setup: keep guest hostname resolvable

CHANGELOG.md

@@ -117,6 +117,18 @@
   and a liveness probe, so a guest whose sshd stops answering fails after ~90s
   — the bound interactive sessions already had.
 
+- **The guest hostname resolves, so `sudo` stops warning** — Instance creation
+  renamed the Firecracker guest to `claude-<name>` in `/etc/hostname` but left
+  the image's `127.0.1.1 claude-vm` entry in `/etc/hosts`, so every `sudo` in
+  the guest printed `sudo: unable to resolve host claude-<name>` before running.
+  Both files are now written together at create and restore, and the guest
+  hostname is clamped to fit the kernel's 64-byte hostname limit so long
+  instance names still get a resolvable name. No image rebuild is needed — the patch is
+  per-instance, and the image's own entry is what gets overwritten — but
+  `patch_guest_network` runs only on create and restore, so an existing VM
+  keeps the stale entry until `coop restore <vm> --image <image>` or a destroy
+  and recreate.
+
 - **Fail closed on an unmanaged `CODEX_HOME` in ChatGPT auth mode** (#441) —
   The guest wrapper now refuses an explicitly set `CODEX_HOME` when coop's
   managed `~/.codex/config.toml` selects keyring storage. This prevents `codex

docs/backends.md

@@ -77,7 +77,7 @@ All three steps are idempotent. If the artifact already exists and is up to date
 Creating an instance (`coop up`) follows this sequence:
 
 1. Copies the template rootfs to the instance directory using `cp --reflink=auto` for copy-on-write on supported filesystems.
-2. Mounts the copy and patches the guest network config with the instance's unique IP address and hostname.
+2. Mounts the copy and patches the guest network config with the instance's unique IP address, plus `/etc/hostname` and the matching `/etc/hosts` alias so the guest can resolve its own name.
 3. Optionally resizes the rootfs if a larger disk was requested (truncate + e2fsck + resize2fs).
 4. Writes a Firecracker JSON config specifying the kernel, rootfs drive, vCPU/memory allocation, network interface, and vsock device.
 5. Creates and attaches a TAP device to the bridge (see TAP networking below).

docs/trust-model.md

@@ -49,6 +49,18 @@ user launched it.
   `tar_pipe_pull` / `rsync_pull` bring guest-authored file contents, filenames,
   and symlinks onto the host filesystem. This is the **widest guest→host
   channel** and the primary place a path-traversal or symlink escape could land.
+- **Rootfs files touched while loop-mounted during setup.** `setup.rs`
+  `patch_guest_network` reads and rewrites the guest's `/etc/hosts`, and `coop
+  commit` turns a guest-mutated rootfs into an image template — so the guest
+  authors both the contents and the directory entry at that path on every later
+  create/restore. Contents are read bounded and best-effort
+  (`bound_guest_hosts` degrades to a default rather than aborting the
+  lifecycle). Hosts-file operations use pinned directory descriptors, reject
+  symlinked `/etc`, and read only regular files checked through an `O_PATH`
+  descriptor. Replacement is atomic; permissions are set on the new file's
+  descriptor. Other paths remain **host** paths: `MountGuard::simple` is a
+  loop mount, not a chroot, so the traversal rule below still applies to the
+  hostname and network-config writes. Those paths are not currently validated.
 - **Guest command output read by the host.** e.g. `check_guest_dirty` reads
   `git status --porcelain` from the guest. Today this only gates control flow /
   is printed to the user — it is never fed into `sh -c` on the host. Keep it

src/lib.rs

@@ -97,6 +97,11 @@ pub(crate) struct Cli {
 
 #[derive(Subcommand)]
 enum Commands {
+    /// Internal privileged helper for a mounted Firecracker rootfs.
+    #[cfg(target_os = "linux")]
+    #[command(name = "__patch-guest-hosts", hide = true)]
+    PatchGuestHosts { mount: PathBuf, hostname: String },
+
     /// Ensure an environment for a project directory exists and is running.
     ///
     /// Re-runnable: if an instance already exists for DIR it is reused
@@ -926,6 +931,15 @@ pub fn run() -> Result<()> {
     let cli = Cli::parse();
     init_tracing(cli.verbose);
 
+    #[cfg(target_os = "linux")]
+    if let Commands::PatchGuestHosts {
+        ref mount,
+        ref hostname,
+    } = cli.command
+    {
+        return setup::patch_guest_hosts(mount, hostname);
+    }
+
     if let Commands::Completions { shell } = cli.command {
         completions::emit_static(shell);
         return Ok(());
@@ -997,6 +1011,8 @@ pub fn run() -> Result<()> {
 
     let raw_args: Vec<String> = std::env::args().collect();
     match cli.command {
+        #[cfg(target_os = "linux")]
+        Commands::PatchGuestHosts { .. } => unreachable!("handled before config loading"),
         Commands::Up {
             dir,
             name,