Tag Archives: DNSSEC

DNS Capture – The Records Edition

Some time ago I published a post called DNS Test Names & Resource Records which lists many different FQDNs with lots of different RRs. You can use those public available DNS names to test your DNS servers or the like. However, I was missing a packet capture showing all these resource records as they appear on the wire. So now, here it is. If you are searching for some packets to test your tools for whatever reason, feel free to download this pcap.

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Dive into delv: DNSSEC Validation

If you’re into DNSSEC, you’ll probably have to troubleshoot or at least to verify it. While there are some good online tools such as DNSViz, there is also a command-line tool to test DNSSEC signatures onsite: delv.

delv will send to a specified name server all queries needed to fetch and validate the requested data; this includes the original requested query, subsequent queries to follow CNAME or DNAME chains, and queries for DNSKEY, DS and DLV records to establish a chain of trust for DNSSEC validation. It does not perform iterative resolution, but simulates the behavior of a name server configured for DNSSEC validating and forwarding.

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DNS Capture: UDP, TCP, IP-Fragmentation, EDNS, ECS, Cookie

It’s not always this simple DNS thing such as “single query – single answer, both via UDP”. Sometimes you have some more options or bigger messages that look and behave differently on the network. For example: IP fragmentation for larger DNS answers that do not fit into a single UDP datagram (hopefully not after the DNS flag day 2020 anymore), or DNS via TCP, or some newer options within the EDNS space such as “EDNS Client Subnet” (ECS) or DNS cookies.

I won’t explain any details about those options, but I am publishing a pcap with that kind of packets along with some Wireshark screenshots. Feel free to dig into it.

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Single DNS Query – Hundreds of Packets

I was interested in how a recursive DNS server resolves DNS queries in detail. That is, not only the mere AAAA or A record, but also DNSSEC keys and signatures, the authority and additional section when testing with dig , and so on. For this I made two simple DNS queries to my recursive DNS server which resulted in more than 100 DNS packets at all. Wow.

In the following I am publishing a downloadable pcap so that you can analyse it by yourself. Furthermore I am showing some listings and screenshots to get an idea of the DNS resolution process.

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Signed DNS Zone with too long-living TTLs

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

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 suggest it, I am showing DNSViz graphs after every step since it greatly reveals the current DNSKEYs etc.

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DNSSEC KSK Key Rollover

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

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

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

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

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

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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BIND Inline-Signing Serial Numbers Cruncher

I know that BIND correctly changes the serial numbers of zones when it is enabled with inline signing and auto-dnssec. However, I got confused one more time as I looked on some of my SOA records. So, just for the record, here is an example how the serial numbers increase while the admin has not changed anything manually on the zone files.

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Idea: SSHFP Validator

The usage of the SSHFP resource record helps admins to authenticate the SSH server before they are exposing their credentials or before a man-in-the-middle attack occurs. This is only one great extension of DNSSEC (beside 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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