This is a simple Go program for serving plain text content over TCP, or UDP, connection, often used during CTF challenges. Options can be supplied either via command line arguments or environment variables. Can be used as a stand-alone program, or via supplied docker images.

By default flagserver will listen on port 9999 on 0.0.0.0, and wil serve content of /home/user/flag.txt to each new TCP connection.

Options can be provided either via command line arguments, or environment variables. Currently environment variables take precedence over CLI arguments (except for the content. And yes, it's backwards)

• --content or FLAG_SERVER_CONTENT - optional way to provide text flag via command line or an environment variable
• --file_path or FLAG_SERVER_FILE_PATH - path to a file containing text flag. Defaults to ~/flag.txt
• --host or FLAG_SERVER_HOST - host/IP used for listening to new traffic. Defaults to 0.0.0.0 (all traffic)
• --port or FLAG_SERVER_PORT - port to listen on. Defaults to a random port
• --protocol or FLAG_SERVER_PROTOCOL - use tcp, udp, or http. Defaults to tcp
• --http-path or FLAG_SERVER_HTTP_PATH - specify what HTTP path to listen on. Used only for http protocol, defaults to /

These options can be stored in .env for use with docker-compose

# name of the container
CONTAINER_NAME=flagserver
# path on the docker host system for the flag file
FLAG_EXTERNAL_FILE_PATH=./flag.txt
# this is location of the flag file inside the container
FLAG_SERVER_FILE_PATH=/home/user/flag.txt
# address to listen to
FLAG_SERVER_HOST=0.0.0.0
# port to listen on
FLAG_SERVER_PORT=9999
# specify which protocol to use: TCP (default), UDP, or HTTP
FLAG_SERVER_PROTOCOL=http
# specify what path should HTTP requests reply to, defaults to /
FLAG_SERVER_HTTP_PATH=/content

# instead of specifying paths for flags, one can use specify content via
FLAG_SERVER_CONTENT=supersecretword

$ cd main
$ go build && ./flagserver --content "foobar" --port 9999 --protocol http
2023/04/21 22:23:40 Starting new flag server instance
2023/04/21 22:23:40 Host: 0.0.0.0
2023/04/21 22:23:40 Port: 9999
2023/04/21 22:23:40 File path: ~/flag.txt
2023/04/21 22:23:40 Protocol: http
2023/04/21 22:23:40 Using content from command line: foobar