pppd [ 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 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.
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.
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.
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 distribution).
Probably the most common use of pppd is to dial out to an ISP. This can be done with a command such as
where the /etc/ppp/peers/isp file is set up by the system administrator to contain something like this:
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:
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 (installed setuid-root) with a command such as
To allow a user to use the PPP facilities, you need to allocate an IP address for that user's machine and create an entry in /etc/ppp/pap-secrets, /etc/ppp/chap-secrets, or /etc/ppp/srp-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" that 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:
(See srp-entry(8) for a means to generate the server's entry when SRP-SHA1 is in use.) 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 syslog daemon using facility LOG_DAEMON. (This can be overridden by recompiling pppd with the macro LOG_PPP defined as the desired facility.) See the syslog(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 received to be logged, that is, all LCP, PAP, CHAP, EAP, or IPCP packets. This can be useful if the PPP negotiation does not succeed or if authentication 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.
The exit status of pppd is set to indicate whether any error was detected, or the reason for the link being terminated. The values used are:
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 (except for the ip-pre-up script). 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 careful 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 environment variables that pppd sets are:
For the ip-down and auth-down scripts, pppd also sets the following variables giving statistics for the connection:
Pppd invokes the following scripts, if they exist. It is not an error if they don't exist.
Some limited degree of control can be exercised over a running pppd process by sending it a signal from the list below.
Paul Mackerras (firstname.lastname@example.org), based on earlier work by Drew Perkins, Brad Clements, Karl Fox, Greg Christy, and Brad Parker.
Pppd is copyrighted and made available under conditions which provide that it may be copied and used in source or binary forms provided that the conditions listed below are met. Portions of pppd are covered by the following copyright notices:
Copyright (c) 1984-2000 Carnegie Mellon University. All rights reserved. Copyright (c) 1993-2004 Paul Mackerras. All rights reserved. Copyright (c) 1995 Pedro Roque Marques. All rights reserved. Copyright (c) 1995 Eric Rosenquist. All rights reserved. Copyright (c) 1999 Tommi Komulainen. All rights reserved. Copyright (C) Andrew Tridgell 1999 Copyright (c) 2000 by Sun Microsystems, Inc. All rights reserved. Copyright (c) 2001 by Sun Microsystems, Inc. All rights reserved. Copyright (c) 2002 Google, Inc. All rights reserved.
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