Let's Encrypt配置域名验证总是出错

参照这篇文章http://www.tuicool.com/articles/V3M3ae 配置https，在第三步:配置域名验证的时候总是出错，错误信息如下最后一行：

[root@i001 ssl]# python acme_tiny.py --account-key ./account.key --csr ./domain.csr --acme-dir /var/www/challenges/ > ./signed.crt
Parsing account key...
Parsing CSR...
Registering account...
Already registered!
Verifying www.aa.com...
Traceback (most recent call last):
  File "acme_tiny.py", line 198, in <module>
    main(sys.argv[1:])
  File "acme_tiny.py", line 194, in main
    signed_crt = get_crt(args.account_key, args.csr, args.acme_dir, log=LOGGER, CA=args.ca)
  File "acme_tiny.py", line 123, in get_crt
    wellknown_path, wellknown_url))
ValueError: Wrote file to /var/www/challenges/qIDWXLRTPubp-HR5SLSIMekQuFq86dl1l4K2ESMfuog, but couldn't download http://www.aa.com/.well-known/acme-challenge/qIDWXLRTPubp-HR5SLSIMekQuFq86dl1l4K2ESMfuog