Fix various XDP bugs (#1072)

.gitignore

@@ -39,3 +39,6 @@ crates/ebpf/target/
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
+
+# Unaccepted snapshots should never be checked in
+**/*.snap.new

crates/test/tests/snapshots/xdp__changes_ip_version-2.snap

@@ -45,7 +45,7 @@ PacketHeaders {
                 ),
                 time_to_live: 63,
                 protocol: 17 (UDP - User Datagram),
-                header_checksum: 0,
+                header_checksum: 28089,
                 source: [
                     2,
                     2,

crates/test/tests/snapshots/xdp__simple_forwarding.snap

@@ -45,7 +45,7 @@ PacketHeaders {
                 ),
                 time_to_live: 63,
                 protocol: 17 (UDP - User Datagram),
-                header_checksum: 0,
+                header_checksum: 30145,
                 source: [
                     2,
                     2,

crates/xdp/src/lib.rs

@@ -249,10 +249,13 @@ pub fn get_default_nic() -> std::io::Result<Option<NicIndex>> {
         };
 
         if let Some(def) = def_iface {
-            return Err(std::io::Error::new(
-                std::io::ErrorKind::Unsupported,
-                format!("unable to determine default interface, found {def:?} and {iface:?}"),
-            ));
+            // A NIC can have multiple routes, so don't error when it comes up again
+            if def != iface {
+                return Err(std::io::Error::new(
+                    std::io::ErrorKind::Unsupported,
+                    format!("unable to determine default interface, found {def:?} and {iface:?}"),
+                ));
+            }
         }
 
         def_iface = Some(iface);

src/net/xdp.rs

@@ -321,6 +321,12 @@ pub fn spawn(workers: XdpWorkers, config: Arc<crate::Config>) -> Result<XdpLoop,
     // Now that all the io loops are running, attach the eBPF program to route
     // packets to the bound sockets
     let mut ebpf_prog = workers.ebpf_prog;
+
+    // We use the default flags here, which means that the program will be attached
+    // in driver mode if the NIC + driver is capable of it, otherwise it will fallback
+    // to SKB mode. This allows maximum compatibility, and we already provide
+    // flags to force zerocopy, which relies on driver mode, so the user can use
+    // that if they don't want the fallback behavior
     let xdp_link =
         ebpf_prog.attach(workers.nic, quilkin_xdp::aya::programs::XdpFlags::default())?;