# We will look at the client side of ‹nc› (netcat) in this script.
# We left off with ‹dig› when we learned that email for ‹fi.muni.cz›
# is handled by ‹relay.muni.cz›. Let's look at what would happen
# next in our hypothetical mail scenario.

# The MTA would learn the destination IP address and initiate an
# SMTP session.  I won't get into much detail, but let's talk about
# ‹nc› for a bit. What ‹nc› does is basically what ‹cat› does, but
# then sends the data over a network. It can both listen to
# connections (with ‹-l›) or connect to existing services. We will
# now use the latter mode.

# In either case, whatever you send to ‹nc›, it will forward into
# the network, and what falls out of the network connection, ‹nc›
# will print to its ‹stdout›.  Let's try with some SMTP:

{
    sleep 5; # wait a bit for the remote server to greet us
    echo HELO aisa # tell them who we are (hostname)
    echo "MAIL FROM:<not@me>" # and our mail address
    echo "RCPT TO:<yes@me>"   # and the recipient of the email

# At this point, the SMTP server will stop talking to us because it
# does not want to send mail to ‹yes@me› -- that is fine, we didn't
# want to actually spam anyone. The rest of the SMTP session would
# mostly involve the actual message.

} | nc relay.muni.cz 25 # 25 is the SMTP port

# Let me just note that ‹nc› is not quite standardized. It's present
# on many/most Unix systems, but the exact behaviour and switches
# might differ. Be careful and read man pages. Let's try looking at
# ‹nc› in server mode, in ‹listen.sh›.