pap-secrets
Usernames, passwords and IP addresses for PAP
authentication.
/etc/ppp/chap-secrets
Names, secrets and IP addresses for CHAP
authentication.
/etc/ppp/options
System default options for pppd, read before user
default options or command-line options.
$HOME/.ppprc
User default options, read before command-line
options.
SEE ALSO
RFC1144
Jacobson, V. Compressing TCP/IP headers for low-
speed serial links. 1990 February.
RFC1321
Rivest, R. The MD5 Message-Digest Algorithm. 1992
April.
RFC1332
McGregor, G. PPP Internet Protocol Control
Protocol (IPCP). 1992 May.
RFC1334
Lloyd, B.; Simpson, W.A. PPP authentication
protocols. 1992 October.
11
PPPD(8) PPPD(8)
RFC1548
Simpson, W.A. The Point-to-Point Protocol (PPP).
1993 December.
RFC1549
Simpson, W.A. PPP in HDLC Framing. 1993 December
NOTES
The following signals have the specified effect when sent
to the pppd process.
SIGINT, SIGTERM
These signals cause pppd to terminate the link (by
closing LCP), restore the serial device settings,
and exit.
SIGHUP Indicates that the physical layer has been
disconnected. pppd will attempt to restore the
serial device settings (this may produce error
messages on Suns), and then exit.
BUGS
The use of the modem control lines and the effects of the
modem and local options are not well defined.
AUTHORS
Drew Perkins, Brad Clements, Karl Fox, Greg Christy, Brad
Parker (brad@fcr.com), Paul Mackerras
(paulus@cs.anu.edu.au)
12