# curl-rust wraps libcurl so you inherit its TLS backends, its bugs and its C dependency

> The curl crate is Rust bindings to libcurl, offering the easy interface for single transfers and the multi interface for concurrent ones, with a set of Cargo features that choose between the platform TLS backend, Rustls, a bundled libcurl and a bundled OpenSSL. It fits a team that wants libcurl's protocol coverage with a Rust API, and it does not fit a team that wants a pure Rust dependency tree.

**sagebind/curl-rust** — Rust bindings to libcurl

- Repository: https://github.com/sagebind/curl-rust
- Website: https://sagebind.github.io/curl-rust/curl/
- Stars: 1,135 · Forks: 264
- Language: Rust
- License: MIT
- Published: 2026-10-19 · Updated: 2026-10-19 · Language: en
- Canonical page: https://hysenlabs.com/projects/sagebind-curl-rust

## The API is libcurl's, with a Rust shape on top

The curl crate is described as Rust bindings to libcurl for making HTTP requests, and the quick start shows the shape of the API in a few lines.

```rust
use std::io::{stdout, Write};

use curl::easy::Easy;

// Print a web page onto stdout
fn main() {
    let mut easy = Easy::new();
    easy.url("https://www.rust-lang.org/").unwrap();
    easy.write_function(|data| {
        stdout().write_all(data).unwrap();
        Ok(data.len())
    }).unwrap();
    easy.perform().unwrap();

    println!("{}", easy.response_code().unwrap());
}
```

The design is a handle, Easy, that you configure with setters and then run with perform. The data callback is the interesting part, because libcurl pushes bytes to you rather than you reading a response body, and the closure returns the number of bytes consumed.

The second example shows the difference between Easy and a Transfer, which is the closer analogue to a request object:

```rust
use curl::easy::Easy;

// Capture output into a local `Vec`.
fn main() {
    let mut dst = Vec::new();
    let mut easy = Easy::new();
    easy.url("https://www.rust-lang.org/").unwrap();

    let mut transfer = easy.transfer();
    transfer.write_function(|data| {
        dst.extend_from_slice(data);
        Ok(data.len())
    }).unwrap();
    transfer.perform().unwrap();
}
```

A Transfer borrows the Easy handle, so the same configured handle can produce several transfers, which is what makes connection reuse natural rather than an optimisation you have to reach for.

The crate is published on crates.io as curl, MIT licensed, at version 0.4.51, with the last push on 2026-10-10 and the 0.4.50 release from 2026-06-11.

## Post, headers and keep alive are the everyday operations

The README documents the operations a real project reaches for, and they map closely onto libcurl's option set.

The put and post methods on Easy configure the method of the HTTP request, and then read_function can be used to specify how data is filled in. The interface works particularly well with types that implement Read, which is why the example passes the body through a closure that calls data.read(buf):

```rust,no_run
use std::io::Read;
use curl::easy::Easy;

fn main() {
    let mut data = "this is the body".as_bytes();

    let mut easy = Easy::new();
    easy.url("http://www.example.com/upload").unwrap();
    easy.post(true).unwrap();
    easy.post_field_size(data.len() as u64).unwrap();

    let mut transfer = easy.transfer();
    transfer.read_function(|buf| {
        Ok(data.read(buf).unwrap_or(0))
    }).unwrap();
    transfer.perform().unwrap();
}
```

Custom headers are set through a List, which is the libcurl slist API:

```rust,no_run
use curl::easy::{Easy, List};

fn main() {
    let mut easy = Easy::new();
    easy.url("http://www.example.com").unwrap();

    let mut list = List::new();
    list.append("Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==").unwrap();
    easy.http_headers(list).unwrap();
    easy.perform().unwrap();
}
```

Connection reuse is documented as a property of the handle rather than a feature you enable. The handle can be re-used across multiple requests, and curl will attempt to keep the connections alive, which the example shows by setting two different URLs on the same handle and performing twice.

For concurrency there is a separate module. The libcurl library provides support for sending multiple requests simultaneously through the multi interface, and this is bound in the multi module of the crate, providing the ability to execute multiple transfers simultaneously.

## Cargo features decide whether you link the system or bundle the world

By default, the crate attempts to dynamically link to the system-wide libcurl and the system-wide SSL library, and some of this behaviour can be customised with various Cargo features.

The ssl feature enables SSL/TLS support using the platform-default TLS backend, which is Schannel on Windows, Secure Transport on macOS, and OpenSSL or equivalent on all other platforms. It is enabled by default. The rustls feature enables SSL/TLS support via Rustls, a TLS backend written in Rust, and Rustls is always statically linked, but it is disabled by default, with a note that Rustls support is experimental within curl itself and may have significant bugs, so no stability guarantee is offered for that feature.

The rest of the list is mostly about what gets bundled. http2 enables HTTP/2 support via libnghttp2 and is disabled by default. static-curl uses a bundled libcurl version and statically links to it. static-ssl uses a bundled OpenSSL version and statically links to it, and only applies on platforms that use OpenSSL. apple-sectrust uses Apple SecTrust for certificate verification with bundled libcurl and OpenSSL on Apple platforms, and it implies both ssl and static-curl.

Two features are version-gated on the C library. upkeep_7_62_0 enables curl_easy_upkeep() support, introduced in curl 7.62.0, and poll_7_68_0 enables curl_multi_poll() and curl_multi_wakeup() support, requiring curl 7.68.0 or later. The spnego feature enables SPNEGO support, and ntlm enables NTLM support in curl.

The Cargo.toml also carries a mesalink feature for the MesaLink TLS backend and a force-system-lib-o option that was cut off in the listing, but the pattern of the feature set is clear: the default is to use what the system provides, and every deviation from that is opt-in.

## Version support is stated as a floor, and the old libcurl is the base

The Version Support section is short and has an important admission in it. The bindings have been developed using curl version 7.24.0, and they should work with any newer version of curl and possibly with older versions, but this has not been tested.

That phrasing is worth parsing carefully. The baseline is 7.24.0, which is from 2012, so anything modern is in range. The real question is the other end, whether the bindings still compile and behave correctly against a current libcurl, and the answer is that it should but that it is not tested for old versions.

The feature list makes the version-gated items explicit, which is how a binding handles the fact that its C library moves. Features keyed to a curl version, such as upkeep_7_62_0 and poll_7_68_0, exist so a user on an old system can enable the older API and a user on a new one can use the newer one without the binding guessing.

The repository layout shows the two-layer structure. The crate itself is at the root with src/, examples/, tests/, build.rs, Cargo.toml and a .gitmodules file, and there is a curl-sys/ directory holding the raw FFI layer, plus a systest/ directory. The Cargo.toml declares curl-sys as a path dependency with default-features = false, and the workspace members are curl-sys and systest.

The examples directory points at what the bindings can actually reach: aws_sigv4.rs, doh.rs for DNS over HTTPS, https.rs, multi-dl.rs, ssl_cert_blob.rs, ssl_client_auth.rs and ssl_proxy.rs. The last three are the interesting ones, because they cover the certificate and proxy paths that the README's feature list implies but does not demonstrate.

## The NSS build is a real failure mode with a documented fix

The Troubleshooting section covers one specific and genuinely confusing error, and it is worth knowing about before you hit it.

If you encounter the message:

```
  [77] Problem with the SSL CA cert (path? access rights?)
```

that means most likely that curl was linked against libcurl-nss.so due to installed libcurl NSS development files, and that the required library libnsspem.so is missing. The README quotes the curl manual page on the point: if curl is built against the NSS SSL library, the NSS PEM PKCS#11 module, libnsspem.so, needs to be available for this option to work properly.

There are two ways out. You can install the missing library, for example the nss-plugin-pem package on Debian. Or you can remove the libcurl NSS development files, for example libcurl4-nss-dev on Debian, and rebuild curl-rust.

That second option is the interesting one. The error is not a bug in the bindings, it is a symptom of the dynamic linking model, where the system libcurl that gets picked up may not be the one your distribution intends you to use. A system with both an NSS-flavored libcurl and an OpenSSL one installed can produce a binary that links against the wrong TLS stack, and the error surfaces as a certificate path problem rather than a linking problem.

This is the general shape of the trade-off. Dynamic linking to the system libcurl means small binaries and automatic picks up of system security updates, and it also means the build's behaviour depends on what the machine happens to have installed. The static-curl and static-ssl features exist for the deployments where that is not acceptable.

## Where curl-rust is the wrong choice

The first wrong-tool case is a project that wants no C in the dependency tree. This crate links libcurl, and libcurl links a TLS library, and the default configuration picks up the system copies of both. The rustls feature reduces one part of that, but the library is still libcurl, and the README is explicit that rustls support inside curl itself is experimental with no stability guarantee. A project whose build pipeline refuses native code should look elsewhere, at a pure Rust HTTP client.

The second is a task where libcurl is more than you need. The crate inherits libcurl's whole option surface, along with the maintenance burden that comes with a C library that has to be kept in step with the system copy. If the requirement is a simple HTTPS GET with a timeout, the standard library and a small TLS stack cover it without a FFI boundary.

The third is the passive maintenance badge. The Cargo.toml records the maintenance status as passively-maintained, which is a crates.io field and an honest signal that the project is not actively being developed. The releases tell the same story, with 0.4.50 in June 2026 and 0.4.51 in October 2026 after a long gap back to 0.4.44 in July 2022. A library that is the HTTP layer of a production service needs a longer-term answer than that, whether that is vendoring the crate, pinning a version, or choosing something else.

The fourth is that the bindings are only as good as the libcurl they wrap. The README says the bindings were developed against curl 7.24.0, and a feature like curl_easy_upkeep() is gated behind a version check because the API genuinely differs across curl releases. Anything new in libcurl is available only when the underlying library supports it, and the crate's release cadence is not the same as libcurl's.

## Conclusion

curl-rust fits a Rust project that needs libcurl's protocol coverage and its HTTP/2 and SPNEGO support without writing unsafe FFI by hand, and it does not fit a project that wants a pure Rust dependency tree. Before adopting it, check which TLS backend your target system provides, since the default ssl feature uses Schannel on Windows, Secure Transport on macOS and OpenSSL elsewhere, and note that the rustls feature is documented as experimental within curl itself with no stability guarantee.

## FAQ

### What is the curl crate for Rust?

It provides Rust bindings to libcurl for making HTTP requests. The easy interface covers single transfers through a configurable Easy handle and a Transfer, and the multi module binds libcurl's multi interface so multiple transfers can execute simultaneously.

### Which TLS backend does curl-rust use by default?

The default ssl feature uses the platform-default TLS backend, which is Schannel on Windows, Secure Transport on macOS, and OpenSSL or equivalent elsewhere. The rustls feature switches to Rustls and is always statically linked, but it is disabled by default and documented as experimental within curl itself with no stability guarantee.

### How do I build curl-rust against a bundled libcurl?

Enable the static-curl feature to use a bundled libcurl version and statically link to it, and static-ssl to bundle OpenSSL as well, which only applies on platforms that use OpenSSL. By default the crate dynamically links to the system-wide libcurl and the system-wide SSL library.

### What does the 'Problem with the SSL CA cert' error in curl-rust mean?

It usually means curl was linked against libcurl-nss.so because libcurl NSS development files were installed, and the required libnsspem.so is missing. You can install the missing module, for example nss-plugin-pem on Debian, or remove the NSS development files and rebuild curl-rust.

## Sources

- [License: MIT](https://github.com/sagebind/curl-rust/blob/main/LICENSE)
- [Project website](https://sagebind.github.io/curl-rust/curl/)
- [README](https://github.com/sagebind/curl-rust/blob/main/README.md)
- [Releases](https://github.com/sagebind/curl-rust/releases)
- [sagebind/curl-rust on GitHub](https://github.com/sagebind/curl-rust)

---

Hysen Labs editorial analysis, written from the project's own repository and release notes. Cite the canonical page: https://hysenlabs.com/projects/sagebind-curl-rust