Dynamic DNS resolver demo

A repository to emulate dynamic DNS resolving using a lua library

The reasoning behind this repository is to understand how this resolver works and, at the same time, have a real project to demonstrate it.

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Special thanks to @juanaugusto and @leandromoreira for helping to create this demo.

Why?

NGINX does not query the DNS again after it starts. If the IP resolved by DNS changes, NGINX won't be aware of it, causing the application to crash. This is a huge problem if you're using upstreams and can't control the IPs behind the DNS. There are plenty of workarounds for this, i.e., other libraries, modules, using static domains (avoiding variables) and even restarting NGINX service.

Usage

Before starting our containers, we must create a network where all our containers are able to communicate to each other.

make create_network

Now, fire up two upstreams. These will be our NGINX upstreams used to emulate invalid IPs being cached by NGINX.

make start_upstreams

Now we can start our DNS server to answer queries from our NGINX edge:

make start_dns

And finally, we need to get our edge up and running to use the DNS server to simulate the dynamic DNS resolver working:

make start_edge

Use http://localhost:8080?backend=resolved for fetching data from an upstream that has its IPs resolved every time the TTL expires.

or

http://localhost:8080?backend=unresolved for fetching data from a backend that does not query DNS after the TTL expires. Note that even if the DNS is changed, you'll still get old IPs.

Now you can change the dns/config/ddnsr-demo.hosts file and check if http://localhost:8080?backend=hello still works.

There are two IPs you can use to simulate this: 172.20.0.3 and 172.20.0.4

And finally, we need to get our edge up and running to use the DNS server to simulate the dynamic DNS resolver.

Testing keep-alive connections

To check if the keepalive feature still works after using dynamic DNS resolver, use tcpdump:

1. open up a terminal in the edge container
2. execute tcpdump -i any -nn -s0 -vvv "dst 172.20.0.3 and tcp[tcpflags] & tcp-syn != 0"
3. make a request to http://localhost:8080 check if received a line containing Flags [S]
4. repeat the request above and check if you are still receiving that line

If you did not receive it twice, keep-alive is working fine.