Restrict service account from direct interactive sessions

UNIX for Advanced & Expert Users
1

Environment: CentOS 7

I would like to have a solution where a service account can access a server in only these ways:

  • ssh non-interactively via password or ssh key; that is, run commands or scripts (but running anything in /etc/shells will not be allowed)
  • not ssh interactively
  • regular users can su $serviceaccount or otherwise get an interactive shell

The purpose is to make users log in to the server as themselves, and then switch user, but also to allow the service account to interact with itself through scripted processes from other servers.

I have tried these steps already

  • sshd_config no ttys

/etc/ssh/sshd_config:

Match User $serviceaccount
   PermitTTY no

This one actually does nothing except prevent a nice-looking terminal. The user still gets an interactive shell.

  • commands in ~/.authorized_keys

/home/serviceaccount/.ssh/authorized_keys:

command="/usr/local/bin/oneshellscripttorulethemall.sh" ssh-rsa AAAAAA....

Users can modify it, plus I cannot guarantee that every connection uses an ssh key.

  • altering default shell in /etc/passwd

/etc/passwd

serviceaccount:x:1500:1500:service account:/home/serviceaccount:/sbin/nologin

/sbin/nologin: prevents all logins, except "sudo -su $serviceaccount"
/bin/false: fails out entirely
/bin/true: does not allow any activity at all
custom wrapper script: A custom script that checks for "$@" and reacts to it might be my only choice and I will continue experimentation. But it could get weird for the local users who su $serviceaccount.

  • restrict logins from certain IPs (the other servers who are using the service account)

Users could just get a shell over there, and ssh in directly to an interactive shell.

In conclusion

I am interested in any and all attempts to meet the goals described above: Paid solutions, free solutions, hacky shell scripts, ssh config customization, custom default shells, wrapper scripts, etc. I would be pleased to see even partial answers, and I can bang away on adding the missing portions.

Is what I'm aiming for reasonable, or even possible?

2

Looks unreasonably complex to implement yes, with requests for functionality overlapping.

Is there another approach for desired outcome ?
Perhaps some web server and actual application....

You will have many issues with implementing your entire functionality using one user and SSH protocol.
If you manage to do that in the end you will have a hacky mess.

Perhaps more service users with separated privileges.
Linux and unix systems are multi user environments in their essence, so exploit that as much as you can.

Hope that helps
Regards
Peasant.

3

I started a possible tool to watch the logs and deal with shell sessions, but it can be easily defeated with a ssh remoteserver /bin/bash.

#!/bin/sh
# startdate: 2018-10-05 13:20
# Purpose: if a service account user logs in interactively, then kill it.
# incomplete. Can be foiled with: ssh -t clonetest210 /bin/bash
# improve: how to retrieve log entries to check

# Sample journalctl output.
# Oct 05 13:12:52 clonetest210 sshd[1868]: Starting session: shell on pts/3 for bgstack15-local from 10.200.18.240 port 59349 id 0

# Dependencies: sshd_config LogLevel VERBOSE
# journalctl -f -u sshd is not sufficient. I cannot tell what unit logs the notice seen above.

BADUSERS="(bgstack15-local|prophetess)"

journalctl -n100 | grep -oE "sshd\[.{1,10}\]: Starting session: shell on .* for ${BADUSERS} from .*" | awk '{print $1,$6,$8,$10}' | while read longpid tty tu srcip ;
do
   pid="$( echo "${longpid}" | tr -dc "[:digit:]" )"
   echo "Found login: ${tu} from pid ${pid} from ip ${srcip} and made terminal ${tty}"

   # investigate that current pid. if it exists and is sshd, kill it
   psout="$( ps -e -o pid:9,ppid:9,user:15,command:90 2>/dev/null | awk "\$1 == $pid" )"
   if test -n "${psout}" && echo "${psout}" | grep -qE "sshd:" ;
   then
      echo "need to warn user ${tu} on tty ${tty} and then kill pid ${pid}"
      printf "\n%s\n" "Interactive sessions are not allowed for user ${tu}." > "/dev/${tty}"
      sleep 0
      kill "${pid}"
   fi
done