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Subject[Regression w/ patch] Restore network resistance to weird ICMP messages
Handle weird ICMP fragmentation needed messages with next hop MTU
equal to (or exceeding) dropped packet size

Fixes: 46517008e116 ("ipv4: Kill ip_rt_frag_needed().")

In a large corporate network, we spotted this weird ICMP message after
a long troubleshooting. See attached capture file. Those ICMP "network
unreachable - fragmentation needed and don't fragment bit set"
messages are sent by a router that drop 1500 bytes IP packets and fill
the next hop MTU ICMP field with 1500.

Those messages cause the TCP connection to stall but only on newer
kernels. Older kernels set path MTU to 1492 and communicates

After checking code and commit history, I spotted how commit
46517008e116 ("ipv4: Kill ip_rt_frag_needed().") from June 2012
changed ICMP messages handling by removing ip_rt_frag_needed function.

The relevant part of the ip_rt_frag_needed function that was removed is:

if (new_mtu < 68 || new_mtu >= old_mtu) {
/* BSD 4.2 derived systems incorrectly adjust
* tot_len by the IP header length, and report
* a zero MTU in the ICMP message.
if (mtu == 0 &&
old_mtu >= 68 + (iph->ihl << 2))
old_mtu -= iph->ihl << 2;
mtu = guess_mtu(old_mtu);

This condition handled the cases when next hop MTU where zero (less
than 68). Now this is handled by the protocol and fixed by commit
68b7107b6298 "ipv4: icmp: Fix pMTU handling for rare case".

But the rarest case when (next hop MTU) new_mtu >= old_mtu (dropped
packet length) was also removed. This commit restores this check.
Instead of using a table lookup like function guess_mtu uses, it just
try to set the path MTU decrementing by 2 bytes the dropped packet

In our case, setting the path MTU to just 1498 (one iteration) worked.
This solution should converge in any case to a good value by small
steps. I don't think there's a need to a more complex solution.

The patched kernel worked perfectly setting the path MTU to 1498 from
the initial default interface value of 1500. This time I don't have a
capture file from inside the affected center, but all received packed
had a maximum size of 1498.

[unhandled content-type:application/octet-stream][unhandled content-type:application/octet-stream]
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