How to fix E: Failed to fetch URL 407 Proxy Authentication Required in APT

Check if apt has proxy settings configured:

sudo grep -r "Acquire::.*::Proxy" /etc/apt/

This will show any existing proxy configurations. If you see a proxy without credentials, that's likely the issue.

Edit the APT configuration file:

sudo nano /etc/apt/apt.conf.d/99apt-proxy

Add proxy settings with credentials in this format (note the trailing slash):

Acquire::http::Proxy "http://username:password@proxyserver:port/";
Acquire::https::Proxy "https://username:password@proxyserver:port/";

Replace:
- username with your proxy username
- password with your proxy password
- proxyserver with the proxy server hostname or IP
- port with the proxy port (usually 8080 or 3128)

Save the file (Ctrl+O, Enter, Ctrl+X).

If your password contains special characters (like @, #, %, etc.), URL-encode them:

- @ becomes %40
- # becomes %23
- % becomes %25
- : becomes %3A
- / becomes %2F

Example: password Pass@123# becomes Pass%40123%23:

Acquire::http::Proxy "http://user:Pass%40123%[email protected]:8080/";

Clear apt cache and attempt an update:

sudo apt-get clean
sudo apt-get update

If it succeeds, the proxy authentication is now configured correctly. Try installing a package to confirm:

sudo apt-get install curl

If you're using a Microsoft ISA or NTLM-based proxy, apt may not support NTLM authentication directly. Use cntlm as a local authentication relay:

sudo apt-get install cntlm
sudo nano /etc/cntlm.conf

Configure with your credentials:

Username username
Domain DOMAIN
Password yourpassword
Proxy proxy.example.com:8080
Listen 3128

Then restart cntlm and point apt to localhost:

sudo systemctl restart cntlm

Add to apt config:

Acquire::http::Proxy "http://localhost:3128/";

No authentication needed for localhost since cntlm handles it.

Test if the proxy is reachable and your credentials work:

curl -x http://username:password@proxyserver:port/ -I http://archive.ubuntu.com

If you get a 407 error, the credentials are incorrect. If you get a 200 response, the proxy is accessible and auth works. This helps isolate whether the issue is with apt configuration or the proxy itself.