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PPPOE(4) Device Drivers Manual PPPOE(4)

pppoe — PPP Over Ethernet protocol network interface

pseudo-device pppoe

The pppoe interface encapsulates 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:

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 ‘up’:

# 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:

option PPPOE_TERM_UNKNOWN_SESSIONS

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. 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).

Alternatively in cases where the remote equipment supports RFC 4638 and the physical interface is configured to support jumbo frames, the MTU of the pppoe interface can be raised and it will attempt to negotiate an increased MTU. For example, in /etc/hostname.pppoe0:

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 must also be configured like so:

# echo "up mtu 1508" > /etc/hostname.em0

With this, the previously mentioned MSS clamping rules in pf.conf(5) are no longer necessary.

If the MTU is set to a value larger than 1492 and the remote endpoint does support RFC 4638, pppoe will write “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.

See pf.conf(5) for more information on MTU, MSS, and NAT.

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.

The pppoe device first appeared in OpenBSD 3.7.

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.

Two pppoe interfaces configured with the same wildcard destination address cannot share a routing table.

March 16, 2021 OpenBSD-7.0