From: "Maciej Żenczykowski" maze@google.com
[ Upstream commit 02a1b175b0e92d9e0fa5df3957ade8d733ceb6a0 ]
Documentation/networking/ip-sysctl.txt:46 says: ip_forward_use_pmtu - BOOLEAN By default we don't trust protocol path MTUs while forwarding because they could be easily forged and can lead to unwanted fragmentation by the router. You only need to enable this if you have user-space software which tries to discover path mtus by itself and depends on the kernel honoring this information. This is normally not the case. Default: 0 (disabled) Possible values: 0 - disabled 1 - enabled
Which makes it pretty clear that setting it to 1 is a potential security/safety/DoS issue, and yet it is entirely reasonable to want forwarded traffic to honour explicitly administrator configured route mtus (instead of defaulting to device mtu).
Indeed, I can't think of a single reason why you wouldn't want to. Since you configured a route mtu you probably know better...
It is pretty common to have a higher device mtu to allow receiving large (jumbo) frames, while having some routes via that interface (potentially including the default route to the internet) specify a lower mtu.
Note that ipv6 forwarding uses device mtu unless the route is locked (in which case it will use the route mtu).
This approach is not usable for IPv4 where an 'mtu lock' on a route also has the side effect of disabling TCP path mtu discovery via disabling the IPv4 DF (don't frag) bit on all outgoing frames.
I'm not aware of a way to lock a route from an IPv6 RA, so that also potentially seems wrong.
Signed-off-by: Maciej Żenczykowski maze@google.com Cc: Eric Dumazet edumazet@google.com Cc: Willem de Bruijn willemb@google.com Cc: Lorenzo Colitti lorenzo@google.com Cc: Sunmeet Gill (Sunny) sgill@quicinc.com Cc: Vinay Paradkar vparadka@qti.qualcomm.com Cc: Tyler Wear twear@quicinc.com Cc: David Ahern dsahern@kernel.org Reviewed-by: Eric Dumazet edumazet@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/net/ip.h | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/include/net/ip.h +++ b/include/net/ip.h @@ -439,12 +439,18 @@ static inline unsigned int ip_dst_mtu_ma bool forwarding) { struct net *net = dev_net(dst->dev); + unsigned int mtu;
if (net->ipv4.sysctl_ip_fwd_use_pmtu || ip_mtu_locked(dst) || !forwarding) return dst_mtu(dst);
+ /* 'forwarding = true' case should always honour route mtu */ + mtu = dst_metric_raw(dst, RTAX_MTU); + if (mtu) + return mtu; + return min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU); }