Tag Archives: DNS

"decisions" by Martin Fisch is licensed under CC BY-SA 2.0

Detect DNS Spoofing: dnstraceroute

Another great tool from Babak Farrokhi is dnstraceroute. It is part of the DNSDiag toolkit from which I already showed the dnsping feature. With dnstraceroute you can verify whether a DNS request is indeed answered by the correct DNS server destination or whether a man-in-the-middle has spoofed/hijacked the DNS reply. It works by using the traceroute trick by incrementing the TTL value within the IP header from 1 to 30.

Beside detecting malicious DNS spoofing attacks, it can also be used to verify security features such as DNS sinkholing. I am showing the usage as well as a test case for verifying a sinkhole feature.

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"Werkzeug" by Andreas Issleib is licensed under CC BY-NC-ND 2.0

Compare & Troubleshoot DNS Servers: dnseval

The third tool out of the DNSDiag toolkit from Babak is dnseval. “dnseval is a bulk ping utility that sends an arbitrary DNS query to a given list of DNS servers. This script is meant for comparing response times of multiple DNS servers at once”. It is not only listing the response times but also further information about the DNS responses such as the TTL and the flags. Really great for comparison and troubleshooting different DNS forwarders as well as own authoritative DNS server responses as seen by others.

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"Füße" by Robert Agthe is licensed under CC BY 2.0

How to walk DNSSEC Zones: dnsrecon

After the implementation of DNS and DNSSEC (see the last posts) it is good to do some reconnaissance attacks against the own DNS servers. Especially to see the NSEC or NSEC3 differences, i.e., whether zone walking (enumeration) is feasible or not.

For many different kinds of DNS reconnaissance the tool dnsrecon can be used. In this post I will focus on the -z  option which is used for DNSSEC zone walking, i.e., walk leaf by leaf of the whole DNS zone.

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"Keys" by Rosa Say is licensed under CC BY-NC-ND 2.0

DNSSEC ZSK Key Rollover

One important maintenance requirement for DNSSEC is the key rollover of the zone signing key (ZSK). With this procedure a new public/private key pair is used for signing the resource records, of course without any problems for the end user, i.e., no falsified signatures, etc.

In fact it is really simply to rollover the ZSK with BIND. It is almost one single CLI command to generate a new key with certain time ranges. BIND will use the correct keys at the appropriate time automatically. Here we go:

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DANE featured image

How to use DANE/TLSA

DNS-based Authentication of Named Entities (DANE) is a great feature that uses the advantages of a DNSSEC signed zone in order to tell the client which TLS certificate he has to expect when connecting to a secure destination over HTTPS or SMTPS. Via a secure channel (DNSSEC) the client can request the public key of the server. This means, that a Man-in-the-Middle attack (MITM) with a spoofed certificate would be exposed directly, i.e., is not possible anymore. Furthermore, the trust to certificate authorities (CAs) is not needed anymore.

In this blog post I will show how to use DANE and its DNS records within an authoritative DNS server to provide enhanced security features for the public.

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BIND DNSSEC Signing

DNSSEC Signing w/ BIND

To solve the chicken-or-egg problem for DNSSEC from the other side, let’s use an authoritative DNS server (BIND) for signing DNS zones. This tutorial describes how to generate the keys and configure the “Berkeley Internet Name Domain” (BIND) server in order to automatically sign zones. I am not explaining many details of DNSSEC at all, but only the configuration and verification steps for a concrete BIND server.

It is really easy to tell BIND to do the inline signing. With this option enabled, the admin can still configure the static database for his zone files without any relation to DNSSEC. Everything with signing and maintaining is fully done by BIND without any user interaction. Great.

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Basic BIND Installation - featured image

Basic BIND Installation

This is a basic tutorial on how to install BIND, the Berkeley Internet Name Domain server, on a Ubuntu server in order to run it as an authoritative DNS server. It differs from other tutorials because I am using three servers (one as a hidden primary and two slaves as the public accessible ones), as well as some security such as denying recursive lookups and public zone transfers, as well as using TSIG for authenticating internal zone transfers. That is, this post is not an absolute beginner’s guide.

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Palo Alto DNS Proxy Rule featured image

Palo Alto DNS Proxy Rule for Reverse DNS

I am using the DNS Proxy on a Palo Alto Networks firewall for some user subnets. Beside the default/primary DNS server it can be configured with proxy rules (sometimes called conditional forwarding) which I am using for reverse DNS lookups, i.e., PTR records, that are answered by a BIND DNS server. While it is easy and well-known to configure the legacy IP (IPv4) reverse records, the IPv6 ones are slightly more difficult. Fortunately there are some good tools on the Internet to help reversing IPv6 addresses.

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Palo Alto FQDN Objects featured image

Palo Alto FQDN Objects

While I tested the FQDN objects with a Palo Alto Networks firewall, I ran into some strange behaviours which I could not reproduce, but have documented them. I furthermore tested the usage of FQDN objects with more than 32 IP addresses, which are the maximum that are supported due to the official Palo Alto documentation. Here we go:

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Advanced Ping: httping, dnsping, smtpping

I really love ping! It is easy to use and directly reveals whether the network works or not. Refer to Why Ping is no Security Flaw! (But your Friend) and Advanced Tracerouting. At least outgoing pings (from trust to untrust) should be allowed without any security concerns. However, many companies are denying these ICMP echo-requests from untrust into the DMZ which makes it difficult to test whether all servers are up and running.

I was sitting at the customer’s site replacing the DMZ firewall. Of course I wanted to know (from the outside) whether all servers are connected correctly (NAT) and whether the firewall permits the connections (policy). However, ping was not allowed. Therefore I used several layer 7 ping tools that generate HTTP, DNS, or SMTP sessions (instead of ICMP echo-requests) and revealed whether the services (and not only the servers) were running. Great!

This post shows the installation and usage of httping, dnsping, and smtpping on a Linux machine, in my case a Ubuntu server 14.04.4 LTS, as well as some Wireshark screenshots from captured sessions.

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IPv6 Dynamic Prefix Problems featured image

IPv6 Dyn Prefix Problems

I am lucky to have a full dual-stack ISP connection at home. However, the ISP only offers a dynamic IPv6 prefix with all of its disadvantages (while no single advantage). In this post, I am summarizing the limitations of a dynamic prefix and some of the ideas on how to overcome them. I am always comparing the “IPv6 dynamic prefix” state with the legacy “dynamic IPv4 address” situation. I suppose that some of these problems will hit many small office / home office locations during the next years.

Of course, IPv6 ISP connections with dynamic prefixes should only be purchased at private home sites. It is no problem to have new IPv6 addresses there because all connections are outbound. However, many small remote offices (SOHO) might rely on such cheap ISP connections, too. If they provide some servers in a DMZ or other components such as network cameras, building components with IPv6 connections, etc., they will run into these kind of problems. (The remote office could even tunnel every outbound IPv6 traffic through a VPN to the headquarter. But if it wants to use a local breakout, this won’t be an alternative.)

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DNS Proxy Featured Image

Palo Alto: DNS Proxy for Management Services

The Palo Alto firewall has a feature called DNS Proxy. Normally it is used for data plane interfaces so that clients can use the interfaces of the Palo for its recursive DNS server. Furthermore, this DNS Proxy Object can be used for the DNS services of the management plane, specified under Device -> Setup -> Services. However, there was a bug in PAN-OS that did not process the proxy rules and static entries when a DNS proxy object was used in the management plane. This bug was fixed in PAN-OS 6.0.0. I tested it in my lab with PAN-OS 6.1.0 running. Here are the successful results.

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IPv6 Dynamic Prefix - Featured Image

Idea: IPv6 Dynamic Prefix

For dynamic IPv4 addresses, dynamic DNS services such as Dyn or No-IP are well-known. If an ISP issues a single dynamic IPv4 address every 24 hours (or the like), the router or any other device registers the IPv4 address for a DNS record. With port-forwardings on the router, several services on different clients can be accessed.

Since there are some ISPs that offer dynamic IPv6 prefixes as well, I have a suggestion on how to optimize the “dynamic DNS” service for several IPv6 addresses and names. The main idea is to update only the IPv6 prefix, while the host IDs are static configured on the DNS server. This limits the DNS updates and expands the usage of DNS names even for devices that have no “DynDNS update client” built-in.

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Palo Alto DNS Proxy malicious request featured image

If only one DNS query is malicious …

… the whole Internet breaks down. So happened on a Palo Alto with a DNS proxy and a (slightly misconfigured) anti-spyware profile.

All network clients had a single DNS server configured, namely the DNS proxy of the Palo Alto. And as a single network client requested an URL that was classified as “spyware”, the Palo correctly (!) blocked the DNS session from its DNS proxy to the Internet. Unluckily, this session stayed active for a long time (with drop-all-packets) since many DNS requests were traversing through it. But since it stayed blocked, the Internet was “unavailable” for all end users

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