https://www.pro-bono-publico.de/openbsd/pppoe/

In-kernel PPPoE for OpenBSD

This page used to describe how to copy the NetBSD PPP subsystem to OpenBSD in order to get in-kernel PPPoE support. However, as there's native PPPoE support in -current since November 29, 2004, I've removed that and other bits of information.

Some hints, in no particular order ...

Stock ifstated works just fine with with pppoe devices. For starters, try the following sample /etc/ifstated.conf:

pppoe_up = "(pppoe0.link.up)"

state auto {
        if $pppoe_up {
		set-state online_ini
	}
        if ! $pppoe_up {
                set-state offline_ini
        }
}

state online_ini {
        if ! $pppoe_up {
                set-state offline
        }
}

state offline_ini {
        if $pppoe_up {
                set-state online
        }
}

state online {
        init {
                run "/etc/ppp/ip-up"
        }
        if ! $pppoe_up {
                set-state offline
        }
}

state offline {
        init {
                run "/etc/ppp/ip-down"
        }
        if $pppoe_up {
                set-state online
        }
}

Use a variation of
```
scrub in all max-mss 1452
```
in your pf configuration to perform MSS clamping.
Set the default MSS to the ADSL-imposed limit:
```
sysctl -w net.inet.tcp.mssdflt=1452
```

The correct value for the maximum segment size would be 1408. Have a look at a full-size sample PPPoE packet:

Ethernet payload, 1500 octets

PPPoE + PPP header, 8 octets

PPPoE/PPP payload, 1492 octets

IP header, 20 octets

IP payload, 1472 octets

TCP header, 20 to 64 octets

TCP payload, 1452 to 1408 octets

Some folks recommend to use 1440 (which gives at least some room for TCP options, e.g. SACK). YMMV, however. Personally, I've never had any trouble with 1452, and, in my experience, none of the other "recommended values" can beat that one.

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