Initial commit: DHCP whitelist service for direct link connections

Features:
- Docker-based DHCP server with MAC address whitelisting
- Binds to specific ethernet interface only
- NO DNS/gateway advertised (direct link only, not a router)
- Configurable network parameters (subnet, DHCP range, lease times)
- Systemd service integration for Arch/Manjaro
- Test environment with isolated network (172.20.0.0/24)
- Auto-configuration script to detect network settings
- Complete Makefile with management targets

Security:
- Only responds to whitelisted MAC addresses
- deny unknown-clients configuration
- Runs in Docker container for isolation

Configuration:
- Copy .example files to create your config
- interface.conf: Network interface to bind to
- whitelist.conf: Allowed MAC addresses
- network.conf: Network parameters (optional)

config/interface.conf.example

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+# Example: Ethernet interface name
+# Find your interface with: ip link show
+eth0

config/network.conf.example

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+# Network configuration for DHCP server
+# These values override automatic detection
+
+# Network subnet (leave empty for auto-detection from interface)
+# Example: SUBNET=192.168.1.0
+SUBNET=
+
+# Netmask in dotted notation (leave empty for auto-detection)
+# Example: NETMASK=255.255.255.0
+NETMASK=
+
+# DHCP range start offset from network base (default: 10)
+# For example, if network is 192.168.1.0, start will be 192.168.1.10
+RANGE_START_OFFSET=10
+
+# DHCP range end offset from network base (default: 100)
+# For example, if network is 192.168.1.0, end will be 192.168.1.100
+RANGE_END_OFFSET=100
+
+# Lease time in seconds (default: 43200 = 12 hours)
+LEASE_TIME=43200
+
+# Max lease time in seconds (default: 86400 = 24 hours)
+MAX_LEASE_TIME=86400

config/whitelist.conf.example

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+# MAC addresses to whitelist (one per line)
+# Format: aa:bb:cc:dd:ee:ff
+# Example:
+# 00:11:22:33:44:55
+# aa:bb:cc:dd:ee:ff