The point-to-point protocol (PPP) provides a method for transmitting datagrams over serial point-to-point links. PPP is composed of three components: a facility 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 several NCPs for establishing and configuring the Internet Protocol (referred to as the IP Control Protocol or “IPCP”) and IPv6 (IPV6CP).
The following sections discuss the pppd options:
Options are taken from files and the command line. pppd reads options from the files /etc/ppp/options, $HOME/.ppprc and /etc/ppp/options.ttyname (in that order) before processing the options on the command line. (Command-line options are scanned for the terminal name before the options.ttyname file is read.) To form the name of the options.ttyname file, the initial /dev/ is removed from the terminal name, and any remaining forward slash characters (/) are replaced with dots. For example, with serial device /dev/cua/a, option file /etc/ppp/options.cua.a is read.
An options file is parsed into a series of words that are delimited by whitespace. Whitespace can be included in a word by enclosing the word in double-quotes ("). A backslash (\) quotes the succeeding 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.
Communicate over the named device. The string /dev/ is prepended if necessary. If no device name is given, or if the name of the terminal connected to the standard input is given, pppd uses that terminal and does not fork to put itself in the background. A value for this option from a privileged source cannot be overridden by a non-privileged user.
Set the baud rate to <speed> (a decimal number). The default is to leave the baud rate unchanged. This option is normally needed for dial-out only.
async character map to <map>. The map describes which control characters cannot be successfully received over the
serial line. pppd asks 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, pppd attempts to negotiate a value of 0. If the peer agrees, this disables escaping of the standard control characters. Use the default-asyncmap option to disable negotiation and escape all control characters.
Require the peer to authenticate itself before allowing network packets to be sent or received. This option is the default if the system has a default route. If the auth or the noauth option is not specified, pppd allows the peer to use only those IP addresses to which the system does not already have a route.
Read options from the file /etc/ppp/peers/name. This file may contain privileged options, including noauth, even if pppd is not being run by root. The name string may not begin with a slash (“/”) or include consecutive periods ("..") as a pathname component.
Request a callback to the given telephone number using Microsoft CBCP.
Use the executable or shell command specified by script to set up the serial line. This script would typically use the chat(1M) program to dial the modem and start the remote PPP session. A value for this option originating from a privileged source cannot be overridden by a non-privileged user.
Use hardware flow control, that is, RTS/CTS, to control the flow of data on the serial port. If the crtscts, nocrtscts, cdtrcts or nocdtrcts option is not provided, the hardware flow control setting for the serial port is left unchanged. Some serial ports lack a true RTS output and use this mode to implement unidirectional flow control. The serial port suspends transmission when requested by the modem by means of CTS but cannot request the modem to stop sending to the computer. This mode allows the use of DTR as a modem control line.
Add a default route to the system routing tables when IPCP negotiation successfully completes, using the peer as the gateway. This entry is removed when the PPP connection is broken. This option is privileged if the nodefaultroute option is specified.
Run the executable or shell command specified by script after pppd terminates the link. Typically, this script is used to command the modem to hang up if hardware modem control signals are not available. disconnect is not run if the modem has already hung up. A value for this option originating from a privileged source cannot be overridden by a non-privileged user.
Specifies that certain characters 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 allows only control characters to be specified. Characters that cannot
be escaped are those containing hex values 0x20 through 0x3f and 0x5e.
Read options from file name. If this option is used on the command line or in $HOME/.ppprc, the file must be readable by the user invoking pppd. See Options Files for a list of files that pppd always reads, regardless of the use of this option.
Run the executable or shell command specified by script to initialize the serial line. This script would typically use the chat(1M) program to configure the modem to enable auto-answer. A value for this option from a privileged source cannot be overridden by a non-privileged user.
Directs pppd to create a UUCP-style lock file for the serial device to ensure exclusive access to the device.
Set the Maximum Receive Unit (MRU) value to n. pppd asks the peer to send packets of no more than n bytes. Minimum MRU value is 128. Default MRU value is 1500. A value of 296 is recommended for slow links (40 bytes for TCP/IP header + 256 bytes of data). For IPv6, MRU must be at least 1280.
Set the Maximum Transmit Unit (MTU) value to n. Unless the peer requests a smaller value via MRU negotiation, pppd requests the kernel networking code to send data packets of no more than n bytes through the PPP network interface. For IPv6, MTU must be at least 1280.
Enables the "passive" option in the LCP. With this option, pppd attempts to initiate a connection; if no reply is received from the peer, pppd waits passively for a valid LCP packet instead of exiting, as it would without this option.
Set the local and/or remote interface IP addresses. Either one may be omitted, but the colon is required. The IP addresses are specified with a host name or in decimal dot notation, for example: :10.1.2.3. The default local address is the first IP address of the system unless the noipdefault option is provided. The remote address is 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.
Set allowable FCS type(s) for data sent to the peer. The fcs-type is a comma-separated list of "crc16", "crc32", "null", or integers. By default, all known types are allowed. If this option is specified and the peer requests a type not listed, a LCP Configure-Nak is sent to request only the listed types.
Allow peers to use the given IP address or subnet without authenticating themselves. The parameter is parsed in the same manner as each element of the list of allowed IP addresses is parsed in the secrets files. See the Authentication section more more details.
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 provide 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 nobsdcomp or bsdcomp 0 to disable BSD-Compress compression entirely. If this option is read from a privileged source, a nonprivileged user may not specify a code size larger than the value from the privileged source.
Use a non-standard hardware flow control such as DTR/CTS to control the flow of data on the serial port. If the crtscts, nocrtscts, cdtrcts or nocdtrcts option is not specified, the hardware flow control setting for the serial port is left unchanged. Some serial ports lack a true RTS output. Such serial ports use this mode to implement true bi-directional flow control. Note that this flow control mode does not permit using DTR as a modem control line.
If this option is given, pppd will rechallenge the peer every n seconds.
Set the maximum number of CHAP challenge transmissions to n (default 10).
Set the CHAP restart interval (retransmission timeout for challenges) to n seconds. The default is 3.
Wait for up to n milliseconds after the connect script finishes for a valid PPP packet from the peer. When the wait period elapses or when a valid PPP packet is received from the peer, pppd begins negotiation by sending its first LCP packet. The default value is 1000 (1 second). A wait period applies only if the connect or pty option is used.
Set maximum data rate to n (in bytes per second) when using the pty, notty, record, or socket options.
Enables connection debugging facilities. If this option is given, pppd logs 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 configuring /etc/syslog.conf appropriately.
asyncmap negotiation, forcing all control characters to be escaped for both the transmit and the receive direction.
Disable FCS Alternatives negotiation entirely. By default, no FCS Alternatives option is sent to the peer, but the option is accepted. If this option is specified by the peer, then LCP Configure-Reject is sent.
Disable MRU [Maximum Receive Unit] negotiation. With this option, pppd uses the default MRU value of 1500 bytes for the transmit and receive directions.
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 and effort level of e (1 to 9). If nt is not specified, it defaults to the value given for nr. If e is not specified, it defaults to 6. Values in the range 9 to 15 may be used for nr and nt; larger values provide better compression but consume more kernel memory for compression dictionaries. (Value 8 is not permitted due to a zlib bug.) 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.) If this option is read from a privileged source, a nonprivileged user may not specify a code size larger than the value from the privileged source.
Initiate the link only on demand, that is, 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 initially configures and enables the interface for IP traffic without connecting to the peer. When traffic is available, pppd connects to the peer and performs negotiation, authentication and other actions. When completed, pppd passes data packets across the link. The demand option implies the persist option. If this behavior is not desired, use the nopersist option after the demand option. The idle and holdoff options can be used in conjunction with the demand option.
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 could specify domain Quotron.COM. With this configuration, pppd uses the name porsche.Quotron.COM for accessing secrets in the secrets file and as the default name when authenticating to the peer. This option is privileged.
Set the endpoint discriminator (normally used for RFC 1990 Multilink PPP operation). The endpoint-value consists of a class identifier and a class-dependent value. The class identifier is one of "null," "local," "IP," "MAC," "magic," "phone," or a decimal integer. If present, the class-dependent value is separated from the identifier by a colon (“:”) or period (“.”) . This value may be a standard dotted-decimal IP address for class "IP," an optionally colon-or-dot separated hex Ethernet address for class "MAC" (must have 6 numbers), or an arbitrary string of bytes specified in hex with optional colon or dot separators between bytes. Although this option is available, this implementation does not support multilink.
Set FCS type(s) desired for data sent by the peer. The fcs-type is a comma-separated list of
integers. By default, an FCS Alternatives option is not specified, and the medium-dependent FCS type is used. If this option is specified and the peer sends an LCP Configure-Nak, only the listed types are used. If none are in common, the FCS Alternatives option is omitted from the next LCP Configure-Request
to drop back to the default.
When logging the contents of PAP packets, this option causes pppd to exclude the password string from the log. This is the default.
Specifies how many seconds to wait before re-initiating the link after it terminates. This option is effective only if the persist or demand option is used. The holdoff period is not applied if the link is terminated because it was idle.
Set the LCP Identification string. The default value is a version string similar to that displayed by the --version option.
Specifies that pppd must 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. Do not use this option with the persist option but without the demand option.
With this option, pppd accepts the peer's idea of the local IP address, even if the local IP address is specified in an option.
With this option, pppd accepts the peer's idea of its remote IP address, even if the remote IP address is specified in an option.
Set the maximum number of IPCP Configure-Request transmissions to n (default 10).
Set the maximum number of IPCP Configure-NAKs sent before sending Configure-Rejects instead to n (default 10).
Set the maximum number of IPCP terminate-request transmissions to n (default 3).
Set the IPCP restart interval (retransmission timeout) to n seconds (default 3).
Provides an extra parameter to the ip-up and ip-down scripts. When this option is given, the string supplied is given as the sixth parameter to those scripts. See the Scripts section.
Set the local and/or remote 64-bit interface identifier. Either one may be omitted. The identifier must be specified in standard ASCII notation of IPv6 addresses (for example: ::dead:beef). If the ipv6cp-use-ipaddr option is given, the local and remote identifiers are derived from the respective IPv4 addresses (see above). The ipv6cp-use-persistent option can be used instead of the ipv6 <local>,<remote> option.
Accept peer's interface identifier for the local link identifier.
Set the maximum number of IPv6CP Configure-Request transmissions to n (default 10).
Set the maximum number of IPv6CP Configure-NAKs sent before sending Configure-Rejects instead to n (default 10).
Set the maximum number of IPv6CP terminate-request transmissions to n (default 3).
Set the IPv6CP restart interval (retransmission timeout) to n seconds (default 3).
If either the local or remote IPv6 address is unspecified, use the corresponding configured IPv4 address as a default interface identifier. (This option uses the configured addresses, not the negotiated addresses. Do not use it with ipcp-accept-local if the local IPv6 identifier is unspecified or with ipcp-accept-remote if the remote IPv6 identifier is unspecified.)
Use uniquely-available persistent value for link local address.
Enable debugging code in the kernel-level PPP driver. Argument n is the sum of the following values: 1 to enable general debug messages, 2 to request that contents of received packets be printed, and 4 to request contents of transmitted packets be printed. Messages printed by the kernel are logged by syslogd(1M) to a file directed in the /etc/syslog.conf configuration file. Do not use the kdebug option to debug failed links. Use the debug option instead.
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. This option requires a non-zero value for the lcp-echo-interval parameter. This option enables pppd to terminate after the physical connection is broken (for example, if 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. Normally the peer responds 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.
Set the maximum number of LCP Configure-Request transmissions to n (default 10).
Set the maximum number of LCP Configure-NAKs sent before starting to send Configure-Rejects instead to n (default 10).
Set the maximum number of LCP Terminate-Request transmissions to n (default 3).
Set the LCP restart interval (retransmission timeout) to n seconds (default 3).
Sets the logical name of the link to name. pppd creates a file named ppp-name.pid in /var/run containing its process ID. This is useful in determining which instance of pppd is responsible for the link to a given peer system. This is a privileged option.
Do not use 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.
Send log messages to file descriptor n. pppd sends log messages to (at most) one file or file descriptor (as well as sending the log messages to syslog), so this option and the logfile option are mutually exclusive. By default pppd sends log messages to stdout (file descriptor 1) unless the serial port is open on stdout.
Append log messages to the file filename (and send the log messages to syslog). The file is opened in append mode with the privileges of the user who invoked pppd.
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 and the system password database to be allowed access.
Terminate the connection after it has been available for network traffic for n seconds (that is, n seconds after the first network control protocol starts). An LCP Time-Remaining message is sent when the first NCP starts, and again when 5, 2, and 0.5 minutes are remaining.
Terminate after n consecutive failed connection attempts. A value of 0 means no limit. The default value is 10.
Use 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 drops the DTR (Data Terminal Ready) signal briefly when the connection is terminated and before executing the connect script.
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. If the first instance specifies a name that resolves to multiple IP addresses, then the first two addresses are used. (This option is present in some older versions of pppd under the name dns-addr.)
If pppd connects as a client to a Microsoft server and uses MS-CHAPv1 for authentication, this option selects the LAN Manager password style instead of Microsoft NT.
If pppd acts 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. As with ms-dns, if the name specified resolves to multiple IP addresses, then the first two will be taken as primary and secondary.
Set the name of the local system for authentication purposes to name. This is a privileged option. With this option, pppd uses lines in the secrets files that have name as the second field to look for a secret to use in authenticating the peer. In addition, unless overridden with the user option, name is used as the name to send to the peer when authenticating the local system. (Note that pppd does not append the domain name to name.)
Disable use of
asyncmap (ACCM) checking using LCP Echo-Request messages. If the lcp-echo-failure is used on an asynchronous line, pppd includes all control characters
in the first n LCP Echo-Request messages. If the
asyncmap is set incorrectly, the link drops rather than continue operation with random failures. This option disables that feature.
Disable HDLC Address/Control compression in both directions (send and receive).
Do not require the peer to authenticate itself. This option is privileged.
Disables BSD-Compress compression; pppd will not request or agree to compress packets using the BSD-Compress scheme. This option is not necessary if noccp is specified.
Disable CCP (Compression Control Protocol) negotiation. This option should only be required if the peer has bugs or becomes confused by requests from pppd for CCP negotiation. If CCP is disabled, then BSD and deflate compression do not need to be separately disabled.
Disable hardware flow control (i.e. RTS/CTS) on the serial port. If the crtscts, nocrtscts, cdtrcts or nocdtrcts options are not given, the hardware flow control setting for the serial port is left unchanged.
This option is a synonym for nocrtscts. Either option will disable both forms of hardware flow control.
Disable the defaultroute option. You can prevent non-root users from creating default routes with pppd by placing this option in the /etc/ppp/options file.
Disables deflate compression; pppd will not request or agree to compress packets using the deflate scheme. This option is not necessary if noccp is specified.
Do not use Internet Draft (incorrectly assigned) algorithm number for deflate compression. This option is not necessary if noccp is specified.
Do not detach from the controlling terminal. Without this option, pppd forks to become a background process if a serial device other than the terminal on the standard input is specified.
Do not send or accept the Multilink Endpoint Discriminator option.
Disable use of LCP Identification. LCP Identification messages will not be sent to the peer, but received messages will be logged. (Specify this option twice to completely disable LCP Identification. In this case, pppd sends LCP Code-Reject in response to received LCP Identification messages.)
Disable IPCP negotiation and IP communication. Use this option only if the peer has bugs or becomes confused by requests from pppd for IPCP negotiation.
Disable IPv6CP negotiation and IPv6 communication. IPv6 is not enabled by default.
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 specified explicitly on the command line or in an options file).
Do not send log messages to a file or file descriptor. This option cancels the logfd and logfile options. nologfd acts as an alias for this option.
Disable magic number negotiation. With this option, pppd cannot detect a looped-back line. Use this option only if the peer has bugs. Do not use this option to work around the “Serial line is looped back” error message.
This privileged option disables use of pluggable authentication modules. If this option is specified, pppd reverts to standard authentication mechanisms. The default is not to use PAM.
Disable protocol field compression negotiation in the receive and the transmit direction.
Exit once a connection has been made and terminated. This is the default unless the persist or demand option is specified.
Cause pppd to use I_LINK instead of I_PLINK. This is the default. When I_LINK is used, the system cleans up terminated interfaces (even when SIGKILL is used) but does not allow ifconfig(1M) to unplumb PPP streams or insert or remove modules dynamically. Use the plink option if ifconfig(1M) modinsert, modremove or unplumb support is needed.
Do not accept or agree to Predictor-1 compression. (This option is accepted for compatibility. The implementation does not support Predictor-1 compression.)
Disable the proxyarp option. If you want to prevent users from creating proxy ARP entries with pppd, place this option in the /etc/ppp/options file.
Normally, pppd requires a terminal device. With this option, pppd allocates itself a pseudo-tty master/slave pair and uses the slave as its terminal device. pppd creates a child process to act as a character shunt to transfer characters between the pseudo-tty master and its standard input and output. Thus, pppd transmits characters on its standard output and receives characters on its standard input even if they are not terminal devices. This option increases the latency and CPU overhead of transferring data over the ppp interface as all of the characters sent and received must flow through the character shunt process. An explicit device name may not be given if this option is used.
Disable Van Jacobson style TCP/IP header compression in both the transmit and the receive direction.
Disable the connection-ID compression option in Van Jacobson style TCP/IP header compression. With this option, pppd does not omit the connection-ID byte from Van Jacobson compressed TCP/IP headers, nor does it ask the peer to do so. This option is unnecessary if novj is specified.
This privileged option enables use of PAM. If this is specified, pppd uses the pam(3PAM) framework for user authentication with a service name of "ppp" if the login option and PAP authentication are used. The default is not to use PAM.
Indicates that pppd should not accept a password which, before encryption, is identical to the secret from the /etc/ppp/pap-secrets file. Use this option if the secrets in the pap-secrets file are in crypt(3C) format.
Set the maximum number of PAP authenticate-request transmissions to n (default 10).
Set the PAP restart interval (retransmission timeout) to n seconds (default 3).
Set the maximum time that pppd waits for the peer to authenticate itself with PAP to n seconds (0= no limit). The default is 30 seconds.
Password string for authentication to the peer.
Do not exit after a connection is terminated; instead try to reopen the connection.
Cause pppd to use I_PLINK instead of I_LINK. The default is to use I_LINK, which cleans up terminated interface (even if SIGKILL is used), but does not allow ifconfig(1M) to unplumb PPP streams or insert or remove modules dynamically. Use this option if ifconfig(1M) modinsert/modremove/unplumb support is needed. See also the plumbed option.
Load the shared library object file filename as a plugin. This is a privileged option. Unless the filename specifies an explicit path, /etc/ppp/plugins and /usr/lib/inet/ppp will be searched for the object to load in that order.
This option indicates that pppd should find a plumbed interface and use that for the session. If IPv4 addresses or IPv6 interface IDs or link MTU are otherwise unspecified, they are copied from the interface selected. This mode mimics some of the functionality of the older aspppd implementation and may be helpful when pppd is used with external applications that use ifconfig(1M).
Enable PPP Multiplexing option negotiation and set transmit multiplexing timeout to timer microseconds.
Allows members of group group-name to use privileged options. This is a privileged option. Because there is no guarantee that members of group-name cannot use pppd to become root themselves, you should be careful using this option. Consider it equivalent to putting the members of group-name in the root or sys group.
Add an entry to the system's Address Resolution Protocol (ARP) table with the IP address of the peer and the Ethernet address of this system. When you use this option, the peer appears to other systems to be on the local Ethernet. The remote address on the PPP link must be in the same subnet as assigned to an Ethernet interface.
Specifies that the command script, and not a specific terminal device is used for serial communication. pppd allocates itself a pseudo-tty master/slave pair and uses the slave as its terminal device. script runs in a child process with the pseudo-tty master as its standard input and output. An explicit device name may not be given if this option is used. (Note: if the record option is used in conjunction with the pty option, the child process will have pipes on its standard input and output.)
With this option, pppd accepts all control characters from the peer, including those marked in the receive
asyncmap. Without this option, pppd discards those characters
as specified in RFC 1662. This option should be used only if the peer has bugs, as is often found with dial-back implementations.
Directs pppd to record all characters sent and received to a file named filename. filename is opened in append mode, using the user's user-ID and permissions. Because this option uses a pseudo-tty and a process to transfer characters between the pseudo-tty and the real serial device, it increases the latency and CPU overhead of transferring data over the PPP interface. Characters are stored in a tagged format with timestamps that can be displayed in readable form using the pppdump(1M) program. This option is generally used when debugging the kernel portion of pppd (especially CCP compression algorithms) and not for debugging link configuration problems. See the debug option.
Set the assumed name of the remote system for authentication purposes to name. Microsoft WindowsNT does not provide a system name in its CHAP Challenge messages, and this option is often used to work around this problem.
With this option, pppd will not agree to authenticate itself to the peer using standard Challenge Handshake Authentication Protocol (CHAP). (MS-CHAP is not affected.)
Do not agree to authenticate to peer with MS-CHAPv1. If this option is specified, requests for MS-CHAPv1 authentication from the peer are declined with LCP Configure-Nak. That option does not disable any other form of CHAP.
Do not agree to authenticate to peer with MS-CHAPv2. If specified, this option requests that MS-CHAPv2 authentication from the peer be declined with LCP Configure-Nak. That option does not disable any other form of CHAP.
With this option, pppd will not agree to authenticate itself to the peer using Password Authentication Protocol (PAP).
Require the peer to authenticate itself using standard CHAP authentication. MS-CHAP is not affected.
Require the peer to authenticate itself using MS-CHAPv1 authentication.
Require the peer to authenticate itself using MS-CHAPv2 authentication.
Require the peer to authenticate itself using PAP authentication.
When logging contents of PAP packets, this option causes pppd to show the password string in the log message.
With this option, pppd will not transmit LCP packets to initiate a connection until a valid LCP packet is received from the peer. This is like the “passive” option with older versions of pppd and is retained for compatibility, but the current passive option is preferred.
When checking the
asyncmap (ACCM) setting, pppd uses all 256 possible values by default. See no-accm-test. This option restricts the test so that only the 32 values
affected by standard ACCM negotiation are tested. This option is useful on very slow links.
Connect to given host and port using TCP and run PPP over this connection.
Use synchronous HDLC serial encoding instead of asynchronous. The device used by pppd with this option must have sync support. Currently supports zs, se, and hsi drivers.
Set PPP interface unit number to n, if possible.
With this option, pppd detaches from its controlling terminal after establishing the PPP connection. When this is specified, messages sent to stderr by the connect script, usually chat(1M), and debugging messages from the debug option are directed to pppd's standard output.
Enforce the use of the hostname with domain name appended, if given, as the name of the local system for authentication purposes. This overrides the name option. Because the name option is privileged, this option is normally not needed.
Ask the peer for up to two DNS server addresses. Addresses supplied by the peer, if any, are passed to the /etc/ppp/ip-up script in the environment variables DNS1 and DNS2. In addition, pppd creates an /etc/ppp/resolv.conf file containing one or two nameserver lines with the address(es) supplied by the peer.
Sets the name used for authenticating the local system to the peer to name.
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).
Run the executable or shell command specified by script before initiating PPP negotiation, after the connect script, if any, has completed. A value for this option from a privileged source cannot be overridden by a non-privileged user.
Use software flow control, that is, XON/XOFF, to control the flow of data on the serial port.
The following options are obsolete:
Read a PAP user name and password from the file name. This file must have two lines for name and password. Name and password are sent to the peer when the peer requests PAP authentication.
Enable IPv6 and IPv6CP without specifying interface identifiers.
Show version number and exit.
Show brief help message and exit.
The following sections discuss miscellaneous features of pppd:
pppd allows system administrators to provide legitimate users with PPP access to a server machine without fear of compromising the security of the server or the network it runs on. Access control is provided by restricting IP addresses the peer may use based on its authenticated identity (if any), and through restrictions on options a non-privileged user may use. Options that permit potentially insecure configurations are privileged. Privileged options are accepted only in files that are under the control of the system administrator or when pppd is being run by root.
By default, pppd allows an unauthenticated peer to use a given IP address only if the system does not already have a route to that IP address. For example, a system with a permanent connection to the wider Internet will normally have a default route, meaning all peers must authenticate themselves to set up a connection. On such a system, the auth option is the default. Conversely, a system with a PPP link that comprises the only connection to the Internet probably does not possess a default route, so the peer can use virtually any IP address without authenticating itself.
Security-sensitive options are privileged and cannot be accessed by a non-privileged user running pppd, either on the command line, in the user's $HOME/.ppprc file, or in an options file read using the file option. Privileged options may be used in /etc/ppp/options file or in an options file read using the call option. If pppd is run by the root user, privileged options can be used without restriction. If the /etc/ppp/options file does not exist, then only root may invoke pppd. The /etc/ppp/options file must be created (but may be empty) to allow ordinary non-root users to access pppd.
When opening the device, pppd uses the invoking user's user ID or the root UID (that is, 0), depending if the device name was specified by the user or the system administrator. If the device name comes from a privileged source, that is, /etc/ppp/options or an options file read using the call option, pppd uses full root privileges when opening the device. Thus, by creating an appropriate file under /etc/ppp/peers, the system administrator can allow users to establish a PPP connection via a device that they would not normally have access to. Otherwise pppd uses the invoking user's real UID when opening the device.
During the authentication process, one peer convinces the other of its identity by sending its name and some secret information to the other. During authentication, the first peer becomes the "client" and the second becomes the "server." Authentication names can (but are not required to) correspond to the peer's Internet hostnames.
pppd supports four authentication protocols: the Password Authentication Protocol (PAP) and three forms of the Challenge Handshake Authentication Protocol (CHAP). With the PAP protocol, the client sends its name and a cleartext password to the server to authenticate itself. With CHAP, the server initiates the authentication exchange by sending a challenge to the client who must respond with its name and a hash value derived from the shared secret and the challenge.
The PPP protocol is symmetrical, meaning that each peer may be required to authenticate itself to the other. Different authentication protocols and names can be used for each exchange.
By default, pppd authenticates if requested and does not require authentication from the peer. However, pppd does not authenticate itself with a specific protocol if it has no secrets that can do so.
pppd stores authentication secrets in the /etc/ppp/pap-secrets (for PAP), and /etc/ppp/chap-secrets (for CHAP) files. Both files use the same format. pppd uses secrets files to authenticate itself to other systems and to authenticate other systems to itself.
Secrets files contain one secret per line. Secrets are specific to a particular combination of client and server and can only be used by that client to authenticate itself to that server. Each line in a secrets file has a minimum of three fields that contain the client and server names followed by the secret. Often, these three fields are followed by IP addresses that are used by clients to connect to a server.
A secrets file is parsed into words, with client name, server name and secrets fields allocated one word each. Embedded spaces or other special characters within a word must be quoted or escaped. Case is significant in all three fields.
A secret beginning with an at sign (“@”) is followed by the name of a file containing the secret. An asterisk (*) as the client or server name matches any name. When choosing a match, pppd selects the one with the fewest wildcards. Succeeding words on a line are interpreted by pppd as acceptable IP addresses for that client. IP Addresses are disallowed if they appear in lines that contain only three words or lines whose first word begins with a hyphen (“-”). To allow any address, use "*". An address starting with an exclamation point (”!”) indicates that the specified address is not acceptable. An address may be followed by "/" and a number n to indicate a whole subnet (all addresses that have the same value in the most significant n bits). In this form, the address may be followed by a plus sign ("+") to indicate that one address from the subnet is authorized, based on the ppp network interface unit number in use. In this case, the host part of the address is set to the unit number, plus one.
When authenticating the peer, pppd chooses a secret with the peer's name in the first field of the secrets file and the name of the local system in the second field. The local system name defaults to the hostname, with the domain name appended if the domain option is used. The default can be overridden with the name option unless the usehostname option is used.
When authenticating to the peer, pppd first determines the name it will use to identify itself to the peer. This name is specified with the user option. If the user option is not used, the name defaults to the host name of the local system. pppd then selects a secret from the secrets file by searching for an entry with a local name in the first field and the peer's name in the second field. pppd will know the name of the peer if standard CHAP authentication is used because the peer will have sent it in the Challenge packet. However, if MS-CHAP or PAP is being used, pppd must 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. If that fails, pppd uses the null string as the peer's name.
When authenticating the peer with PAP, the supplied password is compared with data in the secrets file. If the password and secret do not match, the password is encrypted using crypt() and checked against the secret again. If the papcrypt option is given, the first unencrypted comparison is omitted for better security, and entries must thus be in encrypted crypt(3C) form.
If the login option is specified, the username and password are also checked against the system password database. This allows you to set up the pap-secrets file to enable PPP access only to certain users, and to restrict the set of IP addresses available to users. Typically, when using the login option, the secret in /etc/ppp/pap-secrets would be "", which matches any password supplied by the peer. This makes having the same secret in two places unnecessary. When login is used, the pam option enables access control through pam(3PAM).
Authentication must be completed before IPCP (or other network protocol) can be started. If the peer is required to authenticate itself and fails, pppd closes LCP and terminates the link. If IPCP negotiates an unacceptable IP address for the remote host, IPCP is closed. IP packets are sent or received only when IPCP is open.
To allow hosts that cannot authenticate themselves to connect and use one of a restricted set of IP addresses, add a line to the pap-secrets file specifying the empty string for the client name and secret.
Additional pppd options for a given peer may be specified by placing them at the end of the secrets entry, separated by two dashes (––). For example
peername servername secret ip-address -- novj
When IPCP negotiation is complete, pppd informs the kernel of the local and remote IP addresses for the PPP interface and creates a host route to the remote end of the link that enables peers to exchange IP packets. Communication with other machines generally requires further modification to routing tables and/or Address Resolution Protocol (ARP) tables. In most cases the defaultroute and/or proxyarp options are sufficient for this, but further intervention may be necessary. If further intervention is required, use the /etc/ppp/ip-up script or a routing protocol daemon.
To add a default route through the remote host, use the defaultroute option. This option is typically used for “client” systems; that is, end-nodes that use the PPP link for access to the general Internet.
In some cases it is desirable to use proxy ARP, for example on a server machine connected to a LAN, to allow other hosts to communicate with the remote host. proxyarp instructs pppd to look for a network interface on the same subnet as the remote host. That is, an interface supporting broadcast and ARP that is not a point-to-point or loopback interface and that is currently up. 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.
When the demand option is used, the interface IP addresses are already set at the time when IPCP comes up. If pppd cannot negotiate the same addresses it used to configure the interface, it changes the interface IP addresses to the negotiated addresses. This may disrupt existing connections. Using demand dialing with peers that perform dynamic IP address assignment is not recommended.
pppd invokes scripts at various stages during processing that are used to perform site-specific ancillary processing. These scripts may be shell scripts or executable programs. 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), enabling them to update routing tables, run privileged daemons, or perform other tasks. Be sure 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 pppd environment variables are:
Name of the serial tty device.
Name of the network interface.
IP address for the link's local end. This is set only when IPCP has started.
IP address for the link's remote end. This is set only when IPCP has started.
Authenticated name of the peer. This is set only if the peer authenticates itself.
Baud rate of the tty device.
Real user-id of user who invoked pppd.
Username of the real user-id who invoked pppd. This is always set.
pppd also sets the following variables for the ip-down and auth-down scripts:
Number of seconds between the start of PPP negotiation and connection termination.
Number of bytes sent at the level of the serial port during the connection.
Number of bytes received at the level of the serial port during the connection.
Logical name of the link, set with the linkname option.
If they exist, pppd invokes the following scripts. It is not an error if they do not exist.
Program or script executed after the remote system successfully authenticates itself. It is executed with five command-line arguments: interface-name peer-name user-name tty-device speed. Note that this script is not executed if the peer does not authenticate itself, for example, when the noauth option is used.
Program or script 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.
A program or script that is executed when the link is available for sending and receiving IP packets (that is, IPCP has come up). It is executed with six command-line arguments: interface-name tty-device speed local-IP-address remote-IP-address ipparam.
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.
Similar to /etc/ppp/ip-up, except that it is executed when the link is available for sending and receiving IPv6 packets. Executed with six command-line arguments: interface-name tty-device speed local-link-local-address remote-link-local-address ipparam.
Similar to /etc/ppp/ip-down, but executed when IPv6 packets can no longer be transmitted on the link. Executed with the same parameters as the ipv6-up script.
The following examples assume that the /etc/ppp/options file contains the auth option.
pppd is commonly used to dial out to an ISP. You can do this using the “pppd call isp” command where the /etc/ppp/peers/isp file is set up to contain a line similar to the following:
cua/a 19200 crtscts connect '/usr/bin/chat -f /etc/ppp/chat-isp' noauth
For this example, chat(1M) is used to dial the ISP's modem and process any login sequence required. The /etc/ppp/chat-isp file is used by chat and could contain the following:
ABORT "NO CARRIER" ABORT "NO DIALTONE" ABORT "ERROR" ABORT "NO ANSWER" ABORT "BUSY" ABORT "Username/Password Incorrect" "" "at" OK "at&f&d2&c1" OK "atdt2468135" "name:" "^Umyuserid" "word:" "\qmypassword" "ispts" "\q^Uppp" "~-^Uppp-~"
See the chat(1M) man page for details of chat scripts.
pppd can also 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 as shown in the following example:
example% pppd proxyarp
To provide a user with access to the PPP facilities, allocate an IP address for the user's machine, create an entry in /etc/ppp/pap-secrets or /etc/ppp/chap-secrets. This enables the user's machine to authenticate itself. For example, to enable user “Joe” using machine "joespc" to dial in to machine "server" and use the IP address “joespc.my.net,” add the following entry to the /etc/ppp/pap-secrets or /etc/ppp/chap-secrets files:
joespc server "joe's secret" joespc.my.net
Alternatively, you can create another username, for example “ppp," whose login shell is /usr/bin/pppd and whose home directory is /etc/ppp. If you run pppd this way, add the options to the /etc/ppp/.ppprc file.
If your serial connection is complex, it may be useful to escape such control characters as XON (^Q) and XOFF (^S), using asyncmap a0000. If the path includes a telnet, escape ^] (asyncmap 200a0000). If the path includes a rlogin command, add escape ff option to the options, because rlogin removes the window-size-change sequence [0xff, 0xff, 0x73, 0x73, followed by any 8 bytes] from the stream.
The pppd exit status indicates errors or specifies why a link was terminated. Exit status values are:
pppd has detached or the connection was successfully established and terminated at the peer's request.
An immediately fatal error occurred. For example, an essential system call failed.
An error was detected in the options given. For example, two mutually exclusive options were used, or /etc/ppp/options is missing and the user is not root.
pppd is not setuid-root and the invoking user is not root.
The kernel does not support PPP. For example, the PPP kernel driver is not included or cannot be loaded.
pppd terminated because it was sent a SIGINT, SIGTERM or SIGHUP signal.
The serial port could not be locked.
The serial port could not be opened.
The connect script failed and returned a non-zero exit status.
The command specified as the argument to the pty option could not be run.
The PPP negotiation failed because no network protocols were able to run.
The peer system failed or refused to authenticate itself.
The link was established successfully, but terminated because it was idle.
The link was established successfully, but terminated because the connect time limit was reached.
Callback was negotiated and an incoming call should arrive shortly.
The link was terminated because the peer is not responding to echo requests.
The link was terminated by the modem hanging up.
The PPP negotiation failed because serial loopback was detected.
The init script failed because a non-zero exit status was returned.
Authentication to the peer failed.
Process-ID for pppd process on PPP interface unit n.
Process-ID for pppd process for logical link name (see the linkname option).
Usernames, passwords and IP addresses for PAP authentication. This file should be owned by root and not readable or writable by any other user, otherwise pppd will log a warning.
Names, secrets and IP addresses for all forms of CHAP authentication. The /etc/ppp/pap-secrets file should be owned by root should not readable or writable by any other user, otherwise, pppd will log a warning.
System default options for pppd, read before user default options or command-line options.
User default options, read before /etc/ppp/options.ttyname.
System default options for the serial port in use; read after $HOME/.ppprc. The ttyname component of this filename is formed when the initial /dev/ is stripped from the port name (if present), and slashes (if any) are converted to dots.
Directory with options files that 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 attributes(5) for descriptions of the following attributes:
Haskin, D., Allen, E. RFC 2472 – IP Version 6 Over PPP. Network Working Group. December 1998.
Jacobson, V. RFC 1144, Compressing TCP/IP Headers for Low-Speed Serial Links. Network Working Group. February, 1990
Lloyd, B., Simpson, W. RFC 1334, PPP Authentication Protocols. Network Working Group. October 1992.
McGregor, G. RFC 1332, The PPP Internet Protocol Control Protocol (IPCP). Network Working Group. May 1992.
Rivest, R. RFC 1321, The MD5 Message-Digest Algorithm. Network Working Group. April 1992
Simpson, W. RFC 1661, The Point-to-Point Protocol (PPP). Network Working Group. July 1994.
Simpson, W. RFC 1662, HDLC-like Framing . Network Working Group. July 1994.
These signals affect pppd behavior:
Terminate the link, restore the serial device settings and exit.
Terminate the link, restore the serial device settings and close the serial device. If the persist or demand option is specified, pppd attempts to reopen
the serial device and start another connection after the holdoff period. Otherwise pppd exits. If received during the holdoff period,
SIGHUP causes pppd to end the holdoff period immediately.
Toggles the state of the debug option and prints link status information to the log.
Causes pppd to renegotiate compression. This is 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 an implementation.)
Messages are sent to the syslog daemon using facility LOG_DAEMON. To see error and debug messages, edit the /etc/syslog.conf file to direct the messages to the desired output device or file, or use the updetach or logfile options.
The debug option causes the contents of all LCP, PAP, CHAP or IPCP control packets sent or received to be logged. This is useful if PPP negotiation does not succeed or if authentication fails.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal, which acts as a toggle to the pppd process.