Finding out the hosts blocked by mysql server
Noted in 5.6.5 changelog.

MySQL now provides improved access to the host cache, which contains
client host name and IP address information and is used to avoid DNS
lookups. These improvements have been implemented:


A host_cache Performance Schema table exposes the contents of the
host cache so that it can be examined using SELECT statements. The
Performance Schema must be enabled or this table is empty.

If you upgrade to this release of MySQL from an earlier version, you
must run mysql_upgrade (and restart the server) to incorporate this
change into the performance_schema database.
The cache size is configurable using the host_cache_size system
variable. Setting the size to 0 disables the host cache.This is
similar to starting the server with --skip-host-cache, but
host_cache_size is more flexible because it can also be used to
resize, enable, or disable the host cache at runtime, not just at
server startup. If you start the server with --skip-host-cache to
disable the host cache, it cannot be re-enabled at runtime.
There are Connection_errors_xxx status variables that provide
information about the nature of connection errors and that can help
diagnose connection problems.


Improved access to host cache contents makes it possible to answer
questions such as how many hosts are cached, or how close hosts are
to being blocked (by checking whether the
host_cache.SUM_CONNECT_ERRORS column is approaching the value of the
max_connect_errors system variable).