from datetime import datetime


def pcap_filename(duration: int) -> str:
    """Generates a pcap filename using datetime of the capture start.
    
    The datetime is UTC, and DDDDD is the duration in seconds.

    Returns:
        A string formatted as `capture_YYYYmmddTHHMMSS_DDDDD.pcap`.

    """
    dt = datetime.utcnow().isoformat().replace('-', '').replace(':', '')[0:15]
    filename = f'capture_{dt}_{duration}.pcap'
    return filename