diff --git a/Cargo.toml b/Cargo.toml
index e1ab8ef2..53a7cb61 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -90,13 +90,14 @@ features = ["std", "std-future"]
 env_logger = "0.9"
 flate2 = "1.0.3"
 indicatif = "0.15"
+libc = "0.2.177"
 rayon = "1"
+serde_json = "1"
 static_assertions = "1.1"
 structopt = "0.3"
 tempfile = "3.1"
 test-case = "2.0"
 tracing-subscriber = ">=0.2.12, <0.4.0"
-serde_json = "1"
 
 [dev-dependencies.testserver]
 path = "testserver"
diff --git a/build.rs b/build.rs
index 9649004c..ef6989d1 100644
--- a/build.rs
+++ b/build.rs
@@ -3,6 +3,10 @@ use std::{env, error::Error};
 fn main() -> Result<(), Box<dyn Error>> {
     println!("cargo:rustc-env=ISAHC_FEATURES={}", get_feature_string());
 
+    // Allow conditional compilation for tarpaulin and debug builds.
+    println!("cargo::rustc-check-cfg=cfg(tarpaulin)");
+    println!("cargo::rustc-check-cfg=cfg(debug)");
+
     Ok(())
 }
 
diff --git a/src/client.rs b/src/client.rs
index 835b1bfb..860741ba 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -5,7 +5,7 @@ use crate::{
     body::{AsyncBody, Body},
     config::{
         client::ClientConfig,
-        request::{RequestConfig, SetOpt, WithRequestConfig},
+        request::{NoCustomOpt, RequestConfig, SetOpt, WithRequestConfig},
         *,
     },
     default_headers::DefaultHeadersInterceptor,
@@ -71,9 +71,9 @@ static USER_AGENT: Lazy<String> = Lazy::new(|| {
 /// # Ok::<(), isahc::Error>(())
 /// ```
 #[must_use = "builders have no effect if unused"]
-pub struct HttpClientBuilder {
+pub struct HttpClientBuilder<T: SetOpt> {
     agent_builder: AgentBuilder,
-    client_config: ClientConfig,
+    client_config: ClientConfig<T>,
     request_config: RequestConfig,
     interceptors: Vec<InterceptorObj>,
     default_headers: HeaderMap<HeaderValue>,
@@ -83,13 +83,13 @@ pub struct HttpClientBuilder {
     cookie_jar: Option<crate::cookies::CookieJar>,
 }
 
-impl Default for HttpClientBuilder {
+impl Default for HttpClientBuilder<NoCustomOpt> {
     fn default() -> Self {
         Self::new()
     }
 }
 
-impl HttpClientBuilder {
+impl HttpClientBuilder<NoCustomOpt> {
     /// Create a new builder for building a custom client. All configuration
     /// will start out with the default values.
     ///
@@ -114,6 +114,75 @@ impl HttpClientBuilder {
         }
     }
 
+    /// Set custom curl options.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ptr;
+    /// use std::error;
+    /// use std::fmt;
+    /// use std::mem;
+    /// use std::ffi;
+    ///
+    /// use isahc::{HttpClient, SetOpt, prelude::*};
+    ///
+    /// /// Helper functions for raising nice errors taken from the `curl` crate.
+    /// fn cvt<H>(easy: &mut curl::easy::Easy2<H>, rc: curl_sys::CURLcode) -> Result<(), curl::Error> {
+    ///     if rc == curl_sys::CURLE_OK {
+    ///         return Ok(());
+    ///     }
+    ///     let mut err = curl::Error::new(rc);
+    ///     if let Some(msg) = easy.take_error_buf() {
+    ///         err.set_extra(msg);
+    ///     }
+    ///     Err(err)
+    /// }
+    ///
+    /// /// Import the base function for the open socket callback from the `curl` crate.
+    /// extern "C" {
+    ///     pub fn opensocket_cb(data: *mut libc::c_void, purpose: curl_sys::curlsocktype, address: *mut curl_sys::curl_sockaddr) -> curl_sys::curl_socket_t;
+    /// }
+    ///
+    /// /// Wrap the callback to perform our custom logic before opening the socket.
+    /// extern "C" fn opensocket_cb_custom(data: *mut libc::c_void, purpose: curl_sys::curlsocktype, address: *mut curl_sys::curl_sockaddr) -> curl_sys::curl_socket_t {
+    ///     // TODO: Wrapper the open socket callback to perform some custom logic.
+    ///     unsafe { opensocket_cb(data, purpose, address) }
+    /// }
+    ///
+    /// /// Implement a custom curl option that modifies the open socket function.
+    /// #[derive(Debug)]
+    /// struct CustomOpt {}
+    /// impl SetOpt for CustomOpt {
+    ///     fn set_opt<H>(&self, easy: &mut curl::easy::Easy2<H>) -> Result<(), curl::Error> {
+    ///         let opt = curl_sys::CURLOPT_OPENSOCKETFUNCTION;
+    ///         let cb: curl_sys::curl_opensocket_callback = opensocket_cb_custom;
+    ///         unsafe { cvt(easy, curl_sys::curl_easy_setopt(easy.raw(), opt, cb))?; }
+    ///         Ok(())
+    ///     }
+    /// }
+    ///
+    /// let client = HttpClient::builder()
+    ///     .custom_curl_options(CustomOpt {})
+    ///     .build()?;
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn custom_curl_options<S: SetOpt>(self, custom_curl_options: S) -> HttpClientBuilder<S> {
+        // Similar to the dns_cache option, this operation actually affects all
+        // requests in a multi handle so we do not expose it per-request to
+        // avoid confusing behavior.
+        HttpClientBuilder{
+            agent_builder: self.agent_builder,
+            client_config: ClientConfig { connection_cache_ttl: self.client_config.connection_cache_ttl, close_connections: self.client_config.close_connections, dns_cache: self.client_config.dns_cache, dns_resolve: self.client_config.dns_resolve, custom_curl_options: Some(custom_curl_options), },
+            request_config: self.request_config,
+            interceptors: self.interceptors,
+            default_headers: self.default_headers,
+            error: self.error,
+        }
+    }
+}
+
+impl<T: SetOpt> HttpClientBuilder<T> {
     /// Enable persistent cookie handling for all requests using this client
     /// using a shared cookie jar.
     ///
@@ -436,7 +505,7 @@ impl HttpClientBuilder {
     ///
     /// If the client fails to initialize, an error will be returned.
     #[allow(unused_mut)]
-    pub fn build(mut self) -> Result<HttpClient, Error> {
+    pub fn build(mut self) -> Result<HttpClient<T>, Error> {
         if let Some(err) = self.error {
             return Err(err);
         }
@@ -483,7 +552,7 @@ impl HttpClientBuilder {
     }
 }
 
-impl Configurable for HttpClientBuilder {
+impl<T: SetOpt> Configurable for HttpClientBuilder<T> {
     #[cfg(feature = "cookies")]
     fn cookie_jar(mut self, cookie_jar: crate::cookies::CookieJar) -> Self {
         self.cookie_jar = Some(cookie_jar);
@@ -491,7 +560,7 @@ impl Configurable for HttpClientBuilder {
     }
 }
 
-impl WithRequestConfig for HttpClientBuilder {
+impl<T: SetOpt> WithRequestConfig for HttpClientBuilder<T> {
     #[inline]
     fn with_config(mut self, f: impl FnOnce(&mut RequestConfig)) -> Self {
         f(&mut self.request_config);
@@ -499,7 +568,7 @@ impl WithRequestConfig for HttpClientBuilder {
     }
 }
 
-impl fmt::Debug for HttpClientBuilder {
+impl<T: SetOpt> fmt::Debug for HttpClientBuilder<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("HttpClientBuilder").finish()
     }
@@ -594,17 +663,24 @@ impl<'a, K: Copy, V: Copy> HeaderPair<K, V> for &'a (K, V) {
 ///
 /// See the documentation on [`HttpClientBuilder`] for a comprehensive look at
 /// what can be configured.
-#[derive(Clone)]
-pub struct HttpClient {
-    inner: Arc<Inner>,
+pub struct HttpClient<T: SetOpt> {
+    inner: Arc<Inner<T>>,
 }
 
-struct Inner {
+impl<T: SetOpt> Clone for HttpClient<T> {
+    fn clone(&self) -> Self {
+        Self {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+struct Inner<T: SetOpt> {
     /// This is how we talk to our background agent thread.
     agent: agent::Handle,
 
     /// Client-wide request configuration.
-    client_config: ClientConfig,
+    client_config: ClientConfig<T>,
 
     /// Default request configuration to use if not specified in a request.
     request_config: RequestConfig,
@@ -617,7 +693,7 @@ struct Inner {
     cookie_jar: Option<crate::cookies::CookieJar>,
 }
 
-impl HttpClient {
+impl HttpClient<NoCustomOpt> {
     /// Create a new HTTP client using the default configuration.
     ///
     /// If the client fails to initialize, an error will be returned.
@@ -629,17 +705,19 @@ impl HttpClient {
     ///
     /// TODO: Stabilize.
     pub(crate) fn shared() -> &'static Self {
-        static SHARED: Lazy<HttpClient> =
+        static SHARED: Lazy<HttpClient<NoCustomOpt>> =
             Lazy::new(|| HttpClient::new().expect("shared client failed to initialize"));
 
         &SHARED
     }
 
     /// Create a new [`HttpClientBuilder`] for building a custom client.
-    pub fn builder() -> HttpClientBuilder {
+    pub fn builder() -> HttpClientBuilder<NoCustomOpt> {
         HttpClientBuilder::default()
     }
+}
 
+impl<T: SetOpt + 'static> HttpClient<T> {
     /// Get the configured cookie jar for this HTTP client, if any.
     ///
     /// # Availability
@@ -1064,23 +1142,20 @@ impl HttpClient {
 
         easy.signal(false)?;
 
-        let request_config = request
-            .extensions()
-            .get::<RequestConfig>()
-            .unwrap();
+        let request_config = request.extensions().get::<RequestConfig>().unwrap();
 
         request_config.set_opt(&mut easy)?;
         self.inner.client_config.set_opt(&mut easy)?;
 
         // Check if we need to disable the Expect header.
-        let disable_expect_header = request_config.expect_continue
+        let disable_expect_header = request_config
+            .expect_continue
             .as_ref()
             .map(|x| x.is_disabled())
             .unwrap_or_default();
 
         // Set the HTTP method to use. Curl ties in behavior with the request
         // method, so we need to configure this carefully.
-        #[allow(indirect_structural_match)]
         match (request.method(), has_body) {
             // Normal GET request.
             (&http::Method::GET, false) => {
@@ -1161,7 +1236,7 @@ impl HttpClient {
     }
 }
 
-impl crate::interceptor::Invoke for &HttpClient {
+impl<T: SetOpt + 'static> crate::interceptor::Invoke for &HttpClient<T> {
     fn invoke(
         &self,
         mut request: Request<AsyncBody>,
@@ -1236,7 +1311,7 @@ impl crate::interceptor::Invoke for &HttpClient {
     }
 }
 
-impl fmt::Debug for HttpClient {
+impl<T: SetOpt> fmt::Debug for HttpClient<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("HttpClient").finish()
     }
@@ -1275,12 +1350,12 @@ impl<'c> fmt::Debug for ResponseFuture<'c> {
 
 /// Response body stream. Holds a reference to the agent to ensure it is kept
 /// alive until at least this transfer is complete.
-struct ResponseBody {
+struct ResponseBody<T: SetOpt> {
     inner: ResponseBodyReader,
-    _client: HttpClient,
+    _client: HttpClient<T>,
 }
 
-impl AsyncRead for ResponseBody {
+impl<T: SetOpt> AsyncRead for ResponseBody<T> {
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
@@ -1319,8 +1394,8 @@ fn uri_to_string(uri: &http::Uri) -> String {
 mod tests {
     use super::*;
 
-    static_assertions::assert_impl_all!(HttpClient: Send, Sync);
-    static_assertions::assert_impl_all!(HttpClientBuilder: Send);
+    static_assertions::assert_impl_all!(HttpClient::<NoCustomOpt>: Send, Sync);
+    static_assertions::assert_impl_all!(HttpClientBuilder::<NoCustomOpt>: Send);
 
     #[test]
     fn test_default_header() {
@@ -1335,7 +1410,8 @@ mod tests {
 
     #[test]
     fn test_default_headers_mut() {
-        let mut builder = HttpClientBuilder::new().default_header("some-key", "some-value");
+        let mut builder =
+            HttpClientBuilder::new().default_header("some-key", "some-value");
         let headers_map = &mut builder.default_headers;
         assert!(headers_map.len() == 1);
 
diff --git a/src/config/client.rs b/src/config/client.rs
index 19fc2b94..7d2c5bd1 100644
--- a/src/config/client.rs
+++ b/src/config/client.rs
@@ -1,18 +1,33 @@
+use crate::NoCustomOpt;
+
 use super::{
     dns::{DnsCache, ResolveMap},
     request::SetOpt,
 };
-use std::time::Duration;
+use std::{fmt::Debug, time::Duration};
 
-#[derive(Debug, Default)]
-pub(crate) struct ClientConfig {
+#[derive(Debug)]
+pub(crate) struct ClientConfig<T: SetOpt> {
     pub(crate) connection_cache_ttl: Option<Duration>,
     pub(crate) close_connections: bool,
     pub(crate) dns_cache: Option<DnsCache>,
     pub(crate) dns_resolve: Option<ResolveMap>,
+    pub(crate) custom_curl_options: Option<T>,
 }
 
-impl SetOpt for ClientConfig {
+impl Default for ClientConfig<NoCustomOpt> {
+    fn default() -> Self {
+        Self {
+            connection_cache_ttl: Default::default(),
+            close_connections: Default::default(),
+            dns_cache: Default::default(),
+            dns_resolve: Default::default(),
+            custom_curl_options: None,
+        }
+    }
+}
+
+impl<T: SetOpt> SetOpt for ClientConfig<T> {
     fn set_opt<H>(&self, easy: &mut curl::easy::Easy2<H>) -> Result<(), curl::Error> {
         if let Some(ttl) = self.connection_cache_ttl {
             easy.maxage_conn(ttl)?;
@@ -26,6 +41,10 @@ impl SetOpt for ClientConfig {
             map.set_opt(easy)?;
         }
 
+        if let Some(custom_curl_options) = self.custom_curl_options.as_ref() {
+            custom_curl_options.set_opt(easy)?;
+        }
+
         easy.forbid_reuse(self.close_connections)
     }
 }
diff --git a/src/config/mod.rs b/src/config/mod.rs
index 1f8dc0db..bac58bdc 100644
--- a/src/config/mod.rs
+++ b/src/config/mod.rs
@@ -13,7 +13,7 @@
 // to update the client code to apply the option when configuring an easy
 // handle.
 
-use self::{proxy::Proxy, request::SetOpt};
+use self::{proxy::Proxy};
 use crate::{
     auth::{Authentication, Credentials},
     is_http_version_supported,
@@ -32,6 +32,7 @@ pub(crate) mod ssl;
 pub use dial::{Dialer, DialerParseError};
 pub use dns::{DnsCache, ResolveMap};
 pub use redirect::RedirectPolicy;
+pub use request::SetOpt;
 pub use ssl::{CaCertificate, ClientCertificate, PrivateKey, SslOption};
 
 /// Provides additional methods when building a request for configuring various
diff --git a/src/config/request.rs b/src/config/request.rs
index 6e6551f1..08903a7c 100644
--- a/src/config/request.rs
+++ b/src/config/request.rs
@@ -2,6 +2,7 @@
 
 use super::{proxy::Proxy, *};
 use curl::easy::Easy2;
+use std::fmt::Debug;
 
 /// Base trait for any object that can be configured for requests, such as an
 /// HTTP request builder or an HTTP client.
@@ -12,11 +13,21 @@ pub trait WithRequestConfig: Sized {
 }
 
 /// A helper trait for applying a configuration value to a given curl handle.
-pub(crate) trait SetOpt {
+pub trait SetOpt: Debug + Send + Sync {
     /// Apply this configuration property to the given curl handle.
     fn set_opt<H>(&self, easy: &mut Easy2<H>) -> Result<(), curl::Error>;
 }
 
+/// An type marker that ensures that we carry no custom `SetOpt` code around.
+#[derive(Debug)]
+pub enum NoCustomOpt {}
+
+impl SetOpt for NoCustomOpt {
+    fn set_opt<H>(&self, _easy: &mut Easy2<H>) -> Result<(), curl::Error> {
+        Ok(())
+    }
+}
+
 // Define this struct inside a macro to reduce some boilerplate.
 macro_rules! define_request_config {
     ($($field:ident: $t:ty,)*) => {
diff --git a/src/lib.rs b/src/lib.rs
index 8795881a..dacf9b4d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -272,6 +272,7 @@ pub mod interceptor;
 pub(crate) mod interceptor;
 
 pub use crate::{
+    config::request::{SetOpt, NoCustomOpt},
     body::{AsyncBody, Body},
     client::{HttpClient, HttpClientBuilder, ResponseFuture},
     error::Error,
diff --git a/tests/headers.rs b/tests/headers.rs
index 07049d55..d5abae06 100644
--- a/tests/headers.rs
+++ b/tests/headers.rs
@@ -1,5 +1,5 @@
 use futures_lite::future::block_on;
-use isahc::{prelude::*, HttpClient, Request};
+use isahc::{HttpClient, Request, prelude::*};
 use std::{
     io::{self, Write},
     net::{Shutdown, TcpListener, TcpStream},
diff --git a/tests/metrics.rs b/tests/metrics.rs
index cc6cae56..689b4922 100644
--- a/tests/metrics.rs
+++ b/tests/metrics.rs
@@ -1,4 +1,4 @@
-use isahc::{prelude::*, HttpClient, Request};
+use isahc::{HttpClient, Request, prelude::*};
 use std::{io, time::Duration};
 use testserver::mock;
 
@@ -19,7 +19,10 @@ fn enabling_metrics_causes_metrics_to_be_collected() {
         body: "hello world",
     };
 
-    let client = HttpClient::builder().metrics(true).build().unwrap();
+    let client = HttpClient::builder()
+        .metrics(true)
+        .build()
+        .unwrap();
 
     let mut response = client
         .send(Request::post(m.url()).body("hello server").unwrap())
diff --git a/tests/redirects.rs b/tests/redirects.rs
index 814db254..b786e791 100644
--- a/tests/redirects.rs
+++ b/tests/redirects.rs
@@ -1,4 +1,4 @@
-use isahc::{config::RedirectPolicy, prelude::*, Body, HttpClient, Request};
+use isahc::{Body, HttpClient, Request, config::RedirectPolicy, prelude::*};
 use test_case::test_case;
 use testserver::mock;
 
diff --git a/testserver/src/responder.rs b/testserver/src/responder.rs
index 9f8bfcd3..489eef54 100644
--- a/testserver/src/responder.rs
+++ b/testserver/src/responder.rs
@@ -51,12 +51,3 @@ pub trait Responder: Send + Sync + 'static {
     /// Respond to a request.
     fn respond(&self, ctx: &mut RequestContext<'_>);
 }
-
-/// Simple responder that returns a general response.
-pub struct DefaultResponder;
-
-impl Responder for DefaultResponder {
-    fn respond(&self, ctx: &mut RequestContext<'_>) {
-        ctx.send(Response::default());
-    }
-}