Because the global cookie threshold is higher than the per-IP block
threshold, it was previously possible for an attacker to block a legitimate
user by sending spoofed IKE_SA_INIT packets from that user's IP.
The timespan for requiring cookies is now also not extended anymore with
every IKE_SA_INIT received during the calm down period. Because this
allowed an attacker, after initially triggering the global cookie threshold,
to force cookies for all clients by sending just a single spoofed
IKE_SA_INIT every 10 seconds.
We keep track of reaching the per-IP threshold in segments of the hashed
IP addresses, so only a (random, due to chunk_hash()'s random key) subset
of clients will receive cookies, if single IPs are targeted.
The default global threshold is increased a bit.
Tobias Brunner
d8104b7c69