--- title: "Notes on capturing web server network packets with Fiddler" description: "Walks through how to install and configure Fiddler Classic to capture HTTPS traffic from a web server, making it easier to debug and inspect network request behavior." canonical_url: "https://blog.markkulab.net/en/post/fiddler-debuging-webserver" author: "Mark Ku" author_url: "https://blog.markkulab.net/en/author/mark-ku" site: "Mark Ku's Tech Notes" date_published: "2022-08-10 01:01:01 +0800" category: "Infra" tags: ["fiddler", "network", "debug", "https", "packet capture", "proxy", "infra"] language: "en" license: "CC BY 4.0" license_url: "https://creativecommons.org/licenses/by/4.0/" attribution: "when reusing or quoting, credit the author and link back to the original" --- # Notes on capturing web server network packets with Fiddler ## The problem this solves To verify whether a web server is sending the right requests, we typically reach for a packet-capture tool to see what the other side returned, or what extra requests the site is making. Since I only do this once in a while, I always forget the setup steps, so I figured I would write a note to refresh my memory. ## Installation steps ### Download and install [Free download link for Fiddler Classic](https://www.telerik.com/fiddler/) ### Tool > Options ![Fiddler Classic showing Tools menu with Options highlighted](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/e13g9ZJ.png) ### Switch to the Https tab > Actions to install the certificate > OK ![Fiddler Classic HTTPS options, Trust Root Certificate action highlighted](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/dJQ5dt5.png) ### After configuring, close and restart Fiddler ![Fiddler Classic Options HTTPS tab showing traffic decryption and certificate set](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/jxxaGAj.png) ### You can drag and drop to monitor traffic generated by a specific application id ![Fiddler Classic monitoring network requests from a command prompt application](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/0c0P8h4.png) ### If you know the source of the API, you can also locate it with Ctrl + F5 ![Fiddler Find Sessions dialog showing api search and highlight options](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/99N7BZu.png) ### Search results ![Fiddler Classic interface displaying web server network traffic and JSON respons](https://blog.markkulab.net/content/markku/posts/fiddler-debuging-webserver/images/1mMMQR8.png) ## Notes HTTPS does not let you capture the request body. For local testing, switch to HTTP. This version seems to have a few bugs - only "all processes" mode actually captures data. --- ## About this article and its author Originally published on [Mark Ku's Tech Notes](https://blog.markkulab.net/en/post/fiddler-debuging-webserver) License: [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/) — when reusing or quoting, credit the author and link back to the original ### About the author **[Mark Ku](https://blog.markkulab.net/en/author/mark-ku)** — Software Solution Provider - 10+ years senior software engineer, now an AI Builder - Focused on large-platform architecture — North-American e-commerce, AI SaaS subscription billing - Combining AI Agents and automation to build evolvable product foundations ### Free tools built by the author All of these are free to use: - [Free PDF Sign Tool](https://blog.markkulab.net/en/tools/pdf-sign): Online PDF sign tool — draw, type, or upload a signature, then drag, resize, and download. 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No sign-up, nothing uploaded to a server, and mermaid.live share links work as-is. - [React Intl Phone Number](https://blog.markkulab.net/en/tools/react-intl-phone-number): react-intl-phone-number is an open-source React component: framework-agnostic and antd-free, with E.164 in/out, a searchable flag / country-code dropdown, configurable validation levels (strict / mobile-strict / loose), themeable CSS, and i18n — phone logic powered by google-libphonenumber. Lightweight and fully typed. Free and open source (MIT). - [Uptime Kuma Cluster](https://blog.markkulab.net/en/tools/uptime-kuma-cluster): Turn single-node Uptime Kuma into a highly available cluster: OpenResty + Lua smart load balancing, shared MariaDB state, health checks and automatic failover, plus cluster-management REST APIs. One Docker Compose command to start. 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