app/vmauth: allow discovering backend ips behind shared hostname and spreading load among the discovered ips

This is done with the `discover_backend_ips` option at `user` and `url_map` level.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5707

app/vmauth/auth_config.go

@@ -2,15 +2,17 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"encoding/base64"
 	"flag"
 	"fmt"
+	"math"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
 	"regexp"
 	"sort"
-	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -36,7 +38,11 @@ var (
 	defaultRetryStatusCodes = flagutil.NewArrayInt("retryStatusCodes", 0, "Comma-separated list of default HTTP response status codes when vmauth re-tries the request on other backends. "+
 		"See https://docs.victoriametrics.com/vmauth.html#load-balancing for details")
 	defaultLoadBalancingPolicy = flag.String("loadBalancingPolicy", "least_loaded", "The default load balancing policy to use for backend urls specified inside url_prefix section. "+
-		"Supported policies: least_loaded, first_available. See https://docs.victoriametrics.com/vmauth.html#load-balancing for more details")
+		"Supported policies: least_loaded, first_available. See https://docs.victoriametrics.com/vmauth.html#load-balancing")
+	discoverBackendIPsGlobal = flag.Bool("discoverBackendIPs", false, "Whether to discover backend IPs via periodic DNS queries to hostnames specified in url_prefix. "+
+		"This may be useful when url_prefix points to a hostname with dynamically scaled instances behind it. See https://docs.victoriametrics.com/vmauth.html#discovering-backend-ips")
+	discoverBackendIPsInterval = flag.Duration("discoverBackendIPsInterval", 10*time.Second, "The interval for re-discovering backend IPs if -discoverBackendIPs command-line flag is set. "+
+		"Too low value may lead to DNS errors")
 )
 
 // AuthConfig represents auth config.
@@ -57,6 +63,7 @@ type UserInfo struct {
 	Password    string `yaml:"password,omitempty"`
 
 	URLPrefix              *URLPrefix  `yaml:"url_prefix,omitempty"`
+	DiscoverBackendIPs     *bool       `yaml:"discover_backend_ips,omitempty"`
 	URLMaps                []URLMap    `yaml:"url_map,omitempty"`
 	HeadersConf            HeadersConf `yaml:",inline"`
 	MaxConcurrentRequests  int         `yaml:"max_concurrent_requests,omitempty"`
@@ -111,6 +118,8 @@ func (ui *UserInfo) getMaxConcurrentRequests() int {
 type Header struct {
 	Name  string
 	Value string
+
+	sOriginal string
 }
 
 // UnmarshalYAML unmarshals h from f.
@@ -119,6 +128,8 @@ func (h *Header) UnmarshalYAML(f func(interface{}) error) error {
 	if err := f(&s); err != nil {
 		return err
 	}
+	h.sOriginal = s
+
 	n := strings.IndexByte(s, ':')
 	if n < 0 {
 		return fmt.Errorf("missing speparator char ':' between Name and Value in the header %q; expected format - 'Name: Value'", s)
@@ -130,8 +141,7 @@ func (h *Header) UnmarshalYAML(f func(interface{}) error) error {
 
 // MarshalYAML marshals h to yaml.
 func (h *Header) MarshalYAML() (interface{}, error) {
-	s := fmt.Sprintf("%s: %s", h.Name, h.Value)
+	return h.sOriginal, nil
-	return s, nil
 }
 
 // URLMap is a mapping from source paths to target urls.
@@ -151,6 +161,9 @@ type URLMap struct {
 	// UrlPrefix contains backend url prefixes for the proxied request url.
 	URLPrefix *URLPrefix `yaml:"url_prefix,omitempty"`
 
+	// DiscoverBackendIPs instructs discovering URLPrefix backend IPs via DNS.
+	DiscoverBackendIPs *bool `yaml:"discover_backend_ips,omitempty"`
+
 	// HeadersConf is the config for augumenting request and response headers.
 	HeadersConf HeadersConf `yaml:",inline"`
 
@@ -181,14 +194,16 @@ type QueryArg struct {
 
 // UnmarshalYAML unmarshals up from yaml.
 func (qa *QueryArg) UnmarshalYAML(f func(interface{}) error) error {
-	if err := f(&qa.sOriginal); err != nil {
+	var s string
+	if err := f(&s); err != nil {
 		return err
 	}
+	qa.sOriginal = s
 
-	n := strings.IndexByte(qa.sOriginal, '=')
+	n := strings.IndexByte(s, '=')
 	if n >= 0 {
-		qa.Name = qa.sOriginal[:n]
+		qa.Name = s[:n]
-		qa.Value = qa.sOriginal[n+1:]
+		qa.Value = s[n+1:]
 	}
 	return nil
 }
@@ -200,19 +215,34 @@ func (qa *QueryArg) MarshalYAML() (interface{}, error) {
 
 // URLPrefix represents passed `url_prefix`
 type URLPrefix struct {
-	n atomic.Uint32
-
-	// the list of backend urls
-	bus []*backendURL
-
 	// requests are re-tried on other backend urls for these http response status codes
 	retryStatusCodes []int
 
 	// load balancing policy used
 	loadBalancingPolicy string
 
-	// how many request path prefix parts to drop before routing the request to backendURL.
+	// how many request path prefix parts to drop before routing the request to backendURL
 	dropSrcPathPrefixParts int
+
+	// busOriginal contains the original list of backends specified in yaml config.
+	busOriginal []*url.URL
+
+	// n is an atomic counter, which is used for balancing load among available backends.
+	n atomic.Uint32
+
+	// the list of backend urls
+	//
+	// the list can be dynamically updated if `discover_backend_ips` option is set.
+	bus atomic.Pointer[[]*backendURL]
+
+	// if this option is set, then backend ips for busOriginal are periodically re-discovered and put to bus.
+	discoverBackendIPs bool
+
+	// The next deadline for DNS-based discovery of backend IPs
+	nextDiscoveryDeadline atomic.Uint64
+
+	// vOriginal contains the original yaml value for URLPrefix.
+	vOriginal interface{}
 }
 
 func (up *URLPrefix) setLoadBalancingPolicy(loadBalancingPolicy string) error {
@@ -253,25 +283,121 @@ func (bu *backendURL) put() {
 }
 
 func (up *URLPrefix) getBackendsCount() int {
-	return len(up.bus)
+	pbus := up.bus.Load()
+	return len(*pbus)
 }
 
 // getBackendURL returns the backendURL depending on the load balance policy.
 //
 // backendURL.put() must be called on the returned backendURL after the request is complete.
 func (up *URLPrefix) getBackendURL() *backendURL {
+	up.discoverBackendIPsIfNeeded()
+
+	pbus := up.bus.Load()
+	bus := *pbus
 	if up.loadBalancingPolicy == "first_available" {
-		return up.getFirstAvailableBackendURL()
+		return getFirstAvailableBackendURL(bus)
 	}
-	return up.getLeastLoadedBackendURL()
+	return getLeastLoadedBackendURL(bus, &up.n)
+}
+
+func (up *URLPrefix) discoverBackendIPsIfNeeded() {
+	if !up.discoverBackendIPs {
+		// The discovery is disabled.
+		return
+	}
+
+	ct := fasttime.UnixTimestamp()
+	deadline := up.nextDiscoveryDeadline.Load()
+	if ct < deadline {
+		// There is no need in discovering backends.
+		return
+	}
+
+	intervalSec := math.Ceil(discoverBackendIPsInterval.Seconds())
+	if intervalSec <= 0 {
+		intervalSec = 1
+	}
+	nextDeadline := ct + uint64(intervalSec)
+	if !up.nextDiscoveryDeadline.CompareAndSwap(deadline, nextDeadline) {
+		// Concurrent goroutine already started the discovery.
+		return
+	}
+
+	// Discover ips for all the backendURLs
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*time.Duration(intervalSec))
+	hostToIPs := make(map[string][]string)
+	for _, bu := range up.busOriginal {
+		host := bu.Hostname()
+		if hostToIPs[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()
+			}
+			// sort ips, so they could be compared below in areEqualBackendURLs()
+			sort.Strings(ips)
+		}
+		hostToIPs[host] = ips
+	}
+	cancel()
+
+	// generate new backendURLs for the resolved IPs
+	var busNew []*backendURL
+	for _, bu := range up.busOriginal {
+		host := bu.Hostname()
+		port := bu.Port()
+		for _, ip := range hostToIPs[host] {
+			buCopy := *bu
+			buCopy.Host = ip
+			if port != "" {
+				buCopy.Host += ":" + port
+			}
+			busNew = append(busNew, &backendURL{
+				url: &buCopy,
+			})
+		}
+	}
+
+	pbus := up.bus.Load()
+	if areEqualBackendURLs(*pbus, busNew) {
+		return
+	}
+
+	// Store new backend urls
+	up.bus.Store(&busNew)
+}
+
+func areEqualBackendURLs(a, b []*backendURL) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, aURL := range a {
+		bURL := b[i]
+		if aURL.url.String() != bURL.url.String() {
+			return false
+		}
+	}
+	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.
-func (up *URLPrefix) getFirstAvailableBackendURL() *backendURL {
+func getFirstAvailableBackendURL(bus []*backendURL) *backendURL {
-	bus := up.bus
-
 	bu := bus[0]
 	if !bu.isBroken() {
 		// Fast path - send the request to the first url.
@@ -293,8 +419,7 @@ func (up *URLPrefix) getFirstAvailableBackendURL() *backendURL {
 // getLeastLoadedBackendURL returns the backendURL with the minimum number of concurrent requests.
 //
 // backendURL.put() must be called on the returned backendURL after the request is complete.
-func (up *URLPrefix) getLeastLoadedBackendURL() *backendURL {
+func getLeastLoadedBackendURL(bus []*backendURL, atomicCounter *atomic.Uint32) *backendURL {
-	bus := up.bus
 	if len(bus) == 1 {
 		// Fast path - return the only backend url.
 		bu := bus[0]
@@ -303,7 +428,7 @@ func (up *URLPrefix) getLeastLoadedBackendURL() *backendURL {
 	}
 
 	// Slow path - select other backend urls.
-	n := up.n.Add(1)
+	n := atomicCounter.Add(1)
 
 	for i := uint32(0); i < uint32(len(bus)); i++ {
 		idx := (n + i) % uint32(len(bus))
@@ -341,6 +466,7 @@ func (up *URLPrefix) UnmarshalYAML(f func(interface{}) error) error {
 	if err := f(&v); err != nil {
 		return err
 	}
+	up.vOriginal = v
 
 	var urls []string
 	switch x := v.(type) {
@@ -363,38 +489,21 @@ func (up *URLPrefix) UnmarshalYAML(f func(interface{}) error) error {
 		return fmt.Errorf("unexpected type for `url_prefix`: %T; want string or []string", v)
 	}
 
-	bus := make([]*backendURL, len(urls))
+	bus := make([]*url.URL, len(urls))
 	for i, u := range urls {
 		pu, err := url.Parse(u)
 		if err != nil {
 			return fmt.Errorf("cannot unmarshal %q into url: %w", u, err)
 		}
-		bus[i] = &backendURL{
+		bus[i] = pu
-			url: pu,
-		}
 	}
-	up.bus = bus
+	up.busOriginal = bus
 	return nil
 }
 
 // MarshalYAML marshals up to yaml.
 func (up *URLPrefix) MarshalYAML() (interface{}, error) {
-	var b []byte
+	return up.vOriginal, nil
-	if len(up.bus) == 1 {
-		u := up.bus[0].url.String()
-		b = strconv.AppendQuote(b, u)
-		return string(b), nil
-	}
-	b = append(b, '[')
-	for i, bu := range up.bus {
-		u := bu.url.String()
-		b = strconv.AppendQuote(b, u)
-		if i+1 < len(up.bus) {
-			b = append(b, ',')
-		}
-	}
-	b = append(b, ']')
-	return string(b), nil
 }
 
 func (r *Regex) match(s string) bool {
@@ -415,12 +524,13 @@ func (r *Regex) UnmarshalYAML(f func(interface{}) error) error {
 	if err := f(&s); err != nil {
 		return err
 	}
+	r.sOriginal = s
+
 	sAnchored := "^(?:" + s + ")$"
 	re, err := regexp.Compile(sAnchored)
 	if err != nil {
 		return fmt.Errorf("cannot build regexp from %q: %w", s, err)
 	}
-	r.sOriginal = s
 	r.re = re
 	return nil
 }
@@ -689,8 +799,9 @@ func (ui *UserInfo) initURLs() error {
 	retryStatusCodes := defaultRetryStatusCodes.Values()
 	loadBalancingPolicy := *defaultLoadBalancingPolicy
 	dropSrcPathPrefixParts := 0
+	discoverBackendIPs := *discoverBackendIPsGlobal
 	if ui.URLPrefix != nil {
-		if err := ui.URLPrefix.sanitize(); err != nil {
+		if err := ui.URLPrefix.sanitizeAndInitialize(); err != nil {
 			return err
 		}
 		if ui.RetryStatusCodes != nil {
@@ -702,14 +813,18 @@ func (ui *UserInfo) initURLs() error {
 		if ui.DropSrcPathPrefixParts != nil {
 			dropSrcPathPrefixParts = *ui.DropSrcPathPrefixParts
 		}
+		if ui.DiscoverBackendIPs != nil {
+			discoverBackendIPs = *ui.DiscoverBackendIPs
+		}
 		ui.URLPrefix.retryStatusCodes = retryStatusCodes
 		ui.URLPrefix.dropSrcPathPrefixParts = dropSrcPathPrefixParts
+		ui.URLPrefix.discoverBackendIPs = discoverBackendIPs
 		if err := ui.URLPrefix.setLoadBalancingPolicy(loadBalancingPolicy); err != nil {
 			return err
 		}
 	}
 	if ui.DefaultURL != nil {
-		if err := ui.DefaultURL.sanitize(); err != nil {
+		if err := ui.DefaultURL.sanitizeAndInitialize(); err != nil {
 			return err
 		}
 	}
@@ -720,12 +835,13 @@ func (ui *UserInfo) initURLs() error {
 		if e.URLPrefix == nil {
 			return fmt.Errorf("missing `url_prefix` in `url_map`")
 		}
-		if err := e.URLPrefix.sanitize(); err != nil {
+		if err := e.URLPrefix.sanitizeAndInitialize(); err != nil {
 			return err
 		}
 		rscs := retryStatusCodes
 		lbp := loadBalancingPolicy
 		dsp := dropSrcPathPrefixParts
+		dbd := discoverBackendIPs
 		if e.RetryStatusCodes != nil {
 			rscs = e.RetryStatusCodes
 		}
@@ -735,11 +851,15 @@ func (ui *UserInfo) initURLs() error {
 		if e.DropSrcPathPrefixParts != nil {
 			dsp = *e.DropSrcPathPrefixParts
 		}
+		if e.DiscoverBackendIPs != nil {
+			dbd = *e.DiscoverBackendIPs
+		}
 		e.URLPrefix.retryStatusCodes = rscs
 		if err := e.URLPrefix.setLoadBalancingPolicy(lbp); err != nil {
 			return err
 		}
 		e.URLPrefix.dropSrcPathPrefixParts = dsp
+		e.URLPrefix.discoverBackendIPs = dbd
 	}
 	if len(ui.URLMaps) == 0 && ui.URLPrefix == nil {
 		return fmt.Errorf("missing `url_prefix` or `url_map`")
@@ -805,14 +925,24 @@ func getAuthTokensFromRequest(r *http.Request) []string {
 	return ats
 }
 
-func (up *URLPrefix) sanitize() error {
+func (up *URLPrefix) sanitizeAndInitialize() error {
-	for _, bu := range up.bus {
+	for i, bu := range up.busOriginal {
-		puNew, err := sanitizeURLPrefix(bu.url)
+		puNew, err := sanitizeURLPrefix(bu)
 		if err != nil {
 			return err
 		}
-		bu.url = puNew
+		up.busOriginal[i] = puNew
 	}
+
+	// Initialize up.bus
+	bus := make([]*backendURL, len(up.busOriginal))
+	for i, bu := range up.busOriginal {
+		bus[i] = &backendURL{
+			url: bu,
+		}
+	}
+	up.bus.Store(&bus)
+
 	return nil
 }
 

app/vmauth/auth_config_test.go

@@ -328,7 +328,7 @@ users:
 - username: foo
   url_prefix: http://foo
 - username: bar
-  url_prefix: https://bar/x///
+  url_prefix: https://bar/x/
 `, map[string]*UserInfo{
 		getHTTPAuthBasicToken("foo", ""): {
 			Username:  "foo",
@@ -336,7 +336,7 @@ users:
 		},
 		getHTTPAuthBasicToken("bar", ""): {
 			Username:  "bar",
-			URLPrefix: mustParseURL("https://bar/x"),
+			URLPrefix: mustParseURL("https://bar/x/"),
 		},
 	})
 
@@ -409,7 +409,7 @@ users:
   url_prefix: http://foo
 - username: foo-same
   password: bar
-  url_prefix: https://bar/x///
+  url_prefix: https://bar/x
 `, map[string]*UserInfo{
 		getHTTPAuthBasicToken("foo-same", "baz"): {
 			Username:  "foo-same",
@@ -516,7 +516,7 @@ users:
     team: dev
 - username: foo-same
   password: bar
-  url_prefix: https://bar/x///
+  url_prefix: https://bar/x
   metric_labels:
     backend_env: test
     team: accounting
@@ -710,9 +710,14 @@ func mustParseURLs(us []string) *URLPrefix {
 			url: pu,
 		}
 	}
-	return &URLPrefix{
+	up := &URLPrefix{}
-		bus: bus,
+	if len(us) == 1 {
+		up.vOriginal = us[0]
+	} else {
+		up.vOriginal = us
 	}
+	up.bus.Store(&bus)
+	return up
 }
 
 func intp(n int) *int {

app/vmauth/main.go

@@ -239,7 +239,14 @@ func tryProcessingRequest(w http.ResponseWriter, r *http.Request, targetURL *url
 	// This code has been copied from net/http/httputil/reverseproxy.go
 	req := sanitizeRequestHeaders(r)
 	req.URL = targetURL
-	req.Host = targetURL.Host
+
+	if req.URL.Scheme == "https" {
+		// Override req.Host only for https requests, since https server verifies hostnames during TLS handshake,
+		// so it expects the targetURL.Host in the request.
+		// There is no need in overriding the req.Host for http requests, since it is expected that backend server
+		// may properly process queries with the original req.Host.
+		req.Host = targetURL.Host
+	}
 	updateHeadersByConfig(req.Header, hc.RequestHeaders)
 	res, err := ui.httpTransport.RoundTrip(req)
 	rtb, rtbOK := req.Body.(*readTrackingBody)

app/vmauth/target_url_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/url"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -93,15 +94,17 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 		if up == nil {
 			t.Fatalf("cannot determie backend: %s", err)
 		}
-		bu := up.getLeastLoadedBackendURL()
+		bu := up.getBackendURL()
 		target := mergeURLs(bu.url, u, up.dropSrcPathPrefixParts)
 		bu.put()
 		if target.String() != expectedTarget {
 			t.Fatalf("unexpected target; got %q; want %q", target, expectedTarget)
 		}
-		headersStr := fmt.Sprintf("%q", hc.RequestHeaders)
+		if s := headersToString(hc.RequestHeaders); s != expectedRequestHeaders {
-		if headersStr != expectedRequestHeaders {
+			t.Fatalf("unexpected request headers; got %q; want %q", s, expectedRequestHeaders)
-			t.Fatalf("unexpected request headers; got %s; want %s", headersStr, expectedRequestHeaders)
+		}
+		if s := headersToString(hc.ResponseHeaders); s != expectedResponseHeaders {
+			t.Fatalf("unexpected response headers; got %q; want %q", s, expectedResponseHeaders)
 		}
 		if !reflect.DeepEqual(up.retryStatusCodes, expectedRetryStatusCodes) {
 			t.Fatalf("unexpected retryStatusCodes; got %d; want %d", up.retryStatusCodes, expectedRetryStatusCodes)
@@ -116,34 +119,42 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 	// Simple routing with `url_prefix`
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
-	}, "", "http://foo.bar/.", "[]", "[]", nil, "least_loaded", 0)
+	}, "", "http://foo.bar/.", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
 		HeadersConf: HeadersConf{
-			RequestHeaders: []Header{{
+			RequestHeaders: []Header{
-				Name:  "bb",
+				{
-				Value: "aaa",
+					Name:  "bb",
-			}},
+					Value: "aaa",
+				},
+			},
+			ResponseHeaders: []Header{
+				{
+					Name:  "x",
+					Value: "y",
+				},
+			},
 		},
 		RetryStatusCodes:       []int{503, 501},
 		LoadBalancingPolicy:    "first_available",
 		DropSrcPathPrefixParts: intp(2),
-	}, "/a/b/c", "http://foo.bar/c", `[{"bb" "aaa"}]`, `[]`, []int{503, 501}, "first_available", 2)
+	}, "/a/b/c", "http://foo.bar/c", `bb: aaa`, `x: y`, []int{503, 501}, "first_available", 2)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar/federate"),
-	}, "/", "http://foo.bar/federate", "[]", "[]", nil, "least_loaded", 0)
+	}, "/", "http://foo.bar/federate", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
-	}, "a/b?c=d", "http://foo.bar/a/b?c=d", "[]", "[]", nil, "least_loaded", 0)
+	}, "a/b?c=d", "http://foo.bar/a/b?c=d", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("https://sss:3894/x/y"),
-	}, "/z", "https://sss:3894/x/y/z", "[]", "[]", nil, "least_loaded", 0)
+	}, "/z", "https://sss:3894/x/y/z", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("https://sss:3894/x/y"),
-	}, "/../../aaa", "https://sss:3894/x/y/aaa", "[]", "[]", nil, "least_loaded", 0)
+	}, "/../../aaa", "https://sss:3894/x/y/aaa", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("https://sss:3894/x/y"),
-	}, "/./asd/../../aaa?a=d&s=s/../d", "https://sss:3894/x/y/aaa?a=d&s=s%2F..%2Fd", "[]", "[]", nil, "least_loaded", 0)
+	}, "/./asd/../../aaa?a=d&s=s/../d", "https://sss:3894/x/y/aaa?a=d&s=s%2F..%2Fd", "", "", nil, "least_loaded", 0)
 
 	// Complex routing with `url_map`
 	ui := &UserInfo{
@@ -202,11 +213,11 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 		DropSrcPathPrefixParts: intp(2),
 	}
 	f(ui, "http://host42/vmsingle/api/v1/query?query=up&db=foo", "http://vmselect/0/prometheus/api/v1/query?db=foo&query=up",
-		`[{"xx" "aa"} {"yy" "asdf"}]`, `[{"qwe" "rty"}]`, []int{503, 500, 501}, "first_available", 1)
+		"xx: aa\nyy: asdf", "qwe: rty", []int{503, 500, 501}, "first_available", 1)
 	f(ui, "http://host123/vmsingle/api/v1/query?query=up", "http://default-server/v1/query?query=up",
-		`[{"bb" "aaa"}]`, `[{"x" "y"}]`, []int{502}, "least_loaded", 2)
+		"bb: aaa", "x: y", []int{502}, "least_loaded", 2)
-	f(ui, "https://foo-host/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", []int{}, "least_loaded", 0)
+	f(ui, "https://foo-host/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "", "", []int{}, "least_loaded", 0)
-	f(ui, "https://foo-host/foo/bar/api/v1/query_range", "http://default-server/api/v1/query_range", `[{"bb" "aaa"}]`, `[{"x" "y"}]`, []int{502}, "least_loaded", 2)
+	f(ui, "https://foo-host/foo/bar/api/v1/query_range", "http://default-server/api/v1/query_range", "bb: aaa", "x: y", []int{502}, "least_loaded", 2)
 
 	// Complex routing regexp paths in `url_map`
 	ui = &UserInfo{
@@ -226,19 +237,19 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 		},
 		URLPrefix: mustParseURL("http://default-server"),
 	}
-	f(ui, "/api/v1/query?query=up", "http://vmselect/0/prometheus/api/v1/query?query=up", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "/api/v1/query?query=up", "http://vmselect/0/prometheus/api/v1/query?query=up", "", "", nil, "least_loaded", 0)
-	f(ui, "/api/v1/query_range?query=up", "http://vmselect/0/prometheus/api/v1/query_range?query=up", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "/api/v1/query_range?query=up", "http://vmselect/0/prometheus/api/v1/query_range?query=up", "", "", nil, "least_loaded", 0)
-	f(ui, "/api/v1/label/foo/values", "http://vmselect/0/prometheus/api/v1/label/foo/values", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "/api/v1/label/foo/values", "http://vmselect/0/prometheus/api/v1/label/foo/values", "", "", nil, "least_loaded", 0)
-	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "", "", nil, "least_loaded", 0)
-	f(ui, "/api/v1/foo/bar", "http://default-server/api/v1/foo/bar", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "/api/v1/foo/bar", "http://default-server/api/v1/foo/bar", "", "", nil, "least_loaded", 0)
-	f(ui, "https://vmui.foobar.com/a/b?c=d", "http://vmui.host:1234/vmui/a/b?c=d", "[]", "[]", nil, "least_loaded", 0)
+	f(ui, "https://vmui.foobar.com/a/b?c=d", "http://vmui.host:1234/vmui/a/b?c=d", "", "", nil, "least_loaded", 0)
 
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar?extra_label=team=dev"),
-	}, "/api/v1/query", "http://foo.bar/api/v1/query?extra_label=team=dev", "[]", "[]", nil, "least_loaded", 0)
+	}, "/api/v1/query", "http://foo.bar/api/v1/query?extra_label=team=dev", "", "", nil, "least_loaded", 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar?extra_label=team=mobile"),
-	}, "/api/v1/query?extra_label=team=dev", "http://foo.bar/api/v1/query?extra_label=team%3Dmobile", "[]", "[]", nil, "least_loaded", 0)
+	}, "/api/v1/query?extra_label=team=dev", "http://foo.bar/api/v1/query?extra_label=team%3Dmobile", "", "", nil, "least_loaded", 0)
 }
 
 func TestCreateTargetURLFailure(t *testing.T) {
@@ -270,3 +281,11 @@ func TestCreateTargetURLFailure(t *testing.T) {
 		},
 	}, "/api/v1/write")
 }
+
+func headersToString(hs []Header) string {
+	a := make([]string, len(hs))
+	for i, h := range hs {
+		a[i] = fmt.Sprintf("%s: %s", h.Name, h.Value)
+	}
+	return strings.Join(a, "\n")
+}

docs/CHANGELOG.md

@@ -32,6 +32,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 
 * SECURITY: upgrade Go builder from Go1.21.7 to Go1.22.1. See [the list of issues addressed in Go1.22.1](https://github.com/golang/go/issues?q=milestone%3AGo1.22.1+label%3ACherryPickApproved).
 
+* FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): allow discovering ip addresses for backend instances hidden behind a shared hostname, via `discover_backend_ips: true` option. This allows evenly spreading load among backend instances. See [these docs](https://docs.victoriametrics.com/vmauth/#discovering-backend-ips) and [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5707).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): allow routing incoming requests bassed on HTTP [query args](https://en.wikipedia.org/wiki/Query_string) via `src_query_args` option at `url_map`. See [these docs](https://docs.victoriametrics.com/vmauth/#generic-http-proxy-for-different-backends) and [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5878).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): allow routing incoming requests bassed on HTTP request headers via `src_headers` option at `url_map`. See [these docs](https://docs.victoriametrics.com/vmauth/#generic-http-proxy-for-different-backends).
 * FEATURE: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): reduce memory usage by up to 5x when aggregating over big number of unique [time series](https://docs.victoriametrics.com/keyconcepts/#time-series). The memory usage reduction is most visible when [stream deduplication](https://docs.victoriametrics.com/stream-aggregation/#deduplication) is enabled. The downside is increased CPU usage by up to 30%.

docs/vmauth.md

@@ -401,6 +401,7 @@ Each `url_prefix` in the [-auth.config](#auth-config) can be specified in the fo
 
   In this case `vmauth` spreads requests among the specified urls using least-loaded round-robin policy.
   This guarantees that incoming load is shared uniformly among the specified backends.
+  See also [discovering backend IPs](#discovering-backend-ips).
 
   `vmauth` automatically detects temporarily unavailable backends and spreads incoming queries among the remaining available backends.
   This allows restarting the backends and peforming mantenance tasks on the backends without the need to remove them from the `url_prefix` list.
@@ -466,6 +467,49 @@ Load balancing feature can be used in the following cases:
 
 Load balancig can be configured independently per each `user` entry and per each `url_map` entry. See [auth config docs](#auth-config) for more details.
 
+See also [discovering backend IPs](#discovering-backend-ips).
+
+## Discovering backend IPs
+
+By default `vmauth` spreads load among the listed backends at `url_prefix` as described in [load balancing docs](#load-balancing).
+Sometimes multiple backend instances can be hidden behind a single hostname. For example, `vmselect-service` hostname
+may point to a cluster of `vmselect` instances in [VictoriaMetrics cluster setup](https://docs.victoriametrics.com/cluster-victoriametrics/#architecture-overview).
+So the following config may fail spreading load among available `vmselect` instances, since `vmauth` will send all the requests to the same url, which may end up
+to a single backend instance:
+
+```yaml
+unauthorized_user:
+  url_prefix: http://vmselect-service/select/0/prometheus/
+```
+
+There are the following solutions for this issue:
+
+- To enumerate every `vmselect` hosname or IP in the `url_prefix` list:
+
+  ```yaml
+  unauthorized_user:
+    url_prefix:
+    - http://vmselect-1:8481/select/0/prometheus/
+    - http://vmselect-2:8481/select/0/prometheus/
+    - http://vmselect-3:8481/select/0/prometheus/
+  ```
+
+  This scheme works great, but it needs manual updating of the [`-auth.config`](#auth-config) every time `vmselect` services are restarted,
+  downsaled 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:
+
+  ```yaml
+  unauthorized_user:
+    url_prefix: http://vmselect-service/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
+  via `-discoverBackendIPs` command-line flag.
+
+See also [load balancing docs](#load-balancing).
+
 ## Modifying HTTP headers
 
 `vmauth` supports the ability to set and remove HTTP request headers before sending the requests to backends.