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I'm trying to write a C library that connects and talks to a robot which has a telnet server.

The server info is the following:

  • user: as
  • ip: 192.168.0.1
  • port: 23
  • it has no password

In the robot_init() function I'd like to connect to the robot, and I'd expect the following code to do it:

FILE *telnet = popen("telnet -l as 192.168.0.1 23", "w");

From man telnet:

SYNOPSIS telnet [-468ELadr] [-S tos] [-b address] [-e escapechar] [-l user] [-n tracefile] [host [port]] [...] -l user Specify user as the user to log in as on the remote system. This is accomplished by sending the specified name as the USER envi‐ ronment variable, so it requires that the remote system support the TELNET NEW-ENVIRON option. This option implies the -a option, and may also be used with the open command.

However, before writing the C code, I tried to set up a telnet server in a docker container and talk to it through another docker as a client.

To run the server:

sudo docker container run --name telnet-server --publish 2323:23 --detach --restart unless-stopped secobau/telnetd:alpine-1.1

This server has user = user; ip = 172.17.0.1; port = 2323; and no password.

To run the client:

sudo docker run --interactive --tty --name=telnet-client debian:testing bash apt-get update apt-get install telnet --yes telnet -l user 172.17.0.1 2323

I'd expect this to give me an already logged in telnet conection (or at least ask me for the password directly), but instead it keeps asking for a login.

Is it a bug in telnet? How should I automate this?

If telnet -l doesn't work I think I'll have to write through the pipe something like:

fprintf(telnet, "as\n"); // user fprintf(telnet, "\n"); // password (no password)
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  • apt-get install telnet --yes results in E: Unable to locate package telnet Commented Feb 24, 2020 at 14:43
  • Ups, I forgot to copy the apt-get update line Commented Feb 24, 2020 at 14:45

1 Answer 1

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Interesting question, it shows how easy it is to quickly setup several different services completely isolated from each other with Docker.

I think that telnetd still asks for the password because it's stared with -l /bin/login on the server side as you can see in the Dockerfile for secobau/telnetd:alpine-1.1:

ARG LOGIN=/bin/login ARG PORT=23 ARG USER=user ENV CMD "/usr/sbin/telnetd -p $PORT -b $ADDR -l $LOGIN -F"

You can create a very similar Dockerfile but replace /bin/login with /bin/sh and it would work. For example, new Dockerfile could be:

FROM alpine ARG ADDR=0.0.0.0 ARG LOGIN=/bin/login ARG PORT=23 ARG USER=user ENV CMD "/usr/sbin/telnetd -p $PORT -b $ADDR -l /bin/sh -F" EXPOSE $PORT RUN apk update && apk upgrade && apk add busybox-extras RUN adduser -D $USER && echo -e "\n\n" | passwd $USER RUN echo "$CMD" | tee cmd.sh && chmod +x cmd.sh CMD ./cmd.sh

Build it:

docker build -t my/telnet .

Run it:

docker container run --name my-telnet-server --publish 2323:23 --detach --restart unless-stopped my/telnet

You now should be able to login w/o entering username and password from the client container:

root@e55f8f1f3e55:/# telnet -l user 172.17.0.1 2323 Trying 172.17.0.1... Connected to 172.17.0.1. Escape character is '^]'. / #

BTW, you don't have to run docker with sudo, just add yourself to docker group.

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  • "BTW, you don't have to run docker with sudo, just add yourself to docker group.": Thanks, I didn't know Commented Feb 24, 2020 at 15:06
  • I still have a problem. I then can't log in as "user": # telnet -l user 172.17.0.1 2323 Trying 172.17.0.1... Connected to 172.17.0.1. Escape character is '^]'. / # whoami root / # Commented Feb 24, 2020 at 15:27
  • @CacahueteFrito: indeed, it doesn't work. Sorry. I'm not sure it's possible to achieve that with busybox telnetd implementation. Commented Feb 24, 2020 at 16:16
  • ok, then I'll just write the user and a blank line through the pipe, and maybe a usleep() before and after the password is asked. Commented Feb 24, 2020 at 16:19
  • Can't you do su user after logging in as root? Commented Feb 24, 2020 at 16:28

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