diff --git a/README.md b/README.md
index 3972df0..4ee490a 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ See the specification of the W3C WebSub protocol at
 
  1. Install Racket from <http://download.racket-lang.org/>
  2. Install RacketMQ by running `raco pkg install --auto racketmq`
- 3. `racketmq --canonical-host localhost 7827`
+ 3. `racketmq --baseurl http://localhost:7827/ --listen localhost 7827`
 
 To install from git, replace the `raco pkg install ...` step above
 with an invocation of `make link` from the top directory of your git
@@ -45,9 +45,8 @@ checkout.
 
 ## Configuration
 
-RacketMQ has only one required configuration variable: you must tell
-the hub its primary ("canonical") *host name* and *port number*. These
-are used to build URLs for clients of the Hub to use.
+The most important RacketMQ configuration variable is its canonical
+base URL: the URL prefix used to build URLs for clients to use.
 
 When the RacketMQ startup script is given a "`-f` *filename*" option,
 it loads configuration data from the named file. The option can be
@@ -59,7 +58,8 @@ For a fully-commented example configuration file, see
 Within each file, each configuration entry should be a list (see
 [Racket syntax](https://docs.racket-lang.org/reference/reader.html))
 with a symbol (the "key") as its first item followed by zero or more
-items.
+items. Line comments start with semicolon (`;`) as usual for
+S-expression languages.
 
 Each configuration file is automatically reread by the server when it
 is changed: if you need to make changes, consider doing so atomically
@@ -68,29 +68,30 @@ by producing an updated configuration file and using
 
 ### Required configuration data
 
-    (canonical-host "localhost" 7827)
+    (canonical-baseurl "http://localhost:7827/")
 
-Exactly one "canonical-host" key, containing two values, a string
-hostname, and a number TCP port. This causes an HTTP server to be
-spun up on the named port.
+Exactly one "canonical-baseurl" key, containing a URL string naming
+the base URL used for constructing URLs that are given out to third
+parties, such as subscription endpoints for upstream hubs to use.
 
-Since this is the only mandatory configuration item, RacketMQ can run
-without any configuration file at all if the server is started with
-the `--canonical-host` command-line argument:
+This is *just* for URL construction, and does NOT create any HTTP
+listeners. Those are configured with "http-listener" keys:
 
-    racketmq --canonical-host localhost 7827
-
-### Optional configuration data
-
-    (accepted-host "localhost" 7827)
-    (accepted-host "localhost" 80)
-    (accepted-host "www.example.com" 7827)
+    (http-listener "localhost" 7827)
+    ;; (http-listener "localhost" 80)
+    ;; (http-listener "www.example.com" 7827)
+    ;;
     ;; etc.
 
-If you want your server to appear under several aliases, add them
-here. HTTP servers will be spun up for all mentioned port numbers,
-and within those servers, `Host` headers matching the given host
-names will be accepted.
+At least one "http-listener" key is required. These cause an HTTP
+server to be spun up for each mentioned port number. Traffic will only
+be accepted for HTTP Host headers mentioned in these keys.
+
+Since these are the only mandatory configuration item, RacketMQ can
+run without any configuration file at all if the server is started
+with the `--baseurl` and `--listen` command-line arguments:
+
+    racketmq --baseurl http://localhost:7827/ --listen localhost 7827
 
 ### Fine tuning
 
diff --git a/racketmq/defaults.rktd b/racketmq/defaults.rktd
index 7fc36c7..e9ca01e 100644
--- a/racketmq/defaults.rktd
+++ b/racketmq/defaults.rktd
@@ -15,24 +15,24 @@
 ;;===========================================================================
 
 ;;---------------------------------------------------------------------------
-;; Exactly one "canonical-host" key, containing two values, a string
-;; hostname, and a number TCP port. This causes an HTTP server to be
-;; spun up on the named port.
-(canonical-host "localhost" 7827)
-
-;;===========================================================================
-;; OPTIONAL:
-;;===========================================================================
+;; Exactly one "canonical-baseurl" key, containing a URL string naming
+;; the base URL used for constructing URLs that are given out to third
+;; parties, such as subscription endpoints for upstream hubs to use.
+;;
+;; This is *just* for URL construction, and does NOT create any HTTP
+;; listeners. Those are configured with "http-listener" keys,
+;; documented below.
+;;
+(canonical-baseurl "http://localhost:7827/")
 
 ;;---------------------------------------------------------------------------
-;; If you want your server to appear under several aliases, add them
-;; here. HTTP servers will be spun up for all mentioned port numbers,
-;; and within those servers, `Host` headers matching the given host
-;; names will be accepted.
+;; At least one "http-listener" key. These cause an HTTP server to be
+;; spun up for each mentioned port number. Traffic will only be
+;; accepted for HTTP Host headers mentioned in these keys.
 ;;
-;; (accepted-host "localhost" 7827)
-;; (accepted-host "localhost" 80)
-;; (accepted-host "www.example.com" 7827)
+(http-listener "localhost" 7827)
+;; (http-listener "localhost" 80)
+;; (http-listener "www.example.com" 7827)
 ;;
 ;; etc.
 
diff --git a/racketmq/hub.rkt b/racketmq/hub.rkt
index 8f5f6b8..6c96c1b 100644
--- a/racketmq/hub.rkt
+++ b/racketmq/hub.rkt
@@ -12,17 +12,21 @@
 (require/activate "hub/static-content.rkt")
 (require/activate "hub/topic-demand.rkt")
 (require/activate "hub/local-topic.rkt")
+(require/activate "hub/remote-topic.rkt")
 (require/activate "hub/subscription.rkt")
 (require/activate "hub/websocket.rkt")
 
 (command-line #:program "racketmq"
 
               #:once-each
-              ["--canonical-host" host port "Specify the canonical host and port for this hub"
-               (actor #:name (list 'command-line-canonical-host host port)
-                      (assert (config (list 'canonical-host host (string->number port)))))]
+              ["--baseurl" baseurl "Specify the canonical base URL for this hub"
+               (actor #:name (list 'command-line-canonical-baseurl baseurl)
+                      (assert (config (list 'canonical-baseurl baseurl))))]
 
               #:multi
+              [("-l" "--listen") host port "Specify one HTTP listener"
+               (actor #:name (list 'command-line-http-listener host port)
+                      (assert (config (list 'http-listener host (string->number port)))))]
               [("-o" "--option") key vals "Specify a single configuration option"
                (actor #:name (list 'config-option key vals)
                       (assert (config (cons (string->symbol key)
@@ -32,21 +36,20 @@
 
 (actor #:name 'main
 
-       (during (config (list 'canonical-host $h $p))
-         (assert (canonical-local-host h p))
-         (assert (local-host h p)))
+       (during (config (list 'canonical-baseurl $u))
+         (assert (canonical-baseurl u)))
 
-       (define/query-set canonical-local-hosts ($ c (canonical-local-host _ _)) c)
-       (stop-when (rising-edge (> (set-count (canonical-local-hosts)) 1))
-                  (log-error "Too many canonical-host records in configuration."))
+       (define/query-set canonical-baseurls ($ c (canonical-baseurl _)) c)
+       (stop-when (rising-edge (> (set-count (canonical-baseurls)) 1))
+                  (log-error "Too many canonical-baseurl records in configuration."))
        (on-start (sleep 0.1)
-                 (when (set-empty? (canonical-local-hosts))
-                   (log-error "No canonical-host records specified; try the --canonical-host command line argument")))
-       ;; TODO: Make the too-many-canonical-host-records situation recoverable.
+                 (when (set-empty? (canonical-baseurls))
+                   (log-error "No canonical-baseurl records specified; try the --baseurl command line argument")))
+       ;; TODO: Make the too-many-canonical-baseurl-records situation recoverable.
        ;; TODO: And/or, make the whole application quit when it gets into a bad state.
 
-       (during (config (list 'accepted-host $h $p))
-         (assert (local-host h p)))
+       (during (config (list 'http-listener $h $p))
+         (assert (http-listener h p)))
 
-       (during (local-host $host-name $port)
+       (during (http-listener $host-name $port)
          (assert (vh host-name port))))
diff --git a/racketmq/hub/local-topic.rkt b/racketmq/hub/local-topic.rkt
index 37d534b..1f6abe0 100644
--- a/racketmq/hub/local-topic.rkt
+++ b/racketmq/hub/local-topic.rkt
@@ -11,7 +11,7 @@
 
        (field [topics (set)])
 
-       (during (local-host $host-name $port)
+       (during (http-listener $host-name $port)
          (on (web-request-incoming (id req) (vh host-name port) 'put ("topic" (,$topic ())))
              (when (not (set-member? (topics) topic))
                (topics (set-add (topics) topic))
@@ -51,10 +51,10 @@
               (max-count)))
   (on-stop (log-info "Terminating local topic ~v" topic))
 
-  (during (local-host $host-name $port)
-    (during (canonical-local-host $canonical-host-name $cport)
-      (define hub-url (canonical-url canonical-host-name cport `("hub" ())))
-      (define self-url (canonical-url canonical-host-name cport `("topic" (,topic ()))))
+  (during (http-listener $host-name $port)
+    (during (canonical-baseurl $baseurl)
+      (define hub-url (canonical-url baseurl `("hub" ())))
+      (define self-url (canonical-url baseurl `("topic" (,topic ()))))
       (define discovery-headers (list (cons 'link (format "<~a>; rel=hub" hub-url))
                                       (cons 'link (format "<~a>; rel=self" self-url))))
 
diff --git a/racketmq/hub/remote-topic.rkt b/racketmq/hub/remote-topic.rkt
index 9465301..0c8d51f 100644
--- a/racketmq/hub/remote-topic.rkt
+++ b/racketmq/hub/remote-topic.rkt
@@ -1,7 +1,5 @@
 #lang syndicate/actor
 
-(provide remote-topic-main)
-
 (require racket/dict)
 (require racket/set)
 (require file/sha1)
@@ -34,6 +32,11 @@
   (check-equal? (shrink-lease 90) 81)
   (check-equal? (shrink-lease 50) 45))
 
+(actor #:name 'remote-topic-manager
+       (during/actor (remote-topic-demand $full-topic)
+         #:name (list 'remote-topic full-topic)
+         (remote-topic-main full-topic)))
+
 (define (remote-topic-main full-topic)
   (define sub-id (random-hex-string 16))
   (log-info "Remote sub endpoint ~a for topic ~s" sub-id full-topic)
@@ -62,8 +65,8 @@
                 [#f "disabled"]
                 [n (format "~a seconds" n)])))
 
-  (during (canonical-local-host $canonical-host-name $cport)
-    (define callback (canonical-url canonical-host-name cport `("sub" (,sub-id ()))))
+  (during (canonical-baseurl $baseurl)
+    (define callback (canonical-url baseurl `("sub" (,sub-id ()))))
 
     (define (refresh-subscription!)
       ;; TODO: shared secret
@@ -161,35 +164,36 @@
                                                                 (* 1000 (or (poll-interval-seconds) 0)))))
         (poll-upstream!))
 
-    (on (web-request-get (id req) (vh canonical-host-name cport) ("sub" (,sub-id ())))
-        (log-info "Received verification-of-intent: ~v" (web-request-header-query req))
-        (define challenge (dict-ref (web-request-header-query req) 'hub.challenge ""))
-        (define lease-seconds (dict-ref (web-request-header-query req) 'hub.lease_seconds #f))
-        (if lease-seconds
-            (next-subscription-refresh (+ (current-inexact-milliseconds)
-                                          (* 1000.0 (shrink-lease lease-seconds))))
-            (log-warning "Upstream hub for topic ~s did not supply hub.lease_seconds" full-topic))
-        (web-respond/bytes! id (string->bytes/utf-8 challenge)))
+    (during (http-listener $host-name $port)
+      (on (web-request-get (id req) (vh host-name port) ("sub" (,sub-id ())))
+          (log-info "Received verification-of-intent: ~v" (web-request-header-query req))
+          (define challenge (dict-ref (web-request-header-query req) 'hub.challenge ""))
+          (define lease-seconds (dict-ref (web-request-header-query req) 'hub.lease_seconds #f))
+          (if lease-seconds
+              (next-subscription-refresh (+ (current-inexact-milliseconds)
+                                            (* 1000.0 (shrink-lease lease-seconds))))
+              (log-warning "Upstream hub for topic ~s did not supply hub.lease_seconds" full-topic))
+          (web-respond/bytes! id (string->bytes/utf-8 challenge)))
 
-    (on (web-request-incoming (id req) (vh canonical-host-name cport) 'post ("sub" (,sub-id ())) $body)
-        ;; TODO: verify the use of the shared secret
-        (actor*
-         (define parsed-link-headers (parse-link-headers (web-request-header-headers req)))
-         (define upstream-hub (link-header-ref parsed-link-headers 'hub #f))
-         (define upstream-topic (link-header-ref parsed-link-headers 'self #f))
-         (define content-type (web-request-header-content-type req))
-         (log-info "Remote topic ~a got ~v message ~v; upstream hub ~v, topic ~v"
-                   full-topic
-                   content-type
-                   body
-                   upstream-hub
-                   upstream-topic)
-         (current-content-hash (sha1 (open-input-bytes body)))
-         (current-content-type content-type)
-         (current-upstream-hub upstream-hub)
-         (send! (notification full-topic
-                              upstream-hub
-                              upstream-topic
-                              body
-                              content-type))
-         (web-respond/status! id 201 #"Created")))))
+      (on (web-request-incoming (id req) (vh host-name port) 'post ("sub" (,sub-id ())) $body)
+          ;; TODO: verify the use of the shared secret
+          (actor*
+           (define parsed-link-headers (parse-link-headers (web-request-header-headers req)))
+           (define upstream-hub (link-header-ref parsed-link-headers 'hub #f))
+           (define upstream-topic (link-header-ref parsed-link-headers 'self #f))
+           (define content-type (web-request-header-content-type req))
+           (log-info "Remote topic ~a got ~v message ~v; upstream hub ~v, topic ~v"
+                     full-topic
+                     content-type
+                     body
+                     upstream-hub
+                     upstream-topic)
+           (current-content-hash (sha1 (open-input-bytes body)))
+           (current-content-type content-type)
+           (current-upstream-hub upstream-hub)
+           (send! (notification full-topic
+                                upstream-hub
+                                upstream-topic
+                                body
+                                content-type))
+           (web-respond/status! id 201 #"Created"))))))
diff --git a/racketmq/hub/static-content.rkt b/racketmq/hub/static-content.rkt
index 8e63d4b..ecf6149 100644
--- a/racketmq/hub/static-content.rkt
+++ b/racketmq/hub/static-content.rkt
@@ -16,7 +16,7 @@
 
 (actor #:name 'static-content-server
        (define url->path (make-url->path htdocs-path))
-       (during (local-host $host-name $port)
+       (during (http-listener $host-name $port)
          (on (web-request-get (id req) (vh host-name port) ,_)
              (define-values (path path-pieces)
                (url->path (resource->url (web-request-header-resource req))))
diff --git a/racketmq/hub/subscription.rkt b/racketmq/hub/subscription.rkt
index 6132412..87b3e93 100644
--- a/racketmq/hub/subscription.rkt
+++ b/racketmq/hub/subscription.rkt
@@ -13,10 +13,10 @@
 (require "../protocol.rkt")
 
 (actor #:name 'hub
-       (during (local-host $host-name $port)
-         (during (canonical-local-host $canonical-host-name $cport)
+       (during (http-listener $host-name $port)
+         (during (canonical-baseurl $baseurl)
            (on (web-request-incoming (id req) (vh host-name port) 'post ("hub" ()) $body)
-               (asynchronous-verification-of-intent id req body canonical-host-name cport)
+               (asynchronous-verification-of-intent id req body baseurl)
                (web-respond/status! id 202 #"Accepted"))))
 
        (on (message (update-subscription $topic $callback $settings))
@@ -28,7 +28,7 @@
          #:on-crash (retract! (subscription topic callback ?))
          (subscription-main topic callback)))
 
-(define (asynchronous-verification-of-intent id req body canonical-host-name cport)
+(define (asynchronous-verification-of-intent id req body baseurl)
   (actor* #:name 'verification-of-intent
           (define params (make-immutable-hash (form-urlencoded->alist (bytes->string/utf-8 body))))
           (define callback (hash-ref params 'hub.callback))
@@ -38,9 +38,7 @@
           (define requested-topic (hash-ref params 'hub.topic))
           (define topic
             (url->string
-             (combine-url/relative (string->url (canonical-url canonical-host-name
-                                                               cport
-                                                               `("topic" ("" ()))))
+             (combine-url/relative (string->url (canonical-url baseurl `("topic" ("" ()))))
                                    requested-topic)))
           (define requested-lease-seconds
             (string->number
diff --git a/racketmq/hub/topic-demand.rkt b/racketmq/hub/topic-demand.rkt
index 9731437..228913d 100644
--- a/racketmq/hub/topic-demand.rkt
+++ b/racketmq/hub/topic-demand.rkt
@@ -2,44 +2,32 @@
 
 (require racket/exn)
 (require racket/set)
+(require racket/string)
 (require net/url)
 
 (require/activate syndicate/drivers/web)
-(require/activate "remote-topic.rkt")
 
 (require "../private/util.rkt")
 (require "../protocol.rkt")
 
 (actor #:name 'topic-demand-analyzer
-       (define/query-set local-hosts ($ h (local-host _ _)) h)
        (during/actor (topic-demand $full-topic _)
-         #:name (list 'general-topic full-topic)
-         #:let [(local-hosts-snapshot (local-hosts))]
-         (with-handlers [(exn? (lambda (e)
-                                 (log-error "Topic demand error: ~a" (exn->string e))))]
-           (match-define (web-resource (web-virtual-host _ topic-host topic-port) topic-path)
-             (url->resource (string->url full-topic)))
-           (define maybe-local-topic
-             (match topic-path [`("topic" (,topic ())) topic] [_ #f]))
-           (general-topic-main full-topic
-                               topic-host
-                               topic-port
-                               maybe-local-topic
-                               (set-member? local-hosts-snapshot
-                                            (local-host topic-host topic-port))))))
+         #:name (list 'topic-demand full-topic)
 
-(define (general-topic-main full-topic topic-host topic-port maybe-local-topic start-as-local?)
-  (define (local-state)
-    (react (stop-when (retracted (local-host topic-host topic-port))
-                      (remote-state))
-           (assert (local-topic-demand maybe-local-topic))))
+         (define/query-value topic-baseurl #f (canonical-baseurl $b)
+           (canonical-url b `("topic" ("" ()))))
 
-  (define (remote-state)
-    (react (stop-when #:when maybe-local-topic (asserted (local-host topic-host topic-port))
-                      (local-state))
-           (remote-topic-main full-topic)))
+         (define/dataflow state
+           (cond
+             [(not (topic-baseurl)) 'unknown]
+             [(string-prefix? full-topic (topic-baseurl)) 'local]
+             [else 'remote]))
 
-  (on-start
-   (if (and maybe-local-topic start-as-local?)
-       (local-state)
-       (remote-state))))
+         (begin/dataflow
+           (log-info "Topic-demand for ~s is in state ~s." full-topic (state)))
+
+         (assert #:when (eq? (state) 'local)
+                 (local-topic-demand (substring full-topic (string-length (topic-baseurl)))))
+
+         (assert #:when (eq? (state) 'remote)
+                 (remote-topic-demand full-topic))))
diff --git a/racketmq/hub/websocket.rkt b/racketmq/hub/websocket.rkt
index 8038da7..626e748 100644
--- a/racketmq/hub/websocket.rkt
+++ b/racketmq/hub/websocket.rkt
@@ -13,21 +13,19 @@
 (require "../protocol.rkt")
 
 (actor #:name 'websocket-hub
-       (during (local-host $host-name $port)
-         (during (canonical-local-host $canonical-host-name $cport)
+       (during (http-listener $host-name $port)
+         (during (canonical-baseurl $baseurl)
            (on (web-request-get (id req) (vh host-name port) ("hub" ()))
                (when (equal? (dict-ref (web-request-header-headers req) 'upgrade #f) "websocket")
-                 (websocket-subscription id req canonical-host-name cport))))))
+                 (websocket-subscription id req baseurl))))))
 
-(define (websocket-subscription id req canonical-host-name cport)
+(define (websocket-subscription id req baseurl)
   (actor* #:name (list 'websocket-subscription id)
           (define params (web-request-header-query req))
           (define requested-topic (dict-ref params 'hub.topic))
           (define topic ;; TODO: abstract this expression out (see also subscription.rkt)
             (url->string
-             (combine-url/relative (string->url (canonical-url canonical-host-name
-                                                               cport
-                                                               `("topic" ("" ()))))
+             (combine-url/relative (string->url (canonical-url baseurl `("topic" ("" ()))))
                                    requested-topic)))
           (define poll-interval-seconds
             (match (dict-ref params
diff --git a/racketmq/private/util.rkt b/racketmq/private/util.rkt
index d8a0d68..5a42a8c 100644
--- a/racketmq/private/util.rkt
+++ b/racketmq/private/util.rkt
@@ -144,5 +144,7 @@
 
 (define (vh host-name port) (web-virtual-host "http" host-name port))
 
-(define (canonical-url canonical-host-name cport path)
-  (url->string (resource->url (web-resource (vh canonical-host-name cport) path))))
+(define (canonical-url baseurl path)
+  (define b (url->resource (string->url baseurl)))
+  (url->string (resource->url (struct-copy web-resource b
+                                [path (append-url-path (web-resource-path b) path)]))))
diff --git a/racketmq/protocol.rkt b/racketmq/protocol.rkt
index 51e1dcd..d69b43b 100644
--- a/racketmq/protocol.rkt
+++ b/racketmq/protocol.rkt
@@ -8,8 +8,9 @@
          (struct-out local-topic-config)
          (struct-out topic-demand)
          (struct-out local-topic-demand)
-         (struct-out local-host)
-         (struct-out canonical-local-host))
+         (struct-out remote-topic-demand)
+         (struct-out http-listener)
+         (struct-out canonical-baseurl))
 
 ;; A Topic is a URIString.
 
@@ -53,8 +54,11 @@
 ;; (local-topic-demand String)
 (struct local-topic-demand (name) #:prefab) ;; ASSERTION
 
-;; (local-host String Number)
-(struct local-host (name port) #:prefab) ;; ASSERTION
+;; (remote-topic-demand Topic)
+(struct remote-topic-demand (topic-name) #:prefab) ;; ASSERTION
 
-;; (canonical-local-host String Number)
-(struct canonical-local-host (name port) #:prefab) ;; ASSERTION
+;; (http-listener String Number)
+(struct http-listener (name port) #:prefab) ;; ASSERTION
+
+;; (canonical-baseurl URLString)
+(struct canonical-baseurl (string) #:prefab) ;; ASSERTION