Jumbo in the size mix (MTU 9000 + module jumbo pinned), check-first module config trusting GET reads, cable diag async at startup through the reset path, MODULES table dissolved into SETTINGS/LINKS, per-driver NIC counter sets, comment cleanup

system.go

@@ -214,6 +214,38 @@ type flagsIfreq struct {
 	_     [22]byte
 }
 
+type mtuIfreq struct {
+	name [unix.IFNAMSIZ]byte
+	mtu  int32
+	_    [20]byte
+}
+
+func checkMTU(fd int, ifname string, want int32) checkResult {
+	res := checkResult{item: ifname + " mtu"}
+	var ifr mtuIfreq
+	copy(ifr.name[:], ifname)
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, uintptr(fd),
+		uintptr(unix.SIOCGIFMTU), uintptr(unsafe.Pointer(&ifr))); errno != 0 {
+		res.err = errno
+		return res
+	}
+	if ifr.mtu == want {
+		res.state = fmt.Sprintf("%d", want)
+		return res
+	}
+	was := ifr.mtu
+	ifr.mtu = want
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, uintptr(fd),
+		uintptr(unix.SIOCSIFMTU), uintptr(unsafe.Pointer(&ifr))); errno != 0 {
+		res.err = errno
+		res.state = fmt.Sprintf("could not set %d", want)
+		return res
+	}
+	res.fixed = true
+	res.state = fmt.Sprintf("was %d, now %d (link reset)", was, want)
+	return res
+}
+
 type ethtoolRingparam struct {
 	cmd               uint32
 	rxMaxPending      uint32
@@ -258,6 +290,7 @@ const (
 	wantRxRing        = 8160
 	wantTxRing        = 4096
 	wantIdleLatency   = 10
+	wantMTU           = 9000
 )
 
 type checkResult struct {
@@ -714,6 +747,7 @@ func configureSystem(ifnames []string, ethertypes []uint16) []checkResult {
 		out := []checkResult{checkGovernor(wantGovernor), checkIdleLatency(wantIdleLatency)}
 		for _, ifname := range ifnames {
 			out = append(out, checkLinkUp(fd, ifname))
+			out = append(out, checkMTU(fd, ifname, wantMTU))
 			out = append(out, clearFlowRules(fd, ifname))
 
 			// Ring changes reprogram the queues, so flow rules pointing at those