Free secondary DNS services:

Redundancy is the key to reliability. You need two nameservers minimum, but more than seven is considered gluttonise.

www.everydns.net

• Up to 20 domains for free, unlimited for $15 donation.
• Four geographically diverse (within the USA) servers.
• Individual servers have some down time, but overall they are very reliable.
• Does not use the notify funtionality. Polls the master server once an hour.
• does not allow outgoing axfr transfers for secondary zones.
• review
• Twitter updates
• Recommended.
• DJ Bernstein TinyDNS 1.05

puck.nether.net/dns

• One server in Chicago.
• Secondary DNS only.
• unlimited domains.
• Does not use the notify funtionality.
• does not allow axfr transfers from puck.nether.net.
• Recommended.
• BIND 9.2.3rc1 -- 9.4.0a0

www.zoneedit.com

• 5 domains free.
• 2 nameservers for free, more if you pay (they have 19).
• Once a domain is in their system it counts against your 5 maximum forever, even if you delete it.
• Does not use the notify funtionality.
• axfr transfers can be restricted to specific IP addresses.
• review
• Recommended.
• BIND 8.3.0-RC1 -- 8.4.4

freedns.afraid.org/secondary

• One server in Illinois.
• Uses notify function for instant updating when you make changes.
• BIND 9.2.3rc1 -- 9.4.0a0

xname.org

• Two servers on different networks, both in France.
• Unlimited domains.
• Does not use the notify funtionality.
• BIND 9.2.3rc1 -- 9.4.0a0

www.twisted4life.com

• 10 domains free
• One server, ns1.twisted4life.com, in Malaysia or Singapore.
• Does not accept TCP connections.
• DNS configuration checker
• Not the most reliable.
• DJ Bernstein TinyDNS 1.04

Other DNS services (no secondary DNS):

free.editdns.net

• 2 servers
• Five domains maximum.
• No secondary DNS, primary only

OpenDNS.com

• Two servers.
• They are "forwarders", not "slaves" (caching DNS servers, no zone transfers)

DNSexit.com

• four servers
• does NOT work as a slave DNS.

MiniDNS.net

• two servers
• can not be used as a slave DNS.

web-dns.co.uk

• four servers
• can not be used as a slave DNS.

Other useful DNS links:

• ns2exchange.com A list of folks offering to exchange secondary DNS services.
• Online DNS tools:
  • dnscheck @ squish.net
  • registered nameserver lookup tool
  • DNS configuration checker
  • dnsreport.com
• DNS servers:
  • nsd (name server daemon)
  • maradns
• Some shell scripts I wrote:
  • dnsreport - check the SOA serial numbers at all of the nameservers for a domain.
  • dnscheck - check the SOA serial numbers at all of the nameservers for a list of domains. Emails a report if a NS doesn't respond or has an out of date serial number. Suitable for running from cron as a monitoring script.