In my
previous post, I stated a goal of mimicing the connectivity provided by the microwave T-1 connecting the studio and TV transmitter at my work. I'm pleased to say that I've accomplished that... mostly. There are still a few peculiar kinks to work out.
It all works through the magic of
Proxy ARP. Basically, what Proxy ARP does is allow one machine to answer ARP queries on behalf of another machine, saying, in effect "I'll take that packet; I know how to get to that machine". The answering machine then forwards the packet on to the appropriate machine. This differs from packet routing in that it happens in Layer 2 of IP, rather than Layer 3 (routing).
In practice, what you have to do is set up static routes on the host (which is acting in this case as a bridge) doing Proxy ARP for all hosts the bridge is providing the connection for, then turn on IP forwarding in the kernel, and turn on Proxy ARP for the appropriate interfaces. Since I have an IPsec / L2TP VPN set up, the interfaces are going to be
ppp0 for the VPN (since it's L2TP
and IPsec, PPP is involved, where with straight IPsec it wouldn't be) and
eth0 for the local network. I'm also going to have to do the same thing on the firewall out at the transmitter.
So, first of all, the studio. This is based on
Using Linux as an L2TP/IPsec VPN client by
Jacco de Leeuw.
The xl2tpd configuration file at the studio:
; File: /etc/xl2tpd/xl2tpd.conf at STUDIO
[lac Transmitter]
; transmitter public IP obfuscated for security reasons
lns = X.X.X.10
require chap = yes
refuse pap = yes
require authentication = yes
; Name should be the same as the username in the PPP authentication!
name = bridge
ppp debug = yes
pppoptfile = /etc/ppp/options.l2tpd.client
length bit = yes
Note that I'm not using the
l2tp-secrets file as it really doesn't provide any additional security, as far as I can tell.
The PPP options file at the studio:
# /etc/ppp/options.l2tpd.client at STUDIO
ipcp-accept-local
ipcp-accept-remote
refuse-eap
noccp
noauth
crtscts
mtu 1410
mru 1410
nodefaultroute
debug
lock
connect-delay 5000
PPP authentication is done in
/etc/ppp/chap-secrets:
# File: /etc/ppp/chap-secrets
# Secrets for authentication using CHAP
# client server secret IP addresses
bridge * "supersecret"
* bridge "supersecret"
To set up the static routes, I have set up two scripts in
/etc/ppp/ip-up.d. Scripts in this directory are executed by pppd as
ip-up scripts - scripts that are executed when the PPP interface is brought up. These set up static routing and turn on Proxy ARP.
#!/bin/bash
# File: /etc/ppp/ip-up.d/0001routes
PPP_INTERFACE=$1
LOCAL_ADDR=$4
REMOTE_ADDR=$5
# These are all the hosts that should be accessible both via the VPN
HOSTS=(
10.1.1.10
10.1.1.11
10.1.1.12
10.1.1.13
)
for HOST in ${HOSTS[*]}
do
route add -host $HOST $PPP_INTERFACE
done
#!/bin/bash
# File: /etc/ppp/ip-up.d/0002proxyarp
for INTERFACE in ppp0 eth0 ; do
/sbin/sysctl -w net.ipv4.conf.${INTERFACE}.proxy_arp=1
done
The ipsec (openswan) configuration file at the studio:
# File: /etc/ipsec.conf
#
config setup
protostack=netkey
conn Transmitter
#
# ----------------------------------------------------------
# Use a Preshared Key. Disable Perfect Forward Secrecy.
# Initiate rekeying.
# Connection type _must_ be Transport Mode.
#
authby=secret
pfs=no
rekey=yes
keyingtries=3
type=transport
#
# ----------------------------------------------------------
# The local Linux machine that connects as a client.
#
# The external network interface is used to connect to the server.
# If you want to use a different interface or if there is no
# defaultroute, you can use: left=your.ip.addr.ess
left=%defaultroute
#
leftprotoport=17/1701
#
# ----------------------------------------------------------
# The remote server.
#
# Connect to the server at this IP address. (obfuscated for security)
right=X.X.X.10
#
rightprotoport=17/1701
# ----------------------------------------------------------
#
# Change 'ignore' to 'add' to enable this configuration.
#
auto=add
DPDACTion=restart_by_peer
dpdtimeout=30
dpddelay=3
Next, the transmitter side. Again, starting with xl2tpd:
; File: /etc/xl2tpd/xl2tpd.conf
[global]
ipsec saref = no
listen-addr = X.X.X.10
[lns default]
ip range = 10.1.1.100 - 10.1.1.255
local ip = 10.1.1.1
assign ip = yes
require chap = yes
refuse pap = yes
require authentication = yes
name = Transmitter
ppp debug = no
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
And now PPP:
# File: /etc/ppp/options.xl2tpd
refuse-mschap-v2
refuse-mschap
ms-dns 8.8.8.8
asyncmap 0
auth
lock
hide-password
local
#debug
name l2tpd
#proxyarp
lcp-echo-interval 30
lcp-echo-failure 4
# File: /etc/ppp/chap-secrets
# Secrets for authentication using CHAP
# client server secret IP addresses
user1 * "secret1" 10.1.1.0/24
* user1 "secret1" 10.1.1.0/24
user2 * "secret2" 10.1.1.0/24
* user2 "secret2" 10.1.1.0/24
bridge * "supersecret" 10.1.1.2
* bridge "supersecret" 10.1.1.2
Here I want to pause and note something. I have my regular "road warrior" users set up to get any address in 10.1.1.0/24, and in my xl2tpd config, I have that further restricted to addresses in the range of .100 - .255. The "bridge" user is restricted to 10.1.1.2, which is how I allow my "road warriors" and the bridge at the studio to coexist.
The appropriate scripts in
/etc/ppp/ip-up.d at the transmitter:
#!/bin/bash
# File: /etc/ppp/ip-up.d/0001routes
# The hosts here are made accessible to the transmitter network via the firewall, which is acting as a bridge similar to the one at the studio
PPP_INTERFACE=$1
LOCAL_ADDR=$4
REMOTE_ADDR=$5
# only set up the routes for the VPN bridge
case $REMOTE_ADDR in
10.1.1.2)
for HOST in 10.1.1.20 \
10.1.1.21 \
10.1.1.22 \
10.1.1.23 ; do
route add -host $HOST $1
done
esac
#!/bin/bash
PPP_INTERFACE=$1
LOCAL_ADDR=$4
REMOTE_ADDR=$5
# File: /etc/ppp/ip-up.d/0002proxyarp
# only set up Proxy ARP for the VPN bridge
case $REMOTE_ADDR in
10.1.1.2)
for INTERFACE in $PPP_INTERFACE eth0 ; do
/sbin/sysctl -w net.ipv4.conf.${INTERFACE}.proxy_arp=1
done
esac
And finally the ipsec (openswan) configuration, based on
Configure L2TP/IPSec VPN on Ubuntu by
Riobard Zhan:
#
# File: /etc/ipsec.conf
#
# Transmitter Firewall Side
config setup
oe=off
protostack=netkey
nat_traversal=yes
conn L2TP-PSK-NAT
rightsubnet=vhost:%no
also=L2TP-PSK-noNAT
conn L2TP-PSK-noNAT
authby=secret
pfs=no
auto=add
keyingtries=3
rekey=no
ikelifetime=8h
keylife=1h
type=transport
left=X.X.X.10
leftprotoport=17/1701
right=%any
rightprotoport=17/%any
dpdaction=restart_by_peer
dpdtimeout=30
dpddelay=3
So the way to accomplish this "wire-to-wire" emulation is with
two bridges, one for each network to be bridged. If you want to try something like this, I wish you the best of luck, and I hope my experiences help.
