ssh.reverse.tunnel.txt

# Run this in a blank GitHub Codespace


## set up folders

mkdir -p ssh_client ssh_server http_server


## ssh client

<<'eof' cat > ssh_client/Dockerfile
FROM ubuntu:24.04
RUN apt-get update && apt-get install -y openssh-client iputils-ping curl && rm -rf /var/lib/apt/lists/*
# Create user foobar
RUN useradd -m -s /bin/bash foobar
USER foobar
WORKDIR /home/foobar
RUN mkdir -p /home/foobar/.ssh && chown foobar:foobar /home/foobar/.ssh
CMD ["sleep", "infinity"]
eof


## ssh server

<<'eof' cat > ssh_server/Dockerfile 
FROM ubuntu:24.04
RUN apt-get update && apt-get install -y openssh-server curl && rm -rf /var/lib/apt/lists/*
RUN mkdir /var/run/sshd
# Create user foobar
RUN useradd -m -s /bin/bash foobar
# Configure SSH for GatewayPorts (required for the tunnel to work)
RUN echo "GatewayPorts yes" >> /etc/ssh/sshd_config
# Ensure the user has a .ssh directory with correct ownership
RUN mkdir -p /home/foobar/.ssh && chown foobar:foobar /home/foobar/.ssh
CMD ["/usr/sbin/sshd", "-D"]
eof


## http server

<<'eof' cat > http_server/Dockerfile 
FROM ubuntu:24.04
RUN apt-get update && apt-get install -y nginx && rm -rf /var/lib/apt/lists/*
CMD ["nginx", "-g", "daemon off;"]
eof


## docker compose

<<'eof' cat > docker-compose.yml
services:
  container_a:
    build: ./http_server
    container_name: container_a
    networks:
      - front-tier

  container_b:
    build: ./ssh_client
    container_name: container_b
    networks:
      - front-tier
      - back-tier

  container_c:
    build: ./ssh_server
    container_name: container_c
    networks:
      - back-tier

networks:
  front-tier:
  back-tier:
eof


## Generate keys

ssh-keygen -t rsa -b 4096 -f ./id_rsa -N ""
chmod 600 ./id_rsa


## Start containers

docker compose up -d


## Get Container C's IP

C_IP=$(docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' container_c)
echo ${C_IP}


## Copy keys to Container C (Authorized Keys for login)

docker cp ./id_rsa.pub container_c:/home/foobar/.ssh/authorized_keys
docker exec container_c chown foobar:foobar /home/foobar/.ssh/authorized_keys
docker exec container_c chmod 600 /home/foobar/.ssh/authorized_keys

## Copy keys to Container B (Private key to initiate reverse tunnel)

docker cp ./id_rsa container_b:/home/foobar/.ssh/id_rsa
docker exec -u root container_b chown foobar:foobar /home/foobar/.ssh/id_rsa
docker exec container_b chmod 600 /home/foobar/.ssh/id_rsa


## ssh from B to C

docker exec -u foobar container_b \
  ssh -i /home/foobar/.ssh/id_rsa \
    -N -f -R 8080:container_a:80 \
    foobar@container_c -o StrictHostKeyChecking=no


## ssh from host to C

ssh -i ./id_rsa -N -L 8080:localhost:8080 foobar@$C_IP -o StrictHostKeyChecking=no