reverseproxy: Customizable dial network for SRV upstreams

By request of a sponsor

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -1528,6 +1528,7 @@ func (u *SRVUpstreams) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				return d.Errf("bad delay value '%s': %v", d.Val(), err)
 			}
 			u.FallbackDelay = caddy.Duration(dur)
+
 		case "grace_period":
 			if !d.NextArg() {
 				return d.ArgErr()

modules/caddyhttp/reverseproxy/upstreams.go

@@ -70,6 +70,11 @@ type SRVUpstreams struct {
 	// A negative value disables this.
 	FallbackDelay caddy.Duration `json:"dial_fallback_delay,omitempty"`
 
+	// Specific network to dial when connecting to the upstream(s)
+	// provided by SRV records upstream. See Go's net package for
+	// accepted values. For example, to restrict to IPv4, use "tcp4".
+	DialNetwork string `json:"dial_network,omitempty"`
+
 	resolver *net.Resolver
 
 	logger *zap.Logger
@@ -177,6 +182,9 @@ func (su SRVUpstreams) GetUpstreams(r *http.Request) ([]*Upstream, error) {
 			)
 		}
 		addr := net.JoinHostPort(rec.Target, strconv.Itoa(int(rec.Port)))
+		if su.DialNetwork != "" {
+			addr = su.DialNetwork + "/" + addr
+		}
 		upstreams[i] = Upstream{Dial: addr}
 	}