When dealing with UPS-protected Linux servers, we often configure them to gracefully shut down during prolonged power outages. The common APC UPS setup uses USB communication with apcupsd for this purpose. However, there's a gap in functionality when mains power returns before UPS battery depletion - the server remains offline despite available power.

The typical BIOS "AC Power Recovery" setting works for direct power loss scenarios but fails here because:

• The UPS maintains continuous power to servers
• No actual power interruption occurs to trigger auto-wake
• Wake-on-LAN (WoL) isn't automatically sent after shutdown

Here's a reliable approach using a Raspberry Pi as a power monitoring device:

# Raspberry Pi power monitor script (Python)
import RPi.GPIO as GPIO
import subprocess
import time

GPIO.setmode(GPIO.BCM)
POWER_PIN = 17  # GPIO pin connected to mains power sensor

def send_wol(mac):
    subprocess.call(['wakeonlan', mac])

def main():
    GPIO.setup(POWER_PIN, GPIO.IN)
    last_state = GPIO.input(POWER_PIN)
    
    while True:
        current_state = GPIO.input(POWER_PIN)
        if last_state == 0 and current_state == 1:  # Power restored
            send_wol('00:11:22:33:44:55')  # Replace with server MAC
        last_state = current_state
        time.sleep(5)

if __name__ == "__main__":
    main()

Hardware Requirements:

• Raspberry Pi (any model with GPIO)
• AC power sensor (like HiLetgo ACS712)
• Network connection to target servers

Configuration Steps:

1. Install wakeonlan on the Pi: sudo apt install wakeonlan
2. Enable WoL on target servers: ethtool -s eth0 wol g
3. Configure apcupsd to not shutdown for brief outages

For environments without spare hardware, consider these options:

# Cron job on a always-on device to ping UPS and wake servers
#!/bin/bash
UPS_IP="192.168.1.100"
SERVER_MAC="00:11:22:33:44:55"

if ping -c 1 $UPS_IP &> /dev/null; then
    # UPS is back online
    wakeonlan $SERVER_MAC
fi

Common issues and fixes:

• WoL not working: Verify NIC settings with ethtool eth0
• False triggers: Add debounce logic to power monitoring
• Network issues: Use static ARP entries for target servers

Power Grid → [Monitoring Device] → Network Switch → [Target Servers]
                 ↑
UPS Bypass Line →'

import RPi.GPIO as GPIO
import subprocess
import time

POWER_PIN = 17
SERVERS = ["192.168.1.10", "192.168.1.11"]
MAC_ADDRESSES = ["00:11:22:33:44:55", "00:11:22:33:44:56"]

GPIO.setmode(GPIO.BCM)
GPIO.setup(POWER_PIN, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

def send_wol(mac):
    subprocess.call(["wakeonlan", mac])

while True:
    if GPIO.input(POWER_PIN):
        for i, server in enumerate(SERVERS):
            # Verify server is offline before sending WoL
            response = os.system(f"ping -c 1 {server}")
            if response != 0:
                send_wol(MAC_ADDRESSES[i])
    time.sleep(60)

#!/bin/bash
case $1 in
    "powerback")
        /usr/bin/wakeonlan 00:11:22:33:44:55
        ;;
esac

# Set chassis policy to always-on
ipmitool chassis policy always-on

# Power on via IPMI when power restored
ipmitool -H 192.168.1.10 -U admin -P password power on