How to fix E: Could not connect to proxy server in APT

Check if apt is configured to use a proxy by examining these files:

sudo cat /etc/apt/apt.conf
sudo cat /etc/apt/apt.conf.d/proxy.conf
cat ~/.bashrc | grep -i proxy
cat /etc/environment | grep -i proxy

Look for lines like:

Acquire::http::Proxy "http://proxy:port";
Acquire::https::Proxy "https://proxy:port";
http_proxy=http://proxy:port

Verify the proxy server is reachable using telnet or nc:

telnet proxy.example.com 8080

Or using nc:

nc -zv proxy.example.com 8080

A successful connection shows no error message. If connection is refused, the proxy is not accessible.

If your proxy requires authentication, ensure credentials are properly configured. Special characters in passwords must be URL-encoded:

# Example with credentials:
Acquire::http::Proxy "http://username:[email protected]:8080/";

For special characters like @ or !, use URL encoding:
- @ becomes %40
- ! becomes %21
- $ becomes %24

Test the proxy directly:

curl -x http://username:password@proxy:port http://example.com

Run apt without the proxy to verify connectivity to package repositories:

sudo apt -o Acquire::http::Proxy=false update

If this succeeds, your issue is with the proxy configuration. If it still fails, the problem is network connectivity to repositories.

If proxy is no longer needed, remove it from configuration:

# Remove proxy configuration files
sudo rm /etc/apt/apt.conf.d/proxy.conf

# Or comment out proxy lines in apt.conf
sudo nano /etc/apt/apt.conf

Unset environment variables:

unset http_proxy https_proxy ftp_proxy
unset HTTP_PROXY HTTPS_PROXY FTP_PROXY

Then retry the apt command:

sudo apt update

Some networks have DNS issues that prevent hostname resolution for proxy servers. Try using the IP address directly:

Instead of:

Acquire::http::Proxy "http://proxy.example.com:8080/";

Use:

Acquire::http::Proxy "http://192.168.1.100:8080/";