reverse_proxy: Add support for SRV backends (#3180)

* reverse_proxy: Begin SRV lookup support (WIP)

* reverse_proxy: Finish adding support for SRV-based backends (#3179)

modules/caddyhttp/matchers.go

@@ -41,7 +41,7 @@ type (
 	// especially A/AAAA pointed at your server.
 	//
 	// Automatic HTTPS can be
-	// [customized or disabled](/docs/json/apps/http/servers/automatic_https/).
+	// [customized or disabled](/docs/modules/http#servers/automatic_https).
 	MatchHost []string
 
 	// MatchPath matches requests by the URI's path (case-insensitive). Path

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -177,13 +177,36 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 		return net.JoinHostPort(host, port), nil
 	}
 
+	// appendUpstream creates an upstream for address and adds
+	// it to the list. If the address starts with "srv+" it is
+	// treated as a SRV-based upstream, and any port will be
+	// dropped.
+	appendUpstream := func(address string) error {
+		isSRV := strings.HasPrefix(address, "srv+")
+		if isSRV {
+			address = strings.TrimPrefix(address, "srv+")
+		}
+		dialAddr, err := upstreamDialAddress(address)
+		if err != nil {
+			return err
+		}
+		if isSRV {
+			if host, _, err := net.SplitHostPort(dialAddr); err == nil {
+				dialAddr = host
+			}
+			h.Upstreams = append(h.Upstreams, &Upstream{LookupSRV: dialAddr})
+		} else {
+			h.Upstreams = append(h.Upstreams, &Upstream{Dial: dialAddr})
+		}
+		return nil
+	}
+
 	for d.Next() {
 		for _, up := range d.RemainingArgs() {
-			dialAddr, err := upstreamDialAddress(up)
+			err := appendUpstream(up)
 			if err != nil {
 				return err
 			}
-			h.Upstreams = append(h.Upstreams, &Upstream{Dial: dialAddr})
 		}
 
 		for d.NextBlock(0) {
@@ -194,11 +217,10 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 					return d.ArgErr()
 				}
 				for _, up := range args {
-					dialAddr, err := upstreamDialAddress(up)
+					err := appendUpstream(up)
 					if err != nil {
 						return err
 					}
-					h.Upstreams = append(h.Upstreams, &Upstream{Dial: dialAddr})
 				}
 
 			case "lb_policy":

modules/caddyhttp/reverseproxy/hosts.go

@@ -17,6 +17,8 @@ package reverseproxy
 import (
 	"context"
 	"fmt"
+	"net"
+	"net/http"
 	"strconv"
 	"sync/atomic"
 
@@ -63,10 +65,10 @@ type UpstreamPool []*Upstream
 type Upstream struct {
 	Host `json:"-"`
 
-	// The [network address](/docs/json/apps/http/#servers/listen)
+	// The [network address](/docs/conventions#network-addresses)
 	// to dial to connect to the upstream. Must represent precisely
 	// one socket (i.e. no port ranges). A valid network address
-	// either has a host and port, or is a unix socket address.
+	// either has a host and port or is a unix socket address.
 	//
 	// Placeholders may be used to make the upstream dynamic, but be
 	// aware of the health check implications of this: a single
@@ -75,6 +77,11 @@ type Upstream struct {
 	// backends is down. Also be aware of open proxy vulnerabilities.
 	Dial string `json:"dial,omitempty"`
 
+	// If DNS SRV records are used for service discovery with this
+	// upstream, specify the DNS name for which to look up SRV
+	// records here, instead of specifying a dial address.
+	LookupSRV string `json:"lookup_srv,omitempty"`
+
 	// The maximum number of simultaneous requests to allow to
 	// this upstream. If set, overrides the global passive health
 	// check UnhealthyRequestCount value.
@@ -118,6 +125,47 @@ func (u *Upstream) Full() bool {
 	return u.MaxRequests > 0 && u.Host.NumRequests() >= u.MaxRequests
 }
 
+// fillDialInfo returns a filled DialInfo for upstream u, using the request
+// context. If the upstream has a SRV lookup configured, that is done and a
+// returned address is chosen; otherwise, the upstream's regular dial address
+// field is used. Note that the returned value is not a pointer.
+func (u *Upstream) fillDialInfo(r *http.Request) (DialInfo, error) {
+	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	var addr caddy.ParsedAddress
+
+	if u.LookupSRV != "" {
+		// perform DNS lookup for SRV records and choose one
+		srvName := repl.ReplaceAll(u.LookupSRV, "")
+		_, records, err := net.DefaultResolver.LookupSRV(r.Context(), "", "", srvName)
+		if err != nil {
+			return DialInfo{}, err
+		}
+		addr.Network = "tcp"
+		addr.Host = records[0].Target
+		addr.StartPort, addr.EndPort = uint(records[0].Port), uint(records[0].Port)
+	} else {
+		// use provided dial address
+		var err error
+		dial := repl.ReplaceAll(u.Dial, "")
+		addr, err = caddy.ParseNetworkAddress(dial)
+		if err != nil {
+			return DialInfo{}, fmt.Errorf("upstream %s: invalid dial address %s: %v", u.Dial, dial, err)
+		}
+		if numPorts := addr.PortRangeSize(); numPorts != 1 {
+			return DialInfo{}, fmt.Errorf("upstream %s: dial address must represent precisely one socket: %s represents %d",
+				u.Dial, dial, numPorts)
+		}
+	}
+
+	return DialInfo{
+		Upstream: u,
+		Network:  addr.Network,
+		Address:  addr.JoinHostPort(0),
+		Host:     addr.Host,
+		Port:     strconv.Itoa(int(addr.StartPort)),
+	}, nil
+}
+
 // upstreamHost is the basic, in-memory representation
 // of the state of a remote host. It implements the
 // Host interface.
@@ -204,27 +252,6 @@ func (di DialInfo) String() string {
 	return caddy.JoinNetworkAddress(di.Network, di.Host, di.Port)
 }
 
-// fillDialInfo returns a filled DialInfo for the given upstream, using
-// the given Replacer. Note that the returned value is not a pointer.
-func fillDialInfo(upstream *Upstream, repl *caddy.Replacer) (DialInfo, error) {
-	dial := repl.ReplaceAll(upstream.Dial, "")
-	addr, err := caddy.ParseNetworkAddress(dial)
-	if err != nil {
-		return DialInfo{}, fmt.Errorf("upstream %s: invalid dial address %s: %v", upstream.Dial, dial, err)
-	}
-	if numPorts := addr.PortRangeSize(); numPorts != 1 {
-		return DialInfo{}, fmt.Errorf("upstream %s: dial address must represent precisely one socket: %s represents %d",
-			upstream.Dial, dial, numPorts)
-	}
-	return DialInfo{
-		Upstream: upstream,
-		Network:  addr.Network,
-		Address:  addr.JoinHostPort(0),
-		Host:     addr.Host,
-		Port:     strconv.Itoa(int(addr.StartPort)),
-	}, nil
-}
-
 // GetDialInfo gets the upstream dialing info out of the context,
 // and returns true if there was a valid value; false otherwise.
 func GetDialInfo(ctx context.Context) (DialInfo, bool) {

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -313,7 +313,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyht
 		// the dial address may vary per-request if placeholders are
 		// used, so perform those replacements here; the resulting
 		// DialInfo struct should have valid network address syntax
-		dialInfo, err := fillDialInfo(upstream, repl)
+		dialInfo, err := upstream.fillDialInfo(r)
 		if err != nil {
 			return fmt.Errorf("making dial info: %v", err)
 		}