vsftp – temporary FTP access for a defined list of users

We adminster Opensuse servers for some of our customers at server hosters - e.g. at Strato, a Telekom subsidiary. Sometimes some web server files have to be upgraded either by us or by specific users of the customer. But only sometimes. Under normal operative conditions the FTP access to the server shall not be allowed.

To enable a FTP service we have installed "vsftp". We further want the FTP users of our customers not to be able to access a shell on the server and restrict their FTP access to a certain directory. However, some of our own admins shall be able to access other server directories by FTP also.

Some basics of how you may set up and configure a vsftp server on an Opensuse system have already been described in my article
vsftp unter Opensuse 12.2 und 12.3.
The basic settings are valid for Opensuse 13.1, too. The settings discussed in the named article already restricted FTP users to a specific directory.

Sporadic FTP access

We solve the sporadic access requirement as follows:
The firewall on the server blocks all ports (with the exception of a dedicated SSH-Port for certificate based SSH logins and http/https ports) under normal operation conditions. But a trained user at the customer site can open a SSH connection and is allowed to start (but not change) a shell script (running with root rights) that temporarily opens ports for FTP connections for a defined IP (the WAN IP of the customer) to the vsftp server.

A simplified example for such a shell script that manipulates a IP chain called "zugriff" may look like:

iptables -N zugriff 
iptables -F zugriff 
iptables -A zugriff -p TCP -s $myip --dport ftp -j ACCEPT
iptables -A zugriff -p TCP -s $myip --dport 62050:62100 -j ACCEPT
iptables -t filter -I INPUT 4 -j zugriff
echo "vsftp port and IP rules were set for : " $myip
systemctl start vsftpd.service
echo "vsftp started"

A related simple stop script would be:

iptables -F zugriff
iptables -t filter -D INPUT -j zugriff
systemctl stop vsftpd.service
echo "FTP services stopped. FTP ports blocked in firewall."

The choice of the passive vsftp ports (in our example the range between 62050 and 62200) must of course correspond to the passive port settings in the vsftp configuration. Set the following options in the vsftpd.conf file:


You may also combine the script commands given above with further commands to start a time interval after which the ports are blocked automatically again. (To prevent possible harm if your trained customer user forgets to stop the vsftp service manually by the given stop scipt).

Grant FTP access to users of a defined list

The vsftp configuration given in my previous article vsftp unter Opensuse 12.2 und 12.3. allows for the access of all locally defined users on the server. They get chrooted to a defined chroot directory.

Note that the single FTP user discussed in the named article got the shell "/bin/false". Note further that we did not allow anonymous FTP access.

At our customer several users with different UIDs shall get the right to transfer files to the server. The different UIDs shall give us a chance to distinguish their actions in log protocols. (Note that logging the action of users may require special working contract conditions in Germany).

All of the FTP users become members of a special group. The main FTP directory - let it e.g. be "/srv/www/htdocs/webs" - which becomes their home dir and to which they are restricted by the vsftpd options


- gets group ownership of this special group. In addition the SGID bit is set. All of the customer's FTP users get the shell "bin/false".

Now, how to restrict the FTP access to defined users among others (e.g. from our own company) and how to extend the directory access for our own admins?

To restrict FTP access to users of a defined list requires the following options:


and a file "/etc/vsftpd.user_list" with just the UIDs of the users you want to rant FTP access - each UID in a separate line. The first option tells the vsftpd daemon that the users enlisted in the file "/etc/vsftpd.user_list" get the FTP access granted and shall not be denied it.

Note 1: I had considerable difficulties with my first userlist file, which I had generated and edited with vi on the server. Probably, I did some mistypes whilst editing... Be sure that your userlist file does not contain any special characters not visible on your terminal and that each UID is followed by a line break! Create the file from scratch if you experience unexplainable difficulties or read errors for the file in you FTP client. See: https://groups.google.com/forum/#!msg/alt-f/jtslOMt5aTA/1TW2kGkvmbkJ

Note 2: The file must be readable to the user running the vsftpd process on the server. On Opensuse systems this is presently "root" or "ftpsecure" - depending a bit on how you start the process. You may change ownership of the file to the user "ftpsecure" and drop the standard readability right for "others" - depending on security conditiosn.

Due to the vsftpd option "chroot_local_user=YES" the users get jailed to a certain directory (in our example to directories below "/srv/www/htdocs/webs"). We may want to circumvent this restriction for our own admins whereas for the customer's users it shall hold and work. To define such an exception requires further vsftp options:


If "chroot_local_use" is set to "YES" then UIDs given (one per line) in the file "vsftpd.chroot_list" are NOT jailed to a chroot directory. In this case the file "/etc/vsftpd.chroot_list" defines exceptions from the chroot-rule. Note, however, that if you had set


the opposite would be true: In this case the file "/etc/vsftpd.chroot_list" defines users to be explicitly jailed!

So, now we have combined all our objectives:
We open FTP access when we need it, we grant access to a list of users with most of them jailed to a certain directory
and we grant access to all directories for selected admins.

Have much fun with vsftp on Opensuse!

Strato-V-Server – Opensuse 13.1 – get colors for the "ls" shell command

Today, I tried to work with a freshly installed Strato-V-Server. I had chosen Opensuse 13.1 (64 Bit) as the operative system. When I used the "ls" command I missed the standard coloring of directories and special files.

Using the "env" command and a comparison with a normal Opensuse 13.1 installation showed that required settings for the "ls" coloring were missing. SuSE normally adds these settings via a script "ls.bash" located in the directory "/etc/profile.d". However, this file was missing on the Strato server. [The script in turn uses "/usr/bin/dircolors", which existed on the system.]

You could compensate the lack of coloring by manually adding a line

ls --color=tty

to a local ".bashrc" file. However, this would only give you parts of the SuSE coloring scheme for your shell.

The missing file "ls.bash" is the result of a really minimum Opensuse 13.1 installation provided by Strato on their V-servers. It took me a while to find the right RPM package which installs the required files. It is the RPM aaa_base-extras.

After the installation of this package and a reopening of your shell "env" will show some additional lines that control the "ls" color settings:

user@hxxxxxx:~> env | grep LS
LS_OPTIONS=-N --color=tty -T 0

I hope this information will help others who miss the the usual "ls"-coloring on Strato-V-servers, too.

Hosted Strato server – backups and chained firewall tunneling via SSH and shell

I administer several Linux web servers for partner and customer companies. Most of these servers are V-servers hosted at the German Telekom daughter Strato.

[I have described some reasonable security measures for a Strato server with Opensuse in some articles in this blog. See e.g.:
Strato-V-Server mit Opensuse 12.3 – IV – SSH Key Authentication
Strato-V-Server mit Opensuse 12.3 – III – Stop SSH Root Login
and previous articles referenced there.]

When you work with a hosted server, of course, you want to perform periodic backups. If you - for whatever reasons - do not want to pay for the backup facilities Strato offers, a backup requires that you transfer data from the remote server into your own company network through (encrypted) SSH tunnels. Depending on your specific situation you may wish to create a complete backup or a backup of selected directories on the server. During the backup you want to preserve user-rights and mode settings.

In addition to the topic of creating tunnels across firewalls, the question arises what Linux tool might be convenient to perform the backup task if you work with a shell based connection only. Therefore, I shall briefly discuss what I normally do to perform a manually controlled backup from a hosted Strato server. You can combine the methods described of course with some scripting. The described firewall tunneling by SSH is independent of a specific provider.

Tunneling firewalls with SSH

We presume that our company network has a DMZ with two firewalls - a perimeter firewall "PFW" protecting the DMZ and its hosts against the Internet and one or several inner firewalls "IFW(s)" separating the DMZ from inner network segments. Creating firewall tunnels on the outer and/or inner side of the DMZ is always a critical issue.

Our own backup server system for customer data - let us call it "bup" - is e.g. located in an inner segment of our network. The perimeter firewall "PFW" to the Internet resides on a router and can be configured to forward external WAN ports to ports of special hosts inside the DMZ. Servers inside an inner network segment are allowed to build up SSH connections to hosts in the DMZ across their "IFW(s)"; however the servers in the DMZ are not allowed to initiate SSH connections to the servers in the inner segments. For security reasons we poll data from the servers in the DMZ; we do not forward or push from the DMZ to inner segments. For our scenaio we assume that there is only one inner segment and that we only have to deal with a single "IFW".

I personally prefer a 2-step process to get the data from the hosted server to the backup destination server "bup":

  • Step 1: I open a reverse SSH tunnel from a special host "intmid" in our DMZ to the Strato server on the Internet and transfer the data from the Strato server to "intmid". There, I first of all scan the backup files for viruses and malware.
  • Step 2: Afterwards I stop the reverse tunnel and disallow any incoming connections to "intmid". I then open a ssh reverse tunnel from my destination server "bup" to "intmid" across the IFW, transfer the files to "bup" and encapsulate them there in an encrypted container.

Some readers may think a 2 step procedure may be exaggerated. If you prefer direct connections from the hosted Strato server to "bup" inside your network you may

  • either use a chain of two reverse SSH tunnels (one tunnel per firewall)
  • or a combination of port forwarding on the "PFW" with a reverse tunnel across the "IFW"

Both solutions provide a complete connection from the Strato server to "bup" passing both firewalls of your DMZ. Below I describe both methods briefly.

In both of the following methods we need a reverse SSH tunnel from the backup destination server "bup" (inner segment) to the DMZ host "intmid". I.e., we export a SSH port of server "bup" temporarily to the host "intmid" in the DMZ. This you may e.g. achieve by entering the command

ssh -R 19999:localhost:22 ssh_user_on_intmid@intmid

at your prompt on host "bup". The "-R" option triggers the creation of a "reverse" tunnel. Port 19999 is assumed to be unused on "intmid"; port 22 is your local port on "bup" for the SSH daemon sshd. "ssh_user_on_intmid" represents a user for SSH connections on "intmid" with sufficient privileges.

Note: If you just want to build the tunnel in the background without using a terminal or executing remote commands you may use the following combination of options for SSH (instead of a plain "-R") : ssh -nNTfR ......
Please see the man pages of ssh. These options may be especially helpful when using scripts. So, a more general form of the command could be

ssh -p<sshd port server> -nNTfR <forwarded port server>:localhost:<sshd port local> <username>@<ip server>

Note 1: If and when you really work with background SSH tunnels: Take care not to forget to stop the processes for the background tunnels at the end of your backup operation! In scripts this requires some "pid" determination operations for the running ssh processes.

Note 2: Depending on your sshd configuration you may need to have root rights to build the tunnel.

You can test your reverse tunnel through the inner firewall "IFW" by entering

ssh -p 19999 userid_on_bup@localhost

at the prompt of the "intmid" host. You will of course be asked for the password of a valid user "userid_on_bup" on server "bup" and should then get a prompt on "bup".

Method 1: Chaining two reverse tunnels

The hosted Strato server allows for a ssh connection on a specific port as this is your only way to work on it. You can use this SSH-connection to build up a reverse tunnel from your DMZ host "intmid" to the Strato server. I.e., the SSH port of your DMZ host shall be exported e.g. as port 19999 on the hosted Strato server. By issuing the following command on "intmid" we tunnel the perimeter firewall "PFW":

ssh -R 19999:localhost:19999 strato_user@ip_of_your_Strato-server -p 61111

Note: Here we have assumed that the Strato-Server listens on port 61111 as its configured SSH port. (Normally, I change the standard port 22 to a high port number. See:
Strato-V-Server mit Opensuse 12.3 – II – Installation / SSH-Port ändern)

You will of course have to give the password for your "strato_user" on the hosted Strato server before entering the shell there.

Why did we use port 19999 on the DMZ host "intmid"? Because we want to chain the two tunnels - the one from "bup" to "intmid" in the DMZ across firewall "IFW" and the one from "intmid" to our Strato server on the Internet across firewall "PFW" to get a complete encrypted connection from the Strato server to "bup". Both tunnels "meet" at port 19999 on host "intmid".

Now, at the prompt of the Strato server you can test the combined (reverse) SSH tunnels by entering

ssh -p 19999 userid_on_bup@localhost

Note: Do not get confused by "localhost" in the command above. You must provide the password for user "userid_on_bup" on the server "bup" (!) inside our inner network segment. Our connected tunnels end up there!

You then should get a prompt for user "user_id_on_bup" on our server "bup". Ok, we now can successfully tunnel two firewalls at the same time by making use of 2 chained reverse SSH tunnels!

Method 2: Port forwarding on the perimeter firewall

Port forwarding on the perimeter firewall could be an alternative to chaining two tunnels. Having build our obligatory reverse tunnel from "bup" to "intmid" we could in addition change the behavior of our PFW - provided of course that it is configurable. In my case I can establish port forwarding for incoming connections on an some external (WAN) port from a defined IP address on the Internet to a definable port of any host at the other (DMZ) side of the perimeter firewall. E.g I could forward a connection from the Strato server trying to reach the WAN side of the perimeter firewall on port 19999 to port 19999 on "intmid" in the DMZ.

Note: If you really do something like this on your firewall you have to be careful to restrict the forwarding to a defined server on the Internet and to close this firewall hole again after the backup procedure. [It makes no sense here to give commands or details as the configuration procedure varies with the type of firewall used.]

However, for this approach to work with openSSH you in addition have to set a parameter for your sshd configuration in the file "/etc/ssh/sshd_config" on "intmid":

GatewayPorts yes

Otherwise external hosts will not be allowed to use the reverse tunnel. This setting is somewhat dangerous and you should not forget to change it back.

In this approach we couple (local) port forwarding on the PFW with a reverse tunnel from "bup" to "intmid". The result is basically the same as with two chained reverse tunnels. However, you have to modify your commands on the Strato server somewhat to use port 19999 at the WAN side of your "PFW". To test the forwarding plus reverse tunnel on the Strato server you would enter:

ssh userid_on_bup@ip_address_of_your_routers_WAN_interface -p 19999

You have to provide the IP address of the WAN interface of your router. Depending on your DSL provider this may change on a daily basis. So you have to find out what it presently is - e.g by the admin interface of your PFW (router) or by asking an external service (e.g. curl icanhazip.com).

All in all I regard the second method as more complicated and potentially also more dangerous than method 1 - because you must not forget to change and change back your sshd configuration on the DMZ host "intmid" and also to reset your PFW configuration. Therefore, I shall only follow the first approach below.

Use rsync to perform the backup

As we have successfully build up tunnels across our firewalls one may think of using "scp" to perform a backup of the server directories. Actually this is not a good idea if you want to make backups of all server directories including system directories. The reason is:

"scp" follows all symbolic links! At least on Opensuse systems this may lead to infinite loops for certain directories whilst copying!

Therefore it is better to use "tar" or "rsync". See: http://stackoverflow.com/questions/11030394/is-it-possible-to-make-scp-ignore-symbolic-links-during-copy

Personally, I find "rsync" most convenient. And you can combine it quite easily with SSH! To use the ports defined for our method 1 (with the two chained reverse SSH tunnels) the right syntax for making a backup of e.g. the directory "/var" would be

rsync -avz -e 'ssh -p 19999' /var userid_on_bup@localhost:/backups/bup_strato/server_name/

This command has to be performed on the Strato server. Again: Do not get confused by "localhost" - that's how SSH tunneling works! The "-a" option of the rsync command means "archive mode". This preserves the mode and user settings of all files and directories throughout the data copying and performs recursions. "z" leads to a transfer of compressed data which will save you time. "/backups/bup_strato/server_name/" represents an example directory on server "bup" where you want to place the backup files of your special Strato server. Of course you should replace it by your own path on your backup server.

Note 1: You have to issue this commands as root on your Strato server to get access to all of the directories there.
Note 2: Please, note that slash "/" at the end of the directory path on the destination system! And note the missing "/" after the local directory "/var".
Note 3: Please, note the masking of 'ssh -p 19999' by single quotation marks! [To specify the port by "-p 19999" at the end of the ssh command would not work as you would mix rsync and ssh parameters].
Note 4: The big advantage of rsync naturally is that you only would exchange changed files at the next backup.

Have much fun now with your Strato server and making backups through SSH tunnels!