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.
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.
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.
What failover times do you expect from a network security device that claims to have high availability? 1 ms? Or at least <1 second? Not so for a fully featured Infoblox HA cluster which takes about 1-2 minutes, depending on its configuration. Yep. “Works as designed”. Ouch. Some details:
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.
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.
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.
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.
If you are already familiar with DNSSEC this is quite easy: How to sign a delegated subdomain zone. For the sake of completeness I am showing how to generate and use the appropriate DS record in order to preserve the chain of trust for DNSSEC.
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:
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.
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:
If you’re running your own DNS resolver you’re probably interested in some benchmark tests against it, such as: how fast does my own server (read: Raspberry Pi) answer to common DNS queries compared to 188.8.131.52.
In this blogpost I am showing how to use two tools for testing/benchmarking DNS resolvers: namebench & dnseval. I am listing the defaults, giving some hints about them and showing examples in which I tested some private and public DNS resolvers: a Fritzbox router, a Raspberry Pi with Unbound, Quad9, OpenDNS, and Google Public DNS.
Just a quick glance at the domain_analyzer script from Sebastián García and Verónica Valeros. “Domain analyzer is a security analysis tool which automatically discovers and reports information about the given domain. Its main purpose is to analyze domains in an unattended way.” Nice one. If you’re running your own DNS servers you should check e.g. whether your firewall rules are correct (scanned with Nmap) or whether you’re not allowing zone transfer, etc.
I am testing a lot with my own DNS servers as well as with third-party DNS implementations such as DNS proxies on firewalls, DNSSEC validation on resolvers, etc. While there are a number of free DNS online tools around the Internet I was lacking some DNS test names with certain properties or resource records. Hence I configured a couple of them on my own authoritative DNS servers and its zone weberdns.de.
For example we encountered a bug on the Palo Alto DNS proxy that has not stored the TTL value correctly – hence some test names with different TTL values. Or we had some problems when a single DNS name has more than 15 IPv4/IPv6 addresses – hence some test names with lots of addresses. And many more: Continue reading DNS Test Names & Resource Records