DNS Lookups in Kubernetes
One of the primary advantages of deploying workloads in Kubernetes is seamless application discovery. Intra cluster communication becomes easy with the concept of Service which represents a Virtual IP backing a set of Pod IPs. For example, if
vanilla service needs to talk to a
chocolate service, it can directly use the Virtual IP for
chocolate. Now the question is who resolves the DNS query for
chocolate and how?
DNS resolution is configured in Kubernetes cluster through CoreDNS. The kubelet configures each Pod's
/etc/resolv.conf to use the coredns pod as the
nameserver. You can see the contents of
/etc/resolv.conf inside any pod, they'll look something like:
search hello.svc.cluster.local svc.cluster.local cluster.local nameserver 10.152.183.10 options ndots:5
This config is used by DNS clients to forward the DNS queries to a DNS server.
resolv.conf is the resolver configuration file which has information about:
nameserver : Where the DNS queries are forwarded to. In our case this is the address of
search: Represents the search path for a particular domain. Interestingly
mrkaran.devis not FQDN (fully qualified domain name). A standard convention that most DNS resolvers follow is that if a domain ends with
.(representing the root zone), the domain is considered to be FQDN. Some resolvers try to act smart and append the
mrkaran.dev.is an FQDN but
ndots: This is the most interesting value and is the highlight of this post.
ndotsrepresents the threshold value of the number of dots in a query name to consider it as a "fully qualified" domain name. More on this later, as we discover the flow of DNS lookup.
Let's check what happens when we query for
mrkaran.dev in a pod.
$ nslookup mrkaran.dev Server: 10.152.183.10 Address: 10.152.183.10#53 Non-authoritative answer: Name: mrkaran.dev Address: 126.96.36.199 Name: mrkaran.dev Address: 2400:6180:0:d1::519:6001
For this experiment, I've also turned on CoreDNS logging level to
all which makes it highly verbose. Let's look at the logs of
[INFO] 10.1.28.1:35998 - 11131 "A IN mrkaran.dev.hello.svc.cluster.local. udp 53 false 512" NXDOMAIN qr,aa,rd 146 0.000263728s [INFO] 10.1.28.1:34040 - 36853 "A IN mrkaran.dev.svc.cluster.local. udp 47 false 512" NXDOMAIN qr,aa,rd 140 0.000214201s [INFO] 10.1.28.1:33468 - 29482 "A IN mrkaran.dev.cluster.local. udp 43 false 512" NXDOMAIN qr,aa,rd 136 0.000156107s [INFO] 10.1.28.1:58471 - 45814 "A IN mrkaran.dev. udp 29 false 512" NOERROR qr,rd,ra 56 0.110263459s [INFO] 10.1.28.1:54800 - 2463 "AAAA IN mrkaran.dev. udp 29 false 512" NOERROR qr,rd,ra 68 0.145091744s
Whew. So 2 things piqued my interest here:
The query iterates through all search paths until the answer contains a
NOERRORcode (which the DNS clients understand and store it as the result).
NXDOMAINsimply indicates no record found for that domain name. Since
mrkaran.devisn't an FQDN (according to
ndots=5setting), the resolver looks at search path and determines the order of query.
AAAArecords are fired parallelly. This is because
/etc/resolv.confhas a default configuration to perform parallel IPv4 and IPv6 lookups. You can disable this using
glibc can be configured to send these requests sequentially but
musl cannot, so Alpine users must take note.
Playing around with ndots
Let's play around with
ndots a bit more and see how it behaves. The idea is simple, for the DNS client to know whether a domain is an absolute one or not is through this
ndots setting. For example, if you query for
1, the DNS client will say "oh,
google.com, the search list will be completely ignored since the query name satisfies the
ndots threshold (At least one dot).
We can see this by actually doing it:
$ cat /etc/resolv.conf options ndots:1
$ nslookup mrkaran Server: 10.152.183.10 Address: 10.152.183.10#53 ** server can't find mrkaran: NXDOMAIN
[INFO] 10.1.28.1:52495 - 2606 "A IN mrkaran.hello.svc.cluster.local. udp 49 false 512" NXDOMAIN qr,aa,rd 142 0.000524939s [INFO] 10.1.28.1:59287 - 57522 "A IN mrkaran.svc.cluster.local. udp 43 false 512" NXDOMAIN qr,aa,rd 136 0.000368277s [INFO] 10.1.28.1:53086 - 4863 "A IN mrkaran.cluster.local. udp 39 false 512" NXDOMAIN qr,aa,rd 132 0.000355344s [INFO] 10.1.28.1:56863 - 41678 "A IN mrkaran. udp 25 false 512" NXDOMAIN qr,rd,ra 100 0.034629206s
mrkaran didn't specify any
. so the search list was used to find the answer.
ndots value is silently capped to
15 and is
5 in Kubernetes as default.
Handling this in Production
If your application is of the nature that makes a lot of external network calls, the DNS can become a bottleneck in case of heavy traffic since a lot of extra queries are made before the real DNS query is even fired. It's quite uncommon to see applications appending the root zone in the domain names, but that can be considered as a hack. So instead of using
api.twitter.com, you can hardcode your application to include
api.twitter.com. which would force the DNS clients to do an authoritative lookup directly on the absolute domain.
Alternatively, since K8s 1.14, the
dnsPolicy feature gates have become stable. So while deploying a pod you can specify
ndots setting to something lesser, say
3 or if you want to be really aggressive you can turn it down to
1. The consequences of this will be that every intra-node communication now has to include the full domain. This is one of the classic tradeoffs where you have to choose between performance and portability. If the app doesn't demand super low latencies, I guess you need not worry about this at all since DNS results are cached internally too.
I got to know about this peculiarity first, in a K8s meetup which I went to, last weekend where the folks mentioned about having to deal with this.
Here are some additional links you can read on the web:
- Explainer on why ndots=5 in kubernetes
- Great read on how ndots affects application performance
- musl and glibc resolver inconsistencies
Note: I'm particularly not using
dig in this post.
dig apparently automatically adds a
. (root zone identifier) to make the domain an FQDN one without even first going through the search path. I've mentioned about this briefly in one of my older posts. Nonetheless, it's quite surprising to see that you need to give a flag to make it behave in what seems to be a standard way.
It's always DNS, isn't it ;)
In case you want to play around with
ndots and DNS more, I've worked on a DNS Client which lets you tweak these params on the fly. Feel free to checkout doggo if interested!