| PPPD(8) | 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.
pppd will use that
terminal, and will not fork to put itself in the background. This option
is privileged if the noauth option is used.active-filter
filter-expressionidle
option if there are packets being sent or received regularly over the link
(for example, routing information packets) 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.asyncmap
mappppd will ask the peer to send these characters as
a 2-byte escape sequence. If multiple asyncmap
options are given, the values are ORed together. If no
asyncmap option is given, no async character map
will be negotiated for the receive direction; the peer should then escape
all
control characters. To escape transmitted characters, use the
escape option.authcall
namenoauth, even if pppd is
not being run by root. The name string may not begin
with "/" or include ".." as a pathname component. The
format of the options file is described below.connect
scriptpppd
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.crtsctspppd should set the serial port to
use hardware flow control using the RTS and CTS signals in the RS-232
interface. If neither the crtscts nor the
nocrtscts option is given, the hardware flow
control setting for the serial port is left unchanged.defaultroutenodefaultroute option has been specified.disconnect
scriptpppd 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
disconnect script is not run if the modem has already hung up. This option
is privileged if the noauth option is used.escape
xx,yy,...escape option, unlike the
asyncmap option which only allows control
characters to be specified. The characters which may not be escaped are
those with hex values 0x20 - 0x3f or 0x5e.file
namepppd.lockpppd should create a UUCP-style
lock file for the serial device to ensure exclusive access to the
device.mru
npppd 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
npppd 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.passivepppd 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.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 remote IP
address is specified with this option, pppd will
not accept a different value from the peer in the IPCP negotiation, unless
the ipcp-accept-local and/or
ipcp-accept-remote options are given,
respectively.bsdcomp
nr,ntnobsdcomp or bsdcomp 0
disables BSD-Compress compression entirely.chap-interval
npppd will rechallenge the
peer every n seconds.chap-max-challenge
nchap-restart
ndebugpppd will log the contents of all control packets
sent or received 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 appropriately (see
syslog.conf(5)).default-asyncmapdefault-mrupppd will use the default MRU value of 1500 bytes
for both the transmit and receive direction.deflate
nr,ntnodeflate or deflate 0 to
disable Deflate compression entirely. (Note: pppd
requests Deflate compression in preference to BSD-Compress if the peer can
do either.)demandpppd will
initially configure the interface and enable it for IP traffic without
connecting to the peer. When traffic is available,
pppd will connect to the peer and perform
negotiation, authentication, 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
ddomain 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
npersist or demand option
is used. The holdoff period is not applied if the link was terminated
because it was idle.idle
npppd 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-localpppd will accept the peer's idea
of our local IP address, even if the local IP address was specified in an
option.ipcp-accept-remotepppd 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
nipcp-max-failure
nipcp-max-terminate
nipcp-restart
nipparam
stringkdebug
nlcp-echo-failure
npppd 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
npppd 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
nlcp-max-failure
nlcp-max-terminate
nlcp-restart
nlocalpppd 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.loginmaxconnect
nmodempppd 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.modem_chatms-dns
[addr]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]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.name
namepppd 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
npppd will always use 255.255.255.255 for the
netmask, if that is the only appropriate value for a point-to-point
interface.)noaccompnoauthauth option is specified in
/etc/ppp/options.nobsdcomppppd will not
request or agree to compress packets using the BSD-Compress scheme.noccppppd for CCP negotiation.nocrtsctscrtscts nor the
nocrtscts option is given, the hardware flow
control setting for the serial port is left unchanged.nodefaultroutedefaultroute 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.nodeflatepppd will not
request or agree to compress packets using the Deflate scheme.nodetachpppd will fork to become a background
process.noippppd for IPCP negotiation.noipdefaultnomagicpppd cannot detect a looped-back line. This option
should only be needed if the peer is buggy.nopcompnopersistpersist or
demand option has been specified.nopredictor1noproxyarpproxyarp 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.novjnovjccomppppd will
not omit the connection-ID byte from Van Jacobson compressed TCP/IP
headers, nor ask the peer to do so.papcryptpppd should not accept a password which, before
encryption, is identical to the secret from the
/etc/ppp/pap-secrets file.pap-max-authreq
npap-restart
npap-timeout
npppd will wait for the
peer to authenticate itself with PAP to n seconds (0
means no limit).pass-filter
filter-expressioninbound
and outbound qualifiers.persistpredictor1proxyarpremotename
namerefuse-chappppd will not agree to
authenticate itself to the peer using CHAP.refuse-pappppd will not agree to
authenticate itself to the peer using PAP.require-chaprequire-papsilentpppd 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).usehostnamename option).user
namevj-max-slots
nwelcome
scriptnoauth option is used.xonxoffOptions 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.
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
administrator 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 refuse 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 unauthenticated 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 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 the
/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.
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 two
authentication protocols: the Password Authentication 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 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). 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 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. 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 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.
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 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(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 authenticate itself, and fails to do so, pppd
will terminate 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.
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. 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:
DEVICEIFNAMEIPLOCALIPREMOTEPEERNAMESPEEDUIDpppd.pppd invokes the following scripts, if
they exist. It is not an error if they don't exist.
interface-name peer-name user-name tty-device speed
Note that this script is not executed if the peer doesn't
authenticate itself, for example when the noauth
option is used.
interface-name tty-device speed local-IP-address remote-IP-address ipparam
pppd will log a warning if this is not the
case.pppd will log a warning if this is not the
case.pppd, read before user
default options or command-line options.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.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
pppd call ispwhere 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 (installed setuid-root) with a
command such as
pppd proxyarpTo 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 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.netAlternatively, 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 overridden 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 received 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 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.
cua(4), ppp(4), tty(4), chat(8), syslogd(8), tcpdump(8)
V. Jacobson, Compressing TCP/IP Headers for Low-Speed Serial Links, RFC 1144, February 1990.
R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321, April 1992.
G. McGregor, The PPP Internet Protocol Control Protocol (IPCP), RFC 1332, May 1992.
B. Lloyd and W. Simpson, PPP Authentication Protocols, RFC 1334, October 1992.
W. Simpson, The Point-to-Point Protocol (PPP), RFC 1661, July 1994.
W. Simpson, PPP in HDLC-like Framing, RFC 1662, July 1994.
W. Simpson, PPP Challenge Handshake Authentication Protocol (CHAP), RFC 1994, August 1996.
Some limited degree of control can be exercised over a running
pppd process by sending it a signal from the list
below.
pppd to terminate the link (by
closing LCP), restore the serial device settings, and exit.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.debug
option.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
errors 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.
Scripts should be run as root, but are not.
| December 3, 2020 | OpenBSD-current |