DNR & Proxy

The application of declarativeNetRequest on proxies does not appear to have been discussed.

As an example, it can be useful for passing user/pass via headers, as an (easier!?) alternative to webRequest.onAuthRequired.
Note: It might require a special flag/property in DNR to separate it from other requests.

Scenario: Browser -> proxyurl.com -> target.com

It appears that the urlFilter refers to the target.com.

const removeRuleIds = [1];
const addRules = [
  {
    id: 1,
    priority: 1,
    action: {
      type: 'modifyHeaders',
      responseHeaders: [
        {
          header: 'header-1',
          operation: 'set',
          value: 'header1-added'
        }
      ]
    },
    condition: {urlFilter: '||proxyurl.com' }
  }
];

chrome.declarativeNetRequest.updateDynamicRules({removeRuleIds, addRules});

See Also:
Issue 1318138: Add headers using declarativeNetRequest only if the request is being proxied

Start with issue 367 and the linked Chromium issue 1318138. Review how declarativeNetRequest rules are expected to match proxied requests and how header modification should behave in the Browser -> proxyurl.com -> target.com scenario; done requires an agreed specification for proxy handling.