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 support, 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 terminal, and will not fork to put itself in the background.

<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 successfully 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 number with each bit representing a character to escape. Bit 0 (00000001) represents the character 0x00; bit 31 (80000000) represents the character 0x1f or ^_. If multiple asyncmap options are given, the values are ORed 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 control 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 negotiation 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 control 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 characters 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 packets 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 without this option).

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 old versions of pppd).

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.

-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 disabled).

-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 (otherwise pppd will do so if a serial device is specified).

-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 disabled).

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

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 networking 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 Resolution 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 packets 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 connection 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 timeout) 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 timeout) 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 timeout) 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 timeout for challenges) to <n> seconds (default 3).

chap-max-challenge <n>
    Set the maximum number of CHAP challenge transmissions 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 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 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 acceptable IP addresses for that client. If there are only 3 words on the line, it is assumed that any IP address is OK; to disallow all IP addresses, use "-". 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 hostname,
    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 password 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 certain users, and to restrict the set of IP addresses that each user can use.

Secrets are selected from the CHAP secrets file as follows:

*   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 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 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 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.

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 starting pppd can be automated by using the connect option to run chat, for example:

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

(Note 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 messages to the desired output device or file.

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. If debugging is enabled at compile time, pppd uses facility LOG_LOCAL2 instead of LOG_DAEMON, and the debug option causes additional debugging 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 contents 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 Protocol (IPCP). 1992 May.

RFC1334
    Lloyd, B.; Simpson, W.A. PPP authentication protocols. 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.

SIGHUP
    Indicates that the physical layer has been disconnected. 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)