在HTTP协议中，响应状态码 428 Precondition Required 表示服务器端要求发送条件请求。
一般的，这种情况意味着必要的条件首部——如 If-Match ——的缺失。.
当一个条件首部的值不能匹配服务器端的状态的时候，应答的状态码应该是 412 Precondition Failed，前置条件验证失败。
428 Precondition Required
| Specification | Title |
|---|---|
| RFC 6585, section 3: 428 Precondition Required | Additional HTTP Status Codes |
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