diff --git a/keywordfuncs_bsd.go b/keywordfuncs_bsd.go
index eb5cc3f..8de2608 100644
--- a/keywordfuncs_bsd.go
+++ b/keywordfuncs_bsd.go
@@ -35,7 +35,7 @@ var (
 		}
 
 		stat := info.Sys().(*syscall.Stat_t)
-		g, err := user.LookupGroupId(fmt.Sprintf("%d", stat.Gid))
+		g, err := lookupGroupID(fmt.Sprintf("%d", stat.Gid))
 		if err != nil {
 			return emptyKV, err
 		}
diff --git a/keywordfuncs_linux.go b/keywordfuncs_linux.go
index 1fd843d..c45e1ab 100644
--- a/keywordfuncs_linux.go
+++ b/keywordfuncs_linux.go
@@ -40,7 +40,7 @@ var (
 		}
 
 		stat := info.Sys().(*syscall.Stat_t)
-		g, err := user.LookupGroupId(fmt.Sprintf("%d", stat.Gid))
+		g, err := lookupGroupID(fmt.Sprintf("%d", stat.Gid))
 		if err != nil {
 			return emptyKV, err
 		}
diff --git a/lookup_new.go b/lookup_new.go
new file mode 100644
index 0000000..c8baae7
--- /dev/null
+++ b/lookup_new.go
@@ -0,0 +1,9 @@
+// +build go1.7
+
+package mtree
+
+import (
+	"os/user"
+)
+
+var lookupGroupID = user.LookupGroupId
diff --git a/lookup_old.go b/lookup_old.go
new file mode 100644
index 0000000..8c22e2b
--- /dev/null
+++ b/lookup_old.go
@@ -0,0 +1,102 @@
+// +build !go1.7
+
+package mtree
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+)
+
+const groupFile = "/etc/group"
+
+var colon = []byte{':'}
+
+// Group represents a grouping of users.
+//
+// On POSIX systems Gid contains a decimal number representing the group ID.
+type Group struct {
+	Gid  string // group ID
+	Name string // group name
+}
+
+func lookupGroupID(id string) (*Group, error) {
+	f, err := os.Open(groupFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return findGroupID(id, f)
+}
+
+func findGroupID(id string, r io.Reader) (*Group, error) {
+	if v, err := readColonFile(r, matchGroupIndexValue(id, 2)); err != nil {
+		return nil, err
+	} else if v != nil {
+		return v.(*Group), nil
+	}
+	return nil, UnknownGroupIDError(id)
+}
+
+// lineFunc returns a value, an error, or (nil, nil) to skip the row.
+type lineFunc func(line []byte) (v interface{}, err error)
+
+// readColonFile parses r as an /etc/group or /etc/passwd style file, running
+// fn for each row. readColonFile returns a value, an error, or (nil, nil) if
+// the end of the file is reached without a match.
+func readColonFile(r io.Reader, fn lineFunc) (v interface{}, err error) {
+	bs := bufio.NewScanner(r)
+	for bs.Scan() {
+		line := bs.Bytes()
+		// There's no spec for /etc/passwd or /etc/group, but we try to follow
+		// the same rules as the glibc parser, which allows comments and blank
+		// space at the beginning of a line.
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+		v, err = fn(line)
+		if v != nil || err != nil {
+			return
+		}
+	}
+	return nil, bs.Err()
+}
+
+func matchGroupIndexValue(value string, idx int) lineFunc {
+	var leadColon string
+	if idx > 0 {
+		leadColon = ":"
+	}
+	substr := []byte(leadColon + value + ":")
+	return func(line []byte) (v interface{}, err error) {
+		if !bytes.Contains(line, substr) || bytes.Count(line, colon) < 3 {
+			return
+		}
+		// wheel:*:0:root
+		parts := strings.SplitN(string(line), ":", 4)
+		if len(parts) < 4 || parts[0] == "" || parts[idx] != value ||
+			// If the file contains +foo and you search for "foo", glibc
+			// returns an "invalid argument" error. Similarly, if you search
+			// for a gid for a row where the group name starts with "+" or "-",
+			// glibc fails to find the record.
+			parts[0][0] == '+' || parts[0][0] == '-' {
+			return
+		}
+		if _, err := strconv.Atoi(parts[2]); err != nil {
+			return nil, nil
+		}
+		return &Group{Name: parts[0], Gid: parts[2]}, nil
+	}
+}
+
+// UnknownGroupIDError is returned by LookupGroupId when
+// a group cannot be found.
+type UnknownGroupIDError string
+
+func (e UnknownGroupIDError) Error() string {
+	return "group: unknown groupid " + string(e)
+}