diff --git a/.gitignore b/.gitignore
index 7585238e..78f3c965 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,3 @@
+*.un~
 book
+target/
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 00000000..88e84d43
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,54 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "aho-corasick"
+version = "0.7.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e37cfd5e7657ada45f742d6e99ca5788580b5c529dc78faf11ece6dc702656f"
+dependencies = [
+ "memchr",
+]
+
+[[package]]
+name = "camino"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "52d74260d9bf6944e2208aa46841b4b8f0d7ffc0849a06837b2f510337f86b2b"
+
+[[package]]
+name = "fixlinks"
+version = "0.1.0"
+dependencies = [
+ "camino",
+ "pathdiff",
+ "regex",
+]
+
+[[package]]
+name = "memchr"
+version = "2.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "308cc39be01b73d0d18f82a0e7b2a3df85245f84af96fdddc5d202d27e47b86a"
+
+[[package]]
+name = "pathdiff"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "877630b3de15c0b64cc52f659345724fbf6bdad9bd9566699fc53688f3c34a34"
+
+[[package]]
+name = "regex"
+version = "1.5.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d07a8629359eb56f1e2fb1652bb04212c072a87ba68546a04065d525673ac461"
+dependencies = [
+ "aho-corasick",
+ "memchr",
+ "regex-syntax",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.6.25"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f497285884f3fcff424ffc933e56d7cbca511def0c9831a7f9b5f6153e3cc89b"
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 00000000..98e687a6
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,4 @@
+[workspace]
+members = [
+    "tools/fixlinks",
+]
diff --git a/tools/fixlinks/.gitignore b/tools/fixlinks/.gitignore
new file mode 100644
index 00000000..ea8c4bf7
--- /dev/null
+++ b/tools/fixlinks/.gitignore
@@ -0,0 +1 @@
+/target
diff --git a/tools/fixlinks/Cargo.toml b/tools/fixlinks/Cargo.toml
new file mode 100644
index 00000000..e20a603b
--- /dev/null
+++ b/tools/fixlinks/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "fixlinks"
+version = "0.1.0"
+authors = ["Tyler Mandry <tmandry@gmail.com>"]
+edition = "2018"
+
+[dependencies]
+camino = "1.0"
+regex = "1.5"
+pathdiff = "0.2"
diff --git a/tools/fixlinks/src/main.rs b/tools/fixlinks/src/main.rs
new file mode 100644
index 00000000..8501d682
--- /dev/null
+++ b/tools/fixlinks/src/main.rs
@@ -0,0 +1,186 @@
+//! Searches for broken local paths in markdown files and attempts to repair them.
+//!
+//! Usage: cargo run --bin=fixlinks ./**/*.md
+
+use std::{collections::{BTreeSet, HashMap}, fs, ops::Range, path::Component};
+use camino::*;
+use pathdiff::diff_paths;
+use regex::{Match, Regex};
+
+#[allow(unused)]
+struct FilenameEntry {
+    file: Utf8PathBuf,
+    reported: bool,
+}
+
+fn main() {
+    let files: Vec<Utf8PathBuf> = std::env::args().skip(1).map(Into::into).collect();
+    let mut filenames = HashMap::<String, Vec<Utf8PathBuf>>::new();
+    for file in &files {
+        let name = file.file_name().unwrap();
+        filenames.entry(name.to_owned()).or_default().push(file.clone());
+    }
+
+    for (name, paths) in &filenames {
+        if paths.len() > 1 {
+            eprintln!("Note: Duplicate filename: {}", name);
+            for path in paths {
+                eprintln!("- {}", path);
+            }
+        }
+    }
+
+    let mut cx = Context::new(filenames);
+
+    for file in &files {
+        let contents = fs::read_to_string(&file).unwrap();
+        cx.check_file(file, &contents);
+    }
+
+    eprintln!("{:#?}", cx.count);
+}
+
+struct Context {
+    filenames: HashMap::<String, Vec<Utf8PathBuf>>,
+    count: Counts,
+}
+
+#[derive(Default, Debug)]
+struct Counts {
+    urls: usize,
+    paths: usize,
+    missing: usize,
+    matches: usize,
+    ambiguous: usize,
+    ties: usize,
+}
+
+impl Context {
+    fn new(filenames: HashMap::<String, Vec<Utf8PathBuf>>) -> Self {
+        Context {
+            filenames,
+            count: Default::default(),
+        }
+    }
+
+    fn check_file<'c>(&mut self, file: &Utf8Path, contents: &'c str) {
+        let re_linkref = Regex::new(r"^\[(.*?)\]: (.*)").unwrap();
+        let re_inline = Regex::new(r"\[([^\[\]]*)\]\((.*?)\)").unwrap();
+
+        struct UrlMatch {
+            line_no: usize,
+            source_range: Range<usize>,
+        }
+        let mut matches = vec![];
+        let mut add_match = |line: &'c str, line_no, mat: Match| {
+            // Safety: line and contents are from the same allocation.
+            let line_start_idx = unsafe { line.as_ptr().offset_from(contents.as_ptr()) } as usize;
+            let source_range = (line_start_idx + mat.start())..(line_start_idx + mat.end());
+            matches.push(UrlMatch { line_no, source_range });
+        };
+
+        // Match against linkrefs, then inline links.
+        let mut linkrefs = BTreeSet::new();
+        for (idx, line) in contents.lines().enumerate() {
+            let line_no = idx + 1;
+            for cap in re_linkref.captures_iter(line) {
+                add_match(line, line_no, cap.get(2).unwrap());
+                linkrefs.insert(cap.get(1).unwrap().as_str());
+            }
+        }
+        for (idx, line) in contents.lines().enumerate() {
+            let line_no = idx + 1;
+            for cap in re_inline.captures_iter(line) {
+                if let Some(url) = cap.get(2) {
+                    if !linkrefs.contains(url.as_str()) {
+                        add_match(line, line_no, url);
+                    }
+                }
+            }
+        }
+
+        // We need to process through matches in order so we can build the
+        // modified contents sequentially.
+        matches.sort_by_key(|um| um.source_range.start);
+
+        let mut modified = String::new();
+        let mut source_bytes_written = 0;
+        for UrlMatch { line_no, source_range } in matches {
+            if let Some(replacement) = self.check_url(file, line_no, &contents[source_range.clone()]) {
+                modified.push_str(&contents[source_bytes_written..source_range.start]);
+                modified.push_str(&replacement);
+                source_bytes_written = source_range.end;
+            }
+        };
+
+        if !modified.is_empty() {
+            modified.push_str(&contents[source_bytes_written..]);
+            fs::write(file, modified).unwrap();
+        }
+    }
+
+    fn check_url(&mut self, file: &Utf8Path, line_no: usize, path: &str) -> Option<String> {
+        self.count.urls += 1;
+        if path.starts_with("http:") || path.starts_with("https:") || path.starts_with("#") {
+            return None;
+        }
+        let path = path.split('#').next().unwrap();
+
+        self.count.paths += 1;
+
+        let resolved = file.parent().unwrap().join(Utf8Path::new(path));
+        if !resolved.exists() {
+            self.count.missing += 1;
+            // println!("{}:{}: {}", file, line_no, path);
+
+            let filenames = &self.filenames;
+            if let Some(names) = resolved.file_name().and_then(|f| filenames.get(f)) {
+                self.count.matches += 1;
+                if names.len() > 1 {
+                    self.count.ambiguous += 1;
+                }
+                if let Some(mut replacement) = Self::rank_names(file, names, &mut self.count) {
+                    // mdBook doesn't seem to like "raw" filenames.
+                    if !replacement.starts_with("../") {
+                        replacement.insert_str(0, "./");
+                    }
+                    // println!("- Replacing with: {}", replacement);
+                    return Some(replacement);
+                }
+            } else {
+                eprintln!("Warning: Unable to resolve at {}:{}: {}", file, line_no, path);
+            }
+        }
+        None
+    }
+
+    fn rank_names(file: &Utf8Path, names: &[Utf8PathBuf], count: &mut Counts) -> Option<String> {
+        let parent_dir = file.parent().unwrap();
+        let mut best_score = usize::MAX;
+        let mut best_path = None;
+        let mut ties = 0;
+        for candidate in names {
+            // println!("- Could be: {}", candidate);
+
+            // Candidates are scored by how many differing path components they have.
+            if let Some(relpath) = diff_paths(candidate, parent_dir) {
+                let score = relpath.components().count() +
+                    // Triple count ../ components (prefer candidates in the same directory).
+                    relpath.components().take_while(|c| c == &Component::ParentDir).count() * 2;
+                if score < best_score {
+                    best_score = score;
+                    best_path = Some(relpath.into_os_string().into_string().unwrap());
+                    ties = 0;
+                } else if score == best_score {
+                    ties += 1;
+                }
+            };
+        }
+
+        if best_path.is_some() && ties > 0 {
+            count.ties += 1;
+        }
+
+        best_path
+    }
+}