DNS & Networking

Debug DNS resolution, network connectivity, and HTTP issues. Covers dig/nslookup, port testing, firewall rules, curl diagnostics, /etc/hosts, proxy configuration, and certificate troubleshooting.

When to Use

• DNS name not resolving or resolving to wrong IP
• Connection refused / connection timed out errors
• Diagnosing firewall or security group rules
• HTTP requests failing for unclear reasons
• Proxy configuration issues
• SSL/TLS certificate errors
• Testing connectivity between services

DNS Debugging

Query DNS records

# A record (IP address)
dig example.com
dig +short example.com

# Specific record types
dig example.com MX        # Mail servers
dig example.com CNAME     # Aliases
dig example.com TXT       # Text records (SPF, DKIM, etc.)
dig example.com NS        # Name servers
dig example.com AAAA      # IPv6 address
dig example.com SOA       # Start of Authority

# Query a specific DNS server
dig @8.8.8.8 example.com
dig @1.1.1.1 example.com

# Trace the full resolution path
dig +trace example.com

# Reverse lookup (IP → hostname)
dig -x 93.184.216.34

# nslookup (simpler, works everywhere)
nslookup example.com
nslookup example.com 8.8.8.8    # Query specific server
nslookup -type=MX example.com

# host (simplest)
host example.com
host -t MX example.com

Check DNS propagation

# Query multiple public DNS servers
for dns in 8.8.8.8 1.1.1.1 9.9.9.9 208.67.222.222; do
    echo -n "$dns: "
    dig +short @"$dns" example.com
done

# Check TTL (time to live)
dig example.com | grep -E '^\S+\s+\d+\s+IN\s+A'
# The number is TTL in seconds

Local DNS issues

# Check /etc/resolv.conf (which DNS server the system uses)
cat /etc/resolv.conf

# Check /etc/hosts (local overrides)
cat /etc/hosts

# Flush DNS cache
# macOS:
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder
# Linux (systemd-resolved):
sudo systemd-resolve --flush-caches
# Windows:
ipconfig /flushdns

# Check if systemd-resolved is running (Linux)
resolvectl status

/etc/hosts patterns

# /etc/hosts — local DNS overrides (no TTL, instant)

# Point a domain to localhost (for development)
127.0.0.1    myapp.local
127.0.0.1    api.myapp.local

# Block a domain
0.0.0.0      ads.example.com

# Test a migration (point domain to new server before DNS change)
203.0.113.50    example.com
203.0.113.50    www.example.com

# Multiple names for one IP
192.168.1.100   db.local redis.local cache.local

Port and Connectivity Testing

Test if a port is open

# nc (netcat) — most reliable
nc -zv example.com 443
nc -zv -w 5 example.com 80    # 5 second timeout

# Test multiple ports
for port in 22 80 443 5432 6379; do
    nc -zv -w 2 example.com $port 2>&1
done

# /dev/tcp (bash built-in, no extra tools needed)
timeout 3 bash -c 'echo > /dev/tcp/example.com/443' && echo "Open" || echo "Closed"

# curl (also tests HTTP)
curl -sI -o /dev/null -w "%{http_code}" https://example.com