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# gRPC Name Resolution
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## Overview
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gRPC supports DNS as the default name-system. A number of alternative
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name-systems are used in various deployments. We support an API that is
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general enough to support a range of name-systems and the corresponding
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syntax for names. The gRPC client library in various languages will
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provide a plugin mechanism so resolvers for different name-systems can
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be plugged in.
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## Detailed Design
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### Name Syntax
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A fully qualified, self contained name used for gRPC channel construction
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uses the syntax:
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```
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scheme://authority/endpoint_name
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```
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Here, `scheme` indicates the name-system to be used. Currently, we
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support the following schemes:
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- `dns`
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- `ipv4` (IPv4 address)
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- `ipv6` (IPv6 address)
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- `unix` (path to unix domain socket -- unix systems only)
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In the future, additional schemes such as `etcd` could be added.
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The `authority` indicates some scheme-specific bootstrap information, e.g.,
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for DNS, the authority may include the IP[:port] of the DNS server to
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use. Often, a DNS name may be used as the authority, since the ability to
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resolve DNS names is already built into all gRPC client libraries.
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Finally, the `endpoint_name` indicates a concrete name to be looked up
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in a given name-system identified by the scheme and the authority. The
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syntax of the endpoint name is dictated by the scheme in use.
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### Resolver Plugins
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The gRPC client library will use the specified scheme to pick the right
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resolver plugin and pass it the fully qualified name string.
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Resolvers should be able to contact the authority and get a resolution
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that they return back to the gRPC client library. The returned contents
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include:
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- A list of resolved addresses, each of which has three attributes:
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- The address itself, including both IP address and port.
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- A boolean indicating whether the address is a backend address (i.e.,
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the address to use to contact the server directly) or a balancer
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address (for cases where [external load balancing](load-balancing.md)
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is in use).
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- The name of the balancer, if the address is a balancer address.
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This will be used to perform peer authorization.
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- A [service config](service_config.md).
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The plugin API allows the resolvers to continuously watch an endpoint
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and return updated resolutions as needed.
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