diff --git a/checkpoint.go b/checkpoint.go
index e7c126743..61e58351f 100644
--- a/checkpoint.go
+++ b/checkpoint.go
@@ -44,7 +44,11 @@ checkpointed.`,
 			return err
 		}
 		// XXX: Currently this is untested with rootless containers.
-		if isRootless() {
+		rootless, err := isRootless(context)
+		if err != nil {
+			return err
+		}
+		if rootless {
 			return fmt.Errorf("runc checkpoint requires root")
 		}
 
diff --git a/libcontainer/container_linux.go b/libcontainer/container_linux.go
index 546a007c0..3160f9699 100644
--- a/libcontainer/container_linux.go
+++ b/libcontainer/container_linux.go
@@ -28,7 +28,6 @@ import (
 
 	"github.com/golang/protobuf/proto"
 	"github.com/sirupsen/logrus"
-	"github.com/syndtr/gocapability/capability"
 	"github.com/vishvananda/netlink/nl"
 	"golang.org/x/sys/unix"
 )
@@ -1798,17 +1797,10 @@ func (c *linuxContainer) bootstrapData(cloneFlags uintptr, nsMaps map[configs.Na
 				})
 			}
 			if requiresRootOrMappingTool(c.config) {
-				// check if we have CAP_SETGID to setgroup properly
-				pid, err := capability.NewPid(0)
-				if err != nil {
-					return nil, err
-				}
-				if !pid.Get(capability.EFFECTIVE, capability.CAP_SETGID) {
-					r.AddData(&Boolmsg{
-						Type:  SetgroupAttr,
-						Value: true,
-					})
-				}
+				r.AddData(&Boolmsg{
+					Type:  SetgroupAttr,
+					Value: true,
+				})
 			}
 		}
 	}
diff --git a/libcontainer/system/linux.go b/libcontainer/system/linux.go
index 5f124cd8b..8d353d984 100644
--- a/libcontainer/system/linux.go
+++ b/libcontainer/system/linux.go
@@ -3,13 +3,12 @@
 package system
 
 import (
-	"bufio"
-	"fmt"
 	"os"
 	"os/exec"
 	"syscall" // only for exec
 	"unsafe"
 
+	"github.com/opencontainers/runc/libcontainer/user"
 	"golang.org/x/sys/unix"
 )
 
@@ -102,34 +101,43 @@ func Setctty() error {
 }
 
 // RunningInUserNS detects whether we are currently running in a user namespace.
-// Copied from github.com/lxc/lxd/shared/util.go
+// Originally copied from github.com/lxc/lxd/shared/util.go
 func RunningInUserNS() bool {
-	file, err := os.Open("/proc/self/uid_map")
+	uidmap, err := user.CurrentProcessUIDMap()
 	if err != nil {
 		// This kernel-provided file only exists if user namespaces are supported
 		return false
 	}
-	defer file.Close()
+	return UIDMapInUserNS(uidmap)
+}
 
-	buf := bufio.NewReader(file)
-	l, _, err := buf.ReadLine()
-	if err != nil {
-		return false
-	}
-
-	line := string(l)
-	var a, b, c int64
-	fmt.Sscanf(line, "%d %d %d", &a, &b, &c)
+func UIDMapInUserNS(uidmap []user.IDMap) bool {
 	/*
 	 * We assume we are in the initial user namespace if we have a full
 	 * range - 4294967295 uids starting at uid 0.
 	 */
-	if a == 0 && b == 0 && c == 4294967295 {
+	if len(uidmap) == 1 && uidmap[0].ID == 0 && uidmap[0].ParentID == 0 && uidmap[0].Count == 4294967295 {
 		return false
 	}
 	return true
 }
 
+// GetParentNSeuid returns the euid within the parent user namespace
+func GetParentNSeuid() int {
+	euid := os.Geteuid()
+	uidmap, err := user.CurrentProcessUIDMap()
+	if err != nil {
+		// This kernel-provided file only exists if user namespaces are supported
+		return euid
+	}
+	for _, um := range uidmap {
+		if um.ID <= euid && euid <= um.ID+um.Count-1 {
+			return um.ParentID + euid - um.ID
+		}
+	}
+	return euid
+}
+
 // SetSubreaper sets the value i as the subreaper setting for the calling process
 func SetSubreaper(i int) error {
 	return unix.Prctl(PR_SET_CHILD_SUBREAPER, uintptr(i), 0, 0, 0)
diff --git a/libcontainer/system/linux_test.go b/libcontainer/system/linux_test.go
new file mode 100644
index 000000000..4d613d847
--- /dev/null
+++ b/libcontainer/system/linux_test.go
@@ -0,0 +1,45 @@
+// +build linux
+
+package system
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/opencontainers/runc/libcontainer/user"
+)
+
+func TestUIDMapInUserNS(t *testing.T) {
+	cases := []struct {
+		s        string
+		expected bool
+	}{
+		{
+			s:        "         0          0 4294967295\n",
+			expected: false,
+		},
+		{
+			s:        "         0          0          1\n",
+			expected: true,
+		},
+		{
+			s:        "         0       1001          1\n         1     231072      65536\n",
+			expected: true,
+		},
+		{
+			// file exist but empty (the initial state when userns is created. see man 7 user_namespaces)
+			s:        "",
+			expected: true,
+		},
+	}
+	for _, c := range cases {
+		uidmap, err := user.ParseIDMap(strings.NewReader(c.s))
+		if err != nil {
+			t.Fatal(err)
+		}
+		actual := UIDMapInUserNS(uidmap)
+		if c.expected != actual {
+			t.Fatalf("expected %v, got %v for %q", c.expected, actual, c.s)
+		}
+	}
+}
diff --git a/libcontainer/system/unsupported.go b/libcontainer/system/unsupported.go
index e7cfd62b2..b94be74a6 100644
--- a/libcontainer/system/unsupported.go
+++ b/libcontainer/system/unsupported.go
@@ -2,8 +2,26 @@
 
 package system
 
+import (
+	"os"
+
+	"github.com/opencontainers/runc/libcontainer/user"
+)
+
 // RunningInUserNS is a stub for non-Linux systems
 // Always returns false
 func RunningInUserNS() bool {
 	return false
 }
+
+// UIDMapInUserNS is a stub for non-Linux systems
+// Always returns false
+func UIDMapInUserNS(uidmap []user.IDMap) bool {
+	return false
+}
+
+// GetParentNSeuid returns the euid within the parent user namespace
+// Always returns os.Geteuid on non-linux
+func GetParentNSeuid() int {
+	return os.Geteuid()
+}
diff --git a/libcontainer/user/lookup_unix.go b/libcontainer/user/lookup_unix.go
index c45e30041..c1e634c94 100644
--- a/libcontainer/user/lookup_unix.go
+++ b/libcontainer/user/lookup_unix.go
@@ -114,3 +114,29 @@ func CurrentUser() (User, error) {
 func CurrentGroup() (Group, error) {
 	return LookupGid(unix.Getgid())
 }
+
+func CurrentUserSubUIDs() ([]SubID, error) {
+	u, err := CurrentUser()
+	if err != nil {
+		return nil, err
+	}
+	return ParseSubIDFileFilter("/etc/subuid",
+		func(entry SubID) bool { return entry.Name == u.Name })
+}
+
+func CurrentGroupSubGIDs() ([]SubID, error) {
+	g, err := CurrentGroup()
+	if err != nil {
+		return nil, err
+	}
+	return ParseSubIDFileFilter("/etc/subgid",
+		func(entry SubID) bool { return entry.Name == g.Name })
+}
+
+func CurrentProcessUIDMap() ([]IDMap, error) {
+	return ParseIDMapFile("/proc/self/uid_map")
+}
+
+func CurrentProcessGIDMap() ([]IDMap, error) {
+	return ParseIDMapFile("/proc/self/gid_map")
+}
diff --git a/libcontainer/user/user.go b/libcontainer/user/user.go
index 93414516c..37993da83 100644
--- a/libcontainer/user/user.go
+++ b/libcontainer/user/user.go
@@ -75,12 +75,29 @@ func groupFromOS(g *user.Group) (Group, error) {
 	return newGroup, nil
 }
 
+// SubID represents an entry in /etc/sub{u,g}id
+type SubID struct {
+	Name  string
+	SubID int
+	Count int
+}
+
+// IDMap represents an entry in /proc/PID/{u,g}id_map
+type IDMap struct {
+	ID       int
+	ParentID int
+	Count    int
+}
+
 func parseLine(line string, v ...interface{}) {
-	if line == "" {
+	parseParts(strings.Split(line, ":"), v...)
+}
+
+func parseParts(parts []string, v ...interface{}) {
+	if len(parts) == 0 {
 		return
 	}
 
-	parts := strings.Split(line, ":")
 	for i, p := range parts {
 		// Ignore cases where we don't have enough fields to populate the arguments.
 		// Some configuration files like to misbehave.
@@ -479,3 +496,111 @@ func GetAdditionalGroupsPath(additionalGroups []string, groupPath string) ([]int
 	}
 	return GetAdditionalGroups(additionalGroups, group)
 }
+
+func ParseSubIDFile(path string) ([]SubID, error) {
+	subid, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer subid.Close()
+	return ParseSubID(subid)
+}
+
+func ParseSubID(subid io.Reader) ([]SubID, error) {
+	return ParseSubIDFilter(subid, nil)
+}
+
+func ParseSubIDFileFilter(path string, filter func(SubID) bool) ([]SubID, error) {
+	subid, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer subid.Close()
+	return ParseSubIDFilter(subid, filter)
+}
+
+func ParseSubIDFilter(r io.Reader, filter func(SubID) bool) ([]SubID, error) {
+	if r == nil {
+		return nil, fmt.Errorf("nil source for subid-formatted data")
+	}
+
+	var (
+		s   = bufio.NewScanner(r)
+		out = []SubID{}
+	)
+
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue
+		}
+
+		// see: man 5 subuid
+		p := SubID{}
+		parseLine(line, &p.Name, &p.SubID, &p.Count)
+
+		if filter == nil || filter(p) {
+			out = append(out, p)
+		}
+	}
+
+	return out, nil
+}
+
+func ParseIDMapFile(path string) ([]IDMap, error) {
+	r, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	return ParseIDMap(r)
+}
+
+func ParseIDMap(r io.Reader) ([]IDMap, error) {
+	return ParseIDMapFilter(r, nil)
+}
+
+func ParseIDMapFileFilter(path string, filter func(IDMap) bool) ([]IDMap, error) {
+	r, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	return ParseIDMapFilter(r, filter)
+}
+
+func ParseIDMapFilter(r io.Reader, filter func(IDMap) bool) ([]IDMap, error) {
+	if r == nil {
+		return nil, fmt.Errorf("nil source for idmap-formatted data")
+	}
+
+	var (
+		s   = bufio.NewScanner(r)
+		out = []IDMap{}
+	)
+
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue
+		}
+
+		// see: man 7 user_namespaces
+		p := IDMap{}
+		parseParts(strings.Fields(line), &p.ID, &p.ParentID, &p.Count)
+
+		if filter == nil || filter(p) {
+			out = append(out, p)
+		}
+	}
+
+	return out, nil
+}
diff --git a/main.go b/main.go
index 1b9728c59..278399a56 100644
--- a/main.go
+++ b/main.go
@@ -63,7 +63,11 @@ func main() {
 	app.Version = strings.Join(v, "\n")
 
 	root := "/run/runc"
-	if os.Geteuid() != 0 {
+	rootless, err := isRootless(nil)
+	if err != nil {
+		fatal(err)
+	}
+	if rootless {
 		runtimeDir := os.Getenv("XDG_RUNTIME_DIR")
 		if runtimeDir != "" {
 			root = runtimeDir + "/runc"
@@ -108,6 +112,11 @@ func main() {
 			Name:  "systemd-cgroup",
 			Usage: "enable systemd cgroup support, expects cgroupsPath to be of form \"slice:prefix:name\" for e.g. \"system.slice:runc:434234\"",
 		},
+		cli.StringFlag{
+			Name:  "rootless",
+			Value: "auto",
+			Usage: "enable rootless mode ('true', 'false', or 'auto')",
+		},
 	}
 	app.Commands = []cli.Command{
 		checkpointCommand,
diff --git a/ps.go b/ps.go
index 6e0c7376a..eec9d5f56 100644
--- a/ps.go
+++ b/ps.go
@@ -29,7 +29,11 @@ var psCommand = cli.Command{
 			return err
 		}
 		// XXX: Currently not supported with rootless containers.
-		if isRootless() {
+		rootless, err := isRootless(context)
+		if err != nil {
+			return err
+		}
+		if rootless {
 			return fmt.Errorf("runc ps requires root")
 		}
 
diff --git a/restore.go b/restore.go
index 362be62de..724157da2 100644
--- a/restore.go
+++ b/restore.go
@@ -96,7 +96,11 @@ using the runc checkpoint command.`,
 			return err
 		}
 		// XXX: Currently this is untested with rootless containers.
-		if isRootless() {
+		rootless, err := isRootless(context)
+		if err != nil {
+			return err
+		}
+		if rootless {
 			return fmt.Errorf("runc restore requires root")
 		}
 
diff --git a/utils.go b/utils.go
index 8ed1a88ea..5165336fb 100644
--- a/utils.go
+++ b/utils.go
@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strconv"
+	"strings"
 
 	"github.com/opencontainers/runtime-spec/specs-go"
 
@@ -81,3 +83,12 @@ func revisePidFile(context *cli.Context) error {
 	}
 	return context.Set("pid-file", pidFile)
 }
+
+// parseBoolOrAuto returns (nil, nil) if s is empty or "auto"
+func parseBoolOrAuto(s string) (*bool, error) {
+	if s == "" || strings.ToLower(s) == "auto" {
+		return nil, nil
+	}
+	b, err := strconv.ParseBool(s)
+	return &b, err
+}
diff --git a/utils_linux.go b/utils_linux.go
index c6223f0d0..66fb97c9b 100644
--- a/utils_linux.go
+++ b/utils_linux.go
@@ -16,6 +16,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/intelrdt"
 	"github.com/opencontainers/runc/libcontainer/specconv"
+	"github.com/opencontainers/runc/libcontainer/system"
 	"github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/opencontainers/runtime-spec/specs-go"
 
@@ -220,19 +221,37 @@ func createPidFile(path string, process *libcontainer.Process) error {
 	return os.Rename(tmpName, path)
 }
 
-// XXX: Currently we autodetect rootless mode.
-func isRootless() bool {
-	return os.Geteuid() != 0
+func isRootless(context *cli.Context) (bool, error) {
+	if context != nil {
+		b, err := parseBoolOrAuto(context.GlobalString("rootless"))
+		if err != nil {
+			return false, err
+		}
+		if b != nil {
+			return *b, nil
+		}
+		// nil b stands for "auto detect"
+	}
+	// Even if os.Geteuid() == 0, it might still require rootless mode,
+	// especially when running within userns.
+	// So we use system.GetParentNSeuid() here.
+	//
+	// TODO(AkihiroSuda): how to support nested userns?
+	return system.GetParentNSeuid() != 0 || system.RunningInUserNS(), nil
 }
 
 func createContainer(context *cli.Context, id string, spec *specs.Spec) (libcontainer.Container, error) {
+	rootless, err := isRootless(context)
+	if err != nil {
+		return nil, err
+	}
 	config, err := specconv.CreateLibcontainerConfig(&specconv.CreateOpts{
 		CgroupName:       id,
 		UseSystemdCgroup: context.GlobalBool("systemd-cgroup"),
 		NoPivotRoot:      context.Bool("no-pivot"),
 		NoNewKeyring:     context.Bool("no-new-keyring"),
 		Spec:             spec,
-		Rootless:         isRootless(),
+		Rootless:         rootless,
 	})
 	if err != nil {
 		return nil, err