neulab/tldr huggingface dataset:
dump   With the dump option, pppd will print out all the option values which have been set.  This option is like the dryrun option except that pppd proceeds as normal rather than exiting.
enable-session Enables session accounting via PAM or wtwp/wtmpx, as appropriate. When PAM is enabled, the PAM "account" and "session" module stacks determine behavior, and are enabled for all PPP authentication protocols.  When PAM is disabled, wtmp/wtmpx entries are recorded regardless of whether the peer name identifies a valid user on the local system, making peers visible in the last(1) log.  This feature is automatically enabled when the pppd login option is used.  Session accounting is disabled by default.
endpoint <epdisc> Sets the endpoint discriminator sent by the local machine to the peer during multilink negotiation to <epdisc>.  The default is to use the MAC address of the first ethernet interface on the system, if any, otherwise the IPv4 address corresponding to the hostname, if any, provided it is not in the multicast or locally-assigned IP address ranges, or the localhost address.  The endpoint discriminator can be the string null or of the form type:value, where type is a decimal number or one of the strings local, IP, MAC, magic, or phone.  The value is an IP address in dotted- decimal notation for the IP type, or a string of bytes in hexadecimal, separated by periods or colons for the other types. For the MAC type, the value may also be the name of an ethernet or similar network interface.  This option is currently only available under Linux.
eap-interval n If this option is given and pppd authenticates the peer with EAP (i.e., is the server), pppd will restart EAP authentication every n seconds.  For EAP SRP-SHA1, see also the srp-interval option, which enables lightweight rechallenge.
eap-max-rreq n Set the maximum number of EAP Requests to which pppd will respond (as a client) without hearing EAP Success or Failure.  (Default is 20.)
eap-max-sreq n Set the maximum number of EAP Requests that pppd will issue (as a server) while attempting authentication.  (Default is 10.)
eap-restart n Set the retransmit timeout for EAP Requests when acting as a server (authenticator).  (Default is 3 seconds.)
eap-timeout n Set the maximum time to wait for the peer to send an EAP Request when acting as a client (authenticatee).  (Default is 20 seconds.)
hide-password When logging the contents of PAP packets, this option causes pppd to exclude the password string from the log.  This is the default.
holdoff n Specifies how many seconds to wait before re-initiating the link after it terminates.  This option only has any effect if the persist or demand option is used.  The holdoff period is not applied if the link was terminated because it was idle.
idle n Specifies that pppd should disconnect if the link is idle for n seconds.  The link is idle when no data packets (i.e. IP packets) are being sent or received.  Note: it is not advisable to use this option with the persist option without the demand option.  If the active-filter option is given, data packets which are rejected by the specified activity filter also count as the link being idle.
ipcp-accept-local With this option, pppd will accept the peer's idea of our local IP address, even if the local IP address was specified in an option.
ipcp-accept-remote With this option, pppd will accept the peer's idea of its (remote) IP address, even if the remote IP address was specified in an option.
ipcp-max-configure n Set the maximum number of IPCP configure-request transmissions to n (default 10).
ipcp-max-failure n Set the maximum number of IPCP configure-NAKs returned before starting to send configure-Rejects instead to n (default 10).
ipcp-max-terminate n Set the maximum number of IPCP terminate-request transmissions to n (default 3).
ipcp-restart n Set the IPCP restart interval (retransmission timeout) to n seconds (default 3).
ipparam string Provides an extra parameter to the ip-up, ip-pre-up and ip-down scripts.  If this option is given, the string supplied is given as the 6th parameter to those scripts.
ipv6cp-accept-local With this option, pppd will accept the peer's idea of our local IPv6 interface identifier, even if the local IPv6 interface identifier was specified in an option.
ipv6cp-accept-remote With this option, pppd will accept the peer's idea of its (remote) IPv6 interface identifier, even if the remote IPv6 interface identifier was specified in an option.
ipv6cp-max-configure n Set the maximum number of IPv6CP configure-request transmissions to n (default 10).
ipv6cp-max-failure n Set the maximum number of IPv6CP configure-NAKs returned before starting to send configure-Rejects instead to n (default 10).
ipv6cp-max-terminate n Set the maximum number of IPv6CP terminate-request transmissions to n (default 3).
ipv6cp-restart n Set the IPv6CP restart interval (retransmission timeout) to n seconds (default 3).
ipx    Enable the IPXCP and IPX protocols.  This option is presently only supported under Linux, and only if your kernel has been configured to include IPX support.
ipx-network n Set the IPX network number in the IPXCP configure request frame to n, a hexadecimal number (without a leading 0x).  There is no valid default.  If this option is not specified, the network number is obtained from the peer.  If the peer does not have the network number, the IPX protocol will not be started.
ipx-node n:m Set the IPX node numbers. The two node numbers are separated from each other with a colon character. The first number n is the local node number. The second number m is the peer's node number. Each node number is a hexadecimal number, at most 10 digits long. The node numbers on the ipx-network must be unique. There is no valid default. If this option is not specified then the node numbers are obtained from the peer.
ipx-router-name <string> Set the name of the router. This is a string and is sent to the peer as information data.
ipx-routing n Set the routing protocol to be received by this option. More than one instance of ipx-routing may be specified. The 'none' option (0) may be specified as the only instance of ipx-routing. The values may be 0 for NONE, 2 for RIP/SAP, and 4 for NLSP.
ipxcp-accept-local Accept the peer's NAK for the node number specified in the ipx-node option. If a node number was specified, and non-zero, the default is to insist that the value be used. If you include this option then you will permit the peer to override the entry of the node number.
ipxcp-accept-network Accept the peer's NAK for the network number specified in the ipx-network option. If a network number was specified, and non- zero, the default is to insist that the value be used. If you include this option then you will permit the peer to override the entry of the node number.
ipxcp-accept-remote Use the peer's network number specified in the configure request frame. If a node number was specified for the peer and this option was not specified, the peer will be forced to use the value which you have specified.
ipxcp-max-configure n Set the maximum number of IPXCP configure request frames which the system will send to n. The default is 10.
ipxcp-max-failure n Set the maximum number of IPXCP NAK frames which the local system will send before it rejects the options. The default value is 3.
ipxcp-max-terminate n Set the maximum number of IPXCP terminate request frames before the local system considers that the peer is not listening to them. The default value is 3.
kdebug n Enable debugging code in the kernel-level PPP driver.  The argument values depend on the specific kernel driver, but in general a value of 1 will enable general kernel debug messages. (Note that these messages are usually only useful for debugging the kernel driver itself.)  For the Linux 2.2.x kernel driver, the value is a sum of bits: 1 to enable general debug messages, 2 to request that the contents of received packets be printed, and 4 to request that the contents of transmitted packets be printed.  On most systems, messages printed by the kernel are logged by syslog(1) to a file as directed in the /etc/syslog.conf configuration file.
key keyfile (EAP-TLS) Use the file keyfile as the private key file (in PEM format), needed for setting up an EAP-TLS connection. This option is used on the client-side in conjunction with the ca and cert options.
ktune  Enables pppd to alter kernel settings as appropriate.  Under Linux, pppd will enable IP forwarding (i.e. set /proc/sys/net/ipv4/ip_forward to 1) if the proxyarp option is used, and will enable the dynamic IP address option (i.e. set /proc/sys/net/ipv4/ip_dynaddr to 1) in demand mode if the local address changes.
lcp-echo-adaptive If this option is used with the lcp-echo-failure option then pppd will send LCP echo-request frames only if no traffic was received from the peer since the last echo-request was sent.
lcp-echo-failure n If this option is given, pppd will presume the peer to be dead if n LCP echo-requests are sent without receiving a valid LCP echo-reply.  If this happens, pppd will terminate the connection. Use of this option requires a non-zero value for the lcp-echo-interval parameter.  This option can be used to enable pppd to terminate after the physical connection has been broken (e.g., the modem has hung up) in situations where no hardware modem control lines are available.
lcp-echo-interval n If this option is given, pppd will send an LCP echo-request frame to the peer every n seconds.  Normally the peer should respond to the echo-request by sending an echo-reply.  This option can be used with the lcp-echo-failure option to detect that the peer is no longer connected.
lcp-max-configure n Set the maximum number of LCP configure-request transmissions to n (default 10).
lcp-max-failure n Set the maximum number of LCP configure-NAKs returned before starting to send configure-Rejects instead to n (default 10).
lcp-max-terminate n Set the maximum number of LCP terminate-request transmissions to n (default 3).
lcp-restart n Set the LCP restart interval (retransmission timeout) to n seconds (default 3).
linkname name Sets the logical name of the link to name.  Pppd will create a file named ppp-name.pid in /var/run (or /etc/ppp on some systems) containing its process ID.  This can be useful in determining which instance of pppd is responsible for the link to a given peer system.  This is a privileged option.
local  Don't use the modem control lines.  With this option, pppd will ignore the state of the CD (Carrier Detect) signal from the modem and will not change the state of the DTR (Data Terminal Ready) signal.  This is the opposite of the modem option.
logfd n Send log messages to file descriptor n.  Pppd will send 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.  The default is for pppd to send log messages to stdout (file descriptor 1), unless the serial port is already open on stdout.
logfile filename Append log messages to the file filename (as well as sending the log messages to syslog).  The file is opened with the privileges of the user who invoked pppd, in append mode.
login  Use the system password database for authenticating the peer using PAP, and record the user in the system wtmp file.  Note that the peer must have an entry in the /etc/ppp/pap-secrets file as well as the system password database to be allowed access.  See also the enable-session option.
master_detach If multilink is enabled and this pppd process is the multilink bundle master, and the link controlled by this pppd process terminates, this pppd process continues to run in order to maintain the bundle.  If the master_detach option has been given, pppd will detach from its controlling terminal in this situation, even if the nodetach option has been given.
maxconnect n Terminate the connection when it has been available for network traffic for n seconds (i.e. n seconds after the first network control protocol comes up).
maxfail n Terminate after n consecutive failed connection attempts.  A value of 0 means no limit.  The default value is 10.
modem  Use the modem control lines.  This option is the default.  With this option, pppd will wait for the CD (Carrier Detect) signal from the modem to be asserted when opening the serial device (unless a connect script is specified), and it will drop the DTR (Data Terminal Ready) signal briefly when the connection is terminated and before executing the connect script.  On Ultrix, this option implies hardware flow control, as for the crtscts option.  This is the opposite of the local option.
mp     Enables the use of PPP multilink; this is an alias for the `multilink' option.  This option is currently only available under Linux.
mppe-stateful Allow MPPE to use stateful mode.  Stateless mode is still attempted first.  The default is to disallow stateful mode.
mpshortseq Enables the use of short (12-bit) sequence numbers in multilink headers, as opposed to 24-bit sequence numbers.  This option is only available under Linux, and only has any effect if multilink is enabled (see the multilink option).
mrru n Sets the Maximum Reconstructed Receive Unit to n.  The MRRU is the maximum size for a received packet on a multilink bundle, and is analogous to the MRU for the individual links.  This option is currently only available under Linux, and only has any effect if multilink is enabled (see the multilink option).
ms-dns <addr> If pppd is acting as a server for Microsoft Windows clients, this option allows pppd to supply one or two DNS (Domain Name Server) addresses to the clients.  The first instance of this option specifies the primary DNS address; the second instance (if given) specifies the secondary DNS address.  (This option was present in some older versions of pppd under the name dns-addr.)
ms-wins <addr> If pppd is acting as a server for Microsoft Windows or "Samba" clients, this option allows pppd to supply one or two WINS (Windows Internet Name Services) server addresses to the clients. The first instance of this option specifies the primary WINS address; the second instance (if given) specifies the secondary WINS address.
multilink Enables the use of the PPP multilink protocol.  If the peer also supports multilink, then this link can become part of a bundle between the local system and the peer.  If there is an existing bundle to the peer, pppd will join this link to that bundle, otherwise pppd will create a new bundle.  See the MULTILINK section below.  This option is currently only available under Linux.
name name Set the name of the local system for authentication purposes to name.  This is a privileged option.  With this option, pppd will use lines in the secrets files which have name as the second field when looking for a secret to use in authenticating the peer.  In addition, unless overridden with the user option, name will be used as the name to send to the peer when authenticating the local system to the peer.  (Note that pppd does not append the domain name to name.)
noaccomp Disable Address/Control compression in both directions (send and receive).
need-peer-eap (EAP-TLS) Require the peer to verify our authentication credentials.
noauth Do not require the peer to authenticate itself.  This option is privileged.
nobsdcomp Disables BSD-Compress compression; pppd will not request or agree to compress packets using the BSD-Compress scheme.
noccp  Disable CCP (Compression Control Protocol) negotiation.  This option should only be required if the peer is buggy and gets confused by requests from pppd for CCP negotiation.
nocrtscts Disable hardware flow control (i.e. RTS/CTS) on the serial port. If neither the crtscts nor the nocrtscts nor the cdtrcts nor the nocdtrcts option is given, the hardware flow control setting for the serial port is left unchanged.
nocdtrcts This option is a synonym for nocrtscts. Either of these options will disable both forms of hardware flow control.
nodefaultroute Disable the defaultroute option.  The system administrator who wishes to prevent users from adding a default route with pppd can do so by placing this option in the /etc/ppp/options file.
noreplacedefaultroute Disable the replacedefaultroute option. This allows to disable a replacedefaultroute option set previously in the configuration.
nodefaultroute6 Disable the defaultroute6 option.  The system administrator who wishes to prevent users from adding a default route with pppd can do so by placing this option in the /etc/ppp/options file.
nodeflate Disables Deflate compression; pppd will not request or agree to compress packets using the Deflate scheme.
nodetach Don't detach from the controlling terminal.  Without this option, if a serial device other than the terminal on the standard input is specified, pppd will fork to become a background process.
noendpoint Disables pppd from sending an endpoint discriminator to the peer or accepting one from the peer (see the MULTILINK section below). This option should only be required if the peer is buggy.
noip   Disable IPCP negotiation and IP communication.  This option should only be required if the peer is buggy and gets confused by requests from pppd for IPCP negotiation.
noipv6 Disable IPv6CP negotiation and IPv6 communication. This option should only be required if the peer is buggy and gets confused by requests from pppd for IPv6CP negotiation.
noipdefault Disables the default behaviour when no local IP address is specified, which is to determine (if possible) the local IP address from the hostname.  With this option, the peer will have to supply the local IP address during IPCP negotiation (unless it specified explicitly on the command line or in an options file).
noipx  Disable the IPXCP and IPX protocols.  This option should only be required if the peer is buggy and gets confused by requests from pppd for IPXCP negotiation.
noktune Opposite of the ktune option; disables pppd from changing system settings.
nolock Opposite of the lock option; specifies that pppd should not create a UUCP-style lock file for the serial device.  This option is privileged.
nolog  Do not send log messages to a file or file descriptor.  This option cancels the logfd and logfile options.
nomagic Disable magic number negotiation.  With this option, pppd cannot detect a looped-back line.  This option should only be needed if the peer is buggy.
nomp   Disables the use of PPP multilink.  This option is currently only available under Linux.
nomppe Disables MPPE (Microsoft Point to Point Encryption).  This is the default.
nomppe-40 Disable 40-bit encryption with MPPE.
nomppe-128 Disable 128-bit encryption with MPPE.
nomppe-stateful Disable MPPE stateful mode.  This is the default.
nompshortseq Disables the use of short (12-bit) sequence numbers in the PPP multilink protocol, forcing the use of 24-bit sequence numbers. This option is currently only available under Linux, and only has any effect if multilink is enabled.
nomultilink Disables the use of PPP multilink.  This option is currently only available under Linux.
nopcomp Disable protocol field compression negotiation in both the receive and the transmit direction.
nopersist Exit once a connection has been made and terminated.  This is the default unless the persist or demand option has been specified.
nopredictor1 Do not accept or agree to Predictor-1 compression.
noproxyarp Disable the proxyarp option.  The system administrator who wishes to prevent users from creating proxy ARP entries with pppd can do so by placing this option in the /etc/ppp/options file.
noremoteip Allow pppd to operate without having an IP address for the peer. This option is only available under Linux.  Normally, pppd will request the peer's IP address, and if the peer does not supply it, pppd will use an arbitrary address in the 10.x.x.x subnet.  With this option, if the peer does not supply its IP address, pppd will not ask the peer for it, and will not set the destination address of the ppp interface.  In this situation, the ppp interface can be used for routing by creating device routes, but the peer itself cannot be addressed directly for IP traffic.
notty  Normally, pppd requires a terminal device.  With this option, pppd will allocate itself a pseudo-tty master/slave pair and use the slave as its terminal device.  Pppd will create 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 will transmit characters on its standard output and receive 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.
novj   Disable Van Jacobson style TCP/IP header compression in both the transmit and the receive direction.
novjccomp Disable the connection-ID compression option in Van Jacobson style TCP/IP header compression.  With this option, pppd will not omit the connection-ID byte from Van Jacobson compressed TCP/IP headers, nor ask the peer to do so.
papcrypt Indicates that all secrets in the /etc/ppp/pap-secrets file which are used for checking the identity of the peer are encrypted, and thus pppd should not accept a password which, before encryption, is identical to the secret from the /etc/ppp/pap-secrets file.
pap-max-authreq n Set the maximum number of PAP authenticate-request transmissions to n (default 10).
pap-restart n Set the PAP restart interval (retransmission timeout) to n seconds (default 3).
pap-timeout n Set the maximum time that pppd will wait for the peer to authenticate itself with PAP to n seconds (0 means no limit).
pass-filter filter-expression Specifies a packet filter to applied to data packets being sent or received to determine which packets should be allowed to pass. Packets which are rejected by the filter are silently discarded. This option can be used to prevent specific network daemons (such as routed) using up link bandwidth, or to provide a very basic firewall capability.  The filter-expression syntax is as described for tcpdump(1), except that qualifiers which are inappropriate for a PPP link, such as ether and arp, are not permitted.  Generally the filter expression should be enclosed in single-quotes to prevent whitespace in the expression from being interpreted by the shell.  Note that it is possible to apply different constraints to incoming and outgoing packets using the inbound and outbound qualifiers. This option is currently only available under Linux, and requires that the kernel was configured to include PPP filtering support (CONFIG_PPP_FILTER).
password password-string Specifies the password to use for authenticating to the peer.  Use of this option is discouraged, as the password is likely to be visible to other users on the system (for example, by using ps(1)).
persist Do not exit after a connection is terminated; instead try to reopen the connection. The maxfail option still has an effect on persistent connections.
plugin filename Load the shared library object file filename as a plugin.  This is a privileged option.  If filename does not contain a slash (/), pppd will look in the /usr/lib/pppd/version directory for the plugin, where version is the version number of pppd (for example, 2.4.2).
predictor1 Request that the peer compress frames that it sends using Predictor-1 compression, and agree to compress transmitted frames with Predictor-1 if requested.  This option has no effect unless the kernel driver supports Predictor-1 compression.
privgroup group-name Allows members of group group-name to use privileged options. This is a privileged option.  Use of this option requires care as there is no guarantee that members of group-name cannot use pppd to become root themselves.  Consider it equivalent to putting the members of group-name in the kmem or disk group.
proxyarp Add an entry to this system's ARP [Address Resolution Protocol] table with the IP address of the peer and the Ethernet address of this system.  This will have the effect of making the peer appear to other systems to be on the local ethernet.
pty script Specifies that the command script is to be used to communicate rather than a specific terminal device.  Pppd will allocate itself a pseudo-tty master/slave pair and use the slave as its terminal device.  The script will be run 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.)
receive-all With this option, pppd will accept all control characters from the peer, including those marked in the receive asyncmap.  Without this option, pppd will discard those characters as specified in RFC1662.  This option should only be needed if the peer is buggy.
record filename Specifies that pppd should record all characters sent and received to a file named filename.  This file is opened in append mode, using the user's user-ID and permissions.  This option is implemented using a pseudo-tty and a process to transfer characters between the pseudo-tty and the real serial device, so it will increase the latency and CPU overhead of transferring data over the ppp interface.  The characters are stored in a tagged format with timestamps, which can be displayed in readable form using the pppdump(8) program.
remotename name Set the assumed name of the remote system for authentication purposes to name.
remotenumber number Set the assumed telephone number of the remote system for authentication purposes to number.
refuse-chap With this option, pppd will not agree to authenticate itself to the peer using CHAP.
refuse-mschap With this option, pppd will not agree to authenticate itself to the peer using MS-CHAP.
refuse-mschap-v2 With this option, pppd will not agree to authenticate itself to the peer using MS-CHAPv2.
refuse-eap With this option, pppd will not agree to authenticate itself to the peer using EAP.
refuse-pap With this option, pppd will not agree to authenticate itself to the peer using PAP.
require-chap Require the peer to authenticate itself using CHAP [Challenge Handshake Authentication Protocol] authentication.
require-mppe Require the use of MPPE (Microsoft Point to Point Encryption). This option disables all other compression types.  This option enables both 40-bit and 128-bit encryption.  In order for MPPE to successfully come up, you must have authenticated with either MS-CHAP or MS-CHAPv2.  This option is presently only supported under Linux, and only if your kernel has been configured to include MPPE support.
require-mppe-40 Require the use of MPPE, with 40-bit encryption.
require-mppe-128 Require the use of MPPE, with 128-bit encryption.
require-mschap Require the peer to authenticate itself using MS-CHAP [Microsoft Challenge Handshake Authentication Protocol] authentication.
require-mschap-v2 Require the peer to authenticate itself using MS-CHAPv2 [Microsoft Challenge Handshake Authentication Protocol, Version 2] authentication.
require-eap Require the peer to authenticate itself using EAP [Extensible Authentication Protocol] authentication.
require-pap Require the peer to authenticate itself using PAP [Password Authentication Protocol] authentication.
set name=value Set an environment variable for scripts that are invoked by pppd. When set by a privileged source, the variable specified by name cannot be changed by options contained in an unprivileged source. See also the unset option and the environment described in SCRIPTS.
show-password When logging the contents of PAP packets, this option causes pppd to show the password string in the log message.
silent With this option, pppd will not transmit LCP packets to initiate a connection until a valid LCP packet is received from the peer (as for the `passive' option with ancient versions of pppd).
srp-interval n If this parameter is given and pppd uses EAP SRP-SHA1 to authenticate the peer (i.e., is the server), then pppd will use the optional lightweight SRP rechallenge mechanism at intervals of n seconds.  This option is faster than eap-interval reauthentication because it uses a hash-based mechanism and does not derive a new session key.
srp-pn-secret string Set the long-term pseudonym-generating secret for the server. This value is optional and if set, needs to be known at the server (authenticator) side only, and should be different for each server (or poll of identical servers).  It is used along with the current date to generate a key to encrypt and decrypt the client's identity contained in the pseudonym.
srp-use-pseudonym When operating as an EAP SRP-SHA1 client, attempt to use the pseudonym stored in ~/.ppp_pseudonym first as the identity, and save in this file any pseudonym offered by the peer during authentication.
stop-bits n Set the number of stop bits for the serial port. Valid values are 1 or 2.  The default value is 1.
sync   Use synchronous HDLC serial encoding instead of asynchronous.  The device used by pppd with this option must have sync support. Currently supports Microgate SyncLink adapters under Linux and FreeBSD 2.2.8 and later.
unit num Sets the ppp unit number (for a ppp0 or ppp1 etc interface name) for outbound connections.  If the unit is already in use a dynamically allocated number will be used.
ifname string Set the ppp interface name for outbound connections.  If the interface name is already in use, or if the name cannot be used for any other reason, pppd will terminate.
unset name Remove a variable from the environment variable for scripts that are invoked by pppd.  When specified by a privileged source, the variable name cannot be set by options contained in an unprivileged source.  See also the set option and the environment described in SCRIPTS.
updetach With this option, pppd will detach from its controlling terminal once it has successfully established the ppp connection (to the point where the first network control protocol, usually the IP control protocol, has come up).
up_sdnotify Use this option to run pppd in systemd service units of Type=notify (up_sdnotify implies nodetach).  When up_sdnotify is enabled, pppd will notify systemd once it has successfully established the ppp connection (to the point where the first network control protocl, usually the IP control protocol, has come up). This option is only availble when pppd is compiled with systemd support.
usehostname Enforce the use of the hostname (with domain name appended, if given) as the name of the local system for authentication purposes (overrides the name option).  This option is not normally needed since the name option is privileged.
usepeerdns Ask the peer for up to 2 DNS server addresses.  The addresses supplied by the peer (if any) are passed to the /etc/ppp/ip-up script in the environment variables DNS1 and DNS2, and the environment variable USEPEERDNS will be set to 1.  In addition, pppd will create an /etc/ppp/resolv.conf file containing one or two nameserver lines with the address(es) supplied by the peer.
user name Sets the name used for authenticating the local system to the peer to name.
vj-max-slots n Sets the number of connection slots to be used by the Van Jacobson TCP/IP header compression and decompression code to n, which must be between 2 and 16 (inclusive).
welcome script Run the executable or shell command specified by script before initiating PPP negotiation, after the connect script (if any) has completed.  A value for this option from a privileged source cannot be overridden by a non-privileged user.
xonxoff Use software flow control (i.e. XON/XOFF) to control the flow of data on the serial port.
To establish PPP link over Ethernet (PPPoE) it is needed to load pppd's plugin pppoe.so and then specify option nic-interface instead of modem options ttyname and speed.  Recognized pppd's PPPoE options are:
nic-interface Use the ethernet device interface to communicate with the peer. For example, establishing PPPoE link on eth0 interface is done by specifying ppp'd option nic-eth0. Prefix nic- for this option may be avoided if interface name is unambiguous and does not look like any other pppd's option.
pppoe-service name Connect to specified PPPoE service name. For backward compatibility also rp_pppoe_service option name is supported.
pppoe-ac name Connect to specified PPPoE access concentrator name. For backward compatibility also rp_pppoe_ac option name is supported.
pppoe-sess sessid:macaddr Attach to existing PPPoE session. For backward compatibility also rp_pppoe_sess option name is supported.
pppoe-verbose n Be verbose about discovered access concentrators. For backward compatibility also rp_pppoe_verbose option name is supported.
pppoe-mac macaddr Connect to specified MAC address.
pppoe-host-uniq string Set the PPPoE Host-Uniq tag to the supplied hex string.  By default PPPoE Host-Uniq tag is set to the pppd's process PID.  For backward compatibility this option may be specified without pppoe- prefix.
pppoe-padi-timeout n Initial timeout for discovery packets in seconds (default 5).
pppoe-padi-attempts n Number of discovery attempts (default 3).
Options can be taken from files as well as the command line.  Pppd reads options from the files /etc/ppp/options, ~/.ppprc and /etc/ppp/options.ttyname (in that order) before processing the options on the command line.  (In fact, the command-line options are scanned to find the terminal name before the options.ttyname file is read.)  In forming the name of the options.ttyname file, the initial /dev/ is removed from the terminal name, and any remaining / characters are replaced with dots.
An options file is parsed into a series of words, delimited by whitespace.  Whitespace can be included in a word by enclosing the word in double-quotes (").  A backslash (\) quotes the following character.  A hash (#) starts a comment, which continues until the end of the line. There is no restriction on using the file or call options within an options file.
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.  This control is provided through restrictions on which IP addresses the peer may use, based on its authenticated identity (if any), and through restrictions on which options a non-privileged user may use. Several of pppd's options are privileged, in particular those which permit potentially insecure configurations; these options are only accepted in files which are under the control of the system administrator, or if pppd is being run by root.
The default behaviour of pppd is to allow 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, and thus all peers will have to authenticate themselves in order to set up a connection.  On such a system, the auth option is the default.  On the other hand, a system where the PPP link is the only connection to the internet will not normally have a default route, so the peer will be able to use almost any IP address without authenticating itself.
As indicated above, some security-sensitive options are privileged, which means that they may not be used by an ordinary non-privileged user running a setuid-root pppd, either on the command line, in the user's ~/.ppprc file, or in an options file read using the file option. Privileged options may be used in /etc/ppp/options file or in an options file read using the call option.  If pppd is being run by the root user, privileged options can be used without restriction.
When opening the device, pppd uses either the invoking user's user ID or the root UID (that is, 0), depending on whether 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 which they would not normally have permission to access.  Otherwise pppd uses the invoking user's real UID when opening the device.
Authentication is the process whereby one peer convinces the other of its identity.  This involves the first peer sending its name to the other, together with some kind of secret information which could only come from the genuine authorized user of that name.  In such an exchange, we will call the first peer the "client" and the other the "server".  The client has a name by which it identifies itself to the server, and the server also has a name by which it identifies itself to the client.  Generally the genuine client shares some secret (or password) with the server, and authenticates itself by proving that it knows that secret.  Very often, the names used for authentication correspond to the internet hostnames of the peers, but this is not essential.
At present, pppd supports three authentication protocols: the Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and Extensible Authentication Protocol (EAP).  PAP involves the client sending its name and a cleartext password to the server to authenticate itself.  In contrast, the server initiates the CHAP authentication exchange by sending a challenge to the client (the challenge packet includes the server's name).  The client must respond with a response which includes its name plus a hash value derived from the shared secret and the challenge, in order to prove that it knows the secret.  EAP supports CHAP-style authentication, and also includes the SRP-SHA1 mechanism, which is resistant to dictionary-based attacks and does not require a cleartext password on the server side.
The PPP protocol, being symmetrical, allows both peers to require the other to authenticate itself.  In that case, two separate and independent authentication exchanges will occur.  The two exchanges could use different authentication protocols, and in principle, different names could be used in the two exchanges.
The default behaviour of pppd is to agree to authenticate if requested, and to not require authentication from the peer.  However, pppd will not agree to authenticate itself with a particular protocol if it has no secrets which could be used to do so.
Pppd stores secrets for use in authentication in secrets files (/etc/ppp/pap-secrets for PAP, /etc/ppp/chap-secrets for CHAP, MS-CHAP, MS-CHAPv2, and EAP MD5-Challenge, and /etc/ppp/srp-secrets for EAP SRP-SHA1).  All secrets files have the same format.  The secrets files can contain secrets for pppd to use in authenticating itself to other systems, as well as secrets for pppd to use when authenticating other systems to itself.
Each line in a secrets file contains one secret.  A given secret is specific to a particular combination of client and server - it can only be used by that client to authenticate itself to that server.  Thus each line in a secrets file has at least 3 fields: the name of the client, the name of the server, and the secret.  These fields may be followed by a list of the IP addresses that the specified client may use when connecting to the specified server.
A secrets file is parsed into words as for a options file, so the client name, server name and secrets fields must each be one word, with any embedded spaces or other special characters quoted or escaped.  Note that case is significant in the client and server names and in the secret.
If the secret starts with an `@', what follows is assumed to be the name of a file from which to read the secret.  A "*" as the client or server name matches any name.  When selecting a secret, pppd takes the best match, i.e.  the match with the fewest wildcards.
Any following words on the same line are taken to be a list of acceptable IP addresses for that client.  If there are only 3 words on the line, or if the first word is "-", then all IP addresses are disallowed.  To allow any address, use "*".  A word starting with "!"  indicates that the specified address is not acceptable.  An address may be followed by "/" and a number n, to indicate a whole subnet, i.e. all addresses which have the same value in the most significant n bits.  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 will be set to the unit number plus one.
Thus a secrets file contains both secrets for use in authenticating other hosts, plus secrets which we use for authenticating ourselves to others. When pppd is authenticating the peer (checking the peer's identity), it chooses a secret with the peer's name in the first field and the name of the local system in the second field.  The name of the local system defaults to the hostname, with the domain name appended if the domain option is used.  This default can be overridden with the name option, except when the usehostname option is used.  (For EAP SRP-SHA1, see the srp-entry(8) utility for generating proper validator entries to be used in the "secret" field.)
When pppd is choosing a secret to use in authenticating itself to the peer, it first determines what name it is going to use to identify itself to the peer.  This name can be specified by the user with the user option.  If this option is not used, the name defaults to the name of the local system, determined as described in the previous paragraph.  Then pppd looks for a secret with this name in the first field and the peer's name in the second field.  Pppd will know the name of the peer if CHAP or EAP authentication is being used, because the peer will have sent it in the challenge packet.  However, if PAP is being used, pppd will have to determine the peer's name from the options specified by the user.  The user can specify the peer's name directly with the remotename option. Otherwise, if the remote IP address was specified by a name (rather than in numeric form), that name will be used as the peer's name.  Failing that, pppd will use the null string as the peer's name.
When authenticating the peer with PAP, the supplied password is first compared with the secret from the secrets file.  If the password doesn't match the secret, the password is encrypted using crypt() and checked against the secret again.  Thus secrets for authenticating the peer can be stored in encrypted form if desired.  If the papcrypt option is given, the first (unencrypted) comparison is omitted, for better security.
Furthermore, if the login option was specified, the username and password are also checked against the system password database.  Thus, the system administrator can set up the pap-secrets file to allow PPP access only to certain users, and to restrict the set of IP addresses that each user can use.  Typically, when using the login option, the secret in /etc/ppp/pap-secrets would be "", which will match any password supplied by the peer.  This avoids the need to have the same secret in two places.
Authentication must be satisfactorily completed before IPCP (or any other Network Control Protocol) can be started.  If the peer is required to authenticate itself, and fails to do so, pppd will 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.
When IPCP negotiation is completed successfully, pppd will inform the kernel of the local and remote IP addresses for the ppp interface.  This is sufficient to create a host route to the remote end of the link, which will enable the peers to exchange IP packets.  Communication with other machines generally requires further modification to routing tables and/or ARP (Address Resolution Protocol) tables.  In most cases the defaultroute and/or proxyarp options are sufficient for this, but in some cases further intervention is required.  The /etc/ppp/ip-up script can be used for this.
Sometimes it is desirable to add a default route through the remote host, as in the case of a machine whose only connection to the Internet is through the ppp interface.  The defaultroute option causes pppd to create such a default route when IPCP comes up, and delete it when the link is terminated.
In some cases it is desirable to use proxy ARP, for example on a server machine connected to a LAN, in order to allow other hosts to communicate with the remote host.  The proxyarp option causes pppd to look for a network interface on the same subnet as the remote host (an interface supporting broadcast and ARP, which is up and not a point-to-point or loopback interface).  If found, pppd creates a permanent, published ARP entry with the IP address of the remote host and the hardware address of the network interface found.
When the demand option is used, the interface IP addresses have already been set at the point when IPCP comes up.  If pppd has not been able to negotiate the same addresses that it used to configure the interface (for example when the peer is an ISP that uses dynamic IP address assignment), pppd has to change the interface IP addresses to the negotiated addresses.  This may disrupt existing connections, and the use of demand dialling with peers that do dynamic IP address assignment is not recommended.
Multilink PPP provides the capability to combine two or more PPP links between a pair of machines into a single `bundle', which appears as a single virtual PPP link which has the combined bandwidth of the individual links.  Currently, multilink PPP is only supported under Linux.
Pppd detects that the link it is controlling is connected to the same peer as another link using the peer's endpoint discriminator and the authenticated identity of the peer (if it authenticates itself).  The endpoint discriminator is a block of data which is hopefully unique for each peer.  Several types of data can be used, including locally-assigned strings of bytes, IP addresses, MAC addresses, randomly strings of bytes, or E-164 phone numbers.  The endpoint discriminator sent to the peer by pppd can be set using the endpoint option.
In some circumstances the peer may send no endpoint discriminator or a non-unique value.  The bundle option adds an extra string which is added to the peer's endpoint discriminator and authenticated identity when matching up links to be joined together in a bundle.  The bundle option can also be used to allow the establishment of multiple bundles between the local system and the peer.  Pppd uses a TDB database in /var/run/pppd2.tdb to match up links.
Assuming that multilink is enabled and the peer is willing to negotiate multilink, then when pppd is invoked to bring up the first link to the peer, it will detect that no other link is connected to the peer and create a new bundle, that is, another ppp network interface unit.  When another pppd is invoked to bring up another link to the peer, it will detect the existing bundle and join its link to it.
If the first link terminates (for example, because of a hangup or a received LCP terminate-request) the bundle is not destroyed unless there are no other links remaining in the bundle.  Rather than exiting, the first pppd keeps running after its link terminates, until all the links in the bundle have terminated.  If the first pppd receives a SIGTERM or SIGINT signal, it will destroy the bundle and send a SIGHUP to the pppd processes for each of the links in the bundle.  If the first pppd receives a SIGHUP signal, it will terminate its link but not the bundle.
Note: demand mode is not currently supported with multilink.