.cargo/config

@@ -1 +0,0 @@
-config.toml

Cargo.toml

@@ -1,4 +1,5 @@
 cargo-features = ["codegen-backend"]
+
 [package]
 description = "An HTTP server using HTTP basic auth to make secure calls to nsupdate"
 name = "webnsupdate"
@@ -7,24 +8,26 @@ edition = "2021"
 
 [dependencies]
 axum = "0.7.7"
+axum-auth = { version = "0.7.0", default-features = false, features = [
+  "auth-basic",
+] }
 axum-client-ip = "0.6.1"
 base64 = "0.22.1"
 clap = { version = "4.5.20", features = ["derive", "env"] }
 http = "1.1.0"
-insta = "1.40.0"
 miette = { version = "7.2.0", features = ["fancy"] }
 ring = { version = "0.17.8", features = ["std"] }
+tokio = { version = "1.40.0", features = [
+  "macros",
+  "rt",
+  "process",
+  "io-util",
+] }
 tracing = "0.1.40"
 tracing-subscriber = { version = "0.3.18", features = ["env-filter"] }
 
-[dependencies.axum-auth]
-version = "0.7.0"
-default-features = false
-features = ["auth-basic"]
-
-[dependencies.tokio]
-version = "1.40.0"
-features = ["macros", "rt", "process", "io-util"]
+[dev-dependencies]
+insta = "1.40.0"
 
 [profile.dev]
 debug = 0

default.nix

@@ -19,6 +19,8 @@ let
           ".rs"
           # TOML files are often used to configure cargo based tools (e.g. .cargo/config.toml)
           ".toml"
+          # Snapshot tests
+          ".snap"
         ];
         isCargoLock = base == "Cargo.lock";
       in

src/main.rs

@@ -1,8 +1,7 @@
 use std::{
     ffi::OsStr,
-    io::Write,
+    io::ErrorKind,
     net::{IpAddr, SocketAddr},
-    os::unix::fs::OpenOptionsExt,
     path::{Path, PathBuf},
     process::{ExitStatus, Stdio},
     time::Duration,
@@ -12,14 +11,16 @@ use axum::{extract::State, routing::get, Json, Router};
 use axum_auth::AuthBasic;
 use axum_client_ip::{SecureClientIp, SecureClientIpSource};
 use base64::{engine::general_purpose::URL_SAFE_NO_PAD, Engine};
-use clap::{Args, Parser, Subcommand};
+use clap::{Parser, Subcommand};
 use http::StatusCode;
-use miette::{ensure, miette, Context, IntoDiagnostic, LabeledSpan, NamedSource, Result};
-use ring::digest::Digest;
+use miette::{bail, ensure, Context, IntoDiagnostic, Result};
 use tokio::io::AsyncWriteExt;
 use tracing::{debug, error, info, level_filters::LevelFilter, trace, warn};
 use tracing_subscriber::EnvFilter;
 
+mod password;
+mod records;
+
 const DEFAULT_TTL: Duration = Duration::from_secs(60);
 const DEFAULT_SALT: &str = "UpdateMyDNS";
 
@@ -28,20 +29,29 @@ struct Opts {
     /// Ip address of the server
     #[arg(long, default_value = "127.0.0.1")]
     address: IpAddr,
+
     /// Port of the server
     #[arg(long, default_value_t = 5353)]
     port: u16,
+
     /// File containing password to match against
     ///
     /// Should be of the format `username:password` and contain a single password
     #[arg(long)]
     password_file: Option<PathBuf>,
+
     /// Salt to get more unique hashed passwords and prevent table based attacks
     #[arg(long, default_value = DEFAULT_SALT)]
     salt: String,
+
     /// Time To Live (in seconds) to set on the DNS records
     #[arg(long, default_value_t = DEFAULT_TTL.as_secs())]
     ttl: u64,
+
+    /// Data directory
+    #[arg(long, default_value = ".")]
+    data_dir: PathBuf,
+
     /// File containing the records that should be updated when an update request is made
     ///
     /// There should be one record per line:
@@ -52,70 +62,92 @@ struct Opts {
     /// ```
     #[arg(long)]
     records: PathBuf,
+
     /// Keyfile `nsupdate` should use
     ///
     /// If specified, then `webnsupdate` must have read access to the file
     #[arg(long)]
     key_file: Option<PathBuf>,
+
     /// Allow not setting a password
     #[arg(long)]
     insecure: bool,
+
     /// Set client IP source
     ///
     /// see: https://docs.rs/axum-client-ip/latest/axum_client_ip/enum.SecureClientIpSource.html
     #[clap(long, default_value = "RightmostXForwardedFor")]
     ip_source: SecureClientIpSource,
+
     #[clap(subcommand)]
     subcommand: Option<Cmd>,
 }
 
-#[derive(Debug, Args)]
-struct Mkpasswd {
-    /// The username
-    username: String,
-    /// The password
-    password: String,
-}
-
 #[derive(Debug, Subcommand)]
 enum Cmd {
-    /// Create a password file
-    ///
-    /// If `--password-file` is provided, the password is written to that file
-    Mkpasswd(Mkpasswd),
+    Mkpasswd(password::Mkpasswd),
     /// Verify the records file
     Verify,
 }
 
+impl Cmd {
+    pub fn process(self, args: &Opts) -> Result<()> {
+        match self {
+            Cmd::Mkpasswd(mkpasswd) => mkpasswd.process(args),
+            Cmd::Verify => records::load(&args.records).map(drop),
+        }
+    }
+}
+
 #[derive(Clone)]
 struct AppState<'a> {
     /// TTL set on the Zonefile
     ttl: Duration,
+
     /// Salt added to the password
     salt: &'a str,
+
     /// The IN A/AAAA records that should have their IPs updated
     records: &'a [&'a str],
+
     /// The TSIG key file
     key_file: Option<&'a Path>,
+
     /// The password hash
     password_hash: Option<&'a [u8]>,
+
+    /// The file where the last IP is stored
+    ip_file: &'a Path,
 }
 
-#[tokio::main(flavor = "current_thread")]
-async fn main() -> Result<()> {
+fn load_ip(path: &Path) -> Result<Option<IpAddr>> {
+    let data = match std::fs::read_to_string(path) {
+        Ok(ip) => ip,
+        Err(err) => {
+            return match err.kind() {
+                ErrorKind::NotFound => Ok(None),
+                _ => Err(err).into_diagnostic().wrap_err_with(|| {
+                    format!("failed to load last ip address from {}", path.display())
+                }),
+            }
+        }
+    };
+
+    Ok(Some(
+        data.parse()
+            .into_diagnostic()
+            .wrap_err("failed to parse last ip address")?,
+    ))
+}
+
+fn main() -> Result<()> {
+    // set panic hook to pretty print with miette's formatter
     miette::set_panic_hook();
-    let Opts {
-        address: ip,
-        port,
-        password_file,
-        key_file,
-        insecure,
-        subcommand,
-        records,
-        salt,
-        ttl,
-        ip_source,
-    } = Opts::parse();
+
+    // parse cli arguments
+    let mut args = Opts::parse();
+
+    // configure logger
     let subscriber = tracing_subscriber::FmtSubscriber::builder()
         .without_time()
         .with_env_filter(
@@ -127,58 +159,109 @@ async fn main() -> Result<()> {
     tracing::subscriber::set_global_default(subscriber)
         .into_diagnostic()
         .wrap_err("setting global tracing subscriber")?;
-    match subcommand {
-        Some(Cmd::Mkpasswd(args)) => return mkpasswd(args, password_file.as_deref(), &salt),
-        Some(Cmd::Verify) => {
-            let data = std::fs::read_to_string(&records)
-                .into_diagnostic()
-                .wrap_err_with(|| format!("trying to read {}", records.display()))?;
-            return verify_records(&data, &records);
-        }
-        None => {}
+
+    // process subcommand
+    if let Some(cmd) = args.subcommand.take() {
+        return cmd.process(&args);
     }
+
+    let Opts {
+        address: ip,
+        port,
+        password_file,
+        data_dir,
+        key_file,
+        insecure,
+        subcommand: _,
+        records,
+        salt,
+        ttl,
+        ip_source,
+    } = args;
+
     info!("checking environment");
+
     // Set state
     let ttl = Duration::from_secs(ttl);
-    let mut state = AppState {
+
+    // Use last registered IP address if available
+    let ip_file = data_dir.join("last-ip");
+
+    let state = AppState {
         ttl,
         salt: salt.leak(),
-        records: &[],
-        key_file: None,
-        password_hash: None,
-    };
-    if let Some(path) = password_file {
-        let pass = std::fs::read_to_string(&path).into_diagnostic()?;
+        // Load DNS records
+        records: records::load_no_verify(&records)?,
+        // Load keyfile
+        key_file: key_file
+            .map(|key_file| -> miette::Result<_> {
+                let path = key_file.as_path();
+                std::fs::File::open(path)
+                    .into_diagnostic()
+                    .wrap_err_with(|| {
+                        format!("{} is not readable by the current user", path.display())
+                    })?;
+                Ok(&*Box::leak(key_file.into_boxed_path()))
+            })
+            .transpose()?,
+        // Load password hash
+        password_hash: password_file
+            .map(|path| -> miette::Result<_> {
+                let pass = std::fs::read_to_string(path.as_path()).into_diagnostic()?;
 
                 let pass: Box<[u8]> = URL_SAFE_NO_PAD
                     .decode(pass.trim().as_bytes())
                     .into_diagnostic()
                     .wrap_err_with(|| format!("failed to decode password from {}", path.display()))?
                     .into();
-        state.password_hash = Some(Box::leak(pass));
-    } else {
-        ensure!(insecure, "a password must be used");
-    }
-    if let Some(key_file) = key_file {
-        let path = key_file.as_path();
-        std::fs::File::open(path)
+
+                Ok(&*Box::leak(pass))
+            })
+            .transpose()?,
+        ip_file: Box::leak(ip_file.into_boxed_path()),
+    };
+
+    ensure!(
+        state.password_hash.is_some() || insecure,
+        "a password must be used"
+    );
+
+    ensure!(
+        state.key_file.is_some() || insecure,
+        "a key file must be used"
+    );
+
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
         .into_diagnostic()
-            .wrap_err_with(|| format!("{} is not readable by the current user", path.display()))?;
-        state.key_file = Some(Box::leak(key_file.into_boxed_path()));
-    } else {
-        ensure!(insecure, "a key file must be used");
+        .wrap_err("failed to start the tokio runtime")?;
+
+    rt.block_on(async {
+        // Load previous IP and update DNS record to point to it (if available)
+        match load_ip(state.ip_file) {
+            Ok(Some(ip)) => match nsupdate(ip, ttl, state.key_file, state.records).await {
+                Ok(status) => {
+                    if !status.success() {
+                        error!("nsupdate failed: code {status}");
+                        bail!("nsupdate returned with code {status}");
                     }
-    let data = std::fs::read_to_string(&records)
+                }
+                Err(err) => {
+                    error!("Failed to update records with previous IP: {err}");
+                    return Err(err)
                         .into_diagnostic()
-        .wrap_err_with(|| format!("loading records from {}", records.display()))?;
-    if let Err(err) = verify_records(&data, &records) {
-        warn!("invalid records found: {err}");
+                        .wrap_err("failed to update records with previous IP");
                 }
-    state.records = data
-        .lines()
-        .map(|s| &*s.to_string().leak())
-        .collect::<Vec<&'static str>>()
-        .leak();
+            },
+            Ok(None) => {
+                info!("No previous IP address set");
+            }
+            Err(err) => {
+                error!("Failed to load last ip address: {err}")
+            }
+        };
+
         // Start services
         let app = Router::new()
             .route("/update", get(update_records))
@@ -195,6 +278,7 @@ async fn main() -> Result<()> {
         )
         .await
         .into_diagnostic()
+    })
 }
 
 #[tracing::instrument(skip(state, pass), level = "trace", ret(level = "info"))]
@@ -206,33 +290,39 @@ async fn update_records(
     let Some(pass) = pass else {
         return Err((StatusCode::UNAUTHORIZED, Json::from("no password provided")).into());
     };
+
     if let Some(stored_pass) = state.password_hash {
         let password = pass.trim().to_string();
-        let pass_hash = hash_identity(&username, &password, state.salt);
+        let pass_hash = password::hash_identity(&username, &password, state.salt);
         if pass_hash.as_ref() != stored_pass {
             warn!("rejected update");
             trace!(
                 "mismatched hashes:\n{}\n{}",
                 URL_SAFE_NO_PAD.encode(pass_hash.as_ref()),
-                URL_SAFE_NO_PAD.encode(stored_pass.as_ref()),
+                URL_SAFE_NO_PAD.encode(stored_pass),
             );
             return Err((StatusCode::UNAUTHORIZED, "invalid identity").into());
         }
     }
+
     info!("accepted update");
     match nsupdate(ip, state.ttl, state.key_file, state.records).await {
-        Ok(status) => {
-            if status.success() {
+        Ok(status) if status.success() => {
+            tokio::task::spawn_blocking(move || {
+                if let Err(err) = std::fs::write(state.ip_file, format!("{ip}")) {
+                    error!("Failed to update last IP: {err}");
+                }
+            });
             Ok("successful update")
-            } else {
-                error!("nsupdate failed");
+        }
+        Ok(status) => {
+            error!("nsupdate failed with code {status}");
             Err((
                 StatusCode::INTERNAL_SERVER_ERROR,
                 "nsupdate failed, check server logs",
             )
                 .into())
         }
-        }
         Err(error) => Err((
             StatusCode::INTERNAL_SERVER_ERROR,
             format!("failed to update records: {error}"),
@@ -292,148 +382,11 @@ fn update_ns_records(ip: IpAddr, ttl: Duration, records: &[&str]) -> String {
     cmds
 }
 
-fn hash_identity(username: &str, password: &str, salt: &str) -> Digest {
-    let mut data = Vec::with_capacity(username.len() + password.len() + salt.len() + 1);
-    write!(data, "{username}:{password}{salt}").unwrap();
-    ring::digest::digest(&ring::digest::SHA256, &data)
-}
-
-fn mkpasswd(
-    Mkpasswd { username, password }: Mkpasswd,
-    password_file: Option<&Path>,
-    salt: &str,
-) -> miette::Result<()> {
-    let hash = hash_identity(&username, &password, salt);
-    let encoded = URL_SAFE_NO_PAD.encode(hash.as_ref());
-    let Some(path) = password_file else {
-        println!("{encoded}");
-        return Ok(());
-    };
-    let err = || format!("trying to save password hash to {}", path.display());
-    std::fs::File::options()
-        .mode(0o600)
-        .create_new(true)
-        .open(path)
-        .into_diagnostic()
-        .wrap_err_with(err)?
-        .write_all(encoded.as_bytes())
-        .into_diagnostic()
-        .wrap_err_with(err)?;
-
-    Ok(())
-}
-
-fn verify_records(data: &str, path: &Path) -> miette::Result<()> {
-    let source = || NamedSource::new(path.display().to_string(), data.to_string());
-    let mut byte_offset = 0usize;
-    for line in data.lines() {
-        if line.is_empty() {
-            continue;
-        }
-        ensure!(
-            line.len() <= 255,
-            miette!(
-                labels = [LabeledSpan::new(
-                    Some("this line".to_string()),
-                    byte_offset,
-                    line.len(),
-                )],
-                help = "fully qualified domain names can be at most 255 characters long",
-                url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
-                "hostname too long ({} octets)",
-                line.len(),
-            )
-            .with_source_code(source())
-        );
-        ensure!(
-            line.ends_with('.'),
-            miette!(
-                labels = [LabeledSpan::new(
-                    Some("last character".to_string()),
-                    byte_offset + line.len() - 1,
-                    1,
-                )],
-                help = "hostname should be a fully qualified domain name (end with a '.')",
-                url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
-                "not a fully qualified domain name"
-            )
-            .with_source_code(source())
-        );
-        let mut local_offset = 0usize;
-        for label in line.strip_suffix('.').unwrap_or(line).split('.') {
-            ensure!(
-                !label.is_empty(),
-                miette!(
-                    labels = [LabeledSpan::new(
-                        Some("label".to_string()),
-                        byte_offset + local_offset,
-                        label.len(),
-                    )],
-                    help = "each label should have at least one character",
-                    url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
-                    "empty label",
-                )
-                .with_source_code(source())
-            );
-            ensure!(
-                label.len() <= 63,
-                miette!(
-                    labels = [LabeledSpan::new(
-                        Some("label".to_string()),
-                        byte_offset + local_offset,
-                        label.len(),
-                    )],
-                    help = "labels should be at most 63 octets",
-                    url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
-                    "label too long ({} octets)",
-                    label.len(),
-                )
-                .with_source_code(source())
-            );
-            for (offset, octet) in label.bytes().enumerate() {
-                ensure!(
-                    octet.is_ascii(),
-                    miette!(
-                        labels = [LabeledSpan::new(
-                            Some("octet".to_string()),
-                            byte_offset + local_offset + offset,
-                            1,
-                        )],
-                        help = "we only accept ascii characters",
-                        url = "https://en.wikipedia.org/wiki/Hostname#Syntax",
-                        "'{}' is not ascii",
-                        octet.escape_ascii(),
-                    )
-                    .with_source_code(source())
-                );
-                ensure!(
-                    octet.is_ascii_alphanumeric() || octet == b'-' || octet == b'_',
-                    miette!(
-                        labels = [LabeledSpan::new(
-                            Some("octet".to_string()),
-                            byte_offset + local_offset + offset,
-                            1,
-                        )],
-                        help = "hostnames are only allowed to contain characters in [a-zA-Z0-9_-]",
-                        url = "https://en.wikipedia.org/wiki/Hostname#Syntax",
-                        "invalid octet: '{}'",
-                        octet.escape_ascii(),
-                    )
-                    .with_source_code(source())
-                );
-            }
-            local_offset += label.len() + 1;
-        }
-        byte_offset += line.len() + 1;
-    }
-    Ok(())
-}
-
 #[cfg(test)]
 mod test {
     use insta::assert_snapshot;
 
-    use crate::{update_ns_records, verify_records, DEFAULT_TTL};
+    use crate::{update_ns_records, DEFAULT_TTL};
 
     use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
 
@@ -476,107 +429,4 @@ mod test {
         quit
         "###);
     }
-
-    #[test]
-    fn valid_records() -> miette::Result<()> {
-        verify_records(
-            "\
-            example.com.\n\
-            example.org.\n\
-            example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_valid"),
-        )
-    }
-
-    #[test]
-    fn hostname_too_long() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            example.org.\n\
-            example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @"hostname too long (260 octets)");
-    }
-
-    #[test]
-    fn not_fqd() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            example.org.\n\
-            example.net\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @"not a fully qualified domain name");
-    }
-
-    #[test]
-    fn empty_label() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            name..example.org.\n\
-            example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @"empty label");
-    }
-
-    #[test]
-    fn label_too_long() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            name.an-entremely-long-label-that-should-not-exist-because-it-goes-against-the-spec.example.org.\n\
-            example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @"label too long (78 octets)");
-    }
-
-    #[test]
-    fn invalid_ascii() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            name.this-is-not-aßcii.example.org.\n\
-            example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @r###"'\xc3' is not ascii"###);
-    }
-
-    #[test]
-    fn invalid_octet() {
-        let err = verify_records(
-            "\
-            example.com.\n\
-            name.this-character:-is-not-allowed.example.org.\n\
-            example.net.\n\
-            subdomain.example.com.\n\
-            ",
-            std::path::Path::new("test_records_invalid"),
-        )
-        .unwrap_err();
-        assert_snapshot!(err, @"invalid octet: ':'");
-    }
 }

src/password.rs

@@ -0,0 +1,60 @@
+//! Make a password for use with webnsupdate
+//!
+//! You should call this command an give it's output to the app/script that will update the DNS
+//! records
+use std::io::Write;
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::Path;
+
+use base64::prelude::*;
+use miette::{Context, IntoDiagnostic, Result};
+use ring::digest::Digest;
+
+/// Create a password file
+///
+/// If `--password-file` is provided, the password is written to that file
+#[derive(Debug, clap::Args)]
+pub struct Mkpasswd {
+    /// The username
+    username: String,
+
+    /// The password
+    password: String,
+}
+
+impl Mkpasswd {
+    pub fn process(self, args: &crate::Opts) -> Result<()> {
+        mkpasswd(self, args.password_file.as_deref(), &args.salt)
+    }
+}
+
+pub fn hash_identity(username: &str, password: &str, salt: &str) -> Digest {
+    let mut data = Vec::with_capacity(username.len() + password.len() + salt.len() + 1);
+    write!(data, "{username}:{password}{salt}").unwrap();
+    ring::digest::digest(&ring::digest::SHA256, &data)
+}
+
+pub fn mkpasswd(
+    Mkpasswd { username, password }: Mkpasswd,
+    password_file: Option<&Path>,
+    salt: &str,
+) -> miette::Result<()> {
+    let hash = hash_identity(&username, &password, salt);
+    let encoded = BASE64_URL_SAFE_NO_PAD.encode(hash.as_ref());
+    let Some(path) = password_file else {
+        println!("{encoded}");
+        return Ok(());
+    };
+    let err = || format!("trying to save password hash to {}", path.display());
+    std::fs::File::options()
+        .mode(0o600)
+        .create_new(true)
+        .open(path)
+        .into_diagnostic()
+        .wrap_err_with(err)?
+        .write_all(encoded.as_bytes())
+        .into_diagnostic()
+        .wrap_err_with(err)?;
+
+    Ok(())
+}

src/records.rs

@@ -0,0 +1,283 @@
+//! Deal with the DNS records
+
+use std::path::Path;
+
+use miette::{ensure, miette, Context, IntoDiagnostic, LabeledSpan, NamedSource, Result};
+
+/// Loads and verifies the records from a file
+pub fn load(path: &Path) -> Result<()> {
+    let records = std::fs::read_to_string(path)
+        .into_diagnostic()
+        .wrap_err_with(|| format!("failed to read records from {}", path.display()))?;
+
+    verify(&records, path)?;
+
+    Ok(())
+}
+
+/// Load records without verifying them
+pub fn load_no_verify(path: &Path) -> Result<&'static [&'static str]> {
+    let records = std::fs::read_to_string(path)
+        .into_diagnostic()
+        .wrap_err_with(|| format!("failed to read records from {}", path.display()))?;
+
+    if let Err(err) = verify(&records, path) {
+        tracing::error!("Failed to verify records: {err}");
+    }
+
+    // leak memory: we only do this here and it prevents a bunch of allocations
+    let records: &str = records.leak();
+    let records: Box<[&str]> = records.lines().collect();
+
+    Ok(Box::leak(records))
+}
+
+/// Verifies that a list of records is valid
+pub fn verify(data: &str, path: &Path) -> Result<()> {
+    let mut offset = 0usize;
+    for line in data.lines() {
+        validate_line(offset, line).map_err(|err| {
+            err.with_source_code(NamedSource::new(
+                path.display().to_string(),
+                data.to_string(),
+            ))
+        })?;
+
+        offset += line.len() + 1;
+    }
+
+    Ok(())
+}
+
+fn validate_line(offset: usize, line: &str) -> Result<()> {
+    if line.is_empty() {
+        return Ok(());
+    }
+
+    ensure!(
+        line.len() <= 255,
+        miette!(
+            labels = [LabeledSpan::new(
+                Some("this line".to_string()),
+                offset,
+                line.len(),
+            )],
+            help = "fully qualified domain names can be at most 255 characters long",
+            url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
+            "hostname too long ({} octets)",
+            line.len(),
+        )
+    );
+    ensure!(
+        line.ends_with('.'),
+        miette!(
+            labels = [LabeledSpan::new(
+                Some("last character".to_string()),
+                offset + line.len() - 1,
+                1,
+            )],
+            help = "hostname should be a fully qualified domain name (end with a '.')",
+            url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
+            "not a fully qualified domain name"
+        )
+    );
+
+    let mut label_offset = 0usize;
+    for label in line.strip_suffix('.').unwrap_or(line).split('.') {
+        validate_label(offset + label_offset, label)?;
+        label_offset += label.len() + 1;
+    }
+
+    Ok(())
+}
+
+fn validate_label(offset: usize, label: &str) -> Result<()> {
+    ensure!(
+        !label.is_empty(),
+        miette!(
+            labels = [LabeledSpan::new(
+                Some("label".to_string()),
+                offset,
+                label.len(),
+            )],
+            help = "each label should have at least one character",
+            url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
+            "empty label",
+        )
+    );
+    ensure!(
+        label.len() <= 63,
+        miette!(
+            labels = [LabeledSpan::new(
+                Some("label".to_string()),
+                offset,
+                label.len(),
+            )],
+            help = "labels should be at most 63 octets",
+            url = "https://en.wikipedia.org/wiki/Fully_qualified_domain_name",
+            "label too long ({} octets)",
+            label.len(),
+        )
+    );
+
+    for (octet_offset, octet) in label.bytes().enumerate() {
+        validate_octet(offset + octet_offset, octet)?;
+    }
+
+    Ok(())
+}
+
+fn validate_octet(offset: usize, octet: u8) -> Result<()> {
+    let spans = || [LabeledSpan::new(Some("octet".to_string()), offset, 1)];
+    ensure!(
+        octet.is_ascii(),
+        miette!(
+            labels = spans(),
+            help = "we only accept ascii characters",
+            url = "https://en.wikipedia.org/wiki/Hostname#Syntax",
+            "invalid octet: '{}'",
+            octet.escape_ascii(),
+        )
+    );
+
+    ensure!(
+        octet.is_ascii_alphanumeric() || octet == b'-' || octet == b'_',
+        miette!(
+            labels = spans(),
+            help = "hostnames are only allowed to contain characters in [a-zA-Z0-9_-]",
+            url = "https://en.wikipedia.org/wiki/Hostname#Syntax",
+            "invalid octet: '{}'",
+            octet.escape_ascii(),
+        )
+    );
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use crate::records::verify;
+
+    macro_rules! assert_miette_snapshot {
+        ($diag:expr) => {{
+            use std::borrow::Borrow;
+
+            use insta::{with_settings, assert_snapshot};
+            use miette::{GraphicalReportHandler, GraphicalTheme};
+
+            let mut out = String::new();
+            GraphicalReportHandler::new_themed(GraphicalTheme::unicode_nocolor())
+                .with_width(80)
+                .render_report(&mut out, $diag.borrow())
+                .unwrap();
+            with_settings!({
+                description => stringify!($diag)
+            }, {
+                assert_snapshot!(out);
+            });
+        }};
+    }
+
+    #[test]
+    fn valid_records() -> miette::Result<()> {
+        verify(
+            "\
+            example.com.\n\
+            example.org.\n\
+            example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_valid"),
+        )
+    }
+
+    #[test]
+    fn hostname_too_long() {
+        let err = verify(
+            "\
+            example.com.\n\
+            example.org.\n\
+            example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+
+    #[test]
+    fn not_fqd() {
+        let err = verify(
+            "\
+            example.com.\n\
+            example.org.\n\
+            example.net\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+
+    #[test]
+    fn empty_label() {
+        let err = verify(
+            "\
+            example.com.\n\
+            name..example.org.\n\
+            example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+
+    #[test]
+    fn label_too_long() {
+        let err = verify(
+            "\
+            example.com.\n\
+            name.an-entremely-long-label-that-should-not-exist-because-it-goes-against-the-spec.example.org.\n\
+            example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+
+    #[test]
+    fn invalid_ascii() {
+        let err = verify(
+            "\
+            example.com.\n\
+            name.this-is-not-aßcii.example.org.\n\
+            example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+
+    #[test]
+    fn invalid_octet() {
+        let err = verify(
+            "\
+            example.com.\n\
+            name.this-character:-is-not-allowed.example.org.\n\
+            example.net.\n\
+            subdomain.example.com.\n\
+            ",
+            std::path::Path::new("test_records_invalid"),
+        )
+        .unwrap_err();
+        assert_miette_snapshot!(err);
+    }
+}

src/snapshots/webnsupdate__records__test__empty_label.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Fully_qualified_domain_name\(link)]8;;\
+
+  × empty label
+   ╭─[test_records_invalid:2:6]
+ 1 │ example.com.
+ 2 │ name..example.org.
+   ·      ▲
+   ·      ╰── label
+ 3 │ example.net.
+   ╰────
+  help: each label should have at least one character

src/snapshots/webnsupdate__records__test__hostname_too_long.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Fully_qualified_domain_name\(link)]8;;\
+
+  × hostname too long (260 octets)
+   ╭─[test_records_invalid:3:1]
+ 2 │ example.org.
+ 3 │ example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.example.net.
+   · ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
+   ·                                                                                                                                   ╰── this line
+ 4 │ subdomain.example.com.
+   ╰────
+  help: fully qualified domain names can be at most 255 characters long

src/snapshots/webnsupdate__records__test__invalid_ascii.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Hostname#Syntax\(link)]8;;\
+
+  × invalid octet: '\xc3'
+   ╭─[test_records_invalid:2:19]
+ 1 │ example.com.
+ 2 │ name.this-is-not-aßcii.example.org.
+   ·                   ┬
+   ·                   ╰── octet
+ 3 │ example.net.
+   ╰────
+  help: we only accept ascii characters

src/snapshots/webnsupdate__records__test__invalid_octet.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Hostname#Syntax\(link)]8;;\
+
+  × invalid octet: ':'
+   ╭─[test_records_invalid:2:20]
+ 1 │ example.com.
+ 2 │ name.this-character:-is-not-allowed.example.org.
+   ·                    ┬
+   ·                    ╰── octet
+ 3 │ example.net.
+   ╰────
+  help: hostnames are only allowed to contain characters in [a-zA-Z0-9_-]

src/snapshots/webnsupdate__records__test__label_too_long.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Fully_qualified_domain_name\(link)]8;;\
+
+  × label too long (78 octets)
+   ╭─[test_records_invalid:2:6]
+ 1 │ example.com.
+ 2 │ name.an-entremely-long-label-that-should-not-exist-because-it-goes-against-the-spec.example.org.
+   ·      ───────────────────────────────────────┬──────────────────────────────────────
+   ·                                             ╰── label
+ 3 │ example.net.
+   ╰────
+  help: labels should be at most 63 octets

src/snapshots/webnsupdate__records__test__not_fqd.snap

@@ -0,0 +1,16 @@
+---
+source: src/records.rs
+description: err
+expression: out
+---
+]8;;https://en.wikipedia.org/wiki/Fully_qualified_domain_name\(link)]8;;\
+
+  × not a fully qualified domain name
+   ╭─[test_records_invalid:3:11]
+ 2 │ example.org.
+ 3 │ example.net
+   ·           ┬
+   ·           ╰── last character
+ 4 │ subdomain.example.com.
+   ╰────
+  help: hostname should be a fully qualified domain name (end with a '.')