Add Apple container runtime support

Makefile

@@ -101,6 +101,7 @@ OUTPUT_BIN ?= $(repo_dir)/bin
 DCP_DIR ?= $(home_dir)/.dcp
 DCP_BINARY ?= ${OUTPUT_BIN}/dcp$(bin_exe_suffix)
 DCPTUN_CLIENT_BINARY ?= $(OUTPUT_BIN)/dcptun_c
+DCPDNS_BINARY ?= $(OUTPUT_BIN)/dcpdns_c
 
 # Locations and definitions for tool binaries
 GO_BIN ?= go
@@ -266,11 +267,11 @@ endif
 ##@ Development
 
 ifeq ($(build_os),linux)
-COMMON_BUILD_PREREQS := build-dcp build-dcptun-containerexe
+COMMON_BUILD_PREREQS := build-dcp build-dcptun-containerexe build-dcpdns-containerexe
 COMPILE_PREREQS := $(COMMON_BUILD_PREREQS)
 else
 COMMON_BUILD_PREREQS := build-dcp
-COMPILE_PREREQS := $(COMMON_BUILD_PREREQS) build-dcptun-containerexe
+COMPILE_PREREQS := $(COMMON_BUILD_PREREQS) build-dcptun-containerexe build-dcpdns-containerexe
 endif
 
 # Note: Go runtime is incompatible with C/C++ stack protection feature https://github.com/golang/go/blob/master/src/runtime/cgo/cgo.go#L28 More info/rationale https://github.com/golang/go/issues/21871#issuecomment-329330371
@@ -305,6 +306,15 @@ else
 	GOOS=linux $(GO_BIN) build -o $(DCPTUN_CLIENT_BINARY) $(BUILD_ARGS) ./cmd/dcptun
 endif
 
+.PHONY: build-dcpdns-containerexe
+build-dcpdns-containerexe: $(DCPDNS_BINARY) ## Builds DCP DNS forwarder binary for Linux (used in containers on the Apple container runtime)
+$(DCPDNS_BINARY): $(GO_SOURCES) go.mod | $(OUTPUT_BIN)
+ifeq ($(detected_OS),windows)
+	$$env:GOOS = "linux"; $(GO_BIN) build -o $(DCPDNS_BINARY) $(BUILD_ARGS) ./cmd/dcpdns
+else
+	GOOS=linux $(GO_BIN) build -o $(DCPDNS_BINARY) $(BUILD_ARGS) ./cmd/dcpdns
+endif
+
 .PHONY: clean
 clean: | ${OUTPUT_BIN} ${TOOL_BIN} ## Deletes build output (all binaries), and all cached tool binaries.
 	$(rm_rf) $(OUTPUT_BIN)/*
@@ -318,11 +328,13 @@ lint: golangci-lint generate-grpc ## Runs the linter
 install: compile | $(DCP_DIR) ## Installs all binaries to their destinations
 	$(install) $(DCP_BINARY) $(DCP_DIR)
 	$(install) $(DCPTUN_CLIENT_BINARY) $(DCP_DIR)
+	$(install) $(DCPDNS_BINARY) $(DCP_DIR)
 
 .PHONY: uninstall
 uninstall: ## Uninstalls all binaries from their destinations
 	$(rm_f) $(DCP_DIR)/dcp$(bin_exe_suffix)
 	$(rm_f) $(DCP_DIR)/dcptun_c
+	$(rm_f) $(DCP_DIR)/dcpdns_c
 
 ifneq ($(detected_OS),windows)
 .PHONY: link-dcp

cmd/dcpdns/main.go

@@ -0,0 +1,101 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See LICENSE in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+// dcpdns is the DNS forwarder DCP runs inside a container on the Apple container runtime's
+// network to provide name resolution for container names and network aliases.
+// See internal/dcpdns for details.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"net"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+
+	"github.com/go-logr/logr/funcr"
+
+	"github.com/microsoft/dcp/internal/dcpdns"
+)
+
+func main() {
+	listenAddr := flag.String("listen", ":53", "Address to serve DNS on")
+	hostsPath := flag.String("hosts", "", "Path to the hosts-format records file (required)")
+	upstream := flag.String("upstream", "", "Upstream resolver as host[:port]; defaults to the first nameserver in /etc/resolv.conf")
+	verbose := flag.Bool("v", false, "Enable verbose logging")
+	flag.Parse()
+
+	log := funcr.New(func(prefix, args string) {
+		fmt.Fprintln(os.Stdout, prefix, args)
+	}, funcr.Options{Verbosity: verbosity(*verbose)})
+
+	if *hostsPath == "" {
+		log.Error(nil, "--hosts is required")
+		os.Exit(2)
+	}
+
+	upstreamAddr := *upstream
+	if upstreamAddr == "" {
+		resolvConfUpstream, resolvErr := upstreamFromResolvConf("/etc/resolv.conf")
+		if resolvErr != nil {
+			log.Error(resolvErr, "No --upstream specified and no nameserver found in /etc/resolv.conf")
+			os.Exit(2)
+		}
+		upstreamAddr = resolvConfUpstream
+	}
+	if _, _, splitErr := net.SplitHostPort(upstreamAddr); splitErr != nil {
+		upstreamAddr = net.JoinHostPort(upstreamAddr, "53")
+	}
+
+	udpConn, udpErr := net.ListenPacket("udp", *listenAddr)
+	if udpErr != nil {
+		log.Error(udpErr, "Failed to listen for UDP DNS queries", "Address", *listenAddr)
+		os.Exit(1)
+	}
+
+	tcpListener, tcpErr := net.Listen("tcp", *listenAddr)
+	if tcpErr != nil {
+		log.Error(tcpErr, "Failed to listen for TCP DNS queries", "Address", *listenAddr)
+		os.Exit(1)
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	log.Info("DCP DNS forwarder starting", "Listen", *listenAddr, "Upstream", upstreamAddr, "Hosts", *hostsPath)
+
+	server := dcpdns.NewServer(dcpdns.NewRecords(*hostsPath), upstreamAddr, log)
+	if serveErr := server.Serve(ctx, udpConn, tcpListener); serveErr != nil {
+		log.Error(serveErr, "DNS forwarder failed")
+		os.Exit(1)
+	}
+}
+
+func verbosity(verbose bool) int {
+	if verbose {
+		return 2
+	}
+	return 0
+}
+
+// upstreamFromResolvConf returns the first nameserver listed in the given resolv.conf file.
+func upstreamFromResolvConf(path string) (string, error) {
+	content, readErr := os.ReadFile(path)
+	if readErr != nil {
+		return "", readErr
+	}
+
+	for _, line := range strings.Split(string(content), "\n") {
+		fields := strings.Fields(line)
+		if len(fields) >= 2 && fields[0] == "nameserver" {
+			return fields[1], nil
+		}
+	}
+
+	return "", fmt.Errorf("no nameserver entries in %s", path)
+}