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?

- 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.edu

NAT 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 machine

Inbound 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](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/mYABUNeXTQBu7j6S-screenshot-2026-03-31-at-1-01-03-pm-uxh9jrih.png)

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 pi

Man 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 matters

Multi-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 resolution

Secure 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 well

processes, 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 mercy

processes, 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 above

files, 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 caution

files, 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 file

files, 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.png](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/TfB5hWx4vbbVrEgm-screenshot-2026-03-31-at-1-36-13-pm-cwrnfjhl.png)

Screenshot 2026-03-31 at 1.36.13 PM.png

- 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