Bug Report: Handle DNS timeout gracefully in subnet discovery

Bug Report: DNS Timeout Handling

Description

DNS timeout errors during subnet discovery can crash or hang the scan unexpectedly.

Current Behavior

When DNS servers are unreachable or slow during DC subnet discovery:

1. Scan hangs indefinitely
2. No graceful fallback to next strategy
3. User has to manually kill the process (Ctrl+C)

Expected Behavior

1. Timeout after configured timeout value
2. Log warning message
3. Fallback to next detection strategy (LDAP, port fingerprint)
4. Continue scan without interruption

Reproduction Steps

# Simulate with invalid DNS server
adpentest --target 10.0.0.1 \
  --mode dry-run \
  --scope-confirmed \
  --dns-server 192.0.2.1 \
  --dns-timeout 5.0

Expected Output

[WARNING] DNS timeout during SRV query for _ldap._tcp.dc._msdcs.domain.local
[*] Falling back to LDAP RootDSE probe...
[*] Falling back to port fingerprinting...

Technical Details

• Affected Code: detect_dcs() in core.py (lines ~2500-3200)
• Root Cause: No timeout exception handling in DNS SRV queries
• Impact: Critical - can hang indefinitely

Fix Approach

try:
    results = dns.resolver.resolve(qname, rdtype)
    # process results
except dns.exception.Timeout:
    print(f"[WARNING] DNS timeout for {qname}", file=sys.stderr)
    # Fallback to next strategy
except dns.exception.DNSException as e:
    print(f"[WARNING] DNS error: {e}", file=sys.stderr)
    # Fallback to next strategy

Acceptance Criteria

• All DNS operations have timeout handling
• Clear warning messages on timeout
• Fallback to next DC detection method
• No indefinite hangs
• Documentation updated

Priority

High - affects usability in environments with poor DNS connectivity

Start in core.py at detect_dcs() around lines 2500-3200 and reproduce with the provided adpentest command using an unreachable DNS server. Trace all DNS SRV operations and the existing DC detection strategies, then verify timeout warnings, fallback through LDAP and port fingerprinting, continued scanning, and updated documentation against the acceptance criteria.