DNS Configuration for Oracle SCAN

Complete Guide to Single Client Access Name Configuration in Oracle RAC

1. SCAN Overview and Architecture
2. Prerequisites and Requirements
3. DNS Server Configuration
4. Creating DNS Records
5. Verification and Testing
6. Grid Infrastructure Integration
7. Troubleshooting
8. Best Practices

1. SCAN Overview and Architecture

Single Client Access Name (SCAN) is a critical component of Oracle Real Application Clusters (RAC) that provides a single network name for clients to access any Oracle database running in a cluster. SCAN eliminates the need for clients to change connection strings when nodes are added to or removed from the cluster.

What is SCAN?

SCAN is a fully qualified domain name (FQDN) registered in DNS that resolves to multiple IP addresses. By default, Oracle recommends configuring three SCAN IP addresses to provide high availability and load balancing for client connections.

Key Benefits

Simplified Client Configuration: Single name for all database connections regardless of cluster size
High Availability: Multiple IP addresses provide redundancy and failover capability
Load Balancing: Automatic distribution of client connections across SCAN listeners
Scalability: Add or remove nodes without updating client connection strings
Location Transparency: Clients remain unaware of cluster topology changes

SCAN Architecture

SCAN Architecture Diagram
Client Applications → DNS Server → SCAN Name (3 IPs) → SCAN Listeners → RAC Nodes

Component	Description	Quantity
SCAN Name	Fully qualified domain name (FQDN)	1 per cluster
SCAN IP Addresses	Virtual IP addresses assigned to SCAN	3 (recommended)
SCAN Listeners	Oracle listeners running on SCAN IPs	3 (one per SCAN IP)
DNS Server	Resolves SCAN name to multiple IPs	1 or more (HA)

SCAN Resolution Process

Client application requests connection to database using SCAN name
DNS server resolves SCAN name to 3 IP addresses (round-robin)
Client attempts connection to first IP address returned by DNS
SCAN listener accepts connection and redirects to appropriate node listener
Node listener establishes database connection on local instance
If SCAN listener unavailable, client tries next IP address automatically

2. Prerequisites and Requirements

Network Requirements

Public network configured and operational on all cluster nodes
Three available IP addresses in the same subnet as public network
IP addresses must not be in use or reserved by other systems
Network switches and routers configured to allow SCAN traffic
Firewall rules permitting traffic on port 1521 (or custom listener port)

DNS Requirements

Critical Requirement:
DNS server must support round-robin resolution for multiple A records with the same hostname. This is essential for SCAN functionality.

DNS server installed and operational (BIND, Microsoft DNS, or other)
Forward lookup zone configured for domain
Reverse lookup zones configured for IP subnets (recommended)
Appropriate permissions to create/modify DNS records
DNS server configured in /etc/resolv.conf on all nodes

Oracle Grid Infrastructure Requirements

Oracle Grid Infrastructure 11.2.0.1 or higher
Three SCAN VIP resources will be created during installation
Three SCAN listener resources will be created automatically
Cluster nodes must have DNS resolution working properly

Pre-Configuration Checklist

Item	Requirement	Verification Command
DNS Resolution	Working DNS on all nodes	`nslookup google.com`
Domain Name	FQDN decided for SCAN	Example: rac-scan.example.com
IP Addresses	3 IPs allocated and available	`ping <ip_address>` (should fail)
Subnet Match	Same subnet as public network	`ifconfig` or `ip addr show`
DNS Server Access	Admin credentials	Login to DNS management console

3. DNS Server Configuration

Supported DNS Servers

Oracle SCAN supports any DNS implementation that follows standard DNS protocols. Common implementations include:

BIND (Berkeley Internet Name Domain): Most common on Linux/Unix systems
Microsoft DNS: Integrated with Windows Active Directory
dnsmasq: Lightweight DNS forwarder for smaller deployments
PowerDNS: High-performance authoritative DNS server

BIND DNS Server Configuration

Install BIND (if not already installed)

# RHEL/Oracle Linux/CentOS
sudo yum install bind bind-utils -y

# Ubuntu/Debian
sudo apt-get install bind9 bind9utils -y

# Verify installation
named -v

Configure BIND Main Configuration File

Edit /etc/named.conf to define zones:

# Edit named.conf
sudo vi /etc/named.conf

# Add or verify these options
options {
    directory "/var/named";
    dump-file "/var/named/data/cache_dump.db";
    statistics-file "/var/named/data/named_stats.txt";
    memstatistics-file "/var/named/data/named_mem_stats.txt";
    allow-query { any; };
    recursion yes;

    # Listen on all interfaces
    listen-on port 53 { any; };
    listen-on-v6 port 53 { any; };

    # Forward requests for external domains
    forwarders {
        8.8.8.8;
        8.8.4.4;
    };
};

# Define forward lookup zone
zone "example.com" IN {
    type master;
    file "example.com.zone";
    allow-update { none; };
};

# Define reverse lookup zone (for 192.168.1.0/24)
zone "1.168.192.in-addr.arpa" IN {
    type master;
    file "192.168.1.rev";
    allow-update { none; };
};

Set Proper Ownership and Permissions

# Set ownership
sudo chown named:named /etc/named.conf

# Set permissions
sudo chmod 640 /etc/named.conf

# Verify configuration syntax
sudo named-checkconf /etc/named.conf

Microsoft DNS Server Configuration

Access DNS Manager

Open Server Manager on Windows Server
Navigate to Tools → DNS
Connect to your DNS server
Expand Forward Lookup Zones

Configure Zone

Right-click Forward Lookup Zones and select New Zone
Select Primary Zone and click Next
Enter zone name (e.g., example.com) and click Next
Create new zone file and click Next
Allow dynamic updates if required or select Do not allow dynamic updates
Click Finish

Round-Robin Configuration: Microsoft DNS enables round-robin by default. Verify this setting in DNS server properties under the Advanced tab. Ensure Enable round robin is checked.

4. Creating DNS Records

Planning SCAN Configuration

Parameter	Example Value	Description
SCAN Name	rac-scan	Hostname portion of FQDN
Domain	example.com	DNS domain name
FQDN	rac-scan.example.com	Complete SCAN name
SCAN IP 1	192.168.1.201	First SCAN IP address
SCAN IP 2	192.168.1.202	Second SCAN IP address
SCAN IP 3	192.168.1.203	Third SCAN IP address

BIND DNS – Forward Zone Configuration

Create Forward Zone File

# Create zone file
sudo vi /var/named/example.com.zone

# Add the following content
$TTL 86400
@   IN  SOA     ns1.example.com. admin.example.com. (
                2024011101  ; Serial (YYYYMMDDNN)
                3600        ; Refresh (1 hour)
                1800        ; Retry (30 minutes)
                604800      ; Expire (1 week)
                86400 )     ; Minimum TTL (1 day)

; Name server records
@           IN  NS      ns1.example.com.
ns1         IN  A       192.168.1.10

; SCAN records (critical for Oracle RAC)
rac-scan    IN  A       192.168.1.201
rac-scan    IN  A       192.168.1.202
rac-scan    IN  A       192.168.1.203

; Individual node records
rac1        IN  A       192.168.1.101
rac2        IN  A       192.168.1.102
rac3        IN  A       192.168.1.103

; VIP records
rac1-vip    IN  A       192.168.1.111
rac2-vip    IN  A       192.168.1.112
rac3-vip    IN  A       192.168.1.113

Important: The three rac-scan A records must have the same hostname but different IP addresses. This is what enables round-robin DNS resolution.

Set Permissions and Verify Zone File

# Set ownership
sudo chown named:named /var/named/example.com.zone

# Set permissions
sudo chmod 640 /var/named/example.com.zone

# Verify zone file syntax
sudo named-checkzone example.com /var/named/example.com.zone

# Expected output
zone example.com/IN: loaded serial 2024011101
OK

BIND DNS – Reverse Zone Configuration

Create Reverse Zone File

# Create reverse zone file
sudo vi /var/named/192.168.1.rev

# Add the following content
$TTL 86400
@   IN  SOA     ns1.example.com. admin.example.com. (
                2024011101  ; Serial
                3600        ; Refresh
                1800        ; Retry
                604800      ; Expire
                86400 )     ; Minimum TTL

; Name server
@           IN  NS      ns1.example.com.

; PTR records for SCAN IPs
201         IN  PTR     rac-scan.example.com.
202         IN  PTR     rac-scan.example.com.
203         IN  PTR     rac-scan.example.com.

; PTR records for nodes
101         IN  PTR     rac1.example.com.
102         IN  PTR     rac2.example.com.
103         IN  PTR     rac3.example.com.

; PTR records for VIPs
111         IN  PTR     rac1-vip.example.com.
112         IN  PTR     rac2-vip.example.com.
113         IN  PTR     rac3-vip.example.com.

Verify Reverse Zone File

# Verify reverse zone syntax
sudo named-checkzone 1.168.192.in-addr.arpa /var/named/192.168.1.rev

# Expected output
zone 1.168.192.in-addr.arpa/IN: loaded serial 2024011101
OK

Microsoft DNS – Creating Records

Add Forward (A) Records for SCAN

Open DNS Manager
Expand Forward Lookup Zones
Right-click your zone (e.g., example.com) and select New Host (A or AAAA)
Enter the following for each SCAN IP:
- Name: rac-scan
- IP address: 192.168.1.201 (then 202, 203 for second and third records)
- Uncheck Create associated pointer (PTR) record (we’ll create separately)
- Click Add Host
Repeat for all three SCAN IP addresses

Add Reverse (PTR) Records

Right-click Reverse Lookup Zones and select New Zone
Create reverse lookup zone for your subnet (e.g., 192.168.1.x)
Right-click the reverse zone and select New Pointer (PTR)
Enter IP address and FQDN for each SCAN IP

Restart DNS Service

BIND on Linux

# Restart named service
sudo systemctl restart named

# Enable automatic start on boot
sudo systemctl enable named

# Check service status
sudo systemctl status named

# Verify DNS is listening on port 53
sudo netstat -tulpn | grep :53
sudo ss -tulpn | grep :53

Microsoft DNS

# Restart DNS service via PowerShell
Restart-Service DNS

# Or via Command Prompt
net stop DNS
net start DNS

# Verify service is running
Get-Service DNS

5. Verification and Testing

Test DNS Resolution from Cluster Nodes

Verify /etc/resolv.conf Configuration

# Check DNS configuration on each node
cat /etc/resolv.conf

# Expected content
search example.com
nameserver 192.168.1.10
nameserver 192.168.1.11

Best Practice: Configure at least two nameservers for redundancy. The search domain should match your DNS zone.

Test Forward Resolution

# Test SCAN name resolution using nslookup
nslookup rac-scan.example.com

# Expected output showing 3 IP addresses
Server:         192.168.1.10
Address:        192.168.1.10#53

Name:   rac-scan.example.com
Address: 192.168.1.201
Name:   rac-scan.example.com
Address: 192.168.1.202
Name:   rac-scan.example.com
Address: 192.168.1.203

Sample nslookup Output:
$ nslookup rac-scan.example.com
Server: 192.168.1.10
Address: 192.168.1.10#53Name: rac-scan.example.com
Address: 192.168.1.201
Name: rac-scan.example.com
Address: 192.168.1.202
Name: rac-scan.example.com
Address: 192.168.1.203

Test Round-Robin Behavior

# Test multiple times to verify round-robin
for i in {1..6}; do
    echo "=== Lookup $i ==="
    nslookup rac-scan.example.com | grep -A 1 "Name:"
    sleep 1
done

Expected Behavior: DNS should return all three IP addresses on each query, but the order may vary due to round-robin rotation. This ensures load distribution across SCAN listeners.

Test Using dig Command

# Test with dig (more detailed output)
dig rac-scan.example.com

# Short answer format
dig +short rac-scan.example.com

# Expected output
192.168.1.201
192.168.1.202
192.168.1.203

Test Reverse Resolution

# Test reverse DNS lookup
nslookup 192.168.1.201

# Expected output
Server:         192.168.1.10
Address:        192.168.1.10#53

201.1.168.192.in-addr.arpa      name = rac-scan.example.com

# Test all three SCAN IPs
nslookup 192.168.1.202
nslookup 192.168.1.203

Verify Network Connectivity

Test IP Availability

# SCAN IPs should NOT respond to ping (not yet assigned)
ping -c 3 192.168.1.201
ping -c 3 192.168.1.202
ping -c 3 192.168.1.203

# If any IP responds, it's already in use - choose different IPs

Critical Check: Before Grid Infrastructure installation, SCAN IP addresses must NOT be assigned to any system. If ping succeeds, the IP is in use and cannot be used for SCAN.

Test Name Resolution from All Nodes

# Execute on each cluster node
ssh rac1 "nslookup rac-scan.example.com"
ssh rac2 "nslookup rac-scan.example.com"
ssh rac3 "nslookup rac-scan.example.com"

# All nodes should return the same three IP addresses

Verification Checklist

Test	Command	Expected Result
DNS Service Running	`systemctl status named`	Active (running)
Forward Resolution	`nslookup rac-scan.example.com`	Returns 3 IP addresses
Reverse Resolution	`nslookup 192.168.1.201`	Returns rac-scan.example.com
SCAN IPs Available	`ping 192.168.1.201`	Destination Host Unreachable
All Nodes Resolve	`ssh node "nslookup ..."`	Consistent results across nodes
resolv.conf Correct	`cat /etc/resolv.conf`	DNS server IP present

6. Grid Infrastructure Integration

SCAN Configuration During Grid Installation

When installing Oracle Grid Infrastructure, you’ll be prompted to specify the SCAN configuration:

Grid Installation Wizard – SCAN Settings

Launch Grid Infrastructure installer: ./gridSetup.sh
Proceed through installation wizard to Cluster Configuration page
Enter SCAN Name: rac-scan.example.com
Installer will verify DNS resolution automatically
Installer creates three SCAN VIP resources
Installer creates three SCAN listener resources

Automatic Verification: Grid Infrastructure installer performs DNS resolution check and validates that SCAN name resolves to exactly three IP addresses. Installation will fail if DNS is not configured correctly.

Post-Installation SCAN Verification

Check SCAN VIP Status

# Check SCAN VIP resources
srvctl config scan

# Expected output
SCAN name: rac-scan.example.com, Network: 1
Subnet IPv4: 192.168.1.0/255.255.255.0/eth0
SCAN 1 IPv4 VIP: 192.168.1.201
SCAN VIP is enabled.
SCAN 2 IPv4 VIP: 192.168.1.202
SCAN VIP is enabled.
SCAN 3 IPv4 VIP: 192.168.1.203
SCAN VIP is enabled.

Check SCAN VIP Runtime Status

# Check if SCAN VIPs are online
srvctl status scan

# Expected output
SCAN VIP scan1 is enabled
SCAN VIP scan1 is running on node rac1
SCAN VIP scan2 is enabled
SCAN VIP scan2 is running on node rac2
SCAN VIP scan3 is enabled
SCAN VIP scan3 is running on node rac3

Check SCAN Listener Status

# Check SCAN listener configuration
srvctl config scan_listener

# Expected output
SCAN Listener LISTENER_SCAN1 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.
SCAN Listener LISTENER_SCAN2 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.
SCAN Listener LISTENER_SCAN3 exists. Port: TCP:1521
Registration invited nodes:
Registration invited subnets:
SCAN Listener is enabled.

# Check runtime status
srvctl status scan_listener

# Expected output
SCAN Listener LISTENER_SCAN1 is enabled
SCAN Listener LISTENER_SCAN1 is running on node rac1
SCAN Listener LISTENER_SCAN2 is enabled
SCAN Listener LISTENER_SCAN2 is running on node rac2
SCAN Listener LISTENER_SCAN3 is enabled
SCAN Listener LISTENER_SCAN3 is running on node rac3

Verify SCAN Listeners Are Accepting Connections

# Test connectivity to SCAN listeners
tnsping rac-scan.example.com

# Expected output
TNS Ping Utility for Linux: Version 19.0.0.0.0 - Production on 11-JAN-2026 10:30:00
Copyright (c) 1997, 2019, Oracle.  All rights reserved.
Used parameter files:
Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION=(CONNECT_DATA=(SERVICE_NAME=))(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.1.201)(PORT=1521)))
OK (10 msec)

Client Connection Configuration

TNS Connection String Using SCAN

# tnsnames.ora entry using SCAN
RACDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac-scan.example.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = racdb.example.com)
    )
  )

JDBC Connection String Using SCAN

jdbc:oracle:thin:@//rac-scan.example.com:1521/racdb.example.com

Connection Benefits: With SCAN, clients only need to know one name. Even if cluster topology changes (nodes added/removed), connection string remains unchanged.

7. Troubleshooting

Common Issues and Solutions

Issue 1: DNS Not Resolving SCAN Name

Symptom: nslookup rac-scan.example.com returns “server can’t find” error

Diagnosis:

# Check DNS server is reachable
ping 192.168.1.10

# Verify resolv.conf
cat /etc/resolv.conf

# Check zone file exists
sudo ls -l /var/named/example.com.zone

# Check DNS logs
sudo tail -50 /var/log/messages | grep named

Solutions:

Verify DNS server IP in /etc/resolv.conf
Ensure DNS service is running: systemctl status named
Check zone file syntax: named-checkzone example.com /var/named/example.com.zone
Restart DNS service: systemctl restart named
Verify firewall allows DNS traffic: firewall-cmd --list-all

Issue 2: Only One or Two IPs Returned

Symptom: nslookup returns fewer than 3 IP addresses for SCAN

Diagnosis:

# Check all A records exist in zone file
sudo grep rac-scan /var/named/example.com.zone

# Should show 3 lines with same hostname
rac-scan    IN  A       192.168.1.201
rac-scan    IN  A       192.168.1.202
rac-scan    IN  A       192.168.1.203

# Verify zone serial number was incremented after changes
sudo grep -i serial /var/named/example.com.zone

Solutions:

Ensure all three A records are present in zone file
Increment zone serial number after any changes
Reload zone: rndc reload example.com
Clear DNS cache on client: systemd-resolve --flush-caches

Issue 3: SCAN IPs Already in Use

Symptom: Grid installation fails with message that SCAN IPs are already assigned

Diagnosis:

# Check if IPs respond to ping
ping -c 3 192.168.1.201

# Check ARP table for MAC address
arp -a | grep 192.168.1.201

# Scan network for device using IP
nmap -sP 192.168.1.201

Solutions:

Choose different IP addresses not in use
Update DNS records with new IP addresses
Coordinate with network team to reserve IPs
Document SCAN IP allocation to prevent future conflicts

Issue 4: Reverse DNS Not Working

Symptom: nslookup <ip_address> fails or returns wrong name

Diagnosis:

# Test reverse lookup
nslookup 192.168.1.201

# Check reverse zone file exists
sudo ls -l /var/named/192.168.1.rev

# Verify PTR records
sudo grep "201\|202\|203" /var/named/192.168.1.rev

Solutions:

Create reverse zone if missing
Add PTR records for all three SCAN IPs
Verify reverse zone name matches subnet correctly
Increment serial number and reload zone

Issue 5: Grid Installation Cannot Validate SCAN

Symptom: Grid installer shows “SCAN name resolution failed” error

Diagnosis:

# Test from grid installation user
su - grid
nslookup rac-scan.example.com

# Verify hostname resolution
hostname
hostname -f

# Check /etc/hosts for SCAN (should NOT be there)
grep rac-scan /etc/hosts

Solutions:

Remove any SCAN entries from /etc/hosts on all nodes
Ensure DNS returns exactly 3 IP addresses
Verify all cluster nodes can resolve SCAN
Check network connectivity between nodes and DNS server
Use fully qualified domain name (FQDN) for SCAN

Diagnostic Commands Reference

Command	Purpose
`nslookup <hostname>`	Test DNS forward resolution
`dig <hostname>`	Detailed DNS query information
`host <hostname>`	Simple DNS lookup utility
`named-checkconf`	Verify named.conf syntax
`named-checkzone`	Verify zone file syntax
`rndc reload`	Reload DNS zones without restart
`srvctl config scan`	Display SCAN configuration
`srvctl status scan`	Check SCAN VIP runtime status

8. Best Practices

DNS Server Configuration

High Availability

Deploy at least two DNS servers for redundancy
Configure DNS servers as primary and secondary in /etc/resolv.conf
Use DNS server replication or zone transfers for synchronization
Monitor DNS server health and performance regularly

Security

Restrict zone transfers to authorized servers only
Use TSIG (Transaction Signature) for authenticated zone transfers
Limit recursive queries to internal networks
Regularly update DNS software to patch security vulnerabilities
Enable DNS query logging for audit purposes

Performance

Set appropriate TTL (Time To Live) values (recommended: 3600 seconds)
Enable DNS caching on client systems
Monitor DNS query response times
Place DNS servers on same network segment as cluster for low latency

SCAN Configuration

IP Address Planning

Reserve SCAN IP addresses in IPAM (IP Address Management) system
Document SCAN IP allocation in network documentation
Choose IP addresses from same subnet as public network
Avoid using DHCP for SCAN IP addresses
Use consecutive IP addresses for easier management (optional)

Naming Conventions

Use descriptive SCAN names: <cluster_name>-scan.<domain>
Keep SCAN names short and meaningful
Follow organizational naming standards
Document SCAN name in cluster configuration records

Testing

Test DNS resolution from all cluster nodes before Grid installation
Verify round-robin behavior by querying multiple times
Test both forward and reverse DNS resolution
Document DNS test results in installation checklist
Perform periodic DNS health checks post-installation

Maintenance Procedures

Adding New SCAN IPs

If you need to change SCAN configuration after Grid installation:

# Add new A records to DNS zone file
sudo vi /var/named/example.com.zone

# Increment serial number
# Add or modify rac-scan A records

# Reload DNS zone
sudo rndc reload example.com

# Modify SCAN configuration in Grid
srvctl modify scan -n rac-scan.example.com

# Restart SCAN resources
srvctl stop scan
srvctl start scan

# Verify new configuration
srvctl config scan
srvctl status scan

DNS Zone File Updates

Follow this workflow for any zone file changes:

Backup current zone file: cp /var/named/example.com.zone /var/named/example.com.zone.bak
Edit zone file with required changes
Increment serial number (critical!)
Verify syntax: named-checkzone example.com /var/named/example.com.zone
Reload zone: rndc reload example.com
Test changes: nslookup rac-scan.example.com
Document changes in change management system

Monitoring

Monitor DNS server availability using monitoring tools (Nagios, Zabbix, etc.)
Set up alerts for DNS resolution failures
Monitor SCAN VIP and listener status regularly
Review DNS logs periodically for errors or unusual activity
Perform quarterly DNS resolution tests from all cluster nodes

Documentation

Maintain comprehensive documentation including:

DNS server IP addresses and hostnames
SCAN name and associated IP addresses
DNS zone file locations and backup procedures
Contact information for DNS administrators
Troubleshooting procedures and known issues
Change history for SCAN configuration

Common Pitfalls to Avoid

Critical Mistakes to Avoid:

Adding SCAN entries to /etc/hosts (use DNS only)
Using single IP address for SCAN (always use 3)
Forgetting to increment DNS zone serial number after changes
Using SCAN IP addresses from different subnet than public network
Not testing DNS resolution before Grid installation
Disabling DNS round-robin feature
Using DHCP-assigned IP addresses for SCAN
Not configuring reverse DNS (PTR records)

Pre-Installation Validation Script

Use this script to validate DNS configuration before Grid installation:

#!/bin/bash
# SCAN DNS Validation Script

SCAN_NAME="rac-scan.example.com"
EXPECTED_IPS=("192.168.1.201" "192.168.1.202" "192.168.1.203")

echo "=== SCAN DNS Configuration Validation ==="
echo

# Test 1: Check DNS resolution
echo "Test 1: DNS Forward Resolution"
IPS=$(nslookup $SCAN_NAME | grep -A 10 "Name:" | grep "Address:" | awk '{print $2}' | sort)
COUNT=$(echo "$IPS" | wc -l)

if [ $COUNT -eq 3 ]; then
    echo "✓ PASS: SCAN resolves to 3 IP addresses"
else
    echo "✗ FAIL: SCAN resolves to $COUNT IP addresses (expected 3)"
fi
echo

# Test 2: Check IP availability
echo "Test 2: SCAN IP Availability"
for ip in "${EXPECTED_IPS[@]}"; do
    ping -c 1 -W 1 $ip &>/dev/null
    if [ $? -ne 0 ]; then
        echo "✓ PASS: $ip is not in use"
    else
        echo "✗ FAIL: $ip is already in use"
    fi
done
echo

# Test 3: Check reverse DNS
echo "Test 3: Reverse DNS Resolution"
for ip in "${EXPECTED_IPS[@]}"; do
    REVERSE=$(nslookup $ip | grep "name =" | awk '{print $4}')
    if [[ $REVERSE == $SCAN_NAME* ]]; then
        echo "✓ PASS: $ip resolves to $REVERSE"
    else
        echo "✗ FAIL: $ip does not resolve correctly"
    fi
done
echo

# Test 4: Check /etc/hosts
echo "Test 4: Verify SCAN not in /etc/hosts"
grep -q "rac-scan" /etc/hosts
if [ $? -ne 0 ]; then
    echo "✓ PASS: SCAN not found in /etc/hosts"
else
    echo "✗ FAIL: SCAN found in /etc/hosts (remove it!)"
fi
echo

echo "=== Validation Complete ==="

Implementation Checklist:

✓ DNS server installed and configured
✓ Forward and reverse zones created
✓ Three A records created for SCAN name
✓ PTR records created for SCAN IPs
✓ DNS service running and enabled
✓ All nodes configured to use DNS server
✓ SCAN resolves to exactly 3 IP addresses
✓ SCAN IPs not responding to ping
✓ Reverse DNS working for all SCAN IPs
✓ No SCAN entries in /etc/hosts
✓ Documentation updated
✓ Ready for Grid Infrastructure installation

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