1.2 Linux advanced strand

Advanced strand:  from “running commands” → “thinking like a system admin / engineer”
internal vs. global dns
NAT outbound and inbound port forwarding 
private addresses
look in known_hosts file, repairs
why MTM warnings matter
mulit-hop remote access (cuckoo’s egg situation)
scp
processes, kill -9 
ps aux | grep blah. (pipes)
absolute vs. relative paths
wildcards
file permissions, ownership 
chmod
ls -la
editing config files
reading logs
shell scripts
making scripts executable
crontab -e
updating, upgrading, dpkg
systemctl start, stop, status
ufw status
Private addresses:
any machine on the internal network likely has a "fake" address in the form of 192.168.x.y, or 10.x.y.x, or in some rare cases 172.16.x.y (nobody uses this believe me)
why? look this up
compare the IP4 address for the elab-wiki server (see below) to the IP6 address
many machines can also be reached by elab.local for example, with no DNS involved
challege: look up the default network configuration for several commercial routers: what do you notice?Internal vs. global DNS
for internal network users, you can run an internal DNS that resolves names to numbers
test this by using NSLOOKUP to look up the elab.hpa.edu server
change the DNS lookup server by typing server 8.8.8.8 
repeat the command and compare
challenge: repeat this process for elab-wiki.hpa.edu and xserve.hpa.eduNAT outbound
when you browse the internet, there must be a way to convert your 192.168.x.y address into something else that the web servers can recognize
this is done through network address translation or NAT outbound, where your address is translated into the address of your router on the way out
this router is what the servers see
this process is reversed on the way back in, sending the web page back to your protected internal machineInbound port mapping-firewalls
the reverse of this is inbound port mapping, where an outside user contacts your router and is pointed to your internal machine, usually on a defined port, for example to run a game or host a web page
challenge: look at the page below and describe how outside users might see the elab-wiki server:
Screenshot 2026-03-31 at 1.01.03 PM.png
SSH issues:
when you establish a secure shell connection, two things are added: one to your machine, the other to the host machine.
in your machine, a host file is where these are stored, listing the IP address of the host and a key that the host has shared with you
if the IP address of the host changes, you will be faced with a serious message in the terminal that such a host cannot be trusted
to solve this, you must find the .known_hosts file on your computer, open the file with nano, and delete the previous IP address + key line
in practice, you could just erase the entire file, but any connection you have setup before this will have to be re-established
challenge: find the .known_hosts file on your mac and copy a line from it
challenge: do the same with the piMan in the middle attack (MTM):
it is possible for a hacker or bot to watch the network you are on, and masquerade as the host you are sharing a key with
first they watch the keys being transferred, and copy these
they then kick the host off the network (usually a wireless attack)
they then use the key to interact with your computer, posing as the host
MTM warnings check the .known_hosts file and if the IP address does not match, it refuses to connect
challenge: ssh into the control computer, then change the IP on the computer and repeat
explain why an MTM warning mattersMulti-hop remote access:
ssh into one of the pi units in the lab
from that machine, login to another machine
create a text file in this machine, on the Desktop
enter some text and ctrl-o, return, then ctrl-x
look up "the cuckoo's nest" by Cliff Stoll 
explain how the hacker was tracked and what was the resolutionSecure copy: 
on your pi, create a text file called "payload" in your home directory: /home/pi using nano
secure copy this file over to another computer using this command: scp /home/pi/payload pi@10.14.84.20:/home/pi
go to that machine via ssh and verify that the file has been transferred
verify from the GUI as wellprocesses, kill -9
type ps -aux to see a list of processes
open some app on your pi
look again at the ps command result
kill the process by using the kill -9 <process ID>
n.b. be careful with kill -9 it is like an assassination: it kills with no warning, and no mercyprocesses, pipes
let's say you are looking for a process, but can't find it
piping means sending the result of one command to another like this: ps aux | grep blah
this means: "show me all processes and when you get that list grep (search) for the process called blah"
try this out with the example abovefiles, wildcards
let's say I want to edit the file awesome in the home directory
I could type sudo nano awesome
or, since I am almost as lazy (faul) as a mathematician, I use: sudo nano awe*
this only works if there are no other files in that directory starting with awe, so use this with cautionfiles, listing files
the ls command for list is something you will use constantly 
it is a summary, to see more, use the ls -la flag:
l means long form, or all of the data
a means all of the files, even the hidden ones
challenge: use the ls -la command on your directory, what do you see?
sudo nano .herobrine in your /home/pi directory 
enter some text and ctrl-o, return, then ctrl-x
use ls on your directory
look in the GUI for the file, where is it?
repeat with ls -la
create a hidden file on a remote machine then challenge the user to find the filefiles, who owns what?
in your /home/pi directory, using ls -la you notice lots of other stuff like this:
Screenshot 2026-03-31 at 1.36.13 PM.pngwhat does all this mean?
d means directory or folder
there are then three sets of three letters: r means read, w means write, x means execute
these are for three levels of ownership: owner, user, everyone
read counts at 1, write counts as 2 and execute counts as 4 (notice how these are all base 2 numbers)
we could say a file you could read, write and run as a program was rwx or we could add these to make 7 (1+2+4)
chmod is how we change the permissions on a file, so chmod 777 is the same as chmod rwx
this would be dangerous, as you have made the file read write and execute for owner, user and everyone
you may create a python program you want to run, so you would enter chmod +x program.py
this makes the file a program that can be executed
challenge: create a text file and notice the permissions
cd /var/www/html and notice the permissions on index.html, the home page for your computer
on the browser, go to 127.0.0.1 (me myself and I in computer language)
sudo nano the index.html file and add some words after "It works"
ctrl-o, return, ctrl-x
re-visit the home page
how could you do this as a remote user?files, installing packages
while most apps can be installed using sudo apt get <program name> there are others that download into your computer, usually in the /home/pi/Downloads folder
to open these, use this command: dpkg -i <filename>
you might also look at pi-apps, which is an alternate file download project
to update your computer, use sudo apt upgrade
to upgrade, always add the -y flag: sudo apt upgrade -y
Migrated from Apple Server Wiki. Original page ID: T557x3