DNS Service (BIND)#

Overview#

DNS translates domain names to IP addresses (forward lookup) and IP addresses to domain names (reverse lookup).

• Forward lookup: ncaecybergames.org → 192.168.8.2
• Reverse lookup: 192.168.8.2 → ncaecybergames.org

Service Name#

• named (not bind)

systemctl status named
sudo systemctl start named
sudo systemctl enable named

Configuration Locations#

Ubuntu#

/etc/bind/
├── named.conf                 # Main config (includes other files)
├── named.conf.options         # Server options
├── named.conf.local           # Local zone definitions
├── named.conf.default-zones   # Default zones (localhost, etc.)
├── db.empty                   # Template file to copy for new zones
├── db.local                   # Localhost zone file
├── db.127                     # Localhost reverse zone
└── zones/                     # Your custom zone files (create this)

CentOS/RHEL#

Configuration may be in a different location - check /etc/named/ or /var/named/

Key Concepts#

Zone Files#

• Forward zone: Maps domain names → IP addresses (A records)
• Reverse zone: Maps IP addresses → domain names (PTR records)

Include Structure#

The main named.conf typically includes other config files:

include "/etc/bind/named.conf.options";
include "/etc/bind/named.conf.local";
include "/etc/bind/named.conf.default-zones";

allow-update Directive#

Controls whether dynamic DNS updates are permitted for a zone:

zone "example.org" IN {
    type master;
    file "/etc/bind/zones/forward.example.org";
    allow-update { none; };   # No dynamic updates allowed
};

For competition: Always use { none; }; - prevents attackers from remotely modifying your DNS records.

Setting Up DNS Zones#

Step 1: Add Zone Definitions#

Edit the zones config file (Ubuntu: named.conf.default-zones):

sudo nano /etc/bind/named.conf.default-zones

Add a forward lookup zone:

zone "ncaecybergames.org" IN {
    type master;
    file "/etc/bind/zones/forward.ncaecybergames.org";
    allow-update { none; };
};

Add a reverse lookup zone:

zone "8.168.192.in-addr.arpa" IN {
    type master;
    file "/etc/bind/zones/reverse.ncaecybergames.org";
    allow-update { none; };
};

Important: For reverse zones, write the network portion of the IP backwards:

• IP: 192.168.8.x → Zone: 8.168.192.in-addr.arpa

Step 2: Create Zone Files Directory#

sudo mkdir /etc/bind/zones

Step 3: Copy Template Files#

Copy the empty template (preserves correct ownership and permissions):

sudo cp /etc/bind/db.empty /etc/bind/zones/forward.ncaecybergames.org
sudo cp /etc/bind/db.empty /etc/bind/zones/reverse.ncaecybergames.org

Why copy instead of create from scratch?

• Preserves correct ownership (root:bind)
• Preserves correct permissions (644)
• If permissions/ownership are wrong, bind can't read the files

Check permissions:

ls -l /etc/bind/zones/

Step 4: Configure Forward Zone File#

sudo nano /etc/bind/zones/forward.ncaecybergames.org

Example forward zone file:

$TTL    604800
@       IN      SOA     ncaecybergames.org. root. (
                              2         ; Serial (INCREMENT THIS ON EVERY CHANGE!)
                         604800         ; Refresh
                          86400         ; Retry
                        2419200         ; Expire
                         604800 )       ; Negative Cache TTL

@       IN      NS      sandbox-ubuntu.
sandbox-ubuntu  IN      A       192.168.8.2
www             IN      A       192.168.8.2

Key points:

• Replace localhost with your domain (ncaecybergames.org.)
• Replace localhost after NS with your server hostname (sandbox-ubuntu.)
• Always increment the serial number when making changes (1→2→3...)
• Add A records for each subdomain

Step 5: Configure Reverse Zone File#

sudo nano /etc/bind/zones/reverse.ncaecybergames.org

Example reverse zone file:

$TTL    604800
@       IN      SOA     ncaecybergames.org. root.ncaecybergames.org. (
                              2         ; Serial (INCREMENT THIS ON EVERY CHANGE!)
                         604800         ; Refresh
                          86400         ; Retry
                        2419200         ; Expire
                         604800 )       ; Negative Cache TTL

@       IN      NS      sandbox-ubuntu.
2       IN      PTR     www.ncaecybergames.org.
2       IN      PTR     sandbox-ubuntu.ncaecybergames.org.

Key points:

• For reverse zones, domain names end with a period (.)
• PTR record uses only the host portion of IP (for 192.168.8.2, just use 2)
• Multiple PTR records can point to the same IP

Start and Test DNS#

Start the Service#

sudo systemctl start named
systemctl status named

If it fails to start, check for typos in your config files (missing semicolons, periods, spaces).

Configure Client to Use Your DNS Server#

Edit /etc/resolv.conf:

sudo nano /etc/resolv.conf

Add your DNS server:

nameserver 192.168.8.2

Or add to netplan (/etc/netplan/*.yaml):

nameservers:
  addresses:
    - 192.168.8.2

Testing DNS#

Using nslookup#

Forward lookup:

nslookup www.ncaecybergames.org

Reverse lookup:

nslookup 192.168.8.2

Using Browser#

Navigate to http://www.ncaecybergames.org - should load your web server.

Troubleshooting#

Service Won't Start#

• Check for typos in zone files (missing semicolons, periods)
• Check file permissions (644) and ownership (root:bind)
• Check config syntax: named-checkconf
• Check zone file syntax: named-checkzone ncaecybergames.org /etc/bind/zones/forward.ncaecybergames.org

DNS Not Resolving#

• Verify service is running: systemctl status named
• Check /etc/resolv.conf has your DNS server listed
• Test with nslookup to isolate DNS vs other issues

Common Mistakes#

• Forgetting to increment serial number after changes
• Missing periods at end of domain names in reverse zone files
• Wrong file permissions/ownership
• Typos in IP addresses or domain names
• Missing semicolons in config files

Quick Reference#