RE: large organization nameservers sending icmp packets to dns servers.

All things being equal (which they're usually not) you could use the ACK response time of the TCP handshake if they've got TCP DNS resolution available. Though again most don't for security reasons... Then most are incredibly stupid.

Several anti DoS utilities force unknown hosts to initiate a query via TCP in order to be whitelisted. If the host can't perform a TCP query then they get blacklisted. In addition, any UDP truncated response needs to be retried via TCP- blocking it would cause a variety of problems. -Don