The DNS and FTP Protocols

The DNS and FTP Protocols

1. The DNS System

DNS Overview

DNS (Domain Name System) is a core service of the Internet, responsible for converting domain names into IP addresses.

The Hierarchical Name Space

The structure of a domain name:

• The root domain: the highest level, written as ”.”
• Top-level domains: country codes (such as .cn, .us) or organisation types (such as .com, .org)
• Second-level domains: the name of an organisation or institution
• Host names: the name of a particular host

Examples of domain names:

• www.example.com
• mail.google.com
• ftp.microsoft.com

Name Servers

Root servers:

• There are 13 root server addresses worldwide
• They hold the information about the top-level domain servers
• They answer queries for the top-level domains

Top-level domain servers:

• They hold the information about the second-level domain servers
• They administer a particular top-level domain
• For example .com, .org, .cn

Authoritative servers:

• They hold the IP addresses of the particular domains
• They administer a particular domain
• They provide the final resolution result

Caching servers:

• They store the results of queries
• They reduce repeated queries
• They speed up resolution

The Name Resolution Process

Recursive query:

1. The client queries the local DNS server
2. The local DNS server queries a root server
3. The root server returns the address of a top-level domain server
4. The local DNS server queries the top-level domain server
5. The top-level domain server returns the address of the authoritative server
6. The local DNS server queries the authoritative server
7. The authoritative server returns the IP address
8. The local DNS server returns the result to the client

In a recursive query the local DNS server carries out the whole chain on the client’s behalf and hands back the final answer; the client makes a single request.

Iterative query:

1. The client queries the local DNS server
2. The local DNS server queries a root server
3. The root server returns the address of a top-level domain server
4. The local DNS server queries the top-level domain server
5. The top-level domain server returns the address of the authoritative server
6. The local DNS server queries the authoritative server
7. The authoritative server returns the IP address
8. The local DNS server returns the result to the client

In an iterative query each server returns a referral — the address of the next server to ask — and the local DNS server walks down the chain itself.

The DNS Protocol

Query types:

• A record: a domain mapped to an IPv4 address
• AAAA record: a domain mapped to an IPv6 address
• CNAME record: an alias for a domain
• MX record: the mail exchange record
• NS record: the name server record

Port number: 53 (UDP/TCP)

2. The FTP Protocol

FTP Overview

FTP (File Transfer Protocol) is the standard protocol used for file transfer.

How FTP Works

The two-connection model:

• The control connection: carries the commands and the responses
• The data connection: carries the file data

Establishing the connections:

1. The client establishes a control connection to the server
2. The server responds to the connection request
3. The client sends its credentials
4. The server verifies the user’s identity
5. A data connection is established for the file transfer

The Control Connection

Functions:

• Carries the FTP commands
• Carries the server’s responses
• Manages the FTP session

Common commands:

• USER: the user name
• PASS: the password
• LIST: list the files
• RETR: download a file
• STOR: upload a file
• QUIT: close the connection

Response codes:

• 200: the command succeeded
• 220: the service is ready
• 331: a user name is needed
• 425: the data connection cannot be opened
• 500: syntax error

The Data Connection

Functions:

• Carries the file data
• Carries the directory listings
• Carries the file information

Transfer modes:

• Active mode: the server connects to the client
• Passive mode: the client connects to the server

Data formats:

• ASCII mode: transfer of text files
• Binary mode: transfer of binary files

Characteristics of FTP

Advantages:

• High reliability: it runs over TCP, which guarantees reliable transfer
• Complete function set: it supports upload, download, deletion, and so on
• Broad compatibility: it runs on many operating systems
• Authentication: it supports user name and password authentication

Disadvantages:

• Weak security: the traffic is plaintext and easily eavesdropped
• Firewall problems: the two-connection model is easily restricted by firewalls
• Low efficiency: the protocol overhead is comparatively large

FTP Variants

SFTP:

• Secure FTP based on SSH
• The transfer is encrypted
• It uses a single connection

FTPS:

• FTP based on SSL/TLS
• The transfer is encrypted
• It keeps the original functions of FTP

3. DNS and FTP Applications

DNS Applications

Name resolution:

• Converts domain names into IP addresses
• Supports IPv4 and IPv6
• Provides load balancing

Mail routing:

• MX records designate the mail servers
• They support a priority order among mail servers
• They provide the mail routing service

Security services:

• DNSSEC provides authenticated verification
• It prevents DNS hijacking
• It keeps the resolution of a name secure

FTP Applications

File transfer:

• Uploading files to a server
• Downloading files from a server
• Deleting files on a server

File management:

• Listing the contents of a directory
• Creating and deleting directories
• Renaming files

Batch operations:

• Uploading many files at once
• Downloading many files at once
• Automated scripts

Exercises

Exercise 1

What are the main steps in the DNS name resolution process?

Exercise 2

Why does FTP distinguish between the control connection and the data connection?

---

Glossary