snapshot of https://github.com/grpc/grpc-go.git (master, 2026-09-15; blobs > 131072B omitted)

2026-09-15 20:40:15 +05:00
commit e241b33f26
1365 changed files with 339570 additions and 0 deletions
--- a/internal/resolver/config_selector.go
+++ b/internal/resolver/config_selector.go
@@ -0,0 +1,100 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package resolver provides internal resolver-related functionality.
+package resolver
+
+import (
+	"context"
+	"sync"
+
+	"google.golang.org/grpc/internal/serviceconfig"
+	"google.golang.org/grpc/resolver"
+)
+
+// ConfigSelector controls what configuration to use for every RPC.
+type ConfigSelector interface {
+	// Selects the configuration for the RPC, or terminates it using the error.
+	// This error will be converted by the gRPC library to a status error with
+	// code UNKNOWN if it is not returned as a status error.
+	SelectConfig(RPCInfo) (*RPCConfig, error)
+}
+
+// RPCInfo contains RPC information needed by a ConfigSelector.
+type RPCInfo struct {
+	// Context is the user's context for the RPC and contains headers and
+	// application timeout.  It is passed for interception purposes and for
+	// efficiency reasons.  SelectConfig should not be blocking.
+	Context context.Context
+	Method  string // i.e. "/Service/Method"
+	// Authority is the target authority (host) name of the RPC. This is required
+	// for HTTP filters (such as external processing) to populate request
+	// attributes.
+	Authority string
+}
+
+// RPCConfig describes the configuration to use for each RPC.
+type RPCConfig struct {
+	// The context to use for the remainder of the RPC; can pass info to LB
+	// policy or affect timeout or metadata.
+	Context      context.Context
+	MethodConfig serviceconfig.MethodConfig // configuration to use for this RPC
+	OnCommitted  func()                     // Called when the RPC has been committed (retries no longer possible)
+	Interceptor  any
+}
+
+type csKeyType string
+
+const csKey = csKeyType("grpc.internal.resolver.configSelector")
+
+// SetConfigSelector sets the config selector in state and returns the new
+// state.
+func SetConfigSelector(state resolver.State, cs ConfigSelector) resolver.State {
+	state.Attributes = state.Attributes.WithValue(csKey, cs)
+	return state
+}
+
+// GetConfigSelector retrieves the config selector from state, if present, and
+// returns it or nil if absent.
+func GetConfigSelector(state resolver.State) ConfigSelector {
+	cs, _ := state.Attributes.Value(csKey).(ConfigSelector)
+	return cs
+}
+
+// SafeConfigSelector allows for safe switching of ConfigSelector
+// implementations such that previous values are guaranteed to not be in use
+// when UpdateConfigSelector returns.
+type SafeConfigSelector struct {
+	mu sync.RWMutex
+	cs ConfigSelector
+}
+
+// UpdateConfigSelector swaps to the provided ConfigSelector and blocks until
+// all uses of the previous ConfigSelector have completed.
+func (scs *SafeConfigSelector) UpdateConfigSelector(cs ConfigSelector) {
+	scs.mu.Lock()
+	defer scs.mu.Unlock()
+	scs.cs = cs
+}
+
+// SelectConfig defers to the current ConfigSelector in scs.
+func (scs *SafeConfigSelector) SelectConfig(r RPCInfo) (*RPCConfig, error) {
+	scs.mu.RLock()
+	defer scs.mu.RUnlock()
+	return scs.cs.SelectConfig(r)
+}
--- a/internal/resolver/config_selector_test.go
+++ b/internal/resolver/config_selector_test.go
@@ -0,0 +1,156 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package resolver
+
+import (
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"google.golang.org/grpc/internal/grpctest"
+	"google.golang.org/grpc/internal/serviceconfig"
+)
+
+type s struct {
+	grpctest.Tester
+}
+
+func Test(t *testing.T) {
+	grpctest.RunSubTests(t, s{})
+}
+
+type fakeConfigSelector struct {
+	selectConfig func(RPCInfo) (*RPCConfig, error)
+}
+
+func (f *fakeConfigSelector) SelectConfig(r RPCInfo) (*RPCConfig, error) {
+	return f.selectConfig(r)
+}
+
+func (s) TestSafeConfigSelector(t *testing.T) {
+	testRPCInfo := RPCInfo{Method: "test method"}
+
+	retChan1 := make(chan *RPCConfig)
+	retChan2 := make(chan *RPCConfig)
+	defer close(retChan1)
+	defer close(retChan2)
+
+	one := 1
+	two := 2
+
+	resp1 := &RPCConfig{MethodConfig: serviceconfig.MethodConfig{MaxReqSize: &one}}
+	resp2 := &RPCConfig{MethodConfig: serviceconfig.MethodConfig{MaxReqSize: &two}}
+
+	cs1Called := make(chan struct{}, 1)
+	cs2Called := make(chan struct{}, 1)
+
+	cs1 := &fakeConfigSelector{
+		selectConfig: func(r RPCInfo) (*RPCConfig, error) {
+			cs1Called <- struct{}{}
+			if diff := cmp.Diff(r, testRPCInfo); diff != "" {
+				t.Errorf("SelectConfig(%v) called; want %v\n  Diffs:\n%s", r, testRPCInfo, diff)
+			}
+			return <-retChan1, nil
+		},
+	}
+	cs2 := &fakeConfigSelector{
+		selectConfig: func(r RPCInfo) (*RPCConfig, error) {
+			cs2Called <- struct{}{}
+			if diff := cmp.Diff(r, testRPCInfo); diff != "" {
+				t.Errorf("SelectConfig(%v) called; want %v\n  Diffs:\n%s", r, testRPCInfo, diff)
+			}
+			return <-retChan2, nil
+		},
+	}
+
+	scs := &SafeConfigSelector{}
+	scs.UpdateConfigSelector(cs1)
+
+	cs1Returned := make(chan struct{})
+	go func() {
+		got, err := scs.SelectConfig(testRPCInfo) // blocks until send to retChan1
+		if err != nil || got != resp1 {
+			t.Errorf("SelectConfig(%v) = %v, %v; want %v, nil", testRPCInfo, got, err, resp1)
+		}
+		close(cs1Returned)
+	}()
+
+	// cs1 is blocked but should be called
+	select {
+	case <-time.After(500 * time.Millisecond):
+		t.Fatalf("timed out waiting for cs1 to be called")
+	case <-cs1Called:
+	}
+
+	// swap in cs2 now that cs1 is called
+	csSwapped := make(chan struct{})
+	go func() {
+		// wait awhile first to ensure cs1 could be called below.
+		time.Sleep(50 * time.Millisecond)
+		scs.UpdateConfigSelector(cs2) // Blocks until cs1 done
+		close(csSwapped)
+	}()
+
+	// Allow cs1 to return and cs2 to eventually be swapped in.
+	retChan1 <- resp1
+
+	cs1Done := false // set when cs2 is first called
+	for dl := time.Now().Add(150 * time.Millisecond); !time.Now().After(dl); {
+		gotConfigChan := make(chan *RPCConfig, 1)
+		go func() {
+			cfg, _ := scs.SelectConfig(testRPCInfo)
+			gotConfigChan <- cfg
+		}()
+		select {
+		case <-time.After(500 * time.Millisecond):
+			t.Fatalf("timed out waiting for cs1 or cs2 to be called")
+		case <-cs1Called:
+			// Initially, before swapping to cs2, cs1 should be called
+			retChan1 <- resp1
+			go func() { <-gotConfigChan }()
+			if cs1Done {
+				t.Fatalf("cs1 called after cs2")
+			}
+		case <-cs2Called:
+			// Success! the new config selector is being called
+			if !cs1Done {
+				select {
+				case <-csSwapped:
+				case <-time.After(50 * time.Millisecond):
+					t.Fatalf("timed out waiting for UpdateConfigSelector to return")
+				}
+				select {
+				case <-cs1Returned:
+				case <-time.After(50 * time.Millisecond):
+					t.Fatalf("timed out waiting for cs1 to return")
+				}
+				cs1Done = true
+			}
+			retChan2 <- resp2
+			got := <-gotConfigChan
+			if diff := cmp.Diff(got, resp2); diff != "" {
+				t.Fatalf("SelectConfig(%v) = %v; want %v\n  Diffs:\n%s", testRPCInfo, got, resp2, diff)
+			}
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	if !cs1Done {
+		t.Fatalf("timed out waiting for cs2 to be called")
+	}
+}
--- a/internal/resolver/delegatingresolver/delegatingresolver.go
+++ b/internal/resolver/delegatingresolver/delegatingresolver.go
@@ -0,0 +1,477 @@
+/*
+ *
+ * Copyright 2024 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package delegatingresolver implements a resolver capable of resolving both
+// target URIs and proxy addresses.
+package delegatingresolver
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"sync"
+
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/envconfig"
+	"google.golang.org/grpc/internal/proxyattributes"
+	"google.golang.org/grpc/internal/transport"
+	"google.golang.org/grpc/internal/transport/networktype"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+var (
+	logger = grpclog.Component("delegating-resolver")
+	// HTTPSProxyFromEnvironment will be overwritten in the tests
+	HTTPSProxyFromEnvironment = http.ProxyFromEnvironment
+)
+
+const defaultPort = "443"
+
+// delegatingResolver manages both target URI and proxy address resolution by
+// delegating these tasks to separate child resolvers. Essentially, it acts as
+// an intermediary between the gRPC ClientConn and the child resolvers.
+//
+// It implements the [resolver.Resolver] interface.
+type delegatingResolver struct {
+	target   resolver.Target     // parsed target URI to be resolved
+	cc       resolver.ClientConn // gRPC ClientConn
+	proxyURL *url.URL            // proxy URL, derived from proxy environment and target
+
+	// We do not hold both mu and childMu in the same goroutine. Avoid holding
+	// both locks when calling into the child, as the child resolver may
+	// synchronously callback into the channel.
+	mu                  sync.Mutex         // protects all the fields below
+	targetResolverState *resolver.State    // state of the target resolver
+	proxyAddrs          []resolver.Address // resolved proxy addresses; empty if no proxy is configured
+
+	// childMu serializes calls into child resolvers. It also protects access to
+	// the following fields.
+	childMu        sync.Mutex
+	targetResolver resolver.Resolver // resolver for the target URI, based on its scheme
+	proxyResolver  resolver.Resolver // resolver for the proxy URI; nil if no proxy is configured
+}
+
+// nopResolver is a resolver that does nothing.
+type nopResolver struct{}
+
+func (nopResolver) ResolveNow(resolver.ResolveNowOptions) {}
+
+func (nopResolver) Close() {}
+
+// proxyURLForTarget determines the proxy URL for the given address based on the
+// environment. It can return the following:
+//   - nil URL, nil error: No proxy is configured or the address is excluded
+//     using the `NO_PROXY` environment variable or if req.URL.Host is
+//     "localhost" (with or without // a port number)
+//   - nil URL, non-nil error: An error occurred while retrieving the proxy URL.
+//   - non-nil URL, nil error: A proxy is configured, and the proxy URL was
+//     retrieved successfully without any errors.
+func proxyURLForTarget(address string) (*url.URL, error) {
+	req := &http.Request{URL: &url.URL{
+		Scheme: "https",
+		Host:   address,
+	}}
+	return HTTPSProxyFromEnvironment(req)
+}
+
+// New creates a new delegating resolver that can create up to two child
+// resolvers:
+//   - one to resolve the proxy address specified using the supported
+//     environment variables. This uses the registered resolver for the "dns"
+//     scheme. It is lazily built when a target resolver update contains at least
+//     one TCP address.
+//   - one to resolve the target URI using the resolver specified by the scheme
+//     in the target URI or specified by the user using the WithResolvers dial
+//     option. As a special case, if the target URI's scheme is "dns" and a
+//     proxy is specified using the supported environment variables, the target
+//     URI's path portion is used as the resolved address unless target
+//     resolution is enabled using the dial option.
+func New(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions, targetResolverBuilder resolver.Builder, targetResolutionEnabled bool) (resolver.Resolver, error) {
+	r := &delegatingResolver{
+		target:         target,
+		cc:             cc,
+		proxyResolver:  nopResolver{},
+		targetResolver: nopResolver{},
+	}
+
+	addr := target.Endpoint()
+	var err error
+	if target.URL.Scheme == "dns" && !targetResolutionEnabled && envconfig.EnableDefaultPortForProxyTarget {
+		addr, err = parseTarget(addr)
+		if err != nil {
+			return nil, fmt.Errorf("delegating_resolver: invalid target address %q: %v", target.Endpoint(), err)
+		}
+	}
+
+	r.proxyURL, err = proxyURLForTarget(addr)
+	if err != nil {
+		return nil, fmt.Errorf("delegating_resolver: failed to determine proxy URL for target %q: %v", target, err)
+	}
+
+	// proxy is not configured or proxy address excluded using `NO_PROXY` env
+	// var, so only target resolver is used.
+	if r.proxyURL == nil {
+		return targetResolverBuilder.Build(target, cc, opts)
+	}
+
+	if logger.V(2) {
+		logger.Infof("Proxy URL detected : %s", r.proxyURL)
+	}
+
+	// Resolver updates from one child may trigger calls into the other. Block
+	// updates until the children are initialized.
+	r.childMu.Lock()
+	defer r.childMu.Unlock()
+	// When the scheme is 'dns' and target resolution on client is not enabled,
+	// resolution should be handled by the proxy, not the client. Therefore, we
+	// bypass the target resolver and store the unresolved target address.
+	if target.URL.Scheme == "dns" && !targetResolutionEnabled {
+		r.targetResolverState = &resolver.State{
+			Addresses: []resolver.Address{{Addr: addr}},
+			Endpoints: []resolver.Endpoint{{Addresses: []resolver.Address{{Addr: addr}}}},
+		}
+		r.updateTargetResolverState(*r.targetResolverState)
+		return r, nil
+	}
+	wcc := &wrappingClientConn{
+		stateListener: r.updateTargetResolverState,
+		parent:        r,
+	}
+	if r.targetResolver, err = targetResolverBuilder.Build(target, wcc, opts); err != nil {
+		return nil, fmt.Errorf("delegating_resolver: unable to build the resolver for target %s: %v", target, err)
+	}
+	return r, nil
+}
+
+// proxyURIResolver creates a resolver for resolving proxy URIs using the "dns"
+// scheme. It adjusts the proxyURL to conform to the "dns:///" format and builds
+// a resolver with a wrappingClientConn to capture resolved addresses.
+func (r *delegatingResolver) proxyURIResolver(opts resolver.BuildOptions) (resolver.Resolver, error) {
+	proxyBuilder := resolver.Get("dns")
+	if proxyBuilder == nil {
+		panic("delegating_resolver: resolver for proxy not found for scheme dns")
+	}
+	url := *r.proxyURL
+	url.Scheme = "dns"
+	url.Path = "/" + r.proxyURL.Host
+	url.Host = "" // Clear the Host field to conform to the "dns:///" format
+
+	proxyTarget := resolver.Target{URL: url}
+	wcc := &wrappingClientConn{
+		stateListener: r.updateProxyResolverState,
+		parent:        r,
+	}
+	return proxyBuilder.Build(proxyTarget, wcc, opts)
+}
+
+func (r *delegatingResolver) ResolveNow(o resolver.ResolveNowOptions) {
+	r.childMu.Lock()
+	defer r.childMu.Unlock()
+	r.targetResolver.ResolveNow(o)
+	r.proxyResolver.ResolveNow(o)
+}
+
+func (r *delegatingResolver) Close() {
+	r.childMu.Lock()
+	defer r.childMu.Unlock()
+	r.targetResolver.Close()
+	r.targetResolver = nil
+
+	r.proxyResolver.Close()
+	r.proxyResolver = nil
+}
+
+func needsProxyResolver(state *resolver.State) bool {
+	for _, addr := range state.Addresses {
+		if !skipProxy(addr) {
+			return true
+		}
+	}
+	for _, endpoint := range state.Endpoints {
+		for _, addr := range endpoint.Addresses {
+			if !skipProxy(addr) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// parseTarget takes a target string and ensures it is a valid "host:port" target.
+//
+// It does the following:
+//  1. If the target already has a port (e.g., "host:port", "[ipv6]:port"),
+//     it is returned as is.
+//  2. If the host part is empty (e.g., ":80"), it defaults to "localhost",
+//     returning "localhost:80".
+//  3. If the target is missing a port (e.g., "host", "ipv6"), the defaultPort
+//     is added.
+//
+// An error is returned for empty targets or targets with a trailing colon
+// but no port (e.g., "host:").
+func parseTarget(target string) (string, error) {
+	if target == "" {
+		return "", fmt.Errorf("missing address")
+	}
+
+	host, port, err := net.SplitHostPort(target)
+	if err != nil {
+		// If SplitHostPort fails, it's likely because the port is missing.
+		// We append the default port and return the result.
+		return net.JoinHostPort(target, defaultPort), nil
+	}
+
+	// If SplitHostPort succeeds, we check for edge cases.
+	if port == "" {
+		// A success with an empty port means the target had a trailing colon,
+		// e.g., "host:", which is an error.
+		return "", fmt.Errorf("missing port after port-separator colon")
+	}
+	if host == "" {
+		// A success with an empty host means the target was like ":80".
+		// We default the host to "localhost".
+		host = "localhost"
+	}
+	return net.JoinHostPort(host, port), nil
+}
+
+func skipProxy(address resolver.Address) bool {
+	// Avoid proxy when network is not tcp.
+	networkType, ok := networktype.Get(address)
+	if !ok {
+		networkType, _ = transport.ParseDialTarget(address.Addr)
+	}
+	if networkType != "tcp" {
+		return true
+	}
+
+	req := &http.Request{URL: &url.URL{
+		Scheme: "https",
+		Host:   address.Addr,
+	}}
+	// Avoid proxy when address included in `NO_PROXY` environment variable or
+	// fails to get the proxy address.
+	url, err := HTTPSProxyFromEnvironment(req)
+	if err != nil || url == nil {
+		return true
+	}
+	return false
+}
+
+// updateClientConnStateLocked constructs a combined list of addresses by
+// pairing each proxy address with every target address of type TCP. For each
+// pair, it creates a new [resolver.Address] using the proxy address and
+// attaches the corresponding target address and user info as attributes. Target
+// addresses that are not of type TCP are appended to the list as-is. The
+// function returns nil if either resolver has not yet provided an update, and
+// returns the result of ClientConn.UpdateState once both resolvers have
+// provided at least one update.
+func (r *delegatingResolver) updateClientConnStateLocked() error {
+	if r.targetResolverState == nil || r.proxyAddrs == nil {
+		return nil
+	}
+
+	// If multiple resolved proxy addresses are present, we send only the
+	// unresolved proxy host and let net.Dial handle the proxy host name
+	// resolution when creating the transport. Sending all resolved addresses
+	// would increase the number of addresses passed to the ClientConn and
+	// subsequently to load balancing (LB) policies like Round Robin, leading
+	// to additional TCP connections. However, if there's only one resolved
+	// proxy address, we send it directly, as it doesn't affect the address
+	// count returned by the target resolver and the address count sent to the
+	// ClientConn.
+	var proxyAddr resolver.Address
+	if len(r.proxyAddrs) == 1 {
+		proxyAddr = r.proxyAddrs[0]
+	} else {
+		proxyAddr = resolver.Address{Addr: r.proxyURL.Host}
+	}
+	var addresses []resolver.Address
+	for _, targetAddr := range (*r.targetResolverState).Addresses {
+		if skipProxy(targetAddr) {
+			addresses = append(addresses, targetAddr)
+			continue
+		}
+		addresses = append(addresses, proxyattributes.Set(proxyAddr, proxyattributes.Options{
+			User:        r.proxyURL.User,
+			ConnectAddr: targetAddr.Addr,
+		}))
+	}
+
+	// For each target endpoint, construct a new [resolver.Endpoint] that
+	// includes all addresses from all proxy endpoints and the addresses from
+	// that target endpoint, preserving the number of target endpoints.
+	var endpoints []resolver.Endpoint
+	for _, endpt := range (*r.targetResolverState).Endpoints {
+		var addrs []resolver.Address
+		for _, targetAddr := range endpt.Addresses {
+			// Avoid proxy when network is not tcp.
+			if skipProxy(targetAddr) {
+				addrs = append(addrs, targetAddr)
+				continue
+			}
+			for _, proxyAddr := range r.proxyAddrs {
+				addrs = append(addrs, proxyattributes.Set(proxyAddr, proxyattributes.Options{
+					User:        r.proxyURL.User,
+					ConnectAddr: targetAddr.Addr,
+				}))
+			}
+		}
+		endpoints = append(endpoints, resolver.Endpoint{Addresses: addrs})
+	}
+	// Use the targetResolverState for its service config and attributes
+	// contents. The state update is only sent after both the target and proxy
+	// resolvers have sent their updates, and curState has been updated with the
+	// combined addresses.
+	curState := *r.targetResolverState
+	curState.Addresses = addresses
+	curState.Endpoints = endpoints
+	return r.cc.UpdateState(curState)
+}
+
+// updateProxyResolverState updates the proxy resolver state by storing proxy
+// addresses and endpoints, marking the resolver as ready, and triggering a
+// state update if both proxy and target resolvers are ready. If the ClientConn
+// returns a non-nil error, it calls `ResolveNow()` on the target resolver.  It
+// is a StateListener function of wrappingClientConn passed to the proxy
+// resolver.
+func (r *delegatingResolver) updateProxyResolverState(state resolver.State) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if logger.V(2) {
+		logger.Infof("Addresses received from proxy resolver: %s", state.Addresses)
+	}
+	if len(state.Endpoints) > 0 {
+		// We expect exactly one address per endpoint because the proxy resolver
+		// uses "dns" resolution.
+		r.proxyAddrs = make([]resolver.Address, 0, len(state.Endpoints))
+		for _, endpoint := range state.Endpoints {
+			r.proxyAddrs = append(r.proxyAddrs, endpoint.Addresses...)
+		}
+	} else if state.Addresses != nil {
+		r.proxyAddrs = state.Addresses
+	} else {
+		r.proxyAddrs = []resolver.Address{} // ensure proxyAddrs is non-nil to indicate an update has been received
+	}
+	err := r.updateClientConnStateLocked()
+	// Another possible approach was to block until updates are received from
+	// both resolvers. But this is not used because calling `New()` triggers
+	// `Build()` for the first resolver, which calls `UpdateState()`. And the
+	// second resolver hasn't sent an update yet, so it would cause `New()` to
+	// block indefinitely.
+	if err != nil {
+		go func() {
+			r.childMu.Lock()
+			defer r.childMu.Unlock()
+			if r.targetResolver != nil {
+				r.targetResolver.ResolveNow(resolver.ResolveNowOptions{})
+			}
+		}()
+	}
+	return err
+}
+
+// updateTargetResolverState is the StateListener function provided to the
+// target resolver via wrappingClientConn. It updates the resolver state and
+// marks the target resolver as ready. If the update includes at least one TCP
+// address and the proxy resolver has not yet been constructed, it initializes
+// the proxy resolver. A combined state update is triggered once both resolvers
+// are ready. If all addresses are non-TCP, it proceeds without waiting for the
+// proxy resolver. If ClientConn.UpdateState returns a non-nil error,
+// ResolveNow() is called on the proxy resolver.
+func (r *delegatingResolver) updateTargetResolverState(state resolver.State) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if logger.V(2) {
+		logger.Infof("Addresses received from target resolver: %v", state.Addresses)
+	}
+	r.targetResolverState = &state
+	// If all addresses returned by the target resolver have a non-TCP network
+	// type, or are listed in the `NO_PROXY` environment variable, do not wait
+	// for proxy update.
+	if !needsProxyResolver(r.targetResolverState) {
+		return r.cc.UpdateState(*r.targetResolverState)
+	}
+
+	// The proxy resolver may be rebuilt multiple times, specifically each time
+	// the target resolver sends an update, even if the target resolver is built
+	// successfully but building the proxy resolver fails.
+	if len(r.proxyAddrs) == 0 {
+		go func() {
+			r.childMu.Lock()
+			defer r.childMu.Unlock()
+			if _, ok := r.proxyResolver.(nopResolver); !ok {
+				return
+			}
+			proxyResolver, err := r.proxyURIResolver(resolver.BuildOptions{})
+			if err != nil {
+				r.cc.ReportError(fmt.Errorf("delegating_resolver: unable to build the proxy resolver: %v", err))
+				return
+			}
+			r.proxyResolver = proxyResolver
+		}()
+	}
+
+	err := r.updateClientConnStateLocked()
+	if err != nil {
+		go func() {
+			r.childMu.Lock()
+			defer r.childMu.Unlock()
+			if r.proxyResolver != nil {
+				r.proxyResolver.ResolveNow(resolver.ResolveNowOptions{})
+			}
+		}()
+	}
+	return nil
+}
+
+// wrappingClientConn serves as an intermediary between the parent ClientConn
+// and the child resolvers created here. It implements the resolver.ClientConn
+// interface and is passed in that capacity to the child resolvers.
+type wrappingClientConn struct {
+	// Callback to deliver resolver state updates
+	stateListener func(state resolver.State) error
+	parent        *delegatingResolver
+}
+
+// UpdateState receives resolver state updates and forwards them to the
+// appropriate listener function (either for the proxy or target resolver).
+func (wcc *wrappingClientConn) UpdateState(state resolver.State) error {
+	return wcc.stateListener(state)
+}
+
+// ReportError intercepts errors from the child resolvers and passes them to
+// ClientConn.
+func (wcc *wrappingClientConn) ReportError(err error) {
+	wcc.parent.cc.ReportError(err)
+}
+
+// NewAddress intercepts the new resolved address from the child resolvers and
+// passes them to ClientConn.
+func (wcc *wrappingClientConn) NewAddress(addrs []resolver.Address) {
+	wcc.UpdateState(resolver.State{Addresses: addrs})
+}
+
+// ParseServiceConfig parses the provided service config and returns an object
+// that provides the parsed config.
+func (wcc *wrappingClientConn) ParseServiceConfig(serviceConfigJSON string) *serviceconfig.ParseResult {
+	return wcc.parent.cc.ParseServiceConfig(serviceConfigJSON)
+}
--- a/internal/resolver/delegatingresolver/delegatingresolver_ext_test.go
+++ b/internal/resolver/delegatingresolver/delegatingresolver_ext_test.go
--- a/internal/resolver/delegatingresolver/delegatingresolver_test.go
+++ b/internal/resolver/delegatingresolver/delegatingresolver_test.go
@@ -0,0 +1,107 @@
+/*
+ *
+ * Copyright 2024 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package delegatingresolver
+
+import (
+	"errors"
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"google.golang.org/grpc/internal/grpctest"
+)
+
+type s struct {
+	grpctest.Tester
+}
+
+func Test(t *testing.T) {
+	grpctest.RunSubTests(t, s{})
+}
+
+const targetTestAddr = "test.com"
+
+// overrideHTTPSProxyFromEnvironment function overwrites HTTPSProxyFromEnvironment and
+// returns a function to restore the default values.
+func overrideHTTPSProxyFromEnvironment(hpfe func(req *http.Request) (*url.URL, error)) func() {
+	HTTPSProxyFromEnvironment = hpfe
+	return func() {
+		HTTPSProxyFromEnvironment = nil
+	}
+}
+
+// Tests that the proxyURLForTarget function correctly resolves the proxy URL
+// for a given target address. Tests all the possible output cases.
+func (s) TestproxyURLForTargetEnv(t *testing.T) {
+	err := errors.New("invalid proxy url")
+	tests := []struct {
+		name     string
+		hpfeFunc func(req *http.Request) (*url.URL, error)
+		wantURL  *url.URL
+		wantErr  error
+	}{
+		{
+			name: "valid_proxy_url_and_nil_error",
+			hpfeFunc: func(_ *http.Request) (*url.URL, error) {
+				return &url.URL{
+					Scheme: "https",
+					Host:   "proxy.example.com",
+				}, nil
+			},
+			wantURL: &url.URL{
+				Scheme: "https",
+				Host:   "proxy.example.com",
+			},
+		},
+		{
+			name: "invalid_proxy_url_and_non-nil_error",
+			hpfeFunc: func(_ *http.Request) (*url.URL, error) {
+				return &url.URL{
+					Scheme: "https",
+					Host:   "notproxy.example.com",
+				}, err
+			},
+			wantURL: &url.URL{
+				Scheme: "https",
+				Host:   "notproxy.example.com",
+			},
+			wantErr: err,
+		},
+		{
+			name: "nil_proxy_url_and_nil_error",
+			hpfeFunc: func(_ *http.Request) (*url.URL, error) {
+				return nil, nil
+			},
+			wantURL: nil,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			defer overrideHTTPSProxyFromEnvironment(tt.hpfeFunc)()
+			got, err := proxyURLForTarget(targetTestAddr)
+			if err != tt.wantErr {
+				t.Errorf("parsedProxyURLForProxy(%v) failed with error :%v, want %v\n", targetTestAddr, err, tt.wantErr)
+			}
+			if !cmp.Equal(got, tt.wantURL) {
+				t.Fatalf("parsedProxyURLForProxy(%v) = %v, want %v\n", targetTestAddr, got, tt.wantURL)
+			}
+		})
+	}
+}
--- a/internal/resolver/dns/dns_resolver.go
+++ b/internal/resolver/dns/dns_resolver.go
@@ -0,0 +1,472 @@
+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package dns implements a dns resolver to be installed as the default resolver
+// in grpc.
+package dns
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	rand "math/rand/v2"
+	"net"
+	"net/netip"
+	"os"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	grpclbstate "google.golang.org/grpc/balancer/grpclb/state"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/backoff"
+	"google.golang.org/grpc/internal/envconfig"
+	"google.golang.org/grpc/internal/resolver/dns/internal"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+var (
+	// EnableSRVLookups controls whether the DNS resolver attempts to fetch gRPCLB
+	// addresses from SRV records.  Must not be changed after init time.
+	EnableSRVLookups = false
+
+	// MinResolutionInterval is the minimum interval at which re-resolutions are
+	// allowed. This helps to prevent excessive re-resolution.
+	MinResolutionInterval = 30 * time.Second
+
+	// ResolvingTimeout specifies the maximum duration for a DNS resolution request.
+	// If the timeout expires before a response is received, the request will be canceled.
+	//
+	// It is recommended to set this value at application startup. Avoid modifying this variable
+	// after initialization as it's not thread-safe for concurrent modification.
+	ResolvingTimeout = 30 * time.Second
+
+	logger = grpclog.Component("dns")
+)
+
+func init() {
+	resolver.Register(NewBuilder())
+	internal.TimeAfterFunc = time.After
+	internal.TimeNowFunc = time.Now
+	internal.TimeUntilFunc = time.Until
+	internal.NewNetResolver = newNetResolver
+	internal.AddressDialer = addressDialer
+}
+
+const (
+	defaultPort       = "443"
+	defaultDNSSvrPort = "53"
+	golang            = "GO"
+	// txtPrefix is the prefix string to be prepended to the host name for txt
+	// record lookup.
+	txtPrefix = "_grpc_config."
+	// In DNS, service config is encoded in a TXT record via the mechanism
+	// described in RFC-1464 using the attribute name grpc_config.
+	txtAttribute = "grpc_config="
+)
+
+var addressDialer = func(address string) func(context.Context, string, string) (net.Conn, error) {
+	return func(ctx context.Context, network, _ string) (net.Conn, error) {
+		var dialer net.Dialer
+		return dialer.DialContext(ctx, network, address)
+	}
+}
+
+var newNetResolver = func(authority string) (internal.NetResolver, error) {
+	if authority == "" {
+		return net.DefaultResolver, nil
+	}
+
+	host, port, err := parseTarget(authority, defaultDNSSvrPort)
+	if err != nil {
+		return nil, err
+	}
+
+	authorityWithPort := net.JoinHostPort(host, port)
+
+	return &net.Resolver{
+		PreferGo: true,
+		Dial:     internal.AddressDialer(authorityWithPort),
+	}, nil
+}
+
+// NewBuilder creates a dnsBuilder which is used to factory DNS resolvers.
+func NewBuilder() resolver.Builder {
+	return &dnsBuilder{}
+}
+
+type dnsBuilder struct{}
+
+// Build creates and starts a DNS resolver that watches the name resolution of
+// the target.
+func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
+	host, port, err := parseTarget(target.Endpoint(), defaultPort)
+	if err != nil {
+		return nil, err
+	}
+
+	// IP address.
+	if ipAddr, err := formatIP(host); err == nil {
+		addr := []resolver.Address{{Addr: ipAddr + ":" + port}}
+		cc.UpdateState(resolver.State{
+			Addresses: addr,
+			Endpoints: []resolver.Endpoint{{Addresses: addr}},
+		})
+		return deadResolver{}, nil
+	}
+
+	// DNS address (non-IP).
+	ctx, cancel := context.WithCancel(context.Background())
+	d := &dnsResolver{
+		host:                host,
+		port:                port,
+		ctx:                 ctx,
+		cancel:              cancel,
+		cc:                  cc,
+		rn:                  make(chan struct{}, 1),
+		enableServiceConfig: envconfig.EnableTXTServiceConfig && !opts.DisableServiceConfig,
+	}
+
+	d.resolver, err = internal.NewNetResolver(target.URL.Host)
+	if err != nil {
+		return nil, err
+	}
+
+	d.wg.Add(1)
+	go d.watcher()
+	return d, nil
+}
+
+// Scheme returns the naming scheme of this resolver builder, which is "dns".
+func (b *dnsBuilder) Scheme() string {
+	return "dns"
+}
+
+// deadResolver is a resolver that does nothing.
+type deadResolver struct{}
+
+func (deadResolver) ResolveNow(resolver.ResolveNowOptions) {}
+
+func (deadResolver) Close() {}
+
+// dnsResolver watches for the name resolution update for a non-IP target.
+type dnsResolver struct {
+	host     string
+	port     string
+	resolver internal.NetResolver
+	ctx      context.Context
+	cancel   context.CancelFunc
+	cc       resolver.ClientConn
+	// rn channel is used by ResolveNow() to force an immediate resolution of the
+	// target.
+	rn chan struct{}
+	// wg is used to enforce Close() to return after the watcher() goroutine has
+	// finished. Otherwise, data race will be possible. [Race Example] in
+	// dns_resolver_test we replace the real lookup functions with mocked ones to
+	// facilitate testing. If Close() doesn't wait for watcher() goroutine
+	// finishes, race detector sometimes will warn lookup (READ the lookup
+	// function pointers) inside watcher() goroutine has data race with
+	// replaceNetFunc (WRITE the lookup function pointers).
+	wg                  sync.WaitGroup
+	enableServiceConfig bool
+}
+
+// ResolveNow invoke an immediate resolution of the target that this
+// dnsResolver watches.
+func (d *dnsResolver) ResolveNow(resolver.ResolveNowOptions) {
+	select {
+	case d.rn <- struct{}{}:
+	default:
+	}
+}
+
+// Close closes the dnsResolver.
+func (d *dnsResolver) Close() {
+	d.cancel()
+	d.wg.Wait()
+}
+
+func (d *dnsResolver) watcher() {
+	defer d.wg.Done()
+	backoffIndex := 1
+	for {
+		state, err := d.lookup()
+		if err != nil {
+			// Report error to the underlying grpc.ClientConn.
+			d.cc.ReportError(err)
+		} else {
+			err = d.cc.UpdateState(*state)
+		}
+
+		var nextResolutionTime time.Time
+		if err == nil {
+			// Success resolving, wait for the next ResolveNow. However, also wait 30
+			// seconds at the very least to prevent constantly re-resolving.
+			backoffIndex = 1
+			nextResolutionTime = internal.TimeNowFunc().Add(MinResolutionInterval)
+			select {
+			case <-d.ctx.Done():
+				return
+			case <-d.rn:
+			}
+		} else {
+			// Poll on an error found in DNS Resolver or an error received from
+			// ClientConn.
+			nextResolutionTime = internal.TimeNowFunc().Add(backoff.DefaultExponential.Backoff(backoffIndex))
+			backoffIndex++
+		}
+		select {
+		case <-d.ctx.Done():
+			return
+		case <-internal.TimeAfterFunc(internal.TimeUntilFunc(nextResolutionTime)):
+		}
+	}
+}
+
+func (d *dnsResolver) lookupSRV(ctx context.Context) ([]resolver.Address, error) {
+	// Skip this particular host to avoid timeouts with some versions of
+	// systemd-resolved.
+	if !EnableSRVLookups || d.host == "metadata.google.internal." {
+		return nil, nil
+	}
+	var newAddrs []resolver.Address
+	_, srvs, err := d.resolver.LookupSRV(ctx, "grpclb", "tcp", d.host)
+	if err != nil {
+		err = handleDNSError(err, "SRV") // may become nil
+		return nil, err
+	}
+	for _, s := range srvs {
+		lbAddrs, err := d.resolver.LookupHost(ctx, s.Target)
+		if err != nil {
+			err = handleDNSError(err, "A") // may become nil
+			if err == nil {
+				// If there are other SRV records, look them up and ignore this
+				// one that does not exist.
+				continue
+			}
+			return nil, err
+		}
+		for _, a := range lbAddrs {
+			ip, err := formatIP(a)
+			if err != nil {
+				return nil, fmt.Errorf("dns: error parsing A record IP address %v: %v", a, err)
+			}
+			addr := ip + ":" + strconv.Itoa(int(s.Port))
+			newAddrs = append(newAddrs, resolver.Address{Addr: addr, ServerName: s.Target})
+		}
+	}
+	return newAddrs, nil
+}
+
+func handleDNSError(err error, lookupType string) error {
+	dnsErr, ok := err.(*net.DNSError)
+	if ok && !dnsErr.IsTimeout && !dnsErr.IsTemporary {
+		// Timeouts and temporary errors should be communicated to gRPC to
+		// attempt another DNS query (with backoff).  Other errors should be
+		// suppressed (they may represent the absence of a TXT record).
+		return nil
+	}
+	if err != nil {
+		err = fmt.Errorf("dns: %v record lookup error: %v", lookupType, err)
+		logger.Info(err)
+	}
+	return err
+}
+
+func (d *dnsResolver) lookupTXT(ctx context.Context) *serviceconfig.ParseResult {
+	ss, err := d.resolver.LookupTXT(ctx, txtPrefix+d.host)
+	if err != nil {
+		if envconfig.TXTErrIgnore {
+			return nil
+		}
+		if err = handleDNSError(err, "TXT"); err != nil {
+			return &serviceconfig.ParseResult{Err: err}
+		}
+		return nil
+	}
+	var res string
+	for _, s := range ss {
+		res += s
+	}
+
+	// TXT record must have "grpc_config=" attribute in order to be used as
+	// service config.
+	if !strings.HasPrefix(res, txtAttribute) {
+		logger.Warningf("dns: TXT record %v missing %v attribute", res, txtAttribute)
+		// This is not an error; it is the equivalent of not having a service
+		// config.
+		return nil
+	}
+	sc := canaryingSC(strings.TrimPrefix(res, txtAttribute))
+	return d.cc.ParseServiceConfig(sc)
+}
+
+func (d *dnsResolver) lookupHost(ctx context.Context) ([]resolver.Address, error) {
+	addrs, err := d.resolver.LookupHost(ctx, d.host)
+	if err != nil {
+		err = handleDNSError(err, "A")
+		return nil, err
+	}
+	newAddrs := make([]resolver.Address, 0, len(addrs))
+	for _, a := range addrs {
+		ip, err := formatIP(a)
+		if err != nil {
+			return nil, fmt.Errorf("dns: error parsing A record IP address %v: %v", a, err)
+		}
+		addr := ip + ":" + d.port
+		newAddrs = append(newAddrs, resolver.Address{Addr: addr})
+	}
+	return newAddrs, nil
+}
+
+func (d *dnsResolver) lookup() (*resolver.State, error) {
+	ctx, cancel := context.WithTimeout(d.ctx, ResolvingTimeout)
+	defer cancel()
+	srv, srvErr := d.lookupSRV(ctx)
+	addrs, hostErr := d.lookupHost(ctx)
+	if hostErr != nil && (srvErr != nil || len(srv) == 0) {
+		return nil, hostErr
+	}
+
+	eps := make([]resolver.Endpoint, 0, len(addrs))
+	for _, addr := range addrs {
+		eps = append(eps, resolver.Endpoint{Addresses: []resolver.Address{addr}})
+	}
+
+	state := resolver.State{
+		Addresses: addrs,
+		Endpoints: eps,
+	}
+	if len(srv) > 0 {
+		state = grpclbstate.Set(state, &grpclbstate.State{BalancerAddresses: srv})
+	}
+	if d.enableServiceConfig {
+		state.ServiceConfig = d.lookupTXT(ctx)
+	}
+	return &state, nil
+}
+
+// formatIP returns an error if addr is not a valid textual representation of
+// an IP address. If addr is an IPv4 address, return the addr and error = nil.
+// If addr is an IPv6 address, return the addr enclosed in square brackets and
+// error = nil.
+func formatIP(addr string) (string, error) {
+	ip, err := netip.ParseAddr(addr)
+	if err != nil {
+		return "", err
+	}
+	if ip.Is4() {
+		return addr, nil
+	}
+	return "[" + addr + "]", nil
+}
+
+// parseTarget takes the user input target string and default port, returns
+// formatted host and port info. If target doesn't specify a port, set the port
+// to be the defaultPort. If target is in IPv6 format and host-name is enclosed
+// in square brackets, brackets are stripped when setting the host.
+// examples:
+// target: "www.google.com" defaultPort: "443" returns host: "www.google.com", port: "443"
+// target: "ipv4-host:80" defaultPort: "443" returns host: "ipv4-host", port: "80"
+// target: "[ipv6-host]" defaultPort: "443" returns host: "ipv6-host", port: "443"
+// target: ":80" defaultPort: "443" returns host: "localhost", port: "80"
+func parseTarget(target, defaultPort string) (host, port string, err error) {
+	if target == "" {
+		return "", "", internal.ErrMissingAddr
+	}
+	if _, err := netip.ParseAddr(target); err == nil {
+		// target is an IPv4 or IPv6(without brackets) address
+		return target, defaultPort, nil
+	}
+	if host, port, err = net.SplitHostPort(target); err == nil {
+		if port == "" {
+			// If the port field is empty (target ends with colon), e.g. "[::1]:",
+			// this is an error.
+			return "", "", internal.ErrEndsWithColon
+		}
+		// target has port, i.e ipv4-host:port, [ipv6-host]:port, host-name:port
+		if host == "" {
+			// Keep consistent with net.Dial(): If the host is empty, as in ":80",
+			// the local system is assumed.
+			host = "localhost"
+		}
+		return host, port, nil
+	}
+	if host, port, err = net.SplitHostPort(target + ":" + defaultPort); err == nil {
+		// target doesn't have port
+		return host, port, nil
+	}
+	return "", "", fmt.Errorf("invalid target address %v, error info: %v", target, err)
+}
+
+type rawChoice struct {
+	ClientLanguage *[]string        `json:"clientLanguage,omitempty"`
+	Percentage     *int             `json:"percentage,omitempty"`
+	ClientHostName *[]string        `json:"clientHostName,omitempty"`
+	ServiceConfig  *json.RawMessage `json:"serviceConfig,omitempty"`
+}
+
+func containsString(a *[]string, b string) bool {
+	if a == nil {
+		return true
+	}
+	for _, c := range *a {
+		if c == b {
+			return true
+		}
+	}
+	return false
+}
+
+func chosenByPercentage(a *int) bool {
+	if a == nil {
+		return true
+	}
+	return rand.IntN(100)+1 <= *a
+}
+
+func canaryingSC(js string) string {
+	if js == "" {
+		return ""
+	}
+	var rcs []rawChoice
+	err := json.Unmarshal([]byte(js), &rcs)
+	if err != nil {
+		logger.Warningf("dns: error parsing service config json: %v", err)
+		return ""
+	}
+	cliHostname, err := os.Hostname()
+	if err != nil {
+		logger.Warningf("dns: error getting client hostname: %v", err)
+		return ""
+	}
+	var sc string
+	for _, c := range rcs {
+		if !containsString(c.ClientLanguage, golang) ||
+			!chosenByPercentage(c.Percentage) ||
+			!containsString(c.ClientHostName, cliHostname) ||
+			c.ServiceConfig == nil {
+			continue
+		}
+		sc = string(*c.ServiceConfig)
+		break
+	}
+	return sc
+}
--- a/internal/resolver/dns/dns_resolver_test.go
+++ b/internal/resolver/dns/dns_resolver_test.go
--- a/internal/resolver/dns/fake_net_resolver_test.go
+++ b/internal/resolver/dns/fake_net_resolver_test.go
@@ -0,0 +1,126 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package dns_test
+
+import (
+	"context"
+	"net"
+	"sync"
+
+	"google.golang.org/grpc/internal/testutils"
+)
+
+// A fake implementation of the internal.NetResolver interface for use in tests.
+type testNetResolver struct {
+	// A write to this channel is made when this resolver receives a resolution
+	// request. Tests can rely on reading from this channel to be notified about
+	// resolution requests instead of sleeping for a predefined period of time.
+	lookupHostCh *testutils.Channel
+
+	mu              sync.Mutex
+	hostLookupTable map[string][]string   // Name --> list of addresses
+	srvLookupTable  map[string][]*net.SRV // Name --> list of SRV records
+	txtLookupTable  map[string][]string   // Name --> service config for TXT record
+}
+
+func (tr *testNetResolver) LookupHost(ctx context.Context, host string) ([]string, error) {
+	if tr.lookupHostCh != nil {
+		if err := tr.lookupHostCh.SendContext(ctx, nil); err != nil {
+			return nil, err
+		}
+	}
+
+	tr.mu.Lock()
+	defer tr.mu.Unlock()
+
+	if addrs, ok := tr.hostLookupTable[host]; ok {
+		return addrs, nil
+	}
+
+	return nil, &net.DNSError{
+		Err:         "hostLookup error",
+		Name:        host,
+		Server:      "fake",
+		IsTemporary: true,
+	}
+}
+
+func (tr *testNetResolver) UpdateHostLookupTable(table map[string][]string) {
+	tr.mu.Lock()
+	tr.hostLookupTable = table
+	tr.mu.Unlock()
+}
+
+func (tr *testNetResolver) LookupSRV(_ context.Context, service, proto, name string) (string, []*net.SRV, error) {
+	tr.mu.Lock()
+	defer tr.mu.Unlock()
+
+	cname := "_" + service + "._" + proto + "." + name
+	if srvs, ok := tr.srvLookupTable[cname]; ok {
+		return cname, srvs, nil
+	}
+	return "", nil, &net.DNSError{
+		Err:         "srvLookup error",
+		Name:        cname,
+		Server:      "fake",
+		IsTemporary: true,
+	}
+}
+
+func (tr *testNetResolver) LookupTXT(_ context.Context, host string) ([]string, error) {
+	tr.mu.Lock()
+	defer tr.mu.Unlock()
+
+	if sc, ok := tr.txtLookupTable[host]; ok {
+		return sc, nil
+	}
+	return nil, &net.DNSError{
+		Err:         "txtLookup error",
+		Name:        host,
+		Server:      "fake",
+		IsTemporary: true,
+	}
+}
+
+func (tr *testNetResolver) UpdateTXTLookupTable(table map[string][]string) {
+	tr.mu.Lock()
+	tr.txtLookupTable = table
+	tr.mu.Unlock()
+}
+
+// txtRecordServiceConfig generates a slice of strings (aggregately representing
+// a single service config file) for the input config string, that represents
+// the result from a real DNS TXT record lookup.
+func txtRecordServiceConfig(cfg string) []string {
+	// In DNS, service config is encoded in a TXT record via the mechanism
+	// described in RFC-1464 using the attribute name grpc_config.
+	b := append([]byte("grpc_config="), []byte(cfg)...)
+
+	// Split b into multiple strings, each with a max of 255 bytes, which is
+	// the DNS TXT record limit.
+	var r []string
+	for i := 0; i < len(b); i += txtBytesLimit {
+		if i+txtBytesLimit > len(b) {
+			r = append(r, string(b[i:]))
+		} else {
+			r = append(r, string(b[i:i+txtBytesLimit]))
+		}
+	}
+	return r
+}
--- a/internal/resolver/dns/internal/internal.go
+++ b/internal/resolver/dns/internal/internal.go
@@ -0,0 +1,77 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package internal contains functionality internal to the dns resolver package.
+package internal
+
+import (
+	"context"
+	"errors"
+	"net"
+	"time"
+)
+
+// NetResolver groups the methods on net.Resolver that are used by the DNS
+// resolver implementation. This allows the default net.Resolver instance to be
+// overridden from tests.
+type NetResolver interface {
+	LookupHost(ctx context.Context, host string) (addrs []string, err error)
+	LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*net.SRV, err error)
+	LookupTXT(ctx context.Context, name string) (txts []string, err error)
+}
+
+var (
+	// ErrMissingAddr is the error returned when building a DNS resolver when
+	// the provided target name is empty.
+	ErrMissingAddr = errors.New("dns resolver: missing address")
+
+	// ErrEndsWithColon is the error returned when building a DNS resolver when
+	// the provided target name ends with a colon that is supposed to be the
+	// separator between host and port.  E.g. "::" is a valid address as it is
+	// an IPv6 address (host only) and "[::]:" is invalid as it ends with a
+	// colon as the host and port separator
+	ErrEndsWithColon = errors.New("dns resolver: missing port after port-separator colon")
+)
+
+// The following vars are overridden from tests.
+var (
+	// TimeAfterFunc is used by the DNS resolver to wait for the given duration
+	// to elapse. In non-test code, this is implemented by time.After. In test
+	// code, this can be used to control the amount of time the resolver is
+	// blocked waiting for the duration to elapse.
+	TimeAfterFunc func(time.Duration) <-chan time.Time
+
+	// TimeNowFunc is used by the DNS resolver to get the current time.
+	// In non-test code, this is implemented by time.Now. In test code,
+	// this can be used to control the current time for the resolver.
+	TimeNowFunc func() time.Time
+
+	// TimeUntilFunc is used by the DNS resolver to calculate the remaining
+	// wait time for re-resolution. In non-test code, this is implemented by
+	// time.Until. In test code, this can be used to control the remaining
+	// time for resolver to wait for re-resolution.
+	TimeUntilFunc func(time.Time) time.Duration
+
+	// NewNetResolver returns the net.Resolver instance for the given target.
+	NewNetResolver func(string) (NetResolver, error)
+
+	// AddressDialer is the dialer used to dial the DNS server. It accepts the
+	// Host portion of the URL corresponding to the user's dial target and
+	// returns a dial function.
+	AddressDialer func(address string) func(context.Context, string, string) (net.Conn, error)
+)
--- a/internal/resolver/passthrough/passthrough.go
+++ b/internal/resolver/passthrough/passthrough.go
@@ -0,0 +1,64 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package passthrough implements a pass-through resolver. It sends the target
+// name without scheme back to gRPC as resolved address.
+package passthrough
+
+import (
+	"errors"
+
+	"google.golang.org/grpc/resolver"
+)
+
+const scheme = "passthrough"
+
+type passthroughBuilder struct{}
+
+func (*passthroughBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
+	if target.Endpoint() == "" && opts.Dialer == nil {
+		return nil, errors.New("passthrough: received empty target in Build()")
+	}
+	r := &passthroughResolver{
+		target: target,
+		cc:     cc,
+	}
+	r.start()
+	return r, nil
+}
+
+func (*passthroughBuilder) Scheme() string {
+	return scheme
+}
+
+type passthroughResolver struct {
+	target resolver.Target
+	cc     resolver.ClientConn
+}
+
+func (r *passthroughResolver) start() {
+	r.cc.UpdateState(resolver.State{Addresses: []resolver.Address{{Addr: r.target.Endpoint()}}})
+}
+
+func (*passthroughResolver) ResolveNow(resolver.ResolveNowOptions) {}
+
+func (*passthroughResolver) Close() {}
+
+func init() {
+	resolver.Register(&passthroughBuilder{})
+}
--- a/internal/resolver/unix/unix.go
+++ b/internal/resolver/unix/unix.go
@@ -0,0 +1,78 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package unix implements a resolver for unix targets.
+package unix
+
+import (
+	"fmt"
+
+	"google.golang.org/grpc/internal/transport/networktype"
+	"google.golang.org/grpc/resolver"
+)
+
+const unixScheme = "unix"
+const unixAbstractScheme = "unix-abstract"
+
+type builder struct {
+	scheme string
+}
+
+func (b *builder) Build(target resolver.Target, cc resolver.ClientConn, _ resolver.BuildOptions) (resolver.Resolver, error) {
+	if target.URL.Host != "" {
+		return nil, fmt.Errorf("invalid (non-empty) authority: %v", target.URL.Host)
+	}
+
+	// gRPC was parsing the dial target manually before PR #4817, and we
+	// switched to using url.Parse() in that PR. To avoid breaking existing
+	// resolver implementations we ended up stripping the leading "/" from the
+	// endpoint. This obviously does not work for the "unix" scheme. Hence we
+	// end up using the parsed URL instead.
+	endpoint := target.URL.Path
+	if endpoint == "" {
+		endpoint = target.URL.Opaque
+	}
+	addr := resolver.Address{Addr: endpoint}
+	if b.scheme == unixAbstractScheme {
+		// We can not prepend \0 as c++ gRPC does, as in Golang '@' is used to signify we do
+		// not want trailing \0 in address.
+		addr.Addr = "@" + addr.Addr
+	}
+	cc.UpdateState(resolver.State{Addresses: []resolver.Address{networktype.Set(addr, "unix")}})
+	return &nopResolver{}, nil
+}
+
+func (b *builder) Scheme() string {
+	return b.scheme
+}
+
+func (b *builder) OverrideAuthority(resolver.Target) string {
+	return "localhost"
+}
+
+type nopResolver struct {
+}
+
+func (*nopResolver) ResolveNow(resolver.ResolveNowOptions) {}
+
+func (*nopResolver) Close() {}
+
+func init() {
+	resolver.Register(&builder{scheme: unixScheme})
+	resolver.Register(&builder{scheme: unixAbstractScheme})
+}