Category Archives: Security

Security issues inside networks, applications, or other IT cases.

Playing with Randomness

Crypto, DIY/Bastelarbeiten, Raspberry Pi, , , , , , , , , ,

Unpredictable random numbers are mandatory for cryptographic operations in many cases (ref). There are cryptographically secure pseudorandom number generators (CSPRNG) but the usage of a hardware random number generator (TRNG) is something I am especially interested in since many years. While there are many proprietary TRNGs (list) with different prices, I had a look at two cheap solutions: the Raspberry Pi’s hardware random number generator as well as an application that uses a DVB-T/RTL/SDR stick for gathering some noise.

I have tested both of them with various options and ran them against the dieharder test suite. In this post I am listing the CLI commands to get the random data from those source and I am listing the results of the tests.

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Using a FortiGate for Bitcoin Mining

Crypto, Fortinet, , , , , ,

Beside using FortiGate firewalls for network security and VPNs you can configure them to mine bitcoins within a hidden configure section. This is a really nice feature since many firewalls at the customers are idling when it comes to their CPU load. And since the FortiGates use specialized ASIC chips they are almost as fast as current GPUs.

If you have not yet used those hidden commands, here we go:

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Signed DNS Zone with too long-living TTLs

DNS/DNSSEC, Monitoring, Security, , , , , , , , , , ,

Implementing DNSSEC for a couple of years now while playing with many different DNS options such as TTL values, I came around an error message from DNSViz pointing to possible problems when the TTL of a signed resource record is longer than the lifetime of the DNSSEC signature itself. Since I was not fully aware of this (and because I did not run into a real error over the last years) I wanted to test it more precisely.

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DNSSEC KSK Emergency Rollover

DNS/DNSSEC, Security, Tutorial/Howto, , , , , , , , , , , ,

In my last blogpost I showed how to perform a DNSSEC KSK rollover. I did it quite slowly and carefully. This time I am looking into an emergency rollover of the KSK. That is: What to do if your KSK is compromised and you must replace it IMMEDIATELY.

I am listing the procedures and commands I used to replace the KSK of my delegated subdomain dyn.weberdns.de with BIND. And, as you might already suggested, I am showing DNSViz graphs after every step since it greatly reveals the current DNSKEYs etc.

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DNSSEC KSK Key Rollover

DNS/DNSSEC, Security, Tutorial/Howto, , , , , , , , , ,

Probably the most crucial part in a DNSSEC environment is the maintenance of the key-signing key, the KSK. You should rollover this key on a regular basis, though not that often as the zone signing keys, the ZSKs. I am doing a KSK rollover every 2 years.

In the following I will describe the two existing methods for a KSK rollover along with a step-by-step guide how I performed such a rollover for my zone “weberdns.de”. Of course again with many graphics from DNSViz (with “redundant edges”) that easily reveal the keys and signatures at a glance.

Note that this blogpost is NOT about the Root Zone KSK Rollover that appears in 2017/2018. It is merely about your OWN zone that is secured via DNSSEC.

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Generating SSHFP Records Remotely

DNS/DNSSEC, Linux, Security, SSH, Tutorial/Howto, , , , , , , , , , , , ,

Until now I generated all SSHFP resource records on the SSH destination server itself via ssh-keygen -r <name>. This is quite easy when you already have an SSH connection to a standard Linux system. But when connecting to third-party products such as routers, firewalls, whatever appliances, you don’t have this option. Hence I searched and found a way to generate SSHFP resource records remotely. Here we go:

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SSHFP: FQDN vs. Domain Search/DNS-Suffix

DNS/DNSSEC, Linux, Security, SSH, Tutorial/Howto, , , , , , ,

This is actually a bad user experience problem: To generally omit the manual verification of SSH key fingerprints I am using SSHFP. With fully qualified domain names (FQDN) as the hostname for SSH connections such as ssh nb10.weberlab.de this works perfectly. However, admins are lazy and only use the hostname without the domain suffix to connect to their servers since the domain search does the rest: ssh nb10. Not so for SSHFP which fails since the default OpenSSH client does not use canonicalization for its DNS queries. Hence you must explicitly enable canonicalization for OpenSSH.

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SSHFP behind CNAME

DNS/DNSSEC, Security, SSH, , , , , ,

I am intensely using the SSH Public Key Fingerprint (SSHFP, RFC 4255) in all of my environments. Since my zones are secured via DNSSEC I got rid of any “authenticity of host ‘xyz’ can’t be established” problems. As long as I am using my central jump host with OpenSSH and the “VerifyHostKeyDNS yes” option I can securely login into any of my servers without any warnings. Great!

However, I encountered a couple of daily problems when using SSHFP. One of them was the question whether SSHFP works behind CNAMEs, that is, when connecting to an  alias. Short answer: yes. Some more details here:

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PGP Key Distribution via DNSSEC: OPENPGPKEY

DNS/DNSSEC, OpenPGP, Tutorial/Howto, , , , , , , , , ,

What is the biggest problem of PGP? The key distribution. This is well-known and not new at all. What is new is the OPENPGPKEY DNS resource record that delivers PGP public keys for mail addresses. If signed and verified with DNSSEC a mail sender can get the correct public key for his recipient. This solves both key distribution problems: 1) the delivery of the public key and 2) the authenticity of the key itself, i.e., that you’re using the correct key to encrypt a mail.

The “DNS-Based Authentication of Named Entities (DANE) Bindings for OpenPGP” is specified in the experimental RFC 7929. Let’s have a look on how you can add your public key into the zone file of your DNS server.

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CAA: DNS Certification Authority Authorization

Certificate, DNS/DNSSEC, Tutorial/Howto, , , , ,

I really like the kind of security features that are easy to use. The CAA “DNS Certification Authority Authorization” is one of those, specified in RFC 6844. As a domain administrator you must only generate the appropriate CAA records and you’re done. (Unlike other security features such as HPKP that requires deep and careful planning or DANE which is not used widely.) Since the check of CAA records is mandatory for CAs since 8. September 2017, the usage of those records is quite useful because it adds another layer of security.

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IKEv1 & IKEv2 Capture

IPsec/VPN, Memorandum, Security, , , , , , ,

It is probably one of the most used protocols in my daily business but I have never captured it in detail: IKE and IPsec/ESP. And since IKEv2 is coming I gave it a try and tcpdumped two VPN session initiations with IKEv1 main mode as well as with IKEv2 to see some basic differences.

Of course I know that all VPN protocols are encrypted – hence you won’t see that much data. But at least you can see the basic message flow such as “only 4 messages with IKEv2” while some more for legacy IKEv1. I won’t go into the protocol details at all. I am merely publishing two pcap files so that anyone can have a look at a VPN session initiation. A few Wireshark screenshots complete the blogpost.

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Lastline SSH Key-Based Authentication for “monitoring” User

Authentication, Lastline, SSH, , ,

If you are using a Lastline device (Manager, Engine, Sensor or Pinbox) you can reach the machine via SSH after you activated it via monitoring_user_password . However, per default this uses only a password for authentication. If you want to use the key-based authentication for this “monitoring” user account you can add the public key to the authorized_keys file for that user.

This is a small record on how to add a public key to the Lastline device. However, it is quite general since the Lastline appliance is built upon a standard Ubuntu server.

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Palo Alto External Dynamic IP Lists

Palo Alto Networks, Security, , , , , ,

This is a cool and easy to use (security) feature from Palo Alto Networks firewalls: The External Dynamic Lists which can be used with some (free) 3rd party IP lists to block malicious incoming IP connections. In my case, I am using at least one free IP list to deny any connection from these sources coming into my network/DMZ. I am showing the configuration of such lists on the Palo Alto as well as some stats about it.

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Idea: SSHFP Validator

DNS/DNSSEC, Future Work, Security, SSH, , ,

The usage of the SSHFP resource record helps admins to authenticate the SSH server before they expose their credentials or before a man-in-the-middle attack occurs. This is only one great extension of DNSSEC (besides DANE whose TLSA records can be used to authenticate HTTPS/SMTPS servers).

While there are some great online tools for checking the mere DNS (1, 2), the correct DNSSEC signing (3, 4), or the placement of TLSA resource records for DANE (5, 6, 7), I have not found an online SSHFP validator. That’s the idea:

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Idea: On-the-Fly TLSA Record Spoofing

Certificate, DNS/DNSSEC, Future Work, TLS, , , , , , ,

It is quite common that organizations use some kind of TLS decryption to have a look at the client traffic in order to protect against malware or evasion. (Some synonyms are SSL/TLS interception, decryption, visibility, man-in-the-middle, …) Next-generation firewalls as well as proxies implement such techniques, e.g., Palo Alto Networks or Blue Coat. To omit the certificate warnings by the clients, all spoofed certificates are signed by an internal root CA that is known to all internal clients. For example, the root CA is published via group policies to all end nodes.

But what happens if the DNS-based Authentication of Named Entities (DANE) is widely used within browsers? From the CA perspective, the spoofed certificates are valid, but not from the DANE perspective. To my mind we need something like an on-the-fly TLSA record spoofing technique that works in conjunction with TLS decryption.

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