Host Resolution

This document is a brief overview of how host resolution works in the Chrome network stack.

The stack includes two different implementations: the “system” or “platform” resolver, and the “async” or “built-in” resolver. The higher layers are shared between the implementations, and the lower layers are implemented separately.

Shared layers

Host resolver

The HostResolverImpl is the main interface between DNS and the rest of the network stack. It checks the HostCache, checks if there is an already running Job, and schedules a new Job if there isn't one in progress.

Data collected at this layer:

  • “Net.DNS.TotalTime” (recommended for DNS experiments)
  • “Net.DNS.TotalTimeNotCached”


The HostResolverImpl::Job represents a single DNS resolution from the network (or in some cases the OS‘s DNS cache, which Chrome doesn’t know about). It starts a task depending on which implementation should be used. If a DnsTask fails, it retries using a ProcTask.

Data collected at this layer:

  • “Net.DNS.ResolveSuccessTime” (also by address family)
  • “Net.DNS.ResolveFailureTime” (also by address family)
  • “Net.DNS.ResolveCategory”
  • “Net.DNS.ResolveError.Fast”
  • “Net.DNS.ResolveError.Slow”

System resolver


The entry point for the system resolver is HostResolverImpl::ProcTask. The task runs almost entirely on TaskScheduler. Its main implementation is in SystemHostResolverProc. Other implementations of HostResolverProc can be swapped in for testing.

Data collected at this layer:

  • “Net.DNS.ProcTask.SuccessTime”
  • “Net.DNS.ProcTask.FailureTime”
  • “Net.OSErrorsForGetaddrinfo*”


Attempts in the system resolver are not a separate class. They're implemented as separate tasks posted to TaskScheduler.

Data collected at this layer:

  • “DNS.AttemptFirstSuccess”
  • “DNS.AttemptFirstFailure”
  • “DNS.AttemptSuccess”
  • “DNS.AttemptFailure”
  • “DNS.AttemptDiscarded”
  • “DNS.AttemptCancelled”
  • “DNS.AttemptSuccessDuration”
  • “DNS.AttemptFailDuration”

Async resolver


The entry point for the async resolver is HostResolverImpl::DnsTask. DnsTask starts one DnsTransaction for each lookup needed, which can be one for a single address family or two when both A and AAAA are needed.

Data collected at this layer:

  • “Net.DNS.DnsTask.SuccessTime”
  • “Net.DNS.DnsTask.FailureTime”
  • “Net.DNS.DnsTask.ErrorBeforeFallback.Fast”
  • “Net.DNS.DnsTask.ErrorBeforeFallback.Slow”
  • “Net.DNS.DnsTask.Errors”


The main implementation of the async resolver is in the DnsTransaction. Each transaction represents a single query, which might be tried multiple times or in different ways.


Attempts in the async resolver are an explicit layer, implemented by subclasses of DnsAttempt. In most cases, DnsUDPAttempt is used. DnsTCPAttempt is used instead when the server requests it. DnsHTTPAttempt is experimental.