pppd(8) System Manager's Manual pppd(8)
NAME
pppd - Point-to-Point Protocol (PPP) daemon
SYNOPSIS
/usr/sbin/pppd [options] [tty_name] [speed]
FLAGS
Communicates over the named device. The string /dev/ is prepended if
necessary. If no device name is given or if the name of the control-
ling terminal is given, pppd uses the controlling terminal, and does
not fork to put itself in the background. Sets the baud rate to speed.
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 es-
cape 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 to-
gether. If no asyncmap option is given, no async character map is ne-
gotiated for the receive direction; the peer then escapes all control
characters. Requires the peer to authenticate itself before allowing
network packets to be sent or received. 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. Uses hardware flow control (RTS/CTS) to control the flow
of data on the serial port. 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. 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. Runs the exe-
cutable 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 avail-
able. 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 hexadecimal numbers separated by commas. Note
that almost any character can be specified for the escape option, un-
like 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. Reads options from file f. See the Op-
tions File section for a description of the format. Specifies that
pppd should use a UUCP-style lock on the serial device to ensure exclu-
sive access to the device. Sets the MRU (Maximum Receive Unit) value
to n for negotiation. The pppd daemon will ask the peer to send pack-
ets of no more than n bytes. The minimum MRU value is 128. The de-
fault MRU value is 1500. A value of 296 is recommended for slow links
(40 bytes for TCP/IP header + 256 bytes of data). Sets the interface
netmask to n, a 32-bit netmask in dotted-decimal notation (for example,
255.255.255.0). 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). 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 ex-
tensible 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
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 lo-
cal address is the (first) IP address of the system (unless the noipde-
fault option is given). The remote address will be obtained from the
peer if not specified in any option. Thus, in simple cases, this op-
tion 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-re-
mote options are given, respectively. Does not request or allow nego-
tiation of any options for LCP and IPCP (use default values). Disables
Address/Control compression negotiation (use default, that is, ad-
dress/control field disabled). Disables asyncmap negotiation (use the
default asyncmap, that is, escape all control characters). Same as
asyncmap n. Requires the peer to authenticate itself using CHAP (Cryp-
tographic Handshake Authentication Protocol) authentication. Does not
agree to authenticate using CHAP. Increases debugging level (same as
the debug option). Does not fork to become a background process (oth-
erwise pppd will do so if a serial device is specified). 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.
Disables magic number negotiation. With this option, pppd cannot de-
tect a looped-back line. Disables MRU (Maximum Receive Unit) negotia-
tion (use default, that is, 1500). Same as the passive option. Re-
quires the peer to authenticate itself using PAP. Does not agree to
authenticate using PAP. Disables protocol field compression negotia-
tion (use default, that is, protocol field compression disabled).
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. Disables negotiation of Van Jacob-
son-style IP header compression. Instead, it uses no compression (the
default). Requests the peer to compress all packets that it sends, us-
ing 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. Disables compres-
sion; pppd will not request or agree to compress packets using the BSD-
Compress scheme. If this option is given, pppd challenges the peer
every n seconds. Sets the maximum number of CHAP challenge transmis-
sions to n (default 10). Sets the CHAP restart interval (retransmis-
sion timeout for challenges) to n seconds (default 3). Increases de-
bugging 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 syslogd(8)). (If pppd is compiled
with extra debugging enabled, it will log messages using facility lo-
cal2 instead of daemon). Appends the domain name d to the local host
name for authentication purposes. For example, if gethostname() re-
turns 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. 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. 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.
Sets the maximum number of IPCP configure-request transmissions to n
(default 10). Sets the maximum number of IPCP configure-NAKs returned
before starting to send configure-Rejects instead to n (default 10).
Sets the maximum number of IPCP terminate-request transmissions to n
(default 3). Sets the IPCP restart interval (retransmission timeout)
to n seconds (default 3). Enables debugging code in the kernel-level
PPP driver. The argument n is a number that is the sum of the follow-
ing values: 1 (enables general debug messages), 2 (requests that the
contents of received packets be printed), and 4 (requests that the con-
tents of transmitted packets be printed). If this option is given,
pppd presumes the peer to be dead if n LCP echo-requests are sent with-
out 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. If this option is given, pppd sends an LCP echo-
request frame to the peer every n seconds. Sets the maximum number of
LCP configure-request transmissions to n (default 10). Sets the maxi-
mum number of LCP configure-NAKs returned before starting to send con-
figure-Rejects instead to n (default 10). Sets the maximum number of
LCP terminate-request transmissions to n (default 3). Sets the LCP
restart interval (retransmission timeout) to n seconds (default 3).
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. Uses the
system password database for authenticating the peer using PAP. Uses
the modem control lines. This option is the default. With this op-
tion, 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. 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. Sets the name of the local
system for authentication purposes to n. Disables the default behavior
when no local IP address is specified, which is to determine (if possi-
ble) 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.
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. Sets the maximum number
of PAP authenticate-request transmissions to n (default 10). Sets the
PAP restart interval (retransmission timeout) to n seconds (default 3).
Do not exit after a connection is terminated. Instead, try to reopen
the connection. Adds an entry to this system's ARP (Address Resolution
Protocol) table with the IP address of the peer and the Ethernet ad-
dress of this system. Sets the assumed name of the remote system for
authentication purposes to n. Enforces the use of the hostname as the
name of the local system for authentication purposes (overrides the
name option). Sets the user name to use for authenticating this ma-
chine 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 be-
fore looking at the command line. An options file is parsed into a se-
ries of words, delimited by whitespace. Whitespace can be included in
a word by enclosing the word in double quotation marks ("). A back-
slash (\) 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 ma-
chine 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 indi-
vidual 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 de-
sired.
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 or-
der: 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 re-
mote 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 authen-
ticating the peer, the client must match the username specified in the
PAP authenticate-request and server must match the local name. For au-
thenticating 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 pass-
word 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 (unen-
crypted) 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 admin-
istrator 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 authen-
ticating the peer, the client must match the name specified in the
CHAP-Response message and server must match the local name. For au-
thenticating 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 negoti-
ates 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 authen-
ticate themselves to connect and use one of a restricted set of IP ad-
dresses, 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-se-
crets 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 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 re-
quired.
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.
NOTES
The following signals have the specified effect when sent to the pppd
process: Cause pppd to terminate the link (by closing LCP), restore the
serial device settings, and exit. This signal causes pppd to terminate
the link, restore the serial device settings, and close the serial de-
vice. If the persist option has been specified, pppd tries to reopen
the serial device and start another connection. Otherwise, pppd exits.
Causes pppd to renegotiate compression. This can be useful to re-en-
able compression after it has been disabled as a result of a fatal de-
compression 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 lo-
cal options are not well defined.
EXAMPLES
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 fol-
lowing on each machine: pppd /dev/ttya 9600 passive If one machine has
a getty running, you can log in to the machine from another machine us-
ing kermit or tip, and issue the following command: pppd passive
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
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 par-
ticular, it is often useful to escape XON (^Q) and XOFF (^S), using
asyncmap a0000. If the path includes a telnet, you probably should es-
cape ^] 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.
DIAGNOSTICS
Messages are sent to the syslogd daemon using facility LOG_DAEMON. 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 re-
ceived 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 debug-
ging messages to be logged.
Debugging can also be toggled on and off by sending a SIGUSR1 to the
pppd process.
FILES
Process ID for pppd process on ppp interface unit n. A program or
script that is executed when the link is available for sending and re-
ceiving 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 effec-
tive 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. 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 parame-
ters 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. Usernames, passwords and IP addresses for PAP
authentication. Names, secrets and IP addresses for CHAP au-
thentication. System default options for pppd (read before user
default options or command-line options). User default options
(read before command-line options). System default options for
the serial port being used (read after command-line options).
RELATED INFORMATION
Commands: chat(8), pppstats(8).
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., The PPP Internet Protocol Control Protocol
(IPCP), 1992 May (obsoletes RFC1172).
RFC1334, Lloyd, B.; Simpson, W.A., PPP Authentication Protocols, 1992
October.
RFC1570, Simpson, W.A., PPP LCP Extensions, 1994 January.
RFC1661, Simpson, W.A., The Point-to-Point Protocol (PPP), 1994 July
(obsoletes RFC1548, RFC1331, RFC1171).
RFC1662, Simpson, W.A., PPP in HDLC-like Framing, 1994 July (obsoletes
RFC1549).
ACKNOWLEDGEMENTS
Greg Christy, Brad Clements, Karl Fox, Brad Parker (
[email protected]), Drew
Perkins, Steve Tate (
[email protected]) delim off
pppd(8)