Preface#
After the last DNS error on Ubuntu, the DNS on the Debian server also broke
Why do I have such a connection with DNS issues?
Problem Background#
The Debian server exhibits strange phenomena:
- Using public DNS (like
8.8.8.8or223.5.5.5) cannot resolve domain names - Using a self-built LAN DNS (like
192.168.0.254) resolves normally
Troubleshooting Process#
1. DNS Connectivity Test#
nc -zuv 223.5.5.5 53 # Output: Connection succeeded!
Public DNS's port 53 is reachable
Confused
2. DNS Configuration Check#
cat /etc/resolv.conf
# Output:
nameserver 223.5.5.5
nameserver 8.8.8.8
Quite normal qwq
3. Firewall Rules#
sudo iptables -L -n -v | grep 53
No issues~
4. dig Query#
# UDP Query (failed)
dig @223.5.5.5 aliyun.com +short
# Output: ;; communications error: timed out
# TCP Query (successful!)
dig +tcp @223.5.5.5 aliyun.com +short
# Output: 106.11.253.83 140.205.60.46 ...
UDP DNS queries are intercepted, TCP DNS is normal
5. Network Packet Capture Evidence#
sudo tcpdump -i any port 53 -w dns.pcap
Capture results:
- Only outgoing DNS requests (
A? aliyun.com) - No response packets returned
6. Connection Tracking Verification#
sudo conntrack -L -p udp --dport 53
# Output: 0 flow entries
UDP requests did not establish a connection, response packets were discarded during transmission
Truly unbelievable
Conclusion#
Still not sure what the problem is, maybe it's being intercepted by intermediate network devices?
Docker and similar have also been ruled out
Does anyone know the solution?
Solution#
Force Use of TCP Protocol for Resolution#
# Modify /etc/resolv.conf
echo "options use-vc" | sudo tee -a /etc/resolv.conf
Self-built DNS#
Leaving a gap