# Dig is a somewhat more flexible tool for doing DNS queries from
# the command line. The output is also more DNS-like so to
# understand it, you need to know a little about DNS. For our
# use-case, it should be sufficient to know that there are different
# types of records in DNS (carrying different types of info about
# hostnames): ‹NS› stands for name server (i.e. a DNS server
# responsible for a given subdomain), ‹A› stands for IPv4 address,
# ‹AAAA› stands for IPv6 and ‹MX› stands for mail exchanger. Let's
# try some basic queries:

dig aisa.fi.muni.cz -t A

# The above performs a DNS query using the default system DNS
# recursive server.  It does not perform recursion itself. The
# output is fairly long, so let's look at what's in there. Let's
# ignore the header, all the way until QUESTION SECTION. The
# question section shows us what the question we asked was. In this
# case, we would like the ‹A› record of ‹aisa.fi.muni.cz› filled in
# for us.

# What follows is an ANSWER SECTION, which contains the answer to
# our query: it tells us that the ‹A› record for ‹aisa.fi.muni.cz›
# contains the address ‹147.251.48.1›. What follows after that is
# the list of name servers (‹NS› records) responsible for the domain
# ‹fi.muni.cz.› and finally we get some ‹A› and ‹AAAA› records for
# those name servers too. In case we wanted to directly consult
# them.

# We will actually stop the script here, so you can dig (ha-ha)
# through all the output ; when you are ready to continue, just hit
# enter.

read

# Since output from ‹dig› is so long and tedious, let's only do one
# more. This replicates what I have shown you in the lecture (but
# that was somewhat redacted, to fit in the slide). Let's also ask
# for a different type of record for the fun of it. The ‹+trace›
# tells ‹dig› to perform recursion itself and print all the
# intermediate results too.

dig fi.muni.cz -t MX +trace

# The ‹MX› record tells us who handles mail for the given domain. So
# when you send an email to me (using my ‹@fi.muni.cz› address), the
# MTA that sends email for you will do an ‹MX› lookup for
# ‹fi.muni.cz› to find out which computer it should contact. In this
# case, it'll see:
#
#     fi.muni.cz. 300 IN MX 50 relay.muni.cz.
#
# Which basically means there's only one mail exchanger, and its DNS
# name is ‹relay.muni.cz.› Let's continue in this spirit with
# ‹netcat.sh›.