diff --git a/Makefile b/Makefile
index 24b9a2f289..b63714c124 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,10 @@ CFLAGS=-Wall
 GIT_ANNEX_TMP_BUILD_DIR?=tmp
 IGNORE=-ignore-package monads-fd -ignore-package monads-tf
 BASEFLAGS=-threaded -Wall $(IGNORE) -outputdir $(GIT_ANNEX_TMP_BUILD_DIR) -IUtility
-FEATURES=-DWITH_ASSISTANT -DWITH_S3 -DWITH_WEBAPP -DWITH_OLD_YESOD
+
+# If you get build failures due to missing haskell libraries,
+# you can turn off some of these features.
+FEATURES=-DWITH_ASSISTANT -DWITH_S3 -DWITH_WEBAPP -DWITH_OLD_YESOD -DWITH_MULTICAST
 
 bins=git-annex
 mans=git-annex.1 git-annex-shell.1
diff --git a/debian/control b/debian/control
index 89ca892834..da7ab1e701 100644
--- a/debian/control
+++ b/debian/control
@@ -38,6 +38,7 @@ Build-Depends:
 	libghc-crypto-api-dev,
 	libghc-hamlet-dev,
 	libghc-clientsession-dev,
+	libghc-network-multicast-dev,
 	ikiwiki,
 	perlmagick,
 	git,
diff --git a/doc/design/assistant/pairing.mdwn b/doc/design/assistant/pairing.mdwn
index f33c5e11de..32815698c8 100644
--- a/doc/design/assistant/pairing.mdwn
+++ b/doc/design/assistant/pairing.mdwn
@@ -3,11 +3,23 @@ have some way of pairing devices.
 
 It could work like this:
 
-1. Prompt for the hostname (or do avahi local machine discovery). 
-2. Enable the two hosts to ssh to one-another and run git-annex shell.
-   (A tricky problem, if ssh keys need to be added to do that.)
-3. Push over a clone of the repository. (Using git-annex-shell?)
-4. Start [[syncing]].
+1. Prompt for the hostname, or do avahi local machine discovery, or use
+   ZeroMQ with IP multicast, or use haskell's `network-multicast`. That
+   last option seems to work best!
+2. Let user pick host to pair with. Somehow authenticate that this is
+   the host they expected to pair with, and not an imposter. Probably
+   have the users enter a shared secret and use it to HMAC the ssh public
+   keys.
+3. Exchange a hostname or IP address with the pair. Ideally,
+   use `.local`, as it'll work as long as both are on the same subnet.
+   If mDNS is not available, regular DNS or IP addresses might have
+   to be used, but will result in a more fragile pairing. Or perhaps
+   the assistant could broadcast itself queries for current IP addresses,
+   if connecting to a paired host fails.
+4. Enable the two hosts to ssh to one-another and run git-annex shell.
+   (Set up per-host ssh keys.)
+5. Pull over a clone of the repository.
+6. Start [[syncing]].
 
 Also look into the method used by
 <https://support.mozilla.org/en-US/kb/add-a-device-to-firefox-sync>
diff --git a/doc/install.mdwn b/doc/install.mdwn
index 31a5b15d3e..92b0897f41 100644
--- a/doc/install.mdwn
+++ b/doc/install.mdwn
@@ -65,6 +65,7 @@ libraries. To build and use git-annex by hand, you will need:
   * [crypto-api](http://hackage.haskell.org/package/crypto-api)
   * [hamlet](http://hackage.haskell.org/package/hamlet)
   * [clientsession](http://hackage.haskell.org/package/clientsession)
+  * [network-multicast](http://hackage.haskell.org/package/network-multicast)
 * Shell commands
   * [git](http://git-scm.com/)
   * [uuid](http://www.ossp.org/pkg/lib/uuid/)
diff --git a/git-annex.cabal b/git-annex.cabal
index dbe1806570..4407be3c9e 100644
--- a/git-annex.cabal
+++ b/git-annex.cabal
@@ -40,6 +40,9 @@ Flag Assistant
 Flag Webapp
   Description: Enable git-annex webapp
 
+Flag Multicast
+  Description: Enable multicast pairing
+
 Executable git-annex
   Main-Is: git-annex.hs
   Build-Depends: MissingH, hslogger, directory, filepath,
@@ -75,13 +78,17 @@ Executable git-annex
     Build-Depends: dbus
     CPP-Options: -DWITH_DBUS
 
-  if flag(Webapp)
+  if flag(Webapp) && flag(Assistant)
     Build-Depends: yesod, yesod-static, case-insensitive,
      http-types, transformers, wai, wai-logger, warp, blaze-builder,
      blaze-html, crypto-api, hamlet, clientsession,
      template-haskell, yesod-default (>= 1.1.0), data-default
     CPP-Options: -DWITH_WEBAPP
 
+  if flag(Multicast) && flag(Webapp)
+    Build-Depends: network-multicast
+    CPP-Options: -DWITH_MULTICAST
+
   if os(darwin)
     CPP-Options: -DOSX