Tag Archives: DNS

Server-Verfügbarkeit: Monitoring-Werkzeuge

Angreifer verwenden gern Ping und Traceroute, um Server im Internet ausfindig zu machen. Das bringt viele Security-Admins in Versuchung, den Ping- und Traceroute-Verkehr mittels ihrer Firewall in ihrem Netz zu unterbinden. Doch damit behindern sie nur die Arbeit von Server-Administratoren, denn es gibt noch viel mehr Möglichkeiten, Server aufzuspüren.

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Pi-hole Installation Guide

You probably know already the concept of the Pi-hole. If not: It’s a (forwarding) DNS server that you can install on your private network at home. All your clients, incl. every single smartphone, tablet, laptop, and IoT devices such as smart TVs or light bulb bridges, can use this Pi-hole service as their DNS server. Now here’s the point: it not only caches DNS entries, but blocks certain queries for hostnames that are used for ads, tracking, or even malware. That is: You don’t have to use an ad- or track-blocker on your devices (which is not feasible on smart TVs or smartphone apps, etc.), but you’re blocking this kind of sites entirely. Nice approach!

Yes, there are already some setup tutorials for the Pi-hole out there. However, it’s not only about installing the mere Pi-hole, but setting it up with your own recursive DNS server (since the default installation forwards to public DNS servers), using DNSSEC, and adding some more adlists. That’s why I am listing my installation procedure here as well. However, it’s not a complete beginners guide. You’ll need some basic Linux know-how.

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More Capture Details

In the previous post, I released my Ultimate PCAP which includes every single pcap I had so far on my blog. But that’s not all: I have some packets in there that were not yet published up to now. That is, here are some more details about those (probably well-known) protocols. These are:

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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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My Network Gadgets

This post is not about software but hardware tools for network admins. Which network gadgets am I using during my daily business? At least three, namely the Airconsole, the Pockethernet and the ProfiShark, which help me in connecting to serial ports, testing basic network connectivity, and capturing packets in a high professional way. Come in and have a look at how I’m working.

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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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IPv6 Renumbering: A Pain in the …

If you’re following my blog you probably know that I am using IPv6 everywhere. Everything in my lab is dual-stacked if not already IPv6-only. Great so far.

A few months ago my lab moved to another ISP which required to change all IP addresses (since I don’t have PI space yet). Oh boy! While it was almost no problem to change the legacy IPv4 addresses (only a few NATs), it was a huge pain in the … to change the complete infrastructure with its global unicast IPv6 addresses. It turned out that changing the interface IPv6 addresses was merely the first step, while many modifications at different services were the actual problem. And this was *only* my lab and not a complex company or the like.

Following you find a list of changes I made for IPv6 and for legacy IP. Just an overview to get an idea of differences and stumbling blocks.

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IPv6 Upper Layer Protocol Samples

Some time ago I published a pcap that can be used to study basic IPv6 protocol messages such as ICMPv6 for Router Advertisements, Neighbor Solicitations, etc.: “Basic IPv6 Messages: Wireshark Capture“. You can use it to learn the basic IPv6 address assignment and layer 2 address resolution. However, that pcap does not include any upper layer protocols.

This time I captured a few application layer protocols that I used over IPv6 rather than over legacy IP. Common user protocols such as DNS, HTTP/S, IMAP, SMTP (with STARTTLS), as well as some network administration protocols: SSH, SNMP, and Ping. It is not that interesting at all ;) though you can use it to have some examples for Wireshark to prove that those application protocols are almost the same when run above IPv6 compared to IPv4.

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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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Benchmarking DNS: namebench & dnseval

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 8.8.8.8.

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.

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