How to fix E: Syntax error in apt.conf.d/proxy.conf in APT

First, examine the proxy.conf file to see its current contents:

cat /etc/apt/apt.conf.d/proxy.conf

The error message usually indicates the line number with the problem. Look for:
- Lines missing semicolons at the end
- Mismatched quotes in proxy URLs
- Extra characters or incomplete syntax

APT requires strict syntax with semicolons at the end of each line. The correct format is:

For simple proxy (HTTP/HTTPS):

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

Enter this content (note the semicolon at the end):

Acquire::http::Proxy "http://proxy-server:8080";
Acquire::https::Proxy "http://proxy-server:8080";
Acquire::ftp::Proxy "ftp://proxy-server:8080";

Save with Ctrl+O, Enter, Ctrl+X.

For proxy with authentication:

Acquire::http::Proxy "http://username:password@proxy-server:8080";
Acquire::https::Proxy "http://username:password@proxy-server:8080";

Important: Each line MUST end with a semicolon. Missing semicolons are the most common cause of this error.

After editing, test if apt can parse the configuration:

sudo apt update

If you still get the syntax error, the file still has issues. If it succeeds, the syntax is correct.

Check for common mistakes in your file:

# View the exact file contents with line numbers
cat -n /etc/apt/apt.conf.d/proxy.conf

Look for:
- Lines without semicolons at the end
- Extra spaces or characters after the closing semicolon
- Incomplete or malformed proxy URLs

If your proxy username or password contains special characters, they must be URL-encoded:

Characters that need encoding:
- @ → %40
- : → %3A
- / → %2F
- # → %23
- ? → %3F
- = → %3D
- Space → %20

Example with special characters:

# Original credentials: user@domain:P@ssw0rd
# URL-encoded: user%40domain:P%40ssw0rd

echo 'Acquire::http::Proxy "http://user%40domain:P%40ssw0rd@proxy-server:8080";' | sudo tee /etc/apt/apt.conf.d/proxy.conf

You can use Python to encode:

python3 -c "from urllib.parse import quote; print(quote('P@ssw0rd', safe=''))"

If line-by-line syntax is giving issues, try the block format instead:

sudo tee /etc/apt/apt.conf.d/proxy.conf << EOF
Acquire {
  HTTP::Proxy "http://proxy-server:8080";
  HTTPS::Proxy "http://proxy-server:8080";
  FTP::Proxy "ftp://proxy-server:8080";
};
EOF

Key points for block syntax:
- Opening brace: {
- Each line inside must end with semicolon
- Closing brace must be followed by semicolon: };
- No semicolon after individual proxy lines in some contexts

Then test:

sudo apt update

Sometimes the file contains invisible characters or wrong line endings that cause syntax errors:

# View hidden characters
cat -A /etc/apt/apt.conf.d/proxy.conf

Look for:
- ^M at the end of lines (Windows line endings) - should be $ only
- Extra spaces or tabs
- Strange characters

Convert DOS line endings to Unix:

sudo dos2unix /etc/apt/apt.conf.d/proxy.conf

Or using sed:

sudo sed -i 's/\r$//' /etc/apt/apt.conf.d/proxy.conf

Recreate the file from scratch:
If the file is corrupted, delete and recreate it:

sudo rm /etc/apt/apt.conf.d/proxy.conf
echo 'Acquire::http::Proxy "http://proxy-server:8080";' | sudo tee /etc/apt/apt.conf.d/proxy.conf
sudo apt update

APT provides a way to check configuration syntax before running update:

# Check for syntax errors in all config files
apt-config shell | head

Or dump the entire parsed configuration:

apt-config dump | grep -i proxy

If this succeeds, your configuration is valid. If it fails with a syntax error, the problem is still in your files.

Check individual files:

# List all files in apt.conf.d
ls -la /etc/apt/apt.conf.d/

# Each file should be syntactically valid on its own

If you cannot resolve the syntax issue, temporarily disable the proxy configuration:

# Rename the file to disable it
sudo mv /etc/apt/apt.conf.d/proxy.conf /etc/apt/apt.conf.d/proxy.conf.disabled

# Test if apt works without proxy
sudo apt update

Then either:

Option 1: Use environment variables instead:

# Set proxy environment variables
export http_proxy="http://proxy-server:8080"
export https_proxy="http://proxy-server:8080"
export ftp_proxy="ftp://proxy-server:8080"

sudo -E apt update

Option 2: Recreate the file with minimal syntax:

# Create a simple, clean proxy.conf
sudo tee /etc/apt/apt.conf.d/99proxy << 'EOF'
Acquire::http::Proxy "http://proxy-server:8080";
Acquire::https::Proxy "http://proxy-server:8080";
EOF

sudo apt update

Once you confirm apt works, diagnose the original file to understand what went wrong.