[feature] Provide .well-known/host-meta endpoint (#1604)

* [feature] Provide .well-known/host-meta endpoint

This adds the host-meta endpoint as Mastodon clients use this to
discover the API domain to use when the host and account domains aren't
the same.

* Address review comments

docs/configuration/general.md

@@ -43,6 +43,9 @@ host: "localhost"
 # to "gts.example.org/.well-known/webfinger" so that GtS can handle them properly.
 #
 # You should also redirect requests at "example.org/.well-known/nodeinfo" in the same way.
+#
+# You should also redirect requests at "example.org/.well-known/host-meta" in the same way. This endpoint is used by a number of clients to discover the API endpoint to use when the host and account domain are different.
+#
 # An empty string (ie., not set) means that the same value as 'host' will be used.
 #
 # DO NOT change this after your server has already run once, or you will break things!

docs/installation_guide/advanced.md

@@ -32,6 +32,9 @@ host: "localhost"
 # to "gts.example.org/.well-known/webfinger" so that GtS can handle them properly.
 #
 # You should also redirect requests at "example.org/.well-known/nodeinfo" in the same way.
+#
+# You should also redirect requests at "example.org/.well-known/host-meta" in the same way. This endpoint is used by a number of clients to discover the API endpoint to use when the host and account domain are different.
+#
 # An empty string (ie., not set) means that the same value as 'host' will be used.
 #
 # DO NOT change this after your server has already run once, or you will break things!
@@ -71,6 +74,10 @@ http {
             rewrite ^.*$ https://fedi.example.org/.well-known/webfinger permanent;
         }
 
+        location /.well-known/host-meta {
+            rewrite ^.*$ https://fedi.example.org/.well-known/host-meta permanent;
+        }
+
         location /.well-known/nodeinfo {
             rewrite ^.*$ https://fedi.example.org/.well-known/nodeinfo permanent;
         }
@@ -91,7 +98,7 @@ If `example.org` is running on [Traefik](https://doc.traefik.io/traefik/), we co
     labels:
       - 'traefik.http.routers.myservice.rule=Host(`example.org`)'
       - 'traefik.http.middlewares.myservice-gts.redirectregex.permanent=true'
-      - 'traefik.http.middlewares.myservice-gts.redirectregex.regex=^https://(.*)/.well-known/(webfinger|nodeinfo)$$'
+      - 'traefik.http.middlewares.myservice-gts.redirectregex.regex=^https://(.*)/.well-known/(webfinger|nodeinfo|host-meta)$$'
       - 'traefik.http.middlewares.myservice-gts.redirectregex.replacement=https://fedi.$${1}/.well-known/$${2}'
       - 'traefik.http.routers.myservice.middlewares=myservice-gts@docker'
 ```
@@ -279,9 +286,9 @@ This section contains a number of additional things for configuring nginx.
 
 If you want to harden up your NGINX deployment with advanced configuration options, there are many guides online for doing so ([for example](https://beaglesecurity.com/blog/article/nginx-server-security.html)). Try to find one that's up to date. Mozilla also publishes best-practice ssl configuration [here](https://ssl-config.mozilla.org/).
 
-### Caching Webfinger and Public Key responses
+### Caching Webfinger, Webhost Metadata and Public Key responses
 
-It's possible to use nginx to cache webfinger and public key responses. This may be useful in order to ensure clients still get a response on these endpoints even if your GoToSocial instance is (temporarily) down, or requests are being throttled.
+It's possible to use nginx to cache webfinger, host-meta and public key responses. This may be useful in order to ensure clients still get a response on these endpoints even if your GoToSocial instance is (temporarily) down, or requests are being throttled.
 
 You'll need to configure two things:
 
@@ -311,7 +318,7 @@ server {
   
   ### NEW STUFF STARTS HERE ###
   
-  location /.well-known/webfinger {
+  location ~ /.well-known/(webfinger|host-meta)$ {
     proxy_set_header Host $host;
     proxy_set_header X-Forwarded-For $remote_addr;
     proxy_set_header X-Forwarded-Proto $scheme;

example/config.yaml

@@ -55,6 +55,11 @@ host: "localhost"
 # to "gts.example.org/.well-known/webfinger" so that GtS can handle them properly.
 #
 # You should also redirect requests at "example.org/.well-known/nodeinfo" in the same way.
+#
+# You should also redirect requests at "example.org/.well-known/host-meta" in the same way. This endpoint
+# is used by a number of clients to discover the API endpoint to use when the host and account domain are
+# different.
+#
 # An empty string (ie., not set) means that the same value as 'host' will be used.
 #
 # DO NOT change this after your server has already run once, or you will break things!

internal/api/util/mime.go

@@ -25,6 +25,7 @@
 const (
 	AppJSON           MIME = `application/json`
 	AppXML            MIME = `application/xml`
+	AppXMLXRD         MIME = `application/xrd+xml`
 	AppRSSXML         MIME = `application/rss+xml`
 	AppActivityJSON   MIME = `application/activity+json`
 	AppActivityLDJSON MIME = `application/ld+json; profile="https://www.w3.org/ns/activitystreams"`

internal/api/util/negotiate.go

@@ -58,6 +58,11 @@
 	AppActivityLDJSON,
 }
 
+var HostMetaHeaders = []MIME{
+	AppXMLXRD,
+	AppXML,
+}
+
 // NegotiateAccept takes the *gin.Context from an incoming request, and a
 // slice of Offers, and performs content negotiation for the given request
 // with the given content-type offers. It will return a string representation

internal/api/wellknown.go

@@ -20,6 +20,7 @@
 
 import (
 	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/hostmeta"
 	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/nodeinfo"
 	"github.com/superseriousbusiness/gotosocial/internal/api/wellknown/webfinger"
 	"github.com/superseriousbusiness/gotosocial/internal/middleware"
@@ -30,6 +31,7 @@
 type WellKnown struct {
 	nodeInfo  *nodeinfo.Module
 	webfinger *webfinger.Module
+	hostMeta  *hostmeta.Module
 }
 
 func (w *WellKnown) Route(r router.Router, m ...gin.HandlerFunc) {
@@ -45,11 +47,13 @@ func (w *WellKnown) Route(r router.Router, m ...gin.HandlerFunc) {
 
 	w.nodeInfo.Route(wellKnownGroup.Handle)
 	w.webfinger.Route(wellKnownGroup.Handle)
+	w.hostMeta.Route(wellKnownGroup.Handle)
 }
 
 func NewWellKnown(p *processing.Processor) *WellKnown {
 	return &WellKnown{
 		nodeInfo:  nodeinfo.New(p),
 		webfinger: webfinger.New(p),
+		hostMeta:  hostmeta.New(p),
 	}
 }

internal/api/wellknown/hostmeta/hostmeta.go

@@ -0,0 +1,45 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package hostmeta
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	HostMetaContentType = "application/xrd+xml"
+	HostMetaPath        = "/host-meta"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, HostMetaPath, m.HostMetaGETHandler)
+}

internal/api/wellknown/hostmeta/hostmetaget.go

@@ -0,0 +1,73 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package hostmeta
+
+import (
+	"bytes"
+	"encoding/xml"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+// HostMetaGETHandler swagger:operation GET /.well-known/host-meta hostMetaGet
+//
+// Returns a compliant hostmeta response to web host metadata queries.
+//
+// See: https://www.rfc-editor.org/rfc/rfc6415.html
+//
+//	---
+//	tags:
+//	- .well-known
+//
+//	produces:
+//	- application/xrd+xml"
+//
+//	responses:
+//		'200':
+//			schema:
+//				"$ref": "#/definitions/hostmeta"
+func (m *Module) HostMetaGETHandler(c *gin.Context) {
+	if _, err := apiutil.NegotiateAccept(c, apiutil.HostMetaHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	hostMeta := m.processor.Fedi().HostMetaGet()
+
+	// this setup with a separate buffer we encode into is used because
+	// xml.Marshal does not emit xml.Header by itself
+	var buf bytes.Buffer
+
+	// Preallocate buffer of reasonable length.
+	buf.Grow(len(xml.Header) + 64)
+
+	// No need to check for error on write to buffer.
+	_, _ = buf.WriteString(xml.Header)
+
+	// Encode host-meta as XML to in-memory buffer.
+	if err := xml.NewEncoder(&buf).Encode(hostMeta); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorInternalError(err), m.processor.InstanceGetV1)
+		return
+	}
+
+	c.Data(http.StatusOK, HostMetaContentType, buf.Bytes())
+}

internal/processing/fedi/wellknown.go

@@ -28,6 +28,10 @@
 )
 
 const (
+	hostMetaXMLNS                   = "http://docs.oasis-open.org/ns/xri/xrd-1.0"
+	hostMetaRel                     = "lrdd"
+	hostMetaType                    = "application/xrd+xml"
+	hostMetaTemplate                = ".well-known/webfinger?resource={uri}"
 	nodeInfoVersion                 = "2.0"
 	nodeInfoSoftwareName            = "gotosocial"
 	nodeInfoRel                     = "http://nodeinfo.diaspora.software/ns/schema/" + nodeInfoVersion
@@ -96,6 +100,22 @@ func (p *Processor) NodeInfoGet(ctx context.Context) (*apimodel.Nodeinfo, gtserr
 	}, nil
 }
 
+// HostMetaGet returns a host-meta struct in response to a host-meta request.
+func (p *Processor) HostMetaGet() *apimodel.HostMeta {
+	protocol := config.GetProtocol()
+	host := config.GetHost()
+	return &apimodel.HostMeta{
+		XMLNS: hostMetaXMLNS,
+		Link: []apimodel.Link{
+			{
+				Rel:      hostMetaRel,
+				Type:     hostMetaType,
+				Template: fmt.Sprintf("%s://%s/%s", protocol, host, hostMetaTemplate),
+			},
+		},
+	}
+}
+
 // WebfingerGet handles the GET for a webfinger resource. Most commonly, it will be used for returning account lookups.
 func (p *Processor) WebfingerGet(ctx context.Context, requestedUsername string) (*apimodel.WellKnownResponse, gtserror.WithCode) {
 	// Get the local account the request is referring to.