PPP Over Ethernet protocol network
pppoe interface encapsulates
Protocol (PPP) packets inside Ethernet frames as defined by RFC
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
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.
pppoe interface is usable, it
needs to be configured. The following steps are necessary:
- Create the interface.
- 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.
- Configure authentication. The PPP session needs to identify the client to the peer. For more details on the available options see ifconfig(8).
- If using IPv6, configure a link-local address.
This all is typically accomplished using an /etc/hostname.pppoe0 file. A typical file looks like this:
inet 0.0.0.0 255.255.255.255 NONE \ pppoedev em0 authproto pap \ 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
The physical interface must also be marked
# echo "up" > /etc/hostname.em0
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.
If the local address is set to wildcard address 0.0.0.0, it will be changed to an address suggested by the peer.
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.
Otherwise, PPP negotiation will only agree to exactly the IPv4 addresses which are configured on the interface.
pppoe 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
pppoe 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:
PPPOE AND MTU/MSS
PPPoE has an 8-byte header. When run over a network interface with
the standard Ethernet maximum transmission unit (MTU) of 1500 bytes, this
reduces the maximum available MTU to 1492.
sets the default MTU to this value. Unfortunately issues can occur when the
path between the two endpoints of a TCP connection are not able to carry
same sized packets, leading to possible packet fragmentation and sometimes
packet loss. In that case the maximum packet size can be set using the
max-mss option in
pf.conf(5). For example:
match on pppoe0 scrub (max-mss
When using a PPPoE device configured for a higher MTU ("jumbo
frames"), the MTU for the
pppoe device can also
be raised. In this case
pppoe attempts to negotiate
the higher size with the other PPPoE endpoint using the RFC 4638 protocol.
This can allow standard Ethernet packet sizes (1500 bytes) to be carried
over PPPoE. For example, in
inet 0.0.0.0 255.255.255.255 NONE mtu 1500 \ pppoedev em0 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 would also have to be configured correspondingly:
# echo "up mtu 1508" > /etc/hostname.em0
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.
sppp(4), hostname.if(5), pf.conf(5), ifconfig(8)
L. Mamakos, K. Lidl, J. Evarts, D. Carrel, D. Simone, and R. Wheeler, A Method for Transmitting PPP Over Ethernet (PPPoE), RFC 2516, February 1999.
P. Arberg, D. Kourkouzelis, M. Duckett, T. Anschutz, and J. Moisand, Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE), RFC 4638, September 2006.
pppoe device first appeared in
This implementation is client side only.
It is important to specify “
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
The presence of a mygate(5) file will interfere with the routing table. Make sure this file is either empty or does not exist.
pppoe interfaces configured with the
same wildcard destination address cannot share a routing table.