diff --git a/dendrite-config.yaml b/dendrite-config.yaml
index 25503692b..2a8650db6 100644
--- a/dendrite-config.yaml
+++ b/dendrite-config.yaml
@@ -300,6 +300,11 @@ sync_api:
     max_idle_conns: 2
     conn_max_lifetime: -1
 
+  # This option controls which HTTP header to inspect to find the real remote IP
+  # address of the client. This is likely required if Dendrite is running behind
+  # a reverse proxy server.
+  # real_ip_header: X-Real-IP
+
 # Configuration for the User API.
 user_api:
   internal_api:
diff --git a/internal/config/config_syncapi.go b/internal/config/config_syncapi.go
index 0a96e41ca..fc08f7380 100644
--- a/internal/config/config_syncapi.go
+++ b/internal/config/config_syncapi.go
@@ -7,6 +7,8 @@ type SyncAPI struct {
 	ExternalAPI ExternalAPIOptions `yaml:"external_api"`
 
 	Database DatabaseOptions `yaml:"database"`
+
+	RealIPHeader string `yaml:"real_ip_header"`
 }
 
 func (c *SyncAPI) Defaults() {
diff --git a/internal/httputil/httpapi.go b/internal/httputil/httpapi.go
index 90dcf820d..c69468e6c 100644
--- a/internal/httputil/httpapi.go
+++ b/internal/httputil/httpapi.go
@@ -17,7 +17,6 @@ package httputil
 import (
 	"context"
 	"io"
-	"net"
 	"net/http"
 	"net/http/httptest"
 	"net/http/httputil"
@@ -61,20 +60,6 @@ func MakeAuthAPI(
 		logger = logger.WithField("user_id", device.UserID)
 		req = req.WithContext(util.ContextWithLogger(req.Context(), logger))
 
-		// check if the forwarding proxy, if there is one, has provided a real address.
-		if realIP := req.Header.Get("X-Real-IP"); realIP != "" {
-			if ip := net.ParseIP(realIP); ip != nil {
-				req.RemoteAddr = realIP
-			}
-		} else if forwardedFor := req.Header.Get("X-Forwarded-For"); forwardedFor != "" {
-			addresses := strings.Split(forwardedFor, ",")
-			if len(addresses) > 0 {
-				if ip := net.ParseIP(addresses[0]); ip != nil {
-					req.RemoteAddr = addresses[0]
-				}
-			}
-		}
-
 		return f(req, device)
 	}
 	return MakeExternalAPI(metricsName, h)
diff --git a/syncapi/sync/requestpool.go b/syncapi/sync/requestpool.go
index 30afa20b9..0fa599b1a 100644
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@@ -19,11 +19,13 @@ package sync
 import (
 	"context"
 	"fmt"
+	"net"
 	"net/http"
 	"sync"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/internal/config"
 	keyapi "github.com/matrix-org/dendrite/keyserver/api"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/syncapi/internal"
@@ -38,6 +40,7 @@ import (
 // RequestPool manages HTTP long-poll connections for /sync
 type RequestPool struct {
 	db       storage.Database
+	cfg      *config.SyncAPI
 	userAPI  userapi.UserInternalAPI
 	notifier *Notifier
 	keyAPI   keyapi.KeyInternalAPI
@@ -47,10 +50,11 @@ type RequestPool struct {
 
 // NewRequestPool makes a new RequestPool
 func NewRequestPool(
-	db storage.Database, n *Notifier, userAPI userapi.UserInternalAPI, keyAPI keyapi.KeyInternalAPI,
+	db storage.Database, cfg *config.SyncAPI, n *Notifier,
+	userAPI userapi.UserInternalAPI, keyAPI keyapi.KeyInternalAPI,
 	rsAPI roomserverAPI.RoomserverInternalAPI,
 ) *RequestPool {
-	rp := &RequestPool{db, userAPI, n, keyAPI, rsAPI, sync.Map{}}
+	rp := &RequestPool{db, cfg, userAPI, n, keyAPI, rsAPI, sync.Map{}}
 	go rp.cleanLastSeen()
 	return rp
 }
@@ -75,10 +79,19 @@ func (rp *RequestPool) updateLastSeen(req *http.Request, device *userapi.Device)
 		return
 	}
 
+	remoteAddr := req.RemoteAddr
+	if rp.cfg.RealIPHeader != "" {
+		if realIP := req.Header.Get(rp.cfg.RealIPHeader); realIP != "" {
+			if ip := net.ParseIP(realIP); ip != nil {
+				remoteAddr = realIP
+			}
+		}
+	}
+
 	lsreq := &userapi.PerformLastSeenUpdateRequest{
 		UserID:     device.UserID,
 		DeviceID:   device.ID,
-		RemoteAddr: req.RemoteAddr,
+		RemoteAddr: remoteAddr,
 	}
 	lsres := &userapi.PerformLastSeenUpdateResponse{}
 	go rp.userAPI.PerformLastSeenUpdate(req.Context(), lsreq, lsres) // nolint:errcheck
diff --git a/syncapi/syncapi.go b/syncapi/syncapi.go
index 393a7aa55..7e277ba19 100644
--- a/syncapi/syncapi.go
+++ b/syncapi/syncapi.go
@@ -61,7 +61,7 @@ func AddPublicRoutes(
 		logrus.WithError(err).Panicf("failed to start notifier")
 	}
 
-	requestPool := sync.NewRequestPool(syncDB, notifier, userAPI, keyAPI, rsAPI)
+	requestPool := sync.NewRequestPool(syncDB, cfg, notifier, userAPI, keyAPI, rsAPI)
 
 	keyChangeConsumer := consumers.NewOutputKeyChangeEventConsumer(
 		cfg.Matrix.ServerName, string(cfg.Matrix.Kafka.TopicFor(config.TopicOutputKeyChangeEvent)),