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DNS

The DNS resolver is responsible for resolving human-readable domain names into IP addresses and other type of machine-readable records. This is done by sending DNS queries to a DNS server, which then responds with the associated record of the domain name in question.

Settings​

The DNS resolver is configured with the following attributes, which are available under the resolver key:

  • type: The type of the DNS resolver.
  • preserve-intermediates: Whether to preserve the intermediate name servers in the DNS resolution results.
  • concurrency: The number of concurrent resolution requests that can be made at the same time.
  • timeout: The time after which a resolution request will be timed out if no response is received.
  • attempts: The number of times a resolution request will be retried before it is considered failed.
  • try-tcp-on-error: Whether to try using TCP for resolution requests if an error occurs during a UDP resolution request.

The supported DNS resolver types for the resolver.type value are:

  • system: Use the system's default DNS resolver.
  • cloudflare: Use Cloudflare's public DNS service.
  • cloudflare-tls: Use Cloudflare's public DNS service over a secure TLS connection.
  • quad9: Use Quad9's public DNS service.
  • quad9-tls: Use Quad9's public DNS service over a secure TLS connection.
  • google: Use Google's public DNS service.
  • custom: Use a custom list of DNS servers which are specified under the resolver.custom key.

When using a custom DNS resolver, the list of DNS servers have to be specified under the resolver.custom key using the [protocol://]IP:PORT format. For example:

[resolver]
custom = ["udp://192.0.2.1:55", "tcp://192.0.2.3:55", "8.8.8.8"]

Please note that system or custom should be used when using DNS block lists as lookups from public DNS services are not allowed by most DNSBL services.

Example​

[resolver]
type = "system"
preserve-intermediates = true
concurrency = 2
timeout = "5s"
attempts = 2
try-tcp-on-error = true

Public Suffix List​

The Public Suffix List is a list of top-level domain names (or suffixes) under which Internet users can register domain names. This list is used by the SMTP server to determine the root domain of a given domain name. For example, the root domain of example.com is com and the root domain of a domain ending in .co.uk is co.uk.

The URL to the Public Suffix list is configured under the resolver.public-suffix key. For example:

[resolver]
public-suffix = ["https://publicsuffix.org/list/public_suffix_list.dat", 
                 "https://raw.githubusercontent.com/publicsuffix/list/master/public_suffix_list.dat"]