dns-networking

# 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

```bash
# 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

```bash
# 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

```bash
# 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-resol...