xuhuigithub opened a new issue #2666:
URL: https://github.com/apache/apisix/issues/2666
### Issue description
使用HTTP1.0协议不携带`Host`头部时apisix会返回[400状态码](https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/Host)，在HTTP1.0的标准定义中Host头部是可以缺失的[（定义中没有提到Host头部）](https://tools.ietf.org/html/rfc1945#section-10)，apisix
因为配置文件后端代理默认使用的http1.1协议的问题，在装配负载均衡请求时，[也使用了http的Host头部，链接是apisix的nginx配置模板](https://github.com/apache/apisix/blob/53a8f5a2ed85e4bd00e5889e7d1ab7f7e3228d98/apisix/cli/ngx_tpl.lua#L382)。在负载均衡HTTP1.0到HTTP1.1之间的转换，缺失了Host头部缺失的判断逻辑。
### Environment
* apisix version (cmd: `apisix version`): 1.4
* OS: Centos 7.2
### Minimal test code / Steps to reproduce the issue
1. curl -XGET -I -H 'Host:' -0 http://apisix1:9000/xx2
### What's the actual result? (including assertion message & call stack if
applicable)
```bash
(base) xuhui@g:~$ curl -XGET -I -H 'Host:' -0 http://apisix1:9000/xx2
HTTP/1.1 400
Connection: close
Date: Mon, 09 Nov 2020 07:03:54 GMT
Server: APISIX web server
```
access日志打出："GET /xx2 HTTP/1.0" 400 0 0.001 "-" "curl/7.65.3" "-"
127.0.0.1:8088 400 0.001
### What's the expected result?
```bash
curl -XGET -I -H 'Host:' -0 http://apisix1:9000/xx2
````
返回200，且Host头部不应该缺失，而是转换为后端名称。详情的话可以看一下HTTP1.1的[RFC](https://tools.ietf.org/html/rfc7230#section-5.4)。
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