Reduce advertised ipv6 mtu by 16 to fit ethernet header
This is a temporary hack to workaround the inability of current
kernel's ebpf bpf_skb_change_mode() function to prefix a 14-byte
ethernet header on to a packet without going over the upstream
(source, rawip) interface's mtu *before* we bpf_redirect() to
the downstream (destination, ethernet) interface.
Test: build, atest, atest TetheringTests
Bug: 149816401
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I646148ebfd978a2489c0cd065e4b671b01150add
diff --git a/Tethering/src/android/net/ip/IpServer.java b/Tethering/src/android/net/ip/IpServer.java
index 2653b6d..96da8f5 100644
--- a/Tethering/src/android/net/ip/IpServer.java
+++ b/Tethering/src/android/net/ip/IpServer.java
@@ -541,7 +541,12 @@
if (v6only != null) {
params = new RaParams();
- params.mtu = v6only.getMtu();
+ // We advertise an mtu lower by 16, which is the closest multiple of 8 >= 14,
+ // the ethernet header size. This makes kernel ebpf tethering offload happy.
+ // This hack should be reverted once we have the kernel fixed up.
+ // Note: this will automatically clamp to at least 1280 (ipv6 minimum mtu)
+ // see RouterAdvertisementDaemon.java putMtu()
+ params.mtu = v6only.getMtu() - 16;
params.hasDefaultRoute = v6only.hasIpv6DefaultRoute();
if (params.hasDefaultRoute) params.hopLimit = getHopLimit(v6only.getInterfaceName());