feat: Use turn.delta.chat as fallback TURN server (#7382)

Author: iequidoo

src/calls.rs

@@ -660,24 +660,35 @@ pub(crate) async fn create_fallback_ice_servers(context: &Context) -> Result<Str
     // because of bandwidth costs:
     // <https://github.com/jselbie/stunserver/issues/50>
 
-    // We use nine.testrun.org for a default STUN server.
-    let hostname = "nine.testrun.org";
+    let hostname = "turn.delta.chat";
+    // Do not use cache because there is no TLS.
+    let load_cache = false;
+    let urls: Vec<String> = lookup_host_with_cache(context, hostname, STUN_PORT, "", load_cache)
+        .await?
+        .into_iter()
+        .map(|addr| format!("turn:{addr}"))
+        .collect();
+    let turn_server = IceServer {
+        urls,
+        username: Some("public".to_string()),
+        credential: Some("o4tR7yG4rG2slhXqRUf9zgmHz".to_string()),
+    };
 
+    let hostname = "nine.testrun.org";
     // Do not use cache because there is no TLS.
     let load_cache = false;
     let urls: Vec<String> = lookup_host_with_cache(context, hostname, STUN_PORT, "", load_cache)
         .await?
         .into_iter()
         .map(|addr| format!("stun:{addr}"))
         .collect();
-
-    let ice_server = IceServer {
+    let stun_server = IceServer {
         urls,
         username: None,
         credential: None,
     };
 
-    let json = serde_json::to_string(&[ice_server])?;
+    let json = serde_json::to_string(&[turn_server, stun_server])?;
     Ok(json)
 }
 

src/net/dns.rs

@@ -417,6 +417,13 @@ static DNS_PRELOAD: LazyLock<HashMap<&'static str, Vec<IpAddr>>> = LazyLock::new
             "mail.systemli.org",
             vec![IpAddr::V4(Ipv4Addr::new(93, 190, 126, 36))],
         ),
+        (
+            "turn.delta.chat",
+            vec![
+                IpAddr::V4(Ipv4Addr::new(46, 224, 39, 230)),
+                IpAddr::V6(Ipv6Addr::new(0x2a01, 0x4f8, 0xc014, 0x9593, 0, 0, 0, 1)),
+            ],
+        ),
         ("testrun.org", vec![IpAddr::V4(Ipv4Addr::new(5, 1, 76, 52))]),
         (
             "nine.testrun.org",