On page 5 (life of a DNS query), we saw how resolvers work. This code does the same thing, but it actually works.

def resolve(domain):
# Start at a root nameserver
nameserver = "198.41.0.4"
# A "real" resolver would check its cache here
while True:
reply = query(domain, nameserver)
ip = get_answer(reply)
if ip:
# Best case: we get an answer to our query and we're done return ip
nameserver_ip = get_glue(reply)
if nameserver_ip:
# Second best: we get the IP address* of the nameserver to ask next 
nameserver = nameserver_ip
else:
# Otherwise: we get the domain name* of the nameserver to ask next    
nameserver_domain = get_nameserver(reply) nameserver = resolve(nameserver_domain)
* Actual DNS resolvers are more complicated than this, but this is the core algorithm.

Smiling stick figure with curly hair: You can find the whole program at https://github.com/jvns/tiny-resolver