BT Infinity with my linux router

I recently switched to BT Infinity Broadband which is a FTTC (Fibre To The Cabinet) network using VDSL2 (Very-high-bit-rate Digital Subscriber Line) over the copper telephone line from the street cabinet to your home. What I didn't want however, was to use BT's Home Hub; I have my own linux router and firewall.

The first hurdle was that I recevied a new Home Hub 5 from BT which has an integrated VDSL modem, rather than the combination of Home Hub and separate BT OpenReach VDSL modem. I had to swap equipment with my dad, as my intention was to connect my linux router directly to the modem.

BT Infinity uses PPPoE (Point-to-Point Protocol over Ethernet) which introduces an additional 8 bytes of overhead: 2 bytes for PPP + 6 bytes for PPPoE. The MTU (Maximum Transmission Unit) would be reduced to 1492 bytes from the standard 1500 bytes for ethernet.

Installing the PPPoE client

I installed the pppoe package which includes both a userspace client and a kernal module. I have chosen to use the kernel module as it should perform better.

# apt-get install pppoe

However the PPP package from Debian Squeeze includes an older version of the rp-pppoe.so (Roaring Penguin PPPoE) module, which will not negotiate an MTU of greater than 1492. This now works from version 3.11.

I downloaded and unpacked the latest version of rp-pppoe from Roaring Penguin.

# wget https://www.roaringpenguin.com/files/download/rp-pppoe-3.11.tar.gz
# tar -xzf rp-pppoe-3.11.tar.gz

I then compiled and installed.

# apt-get install ppp-dev
# cd ~/rp-pppoe-3.11/src
# ./configure --enable-plugin
# make
# make install

I renamed the current version of the pppoe plugin being used by ppp and put the new version in its place.

# cd /usr/lib/pppd/2.4.5./
# mv rp-pppoe.so rp-pppoe.so.original
# cp /etc/ppp/plugins/rp-pppoe.so rp-pppoe.so

Configuring the connection

I had been using an old quad port NIC (eth1-4) but the driver would not accept a MTU of 1508.

# ifconfig eth1 mtu 1508
SIOCSIFMTU: Invalid argument

However the more modern Realtek NIC (eth0) on my motherboard managed this without any problems.

The interface must be set for manual configuration, without any IP details and the MTU at 1508 bytes.

/etc/network/interfaces

auto eth0
iface eth0 inet manual
        mtu 1508

auto btbroadband
iface btbroadband inet ppp
        provider btbroadband
        pre-up /sbin/config eth0 up

You can install pppoeconf which will guide you through this configuration but I prefer just editing the files myself.

/etc/ppp/peers/btbroadband

#BT Broadband

# Use the pppoe kernel module.
plugin rp-pppoe.so eth0

# Name this connection as ppp0.
unit 0

user bthomehub@btbroadband.com

# No default IP; allocated dynamically by ISP.
noipdefault

# Try to get name servers from ISP.
usepeerdns

# Use this connection as the default route.
defaultroute

# Replace the default route if already present
replacedefaultroute

# There is an 8 byte overhead for PPPoE; however, 
# we are using baby jumbo frames of 1508 on eth0
# to account for the extra 8 byte overhead.
mtu 1500
mru 1500

# Exclude password string when logging packet contents.
hide-password
noauth
# Try to reopen the connection if it is terminated.
persist

Authentication

BT Infinity uses CHAP (Challenge Handshake Authentication Protocol) and will accept a generic username with any password (or none).

/etc/ppp/chap-secrets

# Secrets for authentication using CHAP
# client        server  secret                  IP addresses
"bthomehub@btbroadband.com"     *       "password"

Bring up the connection

# ifup btbroadband
# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:24:8c:48:6c:e4
          inet6 addr: fe80::224:8cff:fe48:6ce4/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1508  Metric:1
          RX packets:32767508 errors:635 dropped:0 overruns:634 frame:1
          TX packets:19426108 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:45510833119 (42.3 GiB)  TX bytes:2112888191 (1.9 GiB)
          Interrupt:21 Base address:0xe000

ppp0      Link encap:Point-to-Point Protocol
          inet addr:86.144.222.88  P-t-P:172.16.19.19  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:7933730 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4958501 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:10729227377 (9.9 GiB)  TX bytes:409095334 (390.1 MiB)

Note that the MTU is 1500 for ppp0 and 1508 for eth0.

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