[gnome-control-center] Lower WWAN DNS Priority

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 585fe8c94a1d60e1e575dbdc980a9a73d1d6447f
Author: Kyle Rankin <kyle rankin puri sm>
Date:   Fri Feb 21 01:28:13 2020 +0000

    Lower WWAN DNS Priority
    
    The current DNS priority settings for WWAN were set far too low. Most
    connections (including WiFi) do not set DNS priority (set to 0) and per
    https://developer.gnome.org/NetworkManager/stable/nm-settings.html :
    
    "A lower value is better (higher priority). Zero selects a globally
    configured default value. If the latter is missing or zero too, it
    defaults to 50 for VPNs and 100 for other connections."
    
    By setting both the "low" and "high" settings to 15 and 20 respectively,
    the WWAN DNS servers were always appearing above WiFi, even though WiFi
    had routing priority. This caused latency and other problems when the
    wwan connection was slow because the system would query those DNS
    servers before WiFi ones. Beyond that, it would even cause WWAN to
    override VPN DNS settings which isn't what we want.
    
    This change puts the "low priority" setting above the default 100 that
    connections get when they don't otherwise set a priority, and the "high
    priority" slightly below 100. I did this instead of setting the values
    to 0 because I noticed that NM doesn't seem to be aware it should
    prioritize WiFi in that case so WWAN DNS servers were still sometimes
    taking precedence.

 panels/wwan/cc-wwan-data.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)
---
diff --git a/panels/wwan/cc-wwan-data.c b/panels/wwan/cc-wwan-data.c
index 0be8f3403..4062d78fd 100644
--- a/panels/wwan/cc-wwan-data.c
+++ b/panels/wwan/cc-wwan-data.c
@@ -47,9 +47,20 @@
  * of #CcWwanData changes when SIM is changed.
  */
 
-/* Priority for connections. larger the number, lower the priority */
-#define CC_WWAN_DNS_PRIORITY_LOW  (20)
-#define CC_WWAN_DNS_PRIORITY_HIGH (15)
+/*
+ * Priority for connections. The larger the number, the lower the priority
+ * https://developer.gnome.org/NetworkManager/stable/nm-settings.html:
+ *
+ *   A lower value is better (higher priority). Zero selects a globally
+ *   configured default value. If the latter is missing or zero too, it
+ *   defaults to 50 for VPNs and 100 for other connections.
+ *
+ * Since WiFi and other network connections will likely get the default
+ * setting of 100, set WWAN DNS priorities higher than the default, with
+ * room to allow multiple modems to set priority above/below each other.
+ */
+#define CC_WWAN_DNS_PRIORITY_LOW  (120)
+#define CC_WWAN_DNS_PRIORITY_HIGH (115)
 
 /* These are to be set as route metric */
 #define CC_WWAN_ROUTE_PRIORITY_LOW  (1050)