Linux System-level HTTP(S) proxy

linux-https-proxy.txt

Generate "burp.der" certificate

Convert DER to PEM:
openssl x509 -inform der -in burp.der -out burp.pem

Install Burp certificate in Linux:
cp burp.pem /etc/ssl/certs/
update-ca-certificates
cp burp.pem burp.crt
cp burp.crt /usr/local/share/ca-certificates/
cp burp.crt /usr/share/ca-certificates/
update-ca-certificates

Set HTTP/HTTPS proxy:
export HTTP_PROXY=http://burp_ip:8080/
export HTTPS_PROXY=http://burp_ip:8080/
export http_proxy=http://burp_ip:8080/
export https_proxy=http://burp_ip:8080/

Set the above proxy settings in "/etc/environment" to make it permanent throughout the system.

If the system uses another proxy then set it as an upstream proxy in Burp Suite.
If the proxy requires authentication then you can set it in Burp's upstream proxy setting as Basic authentication.

For NTLM authentication you can utilize cntlm:
apt install cntlm
Configure user, password, domain and proxy address in /etc/cntlm.conf
service cntlm start