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::{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])

Usage

Add srv-rs to your dependencies in Cargo.toml, enabling at least one of the DNS resolver backends (see Alternative Resolvers). libresolv is enabled here as an example, but it is not required.

[dependencies]
srv-rs = { version = "0.2.0", features = ["libresolv"] }

Contributing

1. Clone the repo
2. Make some changes
3. Test: cargo test --all-features
4. Format: cargo fmt
5. Clippy: cargo clippy --all-features --tests -- -Dclippy::all
6. Bench: cargo bench --all-features
7. If modifying crate-level docs (src/lib.rs) or README.tpl, update README.md:
   1. cargo install cargo-readme
   2. cargo readme > README.md

History

This project was contributed back to the community by the D. E. Shaw group.