Add support for registry authentication

Updates the docker resolver to support authenticating
with registries.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

cmd/dist/common.go

@@ -1,9 +1,14 @@
 package main
 
 import (
+	"bufio"
 	"context"
+	"crypto/tls"
+	"fmt"
 	"net"
+	"net/http"
 	"path/filepath"
+	"strings"
 	"time"
 
 	imagesapi "github.com/containerd/containerd/api/services/images"
@@ -12,10 +17,31 @@ import (
 	"github.com/containerd/containerd/remotes"
 	"github.com/containerd/containerd/remotes/docker"
 	imagesservice "github.com/containerd/containerd/services/images"
+	"github.com/crosbymichael/console"
+	"github.com/pkg/errors"
 	"github.com/urfave/cli"
 	"google.golang.org/grpc"
 )
 
+var registryFlags = []cli.Flag{
+	cli.BoolFlag{
+		Name:  "skip-verify,k",
+		Usage: "Skip SSL certificate validation",
+	},
+	cli.BoolFlag{
+		Name:  "plain-http",
+		Usage: "Allow connections using plain HTTP",
+	},
+	cli.StringFlag{
+		Name:  "user,u",
+		Usage: "user[:password] Registry user and password",
+	},
+	cli.StringFlag{
+		Name:  "refresh",
+		Usage: "Refresh token for authorization server",
+	},
+}
+
 func resolveContentStore(context *cli.Context) (*content.Store, error) {
 	root := filepath.Join(context.GlobalString("root"), "content")
 	if !filepath.IsAbs(root) {
@@ -50,6 +76,71 @@ func connectGRPC(context *cli.Context) (*grpc.ClientConn, error) {
 }
 
 // getResolver prepares the resolver from the environment and options.
-func getResolver(ctx context.Context) (remotes.Resolver, error) {
-	return docker.NewResolver(), nil
+func getResolver(ctx context.Context, clicontext *cli.Context) (remotes.Resolver, error) {
+	username := clicontext.String("user")
+	var secret string
+	if i := strings.IndexByte(username, ':'); i > 0 {
+		secret = username[i+1:]
+		username = username[0:i]
+	}
+	options := docker.ResolverOptions{
+		PlainHTTP: clicontext.Bool("plain-http"),
+	}
+	if username != "" {
+		if secret == "" {
+			fmt.Printf("Password: ")
+
+			var err error
+			secret, err = passwordPrompt()
+			if err != nil {
+				return nil, err
+			}
+
+			fmt.Print("\n")
+		}
+	} else if rt := clicontext.String("refresh"); rt != "" {
+		secret = rt
+	}
+
+	options.Credentials = func(host string) (string, string, error) {
+		// Only one host
+		return username, secret, nil
+	}
+
+	tr := &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
+		DialContext: (&net.Dialer{
+			Timeout:   30 * time.Second,
+			KeepAlive: 30 * time.Second,
+			DualStack: true,
+		}).DialContext,
+		MaxIdleConns:        10,
+		IdleConnTimeout:     30 * time.Second,
+		TLSHandshakeTimeout: 10 * time.Second,
+		TLSClientConfig: &tls.Config{
+			InsecureSkipVerify: clicontext.Bool("insecure"),
+		},
+		ExpectContinueTimeout: 5 * time.Second,
+	}
+
+	options.Client = &http.Client{
+		Transport: tr,
+	}
+
+	return docker.NewResolver(options), nil
+}
+
+func passwordPrompt() (string, error) {
+	c := console.Current()
+	defer c.Reset()
+
+	if err := c.DisableEcho(); err != nil {
+		return "", errors.Wrap(err, "failed to disable echo")
+	}
+
+	line, _, err := bufio.NewReader(c).ReadLine()
+	if err != nil {
+		return "", errors.Wrap(err, "failed to read line")
+	}
+	return string(line), nil
 }

cmd/dist/fetch.go

@@ -39,7 +39,7 @@ not use this implementation as a guide. The end goal should be having metadata,
 content and snapshots ready for a direct use via the 'ctr run'.
 
 Most of this is experimental and there are few leaps to make this work.`,
-	Flags: []cli.Flag{},
+	Flags: registryFlags,
 	Action: func(clicontext *cli.Context) error {
 		var (
 			ctx = background
@@ -51,7 +51,7 @@ Most of this is experimental and there are few leaps to make this work.`,
 			return err
 		}
 
-		resolver, err := getResolver(ctx)
+		resolver, err := getResolver(ctx, clicontext)
 		if err != nil {
 			return err
 		}

cmd/dist/fetchobject.go

@@ -18,13 +18,13 @@ var fetchObjectCommand = cli.Command{
 	Usage:       "retrieve objects from a remote",
 	ArgsUsage:   "[flags] <remote> <object> [<hint>, ...]",
 	Description: `Fetch objects by identifier from a remote.`,
-	Flags: []cli.Flag{
+	Flags: append([]cli.Flag{
 		cli.DurationFlag{
 			Name:   "timeout",
 			Usage:  "total timeout for fetch",
 			EnvVar: "CONTAINERD_FETCH_TIMEOUT",
 		},
-	},
+	}, registryFlags...),
 	Action: func(context *cli.Context) error {
 		var (
 			ctx     = background
@@ -38,7 +38,7 @@ var fetchObjectCommand = cli.Command{
 			defer cancel()
 		}
 
-		resolver, err := getResolver(ctx)
+		resolver, err := getResolver(ctx, context)
 		if err != nil {
 			return err
 		}

cmd/dist/pull.go

@@ -35,7 +35,7 @@ command. As part of this process, we do the following:
 2. Prepare the snapshot filesystem with the pulled resources.
 3. Register metadata for the image.
 `,
-	Flags: []cli.Flag{},
+	Flags: registryFlags,
 	Action: func(clicontext *cli.Context) error {
 		var (
 			ctx = background
@@ -52,7 +52,7 @@ command. As part of this process, we do the following:
 			return err
 		}
 
-		resolver, err := getResolver(ctx)
+		resolver, err := getResolver(ctx, clicontext)
 		if err != nil {
 			return err
 		}
@@ -75,6 +75,7 @@ command. As part of this process, we do the following:
 			ongoing.add(ref)
 			name, desc, fetcher, err := resolver.Resolve(ctx, ref)
 			if err != nil {
+				log.G(ctx).WithError(err).Error("failed to resolve")
 				return err
 			}
 			log.G(ctx).WithField("image", name).Debug("fetching")

remotes/docker/auth.go

@@ -0,0 +1,182 @@
+package docker
+
+import (
+	"net/http"
+	"sort"
+	"strings"
+)
+
+type authenticationScheme byte
+
+const (
+	basicAuth  authenticationScheme = 1 << iota // Defined in RFC 7617
+	digestAuth                                  // Defined in RFC 7616
+	bearerAuth                                  // Defined in RFC 6750
+)
+
+// challenge carries information from a WWW-Authenticate response header.
+// See RFC 2617.
+type challenge struct {
+	// scheme is the auth-scheme according to RFC 2617
+	scheme authenticationScheme
+
+	// parameters are the auth-params according to RFC 2617
+	parameters map[string]string
+}
+
+type byScheme []challenge
+
+func (bs byScheme) Len() int      { return len(bs) }
+func (bs byScheme) Swap(i, j int) { bs[i], bs[j] = bs[j], bs[i] }
+
+// Sort in priority order: token > digest > basic
+func (bs byScheme) Less(i, j int) bool { return bs[i].scheme > bs[j].scheme }
+
+// Octet types from RFC 2616.
+type octetType byte
+
+var octetTypes [256]octetType
+
+const (
+	isToken octetType = 1 << iota
+	isSpace
+)
+
+func init() {
+	// OCTET      = <any 8-bit sequence of data>
+	// CHAR       = <any US-ASCII character (octets 0 - 127)>
+	// CTL        = <any US-ASCII control character (octets 0 - 31) and DEL (127)>
+	// CR         = <US-ASCII CR, carriage return (13)>
+	// LF         = <US-ASCII LF, linefeed (10)>
+	// SP         = <US-ASCII SP, space (32)>
+	// HT         = <US-ASCII HT, horizontal-tab (9)>
+	// <">        = <US-ASCII double-quote mark (34)>
+	// CRLF       = CR LF
+	// LWS        = [CRLF] 1*( SP | HT )
+	// TEXT       = <any OCTET except CTLs, but including LWS>
+	// separators = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <">
+	//              | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT
+	// token      = 1*<any CHAR except CTLs or separators>
+	// qdtext     = <any TEXT except <">>
+
+	for c := 0; c < 256; c++ {
+		var t octetType
+		isCtl := c <= 31 || c == 127
+		isChar := 0 <= c && c <= 127
+		isSeparator := strings.IndexRune(" \t\"(),/:;<=>?@[]\\{}", rune(c)) >= 0
+		if strings.IndexRune(" \t\r\n", rune(c)) >= 0 {
+			t |= isSpace
+		}
+		if isChar && !isCtl && !isSeparator {
+			t |= isToken
+		}
+		octetTypes[c] = t
+	}
+}
+
+func parseAuthHeader(header http.Header) []challenge {
+	challenges := []challenge{}
+	for _, h := range header[http.CanonicalHeaderKey("WWW-Authenticate")] {
+		v, p := parseValueAndParams(h)
+		var s authenticationScheme
+		switch v {
+		case "basic":
+			s = basicAuth
+		case "digest":
+			s = digestAuth
+		case "bearer":
+			s = bearerAuth
+		default:
+			continue
+		}
+		challenges = append(challenges, challenge{scheme: s, parameters: p})
+	}
+	sort.Stable(byScheme(challenges))
+	return challenges
+}
+
+func parseValueAndParams(header string) (value string, params map[string]string) {
+	params = make(map[string]string)
+	value, s := expectToken(header)
+	if value == "" {
+		return
+	}
+	value = strings.ToLower(value)
+	for {
+		var pkey string
+		pkey, s = expectToken(skipSpace(s))
+		if pkey == "" {
+			return
+		}
+		if !strings.HasPrefix(s, "=") {
+			return
+		}
+		var pvalue string
+		pvalue, s = expectTokenOrQuoted(s[1:])
+		if pvalue == "" {
+			return
+		}
+		pkey = strings.ToLower(pkey)
+		params[pkey] = pvalue
+		s = skipSpace(s)
+		if !strings.HasPrefix(s, ",") {
+			return
+		}
+		s = s[1:]
+	}
+}
+
+func skipSpace(s string) (rest string) {
+	i := 0
+	for ; i < len(s); i++ {
+		if octetTypes[s[i]]&isSpace == 0 {
+			break
+		}
+	}
+	return s[i:]
+}
+
+func expectToken(s string) (token, rest string) {
+	i := 0
+	for ; i < len(s); i++ {
+		if octetTypes[s[i]]&isToken == 0 {
+			break
+		}
+	}
+	return s[:i], s[i:]
+}
+
+func expectTokenOrQuoted(s string) (value string, rest string) {
+	if !strings.HasPrefix(s, "\"") {
+		return expectToken(s)
+	}
+	s = s[1:]
+	for i := 0; i < len(s); i++ {
+		switch s[i] {
+		case '"':
+			return s[:i], s[i+1:]
+		case '\\':
+			p := make([]byte, len(s)-1)
+			j := copy(p, s[:i])
+			escape := true
+			for i = i + 1; i < len(s); i++ {
+				b := s[i]
+				switch {
+				case escape:
+					escape = false
+					p[j] = b
+					j++
+				case b == '\\':
+					escape = true
+				case b == '"':
+					return string(p[:j]), s[i+1:]
+				default:
+					p[j] = b
+					j++
+				}
+			}
+			return "", ""
+		}
+	}
+	return "", ""
+}

remotes/docker/resolver.go

@@ -7,10 +7,12 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"net/textproto"
 	"net/url"
 	"path"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/containerd/containerd/images"
@@ -23,15 +25,43 @@ import (
 	"golang.org/x/net/context/ctxhttp"
 )
 
-// NOTE(stevvooe): Most of the code below this point is prototype code to
-// demonstrate a very simplified docker.io fetcher. We have a lot of hard coded
-// values but we leave many of the details down to the fetcher, creating a lot
-// of room for ways to fetch content.
+var (
+	// ErrNoToken is returned if a request is successful but the body does not
+	// contain an authorization token.
+	ErrNoToken = errors.New("authorization server did not include a token in the response")
 
-type dockerResolver struct{}
+	// ErrInvalidAuthorization is used when credentials are passed to a server but
+	// those credentials are rejected.
+	ErrInvalidAuthorization = errors.New("authorization failed")
+)
 
-func NewResolver() remotes.Resolver {
-	return &dockerResolver{}
+type dockerResolver struct {
+	credentials func(string) (string, string, error)
+	plainHTTP   bool
+	client      *http.Client
+}
+
+// ResolverOptions are used to configured a new Docker register resolver
+type ResolverOptions struct {
+	// Credentials provides username and secret given a host.
+	// If username is empty but a secret is given, that secret
+	// is interpretted as a long lived token.
+	Credentials func(string) (string, string, error)
+
+	// PlainHTTP specifies to use plain http and not https
+	PlainHTTP bool
+
+	// Client is the http client to used when making registry requests
+	Client *http.Client
+}
+
+// NewResolver returns a new resolver to a Docker registry
+func NewResolver(options ResolverOptions) remotes.Resolver {
+	return &dockerResolver{
+		credentials: options.Credentials,
+		plainHTTP:   options.PlainHTTP,
+		client:      options.Client,
+	}
 }
 
 var _ remotes.Resolver = &dockerResolver{}
@@ -44,30 +74,37 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 
 	var (
 		base             url.URL
-		token string
+		username, secret string
 	)
 
-	switch refspec.Hostname() {
-	case "docker.io":
+	host := refspec.Hostname()
 	base.Scheme = "https"
+
+	if host == "docker.io" {
 		base.Host = "registry-1.docker.io"
-		prefix := strings.TrimPrefix(refspec.Locator, "docker.io/")
-		base.Path = path.Join("/v2", prefix)
-		token, err = getToken(ctx, "repository:"+prefix+":pull")
+	} else {
+		base.Host = host
+
+		if r.plainHTTP || strings.HasPrefix(host, "localhost:") {
+			base.Scheme = "http"
+		}
+	}
+
+	if r.credentials != nil {
+		username, secret, err = r.credentials(base.Host)
 		if err != nil {
 			return "", ocispec.Descriptor{}, nil, err
 		}
-	case "localhost:5000":
-		base.Scheme = "http"
-		base.Host = "localhost:5000"
-		base.Path = path.Join("/v2", strings.TrimPrefix(refspec.Locator, "localhost:5000/"))
-	default:
-		return "", ocispec.Descriptor{}, nil, errors.Errorf("unsupported locator: %q", refspec.Locator)
 	}
 
+	prefix := strings.TrimPrefix(refspec.Locator, host+"/")
+	base.Path = path.Join("/v2", prefix)
+
 	fetcher := &dockerFetcher{
 		base:     base,
-		token: token,
+		client:   r.client,
+		username: username,
+		secret:   secret,
 	}
 
 	var (
@@ -125,16 +162,13 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 
 		if dgstHeader != "" {
 			if err := dgstHeader.Validate(); err != nil {
-				if err == nil {
-					return "", ocispec.Descriptor{}, nil, errors.Errorf("%q in header not a valid digest", dgstHeader)
-				}
 				return "", ocispec.Descriptor{}, nil, errors.Wrapf(err, "%q in header not a valid digest", dgstHeader)
 			}
 			dgst = dgstHeader
 		}
 
 		if dgst == "" {
-			return "", ocispec.Descriptor{}, nil, errors.Wrapf(err, "could not resolve digest for %v", ref)
+			return "", ocispec.Descriptor{}, nil, errors.Errorf("could not resolve digest for %v", ref)
 		}
 
 		var (
@@ -143,8 +177,12 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 		)
 
 		size, err = strconv.ParseInt(sizeHeader, 10, 64)
-		if err != nil || size < 0 {
-			return "", ocispec.Descriptor{}, nil, errors.Wrapf(err, "%q in header not a valid size", sizeHeader)
+		if err != nil {
+
+			return "", ocispec.Descriptor{}, nil, errors.Wrapf(err, "invalid size header: %q", sizeHeader)
+		}
+		if size < 0 {
+			return "", ocispec.Descriptor{}, nil, errors.Errorf("%q in header not a valid size", sizeHeader)
 		}
 
 		desc := ocispec.Descriptor{
@@ -163,6 +201,11 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 type dockerFetcher struct {
 	base  url.URL
 	token string
+
+	client   *http.Client
+	useBasic bool
+	username string
+	secret   string
 }
 
 func (r *dockerFetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error) {
@@ -213,13 +256,14 @@ func (r *dockerFetcher) url(ps ...string) string {
 }
 
 func (r *dockerFetcher) doRequest(ctx context.Context, req *http.Request) (*http.Response, error) {
-	ctx = log.WithLogger(ctx, log.G(ctx).WithField("url", req.URL.String()))
-	log.G(ctx).WithField("request.headers", req.Header).Debug("fetch content")
-	if r.token != "" {
-		req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", r.token))
+	return r.doRequestWithRetries(ctx, req, nil)
 }
 
-	resp, err := ctxhttp.Do(ctx, http.DefaultClient, req)
+func (r *dockerFetcher) doRequestWithRetries(ctx context.Context, req *http.Request, responses []*http.Response) (*http.Response, error) {
+	ctx = log.WithLogger(ctx, log.G(ctx).WithField("url", req.URL.String()))
+	log.G(ctx).WithField("request.headers", req.Header).Debug("fetch content")
+	r.authorize(req)
+	resp, err := ctxhttp.Do(ctx, r.client, req)
 	if err != nil {
 		return nil, err
 	}
@@ -228,50 +272,119 @@ func (r *dockerFetcher) doRequest(ctx context.Context, req *http.Request) (*http
 		"response.headers": resp.Header,
 	}).Debug("fetch response received")
 
+	responses = append(responses, resp)
+	req, err = r.retryRequest(ctx, req, responses)
+	if err != nil {
+		return nil, err
+	}
+	if req != nil {
+		return r.doRequestWithRetries(ctx, req, responses)
+	}
 	return resp, err
 }
 
-func getToken(ctx context.Context, scopes ...string) (string, error) {
-	var (
-		u = url.URL{
-			Scheme: "https",
-			Host:   "auth.docker.io",
-			Path:   "/token",
+func (r *dockerFetcher) authorize(req *http.Request) {
+	if r.useBasic {
+		req.SetBasicAuth(r.username, r.secret)
+	} else if r.token != "" {
+		req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", r.token))
+	}
 }
 
-		q = url.Values{
-			"scope":   scopes,
-			"service": []string{"registry.docker.io"}, // usually comes from auth challenge
+func (r *dockerFetcher) retryRequest(ctx context.Context, req *http.Request, responses []*http.Response) (*http.Request, error) {
+	if len(responses) > 5 {
+		return nil, nil
+	}
+	last := responses[len(responses)-1]
+	if last.StatusCode == http.StatusUnauthorized {
+		log.G(ctx).WithField("header", last.Header.Get("WWW-Authenticate")).Debug("Unauthorized")
+		for _, c := range parseAuthHeader(last.Header) {
+			if c.scheme == bearerAuth {
+				if errStr := c.parameters["error"]; errStr != "" {
+					// TODO: handle expired case
+					return nil, errors.Wrapf(ErrInvalidAuthorization, "server message: %s", errStr)
+				}
+				if err := r.setTokenAuth(ctx, c.parameters); err != nil {
+					return nil, err
+				}
+				return req, nil
+			} else if c.scheme == basicAuth {
+				if r.username != "" && r.secret != "" {
+					r.useBasic = true
+				}
+				return req, nil
+			}
+		}
+		return nil, nil
+	} else if last.StatusCode == http.StatusMethodNotAllowed && req.Method == http.MethodHead {
+		// Support registries which have not properly implemented the HEAD method for
+		// manifests endpoint
+		if strings.Contains(req.URL.Path, "/manifests/") {
+			// TODO: copy request?
+			req.Method = http.MethodGet
+			return req, nil
+		}
 	}
-	)
 
-	u.RawQuery = q.Encode()
+	// TODO: Handle 50x errors accounting for attempt history
+	return nil, nil
+}
 
-	log.G(ctx).WithField("token.url", u.String()).Debug("requesting token")
-	resp, err := ctxhttp.Get(ctx, http.DefaultClient, u.String())
+func isManifestAccept(h http.Header) bool {
+	for _, ah := range h[textproto.CanonicalMIMEHeaderKey("Accept")] {
+		switch ah {
+		case images.MediaTypeDockerSchema2Manifest:
+			fallthrough
+		case images.MediaTypeDockerSchema2ManifestList:
+			fallthrough
+		case ocispec.MediaTypeImageManifest:
+			fallthrough
+		case ocispec.MediaTypeImageIndex:
+			return true
+		}
+	}
+	return false
+}
+
+func (r *dockerFetcher) setTokenAuth(ctx context.Context, params map[string]string) error {
+	realm, ok := params["realm"]
+	if !ok {
+		return errors.New("no realm specified for token auth challenge")
+	}
+
+	realmURL, err := url.Parse(realm)
 	if err != nil {
-		return "", err
-	}
-	defer resp.Body.Close()
-
-	if resp.StatusCode > 299 {
-		return "", errors.Errorf("unexpected status code: %v %v", resp.StatusCode, resp.Status)
+		return fmt.Errorf("invalid token auth challenge realm: %s", err)
 	}
 
-	p, err := ioutil.ReadAll(resp.Body)
+	to := tokenOptions{
+		realm:   realmURL.String(),
+		service: params["service"],
+	}
+
+	scope, ok := params["scope"]
+	if !ok {
+		return errors.Errorf("no scope specified for token auth challenge")
+	}
+
+	// TODO: Get added scopes from context
+	to.scopes = []string{scope}
+
+	if r.secret != "" {
+		// Credential information is provided, use oauth POST endpoint
+		r.token, err = r.fetchTokenWithOAuth(ctx, to)
 		if err != nil {
-		return "", err
+			return errors.Wrap(err, "failed to fetch oauth token")
+		}
+	} else {
+		// Do request anonymously
+		r.token, err = r.getToken(ctx, to)
+		if err != nil {
+			return errors.Wrap(err, "failed to fetch anonymous token")
+		}
 	}
 
-	var tokenResponse struct {
-		Token string `json:"token"`
-	}
-
-	if err := json.Unmarshal(p, &tokenResponse); err != nil {
-		return "", err
-	}
-
-	return tokenResponse.Token, nil
+	return nil
 }
 
 // getV2URLPaths generates the candidate urls paths for the object based on the
@@ -291,3 +404,125 @@ func getV2URLPaths(desc ocispec.Descriptor) ([]string, error) {
 
 	return urls, nil
 }
+
+type tokenOptions struct {
+	realm   string
+	service string
+	scopes  []string
+}
+
+type postTokenResponse struct {
+	AccessToken  string    `json:"access_token"`
+	RefreshToken string    `json:"refresh_token"`
+	ExpiresIn    int       `json:"expires_in"`
+	IssuedAt     time.Time `json:"issued_at"`
+	Scope        string    `json:"scope"`
+}
+
+func (r *dockerFetcher) fetchTokenWithOAuth(ctx context.Context, to tokenOptions) (string, error) {
+	form := url.Values{}
+	form.Set("scope", strings.Join(to.scopes, " "))
+	form.Set("service", to.service)
+	// TODO: Allow setting client_id
+	form.Set("client_id", "containerd-dist-tool")
+
+	if r.username == "" {
+		form.Set("grant_type", "refresh_token")
+		form.Set("refresh_token", r.secret)
+	} else {
+		form.Set("grant_type", "password")
+		form.Set("username", r.username)
+		form.Set("password", r.secret)
+	}
+
+	resp, err := ctxhttp.PostForm(ctx, r.client, to.realm, form)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode == 405 && r.username != "" {
+		// It would be nice if registries would implement the specifications
+		return r.getToken(ctx, to)
+	} else if resp.StatusCode < 200 || resp.StatusCode >= 400 {
+		b, _ := ioutil.ReadAll(resp.Body)
+		log.G(ctx).WithFields(logrus.Fields{
+			"status": resp.Status,
+			"body":   string(b),
+		}).Debugf("token request failed")
+		// TODO: handle error body and write debug output
+		return "", errors.Errorf("unexpected status: %s", resp.Status)
+	}
+
+	decoder := json.NewDecoder(resp.Body)
+
+	var tr postTokenResponse
+	if err = decoder.Decode(&tr); err != nil {
+		return "", fmt.Errorf("unable to decode token response: %s", err)
+	}
+
+	return tr.AccessToken, nil
+}
+
+type getTokenResponse struct {
+	Token        string    `json:"token"`
+	AccessToken  string    `json:"access_token"`
+	ExpiresIn    int       `json:"expires_in"`
+	IssuedAt     time.Time `json:"issued_at"`
+	RefreshToken string    `json:"refresh_token"`
+}
+
+// getToken fetches a token using a GET request
+func (r *dockerFetcher) getToken(ctx context.Context, to tokenOptions) (string, error) {
+	req, err := http.NewRequest("GET", to.realm, nil)
+	if err != nil {
+		return "", err
+	}
+
+	reqParams := req.URL.Query()
+
+	if to.service != "" {
+		reqParams.Add("service", to.service)
+	}
+
+	for _, scope := range to.scopes {
+		reqParams.Add("scope", scope)
+	}
+
+	if r.secret != "" {
+		req.SetBasicAuth(r.username, r.secret)
+	}
+
+	req.URL.RawQuery = reqParams.Encode()
+
+	resp, err := ctxhttp.Do(ctx, r.client, req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
+		// TODO: handle error body and write debug output
+		return "", errors.Errorf("unexpected status: %s", resp.Status)
+	}
+
+	decoder := json.NewDecoder(resp.Body)
+
+	var tr getTokenResponse
+	if err = decoder.Decode(&tr); err != nil {
+		return "", fmt.Errorf("unable to decode token response: %s", err)
+	}
+
+	// `access_token` is equivalent to `token` and if both are specified
+	// the choice is undefined.  Canonicalize `access_token` by sticking
+	// things in `token`.
+	if tr.AccessToken != "" {
+		tr.Token = tr.AccessToken
+	}
+
+	if tr.Token == "" {
+		return "", ErrNoToken
+	}
+
+	return tr.Token, nil
+}