Obviously this can be done with more than 2 ISPs or with different speed
ratios than 2:1.

netfilter/iptables project homepage: http://netfilter.org/
iproute2 project homepage: http://linuxfoundation.org/collaborate/workgroups/networking/iproute2/



   1 #!/bin/bash
   2 #
   3 # TCP/UDP Load Balancer
   4 #
   5 # Made possible by the collective wisdom of the Internets
   6 # Compiled by Jimmy Angelakos <vyruss@hellug.gr>
   7 #
   8 # eth0 is the LAN connection.
   9 # eth1 is connected to ISP#1 with IP1 via gateway GW1 on network NET1.
  10 # eth2 is connected to ISP#2 with IP2 via gateway GW2 on network NET2.
  11 # DNS1 is DNS server for ISP#1 and DNS2 is for ISP#2.
  12 
  13 
  14 # Your configuration variables:
  15 IF1=eth0
  16 IF2=eth1
  17 IF3=eth2
  18 IP1=192.168.22.99
  19 IP2=192.168.99.99
  20 GW1=192.168.22.2
  21 GW2=192.168.99.1
  22 NET1=192.168.22.0/24
  23 NET2=192.168.99.0/24
  24 DNS1=193.92.150.3
  25 DNS2=195.170.0.1
  26 
  27 # Create 2 new routing tables
  28 if ! cat /etc/iproute2/rt_tables | grep -q '^251'
  29 then
  30     echo '251   T1' >> /etc/iproute2/rt_tables
  31 fi
  32 if ! cat /etc/iproute2/rt_tables | grep -q '^252'
  33 then
  34     echo '252   T2' >> /etc/iproute2/rt_tables
  35 fi
  36 ip route flush table T1
  37 ip route flush table T2
  38 
  39 # Associate routing tables with the different ISPs.
  40 ip route add $NET1 dev $IF1 src $IP1 table T1
  41 ip route add default via $GW1 table T1
  42 ip route add $NET2 dev $IF2 src $IP2 table T2
  43 ip route add default via $GW2 table T2
  44 ip route add $NET1 dev $IF1 src $IP1
  45 ip route add $NET2 dev $IF2 src $IP2
  46 
  47 # Add your other networks to both tables.
  48 ip route add 10.0.0.0/8 via 192.168.22.4 table T1
  49 ip route add 10.0.0.0/8 via 192.168.22.4 table T2
  50 
  51 # Delete old rules.
  52 ip rule del from all fwmark 0x1 lookup T1 2>/dev/null
  53 ip rule del from all fwmark 0x2 lookup T2 2>/dev/null
  54 ip rule del from all fwmark 0x2 2>/dev/null
  55 ip rule del from all fwmark 0x1 2>/dev/null
  56 ip rule del from $IP1
  57 ip rule del from $IP2
  58 
  59 # Add new rules for connection tracking to our routing tables.
  60 ip rule add from $IP1 table T1
  61 ip rule add from $IP2 table T2
  62 ip rule add fwmark 1 table T1
  63 ip rule add fwmark 2 table T2 
  64 
  65 # You need these system settings.
  66 echo 1 > /proc/sys/net/ipv4/ip_forward
  67 for f in /proc/sys/net/ipv4/conf/*/rp_filter ; do echo 1 > $f ; done
  68 
  69 # Clear iptables/netfilter settings.
  70 iptables -F
  71 iptables -X
  72 iptables -t nat -F
  73 iptables -t nat -X
  74 iptables -t filter -F
  75 iptables -t filter -X
  76 iptables -t mangle -F
  77 iptables -t mangle -X
  78  
  79 # Set up connection tracking and Source NAT.
  80 
  81 # Create two chains that will mark the packet. 
  82 iptables -t mangle -N MARK1
  83 iptables -t mangle -A MARK1 -j MARK --set-mark 1
  84 iptables -t mangle -A MARK1 -j CONNMARK --save-mark
  85 iptables -t mangle -N MARK2
  86 iptables -t mangle -A MARK2 -j MARK --set-mark 2
  87 iptables -t mangle -A MARK2 -j CONNMARK --save-mark
  88  
  89 iptables -t nat -N OUT_IF1
  90 iptables -t nat -A OUT_IF1 -j SNAT --to-source $IP1
  91 iptables -t nat -N OUT_IF2
  92 iptables -t nat -A OUT_IF2 -j SNAT --to-source $IP2
  93 
  94 iptables -t nat -A POSTROUTING -o $IF1 -j OUT_IF1
  95 iptables -t nat -A POSTROUTING -o $IF2 -j OUT_IF2
  96  
  97 iptables -t nat -N IN_IF1
  98 iptables -t nat -A IN_IF1 -j MARK --set-mark 1
  99 iptables -t nat -A IN_IF1 -j CONNMARK --save-mark
 100 iptables -t nat -N IN_IF2
 101 iptables -t nat -A IN_IF2 -j MARK --set-mark 2
 102 iptables -t nat -A IN_IF2 -j CONNMARK --save-mark
 103  
 104 iptables -t nat -A PREROUTING -i $IF1 -j IN_IF1
 105 iptables -t nat -A PREROUTING -i $IF2 -j IN_IF2
 106 
 107 # TCP balancing. ISP#1 has twice the connection speed of ISP#2, so in every 3 packets two should go to ISP#1 and one to ISP#2.
 108 iptables -t mangle -A PREROUTING -i $IF3 -p tcp -m tcp -m state --state NEW -m statistic --mode nth --every 3 --packet 0 -j MARK1
 109 iptables -t mangle -A PREROUTING -i $IF3 -p tcp -m tcp -m state --state NEW -m statistic --mode nth --every 3 --packet 1 -j MARK2
 110 iptables -t mangle -A PREROUTING -i $IF3 -p tcp -m tcp -m state --state NEW -m statistic --mode nth --every 3 --packet 2 -j MARK1
 111 
 112 # Restore mark on packets belonging to existing connections.
 113 iptables -t mangle -A PREROUTING -i $IF3 -p tcp -m tcp -m state --state ESTABLISHED,RELATED -j CONNMARK --restore-mark
 114  
 115 # UDP balancing. Same story as above.
 116 iptables -t mangle -A PREROUTING -i $IF3 -p udp -m udp -m statistic --mode nth --every 3 --packet 0 -j MARK1
 117 iptables -t mangle -A PREROUTING -i $IF3 -p udp -m udp -m statistic --mode nth --every 3 --packet 1 -j MARK2
 118 iptables -t mangle -A PREROUTING -i $IF3 -p udp -m udp -m statistic --mode nth --every 3 --packet 2 -j MARK1
 119  
 120 # Send DNS requests through the correct ISP or they will fail.
 121 iptables -t mangle -A PREROUTING -p tcp --dport 53 -d $DNS1 -j MARK1
 122 iptables -t mangle -A PREROUTING -p udp --dport 53 -d $DNS1 -j MARK1
 123 iptables -t mangle -A PREROUTING -p tcp --dport 53 -d $DNS2 -j MARK2
 124 iptables -t mangle -A PREROUTING -p udp --dport 53 -d $DNS2 -j MARK2
 125 
 126 ip route flush cache
 127 
 128 # You're done! Now you can watch the load balancing taking place with:
 129 # watch -n1 iptables -t mangle -nvL
 130 #
 131 # Enjoy!