On 9/23/20 6:51 AM, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <m...@google.com>
>
> 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 <m...@google.com>
> Cc: Eric Dumazet <m...@google.com>
Note that my email address is more like : <eduma...@google.com>
> Cc: Willem de Bruijn <will...@google.com>
> Cc: Lorenzo Colitti <lore...@google.com>
> Cc: Sunmeet Gill (Sunny) <sg...@quicinc.com>
> Cc: Vinay Paradkar <vpara...@qti.qualcomm.com>
> Cc: Tyler Wear <tw...@quicinc.com>
> Cc: David Ahern <dsah...@kernel.org>
> ---
> include/net/ip.h | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/include/net/ip.h b/include/net/ip.h
> index b09c48d862cc..c2188bebbc54 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -436,12 +436,17 @@ static inline unsigned int
> ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
> 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;
if (mtu)
return mtu;
Apparently route mtu are capped to 65520, not sure where it is done exactly
IP_MAX_MTU being 65535)
# ip ro add 1.1.1.4 dev wlp2s0 mtu 100000
# ip ro get 1.1.1.4
1.1.1.4 dev wlp2s0 src 192.168.8.147 uid 0
cache mtu 65520
> +
> return min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU);
> }
>
>
