Read SNI Hostname from ClientHello SNI Header

sni.js

const snireader = (buffer) => {

  let pos = 0;

  if (buffer[pos] !== 0x16) {
    // SNI: Handshake
    return;
  }

  if (buffer[pos = pos + 5] !== 0x01) {
    // SNI: ClientHello
    return;
  }

  // Skip Length
  pos += 3;

  // Skip TLS Version 
  pos += 2;

  // Skip Random 32 bytes
  pos += 32;

  // Session ID Length
  const sessionIDLength = buffer[++pos];
  pos += sessionIDLength;

  // Cipher Suites Length
  pos++;
  const cipherLength = buffer[++pos];
  pos += cipherLength;

  // Compression Mode Length
  const compressionModeLength = buffer[++pos];
  pos += compressionModeLength;

  // Extension Length
  const extensionLength = buffer[++pos] * 0x1FF + buffer[++pos];

  for (let i = pos; (i || pos) < Buffer.byteLength(buffer); i++) {

    // Extension ServerName
    // Server Name
    if (buffer[pos + 1] === 0x00 && buffer[pos + 2] === 0x00) {

      pos += 2;
      
      // ServerName Extension Length
      pos += 2;

      // Server Name List Length
      pos += 2;


      if (buffer[++pos] !== 0x00) {
        // SNI: Hostname
        continue;
      }

      pos += 2;

      const serverNameLength = buffer[pos];
      const serverName = buffer.slice(++pos, pos + serverNameLength);
      console.log('SNI:Server-Name:', serverName.toString());
      return serverName.toString();

    } else {
      
      pos += 2;
      
      // Skip Extension
      const skipLength = buffer[++pos] * 0x1FF + buffer[++pos];

      pos += skipLength;
      
    }

  }
  
  // fail
  return null;
  
}