TUN-10383: Set edge-ip-version to auto

To allow pre-checks to test both IPv6 and IPv4, we must change the default value of edge-ip-version's from 4 to auto. This will allows the tunnel (and pre-check) to probe both IPv4 and IPv6 addresses by default, respecting the system's DNS preference. Instead of always preferring IPv4, cloudflared will now use whichever address family the system resolver returns first.

CHANGES.md

@@ -1,3 +1,7 @@
+## 2026.4.0
+### Breaking Change
+- The default value of `--edge-ip-version` has changed from `4` to `auto`. This means cloudflared will now use whichever address family (IPv4 or IPv6) the system resolver returns first, instead of always preferring IPv4. Users who require IPv4-only connections should explicitly set `--edge-ip-version 4`.
+
 ## 2026.2.0
 ### Breaking Change
 - Removes the `proxy-dns` feature from cloudflared. This feature allowed running a local DNS over HTTPS (DoH) proxy.

cmd/cloudflared/tunnel/cmd.go

@@ -631,7 +631,7 @@ func tunnelFlags(shouldHide bool) []cli.Flag {
 			Name:    cfdflags.EdgeIpVersion,
 			Usage:   "Cloudflare Edge IP address version to connect with. {4, 6, auto}",
 			EnvVars: []string{"TUNNEL_EDGE_IP_VERSION"},
-			Value:   "4",
+			Value:   "auto",
 			Hidden:  false,
 		}),
 		altsrc.NewStringFlag(&cli.StringFlag{

component-tests/test_edge_discovery.py

@@ -20,12 +20,21 @@ class TestEdgeDiscovery:
     @pytest.mark.parametrize("protocol", protocols())
     def test_default_only(self, tmp_path, component_tests_config, protocol):
         """
-        This test runs a tunnel to connect via IPv4-only edge addresses (default is unset "--edge-ip-version 4")
+        This test runs a tunnel with the default edge-ip-version (auto), which will use
+        whichever address family the system resolver returns first.
         """
         if self.has_ipv6_only():
-            pytest.skip("Host has IPv6 only support and current default is IPv4 only")
-        self.expect_address_connections(
-            tmp_path, component_tests_config, protocol, None, self.expect_ipv4_address)
+            self.expect_address_connections(
+                tmp_path, component_tests_config, protocol, None, self.expect_ipv6_address)
+        elif self.has_ipv4_only():
+            self.expect_address_connections(
+                tmp_path, component_tests_config, protocol, None, self.expect_ipv4_address)
+        elif self.has_dual_stack(address_family_preference=socket.AddressFamily.AF_INET6):
+            self.expect_address_connections(
+                tmp_path, component_tests_config, protocol, None, self.expect_ipv6_address)
+        else:
+            self.expect_address_connections(
+                tmp_path, component_tests_config, protocol, None, self.expect_ipv4_address)
 
     @pytest.mark.parametrize("protocol", protocols())
     def test_ipv4_only(self, tmp_path, component_tests_config, protocol):