Multiple DNS records pointing to a single domain

a-recorddomain-name-system

Our business currently subscribes to a DSL internet connection. We're moving away from this and in fact will have two 3Mbps dedicated fiber connections run into our operations center in the coming weeks. The first fiber connection will be ready in 2 weeks, the second in 2 months.

My question is with regards to DNS records, specifically in publishing multiple DNS records that point to the same address. When the first fiber connection becomes available I will want to have our domain (www.example.com) point to both it and the old dsl service. I will be using a multi-wan router to handle both connections. When the second fiber connection is installed I will essentially do the same configuration as before, cutting the dsl conn out of the loop. I'm fairly new to DNS and am concerned as to how I might achieve this.

Best Answer

Just add multiple IN A records, they will be randomly served by your DNS server.

See Round robin DNS on Wikipedia

Related Solutions

DNS Round Robin – Is It the Only Way to Ensure Instant Fail-Over for Multiple Data Centers?

When I use the term "DNS Round Robin" I generally mean in in the sense of the "cheap load balancing technique" as OP describes it.

But that's not the only way DNS can be used for global high availability. Most of the time, it's just hard for people with different (technology) backgrounds to communicate well.

The best load balancing technique (if money is not a problem) is generally considered to be:

A Anycast'ed global network of 'intelligent' DNS servers,
and a set of globally spread out datacenters,
where each DNS node implements Split Horizon DNS,
and monitoring of availability and traffic flows are available to the 'intelligent' DNS nodes in some fashion,
so that the user DNS request flows to the nearest DNS server via IP Anycast,
and this DNS server hands out a low-TTL A Record / set of A Records for the nearest / best datacenter for this end user via 'intelligent' split horizon DNS.

Using anycast for DNS is generally fine, because DNS responses are stateless and almost extremely short. So if the BGP routes change it's highly unlikely to interrupt a DNS query.

Anycast is less suited for the longer and stateful HTTP conversations, thus this system uses split horizon DNS. A HTTP session between a client and server is kept to one datacenter; it generally cannot fail over to another datacenter without breaking the session.

As I indicated with "set of A Records" what I would call 'DNS Round Robin' can be used together with the setup above. It is typically used to spread the traffic load over multiple highly available load balancers in each datacenter (so that you can get better redundancy, use smaller/cheaper load balancers, not overwhelm the Unix network buffers of a single host server, etc).

So, is it true that, with multiple data centers and HTTP traffic, the use of DNS RR is the ONLY way to assure high availability?

No it's not true, not if by 'DNS Round Robin' we simply mean handing out multiple A records for a domain. But it's true that clever use of DNS is a critical component in any global high availability system. The above illustrates one common (often best) way to go.

Edit: The Google paper "Moving Beyond End-to-End Path Information to Optimize CDN Performance" seems to me to be state-of-the-art in global load distribution for best end-user performance.

Edit 2: I read the article "Why DNS Based .. GSLB .. Doesn't Work" that OP linked to, and it is a good overview -- I recommend looking at it. Read it from the top.

In the section "The solution to the browser caching issue" it advocates DNS responses with multiple A Records pointing to multiple datacenters as the only possible solution for instantaneous fail over.

In the section "Watering it down" near the bottom, it expands on the obvious, that sending multiple A Records is uncool if they point to datacenters on multiple continents, because the client will connect at random and thus quite often get a 'slow' DC on another continent. Thus for this to work really well, multiple datacenters on each continent are needed.

This is a different solution than my steps 1 - 6. I can't provide a perfect answer on this, I think a DNS specialist from the likes of Akamai or Google is needed, because much of this boils down to practical know-how on the limitations of deployed DNS caches and browsers today. AFAIK, my steps 1-6 are what Akamai does with their DNS (can anyone confirm this?).

My feeling -- coming from having worked as a PM on mobile browser portals (cell phones) -- is that the diversity and level of total brokeness of the browsers out there is incredible. I personally would not trust a HA solution that requires the end user terminal to 'do the right thing'; thus I believe that global instantaneous fail over without breaking a session isn't feasible today.

I think my steps 1-6 above are the best that are available with commodity technology. This solution does not have instantaneous fail over.

I'd love for one of those DNS specialists from Akamai, Google etc to come around and prove me wrong. :-)

DNS Records – How to List All DNS Records in a Domain Using Dig

Answer

The short answer to your specific question of listing CNAMEs is that you cannot without permission to do zone transfers (see How to list all CNAME records for a given domain?).

That said, if your company's DNS server still supports the ANY query, you can use dig to list the other records by doing:

dig +noall +answer +multiline yourdomain.yourtld any

These ... +noall +answer +multiline ... are strictly optional and are simply output formatting flags to make the output more easily human readable (see dig man page ).

Example

$ dig +noall +answer +multiline bad.horse any

Returns:

bad.horse.              7200 IN A 162.252.205.157
bad.horse.              7200 IN CAA 0 issue "letsencrypt.org"
bad.horse.              7200 IN CAA 0 iodef "mailto:abuse@sandwich.net"
bad.horse.              7200 IN MX 10 mx.sandwich.net.
bad.horse.              7200 IN NS a.sn1.us.
bad.horse.              7200 IN NS b.sn1.us.
bad.horse.              7200 IN SOA a.sn1.us. n.sn1.us. (
                                2017032202 ; serial
                                1200       ; refresh (20 minutes)
                                180        ; retry (3 minutes)
                                1209600    ; expire (2 weeks)
                                60         ; minimum (1 minute)
                                )

Caveats (RFC8482)

Note that, since around 2019, most public DNS servers have stopped answering most DNS ANY queries usefully. For background on that, see: https://blog.cloudflare.com/rfc8482-saying-goodbye-to-any/

If ANY queries do not enumerate multiple records, the only option is to request each record type (e.g. A, CNAME, or MX) individually.