Phoenix app w/ extra service on port 53

Phoenix
, ,
1

Howdy! I have a phoenix app that also runs a service on port 53 (DNS server) but I’m getting an {:error, :eaccess} when I actually try to get my service to bind to the port. Is my fly.toml or dockerfile wrong?

Also, if helpful, I resolve fly-global-services to an IP in the actual :gen_udp here → elixir-dns/lib/dns/server.ex at efadb43a4c0716eee8b29419f60f893a6ab79816 · mheld/elixir-dns · GitHub

fly.toml:

app = 'xxx'
kill_signal = 'SIGTERM'

[build]

[deploy]
release_command = '/app/bin/migrate'

[env]
PHX_HOST = 'xxx.fly.dev'
PORT = '8080'

[http_service]
internal_port = 8080
force_https = true
auto_stop_machines = 'stop'
auto_start_machines = true
min_machines_running = 0
processes = ['app']

[http_service.concurrency]
type = 'connections'
hard_limit = 1000
soft_limit = 1000

[[services]]
protocol = 'udp'
internal_port = 53

[[services.ports]]
port = 53

[[vm]]
size = 'shared-cpu-2x'

Dockerfile:

# Find eligible builder and runner images on Docker Hub. We use Ubuntu/Debian
# instead of Alpine to avoid DNS resolution issues in production.
#
# https://hub.docker.com/r/hexpm/elixir/tags?page=1&name=ubuntu
# https://hub.docker.com/_/ubuntu?tab=tags
#
# This file is based on these images:
#
#   - https://hub.docker.com/r/hexpm/elixir/tags - for the build image
#   - https://hub.docker.com/_/debian?tab=tags&page=1&name=bullseye-20241111-slim - for the release image
#   - https://pkgs.org/ - resource for finding needed packages
#   - Ex: hexpm/elixir:1.17.3-erlang-27.1.2-debian-bullseye-20241111-slim
#
ARG ELIXIR_VERSION=1.17.3
ARG OTP_VERSION=27.1.2
ARG DEBIAN_VERSION=bullseye-20241111-slim

ARG BUILDER_IMAGE="hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION}-debian-${DEBIAN_VERSION}"
ARG RUNNER_IMAGE="debian:${DEBIAN_VERSION}"

FROM ${BUILDER_IMAGE} as builder

# install build dependencies
RUN apt-get update -y && apt-get install -y build-essential git \
  && apt-get clean && rm -f /var/lib/apt/lists/*_*

# prepare build dir
WORKDIR /app

# install hex + rebar
RUN mix local.hex --force && \
  mix local.rebar --force

# set build ENV
ENV MIX_ENV="prod"

# install mix dependencies
COPY mix.exs mix.lock ./
RUN mix deps.get --only $MIX_ENV
RUN mkdir config

# copy compile-time config files before we compile dependencies
# to ensure any relevant config change will trigger the dependencies
# to be re-compiled.
COPY config/config.exs config/${MIX_ENV}.exs config/
RUN mix deps.compile

COPY priv priv

COPY lib lib

COPY assets assets

# compile assets
RUN mix assets.deploy

# Compile the release
RUN mix compile

# Changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/

COPY rel rel
RUN mix release

# start a new build stage so that the final image will only contain
# the compiled release and other runtime necessities
FROM ${RUNNER_IMAGE}

RUN apt-get update -y && \
  apt-get install -y libstdc++6 openssl libncurses5 locales ca-certificates \
  && apt-get clean && rm -f /var/lib/apt/lists/*_*

# Set the locale
RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen

ENV LANG en_US.UTF-8
ENV LANGUAGE en_US:en
ENV LC_ALL en_US.UTF-8

WORKDIR "/app"
RUN chown nobody /app

# set runner ENV
ENV MIX_ENV="prod"

# Only copy the final release from the build stage
COPY --from=builder --chown=nobody:root /app/_build/${MIX_ENV}/rel/corridor ./

USER nobody

# If using an environment that doesn't automatically reap zombie processes, it is
# advised to add an init process such as tini via `apt-get install`
# above and adding an entrypoint. See https://github.com/krallin/tini for details
# ENTRYPOINT ["/tini", "--"]

EXPOSE 53/udp
EXPOSE 53/tcp

CMD ["/app/bin/server"]
3

Hi… On Linux, you need superuser rights (i.e., root) to bind to port 53.

More generally… UDP is hard-mode Fly.io, so I would suggest minimizing use of privileged ports, auto-stop, etc., at first. Once you have something basic working, then progressively lock things down more and more, :cactus:.

(Maybe even try the super-simple Netcat test, at first.)

Hope this helps a little!

4

it was exactly the USER nobody issue – changed it to USER root and all was good. Will investigate using like authbind or something to make this less scary but thank you!

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