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Securing a Linux Server Part 2: SSH

Wow, it's been a while since I posted something in this series! Last time, I took a look at the Uncomplicated Firewall, and how you can use it to control the traffic coming in (and going out) of your server. This time, I'm going to take a look at steps you can take to secure another vitally important part of most servers: SSH. Used by servers and their administrators across the world to talk to one another, if someone manages to get in who isn't supposed to, they could do all kinds of damage!

The first, and easiest thing we can do it improve security is to prevent the root user logging in. If you haven't done so already, you should create a new user on your server, set a good password, and give it superuser privileges. Login with the new user account, and then edit /etc/ssh/sshd_config, finding the line that says something like

PermitRootLogin yes

....and change it to

PermitRootLogin no

Once done, restart the ssh server. Your config might be slightly different (e.g. it might be PermitRootLogin without-password) - but the principle is the same. This adds an extra barrier to getting into your server, as now attackers must not only guess your password, but your username as well (some won't even bother, and keep trying to login to the root account :P).

Next, we can move SSH to a non-standard port. Some might argue that this isn't a good security measure to take and that it doesn't actually make your server more secure, but I find that it's still a good measure to take for 2 reasons: defence in depth, and preventing excessive CPU load from all the dumb bots that try to get in on the default port. With that, it's make another modification to /etc/ssh/sshd_config. Make sure you test at every step you take, as if you lock yourself out, you'll have a hard time getting back in again....

Port 22

Change 22 in the above to any other number between about 1 and 65535. Next, make sure you've allowed the new port through your firewall! If you're using ufw, my previous post (link above) gives a helpful guide on how to do this. Once done, restart your SSH server again - and try logging in before you close your current session. That way if you make a mistake, you can fix through your existing session.

Once you're confident that you've got it right, you can close port 22 on your firewall.

So we've created a new user account with a secure password (tip: use a password manager if you have trouble remembering it :-)), disabled root login, and moved the ssh port to another port number that's out of the way. Is there anything else we can do? Turns out there is.

Passwords are not the only we can authenticate against an SSH server. Public private keypairs can be used too - and are much more secure - and convenient - than passwords if used correctly. You can generate your own public-private keypair like so:

ssh-keygen -t ed25519

It will ask you a few questions, such as a password to encrypt the private key on disk, and where to save it. Once done, we need to tell ssh to use the new public-private keypair. This is fairly easy to do, actually (though it took me a while to figure out how!). Simply edit ~/.ssh/config (or create it if it doesn't exist), and create (or edit) an entry for your ssh server, making it look something like this:

Host bobsrockets.com
    Port            {port_name}
    IdentityFile    {path/to/private/keyfile}

It's the IdentityFile line that's important. The port line simply makes it such that you can type ssh bobsrockets.com (or whatever your server is called) and it will figure out the port number for you.

With a public-private keypair now in use, there's just one step left: disable password-based logins. I'd recommend trailing it for a while to make sure you haven't messed anything up - because once you disable it, if you lose your private key, you won't be getting back in again any time soon!

Again, open /etc/ssh/sshd_config for editing. Find the line that starts with PasswordAuthentication, and comment it out with a hash symbol (#), if it isn't already. Directly below that line, add PasswordAuthentication no.

Once done, restart ssh for a final time, and check it works. If it does, congratulations! You've successfully secured your SSH server (to the best of my knowledge, of course). Got a tip I haven't covered here? Found a mistake? Let me know in a comment below!

Deep dive: Email, Trust, DKIM, SPF, and more

Lots of parcels (Above: Lots of parcels. Hopefully you won't get this many through the door at once..... Source)

Now that I'm on holiday, I've got some time to write a few blog posts! As I've promised a few people a post on the email system, that's what I'll look at this this post. I'm going to take you on a deep dive through the email system and trust. We'll be journeying though the fields of DKIM signatures, and climb the SPF mountain. We'll also investigate why the internet needs to take this journey in the first place, and look at some of the challenges one faces when setting up their own mail server.

Hang on to your hats, ladies and gentlemen! If you get to the end, give yourself a virtual cookie :D

Before we start though, I'd like to mention that I'll be coming at this from the perspective of my own email server that I set up myself. Let me introduce to you the cast: Postfix (the SMTP MTA), Dovecot (the IMAP MDA), rspamd (the spam filter), and OpenDKIM (the thing that deals with DKIM signatures).

With that out of the way, let's begin! We'll start of our journey by mapping out the journey a typical email undertakes.

The path a typical email takes. See the explanation below.

Let's say Bob Kerman wants to send Bill an email. Here's what happens:

  1. Bill writes the email and hits send. His email client connects to his email server, logs in, and asks the server to deliver a message for him.
  2. The server takes the email and reads the From header (in this case it's bill@billsboosters.com), figures out where the mail server is located, connects to it, and asks it to deliver Bob's message to Bill. mail.billsboosters.com takes the email and files it in Bill's inbox.
  3. Bill connects to his mail server and retrieves Bob's message.

Of course, this is simplified in several places. mail.bobsrockets.com will obviously need to do a few DNS lookups to find billsboosters.com's mail server and fiddle with the headers of Bob's message a bit (such as adding a Received header etc.), and smtp.billsboosters.com won't just accept the message for delivery without checking out the server it came from first. How does it check though? What's preventing seanssatellites.net pretending to be bobsrockets.com and sending an imposter?

Until relatively recently, the answer was, well, nothing really. Anyone could send an email to anyone else without having to prove that they could indeed send email in the name of a domain. Try it out for yourself by telnetting to a mail server on port 25 (unencrypted SMTP) and trying in something like this:

HELO mail.bobsrockets.com
MAIL From: <frank@franksfuel.io>
RCPT TO <bill@billsboosters.com>
DATA
From: sean@seanssatellites.net
To: bill@billsboosters.com

Hello! This is a email to remind you.....
.
QUIT

Oh, my! Frank at franksfuel.io can connect to any mail server and pretend that sean@seanssatellites.net is sending a message to bill@billsboosters.com! Mail servers that allow this are called open relays, and today they usually find themselves on several blacklists within minutes. Ploys like these are easy to foil, thankfully (by only accepting mail for your own domains), but it still leaves the problem of what to do about random people connecting to your mail server delivering spam to your inbox that claims to be from someone they aren't supposed to be sending mail for.

In response, some mail servers demanded things like the IP that connects to send an email must reverse to the domain name that they want to send email from. Clever, but when you remember that anyone can change their own PTR records, you realise that it's just a minor annoyance to the determined spammer, and another hurdle to the legitimate person in setting up their own mail server!

Clearly, a better solution is needed. Time to introduce our first destination: SPF. SPF stands for sender policy framework, and defines a mechanism by which a mail server can determine which IP addresses a domain allows mail to be sent from in it's name. It's a TXT record that sites at the root of a domain. It looks something like this:

v=spf1 a mx ptr ip4:5.196.73.75 ip6:2001:41d0:e:74b::1 a:starbeamrainbowlabs.com a:mail.starbeamrainbowlabs.com -all

The above is my SPF TXT record for starbeamrainbowlabs.com. It's quite simple, really - let's break it down.

v=spf1

This just defines the version of the SPF standard. There's only one version so far, so we include this to state that this record is an SPF version 1 record.

a mx ptr

This says that the domain that the sender claims to be from must have an a and an mx record that matches the IP address that's sending the email. It also says that the ptr record associated with the sender's IP must resolve to the domain the sender claims to be sending from, as described above (it does help with dealing with infected machines and such).

ip4:5.196.73.75 ip6:2001:41d0:e:74b::1

This bit says that the IP addresses 5.196.73.75 and 2001:41d0:e:74d::1 are explicitly allowed to send mail in the name of starbeamrainbowlabs.com.

a:starbeamrainbowlabs.com a:mail.starbeamrainbowlabs.com

After all of the above, this bit isn't strictly necessary, but it says that all the IP addresses found in the a records for starbeamrainbowlabs.com and mail.starbeamrainbowlabs.com are allowed to send mail in the name of starbeamrainbowlabs.com.

-all

Lastly, this says that if you're not on the list, then your message should be rejected! Other variants on this include ~all (which says "put it in the spam box instead"), and +all (which says "accept it anyway", though I can't see how that's useful :P).

As you can see, SPF allows a mail server to verify if a given client is indeed allowed to send an email in the name of any particular domain name. For a while, this worked a treat - until a new problem arose.

Many of the mail servers on the internet don't (and probably still don't!) support encryption when connecting to and delivering mail, as certificates were expensive and difficult to get hold of (nowadays we've got LetsEncrypt who give out certificates for free!). The encryption used when mail servers connect to one another is practically identical to that used in HTTPS - so if done correctly, the identity of the remote server can be verified and the emails exchanged encrypted, if the world's certification authorities aren't corrupted, of course.

Since most emails weren't encrypted when in transit, a new problem arose: man-in-the-middle attacks, whereby an email is altered by one or more servers in the delivery chain. Thinking about it - this could still happen today even with encryption, if any one server along an email's route is compromised. To this end, another mechanism was desperately needed - one that would allow the receiving mail server to verify that an email's content / headers hadn't been surreptitiously altered since it left the origin mail server - potentially preventing awkward misunderstandings.

Enter stage left: DKIM! DKIM stands for Domain Keys Identified Mail - which, in short, means that it provides a method by which a receiving mail server can cryptographically prove that a message hasn't been altered during transit.

It works by having a public-private keypair, in which the public key can only decrypt things, but the private key is capable of encrypting things. A hash of the email's headers / content is computed and encrypted with the private key. Then the encrypted hash is attached to the email in the DKIM-Signature header.

The receiving mail server does a DNS lookup to find the public key, and decrypts the hash. It then computes it's own hash of the email headers / content, and compares it against the decrypted hash. If it matches, then the email hasn't been fiddled with along the way!

Of course, not all the headers in the email are hashed - only a specific subset are included in the hash, since some headers (like Received and X-Spam-Result) are added and altered during transit. If you're interested in implementing DKIM yourself - DigitalOcean have a smashing tutorial on the subject, which should adapt easily to whatever system you're running yourself.

With both of those in place, billsboosters.com's mail server can now verify that mail.bobsrockets.com is allowed to send the email on behalf of bobsrockets.com, and that the message content hasn't been tampered with since it left mail.bobsrockets.com. mail.billsboosters.com can also catch franksfuel.io in the act of trying to deliver spam from seanssatellites.net!

There is, however, one last piece of the puzzle left to reveal. With all this in place, how do you know if your mail was actually delivered? Is it possible to roll SPF and DKIM out gradually so that you can be sure you've done it correctly? This can be a particular issue for businesses and larger email server setups.

This is where DMARC comes in. It's a standard that lets you specify an email address you'd like to receive DMARC reports at, which contain statistics as to how many messages receiving mail servers got that claimed to be from you, and what they did with them. It also lets you specify what percentage of messages should be subject to DMARC filtering, so you can roll everything out slowly. Finally, it lets you specify what should happen to messages that fail either SPF, DKIM, or both - whether they should be allowed anyway (for testing purposes), quarantined, or rejected.

DMARC policies get specified (yep, you guessed it!) in a DNS record. unlike SPF though, they go in _dmarc.megsmicroprocessors.org as a TXT record, substituting megsmicroprocessors.org for your domain name. Here's an example:

v=DMARC1; p=none; rua=mailto:dmarc@megsmicroprocessors.org

This is just a simple example - you can get much more complex ones than this! Let's go through it step by step.

v=DMARC1;

Nothing to see here - just a version number as in SPF.

p=none;

This is the policy of what should happen to messages that fail. In this example we've used none, so messages that fail will still pass right on through. You can set it to quarantine or even reject as you gain confidence in your setup.

rua=mailto:dmarc@megsmicroprocessors.org

This specifies where you want DMARC reports to be sent. Each mail server that receives mail from your mail server will bundle up statistics and send them once a day to this address. The format is in XML (which won't be particularly easy to read), but there are free DMARC record parsers out there on the internet that you can use to decode the reports, like dmarcian.

That completes the puzzle. If you're still reading, then congratulations! Post in the comments and say hi :D We've climbed the SPF mountain and discovered how email servers validate who is allowed to send mail in the name of another domain. We've visited the DKIM signature fields and seen how the content of email can be checked to see if it's been altered during transit. Lastly, we took a stroll down DMARC lane to see how it's possible to be sure what other servers are doing with your mail, and how a large email server setup can implement DMARC, DKIM, and SPF more easily.

Of course, I'm not perfect - if there's something I've missed or got wrong, please let me know! I'll try to correct it as soon as possible.

Lastly, this is, as always, a starting point - not an ending point. An introduction if you will - it's up to you to research each technology more thoroughly - especially if you're thinking of implementing them yourself. I'll leave my sources at the bottom of this post if you'd like somewhere to start looking :-)

Sources and Further Reading

How to set up a WebDav share with Nginx

I've just been setting up a WebDav share on a raspberry pi 3 for my local network (long story), and since it was a bit of a pain to set up (and I had to combine a bunch of different tutorials out there to make mine work), I thought I'd share how I did it here.

I'll assume you have a raspberry pi all set up and up-to-date in headless mode, with a ufw for your firewall (if you need help with this, post in the comments below or check out the Raspberry Pi Stack Exchange). To start with, we need to install the nginx-full package:

sudo apt update
sudo apt install  nginx-full

Note that we need the nginx-full package here, because the nginx-extras or just simply nginx packages don't include the required additional webdav support modules. Next, we need to configure Nginx. Nginx's configuration files live at /etc/nginx/nginx.conf, and in /etc/nginx/conf.d. I did something like this for my nginx.conf:

user www-data;
worker_processes 4;
pid /run/nginx.pid;

events {
    worker_connections 768;
    # multi_accept on;
}

http {

    ##
    # Basic Settings
    ##

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    # server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    ##
    # SSL Settings
    ##

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
    ssl_prefer_server_ciphers on;

    ##
    # Logging Settings
    ##

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    ##
    # Gzip Settings
    ##

    gzip on;

    gzip_vary on;
    gzip_proxied any;
    gzip_comp_level 6;
    gzip_buffers 16 8k;
    gzip_http_version 1.1;
    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

    ##
    # Virtual Host Configs
    ##

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
    # Ref comments below.
    # Only uncomment this if you encounter an issue with a modules not being loaded!
    #include /etc/nginx/modules-enabled/*.conf;
}

Not many changes here. Then, I created a file called 0-webdav.conf in the conf.d directory, and this is what I put in it:

server {
    listen 80;
    listen [::]:80;

    server_name plans.helenshydrogen.be;

    auth_basic              realm_name;
    auth_basic_user_file    /etc/nginx/.passwords.list;

    dav_methods     PUT DELETE MKCOL COPY MOVE;
    dav_ext_methods PROPFIND OPTIONS;
    dav_access      user:rw group:rw all:r;

    # This fixes 403 forbidden errors - thanks zmej! Rref https://starbeamrainbowlabs.com/blog/article.php?article=posts%2F237-WebDav-Nginx-Setup.html#adykboslcbxwacvlbhe1b8zbb97m
    autoindex on;

    # Some systems require you to actually create this directory before starting nginx.
    # If this is the case on your system, you may need to move it outside `/tmp` and create the directory manually (not forgetting to give ownership
    # to www-data with chown).
    # Thanks to @joe for pointing this out in a comment!
    client_body_temp_path   /tmp/nginx/client-bodies;
    client_max_body_size    0;
    create_full_put_path    on;

    root /mnt/hydroplans;
}

Now this is where the magic happens. The dav_access directive tells nginx what permissions to set on new files and directories that are created.

_Edit: Corrected above descrption of dav_access - thanks to the anonymous user in the comments for spotting!_

It's also important to note that I found that Windows (10, at least), didn't like the basic authentication - even though Ubuntu's Nautilus accepted it just fine - so I had to comment that bit out :-(

If you do still want authentication (hey! May you'll have better luck than I :P), then you'll need to set up the passwords file. Here's how you create it:


echo -n 'helen:' | sudo tee /etc/nginx/.passwords.list
openssl passwd -apr1 | sudo tee -a /etc/nginx/.passwords.list 
Password:

The above creates a user called helen, and asks you to type a password. If you're adding another user to the file, simply change the first tee to be tee -a to avoid overwriting the first one.

With that all configured, it's time to test the configuration file, and, if we're lucky, restart nginx!


sudo nginx -t
sudo systemctl restart nginx

That's all you should need to do to set up a simple WebDav share. Remember that this is a starting point, and not an ending point - there are a few big holes in the above that you'll need to address, depending on your use case (for example, I haven't included the setup of https / encryption - try letsencrypt for that).

Here are the connection details for the above for a few different clients:

  • Ubuntu / Nautilus: (Go to "Other Locations" and paste this into the "Connect to Server" box) dav://plans.helenshydrogen.be/
  • Windows: (Go to "Map Network Drive" and paste this in) http://plans.helenshydrogen.be/

Did this work for you? Have any problems? Got instructions for a WebDav client not listed here? Let me know in the comments!

Securing a Linux Server Part 1: Firewall

Welcome to a new tutorial series, where I will show you what I have learnt so far about making sure that your linux server (and desktop too!) are secure so that nobody can get in (easily) and assume control.

Disclaimer: This tutorial series will not cover everything, and should not be taken to. There probably will be some mistakes in this post too. Check other guides online or consult a professional to make sure that your machine is secure. Please suggest improvements or point out mistakes in the comments.

To start this tutorial session off, I will talk about firewalls. Firewalls control how data is allowed to travel in and out of your computer. In Ubuntu, a firewall called ufw, the 'uncomplicated firewall' is already present. It acts as a nice frontend to iptables, which I find to be difficult to understand and use. We will be using that as our firewall.

I have done an asciinema recording on a virtual machine of this whole process:

Enabling the firewall

Ufw by default allows all outgoing connections and denys all incoming connections. This means that if you are using ssh to connect to your server, you will need to open the appropriate ports first before enabling ufw. Do that like this:

~$ sudo ufw allow 22/tcp

Ufw will automatically configure iptables to allow incoming connections on port 22 that use tcp. I will talk more about allowing and denying different connections later.

Just in case ufw blocks your ssh connection and you are unable to get back in, you can use another program called at to schedule the disabling of the ufw so that you can get back in again. If you don't have it installed, you can install it with sudo apt-get install at.

~$ sudo at -vM now +10 minutes
ufw disable
^D

Where ^D stands for CTRL + D. Now that you have it set such that ufw will disable itself in 10 minutes time, we go ahead and turn ufw on:

~$ sudo ufw enable

It will warn you that this may disrupt any existing ssh connections you have open. Reply yes to this. Once it have been enabled successfully, you should check that you can still ssh into your server (if that is the method that you are using to control it). If yes, great! If not, ufw will disable itself in 10 minutes and then you can try again.

Now that we have ufw enabled, we can cancel the at job we created to disable ufw. Type sudo atq to list the jobs you have schedules, and sudo atrm <number> to remove it, where <number> is the number of the jobs that you want to delete.

You may also want to cheeck the status of ufw to make sure that it is enabled, or to get a list of the rules that are currently in force. You can do that like this:

~$ sudo ufw status
Status: active

To                         Action      From
--                         ------      ----
80/tcp                     ALLOW       Anywhere
80/tcp (v6)                ALLOW       Anywhere

Allowing connections

Allowing connections through the firewall is easy. Simply type something like this:

~$ sudo ufw allow 80/tcp

Ufw will automatically configure iptables, in this example, to allow all connections on port 80 that use tcp. It will also configure it appropriately for both ipv4 and ipv6. Replace 80 with the port number you want to allow, and tcp with udp if needed. Ufw also understands several protocol names, and can configure itself accordingly:

~$ sudo ufw allow http
~$ sudo ufw allow imap

Denying connections

Denying all connections on a given port is very similar., Simply type something like this:

~$ sudo ufw deny 4722/tcp

The above would deny all tcp connections on port 4722.

You can also prevent a particular ip from gaining access to your server:

~$ sudo ufw deny from 123.123.123.123

The above would block all packets from the ip address 123.123.123.123. It works with IPv6 addresses too:

~$ sudo ufw deny from 2607:f8b0:4003:c05::65

The above would block all packets from the ip address 2607:f8b0:4003:c05::65, which just happens to belong to Google.

Port Ranges

You can open a range of ports with a colon:

~$ sudo ufw allow 60000:61000/udp

The above will allow udp connections on any port in the range 60,000 - 61,000 (the ports used for mosh).

Deleting Rules

Deleting rules can be done like this:

~$ sudo ufw delete allow 4724/tcp

The above would delete the rule(s) allowing tcp connections on port 4724.

Summary

In this post, I have shown you how to activate and configure a simple firewall that is bundled with Ubuntu. Next time, I will talk about securing you ssh daemon.

If you spotted a mistake in this post, have a suggestion, or are having trouble following along, please leave a comment below.

Other useful posts

These posts helped me to understand and use the uncomplicated firewall:

Art by Mythdael