l.gtld-servers.net gives the delegated name server and my PC ask my resolver for the IP of each one of them. (with RD flag) - I'm really bad at explainging things :), this is what I'm trying to tell you, dig is asking your resolver to do the dirty job for you: resolve ns{1,2,3}.serverfault.com.

Start a tcpdump before launching dig command to see what I mean (Ex: tcpdump -i eth0 -n src 8.8.8.8 \or dst 8.8.8.8).

I'm trying to tell you that dig +trace is acting as recursive name server just partially. It will follow referrals (authority section) from root servers but it will resolve name servers names through your DNS caching server.

It will start from root name servers but it will resolve names using local resolver. Make a simple test, start a tcpdump while running query or just remove temporarily your name servers from /etc/resolv.conf.

dig +trace is using your local DNS resolver to resolve names.

Indeed, DNS is very complex and many have problems understanding it, this is why I developed this tool. I wanted to share with everyone what I've learned about DNS and to help people find their DNS problems. Thank you for your observation!

@JohnGardeniers I don't mind to learn new things, if you have comments or suggestions they are welcomed. :)

@syneticon-dj I've updated the message. Thank you!