Day 5 - Comprehensive Mock Exam 2: Linux Usage and Final Review

📌 Key Takeaways

• Part 2 of the exam has two subjects: Operation and Management + Linux Usage. Today we review Linux Usage — three items: X Window · Internet usage · application fields.
• The weights differ greatly — Internet usage is well over half, X Window comes next, and application fields are a handful of questions.
• Part 2 asks for names and values rather than principles. Product names, protocols, ports, and records are the core.
• Three things to keep until the very end are the port numbers · subnet calculation · layer classification table.
• The 10 questions below wrap up the entire Linux Usage scope.

Where Today's Scope Sits in Part 2

• Part 2 is 80 questions · 100 minutes, the passing line is 60 points, and a score below 40% in any subject is a fail. Doing well only in Usage, or only in Operation and Management, will not get you a pass.
• Counting one past exam round, of the 80 questions, Linux Usage had 32, split within it as X Window 8 · Internet usage 20 · application fields 4. It will differ somewhat by round, but the direction is clear: you should spend the most time on Internet usage (W9, W10).
• Application fields have few questions but a narrow scope. You only need to be sure of three chunks: the 3 cluster types, virtualization products, and Linux-kernel-based operating systems.

Final Checklist — Linux Usage

   W8 X Window     server = the side with the screen / client = the program
                   XFree86 → X.org → Wayland
                   layers  X server ← Xlib, XCB ← toolkit (GTK+ C, Qt C++, FLTK, Motif) ← app
                   window managers KWin, Mutter, Xfwm, Windowmaker, Afterstep
                   display managers XDM, GDM, KDM, SDDM, LightDM  (ends in DM)
                   desktop environments GNOME (GTK+), KDE (Qt), Xfce, LXDE, CDE (Motif)
                   DISPLAY :0.0 · startx · runlevel 3/5 · xhost (dangerous) vs ssh -X
 
   W9 Network      OSI Please Do Not Throw Sausage Pizza Away / hub 1 switch 2 router 3
                   bit, frame, packet, segment / MAC layer 2 IP layer 3 port layer 4
                   TCP reliable · UDP fast
                   circuit (PSTN) vs packet (PSDN) · CSMA/CD wired · CA wireless
                   X.25 → Frame Relay → ATM (53 bytes) · FDDI (optical dual-ring token)
                   IEEE 802.3/802.5/802.6/802.11 · IANA
                   classes A 1-126 B 128-191 C 192-223 D 224-239
                   private 10 / 172.16-31 / 192.168 · loopback 127.0.0.1
                   hosts = (256 − mask) − 2  →  254, 126, 62, 30, 14, 6, 2
 
   W10 Internet    HTTP 80 · HTTPS 443 · SSH 22 · TELNET 23
   services        SMTP 25 (sending, between servers) · POP3 110 (fetch and delete) · IMAP 143 (keep on server)
                   MUA → MTA → MDA · MIME
                   DNS 53 · A, AAAA, MX, CNAME, NS, PTR · BIND
                   FTP 21 control / 20 data · active vs passive · anonymous · bin/ascii
                   NFS (Unix, RPC, /etc/exports) vs Samba (Windows, SMB/CIFS/NetBIOS)
                   Gopher (menu search) · Usenet (NNTP) · Archie (FTP search) · IRC
 
   W11 Application HPC (high-computation) · HA (high availability) · LB (load balancing)
   fields          full virtualization (no modification) vs paravirtualization (modification needed)
                   Type 1 KVM, Xen, ESXi / Type 2 VirtualBox, Workstation
                   VirtualBox = Oracle · GPL · full virtualization · supports VMDK/VHD
                   Linux-kernel-based OS: Android, Tizen, webOS (not iOS)
                   IaaS/PaaS/SaaS · Hadoop (HDFS, MapReduce)

Three Things to Carry to the End

   ① Ports     21 FTP · 22 SSH · 23 TELNET · 25 SMTP · 53 DNS
               80 HTTP · 110 POP3 · 143 IMAP · 443 HTTPS · 631 CUPS
 
   ② Subnets   256 − mask = block,  block − 2 = hosts
               /24 254 · /25 126 · /26 62 · /27 30 · /28 14 · /29 6 · /30 2
 
   ③ Classification  Ends in DM → display manager
               Xfce is an environment · Xfwm is a window manager
               NFS is Unix · Samba is Windows
               TELNET is for logging in, not for transferring

Three Weeks Overlap in a Single Connection

One scene, showing a graphical program from a remote server on my own screen, contains X Window (W8), networking (W9), and Internet services (W10) all at once.

ssh -X alice@www.example.com         # run on my PC
echo $DISPLAY                        # from here on, it is the remote server's shell
# localhost:10.0
gedit &                              # a program on the server, but the window appears on my screen

   My PC (the side sitting in front of the screen)       Remote server www.example.com
 
   ① Name to IP         /etc/hosts → DNS (53)
   ② SSH client  ──── TCP port 22, encrypted ────►    SSH server (sshd)
   ③ X server          ◄─── drawing requests inside the tunnel ──  gedit (X client)
      screen, keyboard, mouse

• Within one connection, server and client get crossed. Seen from SSH, the remote machine is the server, but seen from X, my PC with the screen is the server. The "opposite of the web" from week 8 is exactly this scene.
• A DISPLAY of localhost:10.0 is a value SSH sets automatically. The format is still host:display.screen, and it points to a relay display number that SSH created inside the server (usually numbered from 10). Drawing requests sent there travel through the encrypted tunnel and arrive at the real screen :0 of my PC.
• You can do the same thing with xhost + and export DISPLAY=MY_PC_ADDRESS:0.0, but that is a method that is not encrypted and opens the screen to anyone. The exam asks about this contrast.
• If ② had been TELNET (port 23), even the password would flow in plain text. Pair them up as remote login is SSH, remote GUI is ssh -X.

Who Is the Server and Who Is the Client

Gathering the "direction" questions that appeared once each week into a single table shows they are the same type.

Separating Names with the Same Initial

Linux Usage is an exam of putting names into boxes. Names that could be told apart week by week look clumped together by initial on a single exam paper.

• In particular, not everything starting with X belongs to X Window. X.25 is a communication standard, and Xen is virtualization.
• Groupings like three Ks (KDE, KWin, KDM) and KVM, or three Gs (GNOME, GTK+, GDM) and Gopher, where "three are from one family and one is left over", show up as the four choices exactly as they are.

Picking the One Different from the Other Three

This is the typical question shape for this subject. You must be able to state what the three have in common first to see the one left over. Try solving them with the middle column covered.

Subnets — The Reversed Form

If "given the mask, find the host count" is the basic form, "given the host count, find the mask" is the reversed form. From the memorized line 254 · 126 · 62 · 30 · 14 · 6 · 2, choose the smallest box that is at least the required number.

• The trap is the fourth row. Seeing the block size of 32 and reading it as "up to 32 machines" is wrong. The limit is 30, after subtracting the 2 network and broadcast addresses.
• If asked for the number of subnets, it is 256 ÷ block. For /26, 256 ÷ 64 = 4; for /27, 8. The number of hosts and the number of subnets are such that when one grows, the other shrinks.

Judging Right and Wrong

Pre-Exam Check

• I can state the ten ports (21, 22, 23, 25, 53, 80, 110, 143, 443, 631) in both directions: number → service, service → number
• From the last value of a mask I can immediately state the block and host count, and conversely choose the mask from a host count
• I can write the first-octet ranges of classes A, B, C, and D and the three private IP ranges
• I can write the 7 OSI layers in order and place hubs, switches, and routers on the layers
• I can put three names each into window manager, display manager, and desktop environment
• I have memorized the GNOME row (GTK+, Mutter, GDM) and the KDE row (Qt, KWin, SDDM) across
• I can state the direction and difference of SMTP, POP3, and IMAP in one sentence each
• I know what each DNS record A, AAAA, MX, CNAME, NS, and PTR points to
• I can contrast NFS and Samba in three boxes: the other OS · protocol · configuration file
• I know the 3 cluster types, full and paravirtualization, Type 1 and 2, and the four facts about VirtualBox (Oracle, GPL, full virtualization, VMDK/VHD)

After finishing the check, proceed like this.

• If you get stuck on even one item, go back to that week's Day 5 (comprehensive review). Each weekly review gathered the tables for that area onto a single page.
• At the exam, solve the questions that ask for values first, quickly, and mark the ones that take effort, such as subnet calculations, to come back to. With 80 questions in 100 minutes, that is a little over a minute per question.

📖 Glossary

• X server : The side with the screen and input devices. It is the computer the user is sitting at.
• Display manager : The program that provides the graphical login screen. Its name ends in DM.
• Subnet mask : The value that separates the network part from the host part.
• MX record : A DNS record that specifies the server that receives a domain's mail.
• Samba : Software that shares files with Windows through SMB/CIFS.
• HA cluster : A cluster configured so the service does not stop even when a failure occurs.