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Table of Contents
pppoe - user-space PPPoE client.
pppd pty ’pppoe [pppoe_options]’
[pppd_options]
pppoe -A [pppoe_options]
pppoe is a user-space
client for PPPoE (Point-to-Point Protocol over Ethernet) for Linux and other
UNIX systems. pppoe works in concert with the pppd PPP daemon to provide
a PPP connection over Ethernet, as is used by many DSL service providers.
- -I interface
- The -I option specifies the Ethernet interface to use.
Under Linux, it is typically eth0 or eth1. The interface should be "up"
before you start pppoe, but should not be configured to have an IP address.
- -T timeout
- The -T option causes pppoe to exit if no session traffic is detected
for timeout seconds. I recommend that you use this option as an extra safety
measure, but if you do, you should make sure that PPP generates enough
traffic so the timeout will normally not be triggered. The best way to
do this is to use the lcp-echo-interval option to pppd. You should set the
PPPoE timeout to be about four times the LCP echo interval.
- -D file_name
- The -D option causes every packet to be dumped to the specified file_name.
This is intended for debugging only; it produces huge amounts of output
and greatly reduces performance.
- -V
- The -V option causes pppoe to print its
version number and exit.
- -A
- The -A option causes pppoe to send a PADI packet
and then print the names of access concentrators in each PADO packet it
receives. Do not use this option in conjunction with pppd; the -A option
is meant to be used interactively to give interesting information about
the access concentrator.
- -S service_name
- Specifies the desired service name.
pppoe will only initiate sessions with access concentrators which can
provide the specified service. In most cases, you should not specify this
option. Use it only if you know that there are multiple access concentrators
or know that you need a specific service name.
- -C ac_name
- Specifies the
desired access concentrator name. pppoe will only initiate sessions with
the specified access concentrator. In most cases, you should not specify
this option. Use it only if you know that there are multiple access concentrators.
If both the -S and -C options are specified, they must both match for pppoe
to initiate a session.
- -U
- Causes pppoe to use the Host-Uniq tag in its discovery
packets. This lets you run multiple pppoe daemons without having their
discovery packets interfere with one another. You must supply this option
to all pppoe daemons if you intend to run multiple daemons simultaneously.
- -s
- Causes pppoe to use synchronous PPP encapsulation. If you use this option,
then you must use the sync option with pppd. You are encouraged to use
this option if it works, because it greatly reduces the CPU overhead of
pppoe. However, it MAY be unreliable on slow machines -- there is a race
condition between pppd writing data and pppoe reading it. For this reason,
the default setting is asynchronous. If you encounter bugs or crashes with
Synchronous PPP, turn it off -- don’t e-mail me for support!
- -m MSS
- Causes
pppoe to clamp the TCP maximum segment size at the specified value. Because
of PPPoE overhead, the maximum segment size for PPPoE is smaller than for
normal Ethernet encapsulation. This could cause problems for machines on
a LAN behind a gateway using PPPoE. If you have a LAN behind a gateway,
and the gateway connects to the Internet using PPPoE, you are strongly
recommended to use a -m 1412 option. This avoids having to set the MTU on
all the hosts on the LAN.
- -p file
- Causes pppoe to write its process-ID to
the specified file. This can be used to locate and kill pppoe processes.
- -e sess:mac
- Causes pppoe to skip the discovery phase and move directly
to the session phase. The session is given by sess and the MAC address
of the peer by mac. This mode is not meant for normal use; it is designed
only for pppoe-server(8).
- -n
- Causes pppoe not to open a discovery socket.
This mode is not meant for normal use; it is designed only for pppoe-server(8).
- -k
- Causes pppoe to terminate an existing session by sending a PADT frame,
and then exit. You must use the -e option in conjunction with this option
to specify the session to kill. This may be useful for killing sessions
when a buggy peer does not realize the session has ended.
- -d
- Causes pppoe
to perform discovery and then exit, after printing session information
to standard output. The session information is printed in exactly the format
expected by the -e option. This option lets you initiate a PPPoE discovery,
perform some other work, and then start the actual PPP session. Be careful;
if you use this option in a loop, you can create many sessions, which may
annoy your peer.
- -f disc:sess
- The -f option sets the Ethernet frame types
for PPPoE discovery and session frames. The types are specified as hexadecimal
numbers separated by a colon. Standard PPPoE uses frame types 8863:8864.
You should not use this option unless you are absolutely sure the peer
you are dealing with uses non-standard frame types. If your ISP uses non-standard
frame types, complain!
- -h
- The -h option causes pppoe to print usage information
and exit.
PPPoE (Point-to-Point Protocol over Ethernet)
is described in RFC 2516 and is a protocol which allows the session abstraction
to be maintained over bridged Ethernet networks.
PPPoE works by encapsulating
PPP frames in Ethernet frames. The protocol has two distinct stages: The
discovery and the session stage.
In the discovery stage, the host broadcasts
a special PADI (PPPoE Active Discovery Initiation) frame to discover any
access concentrators. The access concentrators (typically, only one access
concentrator) reply with PADO (PPPoE Active Discovery Offer) packets, announcing
their presence and the services they offer. The host picks one of the access
concentrators and transmits a PADR (PPPoE Active Discovery Request) packet,
asking for a session. The access concentrator replies with a PADS (PPPoE
Active Discovery Session-Confirmation) packet. The protocol then moves to
the session stage.
In the session stage, the host and access concentrator
exchange PPP frames embedded in Ethernet frames. The normal Ethernet MTU
is 1500 bytes, but the PPPoE overhead plus two bytes of overhead for the
encapsulated PPP frame mean that the MTU of the PPP interface is at most
1492 bytes. This causes all kinds of problems if you are using a Linux machine
as a firewall and interfaces behind the firewall have an MTU greater than
1492. In fact, to be safe, I recommend setting the MTU of machines behind
the firewall to 1412, to allow for worst-case TCP and IP options in their
respective headers.
Normally, PPP uses the Link Control Protocol (LCP)
to shut down a PPP link. However, the PPPoE specification allows the link
to be shut down with a special PADT (PPPoE Active Discovery Terminate)
packet. This client recognizes this packet and will correctly terminate
if a terminate request is received for the PPP session.
My
design goals for this PPPoE client were as follows, in descending order
of importance:
- o
- It must work.
- o
- It must be a user-space program and not
a kernel patch.
- o
- The code must be easy to read and maintain.
- o
- It must
be fully compliant with RFC 2516, the proposed PPPoE standard.
- o
- It must
never hang up forever -- if the connection is broken, it must detect this
and exit, allowing a wrapper script to restart the connection.
- o
- It must
be fairly efficient.
I believe I have achieved all of these goals, but
(of course) am open to suggestions, patches and ideas. See my home page,
http://www.roaringpenguin.com, for contact information.
For best results,
you must give pppd an mtu option of 1492. I have observed problems with
excessively-large frames unless I set this option. Also, if pppoe is running
on a firewall machine, all machines behind the firewall should have MTU’s
of 1412.
If you have problems, check your system logs. pppoe logs interesting
things to syslog. You may have to turn on logging of debug-level messages
for complete diagnosis.
pppoe was written by David F. Skoll <dfs@roaringpenguin.com>,
with much inspiration from an earlier version by Luke Stras.
The pppoe
home page is http://www.roaringpenguin.com/pppoe/.
pppoe-start(8),
pppoe-stop(8), pppoe-connect(8), pppd(8), pppoe.conf(5), pppoe-setup(8), pppoe-status(8),
pppoe-sniff(8), pppoe-server(8), pppoe-relay(8)
Table of Contents