PPPD(8) MachTen Programmer’s Manual PPPD(8)

NAME
pppd - Point to Point Protocol daemon

SYNOPSIS
pppd [ options ] [ tty_name ] [ speed ]

DESCRIPTION
The Point-to-Point Protocol (PPP) provides a method for
transmitting datagrams over serial point-to-point links.
PPP is composed of three parts: a method for encapsulating
datagrams over serial links, an extensible Link Control
Protocol (LCP), and a family of Network Control Protocols
(NCP) for establishing and configuring different network-
layer protocols.

The encapsulation scheme is provided by driver code in the
kernel. pppd provides the basic LCP, authentication sup-
port, and an NCP for establishing and configuring the
Internet Protocol (IP) (called the IP Control Protocol,
IPCP).

FREQUENTLY USED OPTIONS
<tty_name>
Communicate over the named device. The string
"/dev/" is prepended if necessary. If no device
name is given, pppd will use the controlling termi-
nal, and will not fork to put itself in the back-
ground.

<speed>
Set the baud rate to <speed>. On systems such as
4.4BSD and NetBSD, any speed can be specified.
Other systems (e.g. SunOS) allow only a limited set
of speeds.

asyncmap <map>
Set the async character map to <map>. This map
describes which control characters cannot be suc-
cessfully received over the serial line. pppd will
ask the peer to send these characters as a 2-byte
escape sequence. The argument is a 32 bit hex num-
ber with each bit representing a character to
escape. Bit 0 (00000001) represents the character
0x00; bit 31 (80000000) represents the character
0x1f or ^_. If multiple asyncmap options are
given, the values are ORed together. If no
asyncmap option is given, no async character map
will be negotiated for the receive direction; the
peer will then escape all control characters.

auth Require the peer to authenticate itself before
allowing network packets to be sent or received.

connect <p>
Use the executable or shell command specified by
<p> to set up the serial line. This script would
typically use the "chat" program to dial the modem
and start the remote ppp session.

crtscts
Use hardware flow control (i.e. RTS/CTS) to control
the flow of data on the serial port.

xonxoff
Use software flow control (i.e. XON/XOFF) to con-
trol the flow of data on the serial port. This
option is not implemented on BSD or Ultrix systems
at present.

-crtscts
A synonym for xonxoff.

defaultroute
Add a default route to the system routing tables,
using the peer as the gateway, when IPCP negotia-
tion is successfully completed. This entry is
removed when the PPP connection is broken.

disconnect <p>
Run the executable or shell command specified by
<p> after pppd has terminated the link. This
script could, for example, issue commands to the
modem to cause it to hang up if hardware modem con-
trol signals were not available.

escape xx,yy,...
Specifies that certain characters should be escaped
on transmission (regardless of whether the peer
requests them to be escaped with its async control
character map). The characters to be escaped are
specified as a list of 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 charac-
ters to be specified. The characters which may not
be escaped are those with hex values 0x20 - 0x3f or
0x5e.

file <f>
Read options from file <f> (the format is described
below).

lock Specifies that pppd should use a UUCP-style lock on
the serial device to ensure exclusive access to the
device.

mru <n>
Set the MRU [Maximum Receive Unit] value to <n> for
negotiation. pppd will ask the peer to send pack-
ets of no more than <n> bytes. The minimum MRU
value is 128. The default MRU value is 1500. A
value of 296 is recommended for slow links (40
bytes for TCP/IP header + 256 bytes of data).

netmask <n>
Set the interface netmask to <n>, a 32 bit netmask
in "decimal dot" notation (e.g. 255.255.255.0).

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 does with-
out this option).

silent With this option, pppd will not transmit LCP pack-
ets to initiate a connection until a valid LCP
packet is received from the peer (as for the "pas-
sive" option with old versions of pppd).

persist
With this option, pppd will restart itself after a
disconnect.

ping <host>
This option causes pppd to send icmp echo packets
to the specified host. If there is no response
with a certain time limit, pppd will disconnect.
Use "-" to ping the remote server.

ping-interval <seconds>
This option sets the number of seconds between icmp
echo packets. The default is 5 seconds.

ping-limit <seconds>
This option sets the number of seconds of icmp echo
failure before pppd disconnects. The default is 30
seconds.

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 nota-
tion (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.

-all Don’t request or allow negotiation of any options
for LCP and IPCP (use default values).

-ac Disable Address/Control compression negotiation
(use default, i.e. address/control field dis-
abled).

-am Disable asyncmap negotiation (use the default
asyncmap, i.e. escape all control characters).

-as <n>
Same as asyncmap <n>

-d Increase debugging level (same as the debug
option).

-detach
Don’t fork to become a background process (other-
wise pppd will do so if a serial device is speci-
fied).

-ip Disable IP address negotiation (with this option,
the remote IP address must be specified with an
option on the command line or in an options file).

-mn Disable magic number negotiation. With this
option, pppd cannot detect a looped-back line.

-mru Disable MRU [Maximum Receive Unit] negotiation (use
default, i.e. 1500).

-p Same as the passive option.

-pc Disable protocol field compression negotiation (use
default, i.e. protocol field compression dis-
abled).

+ua <p>
Agree to authenticate using PAP [Password Authenti-
cation Protocol] if requested by the peer, and use
the data in file <p> for the user and password to
send to the peer. The file contains the remote user
name, followed by a newline, followed by the remote
password, followed by a newline. This option is
obsolescent.

+pap Require the peer to authenticate itself using PAP.

-pap Don’t agree to authenticate using PAP.

+chap Require the peer to authenticate itself using CHAP
[Cryptographic Handshake Authentication Protocol]
authentication.

-chap Don’t agree to authenticate using CHAP.

-vj Disable negotiation of Van Jacobson style IP header
compression (use default, i.e. no compression).

debug Increase debugging level (same as -d). If this
option is given, pppd will log the contents of all
control packets sent or received in a readable
form. The packets are logged through syslog with
facility daemon and level debug. This information
can be directed to a file by setting up
/etc/syslog.conf appropriately (see sys-
log.conf(5)). (If pppd is compiled with extra
debugging enabled, it will log messages using
facility local2 instead of daemon).

domain <d>
Append the domain name <d> to the local host name
for authentication purposes. For example, if geth-
ostname() returns the name porsche, but the fully
qualified domain name is porsche.Quotron.COM, you
would use the domain option to set the domain name
to Quotron.COM.

modem Use the modem control lines. On Ultrix, this
option implies hardware flow control, as for the
crtscts option. (This option is not fully imple-
mented.)

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.

local Don’t use the modem control lines.

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 net-
working code send data packets of no more than n
bytes through the PPP network interface.

name <n>
Set the name of the local system for authentication
purposes to <n>.

user <u>
Set the user name to use for authenticating this
machine with the peer using PAP to <u>.

usehostname
Enforce the use of the hostname as the name of the
local system for authentication purposes (overrides
the name option).

remotename <n>
Set the assumed name of the remote system for
authentication purposes to <n>.

proxyarp
Add an entry to this system’s ARP [Address Resolu-
tion Protocol] table with the IP address of the
peer and the Ethernet address of this system.

login Use the system password database for authenticating
the peer using PAP.

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
specified explicitly on the command line or in an
options file).

lcp-echo-interval <n>
If this option is given, pppd will send an LCP
echo-request frame to the peer every n seconds.
Under Linux, the echo-request is sent when no pack-
ets have been received from the peer for 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-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 connec-
tion has been broken (e.g., the modem has hung up)
in situations where no hardware modem control lines
are available.

lcp-restart <n>
Set the LCP restart interval (retransmission time-
out) to <n> seconds (default 3).

lcp-max-terminate <n>
Set the maximum number of LCP terminate-request
transmissions to <n> (default 3).

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
instead to <n> (default 10).

ipcp-restart <n>
Set the IPCP restart interval (retransmission time-
out) to <n> seconds (default 3).

ipcp-max-terminate <n>
Set the maximum number of IPCP terminate-request
transmissions to <n> (default 3).

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
instead to <n> (default 10).

pap-restart <n>
Set the PAP restart interval (retransmission time-
out) to <n> seconds (default 3).

pap-max-authreq <n>
Set the maximum number of PAP authenticate-request
transmissions to <n> (default 10).

chap-restart <n>
Set the CHAP restart interval (retransmission time-
out for challenges) to <n> seconds (default 3).

chap-max-challenge <n>
Set the maximum number of CHAP challenge transmis-
sions to <n> (default 10).

chap-interval <n>
If this option is given, pppd will rechallenge the
peer every <n> seconds.

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.

OPTIONS FILES
Options can be taken from files as well as the command
line. pppd reads options from the files /etc/ppp/options
and ~/.ppprc before looking at the command line. An
options file is parsed into a series of words, delimited
by whitespace. Whitespace can be included in a word by
enclosing the word in quotes ("). A backslash ( quotes
the following character. A hash (#) starts a comment,
which continues until the end of the line.

AUTHENTICATION
pppd provides system administrators with sufficient access
control that PPP access to a server machine can be pro-
vided 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 require authentication
whenever pppd is run, 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 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.

Authentication is based on secrets, which are selected
from secrets files (/etc/ppp/pap-secrets for PAP,
/etc/ppp/chap-secrets for CHAP). Both secrets files have
the same format, and both can store secrets for several
combinations of server (authenticating peer) and client
(peer being authenticated). Note that pppd can be both a
server and client, and that different protocols can be
used in the two directions if desired.

A secrets file is parsed into words as for a options file.
A secret is specified by a line containing at least 3
words, in the order client, server, secret. Any following
words on the same line are taken to be a list of accept-
able IP addresses for that client. If there are only 3
words on the line, it is assumed that any IP address is
OK; to disallow all IP addresses, use "-". 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. Which secret to use
is chosen based on the names of the host (the ‘local
name’) and its peer (the ‘remote name’). The local name
is set as follows:

if the usehostname option is given,
then the local name is the hostname of this machine
(with the domain appended, if given)

else if the name option is given,
then use the argument of the first name option seen

else if the local IP address is specified with a hostname,
then use that name

else use the hostname of this machine (with the domain
appended, if given)

When authenticating ourselves using PAP, there is also a
‘username’ which is the local name by default, but can be
set with the user option or the +ua option.

The remote name is set as follows:

if the remotename option is given,
then use the argument of the last remotename option
seen

else if the remote IP address is specified with a host-
name,
then use that host name

else the remote name is the null string "".

Secrets are selected from the PAP secrets file as follows:

* For authenticating the peer, look for a secret with
client == username specified in the PAP authenticate-
request, and server == local name.

* For authenticating ourselves to the peer, look for a
secret with client == our username, server == remote
name.

When authenticating the peer with PAP, a secret of ""
matches any password supplied by the peer. If the pass-
word doesn’t match the secret, the password is encrypted
using crypt() and checked against the secret again; thus
secrets for authenticating the peer can be stored in
encrypted form. 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 cer-
tain users, and to restrict the set of IP addresses that
each user can use.

Secrets are selected from the CHAP secrets file as fol-
lows:

* For authenticating the peer, look for a secret with
client == name specified in the CHAP-Response message,
and server == local name.

* For authenticating ourselves to the peer, look for a
secret with client == local name, and server == name
specified in the CHAP-Challenge message.

Authentication must be satisfactorily completed before
IPCP (or any other Network Control Protocol) can be
started. If authentication fails, pppd will terminated
the link (by closing LCP). If IPCP negotiates an unac-
ceptable 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 authenti-
cate 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 rout-
ing tables and/or ARP (Address Resolution Protocol)
tables. In some cases this will be done automatically
through the actions of the routed or gated daemons, but in
most cases some further intervention is required.

Sometimes it is desirable to add a default route through
the remote host, as in the case of a machine whose only
connection to the Internet is through the ppp interface.
The defaultroute option causes pppd to create such a
default route when IPCP comes up, and delete it when the
link is terminated.

In some cases it is desirable to use proxy ARP, for exam-
ple 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 inter-
face 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.

EXAMPLES
In the simplest case, you can connect the serial ports of
two machines and issue a command like

pppd /dev/ttya 9600 passive

to each machine, assuming there is no getty running on the
serial ports. If one machine has a getty running, you can
use kermit or tip on the other machine to log in to the
first machine and issue a command like

pppd passive

Then exit from the communications program (making sure the
connection isn’t dropped), and issue a command like

pppd /dev/ttya 9600

The process of logging in to the other machine and start-
ing pppd can be automated by using the connect option to
run chat, for example:

pppd /dev/ttya 38400 connect ’chat "" "" "login:"
"username" "Password:" "password" "% " "exec pppd
passive"’

(Note however that running chat like this will leave the
password visible in the parameter list of pppd and chat.)

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 syslog daemon using facility
LOG_DAEMON. (This can be overriden by recompiling pppd
with the macro LOG_PPP defined as the desired facility.)
In order to see the error and debug messages, you will
need to edit your /etc/syslog.conf file to direct the mes-
sages to the desired output device or file.

The debug option causes the contents of all control pack-
ets sent or received to be logged, that is, all LCP, PAP,
CHAP or IPCP packets. This can be useful if the PPP nego-
tiation does not succeed. If debugging is enabled at com-
pile time, pppd uses facility LOG_LOCAL2 instead of
LOG_DAEMON, and the debug option causes additional debug-
ging messages to be logged.

Debugging can also be enabled by sending a SIGUSR1 to the
pppd process. Debugging may be disabled by sending a
SIGUSR2 to the pppd process.

FILES
/var/run/pppn.pid (BSD), /etc/ppp/pppn.pid (SunOS)
Process-ID for pppd process on ppp interface unit
n.

/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.

This program or script is executed with the same
real and effective user-ID as pppd, that is, at
least the effective user-ID and possibly the real
user-ID will be root. This is so that it can be
used to manipulate routes, run privileged daemons
(e.g. sendmail), etc. Be careful that the con-
tents of the /etc/ppp/ip-up and /etc/ppp/ip-down
scripts do not compromise your system’s security.

/etc/ppp/ip-down
A program or script which is executed when the link
is no longer available for sending and receiving IP
packets. This script can be used for undoing the
effects of the /etc/ppp/ip-up script. It is
invoked with the same parameters as the ip-up
script, and the same security considerations apply,
since it is executed with the same effective and
real user-IDs as pppd.

/etc/ppp/pap-secrets
Usernames, passwords and IP addresses for PAP
authentication.

/etc/ppp/chap-secrets
Names, secrets and IP addresses for CHAP
authentication.

/etc/ppp/options
System default options for pppd, read before user
default options or command-line options.

~/.ppprc
User default options, read before command-line
options.

/etc/ppp/options.ttyname
System default options for the serial port being
used, read after command-line options.

SEE ALSO
RFC1144
Jacobson, V. Compressing TCP/IP headers for low-
speed serial links. 1990 February.

RFC1321
Rivest, R. The MD5 Message-Digest Algorithm. 1992
April.

RFC1332
McGregor, G. PPP Internet Protocol Control Proto-
col (IPCP). 1992 May.

RFC1334
Lloyd, B.; Simpson, W.A. PPP authentication proto-
cols. 1992 October.

RFC1548
Simpson, W.A. The Point-to-Point Protocol (PPP).
1993 December.

RFC1549
Simpson, W.A. PPP in HDLC Framing. 1993 December

NOTES
The following signals have the specified effect when sent
to the pppd process.

SIGINT, SIGTERM
These signals cause pppd to terminate the link (by
closing LCP), restore the serial device settings,
and exit. Note that SIGTERM will ignore the per-
sist option.

SIGHUP Indicates that the physical layer has been discon-
nected. pppd will attempt to restore the serial
device settings (this may produce error messages on
Suns), and then exit.

BUGS
The use of the modem control lines and the effects of the
modem and local options are not well defined.

AUTHORS
Drew Perkins, Brad Clements, Karl Fox, Greg Christy, Brad
Parker (brad@fcr.com), Paul Mackerras
(paulus@cs.anu.edu.au)

MachTen 11