++blog / nat64 kubernetes
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Nico Schottelius
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title: Automatic A and AAAA DNS entries with NAT64 for kubernetes?
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pub_date: 2021-06-24
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author: ungleich
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twitter_handle: ungleich
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_hidden: no
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_discoverable: yes
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---
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abstract:
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Given a kubernetes cluster and NAT64 - how do you create DNS entries?
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---
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body:
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## The DNS kubernetes quiz
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Today our blog entry does not (yet) show a solution, but more a tricky
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quiz on creating DNS entries. The problem to solve is the following:
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* How to make every IPv6 only service in kubernetes also IPv4
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reachable?
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Let's see who can solve it first or the prettiest. Below are some
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thoughts on how to approach this problem.
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## The situation
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Assume your kubernetes cluster is IPv6 only and all services
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have proper AAAA DNS entries. This allows you
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[to directly receive traffic from the
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Internet](/u/blog/kubernetes-without-ingress/) to
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your kubernetes services.
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Now to make that service also IPv4 reachable, we can deploy NAT64
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service that maps an IPv4 address outside the cluster to an IPv6 service
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address inside the cluster:
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```
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A.B.C.D --> 2001:db8::1
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```
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So all traffic to that IPv4 address is converted to IPv6 by the
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external NAT64 translator.
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## The proxy service
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Let's say the service running on 2001:db8::1 is named "ipv4-proxy" and
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thus reachable at ipv4-proxy.default.svc.example.com.
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What we want to achieve is to expose every possible service
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inside the cluster **also via IPv4**. For this purpose we have created
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an haproxy container that access *.svc.example.com and forwards it via
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IPv6.
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So the actual flow would look like:
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```
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IPv4 client --[ipv4]--> NAT64 -[ipv6]-> proxy service
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v
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IPv6 client ---------------------> kubernetes service
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```
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## The DNS dilemma
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It would be very tempting to create a wildcard DNS entry or to
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configure/patch CoreDNS to also include an A entry for every service
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that is:
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```
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*.svc IN A A.B.C.D
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```
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So essentially all services resolve to the IPv4 address A.B.C.D. That
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however would also influence the kubernetes cluster, as pods
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potentially resolve A entries (not only AAAA) as well.
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As the containers / pods do not have any IPv4 address (nor IPv4
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routing), access to IPv4 is not possible. There are various outcomes
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of this situation:
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1. The software in the container does happy eyeballs and tries both
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A/AAAA and uses the working IPv6 connection.
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2. The software in the container misbehaves and takes the first record
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and uses IPv4 (nodejs is known to have or had a broken resolver
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that did exactly that).
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So adding that wildcard might not be the smartest option. And
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additionally it is unclear whether coreDNS would support that.
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## Alternative automatic DNS entries
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The *.svc names in a kubernetes cluster are special in the sense that
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they are used for connecting internally. What if coreDNS (or any other
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DNS) server would instead of using *.svc, use a second subdomain like
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*abc*.*namespace*.v4andv6.example.com and generate the same AAAA
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record as for the service and a static A record like describe above?
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That could solve the problem. But again, does coreDNS support that?
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## Automated DNS entries in other zones
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Instead of fully automated creating the entries as above, another
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option would be to specify DNS entries via annotations in a totally
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different zone, if coreDNS was supporting this. So let's say we also
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have control over example.org and we could instruct coreDNS to create
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the following entries automatically with an annotation:
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```
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abc.something.example.org AAAA <same as the service IP>
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abc.something.example.org A <a static IPv4 address A.B.C.D>
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```
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In theory this might be solved via some scripting, maybe via a DNS
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server like powerDNS?
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## Other solution?
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As you can see, mixing the dynamic IP generation and coupling it with
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static DNS entries for IPv4 resolution is not the easiest tasks. If
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you have a smart idea on how to solve this without manually creating
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entries for each and every service,
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[give us a shout!](/u/contact)
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