diff --git a/Cargo.toml b/Cargo.toml
index 58116f9..6ba3667 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,7 +9,7 @@ name = "ldns"
 path = "src/bin/ldns.rs"
 
 [dependencies]
-clap = { version = "4", features = ["derive"] }
+clap = { version = "4.3.4", features = ["derive"] }
 domain = "0.10.1"
 lexopt = "0.3.0"
 
diff --git a/src/args.rs b/src/args.rs
index 0659044..3883f51 100644
--- a/src/args.rs
+++ b/src/args.rs
@@ -1,3 +1,5 @@
+use crate::env::Env;
+
 use super::commands::Command;
 use super::error::Error;
 
@@ -9,8 +11,8 @@ pub struct Args {
 }
 
 impl Args {
-    pub fn execute(self) -> Result<(), Error> {
-        self.command.execute()
+    pub fn execute(self, env: impl Env) -> Result<(), Error> {
+        self.command.execute(env)
     }
 }
 
diff --git a/src/bin/ldns.rs b/src/bin/ldns.rs
index 6a8a5cc..beda509 100644
--- a/src/bin/ldns.rs
+++ b/src/bin/ldns.rs
@@ -9,14 +9,17 @@ use std::process::ExitCode;
 use dnst::try_ldns_compatibility;
 
 fn main() -> ExitCode {
+    let env = dnst::env::RealEnv;
+
     let mut args = std::env::args_os();
     args.next().unwrap();
-    let args = try_ldns_compatibility(args).expect("ldns commmand is not recognized");
+    let args =
+        try_ldns_compatibility(args).map(|args| args.expect("ldns commmand is not recognized"));
 
-    match args.execute() {
+    match args.and_then(|args| args.execute(&env)) {
         Ok(()) => ExitCode::SUCCESS,
         Err(err) => {
-            err.pretty_print();
+            err.pretty_print(env);
             ExitCode::FAILURE
         }
     }
diff --git a/src/commands/mod.rs b/src/commands/mod.rs
index c602fe3..b7dbb3d 100644
--- a/src/commands/mod.rs
+++ b/src/commands/mod.rs
@@ -8,6 +8,7 @@ use std::str::FromStr;
 
 use nsec3hash::Nsec3Hash;
 
+use crate::env::Env;
 use crate::Args;
 
 use super::error::Error;
@@ -23,9 +24,9 @@ pub enum Command {
 }
 
 impl Command {
-    pub fn execute(self) -> Result<(), Error> {
+    pub fn execute(self, env: impl Env) -> Result<(), Error> {
         match self {
-            Self::Nsec3Hash(nsec3hash) => nsec3hash.execute(),
+            Self::Nsec3Hash(nsec3hash) => nsec3hash.execute(env),
             Self::Help(help) => help.execute(),
         }
     }
diff --git a/src/commands/nsec3hash.rs b/src/commands/nsec3hash.rs
index b673d50..1b175fe 100644
--- a/src/commands/nsec3hash.rs
+++ b/src/commands/nsec3hash.rs
@@ -1,3 +1,4 @@
+use crate::env::Env;
 use crate::error::Error;
 use clap::builder::ValueParser;
 use domain::base::iana::nsec3::Nsec3HashAlg;
@@ -9,6 +10,7 @@ use lexopt::Arg;
 use octseq::OctetsBuilder;
 use ring::digest;
 use std::ffi::OsString;
+use std::fmt::Write;
 use std::str::FromStr;
 
 use super::{parse_os, parse_os_with, LdnsCommand};
@@ -128,11 +130,13 @@ impl Nsec3Hash {
 }
 
 impl Nsec3Hash {
-    pub fn execute(self) -> Result<(), Error> {
+    pub fn execute(self, env: impl Env) -> Result<(), Error> {
         let hash = nsec3_hash(&self.name, self.algorithm, self.iterations, &self.salt)
             .to_string()
             .to_lowercase();
-        println!("{}.", hash);
+
+        let mut out = env.stdout();
+        writeln!(out, "{}.", hash).unwrap();
         Ok(())
     }
 }
@@ -179,3 +183,36 @@ where
     // For normal hash algorithms this should not fail.
     OwnerHash::from_octets(h.as_ref().to_vec()).expect("should not fail")
 }
+
+#[cfg(test)]
+mod test {
+    use crate::env::fake::FakeCmd;
+
+    #[test]
+    fn dnst_parse() {
+        let cmd = FakeCmd::new(["dnst", "nsec3-hash"]);
+
+        assert!(cmd.parse().is_err());
+        assert!(cmd.args(["-a"]).parse().is_err());
+    }
+
+    #[test]
+    fn dnst_run() {
+        let cmd = FakeCmd::new(["dnst", "nsec3-hash"]);
+
+        let res = cmd.run();
+        assert_eq!(res.exit_code, 2);
+
+        let res = cmd.args(["example.test"]).run();
+        assert_eq!(res.exit_code, 0);
+        assert_eq!(res.stdout, "o09614ibh1cq1rcc86289olr22ea0fso.\n")
+    }
+
+    #[test]
+    fn ldns_parse() {
+        let cmd = FakeCmd::new(["ldns-nsec3-hash"]);
+
+        assert!(cmd.parse().is_err());
+        assert!(cmd.args(["-a"]).parse().is_err());
+    }
+}
diff --git a/src/env/fake.rs b/src/env/fake.rs
new file mode 100644
index 0000000..f02c66a
--- /dev/null
+++ b/src/env/fake.rs
@@ -0,0 +1,134 @@
+use std::ffi::OsString;
+use std::fmt;
+use std::sync::Arc;
+use std::sync::Mutex;
+
+use crate::{error::Error, parse_args, run, Args};
+
+use super::Env;
+
+/// A command to run in a [`FakeEnv`]
+///
+/// This is used for testing the utilities, running the real code in a fake
+/// environment.
+#[derive(Clone)]
+pub struct FakeCmd {
+    /// The command to run, including `argv[0]`
+    cmd: Vec<OsString>,
+}
+
+/// The result of running a [`FakeCmd`]
+///
+/// The fields are public to allow for easy assertions in tests.
+pub struct FakeResult {
+    pub exit_code: u8,
+    pub stdout: String,
+    pub stderr: String,
+}
+
+/// An environment that mocks interaction with the outside world
+pub struct FakeEnv {
+    /// Description of the command being run
+    pub cmd: FakeCmd,
+
+    /// The mocked stdout
+    pub stdout: FakeStream,
+
+    /// The mocked stderr
+    pub stderr: FakeStream,
+    // pub stelline: Option<Stelline>,
+    // pub curr_step_value: Option<Arc<CurrStepValue>>,
+}
+
+impl Env for FakeEnv {
+    fn args_os(&self) -> impl Iterator<Item = OsString> {
+        self.cmd.cmd.iter().map(Into::into)
+    }
+
+    fn stdout(&self) -> impl fmt::Write {
+        self.stdout.clone()
+    }
+
+    fn stderr(&self) -> impl fmt::Write {
+        self.stderr.clone()
+    }
+}
+
+impl FakeCmd {
+    /// Construct a new [`FakeCmd`] with a given command.
+    ///
+    /// The command can consist of multiple strings to specify a subcommand.
+    pub fn new<S: Into<OsString>>(cmd: impl IntoIterator<Item = S>) -> Self {
+        Self {
+            cmd: cmd.into_iter().map(Into::into).collect(),
+        }
+    }
+
+    /// Add arguments to a clone of the [`FakeCmd`]
+    ///
+    /// ```rust,ignore
+    /// let cmd = FakeCmd::new(["dnst"])
+    /// let sub1 = cmd.args(["sub1"]);  // dnst sub1
+    /// let sub2 = cmd.args(["sub2"]);  // dnst sub2
+    /// let sub3 = sub2.args(["sub3"]); // dnst sub2 sub3
+    /// ```
+    pub fn args<S: Into<OsString>>(&self, args: impl IntoIterator<Item = S>) -> Self {
+        let mut new = self.clone();
+        new.cmd.extend(args.into_iter().map(Into::into));
+        new
+    }
+
+    /// Parse the arguments of this [`FakeCmd`] and return the result
+    pub fn parse(&self) -> Result<Args, Error> {
+        let env = FakeEnv {
+            cmd: self.clone(),
+            stdout: Default::default(),
+            stderr: Default::default(),
+        };
+        parse_args(env)
+    }
+
+    /// Run the [`FakeCmd`] in a [`FakeEnv`], returning a [`FakeResult`]
+    pub fn run(&self) -> FakeResult {
+        let env = FakeEnv {
+            cmd: self.clone(),
+            stdout: Default::default(),
+            stderr: Default::default(),
+        };
+
+        let exit_code = run(&env);
+
+        FakeResult {
+            exit_code,
+            stdout: env.get_stdout(),
+            stderr: env.get_stderr(),
+        }
+    }
+}
+
+impl FakeEnv {
+    pub fn get_stdout(&self) -> String {
+        self.stdout.0.lock().unwrap().clone()
+    }
+
+    pub fn get_stderr(&self) -> String {
+        self.stderr.0.lock().unwrap().clone()
+    }
+}
+
+/// A type to used to mock stdout and stderr
+#[derive(Clone, Default)]
+pub struct FakeStream(Arc<Mutex<String>>);
+
+impl fmt::Write for FakeStream {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.0.lock().unwrap().push_str(s);
+        Ok(())
+    }
+}
+
+impl fmt::Display for FakeStream {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.0.lock().unwrap().as_ref())
+    }
+}
diff --git a/src/env/mod.rs b/src/env/mod.rs
new file mode 100644
index 0000000..717554e
--- /dev/null
+++ b/src/env/mod.rs
@@ -0,0 +1,57 @@
+use std::ffi::OsString;
+use std::fmt;
+
+mod real;
+
+#[cfg(test)]
+pub mod fake;
+
+pub use real::RealEnv;
+
+pub trait Env {
+    // /// Make a network connection
+    // fn make_connection(&self);
+
+    // /// Make a new [`StubResolver`]
+    // fn make_stub_resolver(&self);
+
+    /// Get an iterator over the command line arguments passed to the program
+    ///
+    /// Equivalent to [`std::env::args_os`]
+    fn args_os(&self) -> impl Iterator<Item = OsString>;
+
+    /// Get a reference to stdout
+    ///
+    /// Equivalent to [`std::io::stdout`]
+    fn stdout(&self) -> impl fmt::Write;
+
+    /// Get a reference to stderr
+    ///
+    /// Equivalent to [`std::io::stderr`]
+    fn stderr(&self) -> impl fmt::Write;
+
+    // /// Get a reference to stdin
+    // fn stdin(&self) -> impl io::Read;
+}
+
+impl<E: Env> Env for &E {
+    // fn make_connection(&self) {
+    //     todo!()
+    // }
+
+    // fn make_stub_resolver(&self) {
+    //     todo!()
+    // }
+
+    fn args_os(&self) -> impl Iterator<Item = OsString> {
+        (**self).args_os()
+    }
+
+    fn stdout(&self) -> impl fmt::Write {
+        (**self).stdout()
+    }
+
+    fn stderr(&self) -> impl fmt::Write {
+        (**self).stderr()
+    }
+}
diff --git a/src/env/real.rs b/src/env/real.rs
new file mode 100644
index 0000000..69fb5e0
--- /dev/null
+++ b/src/env/real.rs
@@ -0,0 +1,34 @@
+use std::ffi::OsString;
+use std::fmt;
+use std::io;
+
+use super::Env;
+
+/// Use real I/O
+pub struct RealEnv;
+
+impl Env for RealEnv {
+    fn args_os(&self) -> impl Iterator<Item = OsString> {
+        std::env::args_os()
+    }
+
+    fn stdout(&self) -> impl fmt::Write {
+        FmtWriter(io::stdout())
+    }
+
+    fn stderr(&self) -> impl fmt::Write {
+        FmtWriter(io::stderr())
+    }
+}
+
+struct FmtWriter<T: io::Write>(T);
+
+impl<T: io::Write> fmt::Write for FmtWriter<T> {
+    fn write_str(&mut self, s: &str) -> std::fmt::Result {
+        self.0.write_all(s.as_bytes()).map_err(|_| fmt::Error)
+    }
+
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> fmt::Result {
+        self.0.write_fmt(args).map_err(|_| fmt::Error)
+    }
+}
diff --git a/src/error.rs b/src/error.rs
index e2c2470..e800a59 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,18 +1,18 @@
-use std::{error, fmt, io};
+use crate::env::Env;
+use std::fmt::{self, Write};
+use std::{error, io};
 
 //------------ Error ---------------------------------------------------------
 
 /// A program error.
 ///
 /// Such errors are highly likely to halt the program.
-#[derive(Clone)]
 pub struct Error(Box<Information>);
 
 /// Information about an error.
-#[derive(Clone)]
 struct Information {
     /// The primary error message.
-    primary: Box<str>,
+    primary: PrimaryError,
 
     /// Layers of context to the error.
     ///
@@ -20,13 +20,28 @@ struct Information {
     context: Vec<Box<str>>,
 }
 
+#[derive(Debug)]
+enum PrimaryError {
+    Clap(clap::Error),
+    Other(Box<str>),
+}
+
+impl fmt::Display for PrimaryError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            PrimaryError::Clap(e) => e.fmt(f),
+            PrimaryError::Other(e) => e.fmt(f),
+        }
+    }
+}
+
 //--- Interaction
 
 impl Error {
     /// Construct a new error from a string.
     pub fn new(error: &str) -> Self {
         Self(Box::new(Information {
-            primary: error.into(),
+            primary: PrimaryError::Other(error.into()),
             context: Vec::new(),
         }))
     }
@@ -38,8 +53,21 @@ impl Error {
     }
 
     /// Pretty-print this error.
-    pub fn pretty_print(self) {
+    pub fn pretty_print(&self, env: impl Env) {
         use std::io::IsTerminal;
+        let mut err = env.stderr();
+
+        let error = match &self.0.primary {
+            // Clap errors are already styled. We don't want our own pretty
+            // styling around that and context does not make sense for command
+            // line arguments either. So we just print the styled string that
+            // clap produces and return.
+            PrimaryError::Clap(e) => {
+                let _ = writeln!(err, "{}", e.render().ansi());
+                return;
+            }
+            PrimaryError::Other(error) => error,
+        };
 
         // NOTE: This is a multicall binary, so argv[0] is necessary for
         // program operation.  We would fail very early if it didn't exist.
@@ -52,9 +80,24 @@ impl Error {
             "ERROR:"
         };
 
-        eprint!("[{prog}] {error_marker} {}", self.0.primary);
+        let _ = write!(err, "[{prog}] {error_marker} {error}");
         for context in &self.0.context {
-            eprint!("\n... while {context}");
+            let _ = writeln!(err, "\n... while {context}");
+        }
+    }
+
+    pub fn exit_code(&self) -> u8 {
+        // Clap uses the exit code 2 and we want to keep that, but we aren't
+        // actually returning the clap error, so we replicate that behaviour
+        // here.
+        //
+        // Argument parsing errors from the ldns-xxx commands will not be clap
+        // errors and therefore be printed with an exit code of 1. This is
+        // expected because ldns also exits with 1.
+        if let PrimaryError::Clap(e) = &self.0.primary {
+            e.exit_code() as u8
+        } else {
+            1
         }
     }
 }
@@ -85,11 +128,20 @@ impl From<lexopt::Error> for Error {
     }
 }
 
+impl From<clap::Error> for Error {
+    fn from(value: clap::Error) -> Self {
+        Self(Box::new(Information {
+            primary: PrimaryError::Clap(value),
+            context: Vec::new(),
+        }))
+    }
+}
+
 //--- Display, Debug
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.write_str(&self.0.primary)
+        self.0.primary.fmt(f)
     }
 }
 
diff --git a/src/lib.rs b/src/lib.rs
index a329971..8a14999 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,29 +1,53 @@
-use std::{ffi::OsString, path::Path};
+use std::ffi::OsString;
+use std::path::Path;
 
+use clap::Parser;
 use commands::{nsec3hash::Nsec3Hash, LdnsCommand};
+use env::Env;
+use error::Error;
 
 pub use self::args::Args;
 
 pub mod args;
 pub mod commands;
+pub mod env;
 pub mod error;
 
-pub fn try_ldns_compatibility<I: IntoIterator<Item = OsString>>(args: I) -> Option<Args> {
+pub fn try_ldns_compatibility<I: IntoIterator<Item = OsString>>(
+    args: I,
+) -> Result<Option<Args>, Error> {
     let mut args_iter = args.into_iter();
-    let binary_path = args_iter.next()?;
+    let binary_path = args_iter.next().ok_or("Missing binary name")?;
 
-    let binary_name = Path::new(&binary_path).file_name()?.to_str()?;
+    let binary_name = Path::new(&binary_path)
+        .file_name()
+        .ok_or("Missing binary file name")?
+        .to_str()
+        .ok_or("Binary file name is not valid unicode")?;
 
     let res = match binary_name {
         "ldns-nsec3-hash" => Nsec3Hash::parse_ldns_args(args_iter),
-        _ => return None,
+        _ => return Ok(None),
     };
 
+    res.map(Some)
+}
+
+fn parse_args(env: impl Env) -> Result<Args, Error> {
+    if let Some(args) = try_ldns_compatibility(env.args_os())? {
+        return Ok(args);
+    }
+    let args = Args::try_parse_from(env.args_os())?;
+    Ok(args)
+}
+
+pub fn run(env: impl Env) -> u8 {
+    let res = parse_args(&env).and_then(|args| args.execute(&env));
     match res {
-        Ok(args) => Some(args),
+        Ok(()) => 0,
         Err(err) => {
-            err.pretty_print();
-            std::process::exit(1)
+            err.pretty_print(&env);
+            err.exit_code()
         }
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 4a14114..b368f32 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,18 +1,6 @@
 use std::process::ExitCode;
 
-use clap::Parser;
-
 fn main() -> ExitCode {
-    // If none of the ldns-* tools matched, then we continue with clap
-    // argument parsing.
-    let env_args = std::env::args_os();
-    let args = dnst::try_ldns_compatibility(env_args).unwrap_or_else(dnst::Args::parse);
-
-    match args.execute() {
-        Ok(()) => ExitCode::SUCCESS,
-        Err(err) => {
-            err.pretty_print();
-            ExitCode::FAILURE
-        }
-    }
+    let env = dnst::env::RealEnv;
+    dnst::run(env).into()
 }