DNS security

O DNS não foi projetado com a segurança em mente e existem muitos tipos de ataques criados para explorar vulnerabilidades no sistema DNS.

Objetivos de aprendizado

Após ler este artigo, você será capaz de:

  • Entenda o que é o DNSSEC e como ele funciona
  • Familiarize-se com os ataques de DNS mais comuns
  • Diferencie DNSSEC e outras soluções de segurança de DNS

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Por que a segurança de DNS é importante?

As consultas padrão de DNS, que são necessárias para quase todo o tráfego da internet, criam oportunidades de explorações do DNS como o sequestro de DNS e ataques on-path. Esses ataques podem redirecionar o tráfego que chega a um site para uma cópia falsa do site, coletando informações confidenciais do usuário e expondo as empresas a um grande risco de responsabilidade civil. Uma das maneiras mais conhecidas de se proteger contra as ameaças ao DNS é adotar o protocolo DNSSEC.

What is DNSSEC?

Like many Internet protocols, the DNS system was not designed with security in mind and contains several design limitations. These limitations, combined with advances in technology, have made it easy for attackers to hijack a DNS lookup for malicious purposes, such as sending a user to a fraudulent website that can distribute malware or collect personal information.

DNS Security Extensions (DNSSEC) is a security protocol created to mitigate this problem. DNSSEC protects against attacks by digitally signing data to help ensure its validity. In order to ensure a secure lookup, the signing must happen at every level in the DNS lookup process.

Esse processo de assinatura é semelhante a alguém que assina um documento legal com uma caneta: essa pessoa assina de uma forma exclusiva que ninguém consegue copiar, e um perito forense pode examinar a assinatura e confirmar se o documento foi realmente assinado por essa pessoa. Essas assinaturas digitais garantem que os dados não foram adulterados.

O DNSSEC implementa uma política hierárquica de assinatura digital em todas as camadas do DNS. Por exemplo, no caso de uma pesquisa do domínio "google.com", um servidor raiz de DNS assinaria uma chave para o nameserver .COM e, em seguida, o nameserver .COM assinaria uma chave para o nameserver autoritativo do google.com.

While improved security is always preferred, DNSSEC is designed to be backwards-compatible to ensure that traditional DNS lookups still resolve correctly, albeit without the added security. DNSSEC is meant to work with other security measures like SSL/TLS as part of a holistic Internet security strategy.

DNSSEC creates a parent-child train of trust that travels all the way up to the root zone. This chain of trust cannot be compromised at any layer of DNS, or else the request will become open to an on-path attack.

To close the chain of trust, the root zone itself needs to be validated (proven to be free of tampering or fraud), and this is actually done using human intervention. Interestingly, in what’s called a Root Zone Signing Ceremony, selected individuals from around the world meet to sign the root DNSKEY RRset in a public and audited way.

Abaixo uma explicação mais detalhada de como o DNSSEC funciona >>>

Quais são alguns dos ataques comuns envolvendo o DNS?

DNSSEC is a powerful security protocol, but unfortunately it is not currently universally adopted. This lack of adoption coupled with other potential vulnerabilities, on top of the fact that DNS is an integral part of most Internet requests, makes DNS a prime target for malicious attacks. Attackers have found a number of ways to target and exploit DNS servers. Here are some of the most common:

Falsificação de DNS/envenenamento de cache: trata-se de um ataque no qual dados de DNS forjados são introduzidos no cache de um resolvedor de DNS e, como resultado, o resolvedor retorna um endereço IP incorreto para um domínio. Ao invés de ir para o site correto, o tráfego pode ser desviado para uma máquina mal-intencionada ou para qualquer outro lugar que o invasor desejar; frequentemente, será uma réplica do site original usada para fins mal-intencionados, como a distribuição de malware ou a coleta de informações de login.

DNS tunneling: This attack uses other protocols to tunnel through DNS queries and responses. Attackers can use SSH, TCP, or HTTP to pass malware or stolen information into DNS queries, undetected by most firewalls.

DNS hijacking: In DNS hijacking the attacker redirects queries to a different domain name server. This can be done either with malware or with the unauthorized modification of a DNS server. Although the result is similar to that of DNS spoofing, this is a fundamentally different attack because it targets the DNS record of the website on the nameserver, rather than a resolver’s cache.

Sequestro de DNS

NXDOMAIN attack: This is a type of DNS flood attack where an attacker inundates a DNS server with requests, asking for records that do not exist, in an attempt to cause a denial-of-service for legitimate traffic. This can be accomplished using sophisticated attack tools that can auto-generate unique subdomains for each request. NXDOMAIN attacks can also target a recursive resolver with the goal of filling the resolver’s cache with junk requests.

Phantom domain attack: A phantom domain attack has a similar result to an NXDOMAIN attack on a DNS resolver. The attacker sets up a bunch of ‘phantom’ domain servers that either respond to requests very slowly or not at all. The resolver is then hit with a flood of requests to these domains and the resolver gets tied up waiting for responses, leading to slow performance and denial-of-service.

Random subdomain attack: In this case, the attacker sends DNS queries for several random, nonexistent subdomains of one legitimate site. The goal is to create a denial-of-service for the domain’s authoritative nameserver, making it impossible to lookup the website from the nameserver. As a side effect, the ISP serving the attacker may also be impacted, as their recursive resolver's cache will be loaded with bad requests.

Domain lock-up attack: Attackers orchestrate this form of attack by setting up special domains and resolvers to create TCP connections with other legitimate resolvers. When the targeted resolvers send requests, these domains send back slow streams of random packets, tying up the resolver’s resources.

Botnet-based CPE attack: These attacks are carried out using CPE devices (Customer Premise Equipment; this is hardware given out by service providers for use by their customers, such as modems, routers, cable boxes, etc.). The attackers compromise the CPEs and the devices become part of a botnet, used to perform random subdomain attacks against one site or domain.

What is the best way to protect against DNS-based attacks?

In addition to DNSSEC, an operator of a DNS zone can take further measures to secure their servers. Over-provisioning infrastructure is one simple strategy to overcome DDoS attacks. Simply put, if your nameserver can handle several multiples more traffic than you expect, it is harder for a volume-based attack to overwhelm your server.

Anycast routing is another handy tool that can disrupt DDoS attacks. Anycast allows multiple servers to share a single IP address, so even if one DNS server gets shut down, there will still be others up and serving. Another popular strategy for securing DNS servers is a DNS firewall.

O que é um DNS Firewall?

O DNS Firewall é uma ferramenta que pode fornecer diversos serviços de segurança e desempenho para servidores de DNS. Um DNS Firewall fica entre o resolvedor recursivo de um usuário e o nameserver autoritativo do site ou serviço que ele está tentando acessar. O firewall pode fornecer um serviço de Rate Limiting para desligar os invasores que estão tentando sobrecarregar o servidor. Se o servidor tiver um tempo de inatividade como resultado de um ataque ou por qualquer outro motivo, o DNS Firewall poderá manter o site ou o serviço do operador funcionando ao fornecer respostas de DNS a partir do cache.

Além de seus recursos de segurança, um DNS Firewall pode também fornecer soluções de desempenho, como pesquisas de DNS mais rápidas e custos reduzidos de largura de banda para o operador de DNS. Saiba mais sobre o DNS Firewall da Cloudflare.

O DNS como uma ferramenta de segurança

DNS resolvers can also be configured to provide security solutions for their end users (people browsing the Internet). Some DNS resolvers provide features such as content filtering, which can block sites known to distribute malware and spam, and botnet protection, which blocks communication with known botnets. Many of these secured DNS resolvers are free to use and a user can switch to one of these recursive DNS services by changing a single setting in their local router. Cloudflare DNS has an emphasis on security.

Are DNS queries private?

Another important DNS security issue is user privacy. DNS queries are not encrypted. Even if users use a DNS resolver like 1.1.1.1 that does not track their activities, DNS queries travel over the Internet in plaintext. This means anyone who intercepts the query can see which websites the user is visiting.

This lack of privacy has an impact on security and, in some cases, human rights; if DNS queries are not private, then it becomes easier for governments to censor the Internet and for attackers to stalk users' online behavior.

DNS over TLS and DNS over HTTPS are two standards for encrypting DNS queries in order to prevent external parties from being able to read them. Cloudflare DNS supports both of these standards. Cloudflare also partners with other organizations to help improve DNS security — for example, helping Mozilla enable DNS over HTTPS in its Firefox browser in order to protect users.