網絡基本功（二十八）：Wireshark抓包實例分析HTTP問題(下)
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Client errors:
| Code | Status | Explanation | What to do |
400 | Bad request | The request could not be understood by the server due to a syntax problem. The request should be modified by the client before resending to it. | Check the website address. This can also happen due to a site error. |
401 | Authorization required | The client is denied access due to the lack of authentication codes. | Check your username and password. |
402 | Payment required | Reserved for future use. | |
403 | Forbidden | The client is not allowed to see a specific file. This can be due to the server access limit. | Check the credentials. Also, there are fewer chances that the server is loaded. |
404 | Not found | The requested resource could not be found. | This can be because the resource was deleted, or it never existed before. It can also be due to URL misspellings. |
405 | Method not allowed | The method you are using to access the file is not supported or not allowed by the resource. | |
406 | Not acceptable | Content generated by the resource is not acceptable according to the client request. | Check/update your browser. |
407 | Proxy authentication required | Request authentication is required before it can be performed. | The client must first authenticate itself with the proxy. |
408 | Request timed out | It took the server longer than the allowed time to process the request. | Check response time and load on the network. |
409 | Conflict | The request submitted by the client cannot be completed because it conflicts with some established rules. | Can be because you try to upload a file that is older that the existing one or similar problems. Check what the client is trying to do. |
410 | Gone | The URL requested by the client is no longer available from that system. | Usually this is a server problem. It can be due to a file that was deleted or location was forwarded to a new location. |
411 | Content length required | The request is missing itsContent-Length header. | Compatibility issue on a website. Change/update your browser. |
412 | Precondition failed | The client has not set up a configuration that is required for the file to be delivered. | Compatibility issue on a website. Change/update your browser. |
413 | Request entity too long | The requested file was too big to process. | Server limitation. |
414 | Request URI too long | The address you entered was overly long for the server. | Server limitation. |
415 | Unsupported media type | The file type of the request is not supported. | Server limitation. |
以下示例是一個簡單的客戶端報錯。按照以下步驟進行操作：
右鍵有報錯的報文。
選擇Follow TCP stream，會看到以下窗口：
3. 顯示以下內容：
客戶端嘗試瀏覽URI/poker-client/broadcast.htm（如截屏中1和3所示）
URI通過http://www.888poker.com/poker-client/promotions.htm轉發（截屏中2所示）
狀態碼爲404 Not Found（如截屏中4所示）
Client errors:
| Code | Status | Explanation | What to do |
500 | Internal server error | The web server encountered an unexpected condition that prevented it from carrying out the client request for access to the requested URL. | Response that is usually caused by a problem in your Perl code when a CGI program is run. |
501 | Not implemented | The request cannot be executed by the server. | A server problem. |
502 | Bad gateway | The server you're trying to reach is sending back errors. | A server problem. |
503 | Service unavailable | The service or file that is being requested is not currently available. | A server problem. |
504 | Gateway timeout | The gateway has timed out. This message is like the 408 timeout error, but this one occurs at the gateway of the server. | Server is down or nonresponsive. |
505 | HTTP version not supported | The HTTP protocol version that you want to use for communicating with the server is not supported by it. | Server does not support the HTTP version. |
服務器不可用（錯誤代碼503）可能有多種原因。以下示例是一個小辦公室碰到的問題：員工能夠訪問Facebook，但當他們點擊站點上的鏈接，則顯示頁面被攔截。以下截屏中，可看出頁面被防火牆攔截：
工作原理:
標準的HTTP瀏覽模式如下：
TCP打開連接（三路握手信號）
HTTP發送GET命令
數據下載到瀏覽器
在一個網頁打開多個連接的情況下（大多數網頁都是如此）。每個連接需要一個DNS 查詢，響應，TCP SYN-SYN/ACK-ACK，以及HTTP GET。之後數據纔會出現在顯示屏上。
當你在packet detail面板沒有看到顯示內容時，右鍵報文並選擇Follow TCP stream，會看到連接的細節數據。另一個廣泛應用的工具是Fiddler，Fiddler是HTTP故障排查的免費工具。