Issues
In practice, difficulties can arise when using EDNS traversing firewalls, since some firewalls assume a maximum DNS message length of 512 bytes and block longer DNS packets.
The introduction of EDNS made a type of Reflected Denial-of-Service attacks called DNS amplification feasible, since EDNS facilitates very large response packets compared to relatively small request packets.
The IETF DNS Extensions working group (dnsext) is working on a refinement of EDNS0, called rfc2671bis.
Read more about this topic: Extension Mechanisms For DNS
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