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net: ipv4: don't allow setting net.ipv4.route.min_pmtu below 68
According to RFC 1191 sections 3 and 4, ICMP frag-needed messages indicating an MTU below 68 should be rejected: A host MUST never reduce its estimate of the Path MTU below 68 octets. and (talking about ICMP frag-needed's Next-Hop MTU field): This field will never contain a value less than 68, since every router "must be able to forward a datagram of 68 octets without fragmentation". Furthermore, by letting net.ipv4.route.min_pmtu be set to negative values, we can end up with a very large PMTU when (-1) is cast into u32. Let's also make ip_rt_min_pmtu a u32, since it's only ever compared to unsigned ints. Reported-by: Jianlin Shi <[email protected]> Signed-off-by: Sabrina Dubroca <[email protected]> Reviewed-by: Stefano Brivio <[email protected]> Signed-off-by: David S. Miller <[email protected]>
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