Just a heads up, Zerigo doesn't operate its own network for DNS, so as far as mitigating an attack they are at the mercy of upstreams.

a.ns.zerigo.net 64.27.57.11, announced by WeHostWebSites.com

b.ns.zerigo.net 174.37.229.229, SoftLayer

c.ns.zerigo.net 109.74.192.232, Linode

d.ns.zerigo.net 174.36.24.250, Softlayer

e.ns.zerigo.net 72.26.219.150, Voxel.net (Internap)

f.ns.zerigo.net 223.27.170.242, Voxel.net (Internap)