pppd - Point-to-Point Protocol (PPP) daemon
/usr/sbin/pppd [options] [tty_name] [speed]
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. The pppd daemon provides the basic LCP, authentication support, and an NCP for establishing and configuring the Internet Protocol (IP) (called the IP Control Protocol, IPCP).
Sets the local or remote interface IP addresses, or both. Either one may be omitted. The IP addresses can be specified with a host name or in decimal dot notation (for example, 18.104.22.168). 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 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 or ipcp-accept-remote options are given, respectively. Does not request or allow negotiation of any options for LCP and IPCP (use default values). Disables Address/Control compression negotiation (use default, that is, address/control field disabled). Disables asyncmap negotiation (use the default asyncmap, that is, escape all control characters). Same as asyncmap n. Requires the peer to authenticate itself using CHAP (Cryptographic Handshake Authentication Protocol) authentication. Does not agree to authenticate using CHAP. Increases debugging level (same as the debug option). Does not fork to become a background process (otherwise pppd will do so if a serial device is specified). Disables 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. Disables magic number negotiation. With this option, pppd cannot detect a looped-back line. Disables MRU (Maximum Receive Unit) negotiation (use default, that is, 1500). Same as the passive option. Requires the peer to authenticate itself using PAP. Does not agree to authenticate using PAP. Disables protocol field compression negotiation (use default, that is, protocol field compression disabled). Agrees 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 obsolete. Disables negotiation of Van Jacobson-style IP header compression. Instead, it uses no compression (the default). Requests the peer to compress all packets that it sends, using the BSD-Compress scheme, with a maximum code size of nr bits, and agrees to compress all 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. Disables compression; pppd will not request or agree to compress packets using the BSD-Compress scheme. If this option is given, pppd challenges the peer every n seconds. Sets the maximum number of CHAP challenge transmissions to n (default 10). Sets the CHAP restart interval (retransmission timeout for challenges) to n seconds (default 3). Increases 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 syslogd(8)). (If pppd is compiled with extra debugging enabled, it will log messages using facility local2 instead of daemon). Appends 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. With this option, pppd accepts the peer's idea of our local IP address, even if the local IP address was specified in an option. With this option, pppd accepts the peer's idea of its (remote) IP address, even if the remote IP address was specified in an option. Sets the maximum number of IPCP configure-request transmissions to n (default 10). Sets the maximum number of IPCP configure-NAKs returned before starting to send configure-Rejects instead to n (default 10). Sets the maximum number of IPCP terminate-request transmissions to n (default 3). Sets the IPCP restart interval (retransmission timeout) to n seconds (default 3). Enables debugging code in the kernel-level PPP driver. The argument n is a number that is the sum of the following values: 1 (enables general debug messages), 2 (requests that the contents of received packets be printed), and 4 (requests that the contents of transmitted packets be printed). If this option is given, pppd presumes the peer to be dead if n LCP echo-requests are sent without receiving a valid LCP echo-reply. If this happens, pppd terminates 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 (for example, the modem has hung up) in situations where no hardware modem control lines are available. If this option is given, pppd sends an LCP echo-request frame to the peer every n seconds. Sets the maximum number of LCP configure-request transmissions to n (default 10). Sets the maximum number of LCP configure-NAKs returned before starting to send configure-Rejects instead to n (default 10). Sets the maximum number of LCP terminate-request transmissions to n (default 3). Sets the LCP restart interval (retransmission timeout) to n seconds (default 3). Does not use the modem control lines. With this option, pppd ignores the state of the CD (Carrier Detect) signal from the modem and does not change the state of the DTR (Data Terminal Ready) signal. Uses the system password database for authenticating the peer using PAP. Uses the modem control lines. This option is the default. With this option, pppd waits 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 drops the DTR (Data Terminal Ready) signal briefly when the connection is terminated and before executing the connect script. Sets the MTU [Maximum Transmit Unit] value to n. Unless the peer requests a smaller value using MRU negotiation, pppd will request that the kernel networking code send data packets of no more than n bytes through the PPP network interface. Sets the name of the local system for authentication purposes to n. Disables the default behavior 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 must supply the local IP address during IPCP negotiation, unless it is specified explicitly on the command line or in an options file. Indicates that all secrets in the /etc/ppp/pap-secrets file used for checking the identity of the peer are encrypted. The pppd daemon should not accept a password that (before encryption) is identical to the secret from the /etc/ppp/pap-secrets file. Sets the maximum number of PAP authenticate-request transmissions to n (default 10). Sets the PAP restart interval (retransmission timeout) to n seconds (default 3). Do not exit after a connection is terminated. Instead, try to reopen the connection. Adds 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. Sets the assumed name of the remote system for authentication purposes to n. Enforces the use of the hostname as the name of the local system for authentication purposes (overrides the name option). Sets the user name to use for authenticating this machine with the peer using PAP to u.
Options can be taken from files as well as the command line. The pppd daemon 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 double quotation marks ("). A backslash (\) quotes any character that follows it. A hash mark (#) starts a comment, which continues until the end of the line.
The pppd daemon provides system administrators with sufficient access control so that legitimate users can have PPP access to a server machine without fear of compromising the security of the server or the network. In part this is provided by the /etc/ppp/options file, in which the administrator can place options to require authentication whenever pppd is run, and in part by the PAP and CHAP secrets files, in which the administrator can restrict the set of IP addresses that individual users may use.
The default behavior 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 for that protocol.
Authentication is based on secrets that are selected from secrets files (/etc/ppp/pap-secrets for PAP and /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 like an options file. A secret is specified by a line containing at least 3 words, in the following 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 a hyphen (-). If the secret starts with an at sign (@), anything following it is assumed to be the name of a file from which to read the secret. An asterisk (*) as the client or server name matches any name. When selecting a secret, pppd takes the best match, that is the match with the fewest wildcards.
A secrets file contains secrets for use in authenticating other hosts and secrets that 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 determined by the following rules: If the usehostname option is given, the local name is the hostname of this machine (with the domain appended, if given). If the name option is given, the local name is the argument of the first name option. If the local IP address is specified with a hostname, the local name is the hostname. If none of the previous rules applies, the local name is 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 determined by the following rules: If the remotename option is given, the remote name is the argument of the last remotename option. If the remote IP address is specified with a hostname, the remote name is the hostname. If neither of the previous rules applies, the remote name is the null string "".
Secrets are selected from the PAP secrets file as follows: For authenticating the peer, the client must match the username specified in the PAP authenticate-request and server must match the local name. For authenticating ourselves to the peer, the client must match our username and server must match the remote name.
When authenticating the peer with PAP, a secret of "" matches any password supplied by the peer. If the password does not match the secret, the password is encrypted using crypt() and checked against the secret again. Therefore, secrets for authenticating the peer can be stored in encrypted form. If the papcrypt option is given, the first (unencrypted) comparison is omitted for better security.
If the login option was specified, the user name 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 "", to avoid the need to have the same secret in two places.
Secrets are selected from the CHAP secrets file as follows: For authenticating the peer, the client must match the name specified in the CHAP-Response message and server must match the local name. For authenticating ourselves to the peer, the client must match the local name and server must equal the 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 that cannot 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.
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 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.
The following signals have the specified effect when sent to the pppd process: Cause pppd to terminate the link (by closing LCP), restore the serial device settings, and exit. This signal causes pppd to terminate the link, restore the serial device settings, and close the serial device. If the persist option has been specified, pppd tries to reopen the serial device and start another connection. Otherwise, pppd exits. 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. With the BSD Compress scheme, fatal decompression errors generally indicate a severe implementation error.
The use of the modem control lines and the effects of the modem and local options are not well defined.
If you want to connect the serial ports of two machines and there is no getty running on the serial ports, issue a command similar to the following on each machine: pppd /dev/ttya 9600 passive If one machine has a getty running, you can log in to the machine from another machine using kermit or tip, and issue the following command: pppd passive
Then, exit from the communications program (making sure the connection is not dropped), and issue a command similar to the following: 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"'
Running chat in this way leaves the password visible in the parameter list of pppd and chat.
If your serial connection is 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 need to use the escape ff option on the end that is running the rlogin client, since many rlogin implementations are not transparent; they remove the sequence 0xff, 0xff, 0x73, 0x73, followed by any 8 bytes, from the stream.
Messages are sent to the syslogd daemon using facility LOG_DAEMON. To see the error and debug messages, 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 is useful if the PPP negotiation does not succeed. If debugging is enabled at compile time, the debug option causes additional debugging messages to be logged.
Debugging can also be toggled on and off by sending a SIGUSR1 to the pppd process.
Commands: chat(8), pppstats(8).
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., The PPP Internet Protocol Control Protocol (IPCP), 1992 May (obsoletes RFC1172).
RFC1334, Lloyd, B.; Simpson, W.A., PPP Authentication Protocols, 1992 October.
RFC1570, Simpson, W.A., PPP LCP Extensions, 1994 January.
RFC1661, Simpson, W.A., The Point-to-Point Protocol (PPP), 1994 July (obsoletes RFC1548, RFC1331, RFC1171).
RFC1662, Simpson, W.A., PPP in HDLC-like Framing, 1994 July (obsoletes RFC1549).
Greg Christy, Brad Clements, Karl Fox, Brad Parker (email@example.com), Drew Perkins, Steve Tate (firstname.lastname@example.org) delim off