cmd/torrent/main.go  |  9 +++++++++
 config.go            | 11 ++++++-----
 tracker/http/http.go |  9 ++++++++-
 version/version.go   | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++

diff --git a/cmd/torrent/main.go b/cmd/torrent/main.go
index bd5d6f7a80497f2a9c0e005f1dc08933bff1b752..61fd2ab2791cafc5ec80d2f5249181b02fe82f5e 100644
--- a/cmd/torrent/main.go
+++ b/cmd/torrent/main.go
@@ -18,6 +18,7 @@ 	"github.com/alexflint/go-arg"
 	"github.com/anacrolix/envpprof"
 	"github.com/anacrolix/missinggo/v2"
 	"github.com/anacrolix/torrent/bencode"
+	"github.com/anacrolix/torrent/version"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/dustin/go-humanize"
 	"golang.org/x/xerrors"
@@ -164,7 +165,10 @@ 	*DownloadCmd      `arg:"subcommand:download"`
 	*ListFilesCmd     `arg:"subcommand:list-files"`
 	*SpewBencodingCmd `arg:"subcommand:spew-bencoding"`
 	*AnnounceCmd      `arg:"subcommand:announce"`
+	*VersionCmd       `arg:"subcommand:version"`
 }
+
+type VersionCmd struct{}
 
 type SpewBencodingCmd struct{}
 
@@ -267,6 +271,11 @@ 				return fmt.Errorf("decoding message index %d: %w", i, err)
 			}
 			spew.Dump(v)
 		}
+		return nil
+	case flags.VersionCmd != nil:
+		fmt.Printf("HTTP User-Agent: %q\n", version.DefaultHttpUserAgent)
+		fmt.Printf("Torrent client version: %q\n", version.DefaultExtendedHandshakeClientVersion)
+		fmt.Printf("Torrent version prefix: %q\n", version.DefaultBep20Prefix)
 		return nil
 	default:
 		p.Fail(fmt.Sprintf("unexpected subcommand: %v", p.Subcommand()))
diff --git a/config.go b/config.go
index 03d85c06eb4d0d53cc4ab68f20b91acde85382b8..c7720a0d5ea9f0191e8f7dd5284dbde6d693679a 100644
--- a/config.go
+++ b/config.go
@@ -9,8 +9,9 @@
 	"github.com/anacrolix/dht/v2"
 	"github.com/anacrolix/dht/v2/krpc"
 	"github.com/anacrolix/log"
+	"github.com/anacrolix/missinggo/expect"
 	"github.com/anacrolix/missinggo/v2"
-	"github.com/anacrolix/missinggo/expect"
+	"github.com/anacrolix/torrent/version"
 	"golang.org/x/time/rate"
 
 	"github.com/anacrolix/torrent/iplist"
@@ -157,10 +158,10 @@ }
 
 func NewDefaultClientConfig() *ClientConfig {
 	cc := &ClientConfig{
-		HTTPUserAgent:                  "Go-Torrent/1.0",
-		ExtendedHandshakeClientVersion: "go.torrent dev 20181121",
-		Bep20:                          "-GT0002-",
-		UpnpID:                         "anacrolix/torrent",
+		HTTPUserAgent:                  version.DefaultHttpUserAgent,
+		ExtendedHandshakeClientVersion: version.DefaultExtendedHandshakeClientVersion,
+		Bep20:                          version.DefaultBep20Prefix,
+		UpnpID:                         version.DefaultUpnpId,
 		NominalDialTimeout:             20 * time.Second,
 		MinDialTimeout:                 3 * time.Second,
 		EstablishedConnsPerTorrent:     50,
diff --git a/tracker/http/http.go b/tracker/http/http.go
index c4f2fac4463e3a679240e94b98088286f0555b9b..15306ca974eaa0656318ab4eab533dbd9ad4f35c 100644
--- a/tracker/http/http.go
+++ b/tracker/http/http.go
@@ -16,6 +16,7 @@ 	"github.com/anacrolix/missinggo/httptoo"
 	"github.com/anacrolix/torrent/bencode"
 	"github.com/anacrolix/torrent/tracker/shared"
 	"github.com/anacrolix/torrent/tracker/udp"
+	"github.com/anacrolix/torrent/version"
 )
 
 var vars = expvar.NewMap("tracker/http")
@@ -76,7 +77,13 @@ func (cl Client) Announce(ctx context.Context, ar AnnounceRequest, opt AnnounceOpt) (ret AnnounceResponse, err error) {
 	_url := httptoo.CopyURL(cl.url_)
 	setAnnounceParams(_url, &ar, opt)
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, _url.String(), nil)
-	req.Header.Set("User-Agent", opt.UserAgent)
+	userAgent := opt.UserAgent
+	if userAgent == "" {
+		userAgent = version.DefaultHttpUserAgent
+	}
+	if userAgent != "" {
+		req.Header.Set("User-Agent", userAgent)
+	}
 	req.Host = opt.HostHeader
 	resp, err := cl.hc.Do(req)
 	if err != nil {
diff --git a/version/version.go b/version/version.go
new file mode 100644
index 0000000000000000000000000000000000000000..3d3ae550f89c3c1fe09f14d61f17a10269ae4c6a
--- /dev/null
+++ b/version/version.go
@@ -0,0 +1,58 @@
+// Package version provides default versions, user-agents etc. for client identification.
+package version
+
+import (
+	"fmt"
+	"reflect"
+	"runtime/debug"
+)
+
+var (
+	DefaultExtendedHandshakeClientVersion string
+	// This should be updated when client behaviour changes in a way that other peers could care
+	// about.
+	DefaultBep20Prefix   = "-GT0003-"
+	DefaultHttpUserAgent string
+	DefaultUpnpId        string
+)
+
+func init() {
+	const (
+		longNamespace   = "anacrolix"
+		longPackageName = "torrent"
+	)
+	type Newtype struct{}
+	var newtype Newtype
+	thisPkg := reflect.TypeOf(newtype).PkgPath()
+	var (
+		mainPath       = "unknown"
+		mainVersion    = "unknown"
+		torrentVersion = "unknown"
+	)
+	if buildInfo, ok := debug.ReadBuildInfo(); ok {
+		mainPath = buildInfo.Main.Path
+		mainVersion = buildInfo.Main.Version
+		// Note that if the main module is the same as this module, we get a version of "(devel)".
+		for _, dep := range append(buildInfo.Deps, &buildInfo.Main) {
+			if dep.Path == thisPkg {
+				torrentVersion = dep.Version
+			}
+		}
+	}
+	DefaultExtendedHandshakeClientVersion = fmt.Sprintf(
+		"%v %v (%v/%v %v)",
+		mainPath,
+		mainVersion,
+		longNamespace,
+		longPackageName,
+		torrentVersion,
+	)
+	DefaultUpnpId = fmt.Sprintf("%v %v", mainPath, mainVersion)
+	// Per https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent#library_and_net_tool_ua_strings
+	DefaultHttpUserAgent = fmt.Sprintf(
+		"%v-%v/%v",
+		longNamespace,
+		longPackageName,
+		torrentVersion,
+	)
+}