NAME¶
pppoe - user-space PPPoE client.
SYNOPSIS¶
pppd pty 'pppoe [pppoe_options]' [pppd_options]
pppoe -A [pppoe_options]
DESCRIPTION¶
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.
OPTIONS¶
- -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.
- -H MAC
- Causes pppoe to use the indicated Ethernet MAC address as the
source address for sending packets. MAC must be specified in the
AA:BB:CC:DD:EE:FF syntax. If
this option is specified, pppoe puts the interface into promiscuous
mode.
- -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 BACKGROUND¶
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.
DESIGN GOALS¶
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.
NOTES¶
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.
AUTHORS¶
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/.
SEE ALSO¶
pppd(8),
pppoe-sniff(8),
pppoe-server(8),
pppoe-relay(8),
/usr/share/doc/pppoe/README.Debian.gz