man/man4/pppoe.4

.\"	$OpenBSD: pppoe.4,v 1.35 2021/03/16 13:53:39 millert Exp $
.\"	$NetBSD: pppoe.4,v 1.26 2003/10/02 07:06:36 wiz Exp $
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.\" by Martin Husemann <martin@NetBSD.org>.
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.Dd $Mdocdate: March 16 2021 $
.Dt PPPOE 4
.Os
.Sh NAME
.Nm pppoe
.Nd PPP Over Ethernet protocol network interface
.Sh SYNOPSIS
.Cd "pseudo-device pppoe"
.Sh DESCRIPTION
The
.Nm
interface encapsulates
.Em Point-to-Point Protocol (PPP)
packets inside Ethernet frames as defined by RFC 2516.
.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 ifconfig 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 ifconfig 8 .
.It
If using IPv6, configure a link-local address.
.El
.Pp
This all is typically accomplished using an
.Pa /etc/hostname.pppoe0
file.
A typical file looks like this:
.Bd -literal -offset indent
inet 0.0.0.0 255.255.255.255 NONE \e
	pppoedev em0 authproto pap \e
	authname 'testcaller' authkey 'donttell' up
dest 0.0.0.1
inet6 eui64
!/sbin/route add default -ifp pppoe0 0.0.0.1
!/sbin/route add -inet6 default -ifp pppoe0 fe80::%pppoe0
.Ed
.Pp
The physical interface must also be marked
.Ql up :
.Bd -literal -offset indent
# echo "up" > /etc/hostname.em0
.Ed
.Pp
Since this is a PPP interface, the addresses assigned to the interface
may change during PPP negotiation.
In the above example, 0.0.0.0 and 0.0.0.1 serve as placeholders for
dynamic address configuration.
.Pp
If the local address is set to wildcard address 0.0.0.0, it will
be changed to an address suggested by the peer.
.Pp
If the destination address is set to a wildcard address in the range
from 0.0.0.1 to 0.0.0.255, it will be changed to an address suggested
by the peer, and if a default route which uses this interface exists
the gateway will be changed to the suggested address as well.
.Pp
Otherwise, PPP negotiation will only agree to exactly the IPv4 addresses
which are configured on the interface.
.Sh KERNEL OPTIONS
.Nm
does not interfere with other PPPoE implementations
running on the same machine.
However under some circumstances
(such as after a crash or power failure)
the peer device might initially refuse to
reestablish a new PPPoE connection
because there is already an open session.
This would be indicated by the client sending a high number of PADI packets
before successfully connecting.
The
.Nm
driver can be told to kill all unknown PPPoE sessions
by sending a PADT packet to explicitly terminate the old session.
Add the following to the kernel config file:
.Pp
.Dl option PPPOE_TERM_UNKNOWN_SESSIONS
.Sh MTU/MSS ISSUES
Problems can arise on machines with private IPs connecting to the Internet
via a machine running both
Network Address Translation (NAT)
and
.Nm .
Standard Ethernet uses a
maximum transmission unit (MTU)
of 1500 bytes,
whereas PPPoE mechanisms need a further 8 bytes of overhead.
This leaves a maximum MTU of 1492.
.Nm
sets the MTU on its interface to 1492 as a matter of course.
However,
machines connecting on a private LAN will still have their MTUs set to 1500,
causing conflict.
Using a packet filter,
the
maximum segment size (MSS)
can be set (clamped) to the required value.
The following rule in
.Xr pf.conf 5
would set the MSS to 1440:
.Pp
.Dl match on pppoe0 scrub (max-mss 1440)
.Pp
Although in theory the maximum MSS over a PPPoE interface
is 1452 bytes,
1440 appears to be a safer bet.
Note that setting the MSS this way can have undesirable effects,
such as interfering with the OS detection features of
.Xr pf 4 .
.Pp
Alternatively in cases where the remote equipment supports RFC 4638
and the physical interface is configured to support jumbo frames,
the MTU of the
.Nm
interface can be raised and it will attempt to negotiate an increased MTU.
For example, in
.Pa /etc/hostname.pppoe0 :
.Bd -literal -offset indent
inet 0.0.0.0 255.255.255.255 NONE mtu 1500 \e
	pppoedev em0 authproto pap \e
	authname 'testcaller' authkey 'donttell' up
dest 0.0.0.1
!/sbin/route add default -ifp pppoe0 0.0.0.1
.Ed
.Pp
The physical interface must also be configured like so:
.Bd -literal -offset indent
# echo "up mtu 1508" > /etc/hostname.em0
.Ed
.Pp
With this, the previously mentioned MSS clamping rules in
.Xr pf.conf 5
are no longer necessary.
.Pp
If the MTU is set to a value larger than 1492 and the remote endpoint does
.Em not
support RFC 4638,
.Nm
will write
.Dq \&No valid PPP-Max-Payload tag received in PADO
to the kernel message buffer and the MTU will remain at the default value.
However, RFC 4638 negotiation only takes into account the MTU configured
on the endpoints, not the maximum MTU supported on the path between them.
If the path cannot pass the larger Ethernet frames, negotiation will succeed
but the larger frames will be dropped.
For this reason it is important to test the connection with large packets
when enabling a higher MTU.
.Pp
See
.Xr pf.conf 5
for more information on MTU, MSS, and NAT.
.Sh SEE ALSO
.Xr sppp 4 ,
.Xr hostname.if 5 ,
.Xr pf.conf 5 ,
.Xr ifconfig 8
.Sh STANDARDS
.Rs
.%A L. Mamakos
.%A K. Lidl
.%A J. Evarts
.%A D. Carrel
.%A D. Simone
.%A R. Wheeler
.%D February 1999
.%R RFC 2516
.%T A Method for Transmitting PPP Over Ethernet (PPPoE)
.Re
.Pp
.Rs
.%A P. Arberg
.%A D. Kourkouzelis
.%A M. Duckett
.%A T. Anschutz
.%A J. Moisand
.%D September 2006
.%R RFC 4638
.%T Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE)
.Re
.Sh HISTORY
The
.Nm
device first appeared in
.Ox 3.7 .
.Sh BUGS
This implementation is client side only.
.Pp
It is important to specify
.Dq Li netmask 255.255.255.255
to
.Xr ifconfig 8 .
If the netmask is unspecified, it will be set to 8 when 0.0.0.0 is
configured to the interface, and it will persist after negotiation.
.Pp
The presence of a
.Xr mygate 5
file will interfere with the routing table.
Make sure this file is either empty or does not exist.
.Pp
Two
.Nm
interfaces configured with the same wildcard destination address
cannot share a routing table.