So, I have been having some problems with my ISP again. However, I have a 3G modem (Option iCON 225) that works in Linux, but you cannot pass it to DomU directly by creating a bridge and attaching that. So I have a following setup:

Internet<->modem<->
                   Dom0
Internet<->DomU<->

There is several things I need to do:
0. Get DKMS
1. Get hso driver
1.1. Unpack it to /usr/src/hso
1.2. dkms add -m hso -v 1.9; dkms build -m hso -v 1.9; dkms install -m hso -v 1.9
2. Obtain some scripts
2.1. cp -f hso.udev /etc/udev/rules.d/z20_hso-udev.rules; cp -f rezero /usr/sbin/; /etc/init.d/udev restart
2.2. Stick the modem in
2.3. Modify the connect.sh script:
Find line “route add default dev $NETDEV". Modify it to following:

ip route add default dev $NETDEV table 10
ip route add $INTERNALRANGE dev $INTERNALBRIDGE table 10
iptables -t nat -A POSTROUTING -s $BACKUPIP -o hso0 -j SNAT --to-source $PIP
iptables -t nat -A PREROUTING -i $NETDEV -j DNAT --to-dest $BACKUPIP
ip rule add from $BACKUPIP/32 to any table 10

INTERNALRANGE=Your internal IP range, for example 10.49.2.0/24.
INTERNALBRIDGE=Your internal bridge device, for example xenbr0
BACKUPIP=Router’s IP which you will set as source for the packets that you want to pass thru the 3G modem

Find line “ifconfig $NETDEV down". Modify it to following:

ip route flush table 10
iptables -t nat -D POSTROUTING -s $BACKUPIP -o hso0 -j SNAT --to-source $PIP
iptables -t nat -D PREROUTING -i $NETDEV -j DNAT --to-dest $BACKUPIP
ip rule add from $BACKUPIP/32 to any table 10
ifconfig $NETDEV down

You also might to comment out the resolv.conf lines.
2.4. Modify conninfo.ini

Note! Dom0 should have IP routing enabled, so “echo 1 > /proc/sys/net/ipv4/ip_forward". Use sysctl to set it at the boot time

3. Set the DomU
3.1. Add $BACKUPIP to $INTERNALBRIDGE
3.2. Add to boot script (for example, /etc/rc.local):

ip route add $INTERNALRANGE dev $INTERNALBRIDGE
ip route add default via $DOM0IP dev $INTERNALBRIDGE
ip rule add from $BACKUPIP lookup 10

Test with:
traceroute -s $BACKUIP ping.funet.fi
haruhi:~# traceroute -s 10.2.255.1 ping.funet.fi
traceroute to ping.funet.fi (128.214.248.132), 30 hops max, 40 byte packets
1 nanoha.serv.azt (10.2.0.10) 0.000 ms 0.000 ms 0.000 ms
2 ge0-0-1-650.esptnl-pe1.fi.elisa.net (213.161.47.241) 1384.000 ms 1384.000 ms 1416.000 ms
3 ae2.heltli-gw1.fi.elisa.net (139.97.6.246) 1416.000 ms 1456.000 ms 1536.000 ms
4 csc.ficix1-ge.ficix.fi (193.110.226.14) 1616.000 ms 1616.000 ms 1692.000 ms
5 helsinki0-x4100-csc0.funet.fi (193.166.255.154) 1736.000 ms 1784.000 ms *
6 ns-secondary.funet.fi (128.214.248.132) 424.000 ms !X 504.000 ms !X 556.000 ms !X

Now everything sent to your modem will be redirected to DomU. Unfortunally the only way to detect them is to match everything but interal IPs, but you can improve this if you want (for example, by setting ToS or creating new bridge for DomU-Dom0 communication).

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