OpenBSD-4.6/usr.sbin/pppd/pppd.8

.\" $OpenBSD: pppd.8,v 1.35 2008/11/09 12:34:47 tobias Exp $
.\" Id: pppd.8,v 1.27 1998/03/31 04:31:08 paulus Exp $
.\"
.\" Copyright (c) 1993-2003 Paul Mackerras <paulus@samba.org>
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.Dd $Mdocdate: November 9 2008 $
.Dt PPPD 8
.Os
.Sh NAME
.Nm pppd
.Nd Point-to-Point Protocol daemon
.Sh SYNOPSIS
.Nm pppd
.Op Ar tty_name
.Op Ar speed
.Op Ar options
.Sh DESCRIPTION
PPP is the protocol used for establishing internet links over dial-up
modems, DSL connections, and many other types of point-to-point links.
The
.Nm
daemon works together with the kernel
.Xr ppp 4
driver to establish and maintain a PPP link with another system
(called the
.Em peer )
and to negotiate Internet Protocol (IP) addresses for each end of the link.
.Nm
can also authenticate the peer and/or supply authentication information
to the peer.
PPP can be used with other network protocols besides IP, but such use
is becoming increasingly rare.
.Sh FREQUENTLY USED OPTIONS
.Bl -tag -width Ds
.It Ar tty_name
Use the serial port called
.Ar ttyname
to communicate with the peer.
The string
.Dq /dev/
is prepended to
.Ar ttyname
to form the name of the device to open.
If no device name is given, or if the name of the terminal
connected to the standard input is given,
.Nm
will use that terminal, and will not fork to put itself in the background.
This option is privileged if the
.Cm noauth
option is used.
.It Ar speed
An option that is a decimal number is taken as the desired baud rate
for the serial device.
On systems such as
.Bx 4.4
and
.Ox ,
any speed can be specified.
Other systems (e.g., Linux, SunOS) only support the commonly used
baud-rates.
.It Cm active-filter Ar filter-expression
Specifies a packet filter to be applied to data packets to determine
which packets are to be regarded as link activity, and therefore reset
the idle timer, or cause the link to be brought up in demand-dialling
mode.
This option is useful in conjunction with the
.Cm idle
option if there are packets being sent or received regularly over the link
(for example, routing information packets)
which would otherwise prevent the link from ever appearing to be idle.
The
.Ar filter-expression
syntax is as described for
.Xr tcpdump 8 ,
except that qualifiers which are inappropriate for a PPP link, such as
.Ar ether
and
.Ar arp ,
are not permitted.
Generally the filter expression should be enclosed in single quotes to
prevent whitespace in the expression from being interpreted by the shell.
.It Cm asyncmap Ar map
This option sets the Async-Control-Character-Map (ACCM) for this end
of the link.
The ACCM is a set of 32 bits, one for each of the ASCII control characters
with values from 0 to 31, where a 1 bit indicates that the corresponding
control character should not be used in PPP packets sent to this system.
The map is encoded as a hexadecimal number (without a leading 0x) where the
least significant bit (00000001) represents character 0 and the
most significant bit (80000000) represents character 31.
.Nm
will ask the peer to send these characters as a 2-byte escape sequence.
If multiple
.Cm asyncmap
options are given, the values are ORed together.
If no
.Cm asyncmap
option is given, no async character map will be negotiated for the receive
direction; the peer should then escape
.Em all
control characters.
To escape transmitted characters, use the
.Cm escape
option.
.It Cm auth
Require the peer to authenticate itself before allowing network
packets to be sent or received.
.Pp
.It Cm call Ar name
Read options from the file
.Pa /etc/ppp/peers/name .
This file may contain privileged options, such as
.Cm noauth ,
even if
.Nm
is not being run by root.
The
.Ar name
string may not begin with
.Qq /
or include
.Qq ..
as a pathname component.
The format of the options file is described below.
.It Cm connect Ar script
Usually there is something which needs to be done to prepare the link
before the PPP protocol can be started; for instance, with a dial-up
modem, commands need to be sent to the modem to dial the appropriate
phone number.
This option specifies an command for
.Nm
to execute (by passing it to a shell) before attempting to start PPP
negotiation.
The
.Xr chat 8
program is often useful here, as it provides a way to send arbitrary strings
to a modem and respond to received characters.
This option is privileged if the
.Cm noauth
option is used.
.It Cm crtscts
Specifies that
.Nm
should set the serial port to use hardware flow control using the RTS and CTS
signals in the RS-232 interface.
If neither the
.Cm crtscts
nor the
.Cm nocrtscts
option is given, the hardware flow control setting for the serial port
is left unchanged.
.It Cm 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
.Cm nodefaultroute
option has been specified.
.It Cm disconnect Ar script
Execute the command specified by
.Ar script ,
by passing it to a shell,
after
.Nm
has terminated the link.
This command 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
.Cm noauth
option is used.
.It Cm escape Ar 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
.Cm escape
option, unlike the
.Cm 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.
.It Cm file Ar name
Read options from file
.Ar name
(the format is described below).
The file must be readable by the user who has invoked
.Nm pppd .
.It Cm lock
Specifies that
.Nm
should create a UUCP-style lock file for the
serial device to ensure exclusive access to the device.
.It Cm mru Ar n
Set the MRU (Maximum Receive Unit) value to
.Ar n .
.Nm
will ask the peer to send packets of no more than
.Ar n
bytes.
The value of
.Ar n
must be between 128 and 16384; the default is 1500.
A value of 296 works well on very slow links
(40 bytes for TCP/IP header + 256 bytes of data).
Note that for the IPv6 protocol, the MRU must be at least 1280.
.It Cm mtu Ar n
Set the MTU (Maximum Transmit Unit) value to
.Ar n .
Unless the peer requests a smaller value via MRU negotiation,
.Nm
will request that the kernel networking code send data packets of no more than
.Ar n
bytes through the PPP network interface.
Note that for the IPv6 protocol, the MTU must be at least 1280.
.It Cm passive
Enables the
.Qq passive
option in the LCP.
With this option,
.Nm
will attempt to initiate a connection; if no reply is received from the peer,
.Nm
will then just wait passively for a valid LCP packet from the peer,
instead of exiting, as it would without this option.
.El
.Sh OPTIONS
.Bl -tag -width Ds
.It Xo
.Oo Ar local_IP_address Oc : Ns
.Op Ar remote_IP_address
.Xc
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
.Cm 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,
.Nm
will not accept a different value from the peer in the IPCP negotiation,
unless the
.Cm ipcp-accept-local
and/or
.Cm ipcp-accept-remote
options are given, respectively.
.It Cm bsdcomp Ar nr,nt
Request that the peer compress packets that it sends, using the
BSD-Compress scheme, with a maximum code size of
.Ar nr
bits, and agree to compress packets sent to the peer with
a maximum code size of
.Ar nt
bits.
If
.Ar nt
is not specified, it defaults to the value given for
.Ar nr .
Values in the range 9 to 15 may be used for
.Ar nr
and
.Ar nt ;
larger values give better compression but
consume more kernel memory for compression dictionaries.
Alternatively, a value of 0 for
.Ar nr
or
.Ar nt
disables compression in the corresponding direction.
.Cm nobsdcomp
or
.Cm bsdcomp 0
disables BSD-Compress compression entirely.
.It Cm chap-interval Ar n
If this option is given,
.Nm
will rechallenge the peer every
.Ar n
seconds.
.It Cm chap-max-challenge Ar n
Set the maximum number of CHAP challenge transmissions to
.Ar n
(default 10).
.It Cm chap-restart Ar n
Set the CHAP restart interval (retransmission timeout for challenges) to
.Ar n
seconds (default 3).
.It Cm debug
Enables connection debugging facilities.
If this option is given,
.Nm
will log the contents of all control packets sent or received in a
readable form.
The packets are logged through
.Xr syslogd 8
with facility
.Ar daemon
and level
.Ar debug .
This information can be directed to a file by setting up
.Pa /etc/syslog.conf
appropriately (see
.Xr syslog.conf 5 ) .
.It Cm default-asyncmap
Disable asyncmap negotiation, forcing all control characters to be
escaped for both the transmit and the receive direction.
.It Cm default-mru
Disable MRU (Maximum Receive Unit) negotiation.
With this option,
.Nm
will use the default MRU value of 1500 bytes for both the
transmit and receive direction.
.It Cm deflate Ar nr,nt
Request that the peer compress packets that it sends, using the
Deflate scheme, with a maximum window size of
.Ar 2**nr
bytes, and agree to compress packets sent to the peer with
a maximum window size of
.Ar 2**nt
bytes.
If
.Ar nt
is not specified, it defaults to the value given for
.Ar nr .
Values in the range 8 to 15 may be used for
.Ar nr
and
.Ar nt ;
larger values give better compression but consume more kernel memory
for compression dictionaries.
Alternatively, a value of 0 for
.Ar nr
or
.Ar nt
disables compression in the corresponding direction.
Use
.Cm nodeflate
or
.Cm deflate 0
to disable Deflate compression entirely.
(Note:
.Nm
requests Deflate compression in preference to BSD-Compress if the peer
can do either.)
.It Cm demand
Initiate the link only on demand, i.e., when data traffic is present.
With this option, the remote IP address must be specified by the user
on the command line or in an options file.
.Nm
will initially configure the interface and enable it for IP traffic without
connecting to the peer.
When traffic is available,
.Nm
will connect to the peer and perform negotiation, authentication, etc.
When this is completed,
.Nm
will commence passing data packets (i.e., IP packets) across the link.
.Pp
The
.Cm demand
option implies the
.Cm persist
option.
If this behaviour is not desired, use the
.Cm nopersist
option after the
.Cm demand
option.
The
.Cm idle
and
.Cm holdoff
options are also useful in conjunction with the
.Cm demand
option.
.It Cm domain Ar d
Append the domain name
.Ar d
to the local host name for authentication purposes.
For example, if
.Xr gethostname 3
returns the name porsche, but the fully qualified domain name is
porsche.Quotron.COM, you could specify
.Cm domain Quotron.COM .
.Nm
would then use the name
.Ar 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.
.It Cm holdoff Ar n
Specifies how many seconds to wait before re-initiating the link after
it terminates.
This option only has any effect if the
.Cm persist
or
.Cm demand
option is used.
The holdoff period is not applied if the link was terminated
because it was idle.
.It Cm idle Ar n
Specifies that
.Nm
should disconnect if the link is idle for
.Ar n
seconds.
The link is idle when no data packets (i.e., IP packets) are
being sent or received.
Note: it is not advisable to use this option with the
.Cm persist
option without the
.Cm demand
option.
If the
.Cm active-filter
option is given, data packets which are rejected by the specified
activity filter also count as the link being idle.
.It Cm ipcp-accept-local
With this option,
.Nm
will accept the peer's idea of our local IP address,
even if the local IP address was specified in an option.
.It Cm ipcp-accept-remote
With this option,
.Nm
will accept the peer's idea of its (remote) IP address,
even if the remote IP address was specified in an option.
.It Cm ipcp-max-configure Ar n
Set the maximum number of IPCP configure-request transmissions to
.Ar n
(default 10).
.It Cm ipcp-max-failure Ar n
Set the maximum number of IPCP configure-NAKs returned before starting
to send configure-Rejects to
.Ar n
(default 10).
.It Cm ipcp-max-terminate Ar n
Set the maximum number of IPCP terminate-request transmissions to
.Ar n
(default 3).
.It Cm ipcp-restart Ar n
Set the IPCP restart interval (retransmission timeout) to
.Ar n
seconds (default 3).
.It Cm ipparam Ar string
Provides an extra parameter to the ip-up and ip-down scripts.
If this option is given, the
.Ar string
supplied is given as the 6th parameter to those scripts.
.It Cm kdebug Ar n
Enable debugging code in the kernel-level PPP driver.
The argument
.Ar n
is a number which is the sum of the following values:
1 to enable general debug messages,
2 to request that the contents of received packets be printed,
and 4 to request that the contents of transmitted packets be printed.
On most systems, messages printed by the kernel are logged by
.Xr syslogd 8
to a file as directed in the
.Pa /etc/syslog.conf
configuration file.
.It Cm lcp-echo-failure Ar n
If this option is given,
.Nm
will presume the peer to be dead if
.Ar n
LCP echo-requests are sent without receiving a valid LCP echo-reply.
If this happens,
.Nm
will terminate the connection.
Use of this option requires a non-zero value for the
.Cm lcp-echo-interval
parameter.
This option can be used to enable
.Nm
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.
.It Cm lcp-echo-interval Ar n
If this option is given,
.Nm
will send an LCP echo-request frame to the peer every
.Ar n
seconds.
Normally the peer should respond to the echo-request by sending an echo-reply.
This option can be used with the
.Cm lcp-echo-failure
option to detect that the peer is no longer connected.
.It Cm lcp-max-configure Ar n
Set the maximum number of LCP configure-request transmissions to
.Ar n
(default 10).
.It Cm lcp-max-failure Ar n
Set the maximum number of LCP configure-NAKs returned before starting
to send configure-Rejects to
.Ar n
(default 10).
.It Cm lcp-max-terminate Ar n
Set the maximum number of LCP terminate-request transmissions to
.Ar n
(default 3).
.It Cm lcp-restart Ar n
Set the LCP restart interval (retransmission timeout) to
.Ar n
seconds (default 3).
.It Cm local
Don't use the modem control lines.
With this option,
.Nm
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.
.It Cm 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
.Pa /etc/ppp/pap-secrets
file as well as the system password database to be allowed access.
.It Cm maxconnect Ar n
Terminate the connection when it has been available for network
traffic for
.Ar n
seconds (i.e.,
.Ar n
seconds after the first network control protocol comes up).
.It Cm modem
Use the modem control lines.
This option is the default.
With this option,
.Nm
will wait for the CD (Carrier Detect) signal from the
modem to be asserted when opening the serial device (unless a connect
script is specified), and it will drop the DTR (Data Terminal Ready)
signal briefly when the connection is terminated and before executing
the connect script.
On Ultrix, this option implies hardware flow control, as for the
.Cm crtscts
option.
.It Cm modem_chat
Use the modem control lines during the chat script.
The default is to ignore the state of the CD (Carrier Detect) signal
from the modem during the chat script.
If you are using a
.Xr cua 4
device (as opposed to a
.Xr tty 4
device)
you should set this option.
You should not use this option with a dialback setup as it will cause
the chat script to exit when carrier drops.
.It Cm ms-dns Op Ar addr
If
.Nm
is acting as a server for Microsoft Windows clients, this option allows
.Nm
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
.Nm
under the name
.Cm dns-addr . )
.It Cm ms-wins Op Ar addr
If
.Nm
is acting as a server for Microsoft Windows or
.Qq Samba
clients,
this option allows
.Nm
to supply one or two WINS (Windows 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.
.It Cm name Ar name
Set the name of the local system for authentication purposes to
.Ar name .
This is a privileged option.
With this option,
.Nm
will use lines in the secrets files which have
.Ar name
as the second field when looking for a secret to use
in authenticating the peer.
In addition, unless overridden with the
.Cm user
option,
.Ar name
will be used as the name to send to the peer when authenticating the
local system to the peer.
(Note that
.Nm
does not append the domain name to
.Ar name . )
.It Cm netmask Ar n
Set the interface netmask to
.Ar n ,
a 32-bit netmask in
.Dq 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.
(Note: on some platforms,
.Nm
will always use 255.255.255.255 for the netmask, if that is the only
appropriate value for a point-to-point interface.)
.It Cm noaccomp
Disable Address/Control compression in both directions (send and receive).
.It Cm noauth
Do not require the peer to authenticate itself.
This option is privileged if the
.Cm auth
option is specified in
.Pa /etc/ppp/options .
.It Cm nobsdcomp
Disables BSD-Compress compression;
.Nm
will not request or agree to compress packets using the BSD-Compress scheme.
.It Cm noccp
Disable CCP (Compression Control Protocol) negotiation.
This option should only be required if the peer is buggy and gets confused by
requests from
.Nm
for CCP negotiation.
.It Cm nocrtscts
Disable hardware flow control (i.e., RTS/CTS) on the serial port.
If neither the
.Cm crtscts
nor the
.Cm nocrtscts
option is given, the hardware flow control setting for the serial port
is left unchanged.
.It Cm nodefaultroute
Disable the
.Cm defaultroute
option.
The system administrator who wishes to prevent users from creating
default routes with
.Nm
can do so by placing this option in the
.Pa /etc/ppp/options
file.
.It Cm nodeflate
Disables Deflate compression;
.Nm
will not request or agree to compress packets using the Deflate scheme.
.It Cm nodetach
Don't detach from the controlling terminal.
Without this option, if a serial device other than the terminal
on the standard input is specified,
.Nm
will fork to become a background process.
.It Cm noip
Disable IPCP negotiation and IP communication.
This option should only be required if the peer is buggy and gets confused
by requests from
.Nm
for IPCP negotiation.
.It Cm 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 was specified explicitly
on the command line or in an options file).
.It Cm nomagic
Disable magic number negotiation.
With this option,
.Nm
cannot detect a looped-back line.
This option should only be needed if the peer is buggy.
.It Cm nopcomp
Disable protocol field compression negotiation in both the receive and
the transmit direction.
.It Cm nopersist
Exit once a connection has been made and terminated.
This is the default unless the
.Cm persist
or
.Cm demand
option has been specified.
.It Cm nopredictor1
Do not accept or agree to Predictor-1 compression.
.It Cm noproxyarp
Disable the
.Cm proxyarp
option.
The system administrator who wishes to prevent users from creating
proxy ARP entries with
.Nm
can do so by placing this option in the
.Pa /etc/ppp/options
file.
.It Cm novj
Disable Van Jacobson style TCP/IP header compression in both the
transmit and the receive direction.
.It Cm novjccomp
Disable the connection-ID compression option in Van Jacobson style
TCP/IP header compression.
With this option,
.Nm
will not omit the connection-ID byte from Van Jacobson compressed
TCP/IP headers, nor ask the peer to do so.
.It Cm papcrypt
Indicates that all secrets in the
.Pa /etc/ppp/pap-secrets
file which are used for checking the identity of the peer are encrypted,
and thus
.Nm
should not accept a password which, before encryption,
is identical to the secret from the
.Pa /etc/ppp/pap-secrets
file.
.It Cm pap-max-authreq Ar n
Set the maximum number of PAP authenticate-request transmissions to
.Ar n
(default 10).
.It Cm pap-restart Ar n
Set the PAP restart interval (retransmission timeout) to
.Ar n
seconds (default 3).
.It Cm pap-timeout Ar n
Set the maximum time that
.Nm
will wait for the peer to authenticate itself with PAP to
.Ar n
seconds (0 means no limit).
.It Cm pass-filter Ar filter-expression
Specifies a packet filter to apply 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 protocols
using up link bandwidth, or to provide a basic firewall capability.
The
.Ar filter-expression
syntax is as described for
.Xr tcpdump 8 ,
except that qualifiers which are inappropriate for a PPP link, such as
.Ar ether
and
.Ar arp ,
are not permitted.
Generally the filter expression should be enclosed in single quotes to prevent
whitespace in the expression from being interpreted by the shell.
Note that it is possible to apply different constraints to incoming and
outgoing packets using the
.Cm inbound
and
.Cm outbound
qualifiers.
.It Cm persist
Do not exit after a connection is terminated; instead try to reopen
the connection.
.It Cm predictor1
Request that the peer compress frames that it sends using Predictor-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.
.It Cm 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.
.It Cm remotename Ar name
Set the assumed name of the remote system for authentication purposes to
.Ar name .
.It Cm refuse-chap
With this option,
.Nm
will not agree to authenticate itself to the peer using CHAP.
.It Cm refuse-pap
With this option,
.Nm
will not agree to authenticate itself to the peer using PAP.
.It Cm require-chap
Require the peer to authenticate itself using CHAP
(Challenge Handshake Authentication Protocol) authentication.
.It Cm require-pap
Require the peer to authenticate itself using PAP
(Password Authentication Protocol) authentication.
.It Cm silent
With this option,
.Nm
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
.Nm pppd ) .
.It Cm usehostname
Enforce the use of the hostname (with domain name appended, if given)
as the name of the local system for authentication purposes (overrides the
.Cm name
option).
.It Cm user Ar name
Sets the name used for authenticating the local system to the peer to
.Ar name .
.It Cm vj-max-slots Ar n
Sets the number of connection slots to be used by the Van Jacobson
TCP/IP header compression and decompression code to
.Ar n ,
which must be between 2 and 16, inclusive.
.It Cm welcome Ar script
Run the executable or shell command specified by
.Ar script
before initiating PPP negotiation, after the connect script (if any) has
completed.
This option is privileged if the
.Cm noauth
option is used.
.It Cm xonxoff
Use software flow control (i.e., XON/XOFF) to control the flow of data on
the serial port.
.El
.Sh OPTIONS FILES
Options can be taken from files as well as the command line.
.Nm
reads options from the files
.Pa /etc/ppp/options , ~/.ppprc
and
.Pf /etc/ppp/options. Ns Ar 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
.Pf options. Ns Ar ttyname
file is read.)
In forming the name of the
.Pf options. Ns Ar ttyname
file,
the initial /dev/ is removed from the terminal name, and any remaining
/ characters are replaced with dots.
.Pp
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-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
.Cm file
or
.Cm call
options within an options file.
.Sh SECURITY
Users must be in group
.Qq network
to be able to use
.Nm pppd .
.Pp
.Nm
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
.Pa /etc/ppp/options file ,
where the administrator can place options to restrict the ways in which
.Nm
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.
.Pp
The normal way that
.Nm
should be set up is to have the
.Cm auth
option in the
.Pa /etc/ppp/options file .
(This may become the default in later releases.)
If users wish to use
.Nm
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
.Pa /etc/ppp/peers
containing the
.Cm noauth
option, the name of the serial port to use, and the
.Cm connect
option (if required), plus any other appropriate options.
In this way,
.Nm
can be set up to allow non-privileged users to make unauthenticated
connections only to trusted peers.
.Pp
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
.Nm pppd ,
either on the command line, in the user's
.Pa ~/.ppprc
file, or in an options file read using the
.Cm file
option.
Privileged options may be used in the
.Pa /etc/ppp/options
file or in an options file read using the
.Cm call
option.
If
.Nm
is being run by the root user, privileged options can be used without
restriction.
.Sh 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
.Qq client
and the other the
.Qq server .
The client has a name by which it identifies itself to the server,
and the server also has a name by which it identifies itself to the client.
Generally the genuine client shares some secret (or password) with the server,
and authenticates itself by proving that it knows that secret.
Very often, the names used for authentication correspond to the internet
hostnames of the peers, but this is not essential.
.Pp
At present,
.Nm
supports two authentication protocols:
the Password Authentication Protocol (PAP)
and the Challenge Handshake Authentication 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.
.Pp
The PPP protocol, being symmetrical, allows both peers to require the
other to authenticate itself.
In that case, two separate and independent authentication exchanges
will occur.
The two exchanges could use different authentication protocols,
and in principle, different names could be used in the two exchanges.
.Pp
The default behaviour of
.Nm
is to agree to authenticate if requested, and to not require authentication
from the peer.
However,
.Nm
will not agree to authenticate itself with a particular protocol
if it has no secrets which could be used to do so.
.Pp
.Nm
stores secrets for use in authentication in secrets files
.Pf ( Ns Pa /etc/ppp/pap-secrets
for PAP,
.Pa /etc/ppp/chap-secrets
for CHAP).
Both secrets files have the same format.
The secrets files can contain secrets for
.Nm
to use in authenticating itself to other systems, as well as secrets for
.Nm
to use when authenticating other systems to itself.
.Pp
Each line in a secrets file contains one 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, or if the first word is
.Qq \- ,
then all IP addresses are disallowed.
To allow any address, use
.Qq * .
A word starting with
.Qq \&!
indicates that the specified address is
.Em not
acceptable.
An address may be followed by
.Qq /
and a number
.Ar n ,
to indicate a whole subnet, i.e., all addresses which have the same value
in the most significant
.Ar n
bits.
Case is significant in the client and server names and in the secret.
.Pp
If the secret starts with an `@', what follows is assumed to be the
name of a file from which to read the secret.
A
.Qq *
as the client or server name matches any name.
When selecting a secret,
.Nm
takes the best match, i.e., the match with the fewest wildcards.
.Pp
Thus a secrets file contains both secrets for use in authenticating
other hosts, plus secrets which we use for authenticating ourselves to
others.
When
.Nm
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
.Cm domain
option is used.
This default can be overridden with the
.Cm name
option, except when the
.Cm usehostname
option is used.
.Pp
When
.Nm
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
.Cm 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
.Nm
looks for a secret with this name in the first field and the peer's name
in the second field.
.Nm
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,
.Nm
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
.Cm remotename
option.
Otherwise, if the remote IP address was specified by a name
(rather than in numeric form), that name will be used as the peer's name.
Failing that,
.Nm
will use the null string as the peer's name.
.Pp
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
.Xr crypt 3
and checked against the secret again.
Thus secrets for authenticating the peer can be stored in encrypted form
if desired.
If the
.Cm papcrypt
option is given, the first (unencrypted) comparison is omitted,
for better security.
.Pp
Furthermore, if the
.Cm login
option was specified, the username 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
.Cm login
option, the secret in
.Pa /etc/ppp/pap-secrets
would be
.Qq ,
which will match any password supplied by the peer.
This avoids the need to have the same secret in two places.
.Pp
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,
.Nm
will terminate 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.
.Pp
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,
.Nm
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.
.Sh ROUTING
When IPCP negotiation is completed successfully,
.Nm
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 most cases the
.Cm defaultroute
and/or
.Cm proxyarp
options are sufficient for this, but in some cases
further intervention is required.
The
.Pa /etc/ppp/ip-up
script can be used for this.
.Pp
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
.Cm defaultroute
option causes
.Nm
to create such a default route when IPCP comes up, and
delete it when the link is terminated.
.Pp
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
.Cm proxyarp
option causes
.Nm
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,
.Nm
creates a permanent, published ARP entry with the IP address of the remote host
and the hardware address of the network interface found.
.Pp
When the
.Cm demand
option is used, the interface IP addresses have
already been set at the point when IPCP comes up.
If
.Nm
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),
.Nm
has to change the interface IP addresses to the negotiated addresses.
This may disrupt existing connections, and the use of demand dialling with
peers that do dynamic IP address assignment is not recommended.
.Sh EXAMPLES
The following examples assume that the
.Pa /etc/ppp/options
file contains the
.Cm auth
option (as in the default
.Pa /etc/ppp/options
file in the PPP distribution).
.Pp
Probably the most common use of
.Nm
is to dial out to an ISP.
This can be done with a command such as
.Pp
.Dl pppd call isp
.Pp
where the
.Pa /etc/ppp/peers/isp
file is set up by the system administrator to contain something like this:
.Bd -literal -offset indent
ttyS0 19200 crtscts
connect '/usr/sbin/chat -v -f /etc/ppp/chat-isp'
noauth
.Ed
.Pp
In this example, we are using chat to dial the ISP's modem and go
through any logon sequence required.
The
.Pa /etc/ppp/chat-isp
file contains the script used by chat; it could for example contain
something like this:
.Bd -literal -offset indent
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-~"
.Ed
.Pp
See the
.Xr chat 8
man page for details of chat scripts.
.Pp
.Nm
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
.Nm
(installed setuid-root) with a command such as
.Pp
.Dl pppd proxyarp
.Pp
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
.Pa /etc/ppp/pap-secrets
or
.Pa /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
.Qq joespc
which is to be allowed to dial in to the machine called
.Qq server
and use the IP address joespc.my.net, you would add an entry like this to
.Pa /etc/ppp/pap-secrets
or
.Pa /etc/ppp/chap-secrets :
.Pp
.Dl joespc	server	"joe's secret"	joespc.my.net
.Pp
Alternatively, you can create a username called (for example)
.Qq ppp ,
whose login shell is
.Nm
and whose home directory is
.Pa /etc/ppp .
Options to be used when
.Nm
is run this way can be put in
.Pa /etc/ppp/.ppprc .
.Pp
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
.Cm asyncmap a0000 .
If the path includes a telnet, you probably should escape ^] as well
.Pf ( Ns Cm asyncmap 200a0000 ) .
If the path includes an rlogin, you will need to use the
.Cm 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.
.Sh DIAGNOSTICS
Messages are sent to the
.Xr syslogd 8
daemon using facility
.Dv LOG_DAEMON .
(This can be overridden by recompiling
.Nm
with the macro
.Dv LOG_PPP
defined as the desired facility.)
See the
.Xr syslogd 8
documentation for details of where the syslog daemon will write the
messages.
On most systems, the syslog daemon uses the
.Pa /etc/syslog.conf
file to specify the destination(s) for syslog messages.
You may need to edit that file to suit.
.Pp
The
.Cm 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
.Cm debug
option also causes other debugging messages to be logged.
.Pp
Debugging can also be enabled or disabled by sending a SIGUSR1 signal
to the
.Nm
process.
This signal acts as a toggle.
.Sh SCRIPTS
.Nm
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.
.Nm
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.
.Nm
runs the scripts with standard input, output and error redirected to
.Pa /dev/null ,
and with an environment that is empty except for some environment variables
that give information about the link.
The environment variables that
.Nm
sets are:
.Bl -tag -width "PEERNAME"
.It Ev DEVICE
The name of the serial tty device being used.
.It Ev IFNAME
The name of the network interface being used.
.It Ev IPLOCAL
The IP address for the local end of the link.
This is only set when IPCP has come up.
.It Ev IPREMOTE
The IP address for the remote end of the link.
This is only set when IPCP has come up.
.It Ev PEERNAME
The authenticated name of the peer.
This is only set if the peer authenticates itself.
.It Ev SPEED
The baud rate of the tty device.
.It Ev UID
The real user ID of the user who invoked
.Nm pppd .
.El
.Pp
.Nm
invokes the following scripts, if they exist.
It is not an error if they don't exist.
.Bl -tag -width Ds
.It Pa /etc/ppp/auth-up
A program or script which is executed after the remote system
successfully authenticates itself.
It is executed with the parameters
.Pp
.Ar interface-name peer-name user-name tty-device speed
.Pp
Note that this script is not executed if the peer doesn't authenticate
itself, for example when the
.Cm noauth
option is used.
.It Pa /etc/ppp/auth-down
A program or script which is executed when the link goes down, if
.Pa /etc/ppp/auth-up
was previously executed.
It is executed in the same manner with the same parameters as
.Pa /etc/ppp/auth-up .
.It Pa /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
.Pp
.Ar interface-name tty-device speed local-IP-address remote-IP-address ipparam
.It Pa /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
.Pa /etc/ppp/ip-up
script.
It is invoked in the same manner and with the same parameters as the ip-up
script.
.El
.Sh FILES
.Bl -tag -width Ds
.It /var/run/ppp Ns Ar n Ns .pid
.\" (BSD or Linux),
.\" /etc/ppp/ppp Ns Ar n Ns .pid
.\" (others)
Process-ID for
.Nm
process on PPP interface unit
.Ar n .
.It /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.
.Nm
will log a warning if this is not the case.
.It /etc/ppp/chap-secrets
Names, secrets and IP addresses for CHAP authentication.
As for
.Pa /etc/ppp/pap-secrets ,
this file should be owned by root and not readable or writable
by any other user.
.Nm
will log a warning if this is not the case.
.It /etc/ppp/options
System default options for
.Nm pppd ,
read before user default options or command-line options.
.It ~/.ppprc
User default options, read before
.Pf /etc/ppp/options. Ns Ar ttyname .
.It /etc/ppp/options. Ns Ar ttyname
System default options for the serial port being used, read after
.Pa ~/.ppprc .
In forming the
.Ar 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.
.It /etc/ppp/peers
A directory containing options files which may contain privileged
options, even if
.Nm
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.
.El
.Sh SEE ALSO
.Xr cua 4 ,
.Xr ppp 4 ,
.Xr tty 4 ,
.Xr chat 8 ,
.Xr ppp 8 ,
.Xr syslogd 8 ,
.Xr tcpdump 8
.Rs
.%B RFC 1144
.%A Jacobson, V.
.%T Compressing TCP/IP headers for low-speed serial links
.%D February 1990
.Re
.Rs
.%B RFC 1321
.%A Rivest, R.
.%T The MD5 Message-Digest Algorithm
.%D April 1992
.Re
.Rs
.%B RFC 1332
.%A McGregor, G.
.%T PPP Internet Protocol Control Protocol (IPCP)
.%D May 1992
.Re
.Rs
.%B RFC 1334
.%A Lloyd, B.
.%A Simpson, W.A.
.%T PPP authentication protocols
.%D October 1992
.Re
.Rs
.%B RFC 1661
.%A Simpson, W.A.
.%T The Point\-to\-Point Protocol (PPP)
.%D July 1994
.Re
.Rs
.%B RFC 1662
.%A Simpson, W.A.
.%T PPP in HDLC-like Framing
.%D July 1994
.Re
.Sh NOTES
Some limited degree of control can be exercised over a running
.Nm
process by sending it a signal from the list below.
.Bl -tag -width Ds
.It SIGINT , SIGTERM
These signals cause
.Nm
to terminate the link (by closing LCP), restore the serial device settings,
and exit.
.It SIGHUP
This signal causes
.Nm
to terminate the link, restore the serial device settings,
and close the serial device.
If the
.Cm persist
or
.Cm demand
option has been specified,
.Nm
will try to reopen the serial device and start another connection
(after the holdoff period).
Otherwise
.Nm
will exit.
If this signal is received during the holdoff period, it causes
.Nm
to end the holdoff period immediately.
.It SIGUSR1
This signal toggles the state of the
.Cm debug
option.
.It SIGUSR2
This signal causes
.Nm
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.)
.El
.Sh AUTHORS
.An Paul Mackerras Aq Paul.Mackerras@samba.org ,
based on earlier work by Drew Perkins, Brad Clements, Karl Fox, Greg Christy,
and Brad Parker.