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IPTables Magic

Blog Post after a long long time. Will be trying to write most of the crazy stuffs done in the past 1.5 years at the sad server

This post is going to cover a bunch of hacks done with iptables to improve / make the most out of linux systems' network performance

IPTables Tee
We are building a packet analysis team which does deep inspection of packets and determine anomalies in the system and determine the slowest performing component in the pipeline. Now sending the raw packets to centralized packet analysis system without affecting the performance of the production system is one of our requirements. We decided to use the iptables tee feature which takes a copy of the packet matching the rule and pass it on to the requested gateway in the same network by just changing the mac address on the cloned packet. The original packet follows the normal process

So lets create a similar setup, my laptop is going to forward a copy of http traffic to raspberry pi in the same network. Make sure ip_forward is turned off at the gateway box(box to which copy of packets are forwarded), here raspberry pi. With ip_forward on, we can cause multiple copies of  packets to be routed in network, tcp will kick in congestion control mechanisms seeing retransmissions. Packets are to be blackholed at the raspberry pi, it can do processing there but should not inject it to the network back conflicting with the original packet. IPtable rules at my box for the forwarding
sudo iptables -t mangle -A INPUT -p tcp --sport 80 -j TEE --gateway;sudo iptables -t mangle -A OUTPUT -p tcp --dport 80 -j TEE --gateway
Here is the IP of raspberry pi. At Prerouting chain we route the incoming packet, http response and at the output chain we  route the outgoing packet, http request. When I ran tcpdump in the raspberry pi while opening in my laptop this is the flow I got

Raspberry pi became the sniffer, it sees all http traffic flowing to/from my box

IPTABLES as LoadBalancer

This is completely inspired from Kubernetes infrastructure. Lets run two webservers in raspberry pi at port 81 and 82. Lets add iptable rules to roundrobin traffic between these two webservers in my laptop. VIP will be my laptop's ip( and port 80, Backends will be pi's ip( and port 81/82

iptables -t nat -A PREROUTING -p tcp  --dport 80 -m state --state NEW -m statistic --mode nth --every 2 --packet 0 -j DNAT --to-destination
iptables -t nat -A PREROUTING -p tcp  --dport 80 -m state --state NEW  -j DNAT --to-destination
iptables -t nat -A POSTROUTING -k MASQUERADE

First two rules is to DNAT the packets to the  backend. The caveat is first rule matches only every 2nd packet, those packets wont follow prerouting chain then. Second rule will match all packets which don't match 1st rule
3rd rule is to SNAT so that actual client ip is replaced by laptop's ip so that backend don't respond to client directly


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