Crate srv_rs

Rust client for communicating with services located by DNS SRV records.

Introduction

SRV Records, as defined in RFC 2782, are DNS records of the form

_Service._Proto.Name TTL Class SRV Priority Weight Port Target

For instance, a DNS server might respond with the following SRV records for _http._tcp.example.com:

_http._tcp.example.com. 60 IN SRV 1 100 443 test1.example.com.
_http._tcp.example.com. 60 IN SRV 2 50  443 test2.example.com.
_http._tcp.example.com. 60 IN SRV 2 50  443 test3.example.com.

A client wanting to communicate with this example service would first try to communicate with test1.example.com:443 (the record with the lowest priority), then with the other two (in a random order, since they are of the same priority) should the first be unavailable.

srv-rs handles the lookup and caching of SRV records as well as the ordered selection of targets to use for communication with SRV-located services. It presents this service in the following interface:

use srv_rs::{client::{SrvClient, Execution}, resolver::libresolv::LibResolv};
let client = SrvClient::<LibResolv>::new("_http._tcp.example.com");
client.execute(Execution::Serial, |address: http::Uri| async move {
    // Communicate with the service at `address`
    // `hyper` is used here as an example, but it is in no way required
    hyper::Client::new().get(address).await
})
.await;

SrvClient::new creates a client (that should be reused to take advantage of caching) for communicating with the service located by _http._tcp.example.com. SrvClient::execute takes in a future-producing closure (emulating async closures, which are currently unstable) and executes the closure on a series of targets parsed from the discovered SRV records, stopping and returning the first Ok or last Err it obtains.

Alternative Resolvers and Target Selection Policies

srv-rs provides multiple resolver backends for SRV lookup and by default uses a target selection policy that maintains affinity for the last target it has used successfully. Both of these behaviors can be changed by implementing the SrvResolver and Policy traits, respectively.

The provided resolver backends are enabled by the following features:

• libresolv (via LibResolv)
• trust-dns (via [trust_dns_resolver::AsyncResolver])

Modules

client	Clients based on SRV lookups.
record	SRV records.
resolver	SRV resolvers.