PPPOE(4) | Device Drivers Manual | PPPOE(4) |
pppoe
—
pseudo-device pppoe
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 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 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 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.
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 following steps are necessary:
This all is typically accomplished using an /etc/ifconfig.pppoe0 file.
pppoe
interface, you will have an
unusually low MTU for today's 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 net.inet.tcp.mss_ifmtu=1
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 connected 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.
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 during 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 clamping:
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 best.
create ! /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 up
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 add
ifwatchd=YES
to /etc/rc.conf. See ifwatchd(8) for details and parameters passed to these scripts.
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.
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:
options
PPPOE_TERM_UNKNOWN_SESSIONS
and set the value of
sysctl(7) variable
net.pppoe.term_unknown
to true.
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.
To enable pppoe
server support in the
kernel, use
options PPPOE_SERVER
As described above, this allows pppoe
interfaces to be created and configured for incoming connections by setting
the “link0
” flag with
ifconfig(8).
A Method for Transmitting PPP Over Ethernet (PPPoE), RFC, 2516, February 1999.
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 appeared in NetBSD
1.6.
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 to support.
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 and conforms to the
enhancement to the PPPoE standard, RFC4638
, to
request the use of this larger MTU value with the PPPoE server.
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 negotiation.
August 7, 2016 | NetBSD 9.4 |