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 ADSL service providers.
The -I option specifies the Ethernet interface to use. Under Linux, it is typi-
cally eth0 or eth1. The interface should be "up" before you start pppoe, but
should not be configured to have an IP address.
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
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
-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 conjunc-
tion with pppd; the -A option is meant to be used interactively to give interesting
information about the access concentrator.
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.
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 initi-
ate 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
-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.
Causes pppoe to write its process-ID to the specified file. This can be used to
locate and kill pppoe processes.
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, per-
form 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.
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 Initia-
tion) 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-Con-
firmation) 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 Dis-
covery 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 informa-
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 fire-
wall 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 <firstname.lastname@example.org>, with much inspiration from
an earlier version by Luke Stras.
The pppoe home page is http://www.roaringpenguin.com/pppoe/.
adsl-start(8), adsl-stop(8), adsl-connect(8), pppd(8), pppoe.conf(5), adsl-setup(8), adsl-
status(8), pppoe-sniff(8), pppoe-server(8), pppoe-relay(8)
4th Berkeley Distribution 3 July 2000 PPPOE(8)