run.go

// Copyright 2014 The rkt 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.

//+build linux

package main

import (
	"bufio"
	"errors"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"

	"github.com/appc/spec/schema/types"
	cnitypes "github.com/containernetworking/cni/pkg/types"
	"github.com/hashicorp/errwrap"
	"github.com/opencontainers/selinux/go-selinux/label"
	"github.com/rkt/rkt/common"
	"github.com/rkt/rkt/pkg/lock"
	pkgPod "github.com/rkt/rkt/pkg/pod"
	"github.com/rkt/rkt/pkg/user"
	"github.com/rkt/rkt/rkt/image"
	"github.com/rkt/rkt/stage0"
	"github.com/rkt/rkt/store/imagestore"
	"github.com/rkt/rkt/store/treestore"
	"github.com/spf13/cobra"
)

var (
	cmdRun = &cobra.Command{
		Use:   "run [--volume=name,kind=host,...] [--mount volume=VOL,target=PATH] IMAGE [-- image-args...[---]]...",
		Short: "Run image(s) in a pod in rkt",
		Long: `IMAGE should be a string referencing an image; either a hash, local file on
disk, or URL. They will be checked in that order and the first match will be
used.

Volumes are made available to the container via --volume. Mounts bind volumes
into each image's root within the container via --mount. --mount is
position-sensitive; occurring before any images applies to all images, occurring
after any images applies only to the nearest preceding image. Per-app mounts
take precedence over global ones if they have the same path.

An "--" may be used to inhibit rkt run's parsing of subsequent arguments, which
will instead be appended to the preceding image app's exec arguments. End the
image arguments with a lone "---" to resume argument parsing.`,
		Run: ensureSuperuser(runWrapper(runRun)),
	}
	flagPorts        portList
	flagNet          common.NetList
	flagPrivateUsers bool
	flagInheritEnv   bool
	flagExplicitEnv  kvMap
	flagEnvFromFile  envFileMap
	flagInteractive  bool
	flagDNS          flagStringList
	flagDNSSearch    flagStringList
	flagDNSOpt       flagStringList
	flagDNSDomain    string
	flagNoOverlay    bool
	flagStoreOnly    bool
	flagNoStore      bool
	flagPodManifest  string
	flagMDSRegister  bool
	flagUUIDFileSave string
	flagHostname     string
	flagHostsEntries flagStringList
	flagPullPolicy   string
	flagIPCMode      string
)

func addIsolatorFlags(cmd *cobra.Command, compat bool) {
	cmd.Flags().Var((*appMemoryLimit)(&rktApps), "memory", "memory limit for the preceding image (example: '--memory=16Mi', '--memory=50M', '--memory=1G')")
	cmd.Flags().Var((*appCPULimit)(&rktApps), "cpu", "cpu limit for the preceding image (example: '--cpu=500m')")
	cmd.Flags().Var((*appCPUShares)(&rktApps), "cpu-shares", "cpu-shares assigns the specified CPU time share weight (example: '--cpu-shares=2048')")
	cmd.Flags().Var((*appCapsRetain)(&rktApps), "caps-retain", "capability to retain (example: '--caps-retain=CAP_SYS_ADMIN')")
	cmd.Flags().Var((*appCapsRemove)(&rktApps), "caps-remove", "capability to remove (example: '--caps-remove=CAP_MKNOD')")
	cmd.Flags().Var((*appSeccompFilter)(&rktApps), "seccomp", "seccomp filter override (example: '--seccomp mode=retain,errno=EPERM,chmod,chown')")
	cmd.Flags().Var((*appOOMScoreAdj)(&rktApps), "oom-score-adj", "oom-score-adj isolator override")

	// For backwards compatibility
	if compat {
		cmd.Flags().Var((*appCapsRetain)(&rktApps), "cap-retain", "capability to retain (example: '--caps-retain=CAP_SYS_ADMIN')")
		cmd.Flags().Var((*appCapsRemove)(&rktApps), "cap-remove", "capability to remove (example: '--caps-remove=CAP_MKNOD')")
		cmd.Flags().MarkDeprecated("cap-retain", "use --caps-retain instead")
		cmd.Flags().MarkDeprecated("cap-remove", "use --caps-remove instead")
	}
}

func addAppFlags(cmd *cobra.Command) {
	cmd.Flags().Var((*appExec)(&rktApps), "exec", "override the exec command for the preceding image")
	cmd.Flags().Var((*appWorkingDir)(&rktApps), "working-dir", "override the working directory of the preceding image")
	cmd.Flags().Var((*appReadOnlyRootFS)(&rktApps), "readonly-rootfs", "if set, the app's rootfs will be mounted read-only")
	cmd.Flags().Var((*appMount)(&rktApps), "mount", "mount point binding a volume to a path within an app")
	cmd.Flags().Var((*appUser)(&rktApps), "user", "user override for the preceding image (example: '--user=user')")
	cmd.Flags().Var((*appGroup)(&rktApps), "group", "group override for the preceding image (example: '--group=group')")
	cmd.Flags().Var((*appSupplementaryGIDs)(&rktApps), "supplementary-gids", "supplementary group IDs override for the preceding image (examples: '--supplementary-gids=1024,2048'")
	cmd.Flags().Var((*appName)(&rktApps), "name", "set the name of the app (example: '--name=foo'). If not set, then the app name default to the image's name")
	cmd.Flags().Var((*appAnnotation)(&rktApps), "annotation", "set the app's annotations (example: '--annotation=foo=bar')")
	cmd.Flags().Var((*appUserAnnotation)(&rktApps), "user-annotation", "set the app's annotations (example: '--user-annotation=foo=bar')")
	cmd.Flags().Var((*appLabel)(&rktApps), "user-label", "set the app's labels (example: '--user-label=foo=bar')")
	cmd.Flags().Var((*appEnv)(&rktApps), "environment", "set the app's environment variables (example: '--environment=foo=bar')")
	if common.IsExperimentEnabled("attach") {
		cmd.Flags().Var((*appStdin)(&rktApps), "stdin", "stdin mode for the preceding application (example: '--stdin=null')")
		cmd.Flags().MarkHidden("stdin")
		cmd.Flags().Var((*appStdout)(&rktApps), "stdout", "stdout mode for the preceding application (example: '--stdout=log')")
		cmd.Flags().MarkHidden("stdout")
		cmd.Flags().Var((*appStderr)(&rktApps), "stderr", "stderr mode for the preceding application (example: '--stderr=log')")
		cmd.Flags().MarkHidden("stderr")
	}
}

func init() {
	cmdRkt.AddCommand(cmdRun)

	/*
	   Careful!
	   Be sure to add common flags to run-prepared as well!
	*/

	addStage1ImageFlags(cmdRun.Flags())
	cmdRun.Flags().Var(&flagPorts, "port", "ports to expose on the host (requires contained network). Syntax: --port=NAME:[HOSTIP:]HOSTPORT")
	cmdRun.Flags().Var(&flagNet, "net", "configure the pod's networking. Optionally, pass a list of user-configured networks to load and set arguments to pass to each network, respectively. Syntax: --net[=n[:args], ...]")
	cmdRun.Flags().Lookup("net").NoOptDefVal = "default"
	cmdRun.Flags().BoolVar(&flagInheritEnv, "inherit-env", false, "inherit all environment variables not set by apps")
	cmdRun.Flags().BoolVar(&flagNoOverlay, "no-overlay", false, "disable overlay filesystem")
	cmdRun.Flags().BoolVar(&flagPrivateUsers, "private-users", false, "run within user namespaces.")
	cmdRun.Flags().Var(&flagExplicitEnv, "set-env", "environment variable to set for all the apps in the form key=value, this will be overridden by --environment")
	cmdRun.Flags().Var(&flagEnvFromFile, "set-env-file", "path to an environment variables file")
	cmdRun.Flags().BoolVar(&flagInteractive, "interactive", false, "run pod interactively. If true, only one image may be supplied.")
	cmdRun.Flags().Var(&flagDNS, "dns", "name servers to write in /etc/resolv.conf. Pass 'host' to use host's resolv.conf. Pass 'none' to ignore CNI DNS config")
	cmdRun.Flags().Var(&flagDNSSearch, "dns-search", "DNS search domains to write in /etc/resolv.conf")
	cmdRun.Flags().Var(&flagDNSOpt, "dns-opt", "DNS options to write in /etc/resolv.conf")
	cmdRun.Flags().StringVar(&flagDNSDomain, "dns-domain", "", "DNS domain to write in /etc/resolv.conf")
	cmdRun.Flags().Var(&flagHostsEntries, "hosts-entry", "Entries to add to the pod-wide /etc/hosts. Pass 'host' to use the host's /etc/hosts")
	cmdRun.Flags().BoolVar(&flagStoreOnly, "store-only", false, "use only available images in the store (do not discover or download from remote URLs)")
	cmdRun.Flags().MarkDeprecated("store-only", "please use --pull-policy=never")
	cmdRun.Flags().BoolVar(&flagNoStore, "no-store", false, "fetch images ignoring the local store")
	cmdRun.Flags().MarkDeprecated("no-store", "please use --pull-policy=update")
	cmdRun.Flags().StringVar(&flagPullPolicy, "pull-policy", image.PullPolicyNew, "when to pull an image")
	cmdRun.Flags().StringVar(&flagPodManifest, "pod-manifest", "", "the path to the pod manifest. If it's non-empty, then only '--net', '--no-overlay' and '--interactive' will have effect")
	cmdRun.Flags().BoolVar(&flagMDSRegister, "mds-register", false, "register pod with metadata service. needs network connectivity to the host (--net=(default|default-restricted|host)")
	cmdRun.Flags().StringVar(&flagUUIDFileSave, "uuid-file-save", "", "write out pod UUID to specified file")
	cmdRun.Flags().StringVar(&flagHostname, "hostname", "", `pod's hostname. If empty, it will be "rkt-$PODUUID"`)
	cmdRun.Flags().Var((*appsVolume)(&rktApps), "volume", "volumes to make available in the pod")
	cmdRun.Flags().StringVar(&flagIPCMode, "ipc", "", `whether to stay in the host IPC namespace. Syntax: --ipc=[auto|private|parent]`)

	// per-app flags
	cmdRun.Flags().Var((*appAsc)(&rktApps), "signature", "local signature file to use in validating the preceding image")
	addAppFlags(cmdRun)
	addIsolatorFlags(cmdRun, true)

	flagPorts = portList{}
	flagDNS = flagStringList{}
	flagDNSSearch = flagStringList{}
	flagDNSOpt = flagStringList{}
	flagHostsEntries = flagStringList{}

	// Disable interspersed flags to stop parsing after the first non flag
	// argument. All the subsequent parsing will be done by parseApps.
	// This is needed to correctly handle image args
	cmdRun.Flags().SetInterspersed(false)
}

func runRun(cmd *cobra.Command, args []string) (exit int) {
	privateUsers := user.NewBlankUidRange()
	err := parseApps(&rktApps, args, cmd.Flags(), true)
	if err != nil {
		stderr.PrintE("error parsing app image arguments", err)
		return 254
	}

	if flagStoreOnly && flagNoStore {
		stderr.Print("both --store-only and --no-store specified")
		return 254
	}
	if flagStoreOnly {
		flagPullPolicy = image.PullPolicyNever
	}
	if flagNoStore {
		flagPullPolicy = image.PullPolicyUpdate
	}

	if flagPrivateUsers {
		if !common.SupportsUserNS() {
			stderr.Print("--private-users is not supported, kernel compiled without user namespace support")
			return 254
		}
		privateUsers.SetRandomUidRange(user.DefaultRangeCount)
	}

	if len(flagPorts) > 0 && flagNet.None() {
		stderr.Print("--port flag does not work with 'none' networking")
		return 254
	}
	if len(flagPorts) > 0 && flagNet.Host() {
		stderr.Print("--port flag does not work with 'host' networking")
		return 254
	}

	if flagMDSRegister && flagNet.None() {
		stderr.Print("--mds-register flag does not work with --net=none. Please use 'host', 'default' or an equivalent network")
		return 254
	}

	if len(flagPodManifest) > 0 && (rktApps.Count() > 0 ||
		(*appsVolume)(&rktApps).String() != "" || (*appMount)(&rktApps).String() != "" ||
		len(flagPorts) > 0 || flagPullPolicy == image.PullPolicyNever ||
		flagPullPolicy == image.PullPolicyUpdate || flagInheritEnv ||
		!flagExplicitEnv.IsEmpty() || !flagEnvFromFile.IsEmpty()) {
		stderr.Print("conflicting flags set with --pod-manifest (see --help)")
		return 254
	}

	if flagInteractive && rktApps.Count() > 1 {
		stderr.Print("interactive option only supports one image")
		return 254
	}

	if rktApps.Count() < 1 && len(flagPodManifest) == 0 {
		stderr.Print("must provide at least one image or specify the pod manifest")
		return 254
	}

	s, err := imagestore.NewStore(storeDir())
	if err != nil {
		stderr.PrintE("cannot open store", err)
		return 254
	}

	ts, err := treestore.NewStore(treeStoreDir(), s)
	if err != nil {
		stderr.PrintE("cannot open treestore", err)
		return 254
	}

	config, err := getConfig()
	if err != nil {
		stderr.PrintE("cannot get configuration", err)
		return 254
	}

	s1img, err := getStage1Hash(s, ts, config)
	if err != nil {
		stderr.Error(err)
		return 254
	}

	fn := &image.Finder{
		S:                  s,
		Ts:                 ts,
		Ks:                 getKeystore(),
		Headers:            config.AuthPerHost,
		DockerAuth:         config.DockerCredentialsPerRegistry,
		InsecureFlags:      globalFlags.InsecureFlags,
		Debug:              globalFlags.Debug,
		TrustKeysFromHTTPS: globalFlags.TrustKeysFromHTTPS,

		PullPolicy: flagPullPolicy,
		WithDeps:   true,
	}
	if err := fn.FindImages(&rktApps); err != nil {
		stderr.Error(err)
		return 254
	}

	p, err := pkgPod.NewPod(getDataDir())
	if err != nil {
		stderr.PrintE("error creating new pod", err)
		return 254
	}

	// if requested, write out pod UUID early so "rkt rm" can
	// clean it up even if something goes wrong
	if flagUUIDFileSave != "" {
		if err := pkgPod.WriteUUIDToFile(p.UUID, flagUUIDFileSave); err != nil {
			stderr.PrintE("error saving pod UUID to file", err)
			return 254
		}
	}

	processLabel, mountLabel, err := label.InitLabels([]string{})
	if err != nil {
		stderr.PrintE("error initialising SELinux", err)
		return 254
	}
	p.MountLabel = mountLabel

	cfg := stage0.CommonConfig{
		DataDir:      getDataDir(),
		MountLabel:   mountLabel,
		ProcessLabel: processLabel,
		Store:        s,
		TreeStore:    ts,
		Stage1Image:  *s1img,
		UUID:         p.UUID,
		Debug:        globalFlags.Debug,
		Mutable:      false,
	}

	ovlOk := true
	if err := common.PathSupportsOverlay(getDataDir()); err != nil {
		if oerr, ok := err.(common.ErrOverlayUnsupported); ok {
			stderr.Printf("disabling overlay support: %q", oerr.Error())
			ovlOk = false
		} else {
			stderr.PrintE("error determining overlay support", err)
			return 254
		}
	}

	useOverlay := !flagNoOverlay && ovlOk

	pcfg := stage0.PrepareConfig{
		CommonConfig: &cfg,
		UseOverlay:   useOverlay,
		PrivateUsers: privateUsers,
	}

	if len(flagPodManifest) > 0 {
		pcfg.PodManifest = flagPodManifest
	} else {
		pcfg.Ports = []types.ExposedPort(flagPorts)
		pcfg.InheritEnv = flagInheritEnv
		pcfg.ExplicitEnv = flagExplicitEnv.Strings()
		pcfg.EnvFromFile = flagEnvFromFile.Strings()
		pcfg.Apps = &rktApps
	}

	if globalFlags.Debug {
		stage0.InitDebug()
	}

	keyLock, err := lock.SharedKeyLock(lockDir(), common.PrepareLock)
	if err != nil {
		stderr.PrintE("cannot get shared prepare lock", err)
		return 254
	}
	err = stage0.Prepare(pcfg, p.Path(), p.UUID)
	if err != nil {
		stderr.PrintE("error setting up stage0", err)
		keyLock.Close()
		return 254
	}
	keyLock.Close()

	// get the lock fd for run
	lfd, err := p.Fd()
	if err != nil {
		stderr.PrintE("error getting pod lock fd", err)
		return 254
	}

	// skip prepared by jumping directly to run, we own this pod
	if err := p.ToRun(); err != nil {
		stderr.PrintE("unable to transition to run", err)
		return 254
	}

	rktgid, err := common.LookupGid(common.RktGroup)
	if err != nil {
		stderr.Printf("group %q not found, will use default gid when rendering images", common.RktGroup)
		rktgid = -1
	}

	DNSConfMode, DNSConfig, HostsEntries, err := parseDNSFlags(flagHostsEntries, flagDNS, flagDNSSearch, flagDNSOpt, flagDNSDomain)
	if err != nil {
		stderr.PrintE("error with dns flags", err)
		return 254
	}

	rcfg := stage0.RunConfig{
		CommonConfig:         &cfg,
		Net:                  flagNet,
		LockFd:               lfd,
		Interactive:          flagInteractive,
		DNSConfMode:          DNSConfMode,
		DNSConfig:            DNSConfig,
		MDSRegister:          flagMDSRegister,
		LocalConfig:          globalFlags.LocalConfigDir,
		RktGid:               rktgid,
		Hostname:             flagHostname,
		InsecureCapabilities: globalFlags.InsecureFlags.SkipCapabilities(),
		InsecurePaths:        globalFlags.InsecureFlags.SkipPaths(),
		InsecureSeccomp:      globalFlags.InsecureFlags.SkipSeccomp(),
		UseOverlay:           useOverlay,
		HostsEntries:         *HostsEntries,
		IPCMode:              flagIPCMode,
	}

	_, manifest, err := p.PodManifest()
	if err != nil {
		stderr.PrintE("cannot get the pod manifest", err)
		return 254
	}

	if len(manifest.Apps) == 0 {
		stderr.Print("pod must contain at least one application")
		return 254
	}
	rcfg.Apps = manifest.Apps
	stage0.Run(rcfg, p.Path(), getDataDir()) // execs, never returns

	return 254
}

// portList implements the flag.Value interface to contain a set of mappings
// from port name --> host port
type portList []types.ExposedPort

func (pl *portList) Set(s string) error {
	parts := strings.SplitN(s, ":", 3)
	if len(parts) < 2 {
		return fmt.Errorf("%q is not in name:[ip:]port format", s)
	}

	name, err := types.NewACName(parts[0])
	if err != nil {
		return errwrap.Wrap(fmt.Errorf("%q is not a valid port name", parts[0]), err)
	}

	portStr := parts[1]
	var ip net.IP
	if len(parts) == 3 {
		portStr = parts[2]
		ip = net.ParseIP(parts[1])
		if ip == nil {
			return fmt.Errorf("%q is not a valid IP", parts[1])
		}
	}

	port, err := strconv.ParseUint(portStr, 10, 16)
	if err != nil {
		return fmt.Errorf("%q is not a valid port number", parts[1])
	}

	p := types.ExposedPort{
		Name:     *name,
		HostPort: uint(port),
		HostIP:   ip,
	}

	*pl = append(*pl, p)
	return nil
}

func (pl *portList) String() string {
	var ps []string
	for _, p := range []types.ExposedPort(*pl) {
		ps = append(ps, fmt.Sprintf("%v:%v", p.Name, p.HostPort))
	}
	return strings.Join(ps, " ")
}

func (pl *portList) Type() string {
	return "portList"
}

// flagStringList implements the flag.Value interface to contain a set of strings
type flagStringList []string

func (dns *flagStringList) Set(s string) error {
	*dns = append(*dns, s)
	return nil
}

func (dns *flagStringList) String() string {
	return strings.Join(*dns, " ")
}

func (dns *flagStringList) Type() string {
	return "flagStringList"
}

// kvMap implements the flag.Value interface to contain a set of key=value mappings
type kvMap struct {
	mapping map[string]string
}

func (e *kvMap) Set(s string) error {
	if e.mapping == nil {
		e.mapping = make(map[string]string)
	}
	pair := strings.SplitN(s, "=", 2)
	if len(pair) != 2 {
		return fmt.Errorf("must be specified as key=value")
	}
	if _, exists := e.mapping[pair[0]]; exists {
		return fmt.Errorf("key %q already set", pair[0])
	}
	e.mapping[pair[0]] = pair[1]
	return nil
}

func (e *kvMap) IsEmpty() bool {
	return len(e.mapping) == 0
}

func (e *kvMap) String() string {
	return strings.Join(e.Strings(), "\n")
}

func (e *kvMap) Strings() []string {
	var env []string
	for n, v := range e.mapping {
		env = append(env, n+"="+v)
	}
	return env
}

func (e *kvMap) Type() string {
	return "kvMap"
}

// envFileMap
type envFileMap struct {
	mapping map[string]string
}

func commentLine(s string) bool {
	return strings.HasPrefix(s, "#") || strings.HasPrefix(s, ";")
}

func (e *envFileMap) Set(s string) error {
	if e.mapping == nil {
		e.mapping = make(map[string]string)
	}
	file, err := os.Open(s)
	if err != nil {
		return err
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		line := scanner.Text()
		// Skip empty lines
		if len(line) == 0 {
			continue
		}
		// Skip comments
		if commentLine(line) {
			continue
		}
		pair := strings.SplitN(line, "=", 2)
		// Malformed lines
		if len(pair) != 2 || len(pair[0]) == 0 {
			return fmt.Errorf("environment variable must be specified as name=value (file %q)", s)
		}
		if _, exists := e.mapping[pair[0]]; exists {
			return fmt.Errorf("environment variable %q already set (file %q)", pair[0], s)
		}
		e.mapping[pair[0]] = pair[1]
	}
	return nil
}

func (e *envFileMap) IsEmpty() bool {
	return len(e.mapping) == 0
}

func (e *envFileMap) String() string {
	return strings.Join(e.Strings(), "\n")
}

func (e *envFileMap) Strings() []string {
	var env []string
	for n, v := range e.mapping {
		env = append(env, n+"="+v)
	}
	return env
}

func (e *envFileMap) Type() string {
	return "envFileMap"
}

/*
 * Parse out the --hosts-entries, --dns, --dns-search, and --dns-opt flags
 * This includes decoding the "magic" values for hosts-entries and dns.
 * Try to detect any obvious insanity, namely invalid IPs or more than one
 * magic option
 */
func parseDNSFlags(flagHostsEntries, flagDNS, flagDNSSearch, flagDNSOpt []string, flagDNSDomain string) (stage0.DNSConfMode, cnitypes.DNS, *stage0.HostsEntries, error) {
	DNSConfMode := stage0.DNSConfMode{
		Resolv: "default",
		Hosts:  "default",
	}
	DNSConfig := cnitypes.DNS{}
	HostsEntries := make(stage0.HostsEntries)

	// Loop through --dns and look for magic option
	// Check for obvious insanity - only one magic option allowed
	for _, d := range flagDNS {
		// parse magic values
		if d == "host" || d == "none" {
			if len(flagDNS) > 1 {
				return DNSConfMode, DNSConfig, &HostsEntries,
					fmt.Errorf("no other --dns options allowed when --dns=%s is passed", d)
			}
			DNSConfMode.Resolv = d
			break

		} else {
			// parse list of IPS
			for _, d := range strings.Split(d, ",") {
				if net.ParseIP(d) == nil {
					return DNSConfMode, DNSConfig, &HostsEntries,
						fmt.Errorf("Invalid IP passed to --dns: %s", d)
				}
				DNSConfig.Nameservers = append(DNSConfig.Nameservers, d)
			}
		}
	}

	DNSConfig.Search = flagDNSSearch
	DNSConfig.Options = flagDNSOpt
	DNSConfig.Domain = flagDNSDomain

	if !common.IsDNSZero(&DNSConfig) {
		if DNSConfMode.Resolv == "default" {
			DNSConfMode.Resolv = "stage0"
		}

		if DNSConfMode.Resolv != "stage0" {
			return DNSConfMode, DNSConfig, &HostsEntries,
				fmt.Errorf("Cannot call --dns-opt, --dns-search, or --dns-domain with --dns=%v", DNSConfMode.Resolv)
		}
	}

	// Parse out --hosts-entries, also looking for the magic value "host"
	for _, entry := range flagHostsEntries {
		if entry == "host" {
			DNSConfMode.Hosts = "host"
			continue
		}
		for _, entry := range strings.Split(entry, ",") {
			vals := strings.SplitN(entry, "=", 2)
			if len(vals) != 2 {
				return DNSConfMode, DNSConfig, &HostsEntries,
					fmt.Errorf("Did not understand --hosts-entry %s", entry)
			}
			ipStr := vals[0]
			hostname := vals[1]

			// validate IP address
			ip := net.ParseIP(ipStr)
			if ip == nil {
				return DNSConfMode, DNSConfig, &HostsEntries,
					fmt.Errorf("Invalid IP passed to --hosts-entry: %s", ipStr)
			}

			_, exists := HostsEntries[ipStr]
			if !exists {
				HostsEntries[ipStr] = []string{hostname}
			} else {
				HostsEntries[ipStr] = append(HostsEntries[ipStr], hostname)
			}
		}
	}

	if len(HostsEntries) > 0 {
		if DNSConfMode.Hosts == "host" {
			return DNSConfMode, DNSConfig, &HostsEntries,
				errors.New("cannot pass --hosts-entry=host with multiple hosts-entries")
		}
		DNSConfMode.Hosts = "stage0"
	}

	return DNSConfMode, DNSConfig, &HostsEntries, nil
}