reverseproxy: Fix dial placeholders, SRV, active health checks (#3780)

* reverseproxy: Fix dial placeholders, SRV, active health checks

Supercedes #3776
Partially reverts or updates #3756, #3693, and #3695

* reverseproxy: add integration tests

Co-authored-by: Mohammed Al Sahaf <msaa1990@gmail.com>

1 files changed, 31 insertions, 12 deletions

diff --git a/modules/caddyhttp/reverseproxy/healthchecks.go b/modules/caddyhttp/reverseproxy/healthchecks.go
index 410b9d4..4e93320 100644
--- a/modules/caddyhttp/reverseproxy/healthchecks.go
+++ b/modules/caddyhttp/reverseproxy/healthchecks.go
@@ -154,23 +154,42 @@ func (h *Handler) doActiveHealthCheckForAllHosts() {
 				}
 			}()
 
-			portStr := strconv.Itoa(upstream.activeHealthCheckPort)
-			hostAddr := net.JoinHostPort(upstream.networkAddress.Host, portStr)
-			if upstream.networkAddress.IsUnixNetwork() {
+			networkAddr, err := caddy.NewReplacer().ReplaceOrErr(upstream.Dial, true, true)
+			if err != nil {
+				h.HealthChecks.Active.logger.Error("invalid use of placeholders in dial address for active health checks",
+					zap.String("address", networkAddr),
+					zap.Error(err),
+				)
+				return
+			}
+			addr, err := caddy.ParseNetworkAddress(networkAddr)
+			if err != nil {
+				h.HealthChecks.Active.logger.Error("bad network address",
+					zap.String("address", networkAddr),
+					zap.Error(err),
+				)
+				return
+			}
+			if hcp := uint(upstream.activeHealthCheckPort); hcp != 0 {
+				if addr.IsUnixNetwork() {
+					addr.Network = "tcp" // I guess we just assume TCP since we are using a port??
+				}
+				addr.StartPort, addr.EndPort = hcp, hcp
+			}
+			if upstream.LookupSRV == "" && addr.PortRangeSize() != 1 {
+				h.HealthChecks.Active.logger.Error("multiple addresses (upstream must map to only one address)",
+					zap.String("address", networkAddr),
+				)
+				return
+			}
+			hostAddr := addr.JoinHostPort(0)
+			if addr.IsUnixNetwork() {
 				// this will be used as the Host portion of a http.Request URL, and
 				// paths to socket files would produce an error when creating URL,
 				// so use a fake Host value instead; unix sockets are usually local
 				hostAddr = "localhost"
 			}
-
-			dialInfo := DialInfo{
-				Upstream: upstream,
-				Network:  upstream.networkAddress.Network,
-				Host:     upstream.networkAddress.Host,
-				Port:     portStr,
-				Address:  hostAddr,
-			}
-			err := h.doActiveHealthCheck(dialInfo, hostAddr, upstream.Host)
+			err = h.doActiveHealthCheck(DialInfo{Network: addr.Network, Address: hostAddr}, hostAddr, upstream.Host)
 			if err != nil {
 				h.HealthChecks.Active.logger.Error("active health check failed",
 					zap.String("address", hostAddr),