Is it possible to change the DNS TTL of embedded DNS server?

[outsinre]

With Docker Desktop on macOS, I have launched multiple containers via Docker Compose. Those containers can communicate with each other.

Within one of the containers, I can see the default DNS server is 127.0.0.11 as follows.

root@0087d71929bd:/data# cat /etc/resolv.conf
nameserver 127.0.0.11
options ndots:0

When I tried to dig another container (e.g. postgres), the TTL is 600 as shown below.

root@0087d71929bd:/data# dig postgres

; <<>> DiG 9.18.19-1~deb12u1-Debian <<>> postgres
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50693
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;postgres.                      IN      A

;; ANSWER SECTION:
postgres.               600     IN      A       172.18.0.5

;; Query time: 1 msec
;; SERVER: 127.0.0.11#53(127.0.0.11) (UDP)
;; WHEN: Mon Feb 05 14:26:45 UTC 2024
;; MSG SIZE  rcvd: 50

I know that containerd has an embedded DNS server. Can we configure the TTL value? And how?

[ndeloof]

internal DNS service is implemented by Docker engine and can't be configured by Compose. Please ask on https://github.com/moby/moby

Can you please clarify the reason you want to tweak DNS TTL ?

[outsinre]

@ndeloof the current DNS TTL value is fixed to 600s (5m). Container A connects to container B periodically at an interval of 2s, and suddenly container B changes its IP address (crashed and recreated with a new IP). For 600s, container A cannot connect to container B any longer due to the cached deprecated IP address.

If we can tweak the DNS TTL to a lower value, container A can recover the connection to container B quickly.

[ndeloof]

For this scenario, a possible workaround is to access the other service trough a proxy, which can manage load-balancing and new instance discovery, see https://medium.com/@aedemirsen/load-balancing-with-docker-compose-and-nginx-b9077696f624. nginx used in this example can be configured as resolver 127.0.0.11 valid=5s; to discover available replicas with a lower TTL

[outsinre]

@ndeloof brilliant reference.

For future reference:

1. https://nginx.org/en/docs/http/ngx_http_core_module.html#resolver
2. https://nginx.org/en/docs/stream/ngx_stream_upstream_module.html#resolver

By default, nginx caches answers using the TTL value of a response. An optional valid parameter allows overriding it:

resolver 127.0.0.1 [::1]:5353 valid=30s;