Monthly Archives: April 2015

Raspberry Pi

Network File System (NFS)

For a while I have been using Samba to remotely connect Windows computers to my Linux computers and one Linux file server. I can even connect my Linux laptop to my Linux server via Samba. But, recently I bought a Raspberry Pi and I got interested in using NFS for three reasons.

  1. The Raspberry Pi will be running 24/7 and I would like the option to automount the home folder and others from it on my Linux computers when I start them up.
  2. I would like to mount some folders on a server (“big” file storage Linux server) that I can start remotely to the Raspberry Pi so that it will act like expanded storage on the Raspberry Pi. Then I can start the “big” server remotely and mount the folders on the Pi and use the Pi as a proxy. So it is connected to the web and I can navigate to folders that are on the big server via a connection to the Pi with NFS mounted directories.
  3. It would make it easy to backup Linux machines, including the Pi to the big server periodically. Years ago I thought about NFS for mounting folders for backup but I was pretty happy using a scripted FTP system for backups, so I shelved implementing NFS mounts back then.

Implementing NFS was a lot easier than I thought it would be. It was actually much easier than getting Samba to work the way I wanted it to.

The first step ( in my opinion) is to have the machines that you will mount directories from and to on static IP address. On a home network it really does make it easier to have all the machines other than guest machines on static IP’s. This can be done either by setting the machine to have a static IP. Or it may be possible depending on the router, for the router to be configured to hand out the same IP address to a machine with a specific MAC ID. Effectively the results are the same.

Static IP’s are useful as the actual IP addresses will be listed in the export file. It may be possible to use names, however this depends on how DNS is handled on your network. Using the actual IP addresses will make initial setup nearly foolproof. Also an easy way to use names on any machine is to add the static IP’s and names of the machines on the network to the /etc/hosts file.

Install NFS Support

To install support for NFS on the machines run….

sudo apt-get install nfs-kernel-server

Exports File on Server

Modify the servers /etc/exports file to suit your needs. Below is an example from my system on the Raspberry Pi. Remember to restart the NFS server when you have made changes to the file….

sudo service nfs-kernel-server restart
# /etc/exports: the access control list for filesystems which may be exported
 #        to NFS clients.  See exports(5).
 # Example for NFSv2 and NFSv3:
 # /srv/homes       hostname1(rw,sync,no_subtree_check) hostname2(ro,sync,no_subtree_check)
 # Example for NFSv4:
 # /srv/nfs4        gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)
 # /srv/nfs4/homes  gss/krb5i(rw,sync,no_subtree_check)

My initial try at this was to semi follow an example and create a /srv/homes directory and export it to one machine at rw = read write access, sync = change both folders on server and client to keep sync’d, no_subtree_check = keeps the machine from having to check consistency of file names, prevents problems if a file is open and the file is renamed.

Then I decided to export my own home directory to all of the machines on the LAN, using which allows access from This time I am using all_squash which maps all UID’s and GID’s to nobody and nogroup, then setting anonuid=1001, my UID on the Rasp Pi and anongid=1004 my group on the Rasp Pi, they will map over to the correct UID and GID for myself on the other machines. Therefore I have no problem with read write access as the same user on the other machine to the NFS drive.

The next line exports the entire file system to one machine, but has no_root_squash set, which allows the root to access and create files as root on the server. This is one to be careful with, I use it only when I need to mount the entire file system and usually I have to move things around or something as root anyways, but as always be cautious.

Restart Required

After modifying the /etc/exports file the NFS server needs to be restarted using…

service nfs-kernel-server restart

Client Machine

You have to install the common code for NFS on the client machine.

sudo apt-get install nfs-common

Mount Commands

My Home Directory on the Raspberry Pi

In this case I am mounting my home directory from the Pi under /home/erick-pi. I had to use the nolock option because I was getting an error without it, other than that it works fine.

Example of the mount command from the command line…

sudo mount -o nolock /home/erick-pi
Entire Root Directory

Occasionally I want to mount the entire file system of the Raspberry Pi at a location on one PC.

sudo mount -o nolock /mnt/nfs/srv/
Mount Scripts

For situations that only apply occasionally, such as the above example of mounting the entire directory structure, I have created some scripts and placed them in the bin folder under my home folder and made them executable by using chmod +x filename. Then I can run them as needed by running a script with a filename that makes sense to me. Like the one for the code below is,

#! /bin/bash
sudo mount -o nolock /mnt/nfs/srv/
intr option

Note that the intr option for nfs mounts is a good one if the computer loses it’s connection with the server. With the Raspberry Pi this is never an issue for me but it is with other servers. It allows an interupt to stop NFS requests if the server goes down or the connection is lost. If intr is not used NFS will keep trying and the process will hang, requireing a reboot. I have had this occur mostly with servers that are on only part time. Most likely, I started the server and then put my computer on standby. When I start it and the server is off, NFS will hang looking for the mount points that have disappeared. This will hang not only the X window folders, but it will hang any command in a terminal that has to touch NFS, such as df -h which tries to look for something that is not there anymore.

Hard and Soft Options

The hard and soft options are like what they sound like. Soft mounts give up after a timeout and don’t keep trying to write or read from an NFS mount point if it flakes out or goes down. Soft mounts should only be used for read only mounts as data being written can be corrupted if a soft mount gives up where the hard mount will just keep trying until the mount comes back. If there is an issue with a write mount point flaking or dropping it is best to mount hard, the default and use the intr option.

Mounting on Startup

It is possible to set up the /etc/fstab file to mount the NFS drive on start up.  It is as simple as adding the following line to the file…    /home/erick-pi    nolock    0    0

On my laptop this did not work, I remembered that Wireless LAN is handled at the user level and not on during bootup when mountpoints are handed out via the /etc/fstab file. So I got an error about the mount point not being found.

On my desktop running Lubuntu 14.04, connected via Ethernet cable, the line above did not work either but I modified it as follows and it worked. It might be that I left out the part with nfs, although I thought that the OS could tell it was a NFS mount from the format, oh well. I also decided to mount it right under my home folder on the desktop…    /home/erick/erick-pi nfs   auto,nolock    0    0

On the desktop once the drive is mounted it will stay mounted even if I reboot the Rasp Pi while the desktop PC is running.

Delayed Mount on Startup

To mount an NFS drive on a machine that has wireless, you have to mount it after it connects to the router and by then it is already running at the user level. You have to trick the system into waiting. There are multiple ways of doing this. I chose putting a line into to root crontab and used sleep 60 for the delay. After all most mounting has to be done as root anyways.

So I put a 60 second delay in before the mount command executes in the root crontab using the @reboot directive…

@reboot bash -c "sleep 60; mount -o nolock /home/erick-pi"

To edit the root crontab, simple do…

sudo nano crontab -e

To simply list what is in the root crontab, which is how I cut and pasted the code above, simply do…

sudo crontab -l

Using Names

It is entirely possible to use names instead of IP addresses when you mount NFS drives and even in the /etc/exports file. One caution, if DNS is down or flaky on you LAN, it could present a problem with reliably mounting drives.  Therefore I recommend adding the server names to your /etc/hosts file. On my LAN I take it a step further, the servers are all set as static IP and my router has the ability to always hand out the same IP to a machine at a specific MAC address so I use that for laptops & etc that normally connect to the network. So in effect every normally used device has a static IP. Therefore I can put them all in /etc/hosts and I don’t even have to care about DNS on the LAN for the machines on it 99% of the time.

/etc/hosts comes with the top two entries, just add what you want to it. As you can see commenting them out works too. The erick-MS-6183 server is down, probably for good at this point!    localhost    erick-laptop
 #    erick-MS-6183    renee-pc      erick-laptop     ubuntuserver     raspberrypi


One time I was backing up my laptop to a laptop-backup directory under my home folder on the big file server. The problem was that I had my home folder on the big file server set as an NFS mount as a folder under my home folder on the laptop. It copied in circles until the harddrive filled up. Oh well, learned the hard way on that one! Be careful of NFS mounts and even symlinks to places when running backups.

NFS and Users

With users there is the notion of the name and then there is the numerical UID. NFS uses the numerical UID to map across machines. If you plan on using NFS on multiple machines, it pays to keep the UID’s lined up between them. For example, if you set up 2 Linux machines from scratch, there will be a user at UID 1000, that would be you, whatever you called it by name. The first user is at 1000. If you use NFS to mount a directory from one machine to another, no problem it all lines up. The user at UID 1000 is the same on both machines, permissions work out, files can be moved back and forth, no problems.


Used this one to get started with NFS…

Helped to figure out the whole user and group ID mapping

NFS: Overview and Gotchas

 exports(5) – Linux man page

Easy to follow, I hink I might have started with this one,

Setting up an NFS Server and Client on Debian Wheezy

I need to look at this one for a sanity check on the errors when I launch NFS server on Raspberrry Pi,

Problem with NFS network


SSH Keys

On one of my servers once I got it set up right and working smooth there was rarely a need to log into via SSH remotely. So I left the SSH port 22 closed down in the router. When I really needed to log into it, I would log into the router and hit the DMZ button and open up all the ports to the server briefly. Then I would SSH into it using the normal username and password combo do my business and lock down the ports again.

I learned about SSH Keys a few years back while I was doing some volunteer work on a site. The owner of the server had SSH Keys setup on it so that I could use WinSCP to move files up to it. He believed, rightly so in keeping the security beefed up and didn’t bother with FTP at all. Recently (February 2015) I purchased a Raspberry Pi. Eventually it will replace one of the servers I run. For now, it is a test bed and I would like to be able to log right in, no fiddling with the router! Plus why not make it more secure, that is where SSH Keys come in.

I hunted down the method to set up SSH Keys online. Not hard at all. I followed one article that helped set up the keys and it logged in great. But, I still could also still login via username and password, so I had to apply another step beyond what the article explained.

Finally, once you open up port 22, many attempts to login will occur on the port and you can see this in your router log. Mine is setup to email me the router log and I quickly noticed that I was being emailed logs one after the other. I decided to change the default port 22 to map to an obscure number higher than 1024 by adjusting the port forwarding in the router.

Installing SSH Server

In case you haven’t installed the server part of SSH on your machine here is the command line directive…

sudo apt-get install openssh-server

Setting up SSH Keys (Public Key Authentication)

These are examples of the commands that I used to set up the keys while on the client machine. It is best to try this while you are not too far from you machine physically, just in case something goes wrong and you need to get on the machine physically.

Create the RSA Public/Private Key on the client machine

You will be asked where you want the key stored the default is the .ssh directory under your home folder with a filename of id_rsa. Then you will be asked to provide a passphrase, hit enter if you do not want a passphrase. A passphrase provides an extra level of security. If someone gets a hold of your machine or private key, they still need the passphrase to get anything going.

ssh-keygen -t rsa

You will get the following message. Depending on the machine, it may take a few seconds after the first line, while the machine is doing the calculation before you see the second line. The Raspberry Pi took about 3-4 seconds to spit out the second line.

Generating public/private rsa key pair.
Enter file in which to save the key (/home/pi/.ssh/id_rsa):

The file is alright by default, hit enter, unless you have another place that you need it and know what you are doing. I assume some config file in the system expects the key in the .ssh folder.

Next comes the passphrase question…

Enter passphrase (empty for no passphrase):

…and again….

Enter same passphrase again:

Finally the key is generated and a randomart image is generated, interesting looking but nothing we need for this operation…

Your identification has been saved in /home/pi/.ssh/id_rsa.
Your public key has been saved in /home/pi/.ssh/
The key fingerprint is:
d7:33:ed:91:ab:00:a7:bd:15:8d:15:21:fe:ed:6b:df pi@raspberrypi
The key's randomart image is:
+--[ RSA 2048]----+
|            . o. |
|           . . . |
|            . .  |
|           . * o |
|        S o * * .|
|         *   = + |
|        . o . o .|
|           + . .o|
|          . . ..E|

Copy the Public Key to the Server

Next you will copy the key up to the server using the ssh-copy-id command. It will log you in and you will use your normal password that you have for your login and then it will copy the key to the server. The example shows that the user is pi and the ip= Change these to your id and server IP.

ssh-copy-id pi@

In this example I am installing it on the same machine that I created it. So this is what I see…

pi@raspberrypi ~ $ ssh-copy-id pi@
The authenticity of host ' (' can't be established.
ECDSA key fingerprint is 7e:f0:94:8a:bd:f2:95:44:f3:a5:36:ff:e3:64:48:a3.
Are you sure you want to continue connecting (yes/no)?
Warning: Permanently added '' (ECDSA) to the list of known hosts.

…And the key is added.

Test It

Now you can login to your server with the newly created keys. But you still can also login via the username and password combo.

Making it SSH Key Only login

You need to set the sshd_config to explicity allow Public Key Authentication. This step requires editing the sshd_config file. Which I didn’t remember the location of so I used…

sudo find / -name sshd_config

Edit it …

sudo nano /etc/ssh/sshd_config

Find the line that reads PasswordAuthentication which is set to yes by default, as in commented out = yes.

Set it to no and make sure it is uncommented…

PasswordAuthentication no

Check to see that this is set also…

ChallengeResponseAuthentication no

Restart the SSH server…

sudo service ssh restart

Remapping the SSH Port 22 to something less obvious

If you don’t remap to port, lots of hits happen to it. Attempts to login that will fill up your router logs. In theory someone can still find the new port, but they would have to get lucky or scan the ports. So this does cut down on bogus login attempts significantly.

There are two ways of doing this, in the configuration file sshd_config or by setting up the port forwarding in yur router. I left sshd_config set at port 22 and made the change in the router. I care about the port being mapped to something else for the outside world on my LAN it can stay 22. So I can simply use SSH servername and not specify a port.

sshd_config mod method

There is a line near the beginning of the file, change the 22 to something else and restart the sshd server…

# What ports, IPs and protocols we listen for
Port 22

Router Port Forward Mapping Change Method

Or go into your router and find the port forwarding. In my Netgear router it was under Advanced–> Port Forwarding/Port triggering. You will see a list that allows there to be changed…

# Service Name External Start Port External End Port Internal Start Port Internal End Port Internal IP address
External End Port Internal Start Port Internal End Port

Set it up for a port other than 22 for External Start and end Port, 5678 in this example…


SSH 5678 5678 22 22

More Tightening of SSH Security

I have not done any of this yet on my machine but for FYI. Under the spot in sshd_config where the port is set there is a place where you can place a whitelist of IP addresses that the sshd will listen for. This can restrict the IP space that can connect to the machine..

# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::

It is also possible to further restrict the actual users that are allowed or denied access to the machine via SSH. This is accomplished by using the AllowUsers, AllowGroups,DenyUsers,DenyGroups directives.


AllowUsers joe bob naomi
AllowGroups workinggroup powerusergroup
DenyUsers tempuser1 tempuser2
DenyGroups gaming

You can also block the ability to login as root, so that users will have to su to root once logged in.

PermitRootLogin on

Encryption and Keys

Creating keys such as the RSA pair is an interesting mathematical concept. It falls in the realm of one way functions. You can have the public key and have only remote odds of being able to generate the corresponding private key by a brute force search. But, the other way around is easy. It’s kind of like glass breaking or throwing a cup of water in the ocean, in theory it is all still there, but to put it back together is nearly impossible.

In physics this is what makes “time” on the macro scale. On the quantum level, time really doesn’t matter. You can play particles interactions backwards and forwards and it all works out OK. Feynman diagrams, work both ways. But on the macro level, a lot of things just go one way, just like the hash algorithms that generate the encryption keys. The same thing applies to hashes to generate look up tables, it is easy to go one way, to the lookup from the hash, but harder to go the other way. Ratchets, diodes and worm gears, go one way but not the other.


How To Set Up SSH Keys

How do I force SSH to only allow uses with a key to log in?

7 Default OpenSSH Security Options You Should Change in /etc/ssh/sshd_config

Ubuntu Server Guide OpenSSH Server