Instead of treating 0 is a default value, use http.DefaultMaxIdleConnsPerHost

caddyhttp/proxy/proxy.go

@@ -108,7 +108,7 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 		if nameURL, err := url.Parse(host.Name); err == nil {
 			outreq.Host = nameURL.Host
 			if proxy == nil {
-				proxy = NewSingleHostReverseProxy(nameURL, host.WithoutPathPrefix, 0)
+				proxy = NewSingleHostReverseProxy(nameURL, host.WithoutPathPrefix, http.DefaultMaxIdleConnsPerHost)
 			}
 
 			// use upstream credentials by default

caddyhttp/proxy/proxy_test.go

@@ -716,7 +716,7 @@ func newFakeUpstream(name string, insecure bool) *fakeUpstream {
 		from: "/",
 		host: &UpstreamHost{
 			Name:         name,
-			ReverseProxy: NewSingleHostReverseProxy(uri, "", 0),
+			ReverseProxy: NewSingleHostReverseProxy(uri, "", http.DefaultMaxIdleConnsPerHost),
 		},
 	}
 	if insecure {
@@ -744,7 +744,7 @@ func (u *fakeUpstream) Select() *UpstreamHost {
 		}
 		u.host = &UpstreamHost{
 			Name:         u.name,
-			ReverseProxy: NewSingleHostReverseProxy(uri, u.without, 0),
+			ReverseProxy: NewSingleHostReverseProxy(uri, u.without, http.DefaultMaxIdleConnsPerHost),
 		}
 	}
 	return u.host
@@ -785,7 +785,7 @@ func (u *fakeWsUpstream) Select() *UpstreamHost {
 	uri, _ := url.Parse(u.name)
 	return &UpstreamHost{
 		Name:         u.name,
-		ReverseProxy: NewSingleHostReverseProxy(uri, u.without, 0),
+		ReverseProxy: NewSingleHostReverseProxy(uri, u.without, http.DefaultMaxIdleConnsPerHost),
 		UpstreamHeaders: http.Header{
 			"Connection": {"{>Connection}"},
 			"Upgrade":    {"{>Upgrade}"}},

caddyhttp/proxy/reverseproxy.go

@@ -122,7 +122,10 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 		rp.Transport = &http.Transport{
 			Dial: socketDial(target.String()),
 		}
-	} else if keepalive != 0 {
+	} else if keepalive != http.DefaultMaxIdleConnsPerHost {
+		// if keepalive is equal to the default,
+		// just use default transport, to avoid creating
+		// a brand new transport
 		rp.Transport = &http.Transport{
 			Proxy: http.ProxyFromEnvironment,
 			Dial: (&net.Dialer{
@@ -132,7 +135,7 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int) *
 			TLSHandshakeTimeout:   10 * time.Second,
 			ExpectContinueTimeout: 1 * time.Second,
 		}
-		if keepalive < 0 {
+		if keepalive == 0 {
 			rp.Transport.(*http.Transport).DisableKeepAlives = true
 		} else {
 			rp.Transport.(*http.Transport).MaxIdleConnsPerHost = keepalive

caddyhttp/proxy/upstream.go

@@ -55,6 +55,7 @@ func NewStaticUpstreams(c caddyfile.Dispenser) ([]Upstream, error) {
 			FailTimeout:       10 * time.Second,
 			MaxFails:          1,
 			MaxConns:          0,
+			KeepAlive:         http.DefaultMaxIdleConnsPerHost,
 		}
 
 		if !c.Args(&upstream.from) {
@@ -321,6 +322,9 @@ func parseBlock(c *caddyfile.Dispenser, u *staticUpstream) error {
 		if err != nil {
 			return err
 		}
+		if n < 0 {
+			return c.ArgErr()
+		}
 		u.KeepAlive = n
 	default:
 		return c.Errf("unknown property '%s'", c.Val())