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gRPC Name Resolution


gRPC supports DNS as the default name-system. A number of alternative name-systems are used in various deployments. We support an API that is general enough to support a range of name-systems and the corresponding syntax for names. The gRPC client library in various languages will provide a plugin mechanism so resolvers for different name-systems can be plugged in.

Detailed Design

Name Syntax

A fully qualified, self contained name used for gRPC channel construction uses URI syntax as defined in RFC 3986.

The URI scheme indicates what resolver plugin to use. If no scheme prefix is specified or the scheme is unknown, the dns scheme is used by default.

The URI path indicates the name to be resolved.

Most gRPC implementations support the following URI schemes:

  • dns:[//authority/]host[:port] -- DNS (default)

    • host is the host to resolve via DNS.
    • port is the port to return for each address. If not specified, 443 is used (but some implementations default to 80 for insecure channels).
    • authority indicates the DNS server to use, although this is only supported by some implementations. (In C-core, the default DNS resolver does not support this, but the c-ares based resolver supports specifying this in the form "IP:port".)
  • unix:path, unix://absolute_path -- Unix domain sockets (Unix systems only)

    • path indicates the location of the desired socket.
    • In the first form, the path may be relative or absolute; in the second form, the path must be absolute (i.e., there will actually be three slashes, two prior to the path and another to begin the absolute path).
  • unix-abstract:abstract_path -- Unix domain socket in abstract namespace (Unix systems only)

    • abstract_path indicates a name in the abstract namespace.
    • The name has no connection with filesystem pathnames.
    • No permissions will apply to the socket - any process/user may access the socket.
    • The underlying implementation of Abstract sockets uses a null byte ('\0') as the first character; the implementation will prepend this null. Do not include the null in abstract_path.
  • vsock:cid:port -- VSOCK (Linux systems only)

    • cid is 32-bit Context Identifier (CID). It indicates the source or destination, which is either a virtual machine or the host.
    • port is a 32-bit port number. It differentiates between multiple services running on a single machine.

The following schemes are supported by the gRPC C-core implementation, but may not be supported in other languages:

  • ipv4:address[:port][,address[:port],...] -- IPv4 addresses

    • Can specify multiple comma-delimited addresses of the form address[:port]:
      • address is the IPv4 address to use.
      • port is the port to use. If not specified, 443 is used.
  • ipv6:address[:port][,address[:port],...] -- IPv6 addresses

    • Can specify multiple comma-delimited addresses of the form address[:port]:
      • address is the IPv6 address to use. To use with a port the address must enclosed in literal square brackets ([ and ]). Example: ipv6:[2607:f8b0:400e:c00::ef]:443 or ipv6:[::]:1234
      • port is the port to use. If not specified, 443 is used.

In the future, additional schemes such as etcd could be added.

Resolver Plugins

The gRPC client library will use the specified scheme to pick the right resolver plugin and pass it the fully qualified name string.

Resolvers should be able to contact the authority and get a resolution that they return back to the gRPC client library. The returned contents include:

  • A list of resolved addresses (both IP address and port). Each address may have a set of arbitrary attributes (key/value pairs) associated with it, which can be used to communicate information from the resolver to the load balancing policy.
  • A service config.

The plugin API allows the resolvers to continuously watch an endpoint and return updated resolutions as needed.