app/vmauth: add ability to drop the specified number of /-delimited prefix parts from request path

This can be done via `drop_src_path_prefix_parts` option at `url_map` and `user` levels.

See https://docs.victoriametrics.com/vmauth.html#dropping-request-path-prefix

app/vmauth/README.md

@@ -32,6 +32,38 @@ Pass `-help` to `vmauth` in order to see all the supported command-line flags wi
 Feel free [contacting us](mailto:info@victoriametrics.com) if you need customized auth proxy for VictoriaMetrics with the support of LDAP, SSO, RBAC, SAML,
 accounting and rate limiting such as [vmgateway](https://docs.victoriametrics.com/vmgateway.html).
 
+## Dropping request path prefix
+
+By default `vmauth` doesn't drop the path prefix from the original request when proxying the request to the matching backend.
+Sometimes it is needed to drop path prefix before routing the request to the backend. This can be done by specifying the number of `/`-delimited
+prefix parts to drop from the request path via `drop_src_path_prefix_parts` option at `url_map` level or at `user` level.
+
+For example, if you need to serve requests to [vmalert](https://docs.victoriametrics.com/vmalert.html) at `/vmalert/` path prefix,
+while serving requests to [vmagent](https://docs.victoriametrics.com/vmagent.html) at `/vmagent/` path prefix for a particular user,
+then the following [-auth.config](#auth-config) can be used:
+
+```yml
+users:
+- username: foo
+  url_map:
+
+    # proxy all the requests, which start with `/vmagent/`, to vmagent backend
+  - src_paths:
+    - "/vmagent/.+"
+
+    # drop /vmagent/ path prefix from the original request before proxying it to url_prefix.
+    drop_src_path_prefix_parts: 1
+    url_prefix: "http://vmagent-backend:8429/"
+
+    # proxy all the requests, which start with `/vmalert`, to vmalert backend
+  - src_paths:
+    - "/vmalert/.+"
+
+    # drop /vmalert/ path prefix from the original request before proxying it to url_prefix.
+    drop_src_path_prefix_parts: 1
+    url_prefix: "http://vmalert-backend:8880/"
+```
+
 ## Load balancing
 
 Each `url_prefix` in the [-auth.config](#auth-config) may contain either a single url or a list of urls.

app/vmauth/auth_config.go

@@ -40,18 +40,19 @@ type AuthConfig struct {
 
 // UserInfo is user information read from authConfigPath
 type UserInfo struct {
-	Name                  string      `yaml:"name,omitempty"`
+	Name                   string      `yaml:"name,omitempty"`
-	BearerToken           string      `yaml:"bearer_token,omitempty"`
+	BearerToken            string      `yaml:"bearer_token,omitempty"`
-	Username              string      `yaml:"username,omitempty"`
+	Username               string      `yaml:"username,omitempty"`
-	Password              string      `yaml:"password,omitempty"`
+	Password               string      `yaml:"password,omitempty"`
-	URLPrefix             *URLPrefix  `yaml:"url_prefix,omitempty"`
+	URLPrefix              *URLPrefix  `yaml:"url_prefix,omitempty"`
-	URLMaps               []URLMap    `yaml:"url_map,omitempty"`
+	URLMaps                []URLMap    `yaml:"url_map,omitempty"`
-	HeadersConf           HeadersConf `yaml:",inline"`
+	HeadersConf            HeadersConf `yaml:",inline"`
-	MaxConcurrentRequests int         `yaml:"max_concurrent_requests,omitempty"`
+	MaxConcurrentRequests  int         `yaml:"max_concurrent_requests,omitempty"`
-	DefaultURL            *URLPrefix  `yaml:"default_url,omitempty"`
+	DefaultURL             *URLPrefix  `yaml:"default_url,omitempty"`
-	RetryStatusCodes      []int       `yaml:"retry_status_codes,omitempty"`
+	RetryStatusCodes       []int       `yaml:"retry_status_codes,omitempty"`
-	TLSInsecureSkipVerify *bool       `yaml:"tls_insecure_skip_verify,omitempty"`
+	DropSrcPathPrefixParts int         `yaml:"drop_src_path_prefix_parts,omitempty"`
-	TLSCAFile             string      `yaml:"tls_ca_file,omitempty"`
+	TLSInsecureSkipVerify  *bool       `yaml:"tls_insecure_skip_verify,omitempty"`
+	TLSCAFile              string      `yaml:"tls_ca_file,omitempty"`
 
 	concurrencyLimitCh      chan struct{}
 	concurrencyLimitReached *metrics.Counter
@@ -119,10 +120,11 @@ func (h *Header) MarshalYAML() (interface{}, error) {
 
 // URLMap is a mapping from source paths to target urls.
 type URLMap struct {
-	SrcPaths         []*SrcPath  `yaml:"src_paths,omitempty"`
+	SrcPaths               []*SrcPath  `yaml:"src_paths,omitempty"`
-	URLPrefix        *URLPrefix  `yaml:"url_prefix,omitempty"`
+	URLPrefix              *URLPrefix  `yaml:"url_prefix,omitempty"`
-	HeadersConf      HeadersConf `yaml:",inline"`
+	HeadersConf            HeadersConf `yaml:",inline"`
-	RetryStatusCodes []int       `yaml:"retry_status_codes,omitempty"`
+	RetryStatusCodes       []int       `yaml:"retry_status_codes,omitempty"`
+	DropSrcPathPrefixParts int         `yaml:"drop_src_path_prefix_parts,omitempty"`
 }
 
 // SrcPath represents an src path

app/vmauth/auth_config_test.go

@@ -249,6 +249,8 @@ users:
   - http://node1:343/bbb
   - http://node2:343/bbb
   tls_insecure_skip_verify: false
+  retry_status_codes: [500, 501]
+  drop_src_path_prefix_parts: 1
 `, map[string]*UserInfo{
 		getAuthToken("", "foo", "bar"): {
 			Username: "foo",
@@ -257,7 +259,9 @@ users:
 				"http://node1:343/bbb",
 				"http://node2:343/bbb",
 			}),
-			TLSInsecureSkipVerify: &insecureSkipVerifyFalse,
+			TLSInsecureSkipVerify:  &insecureSkipVerifyFalse,
+			RetryStatusCodes:       []int{500, 501},
+			DropSrcPathPrefixParts: 1,
 		},
 	})
 

app/vmauth/main.go

@@ -164,7 +164,7 @@ func processUserRequest(w http.ResponseWriter, r *http.Request, ui *UserInfo) {
 
 func processRequest(w http.ResponseWriter, r *http.Request, ui *UserInfo) {
 	u := normalizeURL(r.URL)
-	up, hc, retryStatusCodes := ui.getURLPrefixAndHeaders(u)
+	up, hc, retryStatusCodes, dropSrcPathPrefixParts := ui.getURLPrefixAndHeaders(u)
 	isDefault := false
 	if up == nil {
 		if ui.DefaultURL == nil {
@@ -198,7 +198,7 @@ func processRequest(w http.ResponseWriter, r *http.Request, ui *UserInfo) {
 			query.Set("request_path", u.Path)
 			targetURL.RawQuery = query.Encode()
 		} else { // Update path for regular routes.
-			targetURL = mergeURLs(targetURL, u)
+			targetURL = mergeURLs(targetURL, u, dropSrcPathPrefixParts)
 		}
 		ok := tryProcessingRequest(w, r, targetURL, hc, retryStatusCodes, ui.httpTransport)
 		bu.put()

app/vmauth/target_url.go

@@ -6,12 +6,13 @@ import (
 	"strings"
 )
 
-func mergeURLs(uiURL, requestURI *url.URL) *url.URL {
+func mergeURLs(uiURL, requestURI *url.URL, dropSrcPathPrefixParts int) *url.URL {
 	targetURL := *uiURL
-	if strings.HasPrefix(requestURI.Path, "/") {
+	srcPath := dropPrefixParts(requestURI.Path, dropSrcPathPrefixParts)
+	if strings.HasPrefix(srcPath, "/") {
 		targetURL.Path = strings.TrimSuffix(targetURL.Path, "/")
 	}
-	targetURL.Path += requestURI.Path
+	targetURL.Path += srcPath
 	requestParams := requestURI.Query()
 	// fast path
 	if len(requestParams) == 0 {
@@ -32,18 +33,34 @@ func mergeURLs(uiURL, requestURI *url.URL) *url.URL {
 	return &targetURL
 }
 
-func (ui *UserInfo) getURLPrefixAndHeaders(u *url.URL) (*URLPrefix, HeadersConf, []int) {
+func dropPrefixParts(path string, parts int) string {
+	if parts <= 0 {
+		return path
+	}
+	for parts > 0 {
+		path = strings.TrimPrefix(path, "/")
+		n := strings.IndexByte(path, '/')
+		if n < 0 {
+			return ""
+		}
+		path = path[n:]
+		parts--
+	}
+	return path
+}
+
+func (ui *UserInfo) getURLPrefixAndHeaders(u *url.URL) (*URLPrefix, HeadersConf, []int, int) {
 	for _, e := range ui.URLMaps {
 		for _, sp := range e.SrcPaths {
 			if sp.match(u.Path) {
-				return e.URLPrefix, e.HeadersConf, e.RetryStatusCodes
+				return e.URLPrefix, e.HeadersConf, e.RetryStatusCodes, e.DropSrcPathPrefixParts
 			}
 		}
 	}
 	if ui.URLPrefix != nil {
-		return ui.URLPrefix, ui.HeadersConf, ui.RetryStatusCodes
+		return ui.URLPrefix, ui.HeadersConf, ui.RetryStatusCodes, ui.DropSrcPathPrefixParts
 	}
-	return nil, HeadersConf{}, nil
+	return nil, HeadersConf{}, nil, 0
 }
 
 func normalizeURL(uOrig *url.URL) *url.URL {

app/vmauth/target_url_test.go

@@ -7,20 +7,91 @@ import (
 	"testing"
 )
 
+func TestDropPrefixParts(t *testing.T) {
+	f := func(path string, parts int, expectedResult string) {
+		t.Helper()
+
+		result := dropPrefixParts(path, parts)
+		if result != expectedResult {
+			t.Fatalf("unexpected result; got %q; want %q", result, expectedResult)
+		}
+	}
+
+	f("", 0, "")
+	f("", 1, "")
+	f("", 10, "")
+	f("foo", 0, "foo")
+	f("foo", -1, "foo")
+	f("foo", 1, "")
+
+	f("/foo", 0, "/foo")
+	f("/foo/bar", 0, "/foo/bar")
+	f("/foo/bar/baz", 0, "/foo/bar/baz")
+
+	f("foo", 0, "foo")
+	f("foo/bar", 0, "foo/bar")
+	f("foo/bar/baz", 0, "foo/bar/baz")
+
+	f("/foo/", 0, "/foo/")
+	f("/foo/bar/", 0, "/foo/bar/")
+	f("/foo/bar/baz/", 0, "/foo/bar/baz/")
+
+	f("/foo", 1, "")
+	f("/foo/bar", 1, "/bar")
+	f("/foo/bar/baz", 1, "/bar/baz")
+
+	f("foo", 1, "")
+	f("foo/bar", 1, "/bar")
+	f("foo/bar/baz", 1, "/bar/baz")
+
+	f("/foo/", 1, "/")
+	f("/foo/bar/", 1, "/bar/")
+	f("/foo/bar/baz/", 1, "/bar/baz/")
+
+	f("/foo", 2, "")
+	f("/foo/bar", 2, "")
+	f("/foo/bar/baz", 2, "/baz")
+
+	f("foo", 2, "")
+	f("foo/bar", 2, "")
+	f("foo/bar/baz", 2, "/baz")
+
+	f("/foo/", 2, "")
+	f("/foo/bar/", 2, "/")
+	f("/foo/bar/baz/", 2, "/baz/")
+
+	f("/foo", 3, "")
+	f("/foo/bar", 3, "")
+	f("/foo/bar/baz", 3, "")
+
+	f("foo", 3, "")
+	f("foo/bar", 3, "")
+	f("foo/bar/baz", 3, "")
+
+	f("/foo/", 3, "")
+	f("/foo/bar/", 3, "")
+	f("/foo/bar/baz/", 3, "/")
+
+	f("/foo/", 4, "")
+	f("/foo/bar/", 4, "")
+	f("/foo/bar/baz/", 4, "")
+}
+
 func TestCreateTargetURLSuccess(t *testing.T) {
-	f := func(ui *UserInfo, requestURI, expectedTarget, expectedRequestHeaders, expectedResponseHeaders string, expectedRetryStatusCodes []int) {
+	f := func(ui *UserInfo, requestURI, expectedTarget, expectedRequestHeaders, expectedResponseHeaders string,
+		expectedRetryStatusCodes []int, expectedDropSrcPathPrefixParts int) {
 		t.Helper()
 		u, err := url.Parse(requestURI)
 		if err != nil {
 			t.Fatalf("cannot parse %q: %s", requestURI, err)
 		}
 		u = normalizeURL(u)
-		up, hc, retryStatusCodes := ui.getURLPrefixAndHeaders(u)
+		up, hc, retryStatusCodes, dropSrcPathPrefixParts := ui.getURLPrefixAndHeaders(u)
 		if up == nil {
 			t.Fatalf("cannot determie backend: %s", err)
 		}
 		bu := up.getLeastLoadedBackendURL()
-		target := mergeURLs(bu.url, u)
+		target := mergeURLs(bu.url, u, dropSrcPathPrefixParts)
 		bu.put()
 		if target.String() != expectedTarget {
 			t.Fatalf("unexpected target; got %q; want %q", target, expectedTarget)
@@ -32,11 +103,14 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 		if !reflect.DeepEqual(retryStatusCodes, expectedRetryStatusCodes) {
 			t.Fatalf("unexpected retryStatusCodes; got %d; want %d", retryStatusCodes, expectedRetryStatusCodes)
 		}
+		if dropSrcPathPrefixParts != expectedDropSrcPathPrefixParts {
+			t.Fatalf("unexpected dropSrcPathPrefixParts; got %d; want %d", dropSrcPathPrefixParts, expectedDropSrcPathPrefixParts)
+		}
 	}
 	// Simple routing with `url_prefix`
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
-	}, "", "http://foo.bar/.", "[]", "[]", nil)
+	}, "", "http://foo.bar/.", "[]", "[]", nil, 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
 		HeadersConf: HeadersConf{
@@ -45,29 +119,30 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 				Value: "aaa",
 			}},
 		},
-		RetryStatusCodes: []int{503, 501},
+		RetryStatusCodes:       []int{503, 501},
-	}, "/", "http://foo.bar", `[{"bb" "aaa"}]`, `[]`, []int{503, 501})
+		DropSrcPathPrefixParts: 2,
+	}, "/a/b/c", "http://foo.bar/c", `[{"bb" "aaa"}]`, `[]`, []int{503, 501}, 2)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar/federate"),
-	}, "/", "http://foo.bar/federate", "[]", "[]", nil)
+	}, "/", "http://foo.bar/federate", "[]", "[]", nil, 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("http://foo.bar"),
-	}, "a/b?c=d", "http://foo.bar/a/b?c=d", "[]", "[]", nil)
+	}, "a/b?c=d", "http://foo.bar/a/b?c=d", "[]", "[]", nil, 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("https://sss:3894/x/y"),
-	}, "/z", "https://sss:3894/x/y/z", "[]", "[]", nil)
+	}, "/z", "https://sss:3894/x/y/z", "[]", "[]", nil, 0)
 	f(&UserInfo{
 		URLPrefix: mustParseURL("https://sss:3894/x/y"),
-	}, "/../../aaa", "https://sss:3894/x/y/aaa", "[]", "[]", nil)
+	}, "/../../aaa", "https://sss:3894/x/y/aaa", "[]", "[]", nil, 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)
+	}, "/./asd/../../aaa?a=d&s=s/../d", "https://sss:3894/x/y/aaa?a=d&s=s%2F..%2Fd", "[]", "[]", nil, 0)
 
 	// Complex routing with `url_map`
 	ui := &UserInfo{
 		URLMaps: []URLMap{
 			{
-				SrcPaths:  getSrcPaths([]string{"/api/v1/query"}),
+				SrcPaths:  getSrcPaths([]string{"/vmsingle/api/v1/query"}),
 				URLPrefix: mustParseURL("http://vmselect/0/prometheus"),
 				HeadersConf: HeadersConf{
 					RequestHeaders: []Header{
@@ -87,7 +162,8 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 						},
 					},
 				},
-				RetryStatusCodes: []int{503, 500, 501},
+				RetryStatusCodes:       []int{503, 500, 501},
+				DropSrcPathPrefixParts: 1,
 			},
 			{
 				SrcPaths:  getSrcPaths([]string{"/api/v1/write"}),
@@ -105,11 +181,12 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 				Value: "y",
 			}},
 		},
-		RetryStatusCodes: []int{502},
+		RetryStatusCodes:       []int{502},
+		DropSrcPathPrefixParts: 2,
 	}
-	f(ui, "/api/v1/query?query=up", "http://vmselect/0/prometheus/api/v1/query?query=up", `[{"xx" "aa"} {"yy" "asdf"}]`, `[{"qwe" "rty"}]`, []int{503, 500, 501})
+	f(ui, "/vmsingle/api/v1/query?query=up", "http://vmselect/0/prometheus/api/v1/query?query=up", `[{"xx" "aa"} {"yy" "asdf"}]`, `[{"qwe" "rty"}]`, []int{503, 500, 501}, 1)
-	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", nil)
+	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", nil, 0)
-	f(ui, "/api/v1/query_range", "http://default-server/api/v1/query_range", `[{"bb" "aaa"}]`, `[{"x" "y"}]`, []int{502})
+	f(ui, "/foo/bar/api/v1/query_range", "http://default-server/api/v1/query_range", `[{"bb" "aaa"}]`, `[{"x" "y"}]`, []int{502}, 2)
 
 	// Complex routing regexp paths in `url_map`
 	ui = &UserInfo{
@@ -125,17 +202,17 @@ 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)
+	f(ui, "/api/v1/query?query=up", "http://vmselect/0/prometheus/api/v1/query?query=up", "[]", "[]", nil, 0)
-	f(ui, "/api/v1/query_range?query=up", "http://vmselect/0/prometheus/api/v1/query_range?query=up", "[]", "[]", nil)
+	f(ui, "/api/v1/query_range?query=up", "http://vmselect/0/prometheus/api/v1/query_range?query=up", "[]", "[]", nil, 0)
-	f(ui, "/api/v1/label/foo/values", "http://vmselect/0/prometheus/api/v1/label/foo/values", "[]", "[]", nil)
+	f(ui, "/api/v1/label/foo/values", "http://vmselect/0/prometheus/api/v1/label/foo/values", "[]", "[]", nil, 0)
-	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", nil)
+	f(ui, "/api/v1/write", "http://vminsert/0/prometheus/api/v1/write", "[]", "[]", nil, 0)
-	f(ui, "/api/v1/foo/bar", "http://default-server/api/v1/foo/bar", "[]", "[]", nil)
+	f(ui, "/api/v1/foo/bar", "http://default-server/api/v1/foo/bar", "[]", "[]", nil, 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)
+	}, "/api/v1/query", "http://foo.bar/api/v1/query?extra_label=team=dev", "[]", "[]", nil, 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)
+	}, "/api/v1/query?extra_label=team=dev", "http://foo.bar/api/v1/query?extra_label=team%3Dmobile", "[]", "[]", nil, 0)
 }
 
 func TestCreateTargetURLFailure(t *testing.T) {
@@ -146,7 +223,7 @@ func TestCreateTargetURLFailure(t *testing.T) {
 			t.Fatalf("cannot parse %q: %s", requestURI, err)
 		}
 		u = normalizeURL(u)
-		up, hc, retryStatusCodes := ui.getURLPrefixAndHeaders(u)
+		up, hc, retryStatusCodes, dropSrcPathPrefixParts := ui.getURLPrefixAndHeaders(u)
 		if up != nil {
 			t.Fatalf("unexpected non-empty up=%#v", up)
 		}
@@ -159,6 +236,9 @@ func TestCreateTargetURLFailure(t *testing.T) {
 		if retryStatusCodes != nil {
 			t.Fatalf("unexpected non-empty retryStatusCodes=%d", retryStatusCodes)
 		}
+		if dropSrcPathPrefixParts != 0 {
+			t.Fatalf("unexpected non-zero dropSrcPathPrefixParts=%d", dropSrcPathPrefixParts)
+		}
 	}
 	f(&UserInfo{}, "/foo/bar")
 	f(&UserInfo{

docs/CHANGELOG.md

@@ -79,6 +79,7 @@ The sandbox cluster installation is running under the constant load generated by
 * FEATURE: [vmalert-tool](https://docs.victoriametrics.com/#vmalert-tool): add `unittest` command to run unittest for alerting and recording rules. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/4789) for details.
 * FEATURE: dashboards/vmalert: add new panel `Missed evaluations` for indicating alerting groups that miss their evaluations.
 * FEATURE: all: track requests with wrong auth key and wrong basic auth at `vm_http_request_errors_total` [metric](https://docs.victoriametrics.com/#monitoring) with `reason="wrong_auth_key"` and `reason="wrong_basic_auth"`. See [this issue](https://github.com/victoriaMetrics/victoriaMetrics/issues/4590). Thanks to @venkatbvc for the [pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5166).
+* FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth.html): add ability to drop the specified number of `/`-delimited prefix parts from the request path before proxying the request to the matching backend. See [these docs](https://docs.victoriametrics.com/vmauth.html#dropping-request-path-prefix).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth.html): add ability to skip TLS verification and to specify TLS Root CA when connecting to backends. See [these docs](https://docs.victoriametrics.com/vmauth.html#backend-tls-setup) and [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5240).
 * FEATURE: `vmstorage`: add `-blockcache.missesBeforeCaching` command-line flag, which can be used for fine-tuning RAM usage for `indexdb/dataBlocks` cache when queries touching big number of time series are executed.
 * FEATURE: add `-loggerMaxArgLen` command-line flag for fine-tuning the maximum lengths of logged args.

docs/vmauth.md

@@ -43,6 +43,38 @@ Pass `-help` to `vmauth` in order to see all the supported command-line flags wi
 Feel free [contacting us](mailto:info@victoriametrics.com) if you need customized auth proxy for VictoriaMetrics with the support of LDAP, SSO, RBAC, SAML,
 accounting and rate limiting such as [vmgateway](https://docs.victoriametrics.com/vmgateway.html).
 
+## Dropping request path prefix
+
+By default `vmauth` doesn't drop the path prefix from the original request when proxying the request to the matching backend.
+Sometimes it is needed to drop path prefix before routing the request to the backend. This can be done by specifying the number of `/`-delimited
+prefix parts to drop from the request path via `drop_src_path_prefix_parts` option at `url_map` level or at `user` level.
+
+For example, if you need to serve requests to [vmalert](https://docs.victoriametrics.com/vmalert.html) at `/vmalert/` path prefix,
+while serving requests to [vmagent](https://docs.victoriametrics.com/vmagent.html) at `/vmagent/` path prefix for a particular user,
+then the following [-auth.config](#auth-config) can be used:
+
+```yml
+users:
+- username: foo
+  url_map:
+
+    # proxy all the requests, which start with `/vmagent/`, to vmagent backend
+  - src_paths:
+    - "/vmagent/.+"
+
+    # drop /vmagent/ path prefix from the original request before proxying it to url_prefix.
+    drop_src_path_prefix_parts: 1
+    url_prefix: "http://vmagent-backend:8429/"
+
+    # proxy all the requests, which start with `/vmalert`, to vmalert backend
+  - src_paths:
+    - "/vmalert/.+"
+
+    # drop /vmalert/ path prefix from the original request before proxying it to url_prefix.
+    drop_src_path_prefix_parts: 1
+    url_prefix: "http://vmalert-backend:8880/"
+```
+
 ## Load balancing
 
 Each `url_prefix` in the [-auth.config](#auth-config) may contain either a single url or a list of urls.