wiki:UnprivilegedUser

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Version 13 (modified by grzeg0rz, 4 years ago) (diff)
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By default, OpenVPN runs as the root user. This page seeks to describe how to instead run as an unprivileged user, "openvpn", instead. This is more secure than the built-in directives(--user and --group) because the openvpn process is never started with root permissions. Additionally, reconnects(including those which push fresh routes and configuration changes) which normally break after privileges are dropped via --user are handled without issue.

Do not use this! Allowing OpenVPN to execute ip as root is like giving it root access. See last section for easier and more secure alternative.

   sudo ip netns add foo
   sudo ip netns exec foo sh

There are options for a more-secure version of the script, but you should make sure you understand all the security ramifications before using it.

Configuration

Init Script

The init script is modifed to invoke the openvpn command via su instead of calling it directly(as root). It is recommended to copy the sample init script to a new one(/etc/rc.d/init.d/openvpn-su)before making these changes. Otherwise, package updates will wipe them out.

First, we must tell the init script which user to run as; insert the following near the top of the init script:

OPENVPN_USER="openvpn"

Next, remove the following line:

            $openvpn --daemon --writepid $piddir/$bn.pid --config $c --cd $work $script_security

....and replace it with:

            if [ -z "$OPENVPN_USER" ]
            then
                $openvpn --daemon --writepid $piddir/$bn.pid --config $c --cd $work $script_security
            else
                su $OPENVPN_USER -s /bin/sh --command="$openvpn --daemon --writepid $piddir/$bn.pid --cd $work --config $c $script_security"
            fi

Optional: If you would like, you could move the OPENVPN_USER variable definition into a sysconfig file, and source that instead of defining it directly. This is more in line with typical init script behavior, where a different user may be desirable. The usage of the if block in the init script is meant to accommodate the possibility of the variable being undefined(in which case, openvpn will be executed as root).

Wrapper for ip

Because openvpn will be running unprivileged, it can't execute the ip command directly. Create a wrapper script, /usr/local/sbin/unpriv-ip (remember to chmod this to 755):

#!/bin/sh
sudo /sbin/ip $*

Next, grant sudo access to the openvpn user so it can use the wrapper script. Use visudo to edit your sudoers list, and insert the first line where convenient(at the end works well). NOTE: If you have previously specified "Defaults requiretty" in your sudoers(a useful additional security measure), you will need the second line as well.

openvpn ALL=(ALL) NOPASSWD: /sbin/ip
Defaults:openvpn !requiretty

Secure Wrapper

Your wrapper script can be created to filter input parameters to only those legitimately used by OpenVPN:

#!/bin/bash

# This script wraps `ip` to allow it to be run as root by the `openvpn` user.
# You can/should extend this script to also filter IP addresses and device names.

# List of allowed commands created by searching openvpn source for iproute_path
# src/openvpn/lladdr.c
# :31      link set addr %s dev %s
#
# src/openvpn/networking_iproute2.c
# :68      link set dev %s up/down
# :83      link set dev %s up mtu %d
#
# :99      addr add dev %s %s/%d
# :116  -6 addr add %s/%d dev %s
# :170     addr add dev %s local %s peer %s
#
# :134     addr del dev %s %s/%d
# :152  -6 addr del %s/%d dev %s
# :188     addr del dev %s local %s peer %s
#
# :206     route add %s/%d (metric %d) (dev %s) (via %s)
# :237  -6 route add %s/%d dev %s
# :266     route del %s/%d (metric %d)
# :287  -6 route del %s/%d dev %s (via %s) (metric %d)

DEBUG=
debug_echo () {
  if [ ! -z "$DEBUG" ]; then
     echo "$1" >&2
  fi
}

CMD_IP=`which ip`
CMD_SUDO=`which sudo`

ORIGINAL_ARGS=$*

if (("x$1" == "x-6")); then
        debug_echo "Using IPv6"
        USING_IPv6=1
        shift
else
        USING_IPv6=0
fi

case "$1" in
"link")
        debug_echo "Allowed first arg: $1"
        case "$2 $3" in
        "set addr"|"set dev")
                debug_echo "Allowed second/third argument: $2 $3"
                ;;
        *)
                echo "Unrecognized second/third argument: $2 $3"
                exit 1
                ;;
        esac
        ;;

"addr")
        debug_echo "Allowed first arg: $1"
        case "$2 $3" in
        "addr add"|"addr del")
                debug_echo "Allowed second/third argument: $2 $3"
                ;;
        *)
                echo "Unrecognized second/third argument: $2 $3"
                exit 1
                ;;
        esac
        ;;

"route")
        debug_echo "Allowed first arg: $1"
        case "$2" in
        "add"|"del")
                debug_echo "Allowed second/third argument: $2 $3"
                ;;
        *)
                echo "Unrecognized second/third argument: $2 $3"
                exit 1
                ;;
        esac
        ;;

*)
        echo "Unrecognized first argument: $1"
        exit 1
        ;;
esac

echo "$CMD_IP $ORIGINAL_ARGS"
$CMD_IP $ORIGINAL_ARGS

Change the wrapper script, /usr/local/sbin/unpriv-ip

#!/bin/sh
sudo /usr/local/sbin/unpriv-ip-filter $*

Grant sudo access to the openvpn user so it can use the wrapper wrapper script, but not the wrapper script or ip command directly.

openvpn ALL=(ALL) NOPASSWD: /usr/local/sbin/unpriv-ip-filter

TUN/TAP Device

Because openvpn will be running as an unprivileged user, a static tun/tap device is needed. The init script already supports running a shell script before executing openvpn, so create one to handle this task(/etc/openvpn/openvpn-startup):

#!/bin/sh
openvpn --rmtun --dev tun0
openvpn --mktun --dev tun0 --dev-type tun --user openvpn --group openvpn

User

If you are using openvpn from a binary distribution(such as that provided by EPEL), there should already be an openvpn user created, but it will need to be modified slightly. If it does not exist, create it.

[root@hostname ~]# mkdir /var/lib/openvpn
[root@hostname ~]# chown openvpn:openvpn /var/lib/openvpn
[root@hostname ~]# usermod -d /var/lib/openvpn -s /sbin/nologin openvpn

Some other directories will need to be set up so that the openvpn user can write to them.

[root@hostname ~]# mkdir /var/log/openvpn
[root@hostname ~]# chown openvpn:openvpn /var/run/openvpn /var/log/openvpn /etc/openvpn -R
[root@hostname ~]# chmod u+w /var/run/openvpn /var/log/openvpn -R

Config Changes

Lastly, you need to modify your openvpn config files to take advantage of all of these changes. Add the following directives to your openvpn configuration file(/etc/openvpn/openvpn.conf):

log /var/log/openvpn/openvpn
iproute /usr/local/sbin/unpriv-ip
dev tun0
persist-tun

Usage

Now, give it a whirl!

[root@hostname ~]# service openvpn-su restart
Shutting down openvpn:                                     [  OK  ]
Starting openvpn: Sun Dec  4 03:42:19 2011 TUN/TAP device tun0 opened
Sun Dec  4 03:42:19 2011 Persist state set to: ON
                                                           [  OK  ]
[root@hostname ~]# ps -ef |grep openvpn
openvpn  25557     1  0 03:42 ?        00:00:00 /usr/sbin/openvpn --daemon --wri
root     25560 25499  0 03:42 pts/0    00:00:00 grep openvpn
[root@hostname ~]#

Troubleshooting

Init Script

The init script changes above only apply to the default OpenVPN init scripts, not those provided by Debian/Ubuntu? and derivatives. These do not have support for the .sh auto-execute which this technique relies upon. You can try copying the default init script from the source distribution into /etc/rc.d/init.d/openvpn-su and then patching as above, but the author has not tested this methodology. Information and suggestions are welcomed.

Logs

Since openvpn is no longer being executed as root, it is unable to write to the syslog. Thus you must use /var/log/openvpn/ and the --log directive. If no files are being created inside this directory, check that the permissions on the directory are correct(it should be owned by the openvpn user, and have a mask of 0755 / drwxr-xr-x).

Sudo

Permissions

You should also look at permissions/ownership for your keydir and /etc/openvpn/. The openvpn user should be able to read these, but not write to them, and no user but openvpn should be able to read your keys.

SELinux

In case you have SELinux enabled (e.g. you're using RHEL), you will need to set up additional user policies to allow the scripts run at startup. Create the following files:

# /tmp/openvpn_unpriv_hack.te

module openvpn_unpriv_hack 1.0;

require {
	type openvpn_t;
	type sudo_exec_t;
	class file { read open execute getattr execute_no_trans };
      	class process setrlimit;
        class capability sys_resource;
}

#============= openvpn_t ==============
allow openvpn_t sudo_exec_t:file { read open execute getattr execute_no_trans};
allow openvpn_t self:process setrlimit;
allow openvpn_t self:capability sys_resource;

then compile and install the security modules:

$ checkmodule -M -m -o /tmp/openvpn_unpriv_hack.mod /tmp/openvpn_unpriv_hack.te
$ semodule_package -o /tmp/openvpn_unpriv_hack.pp -m /tmp/openvpn_unpriv_hack.mod
$ semodule -i /tmp/openvpn_upriv_hack.pp

and check if they have loaded correctly:

$ semodule -l | grep openvpn
openvpn	1.9.1	
openvpn_unpriv_hack    1.0

Run OpenVPN within unprivileged podman container

I was able to run openvpn from within unprivileged podman container.

install podman (https://podman.io/getting-started/installation)
Create Dockerfile (as target unprivileged user)

FROM archlinux
RUN pacman -Sy --noconfirm openvpn

Build image (as target unprivileged user)

podman build -t openvpn .

Create systemd openvpn.service file (as root)

[Unit]
Description=OpenVPN in Podman container
After=syslog.target network-online.target
Wants=network-online.target

[Service]
User=target_unprivileged_user
Group=target_unprivileged_group
ExecStart=/usr/bin/podman run --rm -v /home/target_unprivileged_user/ovpn_config_files/:/ovpn_config_files -p 56787:56787 --device /dev/net/tun --device /dev/null --cap-add CAP_IPC_LOCK,CAP_NET_ADMIN,CAP_NET_BIND_SERVICE,CAP_NET_RAW,CAP_SETGID,CAP_SETUID,CAP_SYS_CHROOT,CAP_DAC_OVERRIDE,CAP_AUDIT_WRITE localhost/openvpn:latest /usr/bin/openvpn --config /ovpn_config_files/openvpn_server.conf
ExecStop=/usr/bin/podman stop -t 0 localhost/openvpn:latest
Capabilities=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_DAC_OVERRIDE CAP_AUDIT_WRITE
RestartSec=5s
Restart=on-failure

[Install]
WantedBy=multi-user.target

Drop configuration (openvpn-server.conf) and other required files under /home/target_unprivileged_user/ovpn_config_files, configuration file should have paths relative to podman container (/ovpn_config_files, not /home/target_unprivileged_user/ovpn_config_files); Also note port exposed within systemd service is 56787 (either keep this in mind when writing your configuration file or change systemd service file)

Start the service (as root) - this will result in podman container running as target_unprivileged_user, openvpn port exposed and openvpn available to receive connections

systemctl start openvpn.service

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