feat(notes): add apt proxy check guide

Author: lzwjava

notes/2025-06-28-check-apt-proxy-en.md

@@ -0,0 +1,66 @@
+---
+title: "Check APT Proxy Settings in Ubuntu"
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+To check if `apt` is using a proxy in Ubuntu, follow these steps:
+
+1. **Check the APT configuration file**:
+   - Open the APT configuration file located at `/etc/apt/apt.conf` or `/etc/apt/apt.conf.d/` (files in this directory, like `70debconf` or `proxy.conf`, may contain proxy settings).
+   - Use a command like:
+     ```bash
+     sudo cat /etc/apt/apt.conf
+     ```
+     or
+     ```bash
+     sudo grep -r Proxy /etc/apt/apt.conf.d/
+     ```
+   - Look for lines like:
+     ```
+     Acquire::http::Proxy "http://proxy-server:port";
+     Acquire::https::Proxy "https://proxy-server:port";
+     Acquire::ftp::Proxy "ftp://proxy-server:port";
+     ```
+     If these lines exist, `apt` is configured to use a proxy.
+
+2. **Check environment variables**:
+   - `apt` may use proxy settings defined in environment variables like `http_proxy`, `https_proxy`, or `ftp_proxy`.
+   - Run:
+     ```bash
+     env | grep -i proxy
+     ```
+   - If you see output like `http_proxy=http://proxy-server:port`, `apt` may be using these settings.
+
+3. **Check system-wide proxy settings**:
+   - If you’re using a desktop environment (e.g., GNOME), check the system proxy settings:
+     - Go to *Settings > Network > Proxy* or similar, depending on your desktop environment.
+     - These settings might apply to `apt` if no specific APT proxy is configured.
+
+4. **Test APT with a command**:
+   - Run an `apt` command and observe the behavior:
+     ```bash
+     sudo apt update
+     ```
+   - If `apt` connects through a proxy, you might see delays, specific error messages, or proxy-related logs. To debug further, use:
+     ```bash
+     sudo apt -o Debug::pkgProblemResolver=yes update
+     ```
+     This provides verbose output, which may indicate proxy usage.
+
+5. **Check for proxy in `/etc/environment`**:
+   - Some systems define proxies in `/etc/environment`. Check with:
+     ```bash
+     cat /etc/environment
+     ```
+   - Look for lines like:
+     ```
+     http_proxy="http://proxy-server:port"
+     ```
+
+If none of these show proxy settings, `apt` is likely not using a proxy. For real-time confirmation, you can monitor network traffic with tools like `tcpdump` or `wireshark` to see if `apt` connections route through a proxy server.
+
+If you need help analyzing specific output or setting up a proxy, let me know!