Automatically set socket read & write buffer sizes (#312)

This is an attempt to port the logic that nftables uses to automatically
adjust the recvmsg & sndmsg buffer sizes. The implementation of setting
sndmsg size is not the same as nftables due to some limitations in the
underlying netlink & socket libraries.

- https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n262
- https://git.netfilter.org/libmnl/tree/include/libmnl/libmnl.h?id=03da98bcd284d55212bc79e91dfb63da0ef7b937#n20
- https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n391
- https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n426

We should not enlarge the socket buffers when:
 - We are using a test dial (there are no buffers to enlarge).
 - A connection has been initialized with socket options which means
   that the user could have specified fixed buffer sizes.

conn.go

@@ -116,7 +116,9 @@ func WithTestDial(f nltest.Func) ConnOption {
 }
 
 // WithSockOptions sets the specified socket options when creating a new netlink
-// connection.
+// connection. Note that when using WithSockOptions, you are responsible for
+// providing a large-enough read and write buffer, whereas normally, the
+// nftables package automatically enlarges the buffers as needed.
 func WithSockOptions(opts ...SockOption) ConnOption {
 	return func(cc *Conn) {
 		cc.sockOptions = append(cc.sockOptions, opts...)
@@ -250,6 +252,13 @@ func (cc *Conn) Flush() error {
 	}
 	defer func() { _ = closer() }()
 
+	if err := cc.enlargeWriteBuffer(conn); err != nil {
+		return err
+	}
+	if err := cc.enlargeReadBuffer(conn); err != nil {
+		return err
+	}
+
 	messages, err := conn.SendMessages(batch(cc.messages))
 	if err != nil {
 		return fmt.Errorf("SendMessages: %w", err)
@@ -423,3 +432,104 @@ func (cc *Conn) allocateTransactionID() uint32 {
 	}
 	return cc.lastID
 }
+
+// getMessageSize returns the total size of all messages in the buffer.
+func (cc *Conn) getMessageSize() int {
+	var total int
+	for _, msg := range cc.messages {
+		total += len(msg.Data) + unix.NLMSG_HDRLEN
+	}
+	return total
+}
+
+// canEnlargeBuffers returns true if the connection can automatically enlarge
+// the write and read buffers of the netlink connection.
+func (cc *Conn) canEnlargeBuffers() bool {
+	// If there are sock options, we assume that the user has already set the
+	// buffers to a fixed size.
+	if len(cc.sockOptions) > 0 {
+		return false
+	}
+
+	if cc.TestDial != nil {
+		return false
+	}
+
+	return true
+}
+
+// enlargeWriteBuffer automatically sets the write buffer of the given
+// connection to the accumulated message size. This is only done if the current
+// write buffer is smaller than the message size.
+//
+// nftables actually handles this differently, it multiplies the number of
+// iovec entries by 2MB. This is not possible to do here as our underlying
+// netlink and socket libraries will only add a single iovec entry and
+// won't expose the number of entries.
+// https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n262
+//
+// TODO: Update this function to mimic the behavior of nftables once our
+// socket library supports multiple iovec entries.
+func (cc *Conn) enlargeWriteBuffer(conn *netlink.Conn) error {
+	if !cc.canEnlargeBuffers() {
+		return nil
+	}
+
+	messageSize := cc.getMessageSize()
+	writeBuffer, err := conn.WriteBuffer()
+	if err != nil {
+		return err
+	}
+	if writeBuffer < messageSize {
+		return conn.SetWriteBuffer(messageSize)
+	}
+
+	return nil
+}
+
+// getDefaultEchoReadBuffer returns the minimum read buffer size for batches
+// with echo messages.
+//
+// See https://git.netfilter.org/libmnl/tree/include/libmnl/libmnl.h?id=03da98bcd284d55212bc79e91dfb63da0ef7b937#n20
+// and https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n391
+func (cc *Conn) getDefaultEchoReadBuffer() int {
+	pageSize := os.Getpagesize()
+	return max(pageSize, 8192) * 1024
+}
+
+// enlargeReadBuffer automatically sets the read buffer of the given connection
+// to the required size. This is only done if the current read buffer is smaller
+// than the required size.
+//
+// See https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n426
+func (cc *Conn) enlargeReadBuffer(conn *netlink.Conn) error {
+	if !cc.canEnlargeBuffers() {
+		return nil
+	}
+
+	var bufferSize int
+
+	// If there are any messages with the Echo flag, we initialize the buffer size
+	// to the default echo read buffer size.
+	for _, msg := range cc.messages {
+		if msg.Header.Flags&netlink.Echo == 0 {
+			bufferSize = cc.getDefaultEchoReadBuffer()
+			break
+		}
+	}
+
+	// Just like nftables, we allocate 1024 bytes for each message in the batch.
+	requiredSize := len(cc.messages) * 1024
+	if bufferSize < requiredSize {
+		bufferSize = requiredSize
+	}
+
+	currSize, err := conn.ReadBuffer()
+	if err != nil {
+		return err
+	}
+	if currSize < bufferSize {
+		return conn.SetReadBuffer(bufferSize)
+	}
+	return nil
+}

go.mod

@@ -4,14 +4,14 @@ go 1.23.0
 
 require (
 	github.com/google/go-cmp v0.6.0
-	github.com/mdlayher/netlink v1.7.3-0.20250113171957-fbb4dce95f42
+	github.com/mdlayher/netlink v1.7.3-0.20250702063131-0f7746f74615
 	github.com/vishvananda/netlink v1.3.0
 	github.com/vishvananda/netns v0.0.4
 	golang.org/x/sys v0.31.0
 )
 
 require (
-	github.com/mdlayher/socket v0.5.0 // indirect
+	github.com/mdlayher/socket v0.5.1 // indirect
 	golang.org/x/net v0.38.0 // indirect
 	golang.org/x/sync v0.6.0 // indirect
 )

go.sum

@@ -1,9 +1,9 @@
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
-github.com/mdlayher/netlink v1.7.3-0.20250113171957-fbb4dce95f42 h1:A1Cq6Ysb0GM0tpKMbdCXCIfBclan4oHk1Jb+Hrejirg=
-github.com/mdlayher/netlink v1.7.3-0.20250113171957-fbb4dce95f42/go.mod h1:BB4YCPDOzfy7FniQ/lxuYQ3dgmM2cZumHbK8RpTjN2o=
-github.com/mdlayher/socket v0.5.0 h1:ilICZmJcQz70vrWVes1MFera4jGiWNocSkykwwoy3XI=
-github.com/mdlayher/socket v0.5.0/go.mod h1:WkcBFfvyG8QENs5+hfQPl1X6Jpd2yeLIYgrGFmJiJxI=
+github.com/mdlayher/netlink v1.7.3-0.20250702063131-0f7746f74615 h1:5T2ai+PpYFKe+tyNj/ZxePZGiYoG5xDOylT30nywJUU=
+github.com/mdlayher/netlink v1.7.3-0.20250702063131-0f7746f74615/go.mod h1:ZlWrPUV9wyD64k5skWrIv4WDQmmiUbkNnCkBtEWYKwU=
+github.com/mdlayher/socket v0.5.1 h1:VZaqt6RkGkt2OE9l3GcC6nZkqD3xKeQLyfleW/uBcos=
+github.com/mdlayher/socket v0.5.1/go.mod h1:TjPLHI1UgwEv5J1B5q0zTZq12A/6H7nKmtTanQE37IQ=
 github.com/vishvananda/netlink v1.3.0 h1:X7l42GfcV4S6E4vHTsw48qbrV+9PVojNfIhZcwQdrZk=
 github.com/vishvananda/netlink v1.3.0/go.mod h1:i6NetklAujEcC6fK0JPjT8qSwWyO0HLn4UKG+hGqeJs=
 github.com/vishvananda/netns v0.0.4 h1:Oeaw1EM2JMxD51g9uhtC0D7erkIjgmj8+JZc26m1YX8=

nftables_test.go

@@ -7396,3 +7396,36 @@ func TestSetElementComment(t *testing.T) {
 		}
 	}
 }
+
+func TestAutoBufferSize(t *testing.T) {
+	conn, newNS := nftest.OpenSystemConn(t, *enableSysTests)
+	defer nftest.CleanupSystemConn(t, newNS)
+	conn.FlushRuleset()
+	defer conn.FlushRuleset()
+
+	table := conn.AddTable(&nftables.Table{
+		Family: nftables.TableFamilyIPv4,
+		Name:   "test-table",
+	})
+
+	chain := conn.AddChain(&nftables.Chain{
+		Name:  "test-chain",
+		Table: table,
+	})
+
+	for range 4096 {
+		conn.AddRule(&nftables.Rule{
+			Table: table,
+			Chain: chain,
+			Exprs: []expr.Any{
+				&expr.Verdict{
+					Kind: expr.VerdictAccept,
+				},
+			},
+		})
+	}
+
+	if err := conn.Flush(); err != nil {
+		t.Fatalf("failed to flush: %v", err)
+	}
+}