Submitted upstream: https://github.com/canonical/lightdm/pull/312

diff --git a/src/vnc-server.c b/src/vnc-server.c
index d3500764..00a2fc02 100644
--- a/src/vnc-server.c
+++ b/src/vnc-server.c
@@ -126,18 +126,10 @@ vnc_server_start (VNCServer *server)
 
     g_return_val_if_fail (server != NULL, FALSE);
 
-    g_autoptr(GError) ipv4_error = NULL;
-    priv->socket = open_tcp_socket (G_SOCKET_FAMILY_IPV4, priv->port, priv->listen_address, &ipv4_error);
-    if (ipv4_error)
-        g_warning ("Failed to create IPv4 VNC socket: %s", ipv4_error->message);
-
-    if (priv->socket)
-    {
-        GSource *source = g_socket_create_source (priv->socket, G_IO_IN, NULL);
-        g_source_set_callback (source, (GSourceFunc) read_cb, server, NULL);
-        g_source_attach (source, NULL);
-    }
-
+    // Bind to IPv6 first, as this implies binding to 0.0.0.0 in the
+    // Linux kernel default configuration, which would otherwise cause
+    // IPv6 clients to fail with "Error binding to address [::]:5900:
+    // Address already in use" (#266).
     g_autoptr(GError) ipv6_error = NULL;
     priv->socket6 = open_tcp_socket (G_SOCKET_FAMILY_IPV6, priv->port, priv->listen_address, &ipv6_error);
     if (ipv6_error)
@@ -150,6 +142,18 @@ vnc_server_start (VNCServer *server)
         g_source_attach (source, NULL);
     }
 
+    g_autoptr(GError) ipv4_error = NULL;
+    priv->socket = open_tcp_socket (G_SOCKET_FAMILY_IPV4, priv->port, priv->listen_address, &ipv4_error);
+    if (ipv4_error)
+        g_warning ("Failed to create IPv4 VNC socket: %s", ipv4_error->message);
+
+    if (priv->socket)
+    {
+        GSource *source = g_socket_create_source (priv->socket, G_IO_IN, NULL);
+        g_source_set_callback (source, (GSourceFunc) read_cb, server, NULL);
+        g_source_attach (source, NULL);
+    }
+
     if (!priv->socket && !priv->socket6)
         return FALSE;
 
lue='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=e2496a814f8ac4b62b91e05fc9899b9bb85b3ac4'>root</a>/<a href='/guix/log/gnu?id=e2496a814f8ac4b62b91e05fc9899b9bb85b3ac4'>gnu</a>/<a href='/guix/log/gnu/services?id=e2496a814f8ac4b62b91e05fc9899b9bb85b3ac4'>services</a>/<a href='/guix/log/gnu/services/networking.scm?id=e2496a814f8ac4b62b91e05fc9899b9bb85b3ac4'>networking.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/networking.scm?id=e2496a814f8ac4b62b91e05fc9899b9bb85b3ac4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-05-19 22:46:52 -0400'>2021-05-19</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=a09cdf1f9d25abd1b3910409787a58386ebc21e2'>services: opendht: Fix hang at boot.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-05-17 23:27:40 -0400'>2021-05-17</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=fd449608eb9016fb705eebbea784901b0242cbf2'>services: Add a service for opendht.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-04-12 18:44:09 +0200'>2021-04-12</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=4d0d6d86263f0701244682e2226c8b539b25f669'>services: ipfs: Tweak description.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-04-12 18:42:22 +0200'>2021-04-12</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=2978832b9264b89325605fe2525bc3992d753135'>services: Add ipfs-service-type</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2021-02-22 10:03:02 -0500'>2021-02-22</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=b309a28678664c295e97a70607def02b0c9b4296'>services: tor: Add control-socket? option.</a><span class='msg-avail'>...</span></td><td>Christopher Lemmer Webber</td></tr>
<tr><td><span title='2021-01-16 23:30:59 +0100'>2021-01-16</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=181f290aa262eae22819c6234b4065786afb14cf'>services: openntpd: Remove support for deprecated "-s" option.</a><span class='msg-avail'>...</span></td><td>Simon South</td></tr>
<tr><td><span title='2021-01-13 22:24:18 +0100'>2021-01-13</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=0d22fc8d36e4efba9748c94cab7cedbbe2bbb5bf'>services: shepherd: 'shepherd-service-type' requires documentation.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-01-12 14:40:36 +0300'>2021-01-12</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=4e511fcfce2c005c615df673836c553f96550b35'>services: Add keepalived service.</a><span class='msg-avail'>...</span></td><td>Oleg Pykhalov</td></tr>
<tr><td><span title='2020-11-05 18:45:40 +0100'>2020-11-05</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=fe1cd098d2b83737e96f19438612291f5a9316e4'>services: Add yggdrasil-service-type.</a><span class='msg-avail'>...</span></td><td>raingloom</td></tr>
<tr><td><span title='2020-09-24 20:18:20 +0200'>2020-09-24</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=d48b17adb91d68acf6fb3f321c05102fcc8c39eb'>services: WPA Supplicant: Conditionally depend on D-Bus.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-08-16 16:33:55 +0300'>2020-08-16</span></td><td><a href='/guix/commit/gnu/services/networking.scm?id=0ba3a38bb2a94dfcf0c422b8b880aa0e4608b5b2'>services: connman-shepherd-service: Don't use short flags.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>