求助：C#HttpWebRequest POST无法成功，但是当fiddler运行时就可以

[奔骝定位]

C#HttpWebRequest POST无法成功，但是当fiddler运行时就可以。
我看https://stackoverflow.com/questions/4801189/httpwebrequest-doesnt-work-except-when-fiddler-is-running?answertab=active#tab-top
这个里面有讨论，可是都解决不了我的问题。
我是往一个硬件设备里面去修改参数，那边是cgi
HttpWebRequest req = WebRequest.Create("http://192.168.1.250/config.cgi") as HttpWebRequest;
req.Method = "POST";
req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299";
req.ContentType = "application/x-www-form-urlencoded";
req.KeepAlive = true;
string param = "addr=88&mode=1&mac=2C:D1:41:40:55:9F&sub=255.255.255.0&gw=192.168.1.1&dns=0.0.0.0&ip=192.168.1.250&lpt=32100&sip=192.168.1.253&spt=32500&tcp=1&dhcp=0";
byte[] requestBytes = System.Text.Encoding.UTF8.GetBytes(param);
req.ContentLength = requestBytes.Length;

[圣殿骑士18]

你要把无法成功的错误，贴出来。

[zj_zwl]

以前也遇到过这种情况, 忘了怎么解决的