PPPD(8) BSD System Manager's Manual PPPD(8)
pppd - Point-to-Point Protocol daemon
pppd [tty_name] [speed] [options]
PPP is the protocol used for establishing internet links over dial-up modems, DSL connections, and many other types of point-to-point links. The pppd daemon works together with the kernel ppp(4) driver to establish and maintain a PPP link with another system (called the peer) and to negotiate Internet Protocol (IP) addresses for each end of the link. pppd can also authenticate the peer and/or supply authentication information to the peer. PPP can be used with other network protocols besides IP, but such use is becoming increasingly rare.
tty_name Use the serial port called ttyname to communicate with the peer. The string "/dev/" is prepended to ttyname to form the name of the device to open. If no device name is given, or if the name of the terminal connected to the standard input is given, pppd will use that terminal, and will not fork to put itself in the back- ground. This option is privileged if the noauth option is used. speed An option that is a decimal number is taken as the desired baud rate for the serial device. On systems such as 4.4BSD and OpenBSD, any speed can be specified. Other systems (e.g., Linux, SunOS) only support the commonly used baud-rates. active-filter filter-expression Specifies a packet filter to be applied to data packets to deter- mine which packets are to be regarded as link activity, and therefore reset the idle timer, or cause the link to be brought up in demand-dialling mode. This option is useful in conjunction with the idle option if there are packets being sent or received regularly over the link (for example, routing information pack- ets) which would otherwise prevent the link from ever appearing to be idle. The filter-expression syntax is as described for tcpdump(8), 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. This option is currently only available under OpenBSD, and then only if both the kernel and pppd were compiled with PPP_FILTER defined. asyncmap map This option sets the Async-Control-Character-Map (ACCM) for this end of the link. The ACCM is a set of 32 bits, one for each of the ASCII control characters with values from 0 to 31, where a 1 bit indicates that the corresponding control character should not be used in PPP packets sent to this system. The map is encoded as a hexadecimal number (without a leading 0x) where the least sig- nificant bit (00000001) represents character 0 and the most sig- nificant bit (80000000) represents character 31. pppd will ask the peer to send these characters as a 2-byte escape sequence. If multiple asyncmap options are given, the values are ORed togeth- er. If no asyncmap option is given, no async character map will be negotiated for the receive direction; the peer should then es- cape all control characters. To escape transmitted characters, use the escape option. auth Require the peer to authenticate itself before allowing network packets to be sent or received. call name Read options from the file /etc/ppp/peers/name. This file may contain privileged options, such as noauth, even if pppd is not being run by root. The name string may not begin with "/" or in- clude ".." as a pathname component. The format of the options file is described below. connect script Usually there is something which needs to be done to prepare the link before the PPP protocol can be started; for instance, with a dial-up modem, commands need to be sent to the modem to dial the appropriate phone number. This option specifies an command for pppd to execute (by passing it to a shell) before attempting to start PPP negotiation. The chat(8) program is often useful here, as it provides a way to send arbitrary strings to a modem and respond to received characters. This option is privileged if the noauth option is used. crtscts Specifies that pppd should set the serial port to use hardware flow control using the RTS and CTS signals in the RS-232 inter- face. If neither the crtscts nor the nocrtscts option is given, the hardware flow control setting for the serial port is left un- changed. defaultroute Add a default route to the system routing tables, using the peer as the gateway, when IPCP negotiation is successfully completed. This entry is removed when the PPP connection is broken. This op- tion is privileged if the nodefaultroute option has been speci- fied. disconnect script Execute the command specified by script, by passing it to a shell, after pppd has terminated the link. This command could, for example, issue commands to the modem to cause it to hang up if hardware modem control signals were not available. The discon- nect script is not run if the modem has already hung up. This op- tion is privileged if the noauth option is used. escape xx,yy,... Specifies that certain characters should be escaped on transmis- sion (regardless of whether the peer requests them to be escaped with its async control character map). The characters to be es- caped are specified as a list of hex numbers separated by commas. Note that almost any character can be specified for the escape option, unlike the asyncmap option which only allows control characters to be specified. The characters which may not be es- caped are those with hex values 0x20 - 0x3f or 0x5e. file name Read options from file name (the format is described below). The file must be readable by the user who has invoked pppd. lock Specifies that pppd should create a UUCP-style lock file for the serial device to ensure exclusive access to the device. mru n Set the MRU (Maximum Receive Unit) value to n. pppd will ask the peer to send packets of no more than n bytes. The value of n must be between 128 and 16384; the default is 1500. A value of 296 works well on very slow links (40 bytes for TCP/IP header + 256 bytes of data). Note that for the IPv6 protocol, the MRU must be at least 1280. mtu n Set the MTU (Maximum Transmit Unit) value to n. Unless the peer requests a smaller value via MRU negotiation, pppd will request that the kernel networking code send data packets of no more than n bytes through the PPP network interface. Note that for the IPv6 protocol, the MTU must be at least 1280. passive Enables the "passive" option in the LCP. With this option, pppd will attempt to initiate a connection; if no reply is received from the peer, pppd will then just wait passively for a valid LCP packet from the peer, instead of exiting, as it would without this option.
[local_IP_address]:[remote_IP_address] Set the local and/or remote interface IP addresses. Either one may be omitted. The IP addresses can be specified with a host name or in decimal dot notation (e.g., 188.8.131.52). The de- fault local address is the (first) IP address of the system (un- less the noipdefault option is given). The remote address will be obtained from the peer if not specified in any option. Thus, in simple cases, this option is not required. If a local and/or re- mote IP address is specified with this option, pppd will not ac- cept a different value from the peer in the IPCP negotiation, un- less the ipcp-accept-local and/or ipcp-accept-remote options are given, respectively. bsdcomp nr,nt 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 give better compression but consume more kernel memory for compression dictionaries. Alternatively, a value of 0 for nr or nt disables compression in the corresponding direction. nobsdcomp or bsdcomp 0 disables BSD-Compress compression entire- ly. chap-interval n If this option is given, pppd will rechallenge the peer every n seconds. chap-max-challenge n Set the maximum number of CHAP challenge transmissions to n (de- fault 10). chap-restart n Set the CHAP restart interval (retransmission timeout for chal- lenges) to n seconds (default 3). debug Enables connection debugging facilities. If this option is given, pppd will log the contents of all control packets sent or re- ceived in a readable form. The packets are logged through syslogd(8) with facility daemon and level debug. This information can be directed to a file by setting up /etc/syslog.conf ap- propriately (see syslog.conf(5)). default-asyncmap Disable asyncmap negotiation, forcing all control characters to be escaped for both the transmit and the receive direction. default-mru Disable MRU (Maximum Receive Unit) negotiation. With this option, pppd will use the default MRU value of 1500 bytes for both the transmit and receive direction. deflate nr,nt 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. If nt is not specified, it defaults to the value given for nr. Values in the range 8 to 15 may be used for nr and nt; larger values give better compression but consume more kernel memory for compression dictionaries. Alternatively, a value of 0 for nr or nt disables compression in the corresponding direction. Use nodeflate or deflate 0 to disable Deflate compres- sion entirely. (Note: pppd requests Deflate compression in preference to BSD-Compress if the peer can do either.) demand Initiate the link only on demand, i.e., when data traffic is present. With this option, the remote IP address must be speci- fied by the user on the command line or in an options file. pppd will initially configure the interface and enable it for IP traffic without connecting to the peer. When traffic is avail- able, pppd will connect to the peer and perform negotiation, au- thentication, etc. When this is completed, pppd will commence passing data packets (i.e., IP packets) across the link. The demand option implies the persist option. If this behaviour is not desired, use the nopersist option after the demand option. The idle and holdoff options are also useful in conjunction with the demand option. domain d Append the domain name d to the local host name for authentica- tion purposes. For example, if gethostname(3) returns the name porsche, but the fully qualified domain name is porsche.Quotron.COM, you could specify domain Quotron.COM. pppd would then use the name porsche.Quotron.COM for looking up secrets in the secrets file, and as the default name to send to the peer when authenticating itself to the peer. This option is privileged. 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 ap- plied 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 op- tion. ipcp-accept-remote With this option, pppd will accept the peer's idea of its (re- mote) 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 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 and ip-down scripts. If this option is given, the string supplied is given as the 6th parameter to those scripts. ipx Enable the IPXCP and IPX protocols. This option is presently only supported under Linux, and only if your kernel has been config- ured 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 lo- cal 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 in- clude 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 op- tion 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 argu- ment n is a number which is the sum of the following values: 1 to enable general debug messages, 2 to request that the contents of received packets be printed, and 4 to request that the contents of transmitted packets be printed. On most systems, messages printed by the kernel are logged by syslogd(8) to a file as directed in the /etc/syslog.conf configuration file. 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 con- trol 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 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). 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. login Use the system password database for authenticating the peer us- ing 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. 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). 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 (un- less a connect script is specified), and it will drop the DTR (Data Terminal Ready) signal briefly when the connection is ter- minated and before executing the connect script. On Ultrix, this option implies hardware flow control, as for the crtscts option. modem_chat Use the modem control lines during the chat script. The default is to ignore the state of the CD (Carrier Detect) signal from the modem during the chat script. If you are using a cua(4) device (as opposed to a tty(4) device) you should set this option. You should not use this option with a dialback setup as it will cause the chat script to exit when carrier drops. 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 (Win- dows 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 ad- dress. 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.) netmask n Set the interface netmask to n, a 32-bit netmask in "decimal dot" notation (e.g. 255.255.255.0). If this option is given, the value specified is ORed with the default netmask. The default netmask is chosen based on the negotiated remote IP address; it is the appropriate network mask for the class of the remote IP address, ORed with the netmasks for any non point-to-point network inter- faces in the system which are on the same network. (Note: on some platforms, pppd will always use 255.255.255.255 for the netmask, if that is the only appropriate value for a point-to-point inter- face.) noaccomp Disable Address/Control compression in both directions (send and receive). noauth Do not require the peer to authenticate itself. This option is privileged if the auth option is specified in /etc/ppp/options. 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 op- tion should only be required if the peer is buggy and gets con- fused 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 option is given, the hardware flow control setting for the serial port is left un- changed. nodefaultroute Disable the defaultroute option. The system administrator who wishes to prevent users from creating default routes 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. noip Disable IPCP negotiation and IP communication. This option should only be required if the peer is buggy and gets confused by re- quests from pppd for IPCP negotiation. noipdefault Disables the default behaviour when no local IP address is speci- fied, 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 was 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. 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. nopcomp Disable protocol field compression negotiation in both the re- ceive 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. 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 authen- ticate itself with PAP to n seconds (0 means no limit). pass-filter filter-expression Specifies a packet filter to apply 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(8)) using up link bandwidth, or to provide a basic firewall capability. The filter-expression syntax is as described for tcpdump(8), except that qualifiers which are inappropriate for a PPP link, such as ether and arp, are not permitted. Gen- erally 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 con- straints to incoming and outgoing packets using the inbound and outbound qualifiers. This option is currently only available under OpenBSD, and then only if both the kernel and pppd were compiled with PPP_FILTER defined. persist Do not exit after a connection is terminated; instead try to reo- pen the connection. 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. 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. remotename name Set the assumed name of the remote system for authentication pur- poses to name. refuse-chap With this option, pppd will not agree to authenticate itself to the peer using CHAP. 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-pap Require the peer to authenticate itself using PAP (Password Au- thentication Protocol) authentication. 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). usehostname Enforce the use of the hostname (with domain name appended, if given) as the name of the local system for authentication pur- poses (overrides the name option). 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 Jacob- son 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. This option is privileged if the noauth option is used. xonxoff Use software flow control (i.e., XON/XOFF) to control the flow of data on the serial port.
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 whi- tespace. 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.
Users must be in group "network" to be able to use pppd. pppd provides system administrators with sufficient access control that PPP access to a server machine can be provided to legitimate users without fear of compromising the security of the server or the network it's on. In part this is provided by the /etc/ppp/options file, where the administrator can place options to restrict the ways in which pppd can be used, and in part by the PAP and CHAP secrets files, where the adminis- trator can restrict the set of IP addresses which individual users may use. The normal way that pppd should be set up is to have the auth option in the /etc/ppp/options file. (This may become the default in later releases.) If users wish to use pppd to dial out to a peer which will re- fuse to authenticate itself (such as an internet service provider), the system administrator should create an options file under /etc/ppp/peers containing the noauth option, the name of the serial port to use, and the connect option (if required), plus any other appropriate options. In this way, pppd can be set up to allow non-privileged users to make unauthenti- cated connections only to trusted peers. As indicated above, some security-sensitive options are privileged, which means that they may not be used by an ordinary non-privileged user run- ning 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 the /etc/ppp/options file or in an op- tions file read using the call option. If pppd is being run by the root user, privileged options can be used without restriction.
Authentication is the process whereby one peer convinces the other of its identity. This involves the first peer sending its name to the other, to- gether 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 two authentication protocols: the Password Au- thentication Protocol (PAP) and the Challenge Handshake Authentication Protocol (CHAP). 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. 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 dif- ferent 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). Both 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. 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 ac- ceptable. 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. 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. 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 de- faults to the hostname, with the domain name appended if the domain op- tion is used. This default can be overridden with the name option, except when the usehostname option is used. 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 authen- tication is being used, because the peer will have sent it in the chal- lenge 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(3) 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 au- thenticate itself, and fails to do so, pppd will terminate the link (by closing LCP). If IPCP negotiates an unacceptable IP address for the re- mote 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 authenti- cate themselves to connect and use one of a restricted set of IP ad- dresses, 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 pos- sible 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 furth- er 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 net- work interface on the same subnet as the remote host (an interface sup- porting broadcast and ARP, which is up and not a point-to-point or loop- back 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 ad- dresses. This may disrupt existing connections, and the use of demand di- alling with peers that do dynamic IP address assignment is not recommend- ed.
The following examples assume that the /etc/ppp/options file contains the auth option (as in the default /etc/ppp/options file in the PPP distribu- tion). Probably the most common use of pppd is to dial out to an ISP. This can be done with a command such as pppd call isp where the /etc/ppp/peers/isp file is set up by the system administrator to contain something like this: ttyS0 19200 crtscts connect '/usr/sbin/chat -v -f /etc/ppp/chat-isp' noauth In this example, we are using chat to dial the ISP's modem and go through any logon sequence required. The /etc/ppp/chat-isp file contains the script used by chat; it could for example contain something like this: ABORT "NO CARRIER" ABORT "NO DIALTONE" ABORT "ERROR" ABORT "NO ANSWER" ABORT "BUSY" ABORT "Username/Password Incorrect" "" "at" OK "at&d0&c1" OK "atdt2468135" "name:" "^Umyuserid" "word:" "\qmypassword" "ispts" "\q^Uppp" "~-^Uppp-~" See the chat(8) man page for details of chat scripts. pppd can also be used to 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 (in- stalled setuid-root) with a command such as pppd proxyarp To allow a user to use the PPP facilities, you need to allocate an IP ad- dress for that user's machine and create an entry in /etc/ppp/pap-secrets or /etc/ppp/chap-secrets (depending on which authentication method the PPP implementation on the user's machine supports), so that the user's machine can authenticate itself. For example, if Joe has a machine called "joespc" which is to be allowed to dial in to the machine called "server" and use the IP address joespc.my.net, you would add an entry like this to /etc/ppp/pap-secrets or /etc/ppp/chap-secrets: joespc server "joe's secret" joespc.my.net Alternatively, you can create a username called (for example) "ppp", whose login shell is pppd and whose home directory is /etc/ppp. Options to be used when pppd is run this way can be put in /etc/ppp/.ppprc. If your serial connection is any more complicated than a piece of wire, you may need to arrange for some control characters to be escaped. In particular, it is often useful to escape XON (^Q) and XOFF (^S), using asyncmap a0000. If the path includes a telnet, you probably should escape ^] as well (asyncmap 200a0000). If the path includes an rlogin, you will need to use the escape ff option on the end which is running the rlogin client, since many rlogin implementations are not transparent; they will remove the sequence (0xff, 0xff, 0x73, 0x73, followed by any 8 bytes) from the stream.
Messages are sent to the syslogd(8) daemon using facility LOG_DAEMON. (This can be overriden by recompiling pppd with the macro LOG_PPP defined as the desired facility.) See the syslogd(8) documentation for details of where the syslog daemon will write the messages. On most systems, the syslog daemon uses the /etc/syslog.conf file to specify the destination(s) for syslog messages. You may need to edit that file to suit. The debug option causes the contents of all control packets sent or re- ceived to be logged, that is, all LCP, PAP, CHAP or IPCP packets. This can be useful if the PPP negotiation does not succeed or if authentica- tion fails. If debugging is enabled at compile time, the debug option also causes other debugging messages to be logged. Debugging can also be enabled or disabled by sending a SIGUSR1 signal to the pppd process. This signal acts as a toggle.
pppd invokes scripts at various stages in its processing which can be used to perform site-specific ancillary processing. These scripts are usually shell scripts, but could be executable code files instead. 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), so that they can do things such as update routing tables or run privileged daemons. Be care- ful 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 en- vironment variables that pppd sets are: DEVICE The name of the serial tty device being used. IFNAME The name of the network interface being used. IPLOCAL The IP address for the local end of the link. This is only set when IPCP has come up. IPREMOTE The IP address for the remote end of the link. This is only set when IPCP has come up. PEERNAME The authenticated name of the peer. This is only set if the peer authenticates itself. SPEED The baud rate of the tty device. UID The real user ID of the user who invoked pppd. pppd invokes the following scripts, if they exist. It is not an error if they don't exist. /etc/ppp/auth-up A program or script which is executed after the remote system successfully authenticates itself. It is executed with the param- eters interface-name peer-name user-name tty-device speed Note that this script is not executed if the peer doesn't authen- ticate itself, for example when the noauth option is used. /etc/ppp/auth-down A program or script which is 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. /etc/ppp/ip-up A program or script which is executed when the link is available for sending and receiving IP packets (that is, IPCP has come up). It is executed with the parameters interface-name tty-device speed local-IP-address remote-IP- address ipparam /etc/ppp/ip-down A program or script which is executed when the link is no longer available for sending and receiving IP packets. This script can be used for undoing the effects of the /etc/ppp/ip-up script. It is invoked in the same manner and with the same parameters as the ip-up script. /etc/ppp/ipx-up A program or script which is executed when the link is available for sending and receiving IPX packets (that is, IPXCP has come up). It is executed with the parameters interface-name tty-device speed network-number local-IPX-node- address remote-IPX-node-address local-IPX-routing-protocol remote-IPX-routing-protocol local-IPX-router-name remote-IPX- router-name ipparam pppd-pid The local-IPX-routing-protocol and remote-IPX-routing-protocol field may be one of the following: NONE to indicate that there is no routing protocol RIP to indicate that RIP/SAP should be used NLSP to indicate that Novell NLSP should be used RIP NLSP to indicate that both RIP/SAP and NLSP should be used /etc/ppp/ipx-down A program or script which is executed when the link is no longer available for sending and receiving IPX packets. This script can be used for undoing the effects of the /etc/ppp/ipx-up script. It is invoked in the same manner and with the same parameters as the ipx-up script.
/var/run/pppn.pid Process-ID for pppd process on PPP interface unit n. /etc/ppp/pap-secrets Usernames, passwords and IP addresses for PAP authentication. This file should be owned by root and not readable or writable by any other user. pppd will log a warning if this is not the case. /etc/ppp/chap-secrets Names, secrets and IP addresses for CHAP authentication. As for /etc/ppp/pap-secrets, this file should be owned by root and not readable or writable by any other user. pppd will log a warning if this is not the case. /etc/ppp/options System default options for pppd, read before user default options or command-line options. ~/.ppprc User default options, read before /etc/ppp/options.ttyname. /etc/ppp/options.ttyname System default options for the serial port being used, read after ~/.ppprc. In forming the ttyname part of this filename, an ini- tial /dev/ is stripped from the port name (if present), and any slashes in the remaining part are converted to dots. /etc/ppp/peers A directory containing options files which 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.
cua(4), ppp(4), tty(4), chat(8), ppp(8), syslogd(8), tcpdump(8) Jacobson, V., "Compressing TCP/IP headers for low-speed serial links", RFC 1144, February 1990. Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. McGregor, G., "PPP Internet Protocol Control Protocol (IPCP)", RFC 1332, May 1992. Lloyd, B. and Simpson, W.A., "PPP authentication protocols", RFC 1334, October 1992. Simpson, W.A., "The Point-to-Point Protocol (PPP)", RFC 1661, July 1994. Simpson, W.A., "PPP in HDLC-like Framing", RFC 1662, July 1994.
Some limited degree of control can be exercised over a running pppd pro- cess by sending it a signal from the list below. SIGINT, SIGTERM These signals cause pppd to terminate the link (by closing LCP), restore the serial device settings, and exit. SIGHUP This signal causes pppd to terminate the link, restore the serial device settings, and close the serial device. If the persist or demand option has been specified, pppd will try to reopen the serial device and start another connection (after the holdoff period). Otherwise pppd will exit. If this signal is received during the holdoff period, it causes pppd to end the holdoff period immediately. SIGUSR1 This signal toggles the state of the debug option. SIGUSR2 This signal causes pppd to renegotiate compression. This can be useful to re-enable compression after it has been disabled as a result of a fatal decompression error. (Fatal decompression er- rors generally indicate a bug in one or other implementation.)
Paul Mackerras <Paul.Mackerras@samba.org>, based on earlier work by Drew Perkins, Brad Clements, Karl Fox, Greg Christy, and Brad Parker. MirOS BSD #10-current March 31, 1998 16
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