PPPOE(4)

NAME
pppoe - PPP Over Ethernet protocol network interface

SYNOPSIS
pseudo-device pppoe

DESCRIPTION
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:

•   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).

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
    !/sbin/route add default -ifp pppoe0 0.0.0.1

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. 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).

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.

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

SEE ALSO
sppp(4), hostname.if(5), pf.conf(5), ifconfig(8), ppp(8), pppoe(8)

STANDARDS
L. Mamakos, K. Lidl, J. Evarts, D. Carrel, D. Simone, R. Wheeler, February 1999, RFC 2516, A Method for Transmitting PPP Over Ethernet (PPPoE).

P. Arberg, D. Kourkouzelis, M. Duckett, T. Anschutz, J. Moisand, September 2006, RFC 4638, Accommodating a Maximum Transit Unit/Maximum Receive Unit (MTU/MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE).

HISTORY
The pppoe device first appeared in OpenBSD 3.7.

CAVEATS
RFC 4638 negotiation is only aware of the MTU configured on the endpoints, but not the maximum MTU supported on the path between them. If the path cannot pass the larger Ethernet frames, negotiation will succeed but the connection will not function correctly.

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.