PPPD(8) OpenBSD System Manager's Manual PPPD(8)NAMEpppd - Point-to-Point Protocol daemon
SYNOPSISpppd [tty_name] [speed] [options]
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 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
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.
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.
OPTIONS
[local_IP_address]:[remote_IP_address]
Set the local and/or remote interface IP addresses. Either one
may be omitted. The IP addresses can be specified with a host
name or in decimal dot notation (e.g., 150.234.56.78). The
default local address is the (first) IP address of the system
(unless the noipdefault option is given). The remote address
will be obtained from the peer if not specified in any option.
Thus, in simple cases, this option is not required. If a local
and/or remote IP address is specified with this option, pppd will
not accept a different value from the peer in the IPCP
negotiation, unless the ipcp-accept-local and/or
ipcp-accept-remote options are given, respectively.
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.
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 echo-
reply. 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 echo-request 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-to-
point 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).
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 protocols
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.
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 FILES
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.
SECURITY
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
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 pap-secrets
file which specifies the empty string for the client and password, it is
possible to allow restricted access to hosts which refuse to authenticate
themselves.
ROUTING
When IPCP negotiation is completed successfully, pppd will inform the
kernel of the local and remote IP addresses for the PPP interface. This
is sufficient to create a host route to the remote end of the link, which
will enable the peers to exchange IP packets. Communication with other
machines generally requires further modification to routing tables and/or
ARP (Address Resolution Protocol) tables. In 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-to-point or
loopback interface). If found, pppd creates a permanent, published ARP
entry with the IP address of the remote host and the hardware address of
the network interface found.
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.
EXAMPLES
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.
DIAGNOSTICS
Messages are sent to the syslogd(8) daemon using facility LOG_DAEMON.
(This can be overridden 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.
SCRIPTSpppd 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.
FILES
/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.
SEE ALSOcua(4), ppp(4), tty(4), chat(8), ppp(8), syslogd(8), tcpdump(8)
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.
NOTES
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.)
AUTHORS
Paul Mackerras <Paul.Mackerras@samba.org>, based on earlier work by Drew
Perkins, Brad Clements, Karl Fox, Greg Christy, and Brad Parker.
OpenBSD 4.9 September 20, 2010 OpenBSD 4.9
