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Linux Master 2· AssociateWeek 10 · Day 5~20 min read

Day 5 - Week 10 Review: Internet Services on One Page

📌 Key Takeaways

Sending is SMTP (25), receiving is POP3 (110) / IMAP (143). Server-to-server relay is also SMTP.
POP3 downloads and deletes; IMAP leaves mail on the server.
DNS records — A (IPv4) · AAAA (IPv6) · MX (mail) · CNAME (alias) · NS · PTR (reverse).
TELNET and SSH are remote access, while FTP, NFS, and Samba are file sharing. Questions mix them and ask you to pick.
NFS is between Unix systems (RPC), Samba is with Windows (SMB, CIFS, NetBIOS).

Services on One Page

   Web     HTTP 80 · HTTPS 443       Servers: Apache · Nginx
                                     Browsers: Firefox · Chrome · Lynx (text)
 
   Mail    SMTP 25   sending + server-to-server relay
           POP3 110  download (deleted from server)
           IMAP 143  keep on server and view (multiple devices)
           Components  MUA (user) → MTA (transfer) → MDA (delivery)
                 MTA examples: Sendmail · Postfix
           MIME  specification for carrying Korean text and attachments
 
   DNS     53   name ↔ IP
           Hierarchy  root (.) → TLD (com·kr) → second level → host
           Records A (IPv4) AAAA (IPv6) MX (mail) CNAME (alias) NS PTR (reverse) SOA
           Server  BIND (named)   Lookup: nslookup · dig · host
           Files  resolv.conf (where to ask) · hosts (the answer)
 
   Remote  TELNET 23  plain text — dangerous
           SSH    22  encrypted · public key · tunneling (OpenSSH)
 
   Transfer FTP   21 control / 20 data   active vs passive
                  anonymous account anonymous · bin/ascii mode
           scp · sftp   SSH-based        rsync changed parts only
 
   Sharing NFS    between Unix systems · RPC · /etc/exports
           Samba  with Windows · SMB/CIFS/NetBIOS · smb.conf · smbd/nmbd
 
   Other   DHCP automatic IP · NTP time · SNMP management · LDAP accounts
   Legacy  Gopher (menu search) · Usenet (newsgroups, NNTP) · Archie (FTP search) · IRC (chat)

Grouping by Nature

Group	Services
File sharing/transfer	FTP · NFS · Samba · SCP · SFTP · rsync
Remote access	TELNET · SSH · rlogin
Information retrieval	WWW · Gopher · Archie · WAIS
Messages/bulletin boards	Mail · Usenet · IRC
Names/addresses	DNS · DHCP

Questions that mix services and ask you to pick the one with a different nature are the main form this week. Learn the table above as a whole.

Practice: Picking "the Odd One Out"

The same word gives a different answer depending on which other four it appears with. First find what the three have in common, then pick the one that falls outside it.

Four choices	Odd one out	What the three share
SMB · CIFS · NetBIOS · RPC	RPC	Components of Samba (RPC belongs to NFS)
FTP · NFS · Samba · TELNET	TELNET	File sharing/transfer
TELNET · SSH · rlogin · FTP	FTP	Remote access
SMTP · POP3 · IMAP · NNTP	NNTP	Mail protocols (NNTP is for newsgroups)
Sendmail · Postfix · qmail · Thunderbird	Thunderbird	MTAs (Thunderbird is an MUA)
Lynx · Firefox · Chrome · Apache	Apache	Web browsers (Apache is a web server)
A · MX · CNAME · SMTP	SMTP	DNS records
nslookup · dig · host · showmount	showmount	DNS lookup commands
scp · sftp · rsync · ssh-keygen	ssh-keygen	File transfer tools
Gopher · Archie · WAIS · IRC	IRC	Information retrieval (IRC is chat)
HTTPS · SSH · SFTP · TELNET	TELNET	They encrypt

TELNET appeared three times in this table. In the second row it is "because it is not file transfer," in the third row it is actually on the grouped side, and in the last row it is "because it does not encrypt." Do not decide the answer from the word alone; find the grouping criterion first.
The criterion is usually one of four — what it does (access/transfer/search), counterpart (Unix/Windows), security (plain text/encrypted), or kind (protocol/program/record).

Encrypted Pairs

Plain text	Encrypted counterpart	Port change
HTTP	HTTPS	80 → 443
TELNET	SSH	23 → 22
FTP	SFTP (SSH-based) · FTPS (SSL-based)	21 → 22 (SFTP)
`rcp`	`scp`	→ 22
SMTP	SMTPS	25 → 465
POP3	POP3S	110 → 995
IMAP	IMAPS	143 → 993

SSH alone is the alternative to TELNET, FTP, and rcp. ssh, sftp, and scp all use port 22.
Look at where the S is attached in the name. When it comes after, as in HTTPS, POP3S, IMAPS, and FTPS, it is the original protocol wrapped in SSL/TLS, and when it comes before, as in SFTP, it is a separate protocol that runs on top of SSH.

Who Asks and Who Answers — Client and Server

Every service this week is split into the side that requests (client) and the side that waits (server). Sorting which side a program name or file belongs to makes the choices easier to organize.

Service	Server side (daemon · product)	Client side (command · program)
Web	`httpd` — Apache, Nginx	Firefox, Lynx, `wget`, `curl`
Mail	Sendmail, Postfix, Dovecot	Thunderbird, Evolution, `mail`
DNS	`named` — BIND	`nslookup`, `dig`, `host`
Remote access	`sshd` — OpenSSH	`ssh`
File transfer	vsftpd, ProFTPD	`ftp`, `sftp`, `scp`
NFS	`nfsd`, `rpcbind`	`mount -t nfs`, `showmount`
Samba	`smbd`, `nmbd`	`smbclient`, `mount -t cifs`

Names ending in d are daemons, that is, the server side — httpd, named, sshd, nfsd, smbd. The d is the initial of daemon.
Take extra care with cases where the product name and daemon name differ — Apache is httpd, BIND is named, and Samba is smbd and nmbd.

File	Which side it belongs to	What goes in it
`/etc/resolv.conf`	Client	The DNS server to ask
`/etc/hosts`	Client	Name ↔ IP
`~/.ssh/known_hosts`	Client	Keys of servers connected to
`~/.ssh/authorized_keys`	Server	Public keys allowed to log in
`/etc/ssh/sshd_config`	Server	SSH server settings
`/etc/exports`	Server	Directories to export over NFS
`/etc/samba/smb.conf`	Server	Samba share settings

TCP or UDP

Transport layer	Services
TCP	HTTP · HTTPS · SMTP · POP3 · IMAP · SSH · TELNET · FTP · NNTP
UDP	DHCP · NTP · SNMP · TFTP
Both	DNS — usually UDP, TCP when the response is large or for zone transfers

Logging in, receiving files, and exchanging mail must not lose anything, so they use TCP. Most of this week's services fall here.
The UDP side is "short question, short answer." Getting an address (DHCP), setting the time (NTP), and asking about device status (SNMP).

Port Number Summary

Port	Service
20 / 21	FTP data / control
22	SSH
23	TELNET
25	SMTP
53	DNS
80	HTTP
110	POP3
143	IMAP
443	HTTPS
123	NTP
161	SNMP
631	CUPS

Contrasting Confusing Pairs

Pair	A	B	Deciding criterion
SMTP vs POP3	Sending	Receiving	Direction
POP3 vs IMAP	Download and delete	Keep on server	Multi-device sync
MUA vs MTA	User program	Server-to-server transfer	The middle letter
A vs MX	IPv4 address	Mail server	What it points to
A vs AAAA	IPv4	IPv6	Address length
Forward vs reverse	Name → IP (A)	IP → name (PTR)	Direction
TELNET vs SSH	Plain text	Encrypted	Security
TELNET vs FTP	Remote access	File transfer	Purpose
FTP 20 vs 21	Data	Control	What flows
Active vs passive	Server connects	Client connects	Firewall traversal
sftp vs FTPS	SSH-based	FTP + SSL	Underlying protocol
NFS vs Samba	Between Unix systems, RPC	With Windows, SMB	Counterpart OS
DNS vs DHCP	Name resolution	Automatic IP assignment	What it does
`hosts` vs `resolv.conf`	Holds the answers	Server to ask	Role

Commonly Missed Points

"Servers exchange mail with POP3" → It is SMTP. POP3 is used only when a user fetches mail.
"IMAP downloads and deletes mail" → That is POP3. IMAP keeps it on the server.
"MX holds a mail address" → It does not hold an address; it points to the mail server.
"AAAA is IPv4" → It is IPv6. A is IPv4.
"FTP uses only one port" → It uses two: 21 (control) and 20 (data).
"You can transfer files with TELNET" → You cannot. It is for remote access only.
"RPC is included in Samba's components" → RPC belongs to NFS.
"Share with Windows using NFS" → For Windows, use Samba.
"sftp and FTPS are the same" → sftp is SSH, FTPS is FTP+SSL, so their foundations differ.
"Gopher is a chat service" → It is menu-based information retrieval. Chat is IRC.

Choosing a Service from a Situation

In this situation	Choose	Do not choose, and why
A Windows PC and a Linux server share a folder	Samba	NFS — between Unix/Linux systems
Two Linux servers share `/data`	NFS	Samba — when sharing with Windows
Log in to a remote server and type commands	SSH	TELNET — plain text, so dangerous
Safely copy one file to a remote server	`scp`	FTP — does not encrypt
Back up a large directory daily, sending only what changed	`rsync`	`scp` — sends everything every time
View the same mailbox from a phone and a PC	IMAP	POP3 — downloads and deletes from the server
Office PCs get their IPs automatically	DHCP	DNS — only turns names into addresses
Set all servers' clocks to the same time	NTP	NNTP — the newsgroup protocol
View web documents without a graphical environment	Lynx	Firefox — a graphical browser
Check where a domain's mail server is	`dig domain MX`	A record — only gives an IP address

If the sentence has "Windows," think Samba; "encrypted / securely," think the SSH family; "multiple devices," think IMAP. A single clue word decides the answer.

Values That Appear on the Exam As-Is

Value	What it is
25 / 110 / 143	SMTP / POP3 / IMAP
20, 21	FTP data, control
anonymous	Anonymous FTP account name
`/etc/exports`	NFS share config file
`/etc/samba/smb.conf`	Samba config file
`smbd` / `nmbd`	The two Samba daemons
NNTP	Usenet newsgroup protocol

Next week is the last one. We cover application areas (cloud, big data, embedded) and the overall wrap-up.

📖 Glossary

SMTP / POP3 / IMAP : Protocols for sending mail / downloading mail / keeping mail on the server and viewing it.
MX record : A DNS record that specifies the server that receives mail for a domain.
SSH : A remote access protocol that encrypts communication.
FTP active/passive mode : The data connection is opened by the server / opened by the client.
NFS : A file sharing system between Unix/Linux systems.
Samba : Software that shares files with Windows over SMB/CIFS.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

Which of the following is a protocol whose default behavior is to download mail from the server and then delete the original on the server?

Question 2

Which of the following is a service that differs in nature from the other three?

Question 3

Which of the following is the config file that specifies the directories to share on an NFS server?

Question 4

Which of the following is the DNS record used to specify an alias for a domain name?

Question 5

Which of the following is the mode that must be specified to transfer images or compressed files in FTP without corruption?

Question 6

Which of the following is DNS server software widely used on Linux?

Official references

Linux Master 2 official exam syllabus(opens in a new tab)