NetBSD/share/man/man4/pppoe.4

.\"	$NetBSD: pppoe.4,v 1.19 2003/02/25 10:35:14 wiz Exp $
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.\" by Martin Husemann <martin@NetBSD.org>.
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.Dd September 1, 2002
.Dt PPPOE 4
.Os
.Sh NAME
.Nm pppoe
.Nd PPP over Ethernet protocol network interface
.Sh SYNOPSIS
.Nm pseudo-device pppoe
.Sh DESCRIPTION
The
.Nm
interface encapsulates
.Em Point-to-Point Protocol (PPP)
packets inside Ethernet frames as defined by
.Li RFC2516 .
.Pp
This is often used to connect a router via a DSL modem to
an access concentrator.  The
.Nm
interface does not by itself transmit or receive frames,
but needs an Ethernet interface to do so.  This Ethernet interface
is connected to the
.Nm
interface via
.Xr pppoectl 8 .
The Ethernet interface needs to be marked UP, but does not need to have an
IP address.
.Pp
There are two basic modes of operation, controlled via the
.Em link1
switch.  The default mode,
.Em link1
not being set, tries to keep the configured session open all the
time.  If the session is disconnected, a new connection attempt is started
immediately.  The
.Dq dial on demand
mode, selected by setting
.Em link1 ,
only establishes a connection when data is being sent to the interface.
.Pp
Before a
.Nm
interface is usable, it needs to be configured.  The following steps
are necessary:
.Bl -bullet
.It
Create the interface.
.It
Connect an Ethernet interface.
This interface is used for the physical communication.  As noted above it
must be marked UP, but need not have an IP address.
.It
Configure authentication.
The PPP session needs to identify the client to the peer.  For more details
on the available options see
.Xr pppoectl 8 .
.El
.Pp
This all is typically accomplished using an
.Pa /etc/ifconfig.pppoe0
file.
.Ss MSS/MTU problems
If you are using a
.Nm
interface, you will have an unusually low MTU for todays Internet.
Combined with a lot of misconfigured sites (host using path MTU discovery
behind a router blocking all ICMP traffic) this will often cause problems.
Connections to these servers will only work if your system advertises the
right MSS in the TCP three way handshake.
To get the right MSS, you need to set
.Bd -literal -offset indent
# Obey interface MTUs when calculating MSS
net.inet.tcp.mss_ifmtu=1
.Ed
.Pp
in your
.Pa /etc/sysctl.conf
file.
This causes the calculated MSS to be based on the MTU of the interface
via which the packet is sent.
This is always the right value if you are sure the answer to this packet
will be received on the same interface (i.e., you only have one interface
connected to the Internet.)
.Pp
Unfortunately this sysctl does not fix the MSS advertised by hosts in
the network behind a
.Nm
connected router.
.Ss Setting up NAT with MSS-clamping
Some systems behind misconfigured firewalls try to use
Path-MTU-Discovery, while their firewall blocks all ICMP messages.
This is an illegal, but not uncommon, setup.
Typically you will have no chance to fix this (remote, outside of your
control) setup.
And sometimes you will have to use such remote systems (to download
data from them, or to do your online banking).
.Pp
Without special care systems as described above will not be able
to send larger chunks of data to a system connected via
.Nm .
But there is a workaround (some may call it cheating): pretend to not
be able to handle large packets, by sending a small MSS (maximum
segment size) option during initial TCP handshake.
.Pp
For connections originating from your
.Nm
connected machines, this is accomplished by setting the sysctl
variable
.Dv net.inet.tcp.mss_ifmtu
to 1 (see above).
For connections originating from systems behind your
.Nm
router, you need to set the
.Dv mssclamp
options in your NAT rules, like in this example of
.Pa /etc/ipnat.conf :
.Bd -literal -offset indent
map pppoe0 192.168.1.0/24 -> 0/32 portmap tcp/udp 44000:49999 mssclamp 1440
map pppoe0 192.168.1.0/24 -> 0/32 mssclamp 1440
.Ed
.Pp
If you do not use NAT, you need to setup a 1:1 NAT rule, just to
get the clamping:
.Bd -literal -offset indent
map pppoe0 x.x.x.x/24 -> 0/0 mssclamp 1440
.Ed
.Pp
The above examples assume a MTU of 1492 bytes. If the MTU on your PPPoE
connection is smaller use the MTU \- 52 bytes for clamping e.g. 1408 bytes
for a MTU of 1460 bytes.
.Em Note :
The theoretically correct value for the above example would be 1452 bytes
(it accounts for the smaller PPPoE MTU, the TCP header and the maximum of
0x40 bytes of TCP options) but it seems to not be sufficient in some cases.
Experiments conducted by various people have shown that clamping to the MSS
values suggested above works best.
.Sh EXAMPLES
A typical
.Pa /etc/ifconfig.pppoe0
file looks like this:
.Bd -literal -offset indent
create
! /sbin/ifconfig ne0 up
! /sbin/pppoectl -e ne0 $int
! /sbin/pppoectl $int myauthproto=pap myauthname=testcaller myauthsecret=donttell
inet 0.0.0.0 0.0.0.1
#! /sbin/route add default -iface 0.0.0.1
up
.Ed
The commented out call to
.Xr route 8
may be omitted and the route added in the ip-up script called by
.Xr ifwatchd 8
when the real IP address is known.  This is easy in the
.Dq connect always
mode (link1 not set), but hard to accomplish in the
.Dq dial on demand
mode (link1 set).  In the latter case adding an iface route is an easy
workaround.
.Pp
The
.Nm
interfaces operate completely inside the kernel, without any userland
support.  Because of this, a special daemon is used to fire ip-up or
down scripts to execute arbitrary code when the PPP session is established
and addresses of the interface become available.  To enable the usage of
.Pa /etc/ppp/ip-up
and
.Pa /etc/ppp/ip-down
for this purpose, simply add
.Bd -literal -offset indent
ifwatchd=YES
.Ed
.Pp
to
.Pa /etc/rc.conf .
See
.Xr ifwatchd 8
for details and parameters passed to these scripts.
.Pp
Since this is a PPP interface, the addresses assigned to the interface
may change during PPP negotiation.  There is no fine grained control available
for deciding which addresses are acceptable and which are not.  For the local
side and the
remote address there is exactly one choice: hard coded address or wildcard.
If a real address is assigned to one side of the connection, PPP negotiation
will only agree to exactly this address.  If one side is wildcarded, every
address suggested by the peer will be accepted.
.Pp
To wildcard the local address set it to 0.0.0.0, to wildcard the remote
address set it to 0.0.0.1. Wildcarding is not available (nor necessary)
for IPv6 operation.
.Sh OPTIONS
A
.Nm
enabled kernel will not interfere with other
.Nm PPPoE
implementations running on the same machine.  Under special circumstances
(details below) this is not desirable, so the
.Nm
driver can be told to kill all unknown
.Nm PPPoE
sessions received by the ethernet interface used for a configured
.Nm
interface.  To do this, add
.Pp
.Nm options
.Ar PPPOE_TERM_UNKNOWN_SESSIONS
.Pp
to your kernel config file.
.Pp
Note that this will break all userland
.Nm PPPoE
implementations using the same ethernet interface!
.Pp
This option is only useful if you have a static IP address assigned and
your ISP does not use LCP echo requests to monitor the link status.  After
a crash or power failure the peer device still tries to send data to the no
longer active session on your computer, and might refuse to reestablish a
new connection, because there already is an open session.  On receipt of such
packets the
.Nm
driver with this option set will send a
.Nm PADT
packet (request to terminate the session).  The peer will immediately disconnect
the orphaned session and allow a new one to be established.
.Sh SEE ALSO
.Xr ifwatchd 8 ,
.Xr pppoectl 8
.Rs
.%R RFC
.%N 2516
.%D February 1999
.%T "A Method for Transmitting PPP Over Ethernet (PPPoE)"
.Re
.Sh HISTORY
The
.Nm
device appeared in
.Nx 1.6 .
.Sh BUGS
This implementation is client side only.