app/vmauth: add support for configuring backends via DNS SRV urls

app/vmauth/auth_config.go

@@ -7,7 +7,6 @@ import (
 	"flag"
 	"fmt"
 	"math"
-	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -27,6 +26,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs/fscore"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/netutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/procutil"
 )
 
@@ -302,7 +302,7 @@ func (up *URLPrefix) getBackendsCount() int {
 //
 // backendURL.put() must be called on the returned backendURL after the request is complete.
 func (up *URLPrefix) getBackendURL() *backendURL {
-	up.discoverBackendIPsIfNeeded()
+	up.discoverBackendAddrsIfNeeded()
 
 	pbus := up.bus.Load()
 	bus := *pbus
@@ -312,7 +312,7 @@ func (up *URLPrefix) getBackendURL() *backendURL {
 	return getLeastLoadedBackendURL(bus, &up.n)
 }
 
-func (up *URLPrefix) discoverBackendIPsIfNeeded() {
+func (up *URLPrefix) discoverBackendAddrsIfNeeded() {
 	if !up.discoverBackendIPs {
 		// The discovery is disabled.
 		return
@@ -337,27 +337,42 @@ func (up *URLPrefix) discoverBackendIPsIfNeeded() {
 
 	// Discover ips for all the backendURLs
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second*time.Duration(intervalSec))
-	hostToIPs := make(map[string][]string)
+	hostToAddrs := make(map[string][]string)
 	for _, bu := range up.busOriginal {
 		host := bu.Hostname()
-		if hostToIPs[host] != nil {
+		if hostToAddrs[host] != nil {
 			// ips for the given host have been already discovered
 			continue
 		}
-		addrs, err := resolver.LookupIPAddr(ctx, host)
-		var ips []string
-		if err != nil {
-			logger.Warnf("cannot discover backend IPs for %s: %s; use it literally", bu, err)
-			ips = []string{host}
-		} else {
-			ips = make([]string, len(addrs))
-			for i, addr := range addrs {
-				ips[i] = addr.String()
+		var resolvedAddrs []string
+		if strings.HasPrefix(host, "srv+") {
+			// The host has the format 'srv+realhost'. Strip 'srv+' prefix before performing the lookup.
+			host = strings.TrimPrefix(host, "srv+")
+			_, addrs, err := netutil.Resolver.LookupSRV(ctx, "", "", host)
+			if err != nil {
+				logger.Warnf("cannot discover backend SRV records for %s: %s; use it literally", bu, err)
+				resolvedAddrs = []string{host}
+			} else {
+				resolvedAddrs := make([]string, len(addrs))
+				for i, addr := range addrs {
+					resolvedAddrs[i] = fmt.Sprintf("%s:%d", addr.Target, addr.Port)
+				}
+			}
+		} else {
+			addrs, err := netutil.Resolver.LookupIPAddr(ctx, host)
+			if err != nil {
+				logger.Warnf("cannot discover backend IPs for %s: %s; use it literally", bu, err)
+				resolvedAddrs = []string{host}
+			} else {
+				resolvedAddrs = make([]string, len(addrs))
+				for i, addr := range addrs {
+					resolvedAddrs[i] = addr.String()
+				}
 			}
-			// sort ips, so they could be compared below in areEqualBackendURLs()
-			sort.Strings(ips)
 		}
-		hostToIPs[host] = ips
+		// sort resolvedAddrs, so they could be compared below in areEqualBackendURLs()
+		sort.Strings(resolvedAddrs)
+		hostToAddrs[host] = resolvedAddrs
 	}
 	cancel()
 
@@ -366,10 +381,14 @@ func (up *URLPrefix) discoverBackendIPsIfNeeded() {
 	for _, bu := range up.busOriginal {
 		host := bu.Hostname()
 		port := bu.Port()
-		for _, ip := range hostToIPs[host] {
+		for _, addr := range hostToAddrs[host] {
 			buCopy := *bu
-			buCopy.Host = ip
+			buCopy.Host = addr
 			if port != "" {
+				if n := strings.IndexByte(buCopy.Host, ':'); n >= 0 {
+					// Drop the discovered port and substitute it the the port specified in bu.
+					buCopy.Host = buCopy.Host[:n]
+				}
 				buCopy.Host += ":" + port
 			}
 			busNew = append(busNew, &backendURL{
@@ -400,11 +419,6 @@ func areEqualBackendURLs(a, b []*backendURL) bool {
 	return true
 }
 
-var resolver = &net.Resolver{
-	PreferGo:     true,
-	StrictErrors: true,
-}
-
 // getFirstAvailableBackendURL returns the first available backendURL, which isn't broken.
 //
 // backendURL.put() must be called on the returned backendURL after the request is complete.

app/vmauth/auth_config_test.go

@@ -316,43 +316,54 @@ users:
 	f(`
 users:
 - auth_token: foo
-  url_prefix: http://aaa:343/bbb
+  url_prefix: https://aaa:343/bbb
   max_concurrent_requests: 5
   tls_insecure_skip_verify: true
+  tls_server_name: "foo.bar"
+  tls_ca_file: "foo/bar"
+  tls_cert_file: "foo/baz"
+  tls_key_file: "foo/foo"
 `, map[string]*UserInfo{
 		getHTTPAuthToken("foo"): {
 			AuthToken:             "foo",
-			URLPrefix:             mustParseURL("http://aaa:343/bbb"),
+			URLPrefix:             mustParseURL("https://aaa:343/bbb"),
 			MaxConcurrentRequests: 5,
 			TLSInsecureSkipVerify: &insecureSkipVerifyTrue,
+			TLSServerName:         "foo.bar",
+			TLSCAFile:             "foo/bar",
+			TLSCertFile:           "foo/baz",
+			TLSKeyFile:            "foo/foo",
 		},
 	})
 
 	// Multiple url_prefix entries
 	insecureSkipVerifyFalse := false
+	discoverBackendIPsTrue := true
 	f(`
 users:
 - username: foo
   password: bar
   url_prefix:
   - http://node1:343/bbb
-  - http://node2:343/bbb
+  - http://srv+node2:343/bbb
   tls_insecure_skip_verify: false
   retry_status_codes: [500, 501]
   load_balancing_policy: first_available
   drop_src_path_prefix_parts: 1
+  discover_backend_ips: true
 `, map[string]*UserInfo{
 		getHTTPAuthBasicToken("foo", "bar"): {
 			Username: "foo",
 			Password: "bar",
 			URLPrefix: mustParseURLs([]string{
 				"http://node1:343/bbb",
-				"http://node2:343/bbb",
+				"http://srv+node2:343/bbb",
 			}),
 			TLSInsecureSkipVerify:  &insecureSkipVerifyFalse,
 			RetryStatusCodes:       []int{500, 501},
 			LoadBalancingPolicy:    "first_available",
 			DropSrcPathPrefixParts: intp(1),
+			DiscoverBackendIPs:     &discoverBackendIPsTrue,
 		},
 	})
 

app/vmauth/main.go

@@ -438,6 +438,8 @@ func newRoundTripper(caFileOpt, certFileOpt, keyFileOpt, serverNameOpt string, i
 	if tr.MaxIdleConns != 0 && tr.MaxIdleConns < tr.MaxIdleConnsPerHost {
 		tr.MaxIdleConns = tr.MaxIdleConnsPerHost
 	}
+	tr.DialContext = netutil.DialMaybeSRV
+
 	rt := cfg.NewRoundTripper(tr)
 	return rt, nil
 }

docs/vmauth.md

@@ -125,7 +125,7 @@ See also [authorization](#authorization) and [routing](#routing) docs.
 
 If [vmagent](https://docs.victoriametrics.com/vmagent.html) is used for processing [data push requests](https://docs.victoriametrics.com/vmagent.html#how-to-push-data-to-vmagent),
 then it is possible to scale the performance of data processing at `vmagent` by spreading load among multiple identically configured `vmagent` instances.
-This can be done with the following [config](#auth-config) for `vmagent`:
+This can be done with the following [config](#auth-config) for `vmauth`:
 
 ```yaml
 unauthorized_user:
@@ -596,19 +596,41 @@ There are the following solutions for this issue:
   This scheme works great, but it needs manual updating of the [`-auth.config`](#auth-config) every time `vmselect` services are restarted,
   downscaled or upscaled.
 
-- To set `discover_backend_ips: true` option, so `vmagent` automatically discovers IPs behind the given hostname and then spreads load among the discovered IPs:
+- To set `discover_backend_ips: true` option, so `vmauth` automatically discovers IPs behind the given hostname and then spreads load among the discovered IPs:
 
   ```yaml
   unauthorized_user:
-    url_prefix: http://vmselect-service/select/0/prometheus/
+    url_prefix: http://vmselect-service:8481/select/0/prometheus/
     discover_backend_ips: true
   ```
 
-  The `discover_backend_ips` can be specified at `user` and `url_map` level in the [`-auth.config`](#auth-config). It can also be enabled globally
+  If the `url_prefix` contains hostname with `srv+` prefix, then the hostname without `srv+` prefix is automatically resolved via [DNS SRV](https://en.wikipedia.org/wiki/SRV_record)
+  to the list of hostnames with TCP ports, and `vmauth` balances load among the discovered TCP addresses:
+
+  ```yaml
+  unauthorized_user:
+    url_prefix: "http://srv+vmselect/select/0/prometheus"
+    discover_backend_ips: true
+  ```
+
+  This functionality is useful for balancing load among backend instances, which run on different TCP ports, since DNS SRV records contain TCP ports.
+
+  The `discover_backend_ips` option can be specified at `user` and `url_map` level in the [`-auth.config`](#auth-config). It can also be enabled globally
   via `-discoverBackendIPs` command-line flag.
 
 See also [load balancing docs](#load-balancing).
 
+## SRV urls
+
+If `url_prefix` contains url with the hostname starting with `srv+` prefix, then `vmauth` uses [DNS SRV](https://en.wikipedia.org/wiki/SRV_record) lookup
+for the hostname without the `srv+` prefix and selects random TCP address (e.g. hostname plus TCP port) form the resolved results.
+
+For example, if `some-addr` [DNS SRV](https://en.wikipedia.org/wiki/SRV_record) record contains `some-host:12345` TCP address,
+then `url_prefix: http://srv+some-addr/some/path` is automatically resolved into `url_prefix: http://some-host:12345/some/path`.
+The DNS SRV resolution is performed every time new connection to the `url_prefix` backend is established.
+
+See also [discovering backend addressess](#discovering-backend-ips).
+
 ## Modifying HTTP headers
 
 `vmauth` supports the ability to set and remove HTTP request headers before sending the requests to backends.

lib/netutil/netutil.go

@@ -1,7 +1,12 @@
 package netutil
 
 import (
+	"context"
+	"fmt"
+	"math/rand"
+	"net"
 	"strings"
+	"time"
 )
 
 // IsTrivialNetworkError returns true if the err can be ignored during logging.
@@ -13,3 +18,36 @@ func IsTrivialNetworkError(err error) bool {
 	}
 	return false
 }
+
+// DialMaybeSRV dials the given addr.
+//
+// The addr may be either the usual TCP address or srv+host form, where host is SRV addr.
+// If the addr has srv+host form, then the host is resolved with SRV into randomly chosen TCP address for the connection.
+func DialMaybeSRV(ctx context.Context, network, addr string) (net.Conn, error) {
+	if strings.HasPrefix(addr, "srv+") {
+		addr = strings.TrimPrefix(addr, "srv+")
+		_, addrs, err := Resolver.LookupSRV(ctx, "", "", addr)
+		if err != nil {
+			return nil, fmt.Errorf("cannot resolve SRV addr %s: %w", addr, err)
+		}
+		if len(addrs) == 0 {
+			return nil, fmt.Errorf("missing SRV records for %s", addr)
+		}
+		n := rand.Intn(len(addrs))
+		addr = fmt.Sprintf("%s:%d", addrs[n].Target, addrs[n].Port)
+	}
+	return Dialer.DialContext(ctx, network, addr)
+}
+
+// Resolver is default DNS resolver.
+var Resolver = &net.Resolver{
+	PreferGo:     true,
+	StrictErrors: true,
+}
+
+// Dialer is default network dialer.
+var Dialer = &net.Dialer{
+	Timeout:   30 * time.Second,
+	KeepAlive: 30 * time.Second,
+	DualStack: TCP6Enabled(),
+}