mcping

#!/usr/bin/env bash

# mcping - Minecraft server ping written in Bash
# This Bash script uses only `netcat` to ping a Minecraft server. (because my device doesn't support /dev/tcp)
# Unfortunately, netcat is not capable of closing TCP connection when it recieves the data from the server
# so either you Ctrl-C this script or wait for 30 seconds until netcat automatically end itself. (because of "timed out")
# Author: Nguyen Hong Son <cris@crischu07.is-a.dev>

if [ -z "$@" ]; then
	echo "Usage: $0 [<address>] <port> <version>"
	exit 2
fi

ADDR="$1"
if [ ! -z "$2" ]; then
	PORT="$2"
else
	PORT=25565
fi

if [ ! -z "$3" ]; then
	VERSION="$3"
else
	VERSION="-1"
fi

varint_encode() {
	local value=$1

	# handle overflow on negative number
	if (( value < 0 )); then
		value=$(( value & 0xFFFFFFFF ))
	fi

	while true; do
		local byte=$(( value & 0x7F ))
		value=$(( value >> 7 ))
		if [ $value -ne 0 ]; then
			byte=$(( byte | 0x80 ))
		fi
		printf '%b' "$(printf '\\x%02x' $byte)"
		[ $value -eq 0 ] && break
	done
}

ushort_encode() {
	printf "$(printf '\\x%02x\\x%02x' $(( ($1 >> 8) & 0xFF )) $(( $1 & 0xFF )))"
}

# base handshake
{
	# packet id: 0x00
	varint_encode 0
	# encode our client version
	varint_encode "$VERSION"
	# encode server addr, port
	varint_encode "${#ADDR}"
	printf "%s" "$ADDR"
	ushort_encode "$PORT"
	# next state: status (0x01)
	varint_encode 1
} > .body.bin

# main handshake payload
{
	# add the length of the handshake
	varint_encode $(wc -c < .body.bin)
	cat .body.bin
	# status request packet
	varint_encode 1
	varint_encode 0
} > handshake.bin
rm .body.bin

# pipe stderr to /dev/null
# because when netcat timed out, it will warn us
# in the stderr output.
nc -q 30 -w 30 $ADDR $PORT 2>/dev/null < handshake.bin
rm handshake.bin
echo