pppd - Point-to-Point Protocol daemon
pppd [tty_name] [speed] [options]
PPP is the protocol used for establishing internet links
over dial-up
modems, DSL connections, and many other types of point-topoint links.
The pppd daemon works together with the kernel ppp(4) driver
to establish
and maintain a PPP link with another system (called the
peer) and to negotiate
Internet Protocol (IP) addresses for each end of the
link. pppd
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.
FREQUENTLY USED OPTIONS [Toc] [Back] tty_name
Use the serial port called ttyname to communicate
with the peer.
The string ``/dev/'' is prepended to 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, pppd
will use that terminal, and will not fork to put itself in the
background. This option is privileged if the noauth
option is
used.
speed An option that is a decimal number is taken as the
desired baud
rate for the serial device. On systems such as
4.4BSD and OpenBSD,
any speed can be specified. Other systems
(e.g., Linux,
SunOS) only support the commonly used baud-rates.
active-filter filter-expression
Specifies a packet filter to be applied to data
packets to determine
which packets are to be regarded as link activity, and
therefore reset the idle timer, or cause the link to
be brought
up in demand-dialling mode. This option is useful
in conjunction
with the 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 filter-expression syntax is as described for
tcpdump(8), except that qualifiers which are inappropriate for a
PPP link, such as ether and 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. This option is currently only available under OpenBSD,
and then only if both the kernel and pppd were compiled with
PPP_FILTER defined.
asyncmap 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.
pppd will
ask the peer to send these characters as a 2-byte
escape sequence.
If multiple asyncmap options are given, the
values are
ORed together. If no asyncmap option is given, no
async character
map will be negotiated for the receive direction; the peer
should then escape all control characters. To escape transmitted
characters, use the escape option.
auth Require the peer to authenticate itself before allowing network
packets to be sent or received.
call name
Read options from the file /etc/ppp/peers/name.
This file may
contain privileged options, such as noauth, even if
pppd is not
being run by root. The name string may not begin
with "/" or include
".." as a pathname component. The format of
the options
file is described below.
connect script
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
pppd to execute (by passing it to a shell) before
attempting to
start PPP negotiation. The 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
noauth option is used.
crtscts
Specifies that pppd should set the serial port to
use hardware
flow control using the RTS and CTS signals in the
RS-232 interface.
If neither the crtscts nor the nocrtscts option is given,
the hardware flow control setting for the serial
port is left unchanged.
defaultroute
Add a default route to the system routing tables,
using the peer
as the gateway, when IPCP negotiation is successfully completed.
This entry is removed when the PPP connection is
broken. This
option is privileged if the nodefaultroute option
has been specified.
disconnect script
Execute the command specified by script, by passing
it to a
shell, after pppd 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 noauth option is used.
escape xx,yy,...
Specifies that certain characters should be escaped
on transmission
(regardless of whether the peer requests them
to be escaped
with its async control character map). The characters to be escaped
are specified as a list of hex numbers separated by commas.
Note that almost any character can be specified for
the escape
option, unlike the asyncmap option which only allows
control
characters to be specified. The characters which
may not be escaped
are those with hex values 0x20 - 0x3f or 0x5e.
file name
Read options from file name (the format is described
below). The
file must be readable by the user who has invoked
pppd.
lock Specifies that pppd should create a UUCP-style lock
file for the
serial device to ensure exclusive access to the device.
mru n Set the MRU (Maximum Receive Unit) value to n. pppd
will ask the
peer to send packets of no more than n bytes. The
value of 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.
mtu n Set the MTU (Maximum Transmit Unit) value to n. Unless the peer
requests a smaller value via MRU negotiation, pppd
will request
that the kernel networking code send data packets of
no more than
n bytes through the PPP network interface. Note
that for the
IPv6 protocol, the MTU must be at least 1280.
passive
Enables the "passive" option in the LCP. With this
option, pppd
will attempt to initiate a connection; if no reply
is received
from the peer, pppd will then just wait passively
for a valid LCP
packet from the peer, instead of exiting, as it
would without
this option.
[local_IP_address]:[remote_IP_address]
Set the local and/or remote interface IP addresses.
Either one
may be omitted. The IP addresses can be specified
with a host
name or in decimal dot notation (e.g.,
150.234.56.78). The default
local address is the (first) IP address of the
system (unless
the noipdefault option is given). The remote
address will
be obtained from the peer if not specified in any
option. Thus,
in simple cases, this option is not required. If a
local and/or
remote IP address is specified with this option,
pppd will not
accept a different value from the peer in the IPCP
negotiation,
unless the ipcp-accept-local and/or
ipcp-accept-remote options
are given, respectively.
bsdcomp nr,nt
Request that the peer compress packets that it
sends, using the
BSD-Compress scheme, with a maximum code size of nr
bits, and
agree to compress packets sent to the peer with a
maximum code
size of nt bits. If nt is not specified, it defaults to the value
given for nr. Values in the range 9 to 15 may be
used for nr
and nt; larger values give better compression but
consume more
kernel memory for compression dictionaries. Alternatively, a
value of 0 for nr or nt disables compression in the
corresponding
direction. nobsdcomp or bsdcomp 0 disables BSD-Compress compression
entirely.
chap-interval n
If this option is given, pppd will rechallenge the
peer every n
seconds.
chap-max-challenge n
Set the maximum number of CHAP challenge transmissions to n (default
10).
chap-restart n
Set the CHAP restart interval (retransmission timeout for challenges)
to n seconds (default 3).
debug Enables connection debugging facilities. If this
option is given,
pppd will log the contents of all control packets sent or received
in a readable form. The packets are logged
through
syslogd(8) with facility daemon and level debug.
This information
can be directed to a file by setting up
/etc/syslog.conf appropriately
(see syslog.conf(5)).
default-asyncmap
Disable asyncmap negotiation, forcing all control
characters to
be escaped for both the transmit and the receive direction.
default-mru
Disable MRU (Maximum Receive Unit) negotiation.
With this option,
pppd will use the default MRU value of 1500
bytes for both
the transmit and receive direction.
deflate nr,nt
Request that the peer compress packets that it
sends, using the
Deflate scheme, with a maximum window size of 2**nr
bytes, and
agree to compress packets sent to the peer with a
maximum window
size of 2**nt bytes. If nt is not specified, it defaults to the
value given for nr. Values in the range 8 to 15 may
be used for
nr and nt; larger values give better compression but
consume more
kernel memory for compression dictionaries. Alternatively, a
value of 0 for nr or nt disables compression in the
corresponding
direction. Use nodeflate or deflate 0 to disable
Deflate compression
entirely. (Note: pppd requests Deflate
compression in
preference to BSD-Compress if the peer can do either.)
demand Initiate the link only on demand, i.e., when data
traffic is present.
With this option, the remote IP address must
be specified
by the user on the command line or in an options
file. pppd will
initially configure the interface and enable it for
IP traffic
without connecting to the peer. When traffic is
available, pppd
will connect to the peer and perform negotiation,
authentication,
etc. When this is completed, pppd will commence
passing data
packets (i.e., IP packets) across the link.
The demand option implies the persist option. If
this behaviour
is not desired, use the nopersist option after the
demand option.
The idle and holdoff options are also useful in conjunction with
the demand option.
domain d
Append the domain name d to the local host name for
authentication
purposes. For example, if gethostname(3) returns the name
porsche, but the fully qualified domain name is
porsche.Quotron.COM, you could specify domain
Quotron.COM. pppd
would then use the name porsche.Quotron.COM for
looking up secrets
in the secrets file, and as the default name
to send to the
peer when authenticating itself to the peer. This
option is
privileged.
holdoff n
Specifies how many seconds to wait before re-initiating the link
after it terminates. This option only has any effect if the
persist or demand option is used. The holdoff period is not applied
if the link was terminated because it was
idle.
idle n Specifies that pppd should disconnect if the link is
idle for n
seconds. The link is idle when no data packets
(i.e., IP packets)
are being sent or received. Note: it is not
advisable to
use this option with the persist option without the
demand option.
If the active-filter option is given, data
packets which
are rejected by the specified activity filter also
count as the
link being idle.
ipcp-accept-local
With this option, pppd will accept the peer's idea
of our local
IP address, even if the local IP address was specified in an option.
ipcp-accept-remote
With this option, pppd will accept the peer's idea
of its (remote)
IP address, even if the remote IP address was
specified in
an option.
ipcp-max-configure n
Set the maximum number of IPCP configure-request
transmissions to
n (default 10).
ipcp-max-failure n
Set the maximum number of IPCP configure-NAKs returned before
starting to send configure-Rejects to n (default
10).
ipcp-max-terminate n
Set the maximum number of IPCP terminate-request
transmissions to
n (default 3).
ipcp-restart n
Set the IPCP restart interval (retransmission timeout) to n seconds
(default 3).
ipparam string
Provides an extra parameter to the ip-up and ip-down
scripts. If
this option is given, the string supplied is given
as the 6th parameter
to those scripts.
ipx Enable the IPXCP and IPX protocols. This option is
presently only
supported under Linux, and only if your kernel
has been configured
to include IPX support.
ipx-network n
Set the IPX network number in the IPXCP configure
request frame
to n, a hexadecimal number (without a leading 0x).
There is no
valid default. If this option is not specified, the
network number
is obtained from the peer. If the peer does not
have the
network number, the IPX protocol will not be started.
ipx-node n:m
Set the IPX node numbers. The two node numbers are
separated
from each other with a colon character. The first
number n is
the local node number. The second number m is the
peer's node
number. Each node number is a hexadecimal number,
at most 10
digits long. The node numbers on the ipx-network
must be unique.
There is no valid default. If this option is not
specified then
the node numbers are obtained from the peer.
ipx-router-name [string]
Set the name of the router. This is a string and is
sent to the
peer as information data.
ipx-routing n
Set the routing protocol to be received by this option. More
than one instance of ipx-routing may be specified.
The `none'
option (0) may be specified as the only instance of
ipx-routing.
The values may be 0 for NONE, 2 for RIP/SAP, and 4
for NLSP.
ipxcp-accept-local
Accept the peer's NAK for the node number specified
in the ipxnode
option. If a node number was specified, and
non-zero, the
default is to insist that the value be used. If you
include this
option then you will permit the peer to override the
entry of the
node number.
ipxcp-accept-network
Accept the peer's NAK for the network number specified in the
ipx-network option. If a network number was specified, and nonzero,
the default is to insist that the value be
used. If you
include this option then you will permit the peer to
override the
entry of the node number.
ipxcp-accept-remote
Use the peer's network number specified in the configure request
frame. If a node number was specified for the peer
and this option
was not specified, the peer will be forced to
use the value
which you have specified.
ipxcp-max-configure n
Set the maximum number of IPXCP configure request
frames which
the system will send to n. The default is 10.
ipxcp-max-failure n
Set the maximum number of IPXCP NAK frames which the
local system
will send before it rejects the options. The default value is 3.
ipxcp-max-terminate n
Set the maximum number of IPXCP terminate request
frames before
the local system considers that the peer is not listening to
them. The default value is 3.
kdebug n
Enable debugging code in the kernel-level PPP driver. The argument
n is a number which is the sum of the following
values: 1 to
enable general debug messages, 2 to request that the
contents of
received packets be printed, and 4 to request that
the contents
of transmitted packets be printed. On most systems,
messages
printed by the kernel are logged by syslogd(8) to a
file as directed
in the /etc/syslog.conf configuration file.
lcp-echo-failure n
If this option is given, pppd will presume the peer
to be dead if
n LCP echo-requests are sent without receiving a
valid LCP echoreply.
If this happens, pppd will terminate 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 (e.g.,
the modem has hung up) in situations where no hardware modem control
lines are available.
lcp-echo-interval n
If this option is given, pppd will send an LCP echorequest frame
to the peer every n seconds. Normally the peer
should respond to
the echo-request by sending an echo-reply. This option can be
used with the lcp-echo-failure option to detect that
the peer is
no longer connected.
lcp-max-configure n
Set the maximum number of LCP configure-request
transmissions to
n (default 10).
lcp-max-failure n
Set the maximum number of LCP configure-NAKs returned before
starting to send configure-Rejects to n (default
10).
lcp-max-terminate n
Set the maximum number of LCP terminate-request
transmissions to
n (default 3).
lcp-restart n
Set the LCP restart interval (retransmission timeout) to n seconds
(default 3).
local Don't use the modem control lines. With this option, pppd will
ignore the state of the CD (Carrier Detect) signal
from the modem
and will not change the state of the DTR (Data Terminal Ready)
signal.
login Use the system password database for authenticating
the peer using
PAP, and record the user in the system wtmp
file. Note that
the peer must have an entry in the
/etc/ppp/pap-secrets file as
well as the system password database to be allowed
access.
maxconnect n
Terminate the connection when it has been available
for network
traffic for n seconds (i.e., n seconds after the
first network
control protocol comes up).
modem Use the modem control lines. This option is the default. With
this option, pppd will wait for the CD (Carrier Detect) signal
from the modem to be asserted when opening the serial device (unless
a connect script is specified), and it will
drop the DTR
(Data Terminal Ready) signal briefly when the connection is terminated
and before executing the connect script. On
Ultrix, this
option implies hardware flow control, as for the
crtscts option.
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
cua(4) device
(as opposed to a 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.
ms-dns [addr]
If pppd is acting as a server for Microsoft Windows
clients, this
option allows pppd to supply one or two DNS (Domain
Name Server)
addresses to the clients. The first instance of
this option
specifies the primary DNS address; the second instance (if given)
specifies the secondary DNS address. (This option
was present in
some older versions of pppd under the name
dns-addr.)
ms-wins [addr]
If pppd is acting as a server for Microsoft Windows
or "Samba"
clients, this option allows pppd to supply one or
two WINS (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.
name name
Set the name of the local system for authentication
purposes to
name. This is a privileged option. With this option, pppd will
use lines in the secrets files which have name as
the second
field when looking for a secret to use in authenticating the
peer. In addition, unless overridden with the user
option, name
will be used as the name to send to the peer when
authenticating
the local system to the peer. (Note that pppd does
not append
the domain name to name.)
netmask n
Set the interface netmask to n, a 32-bit netmask in
``decimal
dot'' notation (e.g. 255.255.255.0). If this option
is given,
the value specified is ORed with the default netmask. The default
netmask is chosen based on the negotiated remote IP address;
it is the appropriate network mask for the
class of the
remote IP address, ORed with the netmasks for any
non point-topoint
network interfaces in the system which are on
the same network.
(Note: on some platforms, pppd will always
use
255.255.255.255 for the netmask, if that is the only
appropriate
value for a point-to-point interface.)
noaccomp
Disable Address/Control compression in both directions (send and
receive).
noauth Do not require the peer to authenticate itself.
This option is
privileged if the auth option is specified in
/etc/ppp/options.
nobsdcomp
Disables BSD-Compress compression; pppd will not request or agree
to compress packets using the BSD-Compress scheme.
noccp Disable CCP (Compression Control Protocol) negotiation. This option
should only be required if the peer is buggy
and gets confused
by requests from pppd for CCP negotiation.
nocrtscts
Disable hardware flow control (i.e., RTS/CTS) on the
serial port.
If neither the crtscts nor the nocrtscts option is
given, the
hardware flow control setting for the serial port is
left unchanged.
nodefaultroute
Disable the defaultroute option. The system administrator who
wishes to prevent users from creating default routes
with pppd
can do so by placing this option in the
/etc/ppp/options file.
nodeflate
Disables Deflate compression; pppd will not request
or agree to
compress packets using the Deflate scheme.
nodetach
Don't detach from the controlling terminal. Without
this option,
if a serial device other than the terminal on the
standard input
is specified, pppd will fork to become a background
process.
noip Disable IPCP negotiation and IP communication. This
option
should only be required if the peer is buggy and
gets confused by
requests from pppd for IPCP negotiation.
noipdefault
Disables the default behaviour when no local IP address is 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).
noipx Disable the IPXCP and IPX protocols. This option
should only be
required if the peer is buggy and gets confused by
requests from
pppd for IPXCP negotiation.
nomagic
Disable magic number negotiation. With this option,
pppd cannot
detect a looped-back line. This option should only
be needed if
the peer is buggy.
nopcomp
Disable protocol field compression negotiation in
both the receive
and the transmit direction.
nopersist
Exit once a connection has been made and terminated.
This is the
default unless the persist or demand option has been
specified.
nopredictor1
Do not accept or agree to Predictor-1 compression.
noproxyarp
Disable the proxyarp option. The system administrator who wishes
to prevent users from creating proxy ARP entries
with pppd can do
so by placing this option in the /etc/ppp/options
file.
novj Disable Van Jacobson style TCP/IP header compression
in both the
transmit and the receive direction.
novjccomp
Disable the connection-ID compression option in Van
Jacobson
style TCP/IP header compression. With this option,
pppd will not
omit the connection-ID byte from Van Jacobson compressed TCP/IP
headers, nor ask the peer to do so.
papcrypt
Indicates that all secrets in the
/etc/ppp/pap-secrets file which
are used for checking the identity of the peer are
encrypted, and
thus pppd should not accept a password which, before
encryption,
is identical to the secret from the
/etc/ppp/pap-secrets file.
pap-max-authreq n
Set the maximum number of PAP authenticate-request
transmissions
to n (default 10).
pap-restart n
Set the PAP restart interval (retransmission timeout) to n seconds
(default 3).
pap-timeout n
Set the maximum time that pppd will wait for the
peer to authenticate
itself with PAP to n seconds (0 means no limit).
pass-filter 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
daemons (such
as routed(8)) using up link bandwidth, or to provide
a basic
firewall capability. The filter-expression syntax
is as described
for tcpdump(8), except that qualifiers which
are inappropriate
for a PPP link, such as ether and 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 inbound
and outbound qualifiers. This option is currently
only available
under OpenBSD, and then only if both the kernel and
pppd were
compiled with PPP_FILTER defined.
persist
Do not exit after a connection is terminated; instead try to reopen
the connection.
predictor1
Request that the peer compress frames that it sends
using 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.
proxyarp
Add an entry to this system's ARP (Address Resolution Protocol)
table with the IP address of the peer and the Ethernet address of
this system. This will have the effect of making
the peer appear
to other systems to be on the local Ethernet.
remotename name
Set the assumed name of the remote system for authentication purposes
to name.
refuse-chap
With this option, pppd will not agree to authenticate itself to
the peer using CHAP.
refuse-pap
With this option, pppd will not agree to authenticate itself to
the peer using PAP.
require-chap
Require the peer to authenticate itself using CHAP
(Challenge
Handshake Authentication Protocol) authentication.
require-pap
Require the peer to authenticate itself using PAP
(Password Authentication
Protocol) authentication.
silent With this option, pppd will not transmit LCP packets
to initiate
a connection until a valid LCP packet is received
from the peer
(as for the `passive' option with ancient versions
of pppd).
usehostname
Enforce the use of the hostname (with domain name
appended, if
given) as the name of the local system for authentication purposes
(overrides the name option).
user name
Sets the name used for authenticating the local system to the
peer to name.
vj-max-slots n
Sets the number of connection slots to be used by
the Van Jacobson
TCP/IP header compression and decompression code
to n, which
must be between 2 and 16, inclusive.
welcome script
Run the executable or shell command specified by
script before
initiating PPP negotiation, after the connect script
(if any) has
completed. This option is privileged if the noauth
option is
used.
xonxoff
Use software flow control (i.e., XON/XOFF) to control the flow of
data on the serial port.
Options can be taken from files as well as the command line.
pppd reads
options from the files /etc/ppp/options, ~/.ppprc and
/etc/ppp/options.ttyname (in that order) before processing
the options on
the command line. (In fact, the command-line options are
scanned to find
the terminal name before the options.ttyname file is read.)
In forming
the name of the options.ttyname file, the initial /dev/ is
removed from
the terminal name, and any remaining / characters are replaced with dots.
An options file is parsed into a series of words, delimited
by 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 file or call options within
an options
file.
Users must be in group "network" to be able to use pppd.
pppd provides system administrators with sufficient access
control that
PPP access to a server machine can be provided to legitimate
users without
fear of compromising the security of the server or the
network it's
on. In part this is provided by the /etc/ppp/options file,
where the administrator
can place options to restrict the ways in which
pppd can be
used, and in part by the PAP and CHAP secrets files, where
the administrator
can restrict the set of IP addresses which individual
users may
use.
The normal way that pppd should be set up is to have the
auth option in
the /etc/ppp/options file. (This may become the default in
later releases.)
If users wish to use pppd to dial out to a peer which
will refuse
to authenticate itself (such as an internet service
provider), the system
administrator should create an options file under
/etc/ppp/peers containing
the noauth option, the name of the serial port to use,
and the
connect option (if required), plus any other appropriate options. In
this way, pppd can be set up to allow non-privileged users
to make unauthenticated
connections only to trusted peers.
As indicated above, some security-sensitive options are
privileged, which
means that they may not be used by an ordinary non-privileged user running
a setuid-root pppd, either on the command line, in the
user's
~/.ppprc file, or in an options file read using the file option. Privileged
options may be used in the /etc/ppp/options file or in
an options
file read using the call option. If pppd is being run by
the root user,
privileged options can be used without restriction.
Authentication is the process whereby one peer convinces the
other of its
identity. This involves the first peer sending its name to
the other,
together with some kind of secret information which could
only come from
the genuine authorized user of that name. In such an exchange, we will
call the first peer the "client" and the other the "server".
The client
has a name by which it identifies itself to the server, and
the server
also has a name by which it identifies itself to the client.
Generally
the genuine client shares some secret (or 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.
At present, pppd 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.
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.
The default behaviour of pppd is to agree to authenticate if
requested,
and to not require authentication from the peer. However,
pppd will not
agree to authenticate itself with a particular protocol if
it has no secrets
which could be used to do so.
pppd stores secrets for use in authentication in secrets
files
(/etc/ppp/pap-secrets for PAP, /etc/ppp/chap-secrets for
CHAP). Both secrets
files have the same format. The secrets files can
contain secrets
for pppd to use in authenticating itself to other systems,
as well as secrets
for pppd to use when authenticating other systems to
itself.
Each line in a secrets file contains one secret. 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
"-", then all IP addresses are disallowed. To allow any address, use
"*". A word starting with "!" indicates that the specified
address is
not acceptable. An address may be followed by "/" and a
number n, to indicate
a whole subnet, i.e., all addresses which have the
same value in
the most significant n bits. Case is significant in the
client and server
names and in the secret.
If the secret starts with an `@', what follows is assumed to
be the name
of a file from which to read the secret. A "*" as the
client or server
name matches any name. When selecting a secret, pppd takes
the best
match, i.e., the match with the fewest wildcards.
Thus a secrets file contains both secrets for use in authenticating other
hosts, plus secrets which we use for authenticating ourselves to others.
When pppd is authenticating the peer (checking the peer's
identity), it
chooses a secret with the peer's name in the first field and
the name of
the local system in the second field. The name of the local
system defaults
to the hostname, with the domain name appended if the
domain option
is used. This default can be overridden with the name
option, except
when the usehostname option is used.
When pppd is choosing a secret to use in authenticating itself to the
peer, it first determines what name it is going to use to
identify itself
to the peer. This name can be specified by the user with
the user option.
If this option is not used, the name defaults to the
name of the
local system, determined as described in the previous paragraph. Then
pppd looks for a secret with this name in the first field
and the peer's
name in the second field. pppd will know the name of the
peer if CHAP
authentication is being used, because the peer will have
sent it in the
challenge packet. However, if PAP is being used, pppd will
have to determine
the peer's name from the options specified by the
user. The user
can specify the peer's name directly with the 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,
pppd will use the null string as the peer's name.
When authenticating the peer with PAP, the supplied password
is first
compared with the secret from the secrets file. If the
password doesn't
match the secret, the password is encrypted using crypt(3)
and checked
against the secret again. Thus secrets for authenticating
the peer can
be stored in encrypted form if desired. If the papcrypt option is given,
the first (unencrypted) comparison is omitted, for better
security.
Furthermore, if the login option was specified, the username
and 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 "", which will match any password supplied
by the peer.
This avoids the need to have the same secret in two places.
Authentication must be satisfactorily completed before IPCP
(or any other
Network Control Protocol) can be started. If the peer is
required to authenticate
itself, and fails to do so, pppd will 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.
In some cases it is desirable to allow some hosts which
can't authenticate
themselves to connect and use one of a restricted set
of IP addresses,
even when the local host generally requires authentication. If the
peer refuses to authenticate itself when requested, pppd
takes that as
equivalent to authenticating with PAP using the empty string
for the
username and password. Thus, by adding a line to the papsecrets 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.
When IPCP negotiation is completed successfully, pppd will
inform the
kernel of the local and remote IP addresses for the PPP interface. This
is sufficient to create a host route to the remote end of
the link, which
will enable the peers to exchange IP packets. Communication
with other
machines generally requires further modification to routing
tables and/or
ARP (Address Resolution Protocol) tables. In most cases the
defaultroute
and/or proxyarp options are sufficient for this, but in some
cases further
intervention is required. The /etc/ppp/ip-up script
can be used for
this.
Sometimes it is desirable to add a default route through the
remote host,
as in the case of a machine whose only connection to the Internet is
through the PPP interface. The defaultroute option causes
pppd to create
such a default route when IPCP comes up, and delete it when
the link is
terminated.
In some cases it is desirable to use proxy ARP, for example
on a server
machine connected to a LAN, in order to allow other hosts to
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-topoint or loopback
interface). If found, pppd creates a permanent, published ARP entry
with the IP address of the remote host and the hardware address of the
network interface found.
When the demand option is used, the interface IP addresses
have already
been set at the point when IPCP comes up. If pppd has not
been able to
negotiate the same addresses that it used to configure the
interface (for
example when the peer is an ISP that uses dynamic IP address
assignment),
pppd has to change the interface IP addresses to the negotiated addresses.
This may disrupt existing connections, and the use of
demand dialling
with peers that do dynamic IP address assignment is
not recommended.
The following examples assume that the /etc/ppp/options file
contains the
auth option (as in the default /etc/ppp/options file in the
PPP distribution).
Probably the most common use of pppd is to dial out to an
ISP. This can
be done with a command such as
pppd call isp
where the /etc/ppp/peers/isp file is set up by the system
administrator
to contain something like this:
ttyS0 19200 crtscts
connect '/usr/sbin/chat -v -f /etc/ppp/chat-isp'
noauth
In this example, we are using chat to dial the ISP's modem
and go through
any logon sequence required. The /etc/ppp/chat-isp file
contains the
script used by chat; it could for example contain something
like this:
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "ERROR"
ABORT "NO ANSWER"
ABORT "BUSY"
ABORT "Username/Password Incorrect"
"" "at"
OK "at&d0&c1"
OK "atdt2468135"
"name:" "^Umyuserid"
"word:" "qmypassword"
"ispts" "q^Uppp"
"~-^Uppp-~"
See the chat(8) man page for details of chat scripts.
pppd can also be used to provide a dial-in PPP service for
users. If the
users already have login accounts, the simplest way to set
up the PPP
service is to let the users log in to their accounts and run
pppd (installed
setuid-root) with a command such as
pppd proxyarp
To allow a user to use the PPP facilities, you need to allocate an IP address
for that user's machine and create an entry in
/etc/ppp/pap-secrets
or /etc/ppp/chap-secrets (depending on which authentication
method the
PPP implementation on the user's machine supports), so that
the user's
machine can authenticate itself. For example, if Joe has a
machine
called "joespc" which is to be allowed to dial in to the machine called
"server" and use the IP address joespc.my.net, you would add
an entry
like this to /etc/ppp/pap-secrets or /etc/ppp/chap-secrets:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create a username called (for example) "ppp",
whose login shell is pppd and whose home directory is
/etc/ppp. Options
to be used when pppd is run this way can be put in
/etc/ppp/.ppprc.
If your serial connection is any more complicated than a
piece of wire,
you may need to arrange for some control characters to be
escaped. In
particular, it is often useful to escape XON (^Q) and XOFF
(^S), using
asyncmap a0000. If the path includes a telnet, you probably
should escape
^] as well (asyncmap 200a0000). If the path includes
an rlogin, you
will need to use the escape ff option on the end which is
running the
rlogin client, since many rlogin implementations are not
transparent;
they will remove the sequence (0xff, 0xff, 0x73, 0x73, followed by any 8
bytes) from the stream.
Messages are sent to the syslogd(8) daemon using facility
LOG_DAEMON.
(This can be overriden by recompiling pppd with the macro
LOG_PPP defined
as the desired facility.) See the syslogd(8) documentation
for details
of where the syslog daemon will write the messages. On most
systems, the
syslog daemon uses the /etc/syslog.conf file to specify the
destination(s)
for syslog messages. You may need to edit that file
to suit.
The debug option causes the contents of all control packets
sent or received
to be logged, that is, all LCP, PAP, CHAP or IPCP
packets. This
can be useful if the PPP negotiation does not succeed or if
authentication
fails. If debugging is enabled at compile time, the
debug option
also causes other debugging messages to be logged.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal to
the pppd process. This signal acts as a toggle.
pppd invokes scripts at various stages in its processing
which can be
used to perform site-specific ancillary processing. These
scripts are
usually shell scripts, but could be executable code files
instead. pppd
does not wait for the scripts to finish. The scripts are
executed as
root (with the real and effective user ID set to 0), so that
they can do
things such as update routing tables or run privileged daemons. Be careful
that the contents of these scripts do not compromise
your system's
security. pppd runs the scripts with standard input, output
and error
redirected to /dev/null, and with an environment that is
empty except for
some environment variables that give information about the
link. The environment
variables that pppd sets are:
DEVICE The name of the serial tty device being used.
IFNAME The name of the network interface being used.
IPLOCAL The IP address for the local end of the link.
This is only set
when IPCP has come up.
IPREMOTE The IP address for the remote end of the link.
This is only
set when IPCP has come up.
PEERNAME The authenticated name of the peer. This is only
set if the
peer authenticates itself.
SPEED The baud rate of the tty device.
UID The real user ID of the user who invoked pppd.
pppd invokes the following scripts, if they exist. It is
not an error if
they don't exist.
/etc/ppp/auth-up
A program or script which is executed after the remote system
successfully authenticates itself. It is executed
with the parameters
interface-name peer-name user-name tty-device speed
Note that this script is not executed if the peer
doesn't authenticate
itself, for example when the noauth option is
used.
/etc/ppp/auth-down
A program or script which is executed when the link
goes down, if
/etc/ppp/auth-up was previously executed. It is executed in the
same manner with the same parameters as
/etc/ppp/auth-up.
/etc/ppp/ip-up
A program or script which is executed when the link
is available
for sending and receiving IP packets (that is, IPCP
has come up).
It is executed with the parameters
interface-name tty-device speed local-IP-address
remote-IP-
address ipparam
/etc/ppp/ip-down
A program or script which is executed when the link
is no longer
available for sending and receiving IP packets.
This script can
be used for undoing the effects of the
/etc/ppp/ip-up script. It
is invoked in the same manner and with the same parameters as the
ip-up script.
/etc/ppp/ipx-up
A program or script which is executed when the link
is available
for sending and receiving IPX packets (that is, IPXCP has come
up). It is executed with the parameters
interface-name tty-device speed network-number
local-IPX-node-
address remote-IPX-node-address
local-IPX-routing-protocol
remote-IPX-routing-protocol local-IPX-router-name
remote-IPX-
router-name ipparam pppd-pid
The local-IPX-routing-protocol and remote-IPX-routing-protocol
field may be one of the following:
NONE to indicate that there is no routing protocol
RIP to indicate that RIP/SAP should be used
NLSP to indicate that Novell NLSP should be
used
RIP NLSP to indicate that both RIP/SAP and NLSP
should be used
/etc/ppp/ipx-down
A program or script which is executed when the link
is no longer
available for sending and receiving IPX packets.
This script can
be used for undoing the effects of the
/etc/ppp/ipx-up script.
It is invoked in the same manner and with the same
parameters as
the ipx-up script.
/var/run/pppn.pid
Process-ID for pppd process on PPP interface unit n.
/etc/ppp/pap-secrets
Usernames, passwords and IP addresses for PAP authentication.
This file should be owned by root and not readable
or writable by
any other user. pppd will log a warning if this is
not the case.
/etc/ppp/chap-secrets
Names, secrets and IP addresses for CHAP authentication. As for
/etc/ppp/pap-secrets, this file should be owned by
root and not
readable or writable by any other user. pppd will
log a warning
if this is not the case.
/etc/ppp/options
System default options for pppd, read before user
default options
or command-line options.
~/.ppprc
User default options, read before /etc/ppp/options.ttyname.
/etc/ppp/options.ttyname
System default options for the serial port being
used, read after
~/.ppprc. In forming the ttyname part of this filename, an initial
/dev/ is stripped from the port name (if present), and any
slashes in the remaining part are converted to dots.
/etc/ppp/peers
A directory containing options files which may contain privileged
options, even if pppd was invoked by a user other
than root. The
system administrator can create options files in
this directory
to permit non-privileged users to dial out without
requiring the
peer to authenticate, but only to certain trusted
peers.
Jacobson, V., "Compressing TCP/IP headers for low-speed serial links",
RFC 1144, February 1990.
Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
McGregor, G., "PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332,
May 1992.
Lloyd, B. and Simpson, W.A., "PPP authentication protocols",
RFC 1334,
October 1992.
Simpson, W.A., "The Point-to-Point Protocol (PPP)", RFC
1661, July 1994.
Simpson, W.A., "PPP in HDLC-like Framing", RFC 1662, July
1994.
Some limited degree of control can be exercised over a running pppd process
by sending it a signal from the list below.
SIGINT, SIGTERM
These signals cause pppd to terminate the link (by
closing LCP),
restore the serial device settings, and exit.
SIGHUP This signal causes pppd to terminate the link, restore the serial
device settings, and close the serial device. If
the persist or
demand option has been specified, pppd will try to
reopen the serial
device and start another connection (after the
holdoff period).
Otherwise pppd will exit. If this signal is
received during
the holdoff period, it causes pppd to end the
holdoff period
immediately.
SIGUSR1
This signal toggles the state of the debug option.
SIGUSR2
This signal causes pppd to renegotiate compression.
This can be
useful to re-enable compression after it has been
disabled as a
result of a fatal decompression error. (Fatal decompression errors
generally indicate a bug in one or other implementation.)
Paul Mackerras <[email protected]>, based on earlier
work by Drew
Perkins, Brad Clements, Karl Fox, Greg Christy, and Brad
Parker.
OpenBSD 3.6 March 31, 1998
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