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pppd(8)
NAME
pppd - Point-to-Point Protocol (PPP) daemon
SYNOPSIS
/usr/sbin/pppd [options] [tty_name] [speed]
OPTIONS
tty_name
Communicates over the named device. The string /dev/ is prepended if
necessary. If no device name is given or if the name of the
controlling terminal is given, pppd uses the controlling terminal, and
does not fork to put itself in the background.
speed
Sets the baud rate to speed.
asyncmap map
Sets the async character map to map. This map describes those control
characters that cannot be successfully received over the serial line.
The pppd daemon asks the peer to send these characters as a 2-byte
escape sequence. The argument is a 32-bit hexadecimal 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 is negotiated
for the receive direction; the peer then escapes all control
characters.
auth
Requires the peer to authenticate itself before allowing network
packets to be sent or received.
connect p
Uses 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
Uses hardware flow control (RTS/CTS) to control the flow of data on the
serial port.
-crtscts
Disables hardware flow control (RTS/CTS) on the serial port. If
neither the crtscts nor the -crtscts option is given, the hardware flow
control setting for the serial port is not changed.
defaultroute
Adds 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
Runs 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 if hardware modem control signals were
not available.
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 hexadecimal 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
Reads options from file f. See the Options Files section for a
description of the format.
lock
Specifies that pppd should use a UUCP-style lock on the serial device
to ensure exclusive access to the device.
mru n
Sets the MRU (Maximum Receive Unit) value to n for negotiation. The
pppd daemon 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
Sets the interface netmask to n, a 32-bit netmask in dotted-decimal
notation (for example, 255.255.255.0).
passive
Enables the "passive" option in the LCP. With this option, pppd
attempts to initiate a connection; if no reply is received from the
peer, pppd waits for a valid LCP packet from the peer (instead of
exiting, as it does without this option).
silent
With this option, pppd does 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).
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. The pppd
daemon provides the basic LCP, authentication support, and an NCP for
establishing and configuring the Internet Protocol (IP) (called the IP
Control Protocol, IPCP).
OPTIONS
local_IP_address:remote_IP_address
Sets the local or remote interface IP addresses, or both. Either one
may be omitted. The IP addresses can be specified with a host name or
in decimal dot notation (for example, 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 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 or ipcp-accept-
remote options are given, respectively.
-all
Does not request or allow negotiation of any options for LCP and IPCP
(use default values).
-ac Disables Address/Control compression negotiation (use default, that is,
address/control field disabled).
-am Disables asyncmap negotiation (use the default asyncmap, that is,
escape all control characters).
-as <n>
Same as asyncmap n.
+chap
Requires the peer to authenticate itself using CHAP (Cryptographic
Handshake Authentication Protocol) authentication.
-chap
Does not agree to authenticate using CHAP.
-d Increases debugging level (same as the debug option).
-detach
Does not fork to become a background process (otherwise pppd will do so
if a serial device is specified).
-ip Disables 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.
-mn Disables magic number negotiation. With this option, pppd cannot
detect a looped-back line.
-mru
Disables MRU (Maximum Receive Unit) negotiation (use default, that is,
1500).
-p Same as the passive option.
+pap
Requires the peer to authenticate itself using PAP.
-pap
Does not agree to authenticate using PAP.
-pc Disables protocol field compression negotiation (use default, that is,
protocol field compression disabled).
+ua p
Agrees 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 obsolete.
-vj Disables negotiation of Van Jacobson-style IP header compression.
Instead, it uses no compression (the default).
bsdcomp nr,nt
Requests the peer to compress all packets that it sends, using the
BSD-Compress scheme, with a maximum code size of nr bits, and agrees to
compress all 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.
-bsdcomp
Disables compression; pppd will not request or agree to compress
packets using the BSD-Compress scheme.
chap-interval n
If this option is given, pppd challenges the peer every n seconds.
chap-max-challenge n
Sets the maximum number of CHAP challenge transmissions to n (default
10).
chap-restart n
Sets the CHAP restart interval (retransmission timeout for challenges)
to n seconds (default 3).
debug
Enables connection debugging facilities (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 local2 and level debug. This information can be directed
to a file by setting up /etc/syslog.conf appropriately (see
syslog.conf(4)).
domain d
Appends 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.
ipcp-accept-local
With this option, pppd accepts 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 accepts the peer's idea of its (remote) IP
address, even if the remote IP address was specified in an option.
ipcp-max-configure n
Sets the maximum number of IPCP configure-request transmissions to n
(default 10).
ipcp-max-failure n
Sets the maximum number of IPCP configure-NAKs returned before starting
to send configure-Rejects instead to n (default 10).
ipcp-max-terminate n
Sets the maximum number of IPCP terminate-request transmissions to n
(default 3).
ipcp-restart n
Sets the IPCP restart interval (retransmission timeout) to n seconds
(default 3).
kdebug n
Enables debugging code in the kernel-level PPP driver. The argument n
is a number that is the sum of the following values: 1 (enables general
debug messages), 2 (requests that the contents of received packets be
printed), and 4 (requests that the contents of transmitted packets be
printed).
lcp-echo-failure n
If this option is given, pppd presumes the peer to be dead if n LCP
echo-requests are sent without receiving a valid LCP echo-reply. If
this happens, pppd terminates the connection. 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 (for example, the modem has hung up) in
situations where no hardware modem control lines are available.
lcp-echo-interval n
If this option is given, pppd sends an LCP echo-request frame to the
peer every n seconds.
lcp-max-configure n
Sets the maximum number of LCP configure-request transmissions to n
(default 10).
lcp-max-failure n
Sets the maximum number of LCP configure-NAKs returned before starting
to send configure-Rejects instead to n (default 10).
lcp-max-terminate n
Sets the maximum number of LCP terminate-request transmissions to n
(default 3).
lcp-restart n
Sets the LCP restart interval (retransmission timeout) to n seconds
(default 3).
local
Does not use the modem control lines. With this option, pppd ignores
the state of the CD (Carrier Detect) signal from the modem and does not
change the state of the DTR (Data Terminal Ready) signal.
login
Uses the system password database for authenticating the peer using
PAP.
modem
Uses the modem control lines. This option is the default. With this
option, pppd waits for the CD (Carrier Detect) signal from the modem to
be asserted when opening the serial device (unless a connect script is
specified), and it drops the DTR (Data Terminal Ready) signal briefly
when the connection is terminated and before executing the connect
script.
mtu n
Sets the MTU (Maximum Transmit Unit) value to n. Unless the peer
requests a smaller value using 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
Sets the name of the local system for authentication purposes to n.
noipdefault
Disables the default behavior 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 must supply the local IP address
during IPCP negotiation, unless it is specified explicitly on the
command line or in an options file.
papcrypt
Indicates that all secrets in the /etc/ppp/pap-secrets file used for
checking the identity of the peer are encrypted. The pppd daemon
should not accept a password that (before encryption) is identical to
the secret from the /etc/ppp/pap-secrets file.
pap-max-authreq n
Sets the maximum number of PAP authenticate-request transmissions to n
(default 10).
pap-restart n
Sets the PAP restart interval (retransmission timeout) to n seconds
(default 3).
persist
Do not exit after a connection is terminated. Instead, try to reopen
the connection.
proxyarp
Adds 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.
remotename n
Sets the assumed name of the remote system for authentication purposes
to n.
usehostname
Enforces the use of the hostname as the name of the local system for
authentication purposes (overrides the name option).
user u
Sets the user name to use for authenticating this machine with the peer
using PAP to u.
Options Files
Options can be taken from files as well as the command line. The pppd
daemon reads options from the files /etc/ppp/options and ~/.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 double quotation marks ("). A backslash (\) quotes
any character that follows it. A hash mark (#) starts a comment, which
continues until the end of the line.
Authentication
The pppd daemon provides system administrators with sufficient access
control so that legitimate users can have PPP access to a server machine
without fear of compromising the security of the server or the network. In
part this is provided by the /etc/ppp/options file, in which the
administrator can place options to require authentication whenever pppd is
run, and in part by the PAP and CHAP secrets files, in which the
administrator can restrict the set of IP addresses that individual users
may use.
The default behavior 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 for
that protocol.
Authentication is based on secrets that are selected from secrets files
(/etc/ppp/pap-secrets for PAP and /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 like an options file. A secret is
specified by a line containing at least 3 words, in the following 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 a hyphen (-). If the secret starts with an at sign (@),
anything following it is assumed to be the name of a file from which to
read the secret. An asterisk (*) as the client or server name matches any
name. When selecting a secret, pppd takes the best match, that is the
match with the fewest wildcards.
A secrets file contains secrets for use in authenticating other hosts and
secrets that 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 determined by the following
rules:
· If the usehostname option is given, the local name is the hostname of
this machine (with the domain appended, if given).
· If the name option is given, the local name is the argument of the
first name option.
· If the local IP address is specified with a hostname, the local name
is the hostname.
· If none of the previous rules applies, the local name is 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 determined by the following rules:
· If the remotename option is given, the remote name is the argument of
the last remotename option.
· If the remote IP address is specified with a hostname, the remote name
is the hostname.
· If neither of the previous rules applies, the remote name is the null
string "".
Secrets are selected from the PAP secrets file as follows:
· For authenticating the peer, the client must match the username
specified in the PAP authenticate-request and server must match the
local name.
· For authenticating ourselves to the peer, the client must match our
username and server must match the remote name.
When authenticating the peer with PAP, a secret of "" matches any password
supplied by the peer. If the password does not match the secret, the
password is encrypted using crypt() and checked against the secret again.
Therefore, secrets for authenticating the peer can be stored in encrypted
form. If the papcrypt option is given, the first (unencrypted) comparison
is omitted for better security.
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
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 , to avoid the need to have the same secret in two places.
Secrets are selected from the CHAP secrets file as follows:
· For authenticating the peer, the client must match the name specified
in the CHAP-Response message and server must match the local name.
· For authenticating ourselves to the peer, the client must match the
local name and server must equal the 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 that cannot 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 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.
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.
NOTES
The following signals have the specified effect when sent to the pppd
process:
SIGINT, SIGTERM
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 option
has been specified, pppd tries to reopen the serial device and start
another connection. Otherwise, pppd exits.
SIGUSR2
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. With the BSD Compress scheme, fatal decompression
errors generally indicate a severe implementation error.
RESTRICTIONS
The use of the modem control lines and the effects of the modem and local
options are not well defined.
DIAGNOSTICS
Messages are sent to the syslogd daemon using facility LOG_LOCAL2. To see
the error and debug messages, 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
is useful if the PPP negotiation does not succeed. If debugging is enabled
at compile time, the debug option causes additional debugging messages to
be logged.
Debugging can also be toggled on and off by sending a SIGUSR1 to the pppd
process.
EXAMPLES
1. If you want to connect the serial ports of two machines and there is
no getty running on the serial ports, issue a command similar to the
following on each machine:
pppd /dev/ttya 9600 passive
2. If one machine has a getty running, you can log in to the machine from
another machine using kermit or tip, and issue the following command:
pppd passive
3. Then, exit from the communications program (making sure the connection
is not dropped), and issue a command similar to the following:
pppd /dev/ttya 9600
4. 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
Running chat in this way leaves the password visible in the
parameter list of pppd and chat.
If your serial connection is 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 need to use the escape ff option on the end that is
running the rlogin client, since many rlogin implementations are not
transparent; they remove the sequence 0xff, 0xff, 0x73, 0x73, followed
by any 8 bytes, from the stream.
FILES
/etc/ppp/pppn.pid
Process ID for pppd process on ppp interface unit n.
/etc/ppp/ip-up
A program or script that is executed when the link is available for
sending and receiving IP packets (IPCP is up). It is executed with the
parameters interface-name tty-device speed local-IP-address remote-IP-
address and with its standard input, output and error streams
redirected to /dev/null.
This program or script is executed with the same real and effective
user-ID as pppd, that is, at least the effective user-ID and possibly
the real user-ID will be root. This is so that it can be used to
manipulate routes, run privileged daemons (for example, sendmail). Be
careful that the contents of the /etc/ppp/ip-up and /etc/ppp/ip-down
scripts do not compromise your system's security.
/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 with the same parameters as the ip-up script, and the same
security considerations apply, since it is executed with the same
effective and real user-IDs as pppd.
/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).
~/.ppprc
User default options (read before command-line options).
/etc/ppp/options.ttyname
System default options for the serial port being used (read after
command-line options).
SEE ALSO
Commands: chat(8), pppstats(8)
RFC 1144, Jacobson, V., Compressing TCP/IP Headers for Low-speed Serial
Links, 1990 February.
RFC 1321, Rivest, R., The MD5 Message-Digest Algorithm, 1992 April.
RFC 1332, McGregor, G., The PPP Internet Protocol Control Protocol (IPCP),
1992 May (obsoletes RFC 1172).
RFC 1334, Lloyd, B.; Simpson, W.A., PPP Authentication Protocols, 1992
October.
RFC 1570, Simpson, W.A., PPP LCP Extensions, 1994 January.
RFC 1661, Simpson, W.A., The Point-to-Point Protocol (PPP), 1994 July
(obsoletes RFC 1548, RFC 1331, RFC 1171).
RFC 1662, Simpson, W.A., PPP in HDLC-like Framing, 1994 July (obsoletes RFC
1549).
ACKNOWLEDGEMENTS
Greg Christy, Brad Clements, Karl Fox, Brad Parker (brad@fcr.com), Drew
Perkins, Steve Tate (srt@cs.unt.edu)
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Alphabetical
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