Networking Basics and DNS

Networking Basics

IP Addressing

• Command to Add IP Address:

  • To add an IP address to an interface use:
    bash ip addr add 192.168.1.10/24 dev eth0

  • Example:
    bash ip addr add 192.168.1.11/24 dev eth0

• Testing Connectivity:

  • Use ping to test if the host is reachable:
    bash ping 192.168.1.11

  • Expected output example:
    plaintext Reply from 192.168.1.11: bytes=32 time=4ms TTL=117

Network Interface Details

• Command for Interface Info:

  • To view the state of network interfaces:
    bash ip link show eth0

• Output Breakdown:

  • Example output:
    plaintext eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP

Routing Basics

• Routing Example:

  • For routing between two networks (192.168.1.0 and 192.168.2.0):
    plaintext 192.168.1.10 <--> 192.168.1.11 <--> 192.168.2.10

• Add Route Command:

  • To add a route for a network:
    bash ip route add 192.168.2.0/24 via 192.168.1.1

Default Gateway Configuration

• Setting Up Default Gateway:

  • Command to set the default route through a gateway:
    bash ip route add default via 192.168.2.1

• Kernel IP Routing Table:

  • Example output of routing table:
    plaintext Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 192.168.2.1 255.255.255.0 UG 0 0 0 eth0

Network Address Translation (NAT)

• IP Forwarding:

  • Enabling IP forwarding is crucial for routing traffic between networks:
    bash echo 1 > /proc/sys/net/ipv4/ip_forward

• Check IP Forwarding Status: bash cat /proc/sys/net/ipv4/ip_forward

  • Expected output if enabled: 1

  • If disabled, use application configuration files to enable it permanently.

Domain Name System (DNS) Basics

• Testing Name Resolution:

  • Ping using hostname instead of IP:
    bash ping db

• Adding Entries to Hosts File:

  • To resolve local hostnames, edit /etc/hosts:
    bash echo '192.168.1.11 db' >> /etc/hosts

• Using DNS for Name Resolution:

  • Check DNS resolution using:
    bash nslookup www.google.com

  • Expected output includes server address and resolved IP address.

Basic Commands Summary

• Add IP address:

  • ip addr add

• Add route:

  • ip route add

• View network interfaces:

  • ip link show

• Ping:

  • ping [IP/Hostname]

• Enable IP forwarding:

  • echo 1 > /proc/sys/net/ipv4/ip_forward

Important Tools

• nslookup: For querying the DNS to obtain domain name or IP address mapping.

• dig: Providing detailed DNS query results.

Domain Names Structure

• Components of a Domain Name:

  • Subdomain: The prefix before the main domain (e.g., www).

  • Domain Name: The registered domain (e.g., example.com).

  • Top Level Domain (TLD): Last part of a domain name (e.g., .com, .org).

• Example:
  ```plaintext
  www.google.com =
  www (subdomain)
  google (domain)
  com (TLD)

- **Resource Records Types:**  
  - **A Record**: Maps a hostname to an IPv4 address.
  - **AAAA Record**: Maps a hostname to an IPv6 address.
  - **CNAME Record**: Alias of one domain to another, e.g., food.web-server to hungry.web-server.

## DNS Configuration Best Practices
- Set up local DNS servers to handle internal requests efficiently.
- Maintain an accurate `/etc/resolv.conf` file for proper DNS resolution.  

  - Example content of `/etc/resolv.conf`:  

plaintext
nameserver 192.168.1.100

- Use `ping` to verify hostname resolution both before and after configuration adjustments.  

## Summary of Domain Names
- Example domains to understand hierarchy:  
  - www.google.com  
  - www.facebook.com  
  - www.un.org
- **Scripting Examples:**  

bash
cat >> /etc/hosts <<EOF
192.168.1.10 web
192.168.1.11 db
EOF
```