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103 lines
3.1 KiB
Markdown
103 lines
3.1 KiB
Markdown
# Host Resolution
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This document is a brief overview of how host resolution works in the Chrome
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network stack.
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The stack includes two different implementations: the "system" or "platform"
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resolver, and the "async" or "built-in" resolver. The higher layers are shared
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between the implementations, and the lower layers are implemented separately.
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## Shared layers
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### Host resolver
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The HostResolverImpl is the main interface between DNS and the rest of the
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network stack. It checks the HostCache, checks if there is an already running
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Job, and schedules a new Job if there isn't one in progress.
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Data collected at this layer:
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* "Net.DNS.TotalTime" (recommended for DNS experiments)
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* "Net.DNS.TotalTimeNotCached"
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### Job
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The HostResolverImpl::Job represents a single DNS resolution from the network
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(or in some cases the OS's DNS cache, which Chrome doesn't know about). It
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starts a task depending on which implementation should be used. If a DnsTask
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fails, it retries using a ProcTask.
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Data collected at this layer:
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* "Net.DNS.ResolveSuccessTime" (also by address family)
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* "Net.DNS.ResolveFailureTime" (also by address family)
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* "Net.DNS.ResolveCategory"
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* "Net.DNS.ResolveError.Fast"
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* "Net.DNS.ResolveError.Slow"
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## System resolver
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### Task
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The entry point for the system resolver is HostResolverImpl::ProcTask. The task
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runs almost entirely on TaskScheduler. Its main implementation is in
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SystemHostResolverProc. Other implementations of HostResolverProc can be swapped
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in for testing.
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Data collected at this layer:
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* "Net.DNS.ProcTask.SuccessTime"
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* "Net.DNS.ProcTask.FailureTime"
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* "Net.OSErrorsForGetaddrinfo*"
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### Attempt
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Attempts in the system resolver are not a separate class. They're implemented as
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separate tasks posted to TaskScheduler.
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Data collected at this layer:
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* "DNS.AttemptFirstSuccess"
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* "DNS.AttemptFirstFailure"
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* "DNS.AttemptSuccess"
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* "DNS.AttemptFailure"
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* "DNS.AttemptDiscarded"
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* "DNS.AttemptCancelled"
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* "DNS.AttemptSuccessDuration"
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* "DNS.AttemptFailDuration"
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## Async resolver
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### Task
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The entry point for the async resolver is HostResolverImpl::DnsTask. DnsTask
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starts one DnsTransaction for each lookup needed, which can be one for a single
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address family or two when both A and AAAA are needed.
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Data collected at this layer:
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* "Net.DNS.DnsTask.SuccessTime"
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* "Net.DNS.DnsTask.FailureTime"
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* "Net.DNS.DnsTask.ErrorBeforeFallback.Fast"
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* "Net.DNS.DnsTask.ErrorBeforeFallback.Slow"
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* "Net.DNS.DnsTask.Errors"
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### Transaction
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The main implementation of the async resolver is in the DnsTransaction. Each
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transaction represents a single query, which might be tried multiple times or in
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different ways.
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Data collected at this layer:
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* "AsyncDNS.TransactionFailure"
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* "AsyncDNS.TransactionSuccess" (and by query type)
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### Attempt
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Attempts in the async resolver are an explicit layer, implemented by subclasses
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of DnsAttempt. In most cases, DnsUDPAttempt is used. DnsTCPAttempt is used
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instead when the server requests it. DnsHTTPAttempt is experimental.
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Data collected at this layer:
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* "AsyncDNS.UDPAttemptSuccess"
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* "AsyncDNS.UDPAttemptFail"
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* "AsyncDNS.TCPAttemptSuccess"
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* "AsyncDNS.TCPAttemptFail"
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* "AsyncDNS.AttemptCountSuccess"
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* "AsyncDNS.AttemptCountFail"
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