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PPPOE(4) NetBSD Kernel Interfaces Manual PPPOE(4)
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pppoe -- PPP over Ethernet protocol network interface
The pppoe interface encapsulates Point-to-Point Protocol (PPP) packets
inside Ethernet frames as defined by RFC2516.
This is often used to connect a router via a DSL modem to an access con-
centrator. The pppoe interface does not by itself transmit or receive
frames, but needs an Ethernet interface to do so. This Ethernet inter-
face is connected to the pppoe interface via pppoectl(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 ses-
sion 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
If the kernel is compiled with options 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 session.
Before a pppoe interface is usable, it needs to be configured. The fol-
lowing steps are necessary:
· Create the interface.
· Connect an Ethernet interface. This interface is used for the physi-
cal communication. As noted above it must be marked UP, but need not
have an IP address.
· Configure authentication. The PPP session needs to identify the
client to the peer. For more details on the available options see
This all is typically accomplished using an /etc/ifconfig.pppoe0 file.
If you are using a pppoe 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
# Obey interface MTUs when calculating MSS
in your /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 con-
nected to the Internet.)
Unfortunately this sysctl does not fix the MSS advertised by hosts in the
network behind a pppoe connected router. To fix this you need
MSS-clamping, explained below.
Setting up NAT with MSS-clamping
Some systems behind misconfigured firewalls try to use Path-MTU-Discov-
ery, 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
Without special care systems as described above will not be able to send
larger chunks of data to a system connected via pppoe. 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 dur-
ing initial TCP handshake.
For connections originating from your pppoe connected machines, this is
accomplished by setting the sysctl variable net.inet.tcp.mss_ifmtu to 1
(see above). For connections originating from systems behind your pppoe
router, you need to set the mssclamp options in your NAT rules, like in
this example of /etc/ipnat.conf:
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
If you do not use NAT, you need to set up a 1:1 NAT rule, just to get the
map pppoe0 x.x.x.x/24 -> 0/0 mssclamp 1440
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. 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
A typical /etc/ifconfig.pppoe0 file looks like this:
! /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 netmask 0xffffffff
#! /sbin/route add default -iface 0.0.0.1
The commented out call to route(8) may be omitted and the route added in
the ip-up script called by ifwatchd(8) when the real IP address is known.
This is easy in the ``connect always'' mode (link1 not set), but hard to
accomplish in the ``dial on demand'' mode (link1 set). In the latter
case adding an iface route is an easy workaround.
The pppoe 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 /etc/ppp/ip-up and /etc/ppp/ip-down for this purpose, simply
to /etc/rc.conf. See ifwatchd(8) for details and parameters passed to
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. Wildcarding is not available (nor necessary)
for IPv6 operation.
A pppoe enabled kernel will not interfere with other PPPoE implementa-
tions 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 con-
figured pppoe interface. To do this, add the following to your kernel
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
To enable pppoe server support in the kernel, use
As described above, this allows pppoe interfaces to be created and con-
figured for incoming connections by setting the ``link0'' flag with
A Method for Transmitting PPP Over Ethernet (PPPoE), RFC, 2516, February
The pppoe device appeared in NetBSD 1.6.
DEVIATIONS FROM STANDARD
The PPPoE standard, RFC2516, requires a maximal MTU of 1492 octets. This
value is the maximum conservative value possible, based on the PPPoE
header size and the minimum frame size Ethernet interfaces are required
In practice most modern Ethernet interfaces support bigger frames, and
many PPPoE services allow the use of (slightly) larger MTUs, to avoid the
problems described above.
This implementation allows MTU values as large as possible with the
actual MTU of the used Ethernet interface.
When using the wildcard address 0.0.0.0 (as described above) it is impor-
tant to specify the proper ``netmask'' to ifconfig(8), in most setups
``0xffffffff''. 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 negoti-
NetBSD 5.1 July 4, 2006 NetBSD 5.1