PPPOE(4) OpenBSD Programmer's Manual PPPOE(4)NAMEpppoe - PPP Over Ethernet protocol network interface
SYNOPSIS
pseudo-device pppoeDESCRIPTION
The pppoe interface encapsulates Point-to-Point Protocol (PPP) packets
inside Ethernet frames as defined by RFC 2516.
This is often used to connect a router via a DSL modem to an access
concentrator. The pppoe interface does not by itself transmit or receive
frames, but needs an Ethernet interface to do so. This Ethernet
interface is connected to the pppoe interface via ifconfig(8). The
Ethernet interface needs to be marked UP, but does not need to have an IP
address.
There are two basic modes of operation, controlled via the link1 switch.
The default mode, 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 ``dial on demand'' mode,
selected by setting link1, only establishes a connection when data is
being sent to the interface.
Before a pppoe interface is usable, it needs to be configured. The
following steps are necessary:
o Create the interface.
o 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.
o Configure authentication. The PPP session needs to identify the
client to the peer. For more details on the available options see
ifconfig(8).
This all is typically accomplished using an /etc/hostname.pppoe0 file.
EXAMPLES
A typical /etc/hostname.pppoe0 file looks like this:
inet 0.0.0.0 255.255.255.255 NONE \
pppoedev ne0 authproto pap \
authname 'testcaller' authkey 'donttell' up
dest 0.0.0.1
!/sbin/route add default -ifp pppoe0 0.0.0.1
The physical interface must also be marked `up':
# echo "up" > /etc/hostname.ne0
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.
To wildcard the local address set it to 0.0.0.0; to wildcard the remote
address set it to 0.0.0.1.
KERNEL OPTIONS
A pppoe enabled kernel will not interfere with other PPPoE
implementations running on the same machine. Under special circumstances
(details below) this is not desirable, so the pppoe driver can be told to
kill all unknown PPPoE sessions received by the Ethernet interface used
for a configured pppoe interface. To do this, add the following to your
kernel config file:
option PPPOE_TERM_UNKNOWN_SESSIONS
Note that this will break all userland PPPoE implementations using the
same Ethernet interface!
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 pppoe driver with this option set will
send a PADT packet (request to terminate the session). The peer will
immediately disconnect the orphaned session and allow a new one to be
established.
If the kernel is compiled with option PPPOE_SERVER, there are two modes
of connection, controlled via the link0 switch. The default mode, link0
not being set, is client mode. The ``PPPoE server'' mode, selected by
setting link0, is to wait for incoming PPPoE sessions.
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
pppoe. 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. pppoe 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.
While pppoe(8) has an internal option, ``mssfixup'', which is enabled by
default and takes care of this, pppoe users have to rely on other
methods. Using a packet filter, the maximum segment size (MSS) can be
set (clamped) to the required value. The following rule in pf.conf(5)
would set the MSS to 1440:
match on pppoe0 scrub (max-mss 1440)
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 pf(4).
See pf.conf(5) for more information on MTU, MSS, and NAT.
SEE ALSOsppp(4), hostname.if(5), pf.conf(5), ifconfig(8), ppp(8), pppoe(8)
A Method for Transmitting PPP Over Ethernet (PPPoE), RFC 2516, February
1999.
HISTORY
The pppoe device first appeared in OpenBSD 3.7.
BUGS
This implementation is client side only.
It is important to specify ``netmask 255.255.255.255'' to 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.
The presence of a mygate(5) file will interfere with the routing table.
Make sure this file is either empty or does not exist.
OpenBSD 4.9 January 12, 2010 OpenBSD 4.9
