Javascript function to decrypt the BIGipServer cookie. Inspired from https://github.com/psmet/BIGip-cookie-decoder and https://www.jardinesoftware.net/2015/09/18/f5-bigip-decode-with-fiddler/

index.js

function decryptBigIpCookie(cookieValue) {
  const [ipPart, portPart] = cookieValue.split('.')

  const hexIp = parseInt(ipPart, 10)
    .toString(16)
    .padStart(8, '0')
  const ip1 = parseInt(hexIp.toString().substring(6, 8), 16)
  const ip2 = parseInt(hexIp.toString().substring(4, 6), 16)
  const ip3 = parseInt(hexIp.toString().substring(2, 4), 16)
  const ip4 = parseInt(hexIp.toString().substring(0, 2), 16)
  const ip = `${ip1}.${ip2}.${ip3}.${ip4}`

  const hexPort = parseInt(portPart, 10)
    .toString(16)
    .padStart(4, '0')
  const port1 = hexPort.toString().substring(2, 4)
  const port2 = hexPort.toString().substring(0, 2)
  const port = parseInt(`${port1}${port2}`, 16)

  return {
    ip,
    port,
  }
}