diff --git a/cmd/gomodproxy/main.go b/cmd/gomodproxy/main.go
index f3cd2a7..cca2a24 100644
--- a/cmd/gomodproxy/main.go
+++ b/cmd/gomodproxy/main.go
@@ -97,6 +97,7 @@ func (f *listFlag) Set(s string) error { *f = append(*f, s); return nil }
 
 func main() {
 	gitPaths := listFlag{}
+	vcsPaths := listFlag{}
 
 	addr := flag.String("addr", ":0", "http server address")
 	verbose := flag.Bool("v", false, "verbose logging")
@@ -108,6 +109,7 @@ func main() {
 	memLimit := flag.Int64("mem", 256, "in-memory cache size in MB")
 	workers := flag.Int("workers", 1, "number of parallel VCS workers")
 	flag.Var(&gitPaths, "git", "list of git settings")
+	flag.Var(&vcsPaths, "vcs", "list of custom VCS handlers")
 
 	flag.Parse()
 
@@ -130,6 +132,14 @@ func main() {
 	}
 	options = append(options, api.Log(logger))
 
+	for _, path := range vcsPaths {
+		kv := strings.SplitN(path, ":", 2)
+		if len(kv) != 2 {
+			log.Fatal("bad VCS syntax:", path)
+		}
+		options = append(options, api.CustomVCS(kv[0], kv[1]))
+	}
+
 	for _, path := range gitPaths {
 		kv := strings.SplitN(path, ":", 2)
 		if len(kv) != 2 {
diff --git a/go.sum b/go.sum
index c80e604..9926502 100644
--- a/go.sum
+++ b/go.sum
@@ -19,6 +19,7 @@ github.com/mitchellh/go-homedir v1.0.0 h1:vKb8ShqSby24Yrqr/yDYkuFz8d0WUjys40rvnG
 github.com/mitchellh/go-homedir v1.0.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/pelletier/go-buffruneio v0.2.0 h1:U4t4R6YkofJ5xHm3dJzuRpPZ0mr5MMCoAWooScCR7aA=
 github.com/pelletier/go-buffruneio v0.2.0/go.mod h1:JkE26KsDizTr40EUHkXVtNPvgGtbSNq5BcowyYOWdKo=
+github.com/pkg/errors v0.8.0 h1:WdK/asTD0HN+q6hsWO3/vpuAkAr+tw6aNJNDFFf0+qw=
 github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/sergi/go-diff v1.0.0 h1:Kpca3qRNrduNnOQeazBd0ysaKrUJiIuISHxogkT9RPQ=
diff --git a/pkg/api/api.go b/pkg/api/api.go
index b533cd8..d65054f 100644
--- a/pkg/api/api.go
+++ b/pkg/api/api.go
@@ -86,6 +86,17 @@ func Git(prefix string, auth string) Option {
 	}
 }
 
+func CustomVCS(prefix string, cmd string) Option {
+	return func(api *api) {
+		api.vcsPaths = append(api.vcsPaths, vcsPath{
+			prefix: prefix,
+			vcs: func(module string) vcs.VCS {
+				return vcs.NewCommand(api.log, cmd, module)
+			},
+		})
+	}
+}
+
 // Memory configures API to use in-memory cache for downloaded modules.
 func Memory(log logger, limit int64) Option {
 	return func(api *api) {
diff --git a/pkg/vcs/cmd.go b/pkg/vcs/cmd.go
new file mode 100644
index 0000000..07b90d6
--- /dev/null
+++ b/pkg/vcs/cmd.go
@@ -0,0 +1,75 @@
+package vcs
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type cmdVCS struct {
+	log    logger
+	module string
+	cmd    string
+}
+
+func NewCommand(l logger, cmd string, module string) VCS {
+	return &cmdVCS{log: l, cmd: cmd, module: module}
+}
+
+func (c *cmdVCS) List(ctx context.Context) ([]Version, error) {
+	b, err := c.exec(ctx, "MODULE="+c.module, "ACTION=list", "VERSION=latest", "FILEPATH="+c.module+"/@v/list")
+	if err != nil {
+		return nil, err
+	}
+	versions := []Version{}
+	for _, line := range strings.Split(string(b), "\n") {
+		versions = append(versions, Version(line))
+	}
+	return versions, nil
+}
+
+func (c *cmdVCS) Timestamp(ctx context.Context, version Version) (time.Time, error) {
+	b, err := c.exec(ctx, "MODULE="+c.module, "ACTION=timestamp", "VERSION="+version.String(),
+		"FILEPATH="+c.module+"/@v/"+version.String()+".info")
+	if err != nil {
+		return time.Time{}, err
+	}
+	info := struct {
+		Version string
+		Time    time.Time
+	}{}
+	if json.Unmarshal(b, &info) == nil {
+		return info.Time, nil
+	}
+	if t, err := time.Parse(time.RFC3339, string(b)); err == nil {
+		return t, nil
+	}
+	if sec, err := strconv.ParseInt(string(b), 10, 64); err == nil {
+		return time.Unix(sec, 0), nil
+	}
+	return time.Time{}, errors.New("unknown time format")
+}
+
+func (c *cmdVCS) Zip(ctx context.Context, version Version) (io.ReadCloser, error) {
+	b, err := c.exec(ctx, "MODULE="+c.module, "ACTION=zip", "VERSION="+version.String(),
+		"FILEPATH="+c.module+"/@v/"+version.String()+".zip")
+	if err != nil {
+		return nil, err
+	}
+	return ioutil.NopCloser(bytes.NewReader(b)), nil
+}
+
+func (c *cmdVCS) exec(ctx context.Context, env ...string) ([]byte, error) {
+	cmd := exec.Command("sh", "-c", c.cmd)
+	cmd.Env = append(os.Environ(), env...)
+	cmd.Stderr = os.Stderr
+	return cmd.Output()
+}
diff --git a/pkg/vcs/vcs.go b/pkg/vcs/vcs.go
index 0ce8eae..dd2ef8f 100644
--- a/pkg/vcs/vcs.go
+++ b/pkg/vcs/vcs.go
@@ -27,6 +27,11 @@ func (v Version) Hash() string {
 	return fields[2]
 }
 
+// String returns a string representation of a version
+func (v Version) String() string {
+	return string(v)
+}
+
 // Module is a source code snapshot for which one can get the commit timestamp
 // or the actual ZIP with the source code in it.
 type Module interface {