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Stong SSL Security on lighttpd


This tutorial shows you how to set up strong SSL security on the Lighttpd webserver. We do this by disabling SSL Compression to mitigate the CRIME attack, disable SSLv3 and below because of vulnerabilities in the protocol and we will set up a strong ciphersuite that enables Forward Secrecy when possible. We also set up HSTS and HPKP. This way we have a strong and future proof ssl configuration and we get an A on the Qually Labs SSL Test.

TL;DR: Copy-pastable strong cipherssuites for NGINX, Apache and Lighttpd: https://cipherli.st

This tutorial is tested on a Digital Ocean VPS. If you like this tutorial and want to support my website, use this link to order a Digital Ocean VPS: https://www.digitalocean.com

This tutorial works with the stricter requirements of the SSL Labs test announced on the 21st of January 2014 (It already did before that, if you follow(ed) it you get an A+)

This tutorial is also available for Apache2 This tutorial is also available for NGINX
This tutorial is also available for FreeBSD, NetBSD and OpenBSD over at the BSD Now podcast: http://www.bsdnow.tv/tutorials/nginx

You can find more info on the topics by following the links below:

Make sure you backup the files before editing them!

I’m using lighttpd 1.4.31 from the Debian Wheezy repositories on this website. The CentOS 5/6 EPEL versions wouldn’t work for me because either lighttpd or OpenSSL being to old. Debian Squeeze also failed.

Mitigate the BEAST attack

In short, by tampering with an encryption algorithm’s CBC – cipher block chaining – mode’s, portions of the encrypted traffic can be secretly decrypted. More info on the above link.

Recent browser versions have enabled client side mitigation for the beast attack. The recommendation was to disable all TLS 1.0 ciphers and only offer RC4. However, [RC4 has a growing list of attacks against it],(http://www.isg.rhul.ac.uk/tls/) many of which have crossed the line from theoretical to practical. Moreover, there is reason to believe that the NSA has broken RC4, their so-called “big breakthrough.”

Disabling RC4 has several ramifications. One, users with shitty browsers such as Internet Explorer on Windows XP will use 3DES in lieu. Triple-DES is more secure than RC4, but it is significantly more expensive. Your server will pay the cost for these users. Two, RC4 mitigates BEAST. Thus, disabling RC4 makes TLS 1.0 users susceptible to that attack, by moving them to AES-CBC (the usual server-side BEAST “fix” is to prioritize RC4 above all else). I am confident that the flaws in RC4 significantly outweigh the risks from BEAST. Indeed, with client-side mitigation (which Chrome and Firefox both provide), BEAST is a nonissue. But the risk from RC4 only grows: More cryptanalysis will surface over time.

Factoring RSA-EXPORT Keys (FREAK)

FREAK is a man-in-the-middle (MITM) vulnerability discovered by a group of cryptographers at INRIA, Microsoft Research and IMDEA. FREAK stands for “Factoring RSA-EXPORT Keys.”

The vulnerability dates back to the 1990s, when the US government banned selling crypto software overseas, unless it used export cipher suites which involved encryption keys no longer than 512-bits.

It turns out that some modern TLS clients – including Apple’s SecureTransport and OpenSSL – have a bug in them. This bug causes them to accept RSA export-grade keys even when the client didn’t ask for export-grade RSA. The impact of this bug can be quite nasty: it admits a ‘man in the middle’ attack whereby an active attacker can force down the quality of a connection, provided that the client is vulnerable and the server supports export RSA.

There are two parts of the attack as the server must also accept “export grade RSA.”

The MITM attack works as follows:

  • In the client’s Hello message, it asks for a standard ‘RSA’ ciphersuite.
  • The MITM attacker changes this message to ask for ‘export RSA’.
  • The server responds with a 512-bit export RSA key, signed with its long-term key.
  • The client accepts this weak key due to the OpenSSL/SecureTransport bug.
  • The attacker factors the RSA modulus to recover the corresponding RSA decryption key.
  • When the client encrypts the ‘pre-master secret’ to the server, the attacker can now decrypt it to recover the TLS ‘master secret’.
  • From here on out, the attacker sees plaintext and can inject anything it wants.

The ciphersuite offered here on this page does not enable EXPORT grade ciphers. Make sure your OpenSSL is updated to the latest available version and urge your clients to also use upgraded software.

Heartbleed

Heartbleed is a security bug disclosed in April 2014 in the OpenSSL cryptography library, which is a widely used implementation of the Transport Layer Security (TLS) protocol. Heartbleed may be exploited regardless of whether the party using a vulnerable OpenSSL instance for TLS is a server or a client. It results from improper input validation (due to a missing bounds check) in the implementation of the DTLS heartbeat extension (RFC6520), thus the bug’s name derives from “heartbeat”. The vulnerability is classified as a buffer over-read, a situation where more data can be read than should be allowed.

What versions of the OpenSSL are affected by Heartbleed?

Status of different versions:

  • OpenSSL 1.0.1 through 1.0.1f (inclusive) are vulnerable
  • OpenSSL 1.0.1g is NOT vulnerable
  • OpenSSL 1.0.0 branch is NOT vulnerable
  • OpenSSL 0.9.8 branch is NOT vulnerable

The bug was introduced to OpenSSL in December 2011 and has been out in the wild since OpenSSL release 1.0.1 on 14th of March 2012. OpenSSL 1.0.1g released on 7th of April 2014 fixes the bug.

By updating OpenSSL you are not vulnerable to this bug.

SSL Compression

The CRIME attack uses SSL Compression to do its magic, so we need to disable that. The following option disables SSL compression:

ssl.use-compression = "disable"

By default lighttpd disables SSL compression at compile time. If you find it to be enabled, either use the above option, or recompile OpenSSL without ZLIB support. This will disable the use of OpenSSL using the DEFLATE compression method. If you do this then you can still use regular HTML DEFLATE compression.

SSLv2 and SSLv3

SSL v2 is insecure, so we need to disable it. We also disable SSLv3, as TLS 1.0 suffers a downgrade attack, allowing an attacker to force a connection to use SSLv3 and therefore disable forward secrecy.

Again edit the config file:

ssl.use-sslv2 = "disable"
ssl.use-sslv3 = "disable"

Poodle and TLS-FALLBACK-SCSV

SSLv3 allows exploiting of the POODLE bug. This is one more major reason to disable this.

Google have proposed an extension to SSL/TLS named TLSFALLBACKSCSV that seeks to prevent forced SSL downgrades. This is automatically enabled if you upgrade OpenSSL to the following versions:

  • OpenSSL 1.0.1 has TLSFALLBACKSCSV in 1.0.1j and higher.
  • OpenSSL 1.0.0 has TLSFALLBACKSCSV in 1.0.0o and higher.
  • OpenSSL 0.9.8 has TLSFALLBACKSCSV in 0.9.8zc and higher.

The Cipher Suite

Forward Secrecy ensures the integrity of a session key in the event that a long-term key is compromised. PFS accomplishes this by enforcing the derivation of a new key for each and every session.

This means that when the private key gets compromised it cannot be used to decrypt recorded SSL traffic.

The cipher suites that provide Perfect Forward Secrecy are those that use an ephemeral form of the Diffie-Hellman key exchange. Their disadvantage is their overhead, which can be improved by using the elliptic curve variants.

The following two ciphersuites are recommended by me, and the latter by the Mozilla Foundation.

The recommended cipher suite:

ssl.cipher-list = "AES128+EECDH:AES128+EDH"

The recommended cipher suite for backwards compatibility (IE6/WinXP):

ssl.cipher-list = "ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DES-CBC3-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4"

If your version of OpenSSL is old, unavailable ciphers will be discarded automatically. Always use the full ciphersuite above and let OpenSSL pick the ones it supports.

The ordering of a ciphersuite is very important because it decides which algorithms are going to be selected in priority. The recommendation above prioritizes algorithms that provide perfect forward secrecy.

Older versions of OpenSSL may not return the full list of algorithms. AES-GCM and some ECDHE are fairly recent, and not present on most versions of OpenSSL shipped with Ubuntu or RHEL.

Prioritization logic

  • ECDHE+AESGCM ciphers are selected first. These are TLS 1.2 ciphers and not widely supported at the moment. No known attack currently target these ciphers.
  • PFS ciphersuites are preferred, with ECDHE first, then DHE.
  • AES 128 is preferred to AES 256. There has been discussions on whether AES256 extra security was worth the cost, and the result is far from obvious. At the moment, AES128 is preferred, because it provides good security, is really fast, and seems to be more resistant to timing attacks.
  • In the backward compatible ciphersuite, AES is preferred to 3DES. BEAST attacks on AES are mitigated in TLS 1.1 and above, and difficult to achieve in TLS 1.0. In the non-backward compatible ciphersuite, 3DES is not present.
  • RC4 is removed entirely. 3DES is used for backward compatibility. See discussion in #RC4_weaknesses

Mandatory discards

  • aNULL contains non-authenticated Diffie-Hellman key exchanges, that are subject to Man-In-The-Middle (MITM) attacks
  • eNULL contains null-encryption ciphers (cleartext)
  • EXPORT are legacy weak ciphers that were marked as exportable by US law
  • RC4 contains ciphers that use the deprecated ARCFOUR algorithm
  • DES contains ciphers that use the deprecated Data Encryption Standard
  • SSLv2 contains all ciphers that were defined in the old version of the SSL standard, now deprecated
  • MD5 contains all the ciphers that use the deprecated message digest 5 as the hashing algorithm

HTTP Strict Transport Security

When possible, you should enable HTTP Strict Transport Security (HSTS), which instructs browsers to communicate with your site only over HTTPS.

View my article on HTST to see how to configure it.

HTTP Public Key Pinning Extension

You should also enable the HTTP Public Key Pinning Extension.

Public Key Pinning means that a certificate chain must include a whitelisted public key. It ensures only whitelisted Certificate Authorities (CA) can sign certificates for *.example.com, and not any CA in your browser store.

I’ve written an article about it that has background theory and configuration examples for Apache, Lighttpd and NGINX:https://raymii.org/s/articles/HTTPPublicKeyPinningExtension_HPKP.html

Config Example

var.confdir = "/etc/ssl/certs"
$SERVER["socket"] == ":443" {
  ssl.engine = "enable"
  ssl.pemfile = var.confdir + "/example.org.pem"
  ssl.ca-file = var.confdir + "/example.org.bundle.crt"
  server.name = var.confdir + "/example.org"
  server.document-root = "/srv/html"
  ssl.use-sslv2 = "disable"
  ssl.use-sslv3 = "disable"
  ssl.use-compression = "disable"
  ssl.honor-cipher-order = "enable"
  ssl.cipher-list = "AES128+EECDH:AES128+EDH:!aNULL:!eNULL"
}

Conclusion

If you have applied the above config lines you need to restart lighttpd:

/etc/init.d/lighttpd restart

Now use the SSL Labs test to see if you get a nice A. And of course have a safe and future proof SSL configuration!

HTTP Strict Transport Security for Apache, NGINX and Lighttpd


HTTP Strict Transport Security (often abbreviated as HSTS) is a security feature that lets a web site tell browsers that it should only be communicated with using HTTPS, instead of using HTTP. This tutorial will show you how to set up HSTS in Apache2, NGINX and Lighttpd. It is tested with all mentioned webservers, NGINX 1.1.19, Lighttpd 1.4.28 and Apache 2.2.22 on Ubuntu 12.04, Debian 6 & 7 and CentOS 6.It should work on other distro’s however, these are just reference values.

What is HTTP Strict Transport Security?

Quoting the Mozilla Developer Network:

If a web site accepts a connection through HTTP and redirects to HTTPS, the user in this case may initially talk to the non-encrypted version of the site before being redirected, if, for example, the user types http://www.foo.com/ or even just foo.com.

This opens up the potential for a man-in-the-middle attack, where the redirect could be exploited to direct a user to a malicious site instead of the secure version of the original page.

The HTTP Strict Transport Security feature lets a web site inform the browser that it should never load the site using HTTP, and should automatically convert all attempts to access the site using HTTP to HTTPS requests instead.

An example scenario:

You log into a free WiFi access point at an airport and start surfing the web, visiting your online banking service to check your balance and pay a couple of bills. Unfortunately, the access point you're using is actually a hacker's laptop, and they're intercepting your original HTTP request and redirecting you to a clone of your bank's site instead of the real thing. Now your private data is exposed to the hacker.

Strict Transport Security resolves this problem; as long as you've accessed your bank's web site once using HTTPS, and the bank's web site uses Strict Transport Security, your browser will know to automatically use only HTTPS, which prevents hackers from performing this sort of man-in-the-middle attack.

Do note that HSTS does not work if you’ve never visited the website before. A website needs to tell you it is HTTPS only.

Set up HSTS in Apache2

Edit your apache configuration file (/etc/apache2/sites-enabled/website.conf and /etc/apache2/httpd.conf for example) and add the following to your VirtualHost:

# Optionally load the headers module:
LoadModule headers_module modules/mod_headers.so

<VirtualHost 67.89.123.45:443>
    Header always set Strict-Transport-Security "max-age=63072000; includeSubdomains; preload"
</VirtualHost>

Now your website will set the header every time someone visits, with an expiration date of two years (in seconds). It sets it at every visit. So tomorrow, it will say two years again.
You do have to set it on the HTTPS vhost only. It cannot be in the http vhost.

To redirect your visitors to the HTTPS version of your website, use the following configuration:

<VirtualHost *:80>
  [...]
  ServerName example.com
  Redirect permanent / https://example.com/
</VirtualHost>

If you only redirect, you dont even need a document root.

You can also use modrewrite, however the above method is simpler and safer. However, modrewrite below redirects the user to the page they were visiting over https, the above config just redirects to /:

<VirtualHost *:80>
  [...]
  <IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTPS} off
    RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
  </IfModule>
</VirtualHost>

And don’t forget to restart Apache.

Lighttpd

The lighttpd variant is just as simple. Add it to your Lighttpd configuration file (/etc/lighttpd/lighttpd.conf for example):

server.modules += ( "mod_setenv" )
$HTTP["scheme"] == "https" {
    setenv.add-response-header  = ( "Strict-Transport-Security" => "max-age=63072000; includeSubdomains; preload")
}

And restart Lighttpd. Here the time is also two years.

NGINX

NGINX is even shorter with its config. Add this in the server block for your HTTPS configuration:

add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";

Don’t forget to restart NGINX.

X-Frame-Options header

The last tip I’ll give you is the X-Frame-Options header, which you can add to your HTTPS website to make sure it is not embedded in a frame or iframe. This avoids clickjacking, and might be helpfull for HTTPS websites. Quoting the Mozilla Developer Network again:

The X-Frame-Options HTTP response header can be used to indicate whether or not a browser should be allowed to render a page in a `<frame>` or `<iframe>`. Sites can use this to avoid clickjacking attacks, by ensuring that their content is not embedded into other sites.

You can change DENY to SAMEORIGIN or ALLOW-FROM uri, see the Mozilla link above for more information on that. (Or the RFC.)

X-Frame-Options for Apache2

As above, add this to the apache config file:

Header always set X-Frame-Options DENY

Lighttpd

This goes in the lighttpd config. Make sure you don’t double the above set config, if you have that, just add the rule it to it.

server.modules += ( "mod_setenv" )
$HTTP["scheme"] == "https" {
    setenv.add-response-header  = ( "X-Frame-Options" => "DENY")
}

NGINX

Yet again, in a server block:

add_header X-Frame-Options "DENY";

HTTP Public Key Pinning Extension HPKP for Apache, NGINX and Lighttpd


Public Key Pinning means that a certificate chain must include a whitelisted public key. It ensures only whitelisted Certificate Authorities (CA) can sign certificates for *.example.com, and not any CA in your browser store. This article has background theory and configuration examples for Apache, Lighttpd and NGINX.

HTTP Public Key Pinning Extension

An example might be your bank, which always have their certificate from CA Company A. With the current certificate system, CA Company B, CA Company C and the NSA CA can all create a certificate for your bank, which your browser will hapily accept because those companies are also trusted root CA’s.

If the bank implements HPKP and pin’s their first intermidiate certificate (from CA Company A), browsers will not accept certificates from CA Company B and CA Company C, even if they have a valid trust path. HPKP also allows your browser to report back the failure to the bank, so that they know they are under attack.

Public Key Pinning Extension for HTTP (HPKP) is a standard for public key pinning for HTTP user agents that’s been in development since 2011. It was started by Google, which, even though it had implemented pinning in Chrome, understood that manually maintaining a list of pinned sites can’t scale.

Here is a quick feature overview of HPKP:

  • HPKP is set at the HTTP level, using the Public-Key-Pins response header.
  • The policy retention period is set with the max-age parameter, it specifies duration in seconds.
  • The PKP header can only be used over an error-free secure encryption.
  • If multiple headers are seen, only the first one is processed.
  • Pinning can be extended to subdomains with the includeSubDomains parameter.
  • When a new PKP header is received, it overwrites previously stored pins and metadata.
  • A pin consists out of the hashing algorithm and an “Subject Public Key Info” fingerprint.

This article first has some theory about the workings of HPKP, down below you’ll find the part which shows you how to get the required fingerprints and has web server configuration.

SPKI Fingerprint – Theory

As explained by Adam Langley in his post, we hash a public key, not a certificate:

In general, hashing certificates is the obvious solution, but the wrong one. The problem is that CA certificates are often reissued: there are multiple certificates with the same public key, subject name etc but different extensions or expiry dates. Browsers build certificates chains from a pool of certificates, bottom up, and an alternative version of a certificate might be substituted for the one that you expect.

For example, StartSSL has two root certificates: one signed with SHA1 and the other with SHA256. If you wished to pin to StartSSL as your CA, which certificate hash would you use? You would have to use both, but how would you know about the other root if I hadn’t just told you?

Conversely, public key hashes must be correct:

Browsers assume that the leaf certificate is fixed: it’s always the starting point of the chain. The leaf certificate contains a signature which must be a valid signature, from its parent, for that certificate. That implies that the public key of the parent is fixed by the leaf certificate. So, inductively, the chain of public keys is fixed, modulo truncation.

The only sharp edge is that you mustn’t pin to a cross-certifying root. For example, GoDaddy’s root is signed by Valicert so that older clients, which don’t recognise GoDaddy as a root, still trust those certificates. However, you wouldn’t want to pin to Valicert because newer clients will stop their chain at GoDaddy.

Also, we’re hashing the SubjectPublicKeyInfo not the public key bit string. The SPKI includes the type of the public key and some parameters along with the public key itself. This is important because just hashing the public key leaves one open to misinterpretation attacks. Consider a Diffie-Hellman public key: if one only hashes the public key, not the full SPKI, then an attacker can use the same public key but make the client interpret it in a different group. Likewise one could force an RSA key to be interpreted as a DSA key etc.

Where to Pin

Where should you pin? Pinning your own public key is not the best idea. The key might change or get compromised. You might have multiple certificates in use. The key might change because you rotate your certificates every so often. It might key compromised because the web server was hacked.

The easiest, but not most secure place to pin is the first intermediate CA certificate. The signature of that certificate is on your websites certificate so the issuing CA’s public key must always be in the chain.

This way you can renew your end certificate from the same CA and have no pinning issues. If the CA issues a different root, then you have a problem, there is no clear solution for this yet. There is one thing you can do to mitigate this:

  • Always have a backup pin and a spare certificate from a different CA.

The RFC states that you need to provide at least two pins. One of the pins must be present in the chain used in the connection over which the pins were received, the other pin must not be present.

This other pin is your backup public key. It can also be the SPKI fingerprint of a different CA where you have a certificate issued.

An alternative and more secure take on this issue is to create at least three seperate public keys beforehand (using OpenSSL, see this pagefor a Javascript OpenSSL command generator) and to keep two of those keys as a backup in a safe place, offline and off-site.

You create the SPKI hashes for the three certificates and pin those. You only use the first key as the active certificate. When it is needed, you can then use one of the alternative keys. You do however need to let that certificate sign by a CA to create a certificate pair and that process can take a few days depending on the certificate.

This is not a problem for the HPKP because we take the SPKI hash of the public key, and not of the certificate. Expiration or a different chain of CA signer do not matter in this case.

If you have the means and procedures to create and securely save at least three seperate keys as described above and pin those, it would also protect you from your CA provider getting compromised and giving out a fake certificate for your specific website.

SPKI Fingerprint

To get the SPKI fingerprint from a certificate we can use the following OpenSSL command, as shown in the RFC draft:

openssl x509 -noout -in certificate.pem -pubkey | \
openssl asn1parse -noout -inform pem -out public.key;
openssl dgst -sha256 -binary public.key | openssl enc -base64

Result:

klO23nT2ehFDXCfx3eHTDRESMz3asj1muO+4aIdjiuY=

The input certificate.pem file is the first certificate in the chain for this website. (At the time of writing,COMODO RSA Domain Validation Secure Server CA, Serial 2B:2E:6E:EA:D9:75:36:6C:14:8A:6E:DB:A3:7C:8C:07.)

You need to also do this with your backup public key, ending up with two fingerprints.

Bugs

At the time of writing this article (2015-Jan) the only browser supporting HPKP (Chrome) has a serious issue where Chrome doesn’t treat the max-age and includeSubdomains directives from HSTS and HPKP headers as mutually exclusive. This means that if you have HSTS and HPKP with different policiesfor max-age or includeSubdomains they will be interchanged. See this bug for more info:https://code.google.com/p/chromium/issues/detail?id=444511. Thanks to Scott Helme from https://scotthelme.co.uk for finding and notifying me and the Chromium project about it.

Webserver configuration

Below you’ll find configuration instructions for the three most populair web servers. Since this is just a HTTP header, almost all web servers will allow you to set this. It needs to be set for the HTTPS website.

The example below pins the COMODO RSA Domain Validation Secure Server CA and the Comodo PositiveSSL CA 2 as a backup, with a 30 day expire time including all subdomains.

Apache

Edit your apache configuration file (/etc/apache2/sites-enabled/website.conf or /etc/apache2/httpd.conf for example) and add the following to your VirtualHost:

# Optionally load the headers module:
LoadModule headers_module modules/mod_headers.so

Header set Public-Key-Pins "pin-sha256=\"klO23nT2ehFDXCfx3eHTDRESMz3asj1muO+4aIdjiuY=\"; pin-sha256=\"633lt352PKRXbOwf4xSEa1M517scpD3l5f79xMD9r9Q=\"; max-age=2592000; includeSubDomains"

Lighttpd

The lighttpd variant is just as simple. Add it to your Lighttpd configuration file (/etc/lighttpd/lighttpd.conf for example):

server.modules += ( "mod_setenv" )
$HTTP["scheme"] == "https" {
    setenv.add-response-header  = ( "Public-Key-Pins" => "pin-sha256=\"klO23nT2ehFDXCfx3eHTDRESMz3asj1muO+4aIdjiuY=\"; pin-sha256=\"633lt352PKRXbOwf4xSEa1M517scpD3l5f79xMD9r9Q=\"; max-age=2592000; includeSubDomains")
}

NGINX

NGINX is even shorter with its config. Add this in the server block for your HTTPS configuration:

add_header Public-Key-Pins 'pin-sha256="klO23nT2ehFDXCfx3eHTDRESMz3asj1muO+4aIdjiuY="; pin-sha256="633lt352PKRXbOwf4xSEa1M517scpD3l5f79xMD9r9Q="; max-age=2592000; includeSubDomains';

Reporting

HPKP reporting allows the user-agent to report any failures back to you.

If you add an aditional report-uri="http://example.org/hpkp-report" parameter to the header and set up a listener there, clients will send reports if they encounter a failure. A report is sent as a POST request to the report-uri with a JSON body like this:

{
    "date-time": "2014-12-26T11:52:10Z",
    "hostname": "www.example.org",
    "port": 443,
    "effective-expiration-date": "2014-12-31T12:59:59",
    "include-subdomains": true,
    "served-certificate-chain": [
        "-----BEGINCERTIFICATE-----\nMIIAuyg[...]tqU0CkVDNx\n-----ENDCERTIFICATE-----"
    ],
    "validated-certificate-chain": [
        "-----BEGINCERTIFICATE-----\nEBDCCygAwIBA[...]PX4WecNx\n-----ENDCERTIFICATE-----"
    ],
    "known-pins": [
        "pin-sha256=\"dUezRu9zOECb901Md727xWltNsj0e6qzGk\"",
        "pin-sha256=\"E9CqVKB9+xZ9INDbd+2eRQozqbQ2yXLYc\""
    ]
}

No Enforcment, report only

HPKP can be set up without enforcement, in reporting mode by using the Public-Key-Pins-Report-Only response header.

This approach allows you to set up pinning without your site being unreachable or HPKP being configured incorrectly. You can later move to enforcement by changing the header back to Public-Key-Pins.