I have a server running on localhost:3440 on the host machine. In a Dockerfile I have:

RUN apt-get install -y netcat
RUN nc localhost 3440 | tar -x -O > ores-$(date +%s.%N).tgz

I've read that using --net="host" with docker run will allow a running container to connect to a server running on the host. But my question is - when building an image, how can I connect to a server on localhost?

The error I get is simply:

localhost [] 3440 (?) : Connection refused

anyone know what I might be doing wrong?

  • 2
    You have to change localhost to the host machine IP address as nc is run inside the container localhost will refer to the container local address
    – storm
    May 15, 2018 at 8:23

1 Answer 1


First you have to start netcat listening in the background. $ netcat -l 3440 &

Then build your Dockerfile.

# Ubuntu Dockerfile
# https://github.com/dockerfile/ubuntu

# Pull base image.
FROM ubuntu:14.04

# Install.
  sed -i 's/# \(.*multiverse$\)/\1/g' /etc/apt/sources.list && \
  apt-get update && \
  apt-get -y upgrade && \
  apt-get install -y build-essential && \
  apt-get install -y software-properties-common && \
  apt-get install -y byobu curl git htop man unzip vim wget && \
  rm -rf /var/lib/apt/lists/*

# Set environment variables.
ENV HOME /root

# Define working directory.
RUN apt-get install -y netcat
RUN nc localhost 3440 | tar -x -O > ores-$(date +%s.%N).tgz
# Define default command.
CMD ["bash"]

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