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<div class="title">gRPC Name Resolution </div>  </div>
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<div class="textblock"><h1><a class="anchor" id="autotoc_md237"></a>
Overview</h1>
<p>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.</p>
<h1><a class="anchor" id="autotoc_md238"></a>
Detailed Design</h1>
<h2><a class="anchor" id="autotoc_md239"></a>
Name Syntax</h2>
<p>A fully qualified, self contained name used for gRPC channel construction uses URI syntax as defined in <a href="https://tools.ietf.org/html/rfc3986">RFC 3986</a>.</p>
<p>The URI scheme indicates what resolver plugin to use. If no scheme prefix is specified or the scheme is unknown, the <code>dns</code> scheme is used by default.</p>
<p>The URI path indicates the name to be resolved.</p>
<p>Most gRPC implementations support the following URI schemes:</p>
<ul>
<li><code>dns:[//authority/]host[:port]</code> &ndash; DNS (default)<ul>
<li><code>host</code> is the host to resolve via DNS.</li>
<li><code>port</code> is the port to return for each address. If not specified, 443 is used (but some implementations default to 80 for insecure channels).</li>
<li><code>authority</code> 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".)</li>
</ul>
</li>
<li><code>unix:path</code>, <code>unix://absolute_path</code> &ndash; Unix domain sockets (Unix systems only)<ul>
<li><code>path</code> indicates the location of the desired socket.</li>
<li>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).</li>
</ul>
</li>
<li><code>unix-abstract:abstract_path</code> &ndash; Unix domain socket in abstract namespace (Unix systems only)<ul>
<li><code>abstract_path</code> indicates a name in the abstract namespace.</li>
<li>The name has no connection with filesystem pathnames.</li>
<li>No permissions will apply to the socket - any process/user may access the socket.</li>
<li>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 <code>abstract_path</code>.</li>
<li><code>abstract_path</code> cannot contain null bytes.<ul>
<li>TODO(<a href="https://github.com/grpc/grpc/issues/24638">https://github.com/grpc/grpc/issues/24638</a>): Unix allows abstract socket names to contain null bytes, but this is not supported by the gRPC C-core implementation.</li>
</ul>
</li>
</ul>
</li>
</ul>
<p>The following schemes are supported by the gRPC C-core implementation, but may not be supported in other languages:</p>
<ul>
<li><code>ipv4:address[:port][,address[:port],...]</code> &ndash; IPv4 addresses<ul>
<li>Can specify multiple comma-delimited addresses of the form <code>address[:port]</code>:<ul>
<li><code>address</code> is the IPv4 address to use.</li>
<li><code>port</code> is the port to use. If not specified, 443 is used.</li>
</ul>
</li>
</ul>
</li>
<li><code>ipv6:address[:port][,address[:port],...]</code> &ndash; IPv6 addresses<ul>
<li>Can specify multiple comma-delimited addresses of the form <code>address[:port]</code>:<ul>
<li><code>address</code> is the IPv6 address to use. To use with a <code>port</code> the <code>address</code> must enclosed in literal square brackets (<code>[</code> and <code>]</code>). Example: <code>ipv6:[2607:f8b0:400e:c00::ef]:443</code> or <code>ipv6:[::]:1234</code></li>
<li><code>port</code> is the port to use. If not specified, 443 is used.</li>
</ul>
</li>
</ul>
</li>
</ul>
<p>In the future, additional schemes such as <code>etcd</code> could be added.</p>
<h2><a class="anchor" id="autotoc_md240"></a>
Resolver Plugins</h2>
<p>The gRPC client library will use the specified scheme to pick the right resolver plugin and pass it the fully qualified name string.</p>
<p>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:</p>
<ul>
<li>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 <a class="el" href="load-balancing_8md.html">load balancing</a> policy.</li>
<li>A <a class="el" href="service__config_8md.html">service config</a>.</li>
</ul>
<p>The plugin API allows the resolvers to continuously watch an endpoint and return updated resolutions as needed. </p>
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