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Janak Shrestha
Janak Shrestha

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DNS Troubleshooting

#linux

The system admins team of xFusionCorp Industries has noticed intermittent issues with DNS resolution in several apps . App Server 2 in Stratos Datacenter is having some DNS resolution issues, so we want to add some additional DNS nameservers on this server.

As a temporary fix we have decided to go with Google public DNS (ipv4). Please make appropriate changes on this server.

Solution

Step 1: Connect to App Server 2 (stapp02) 

ssh steve@stapp02
# Password: Am3ric@
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Step 2: Switch to root 

sudo su -
# Password: Am3ric@
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Step 3: Check current DNS configuration 

cat /etc/resolv.conf
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Step 4: Add Google Public DNS nameservers

Google Public DNS IPv4 addresses:

  • 8.8.8.8 (Primary)
  • 8.8.4.4 (Secondary) 
# Add Google DNS servers to resolv.conf
echo "nameserver 8.8.8.8" >> /etc/resolv.conf
echo "nameserver 8.8.4.4" >> /etc/resolv.conf
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Step 5: Verify the configuration 

cat /etc/resolv.conf
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Step 6: Test DNS resolution 

# Test with ping
ping -c 2 google.com

# Test with nslookup
nslookup google.com

# Test with dig
dig google.com
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One-Line Commands

From jump host with password: 

echo 'Am3ric@' | ssh steve@stapp02 "sudo -S bash -c 'echo \"nameserver 8.8.8.8\" >> /etc/resolv.conf && echo \"nameserver 8.8.4.4\" >> /etc/resolv.conf && cat /etc/resolv.conf'"
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Using heredoc (Recommended): 

ssh steve@stapp02 << 'EOF'
echo 'Am3ric@' | sudo -S bash -c '
echo "=== Current DNS configuration ==="
cat /etc/resolv.conf

echo ""
echo "=== Adding Google Public DNS ==="
echo "nameserver 8.8.8.8" >> /etc/resolv.conf
echo "nameserver 8.8.4.4" >> /etc/resolv.conf

echo ""
echo "=== Updated DNS configuration ==="
cat /etc/resolv.conf

echo ""
echo "=== Testing DNS resolution ==="
ping -c 2 google.com
'
EOF
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Step-by-Step Interactive Commands 

# Connect to stapp02
ssh steve@stapp02
# Password: Am3ric@

# Become root
sudo su -
# Password: Am3ric@

# Step 1: Check current DNS configuration
echo "=== Current DNS configuration ==="
cat /etc/resolv.conf

# Step 2: Backup existing resolv.conf (optional but recommended)
echo "=== Backing up resolv.conf ==="
cp /etc/resolv.conf /etc/resolv.conf.backup

# Step 3: Add Google DNS servers
echo "=== Adding Google Public DNS ==="
echo "nameserver 8.8.8.8" >> /etc/resolv.conf
echo "nameserver 8.8.4.4" >> /etc/resolv.conf

# Step 4: Verify the changes
echo "=== Updated DNS configuration ==="
cat /etc/resolv.conf

# Step 5: Test DNS resolution
echo "=== Testing DNS resolution ==="
ping -c 2 google.com
nslookup google.com

# Exit
exit
exit
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Expected Output 

[root@stapp02 ~]# cat /etc/resolv.conf
nameserver 10.0.0.2

[root@stapp02 ~]# echo "nameserver 8.8.8.8" >> /etc/resolv.conf
[root@stapp02 ~]# echo "nameserver 8.8.4.4" >> /etc/resolv.conf

[root@stapp02 ~]# cat /etc/resolv.conf
nameserver 10.0.0.2
nameserver 8.8.8.8
nameserver 8.8.4.4

[root@stapp02 ~]# ping -c 2 google.com
PING google.com (142.250.192.46) 56(84) bytes of data.
64 bytes from 142.250.192.46: icmp_seq=1 ttl=115 time=10.2 ms
64 bytes from 142.250.192.46: icmp_seq=2 ttl=115 time=10.5 ms

--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms

[root@stapp02 ~]# nslookup google.com
Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   google.com
Address: 142.250.192.46
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Alternative Methods

Method 1: Using tee command 

echo "nameserver 8.8.8.8" | sudo tee -a /etc/resolv.conf
echo "nameserver 8.8.4.4" | sudo tee -a /etc/resolv.conf
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Method 2: Using cat with heredoc 

cat >> /etc/resolv.conf << EOF
nameserver 8.8.8.8
nameserver 8.8.4.4
EOF
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Method 3: Using sed to replace existing nameservers 

sed -i 's/^nameserver.*/nameserver 8.8.8.8/' /etc/resolv.conf
echo "nameserver 8.8.4.4" >> /etc/resolv.conf
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Method 4: Using nmcli (NetworkManager) 

# Check connection name
nmcli connection show

# Add DNS servers
nmcli connection modify <connection-name> ipv4.dns "8.8.8.8 8.8.4.4"
nmcli connection up <connection-name>
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Method 5: Using nmtui (Text User Interface) 

nmtui
# Navigate to Edit a connection
# Select the connection
# Add DNS servers
# Save and exit
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Verification Commands 

# 1. Check resolv.conf
cat /etc/resolv.conf

# 2. Test DNS resolution with ping
ping -c 2 google.com

# 3. Test with nslookup
nslookup google.com

# 4. Test with dig
dig google.com

# 5. Test with host
host google.com

# 6. Check DNS resolution order
getent hosts google.com

# 7. Check if DNS servers are reachable
ping -c 2 8.8.8.8
ping -c 2 8.8.4.4
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Understanding Google Public DNS

Google Public DNS IPv4 Addresses

Address Description
8.8.8.8 Primary DNS
8.8.4.4 Secondary DNS

Google Public DNS IPv6 Addresses

Address Description
2001:4860:4860::8888 Primary DNS
2001:4860:4860::8844 Secondary DNS

Why Use Google Public DNS

  • Reliability: Highly available and globally distributed
  • Speed: Fast resolution times
  • Security: DNSSEC validation and protection against DNS poisoning
  • Privacy: No logging of personal information
  • Free: Available to everyone at no cost

Important Notes

resolv.conf May Be Overwritten

On systems using NetworkManager or DHCP, /etc/resolv.conf may be automatically overwritten. To make changes permanent, consider these approaches.

Option 1: Use NetworkManager

nmcli connection modify <connection-name> ipv4.dns "8.8.8.8 8.8.4.4"
nmcli connection modify <connection-name> ipv4.ignore-auto-dns yes
nmcli connection up <connection-name>
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Option 2: Make resolv.conf immutable

chattr +i /etc/resolv.conf
# To undo: chattr -i /etc/resolv.conf
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Option 3: Configure in network interface file

# For RHEL/CentOS
vi /etc/sysconfig/network-scripts/ifcfg-<interface>
# Add: DNS1=8.8.8.8
# Add: DNS2=8.8.4.4
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Troubleshooting

  1. "Permission denied" when editing resolv.conf: 
   # Use sudo or become root
   sudo su -
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  1. DNS still not resolving: 
   # Check if DNS servers are reachable
   ping -c 2 8.8.8.8

   # Check network connectivity
   ping -c 2 8.8.4.4

   # Restart network service
   systemctl restart NetworkManager
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  1. resolv.conf changes not persisting: 
   # Check if NetworkManager is managing resolv.conf
   ls -la /etc/resolv.conf

   # If symlink to NetworkManager, configure via nmcli
   nmcli connection modify <connection> ipv4.dns "8.8.8.8 8.8.4.4"
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  1. DNS resolution slow: 
   # Check which DNS server is being used
   nslookup google.com

   # Check DNS response time
   dig google.com | grep "Query time"
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  1. Cannot reach Google DNS: 
   # Check firewall rules
   firewall-cmd --list-all

   # Allow DNS traffic
   firewall-cmd --add-service=dns --permanent
   firewall-cmd --reload
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Complete Script (Run from Jump Host) 

#!/bin/bash

echo "========================================="
echo "Configuring Google Public DNS"
echo "========================================="

ssh steve@stapp02 << 'EOF'
echo 'Am3ric@' | sudo -S bash -c '
echo ""
echo "=== Current DNS Configuration ==="
cat /etc/resolv.conf

echo ""
echo "=== Backing up resolv.conf ==="
cp /etc/resolv.conf /etc/resolv.conf.backup

echo ""
echo "=== Adding Google Public DNS ==="
echo "nameserver 8.8.8.8" >> /etc/resolv.conf
echo "nameserver 8.8.4.4" >> /etc/resolv.conf

echo ""
echo "=== Updated DNS Configuration ==="
cat /etc/resolv.conf

echo ""
echo "=== Testing DNS Resolution ==="
ping -c 2 google.com

echo ""
echo "========================================="
echo "✅ Google Public DNS configured successfully!"
echo "📌 Primary DNS: 8.8.8.8"
echo "📌 Secondary DNS: 8.8.4.4"
echo "========================================="
'
EOF
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Summary

  • ✅ Google Public DNS added: 8.8.8.8 and 8.8.4.4
  • ✅ Configuration file: /etc/resolv.conf
  • ✅ DNS resolution tested: Working correctly
  • ✅ Backup created: /etc/resolv.conf.backup

The DNS configuration on App Server 2 has been updated to use Google Public DNS as a temporary fix for the intermittent DNS resolution issues.

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