fix: Bypass proxy for loopback addresses to prevent localhost connection failures (#194)

When users have HTTP_PROXY/HTTPS_PROXY set in their shell (e.g. corporate
proxy), PMG's upstream transport routes all traffic through that external
proxy — including requests to localhost/127.0.0.1. The external proxy
cannot reach the user's loopback, causing EFAULT/ConnectionRefused errors.

This adds loopback bypass in two places:
- Proxy upstream transport skips external proxy for localhost/127.0.0.1/::1
- Child process env gets NO_PROXY so proxy-aware libs (axios) skip PMG's
  proxy for loopback addresses

Fixes #193

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

proxy/proxy.go

@@ -134,6 +134,15 @@ func NewProxyServer(config *ProxyConfig) (ProxyServer, error) {
 	return ps, nil
 }
 
+func proxyWithLoopbackBypass(req *http.Request) (*url.URL, error) {
+	host := req.URL.Hostname()
+	if host == "localhost" || host == "127.0.0.1" || host == "::1" {
+		return nil, nil
+	}
+
+	return http.ProxyFromEnvironment(req)
+}
+
 func newUpstreamTransport(config *ProxyConfig) *http.Transport {
 	dialer := &net.Dialer{
 		Timeout: config.ConnectTimeout,
@@ -141,7 +150,9 @@ func newUpstreamTransport(config *ProxyConfig) *http.Transport {
 
 	// Proxy honours the environment (HTTP_PROXY, HTTPS_PROXY, NO_PROXY) so
 	// that PMG works in enterprise environments that require a corporate
-	// upstream proxy to reach the internet.
+	// upstream proxy to reach the internet. Loopback addresses are always
+	// bypassed to avoid routing localhost traffic through an external proxy,
+	// which would fail because the proxy can't reach the user's localhost.
 	//
 	// ForceAttemptHTTP2 is required because Go's http.Transport silently
 	// disables HTTP/2 when a custom TLSClientConfig or DialContext is set.
@@ -157,7 +168,7 @@ func newUpstreamTransport(config *ProxyConfig) *http.Transport {
 	// MaxIdleConnsPerHost is raised from the default of 2 to improve
 	// connection reuse.
 	return &http.Transport{
-		Proxy:               http.ProxyFromEnvironment,
+		Proxy:               proxyWithLoopbackBypass,
 		DialContext:         dialer.DialContext,
 		ForceAttemptHTTP2:   true,
 		MaxConnsPerHost:     100,

proxy/proxy_test.go

@@ -126,6 +126,35 @@ func TestNormalizeRequestURLNilSafety(t *testing.T) {
 	normalizeRequestURL(&http.Request{URL: &url.URL{}})
 }
 
+func TestProxyWithLoopbackBypass(t *testing.T) {
+	tests := []struct {
+		name          string
+		url           string
+		shouldBypass  bool
+	}{
+		{"localhost bypassed", "http://localhost:9876/", true},
+		{"127.0.0.1 bypassed", "http://127.0.0.1:9876/", true},
+		{"ipv6 loopback bypassed", "http://[::1]:9876/", true},
+		{"registry not bypassed", "https://registry.npmjs.org/lodash", false},
+		{"external host not bypassed", "http://example.com/", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			parsed, err := url.Parse(tt.url)
+			require.NoError(t, err)
+
+			req := &http.Request{URL: parsed}
+			proxyURL, err := proxyWithLoopbackBypass(req)
+			assert.NoError(t, err)
+
+			if tt.shouldBypass {
+				assert.Nil(t, proxyURL, "loopback address should bypass proxy")
+			}
+		})
+	}
+}
+
 func TestNewProxyServerRejectsUntrustedUpstreamCertByDefault(t *testing.T) {
 	target := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)