diff --git a/builder/virtualbox/common/step_download_guest_additions.go b/builder/virtualbox/common/step_download_guest_additions.go
index facc60b6a..706d5f466 100644
--- a/builder/virtualbox/common/step_download_guest_additions.go
+++ b/builder/virtualbox/common/step_download_guest_additions.go
@@ -122,7 +122,7 @@ func (s *StepDownloadGuestAdditions) Run(ctx context.Context, state multistep.St
 	}
 
 	// Convert the file/url to an actual URL for step_download to process.
-	url, err = common.DownloadableURL(url)
+	url, err = common.ValidatedURL(url)
 	if err != nil {
 		err := fmt.Errorf("Error preparing guest additions url: %s", err)
 		state.Put("error", err)
diff --git a/builder/virtualbox/ovf/config.go b/builder/virtualbox/ovf/config.go
index 41a013ebd..5eef4507c 100644
--- a/builder/virtualbox/ovf/config.go
+++ b/builder/virtualbox/ovf/config.go
@@ -97,7 +97,7 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	if c.SourcePath == "" {
 		errs = packer.MultiErrorAppend(errs, fmt.Errorf("source_path is required"))
 	} else {
-		c.SourcePath, err = common.DownloadableURL(c.SourcePath)
+		c.SourcePath, err = common.ValidatedURL(c.SourcePath)
 		if err != nil {
 			errs = packer.MultiErrorAppend(errs, fmt.Errorf("source_path is invalid: %s", err))
 		}
diff --git a/builder/virtualbox/ovf/config_test.go b/builder/virtualbox/ovf/config_test.go
index 5e67b242d..51412a0c5 100644
--- a/builder/virtualbox/ovf/config_test.go
+++ b/builder/virtualbox/ovf/config_test.go
@@ -84,7 +84,7 @@ func TestNewConfig_sourcePath(t *testing.T) {
 		t.Fatalf("bad: %#v", warns)
 	}
 	if err == nil {
-		t.Fatal("should error")
+		t.Fatalf("should error")
 	}
 
 	// Good
diff --git a/common/config.go b/common/config.go
index 7bef253ee..aa632ceae 100644
--- a/common/config.go
+++ b/common/config.go
@@ -5,6 +5,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strings"
 	"time"
@@ -43,89 +44,126 @@ func ChooseString(vals ...string) string {
 	return ""
 }
 
-// DownloadableURL processes a URL that may also be a file path and returns
-// a completely valid URL. For example, the original URL might be "local/file.iso"
-// which isn't a valid URL. DownloadableURL will return "file:///local/file.iso"
-func DownloadableURL(original string) (string, error) {
-	if runtime.GOOS == "windows" {
-		// If the distance to the first ":" is just one character, assume
-		// we're dealing with a drive letter and thus a file path.
-		// prepend with "file:///"" now so that url.Parse won't accidentally
-		// parse the drive letter into the url scheme.
-		// See https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/
-		// for more info about valid windows URIs
-		idx := strings.Index(original, ":")
-		if idx == 1 {
-			original = "file:///" + original
+// SupportedProtocol verifies that the url passed is actually supported or not
+// This will also validate that the protocol is one that's actually implemented.
+func SupportedProtocol(u *url.URL) bool {
+	// url.Parse shouldn't return nil except on error....but it can.
+	if u == nil {
+		return false
+	}
+
+	// build a dummy NewDownloadClient since this is the only place that valid
+	// protocols are actually exposed.
+	cli := NewDownloadClient(&DownloadConfig{})
+
+	// Iterate through each downloader to see if a protocol was found.
+	ok := false
+	for scheme := range cli.config.DownloaderMap {
+		if strings.ToLower(u.Scheme) == strings.ToLower(scheme) {
+			ok = true
 		}
 	}
+	return ok
+}
+
+// DownloadableURL processes a URL that may also be a file path and returns
+// a completely valid URL representing the requested file. For example,
+// the original URL might be "local/file.iso" which isn't a valid URL,
+// and so DownloadableURL will return "file://local/file.iso"
+// No other transformations are done to the path.
+func DownloadableURL(original string) (string, error) {
+	var absPrefix, result string
+
+	absPrefix = ""
+	if runtime.GOOS == "windows" {
+		absPrefix = "/"
+	}
+
+	// Check that the user specified a UNC path, and promote it to an smb:// uri.
+	if strings.HasPrefix(original, "\\\\") && len(original) > 2 && original[2] != '?' {
+		result = filepath.ToSlash(original[2:])
+		return fmt.Sprintf("smb://%s", result), nil
+	}
+
+	// Fix the url if it's using bad characters commonly mistaken with a path.
+	original = filepath.ToSlash(original)
+
+	// Check to see that this is a parseable URL with a scheme and a host.
+	// If so, then just pass it through.
+	if u, err := url.Parse(original); err == nil && u.Scheme != "" && u.Host != "" {
+		return original, nil
+	}
+
+	// If it's a file scheme, then convert it back to a regular path so the next
+	// case which forces it to an absolute path, will correct it.
+	if u, err := url.Parse(original); err == nil && strings.ToLower(u.Scheme) == "file" {
+		original = u.Path
+	}
+
+	// If we're on Windows and we start with a slash, then this absolute path
+	// is wrong. Fix it up, so the next case can figure out the absolute path.
+	if rpath := strings.SplitN(original, "/", 2); rpath[0] == "" && runtime.GOOS == "windows" {
+		result = rpath[1]
+	} else {
+		result = original
+	}
+
+	// Since we should be some kind of path (relative or absolute), check
+	// that the file exists, then make it an absolute path so we can return an
+	// absolute uri.
+	if _, err := os.Stat(result); err == nil {
+		result, err = filepath.Abs(filepath.FromSlash(result))
+		if err != nil {
+			return "", err
+		}
+
+		result, err = filepath.EvalSymlinks(result)
+		if err != nil {
+			return "", err
+		}
+
+		result = filepath.Clean(result)
+		return fmt.Sprintf("file://%s%s", absPrefix, filepath.ToSlash(result)), nil
+	}
+
+	// Otherwise, check if it was originally an absolute path, and fix it if so.
+	if strings.HasPrefix(original, "/") {
+		return fmt.Sprintf("file://%s%s", absPrefix, result), nil
+	}
+
+	// Anything left should be a non-existent relative path. So fix it up here.
+	result = filepath.ToSlash(filepath.Clean(result))
+	return fmt.Sprintf("file://./%s", result), nil
+}
+
+// Force the parameter into a url. This will transform the parameter into
+// a proper url, removing slashes, adding the proper prefix, etc.
+func ValidatedURL(original string) (string, error) {
+
+	// See if the user failed to give a url
+	if ok, _ := regexp.MatchString("(?m)^[^[:punct:]]+://", original); !ok {
+
+		// So since no magic was found, this must be a path.
+		result, err := DownloadableURL(original)
+		if err == nil {
+			return ValidatedURL(result)
+		}
+
+		return "", err
+	}
+
+	// Verify that the url is parseable...just in case.
 	u, err := url.Parse(original)
 	if err != nil {
 		return "", err
 	}
 
-	if u.Scheme == "" {
-		u.Scheme = "file"
+	// We should now have a url, so verify that it's a protocol we support.
+	if !SupportedProtocol(u) {
+		return "", fmt.Errorf("Unsupported protocol scheme! (%#v)", u)
 	}
 
-	if u.Scheme == "file" {
-		// Windows file handling is all sorts of tricky...
-		if runtime.GOOS == "windows" {
-			// If the path is using Windows-style slashes, URL parses
-			// it into the host field.
-			if u.Path == "" && strings.Contains(u.Host, `\`) {
-				u.Path = u.Host
-				u.Host = ""
-			}
-		}
-		// Only do the filepath transformations if the file appears
-		// to actually exist.
-		if _, err := os.Stat(u.Path); err == nil {
-			u.Path, err = filepath.Abs(u.Path)
-			if err != nil {
-				return "", err
-			}
-
-			u.Path, err = filepath.EvalSymlinks(u.Path)
-			if err != nil {
-				return "", err
-			}
-
-			u.Path = filepath.Clean(u.Path)
-		}
-
-		if runtime.GOOS == "windows" {
-			// Also replace all backslashes with forwardslashes since Windows
-			// users are likely to do this but the URL should actually only
-			// contain forward slashes.
-			u.Path = strings.Replace(u.Path, `\`, `/`, -1)
-			// prepend absolute windows paths with "/" so that when we
-			// compose u.String() below the outcome will be correct
-			// file:///c/blah syntax; otherwise u.String() will only add
-			// file:// which is not technically a correct windows URI
-			if filepath.IsAbs(u.Path) && !strings.HasPrefix(u.Path, "/") {
-				u.Path = "/" + u.Path
-			}
-
-		}
-	}
-
-	// Make sure it is lowercased
-	u.Scheme = strings.ToLower(u.Scheme)
-
-	// Verify that the scheme is something we support in our common downloader.
-	supported := []string{"file", "http", "https"}
-	found := false
-	for _, s := range supported {
-		if u.Scheme == s {
-			found = true
-			break
-		}
-	}
-
-	if !found {
-		return "", fmt.Errorf("Unsupported URL scheme: %s", u.Scheme)
-	}
+	// We should now have a properly formatted and supported url
 	return u.String(), nil
 }
 
@@ -144,27 +182,38 @@ func DownloadableURL(original string) (string, error) {
 
 func FileExistsLocally(original string) bool {
 	// original should be something like file://C:/my/path.iso
+	u, _ := url.Parse(original)
 
-	fileURL, _ := url.Parse(original)
-	fileExists := false
-
-	if fileURL.Scheme == "file" {
-		// on windows, correct URI is file:///c:/blah/blah.iso.
-		// url.Parse will pull out the scheme "file://" and leave the path as
-		// "/c:/blah/blah/iso".  Here we remove this forward slash on absolute
-		// Windows file URLs before processing
-		// see https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/
-		// for more info about valid windows URIs
-		filePath := fileURL.Path
-		if runtime.GOOS == "windows" && len(filePath) > 0 && filePath[0] == '/' {
-			filePath = filePath[1:]
-		}
-		_, err := os.Stat(filePath)
-		if err != nil {
-			return fileExists
-		} else {
-			fileExists = true
-		}
+	// First create a dummy downloader so we can figure out which
+	// protocol to use.
+	cli := NewDownloadClient(&DownloadConfig{})
+	d, ok := cli.config.DownloaderMap[u.Scheme]
+	if !ok {
+		return false
 	}
-	return fileExists
+
+	// Check to see that it's got a Local way of doing things.
+	local, ok := d.(LocalDownloader)
+	if !ok {
+		return true // XXX: Remote URLs short-circuit this logic.
+	}
+
+	// Figure out where we're at.
+	wd, err := os.Getwd()
+	if err != nil {
+		return false
+	}
+
+	// Now figure out the real path to the file.
+	realpath, err := local.toPath(wd, *u)
+	if err != nil {
+		return false
+	}
+
+	// Finally we can seek the truth via os.Stat.
+	_, err = os.Stat(realpath)
+	if err != nil {
+		return false
+	}
+	return true
 }
diff --git a/common/config_test.go b/common/config_test.go
index 365515109..8f170fe1d 100644
--- a/common/config_test.go
+++ b/common/config_test.go
@@ -37,7 +37,28 @@ func TestChooseString(t *testing.T) {
 	}
 }
 
-func TestDownloadableURL(t *testing.T) {
+func TestValidatedURL(t *testing.T) {
+	// Invalid URL: has hex code in host
+	_, err := ValidatedURL("http://what%20.com")
+	if err == nil {
+		t.Fatalf("expected err : %s", err)
+	}
+
+	// Invalid: unsupported scheme
+	_, err = ValidatedURL("ftp://host.com/path")
+	if err == nil {
+		t.Fatalf("expected err : %s", err)
+	}
+
+	// Valid: http
+	u, err := ValidatedURL("HTTP://packer.io/path")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if u != "http://packer.io/path" {
+		t.Fatalf("bad: %s", u)
+	}
 
 	cases := []struct {
 		InputString string
@@ -55,7 +76,7 @@ func TestDownloadableURL(t *testing.T) {
 	}
 
 	for _, tc := range cases {
-		u, err := DownloadableURL(tc.InputString)
+		u, err := ValidatedURL(tc.InputString)
 		if u != tc.OutputURL {
 			t.Fatal(fmt.Sprintf("Error with URL %s: got %s but expected %s",
 				tc.InputString, tc.OutputURL, u))
@@ -63,78 +84,127 @@ func TestDownloadableURL(t *testing.T) {
 		if (err != nil) != tc.ErrExpected {
 			if tc.ErrExpected == true {
 				t.Fatal(fmt.Sprintf("Error with URL %s: we expected "+
-					"DownloadableURL to return an error but didn't get one.",
+					"ValidatedURL to return an error but didn't get one.",
 					tc.InputString))
 			} else {
 				t.Fatal(fmt.Sprintf("Error with URL %s: we did not expect an "+
-					" error from DownloadableURL but we got: %s",
+					" error from ValidatedURL but we got: %s",
 					tc.InputString, err))
 			}
 		}
 	}
 }
 
+func GetNativePathToTestFixtures(t *testing.T) string {
+	const path = "./test-fixtures"
+	res, err := filepath.Abs(path)
+	if err != nil {
+		t.Fatalf("err converting test-fixtures path into an absolute path : %s", err)
+	}
+	return res
+}
+
+func GetPortablePathToTestFixtures(t *testing.T) string {
+	res := GetNativePathToTestFixtures(t)
+	return filepath.ToSlash(res)
+}
+
 func TestDownloadableURL_WindowsFiles(t *testing.T) {
 	if runtime.GOOS == "windows" {
+		portablepath := GetPortablePathToTestFixtures(t)
+		nativepath := GetNativePathToTestFixtures(t)
+
 		dirCases := []struct {
 			InputString string
 			OutputURL   string
 			ErrExpected bool
 		}{ // TODO: add different directories
 			{
-				"C:\\Temp\\SomeDir\\myfile.txt",
-				"file:///C:/Temp/SomeDir/myfile.txt",
+				fmt.Sprintf("%s\\SomeDir\\myfile.txt", nativepath),
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
 				false,
 			},
-			{ // need windows drive
-				"\\Temp\\SomeDir\\myfile.txt",
-				"",
-				true,
-			},
-			{ // need windows drive
-				"/Temp/SomeDir/myfile.txt",
-				"",
-				true,
-			},
-			{ // UNC paths; why not?
-				"\\\\?\\c:\\Temp\\SomeDir\\myfile.txt",
-				"",
-				true,
-			},
-			{
-				"file:///C:\\Temp\\SomeDir\\myfile.txt",
-				"file:///c:/Temp/SomeDir/myfile.txt",
+			{ // without the drive makes this native path a relative file:// uri
+				"test-fixtures\\SomeDir\\myfile.txt",
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
 				false,
 			},
-			{
-				"file:///c:/Temp/Somedir/myfile.txt",
-				"file:///c:/Temp/SomeDir/myfile.txt",
+			{ // without the drive makes this native path a relative file:// uri
+				"test-fixtures/SomeDir/myfile.txt",
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // UNC paths being promoted to smb:// uri scheme.
+				fmt.Sprintf("\\\\localhost\\C$\\%s\\SomeDir\\myfile.txt", nativepath),
+				fmt.Sprintf("smb://localhost/C$/%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Absolute uri (incorrect slash type)
+				fmt.Sprintf("file:///%s\\SomeDir\\myfile.txt", nativepath),
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Absolute uri (existing and mis-spelled)
+				fmt.Sprintf("file:///%s/Somedir/myfile.txt", nativepath),
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Absolute path (non-existing)
+				"\\absolute\\path\\to\\non-existing\\file.txt",
+				"file:///absolute/path/to/non-existing/file.txt",
+				false,
+			},
+			{ // Absolute paths (existing)
+				fmt.Sprintf("%s/SomeDir/myfile.txt", nativepath),
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Relative path (non-existing)
+				"./nonexisting/relative/path/to/file.txt",
+				"file://./nonexisting/relative/path/to/file.txt",
+				false,
+			},
+			{ // Relative path (existing)
+				"./test-fixtures/SomeDir/myfile.txt",
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Absolute uri (existing and with `/` prefix)
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				fmt.Sprintf("file:///%s/SomeDir/myfile.txt", portablepath),
+				false,
+			},
+			{ // Absolute uri (non-existing and with `/` prefix)
+				"file:///path/to/non-existing/file.txt",
+				"file:///path/to/non-existing/file.txt",
+				false,
+			},
+			{ // Absolute uri (non-existing and missing `/` prefix)
+				"file://path/to/non-existing/file.txt",
+				"file://path/to/non-existing/file.txt",
+				false,
+			},
+			{ // Absolute uri and volume (non-existing and with `/` prefix)
+				"file:///T:/path/to/non-existing/file.txt",
+				"file:///T:/path/to/non-existing/file.txt",
+				false,
+			},
+			{ // Absolute uri and volume (non-existing and missing `/` prefix)
+				"file://T:/path/to/non-existing/file.txt",
+				"file://T:/path/to/non-existing/file.txt",
 				false,
 			},
 		}
-		// create absolute-pathed tempfile to play with
-		err := os.Mkdir("C:\\Temp\\SomeDir", 0755)
-		if err != nil {
-			t.Fatalf("err creating test dir: %s", err)
-		}
-		fi, err := os.Create("C:\\Temp\\SomeDir\\myfile.txt")
-		if err != nil {
-			t.Fatalf("err creating test file: %s", err)
-		}
-		fi.Close()
-		defer os.Remove("C:\\Temp\\SomeDir\\myfile.txt")
-		defer os.Remove("C:\\Temp\\SomeDir")
-
 		// Run through test cases to make sure they all parse correctly
-		for _, tc := range dirCases {
+		for idx, tc := range dirCases {
 			u, err := DownloadableURL(tc.InputString)
 			if (err != nil) != tc.ErrExpected {
-				t.Fatalf("Test Case failed: Expected err = %#v, err = %#v, input = %s",
-					tc.ErrExpected, err, tc.InputString)
+				t.Fatalf("Test Case %d failed: Expected err = %#v, err = %#v, input = %s",
+					idx, tc.ErrExpected, err, tc.InputString)
 			}
 			if u != tc.OutputURL {
-				t.Fatalf("Test Case failed: Expected %s but received %s from input %s",
-					tc.OutputURL, u, tc.InputString)
+				t.Fatalf("Test Case %d failed: Expected %s but received %s from input %s",
+					idx, tc.OutputURL, u, tc.InputString)
 			}
 		}
 	}
@@ -154,10 +224,12 @@ func TestDownloadableURL_FilePaths(t *testing.T) {
 	}
 
 	tfPath = filepath.Clean(tfPath)
-
 	filePrefix := "file://"
+
+	// If we're running windows, then absolute URIs are `/`-prefixed.
+	platformPrefix := ""
 	if runtime.GOOS == "windows" {
-		filePrefix += "/"
+		platformPrefix = "/"
 	}
 
 	// Relative filepath. We run this test in a func so that
@@ -180,8 +252,9 @@ func TestDownloadableURL_FilePaths(t *testing.T) {
 			t.Fatalf("err: %s", err)
 		}
 
-		expected := fmt.Sprintf("%s%s",
+		expected := fmt.Sprintf("%s%s%s",
 			filePrefix,
+			platformPrefix,
 			strings.Replace(tfPath, `\`, `/`, -1))
 		if u != expected {
 			t.Fatalf("unexpected: %#v != %#v", u, expected)
@@ -189,21 +262,22 @@ func TestDownloadableURL_FilePaths(t *testing.T) {
 	}()
 
 	// Test some cases with and without a schema prefix
-	for _, prefix := range []string{"", filePrefix} {
+	for _, prefix := range []string{"", filePrefix + platformPrefix} {
 		// Nonexistent file
 		_, err = DownloadableURL(prefix + "i/dont/exist")
 		if err != nil {
 			t.Fatalf("err: %s", err)
 		}
 
-		// Good file
+		// Good file (absolute)
 		u, err := DownloadableURL(prefix + tfPath)
 		if err != nil {
 			t.Fatalf("err: %s", err)
 		}
 
-		expected := fmt.Sprintf("%s%s",
+		expected := fmt.Sprintf("%s%s%s",
 			filePrefix,
+			platformPrefix,
 			strings.Replace(tfPath, `\`, `/`, -1))
 		if u != expected {
 			t.Fatalf("unexpected: %s != %s", u, expected)
@@ -211,39 +285,28 @@ func TestDownloadableURL_FilePaths(t *testing.T) {
 	}
 }
 
-func test_FileExistsLocally(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		dirCases := []struct {
-			Input  string
-			Output bool
-		}{
-			// file exists locally
-			{"file:///C:/Temp/SomeDir/myfile.txt", true},
-			// file is not supposed to exist locally
-			{"https://myfile.iso", true},
-			// file does not exist locally
-			{"file:///C/i/dont/exist", false},
-		}
-		// create absolute-pathed tempfile to play with
-		err := os.Mkdir("C:\\Temp\\SomeDir", 0755)
-		if err != nil {
-			t.Fatalf("err creating test dir: %s", err)
-		}
-		fi, err := os.Create("C:\\Temp\\SomeDir\\myfile.txt")
-		if err != nil {
-			t.Fatalf("err creating test file: %s", err)
-		}
-		fi.Close()
-		defer os.Remove("C:\\Temp\\SomeDir\\myfile.txt")
-		defer os.Remove("C:\\Temp\\SomeDir")
+func TestFileExistsLocally(t *testing.T) {
+	portablepath := GetPortablePathToTestFixtures(t)
 
-		// Run through test cases to make sure they all parse correctly
-		for _, tc := range dirCases {
-			fileOK := FileExistsLocally(tc.Input)
-			if !fileOK {
-				t.Fatalf("Test Case failed: Expected %#v, received = %#v, input = %s",
-					tc.Output, fileOK, tc.Input)
-			}
+	dirCases := []struct {
+		Input  string
+		Output bool
+	}{
+		// file exists locally
+		{fmt.Sprintf("file://%s/SomeDir/myfile.txt", portablepath), true},
+		// remote protocols short-circuit and are considered to exist locally
+		{"https://myfile.iso", true},
+		// non-existent protocols do not exist and hence fail
+		{"nonexistent-protocol://myfile.iso", false},
+		// file does not exist locally
+		{"file:///C/i/dont/exist", false},
+	}
+	// Run through test cases to make sure they all parse correctly
+	for _, tc := range dirCases {
+		fileOK := FileExistsLocally(tc.Input)
+		if fileOK != tc.Output {
+			t.Fatalf("Test Case failed: Expected %#v, received = %#v, input = %s",
+				tc.Output, fileOK, tc.Input)
 		}
 	}
 }
diff --git a/common/download.go b/common/download.go
index ada4c9756..6d3d2a685 100644
--- a/common/download.go
+++ b/common/download.go
@@ -10,12 +10,19 @@ import (
 	"errors"
 	"fmt"
 	"hash"
-	"io"
 	"log"
-	"net/http"
 	"net/url"
 	"os"
+	"path"
 	"runtime"
+	"strings"
+)
+
+// imports related to each Downloader implementation
+import (
+	"io"
+	"net/http"
+	"path/filepath"
 )
 
 // DownloadConfig is the configuration given to instantiate a new
@@ -75,23 +82,38 @@ func HashForType(t string) hash.Hash {
 // NewDownloadClient returns a new DownloadClient for the given
 // configuration.
 func NewDownloadClient(c *DownloadConfig) *DownloadClient {
+	const mtu = 1500 /* ethernet */ - 20 /* ipv4 */ - 20 /* tcp */
+
+	// Create downloader map if it hasn't been specified already.
 	if c.DownloaderMap == nil {
 		c.DownloaderMap = map[string]Downloader{
+			"file":  &FileDownloader{bufferSize: nil},
 			"http":  &HTTPDownloader{userAgent: c.UserAgent},
 			"https": &HTTPDownloader{userAgent: c.UserAgent},
+			"smb":   &SMBDownloader{bufferSize: nil},
 		}
 	}
-
 	return &DownloadClient{config: c}
 }
 
-// A downloader is responsible for actually taking a remote URL and
-// downloading it.
+// A downloader implements the ability to transfer, cancel, or resume a file.
 type Downloader interface {
+	Resume()
 	Cancel()
+	Progress() uint64
+	Total() uint64
+}
+
+// A LocalDownloader is responsible for converting a uri to a local path
+//	that the platform can open directly.
+type LocalDownloader interface {
+	toPath(string, url.URL) (string, error)
+}
+
+// A RemoteDownloader is responsible for actually taking a remote URL and
+//	downloading it.
+type RemoteDownloader interface {
 	Download(*os.File, *url.URL) error
-	Progress() uint
-	Total() uint
 }
 
 func (d *DownloadClient) Cancel() {
@@ -105,62 +127,64 @@ func (d *DownloadClient) Get() (string, error) {
 		return d.config.TargetPath, nil
 	}
 
+	/* parse the configuration url into a net/url object */
 	u, err := url.Parse(d.config.Url)
 	if err != nil {
 		return "", err
 	}
-
 	log.Printf("Parsed URL: %#v", u)
 
-	// Files when we don't copy the file are special cased.
-	var f *os.File
+	/* use the current working directory as the base for relative uri's */
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+
+	// Determine which is the correct downloader to use
 	var finalPath string
-	sourcePath := ""
-	if u.Scheme == "file" && !d.config.CopyFile {
-		// This is special case for relative path in this case user specify
-		// file:../ and after parse destination goes to Opaque
-		if u.Path != "" {
-			// If url.Path is set just use this
-			finalPath = u.Path
-		} else if u.Opaque != "" {
-			// otherwise try url.Opaque
-			finalPath = u.Opaque
-		}
-		// This is a special case where we use a source file that already exists
-		// locally and we don't make a copy. Normally we would copy or download.
-		log.Printf("[DEBUG] Using local file: %s", finalPath)
 
-		// Remove forward slash on absolute Windows file URLs before processing
-		if runtime.GOOS == "windows" && len(finalPath) > 0 && finalPath[0] == '/' {
-			finalPath = finalPath[1:]
-		}
+	var ok bool
+	d.downloader, ok = d.config.DownloaderMap[u.Scheme]
+	if !ok {
+		return "", fmt.Errorf("No downloader for scheme: %s", u.Scheme)
+	}
 
-		// Keep track of the source so we can make sure not to delete this later
-		sourcePath = finalPath
-		if _, err = os.Stat(finalPath); err != nil {
-			return "", err
-		}
-	} else {
+	remote, ok := d.downloader.(RemoteDownloader)
+	if !ok {
+		return "", fmt.Errorf("Unable to treat uri scheme %s as a Downloader. : %T", u.Scheme, d.downloader)
+	}
+
+	local, ok := d.downloader.(LocalDownloader)
+	if !ok && !d.config.CopyFile {
+		d.config.CopyFile = true
+	}
+
+	// If we're copying the file, then just use the actual downloader
+	if d.config.CopyFile {
+		var f *os.File
 		finalPath = d.config.TargetPath
 
-		var ok bool
-		d.downloader, ok = d.config.DownloaderMap[u.Scheme]
-		if !ok {
-			return "", fmt.Errorf("No downloader for scheme: %s", u.Scheme)
-		}
-
-		// Otherwise, download using the downloader.
 		f, err = os.OpenFile(finalPath, os.O_RDWR|os.O_CREATE, os.FileMode(0666))
 		if err != nil {
 			return "", err
 		}
 
 		log.Printf("[DEBUG] Downloading: %s", u.String())
-		err = d.downloader.Download(f, u)
+		err = remote.Download(f, u)
 		f.Close()
 		if err != nil {
 			return "", err
 		}
+
+		// Otherwise if our Downloader is a LocalDownloader we can just use the
+		//	path after transforming it.
+	} else {
+		finalPath, err = local.toPath(cwd, *u)
+		if err != nil {
+			return "", err
+		}
+
+		log.Printf("[DEBUG] Using local file: %s", finalPath)
 	}
 
 	if d.config.Hash != nil {
@@ -168,7 +192,7 @@ func (d *DownloadClient) Get() (string, error) {
 		verify, err = d.VerifyChecksum(finalPath)
 		if err == nil && !verify {
 			// Only delete the file if we made a copy or downloaded it
-			if sourcePath != finalPath {
+			if d.config.CopyFile {
 				os.Remove(finalPath)
 			}
 
@@ -181,7 +205,6 @@ func (d *DownloadClient) Get() (string, error) {
 	return finalPath, err
 }
 
-// PercentProgress returns the download progress as a percentage.
 func (d *DownloadClient) PercentProgress() int {
 	if d.downloader == nil {
 		return -1
@@ -212,17 +235,21 @@ func (d *DownloadClient) VerifyChecksum(path string) (bool, error) {
 // HTTPDownloader is an implementation of Downloader that downloads
 // files over HTTP.
 type HTTPDownloader struct {
-	progress  uint
-	total     uint
+	current   uint64
+	total     uint64
 	userAgent string
 }
 
-func (*HTTPDownloader) Cancel() {
+func (d *HTTPDownloader) Cancel() {
+	// TODO(mitchellh): Implement
+}
+
+func (d *HTTPDownloader) Resume() {
 	// TODO(mitchellh): Implement
 }
 
 func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
-	log.Printf("Starting download: %s", src.String())
+	log.Printf("Starting download over HTTP: %s", src.String())
 
 	// Seek to the beginning by default
 	if _, err := dst.Seek(0, 0); err != nil {
@@ -230,7 +257,7 @@ func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 	}
 
 	// Reset our progress
-	d.progress = 0
+	d.current = 0
 
 	// Make the request. We first make a HEAD request so we can check
 	// if the server supports range queries. If the server/URL doesn't
@@ -258,7 +285,8 @@ func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 			if fi, err := dst.Stat(); err == nil {
 				if _, err = dst.Seek(0, os.SEEK_END); err == nil {
 					req.Header.Set("Range", fmt.Sprintf("bytes=%d-", fi.Size()))
-					d.progress = uint(fi.Size())
+
+					d.current = uint64(fi.Size())
 				}
 			}
 		}
@@ -272,7 +300,8 @@ func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 		return err
 	}
 
-	d.total = d.progress + uint(resp.ContentLength)
+	d.total = d.current + uint64(resp.ContentLength)
+
 	var buffer [4096]byte
 	for {
 		n, err := resp.Body.Read(buffer[:])
@@ -280,7 +309,7 @@ func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 			return err
 		}
 
-		d.progress += uint(n)
+		d.current += uint64(n)
 
 		if _, werr := dst.Write(buffer[:n]); werr != nil {
 			return werr
@@ -290,14 +319,253 @@ func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 			break
 		}
 	}
-
 	return nil
 }
 
-func (d *HTTPDownloader) Progress() uint {
-	return d.progress
+func (d *HTTPDownloader) Progress() uint64 {
+	return d.current
 }
 
-func (d *HTTPDownloader) Total() uint {
+func (d *HTTPDownloader) Total() uint64 {
 	return d.total
 }
+
+// FileDownloader is an implementation of Downloader that downloads
+// files using the regular filesystem.
+type FileDownloader struct {
+	bufferSize *uint
+
+	active  bool
+	current uint64
+	total   uint64
+}
+
+func (d *FileDownloader) Progress() uint64 {
+	return d.current
+}
+
+func (d *FileDownloader) Total() uint64 {
+	return d.total
+}
+
+func (d *FileDownloader) Cancel() {
+	d.active = false
+}
+
+func (d *FileDownloader) Resume() {
+	// TODO: Implement
+}
+
+func (d *FileDownloader) toPath(base string, uri url.URL) (string, error) {
+	var result string
+
+	// absolute path -- file://c:/absolute/path -> c:/absolute/path
+	if strings.HasSuffix(uri.Host, ":") {
+		result = path.Join(uri.Host, uri.Path)
+
+		// semi-absolute path (current drive letter)
+		//	-- file:///absolute/path -> drive:/absolute/path
+	} else if uri.Host == "" && strings.HasPrefix(uri.Path, "/") {
+		apath := uri.Path
+		components := strings.Split(apath, "/")
+		volume := filepath.VolumeName(base)
+
+		// semi-absolute absolute path (includes volume letter)
+		// -- file://drive:/path -> drive:/absolute/path
+		if len(components) > 1 && strings.HasSuffix(components[1], ":") {
+			volume = components[1]
+			apath = path.Join(components[2:]...)
+		}
+
+		result = path.Join(volume, apath)
+
+		// relative path -- file://./relative/path -> ./relative/path
+	} else if uri.Host == "." {
+		result = path.Join(base, uri.Path)
+
+		// relative path -- file://relative/path -> ./relative/path
+	} else {
+		result = path.Join(base, uri.Host, uri.Path)
+	}
+	return filepath.ToSlash(result), nil
+}
+
+func (d *FileDownloader) Download(dst *os.File, src *url.URL) error {
+	d.active = false
+
+	/* check the uri's scheme to make sure it matches */
+	if src == nil || src.Scheme != "file" {
+		return fmt.Errorf("Unexpected uri scheme: %s", src.Scheme)
+	}
+	uri := src
+
+	/* use the current working directory as the base for relative uri's */
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	/* determine which uri format is being used and convert to a real path */
+	realpath, err := d.toPath(cwd, *uri)
+	if err != nil {
+		return err
+	}
+
+	/* download the file using the operating system's facilities */
+	d.current = 0
+	d.active = true
+
+	f, err := os.Open(realpath)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	// get the file size
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	d.total = uint64(fi.Size())
+
+	// no bufferSize specified, so copy synchronously.
+	if d.bufferSize == nil {
+		var n int64
+		n, err = io.Copy(dst, f)
+		d.active = false
+
+		d.current += uint64(n)
+
+		// use a goro in case someone else wants to enable cancel/resume
+	} else {
+		errch := make(chan error)
+		go func(d *FileDownloader, r io.Reader, w io.Writer, e chan error) {
+			for d.active {
+				n, err := io.CopyN(w, r, int64(*d.bufferSize))
+				if err != nil {
+					break
+				}
+
+				d.current += uint64(n)
+			}
+			d.active = false
+			e <- err
+		}(d, f, dst, errch)
+
+		// ...and we spin until it's done
+		err = <-errch
+	}
+	f.Close()
+	return err
+}
+
+// SMBDownloader is an implementation of Downloader that downloads
+// files using the "\\" path format on Windows
+type SMBDownloader struct {
+	bufferSize *uint
+
+	active  bool
+	current uint64
+	total   uint64
+}
+
+func (d *SMBDownloader) Progress() uint64 {
+	return d.current
+}
+
+func (d *SMBDownloader) Total() uint64 {
+	return d.total
+}
+
+func (d *SMBDownloader) Cancel() {
+	d.active = false
+}
+
+func (d *SMBDownloader) Resume() {
+	// TODO: Implement
+}
+
+func (d *SMBDownloader) toPath(base string, uri url.URL) (string, error) {
+	const UNCPrefix = string(os.PathSeparator) + string(os.PathSeparator)
+
+	if runtime.GOOS != "windows" {
+		return "", fmt.Errorf("Support for SMB based uri's are not supported on %s", runtime.GOOS)
+	}
+
+	return UNCPrefix + filepath.ToSlash(path.Join(uri.Host, uri.Path)), nil
+}
+
+func (d *SMBDownloader) Download(dst *os.File, src *url.URL) error {
+
+	/* first we warn the world if we're not running windows */
+	if runtime.GOOS != "windows" {
+		return fmt.Errorf("Support for SMB based uri's are not supported on %s", runtime.GOOS)
+	}
+
+	d.active = false
+
+	/* convert the uri using the net/url module to a UNC path */
+	if src == nil || src.Scheme != "smb" {
+		return fmt.Errorf("Unexpected uri scheme: %s", src.Scheme)
+	}
+	uri := src
+
+	/* use the current working directory as the base for relative uri's */
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	/* convert uri to an smb-path */
+	realpath, err := d.toPath(cwd, *uri)
+	if err != nil {
+		return err
+	}
+
+	/* Open up the "\\"-prefixed path using the Windows filesystem */
+	d.current = 0
+	d.active = true
+
+	f, err := os.Open(realpath)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	// get the file size (at the risk of performance)
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	d.total = uint64(fi.Size())
+
+	// no bufferSize specified, so copy synchronously.
+	if d.bufferSize == nil {
+		var n int64
+		n, err = io.Copy(dst, f)
+		d.active = false
+
+		d.current += uint64(n)
+
+		// use a goro in case someone else wants to enable cancel/resume
+	} else {
+		errch := make(chan error)
+		go func(d *SMBDownloader, r io.Reader, w io.Writer, e chan error) {
+			for d.active {
+				n, err := io.CopyN(w, r, int64(*d.bufferSize))
+				if err != nil {
+					break
+				}
+
+				d.current += uint64(n)
+			}
+			d.active = false
+			e <- err
+		}(d, f, dst, errch)
+
+		// ...and as usual we spin until it's done
+		err = <-errch
+	}
+	f.Close()
+	return err
+}
diff --git a/common/download_test.go b/common/download_test.go
index 497a05649..beac9cbab 100644
--- a/common/download_test.go
+++ b/common/download_test.go
@@ -8,7 +8,9 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 )
 
@@ -56,6 +58,7 @@ func TestDownloadClient_basic(t *testing.T) {
 	client := NewDownloadClient(&DownloadConfig{
 		Url:        ts.URL + "/basic.txt",
 		TargetPath: tf.Name(),
+		CopyFile:   true,
 	})
 
 	path, err := client.Get()
@@ -91,6 +94,7 @@ func TestDownloadClient_checksumBad(t *testing.T) {
 		TargetPath: tf.Name(),
 		Hash:       HashForType("md5"),
 		Checksum:   checksum,
+		CopyFile:   true,
 	})
 	if _, err := client.Get(); err == nil {
 		t.Fatal("should error")
@@ -115,6 +119,7 @@ func TestDownloadClient_checksumGood(t *testing.T) {
 		TargetPath: tf.Name(),
 		Hash:       HashForType("md5"),
 		Checksum:   checksum,
+		CopyFile:   true,
 	})
 	path, err := client.Get()
 	if err != nil {
@@ -145,6 +150,7 @@ func TestDownloadClient_checksumNoDownload(t *testing.T) {
 		TargetPath: "./test-fixtures/root/another.txt",
 		Hash:       HashForType("md5"),
 		Checksum:   checksum,
+		CopyFile:   true,
 	})
 	path, err := client.Get()
 	if err != nil {
@@ -183,6 +189,7 @@ func TestDownloadClient_resume(t *testing.T) {
 	client := NewDownloadClient(&DownloadConfig{
 		Url:        ts.URL,
 		TargetPath: tf.Name(),
+		CopyFile:   true,
 	})
 	path, err := client.Get()
 	if err != nil {
@@ -240,6 +247,7 @@ func TestDownloadClient_usesDefaultUserAgent(t *testing.T) {
 	config := &DownloadConfig{
 		Url:        server.URL,
 		TargetPath: tf.Name(),
+		CopyFile:   true,
 	}
 
 	client := NewDownloadClient(config)
@@ -271,6 +279,7 @@ func TestDownloadClient_setsUserAgent(t *testing.T) {
 		Url:        server.URL,
 		TargetPath: tf.Name(),
 		UserAgent:  "fancy user agent",
+		CopyFile:   true,
 	}
 
 	client := NewDownloadClient(config)
@@ -351,6 +360,7 @@ func TestDownloadFileUrl(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to detect working directory: %s", err)
 	}
+	cwd = filepath.ToSlash(cwd)
 
 	// source_path is a file path and source is a network path
 	sourcePath := fmt.Sprintf("%s/test-fixtures/fileurl/%s", cwd, "cake")
@@ -376,11 +386,116 @@ func TestDownloadFileUrl(t *testing.T) {
 	// Verify that we fail to match the checksum
 	_, err = client.Get()
 	if err.Error() != "checksums didn't match expected: 6e6f7065" {
-		t.Fatalf("Unexpected failure; expected checksum not to match")
+		t.Fatalf("Unexpected failure; expected checksum not to match. Error was \"%v\"", err)
 	}
 
 	if _, err = os.Stat(sourcePath); err != nil {
 		t.Errorf("Could not stat source file: %s", sourcePath)
 	}
-
+}
+
+// SimulateFileUriDownload is a simple utility function that converts a uri
+// into a testable file path whilst ignoring a correct checksum match, stripping
+// UNC path info, and then calling stat to ensure the correct file exists.
+//    (used by TestFileUriTransforms)
+func SimulateFileUriDownload(t *testing.T, uri string) (string, error) {
+	// source_path is a file path and source is a network path
+	source := fmt.Sprintf(uri)
+	t.Logf("Trying to download %s", source)
+
+	config := &DownloadConfig{
+		Url: source,
+		// This should be wrong. We want to make sure we don't delete
+		Checksum: []byte("nope"),
+		Hash:     HashForType("sha256"),
+		CopyFile: false,
+	}
+
+	// go go go
+	client := NewDownloadClient(config)
+	path, err := client.Get()
+
+	// ignore any non-important checksum errors if it's not a unc path
+	if !strings.HasPrefix(path, "\\\\") && err.Error() != "checksums didn't match expected: 6e6f7065" {
+		t.Fatalf("Unexpected failure; expected checksum not to match")
+	}
+
+	// if it's a unc path, then remove the host and share name so we don't have
+	// to force the user to enable ADMIN$ and Windows File Sharing
+	if strings.HasPrefix(path, "\\\\") {
+		res := strings.SplitN(path, "/", 3)
+		path = "/" + res[2]
+	}
+
+	if _, err = os.Stat(path); err != nil {
+		t.Errorf("Could not stat source file: %s", path)
+	}
+	return path, err
+}
+
+// TestFileUriTransforms tests the case where we use a local file uri
+// for iso_url. There's a few different formats that a file uri can exist as
+// and so we try to test the most useful and common ones.
+func TestFileUriTransforms(t *testing.T) {
+	const testpath = /* have your */ "test-fixtures/fileurl/cake" /* and eat it too */
+	const host = "localhost"
+
+	var cwd string
+	var volume string
+	var share string
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("Unable to detect working directory: %s", err)
+		return
+	}
+	cwd = filepath.ToSlash(cwd)
+	volume = filepath.VolumeName(cwd)
+	share = volume
+
+	// if a volume was found (on windows), replace the ':' from
+	// C: to C$ to convert it into a hidden windows share.
+	if len(share) > 1 && share[len(share)-1] == ':' {
+		share = share[:len(share)-1] + "$"
+	}
+	cwd = cwd[len(volume):]
+
+	t.Logf("TestFileUriTransforms : Running with cwd : '%s'", cwd)
+	t.Logf("TestFileUriTransforms : Running with volume : '%s'", volume)
+
+	// ./relative/path -> ./relative/path
+	// /absolute/path -> /absolute/path
+	// c:/windows/absolute -> c:/windows/absolute
+	testcases := []string{
+		"./%s",
+		cwd + "/%s",
+		volume + cwd + "/%s",
+	}
+
+	// all regular slashed testcases
+	for _, testcase := range testcases {
+		uri := "file://" + fmt.Sprintf(testcase, testpath)
+		t.Logf("TestFileUriTransforms : Trying Uri '%s'", uri)
+		res, err := SimulateFileUriDownload(t, uri)
+		if err != nil {
+			t.Errorf("Unable to transform uri '%s' into a path : %v", uri, err)
+		}
+		t.Logf("TestFileUriTransforms : Result Path '%s'", res)
+	}
+
+	// smb protocol depends on platform support which currently
+	// only exists on windows.
+	if runtime.GOOS == "windows" {
+		// ...and finally the oddball windows native path
+		// smb://host/sharename/file -> \\host\sharename\file
+		testcase := host + "/" + share + "/" + cwd[1:] + "/%s"
+		uri := "smb://" + fmt.Sprintf(testcase, testpath)
+		t.Logf("TestFileUriTransforms : Trying Uri '%s'", uri)
+		res, err := SimulateFileUriDownload(t, uri)
+		if err != nil {
+			t.Errorf("Unable to transform uri '%s' into a path", uri)
+			return
+		}
+		t.Logf("TestFileUriTransforms : Result Path '%s'", res)
+	}
 }
diff --git a/common/iso_config.go b/common/iso_config.go
index 5216dc458..e5a1e4e90 100644
--- a/common/iso_config.go
+++ b/common/iso_config.go
@@ -111,7 +111,7 @@ func (c *ISOConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs [
 	c.ISOChecksum = strings.ToLower(c.ISOChecksum)
 
 	for i, url := range c.ISOUrls {
-		url, err := DownloadableURL(url)
+		url, err := ValidatedURL(url)
 		if err != nil {
 			errs = append(
 				errs, fmt.Errorf("Failed to parse iso_url %d: %s", i+1, err))
diff --git a/common/test-fixtures/SomeDir/myfile.txt b/common/test-fixtures/SomeDir/myfile.txt
new file mode 100644
index 000000000..e69de29bb