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Simple Stateful Load Balancer with iptables and NAT

27/11/2015 Comments off

To demonstrate how iptables can perform network address translation this how-to shows how to use it to implement a over-simplified load balancer. In practice we would use a daemon such as HAProxy allowing IP tables to check packets before forwarding them.

Using the method presented in this tutorial packets get forwarded without going through the INPUT, FORWARD and OUTPUT chains.

iptables is a powerful tool that is used to create rules for how incoming or outgoing packets are handled. It keeps track of a packets state – there is NEW, ESTABLISHED, RELATED, INVALID and UNTRACKED. It can make filtering decisions based on the packets header data and the payload section of the packet, for these purposes iptables even has regular expression matching.

On top of that iptables has extensions that can be used to filter packets based on a packets history so we can keep track of packets and sessions. We can set filters to only trigger at specific times, parse the packet contents and header information searching for specific patterns, differentiate protocols such as tcp, udp, icmp, etc.

For load balancing behavior we want the incoming packets on one machine to be routed to another machine. iptables has extentions that helps us achieve this aim but we also need to muck around with its internal PREROUTING and POSTROUTING table, which is not recommended as this could potentially pose a security risk. lets use iptables to route all traffic coming in on an interface eth0 with a destination port 80 and route it to another IP address:

Allow IP forwarding

(Note: if your testing this on the same box your doing this on it won’t work, you need at least 3 machines to test this out, virtual ones work nicely)

First we enable ipv4 forwarding or this will not work:
# echo "1" > /proc/sys/net/ipv4/ip_forward

XOR

# sysctl net.ipv4.ip_forward=1

next we add a filter that changes the packets destination ip and allows us to masquerade:

# iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination 10.0.0.3:80
# iptables -t nat -A POSTROUTING -j MASQUERADE

The above filter gets added to iptables PREROUTING chain. The packets first go through the filters in the PREROUTING chain before iptables decides where they go. The above filter says all packets input into eth0 that use tcp protocol and have a destination port 80 will have their destination address changed to 1.2.3.4 port 80. The DNAT target in this case is responsible for changing the packets Destination IP address. Variations of this might include mapping to a different port on the same machine or perhaps to another interface all together, that is how one could implement a simple stateful vlan (in theory).

The masquerade option acts as a one to many NAT server allowing one machine to route traffic with one centralized point of access. This is similar to how many commercial firewalls and network routers function.

The above ruleset results in all incoming packets to dport 80 traversing the iptables chains in a straight line from INCOMING to OUTGOING in the image below, effectively bypassing any rules we might have had in our INPUT chain. If we were to choose to implement nat like this we would need to implement those – our desired INPUT filter rules – on the machines where traffic is forwarded OR add them to the FORWARD chain if we want to block things before they are forwarded (Note: packets might go through FORWARD chain in both directions so direction needs to be considered when writing filters for this chain).

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Path incoming packets take through iptables chains

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