# In this script, we will first learn about the ‹host› utility,
# which does basic DNS resolution for us. For instance, we can ask
# what is the IP address of ‹aisa› like this:

host aisa.fi.muni.cz.

# We can also use the shorthand, since we are in the same domain:

host aisa

# This has some interesting consequences:

host www

# What do you think the last query means?

# Besides translating names to addresses, DNS can also go the other
# way: give a hostname for a numeric address. We can simply pass the
# IP address to the ‹host› utility, like this:

host 4.2.2.2

# This uses some magic behind the curtain (though the magic also
# leaks into the user interface some). There is a special domain
# called in-addr.arpa, which has subdomains for all the possible IP
# addresses -- in reverse. So when we ask ‹host› to find the name
# for ‹4.2.2.2›, it is in fact looking for ‹2.2.2.4.in-addr.arpa.›
# There's magic involved because the normal recursion rules don't
# quite work in this domain. Nonetheless, there are DNS records (of
# type ‹PTR›) and they contain the ‘canonic‘ hostname of the
# computer that lives at the given IP address. You usually want the
# name to resolve back to the same IP address.

# In fact, let's refresh our shell scripting skills with an exercise
# (in ‹1.txt›). After that, we'll look at ‹dig› which is another DNS
# tool and then we'll move on to ‹nc›.