diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 00000000..ece1cc11
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,15 @@
+# Keep the build context small.
+#
+# Several test harnesses build images with the repository root as the context
+# (testing/deb-install/test.sh and testing/dns-resolver/test.sh among them), and
+# without this every one of them uploads the whole Cargo target directory to the
+# daemon before running a build that does not use a single file from it. On a
+# developer's machine that directory reaches double-digit gigabytes.
+#
+# Deliberately narrow. Nothing here excludes testing/**/.cache, which
+# testing/deb-install/test.sh copies a package out of, and no Dockerfile in the
+# tree copies from target/ on the host: examples/k8s-sidecar/Dockerfile builds
+# its own inside the image with a multi-stage COPY --from.
+target/
+.git/
+deploy/
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 26677706..212b7a80 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,7 +27,8 @@ env:
 # ─────────────────────────────────────────────────────────────────────────────
 # CI parity invariant
 #
-# This GitHub integration matrix and the local default suite set
+# This workflow's integration matrices — the `integration:` job and the
+# `deb-install:` job — and the local default suite set
 # (testing/ci-local.sh) MUST run the same integration suites, EXCEPT for the
 # deliberate local-only entries below. Adding a suite to one runner without
 # the other means "local green" and "GitHub green" stop being equivalent.
@@ -504,29 +505,6 @@ jobs:
           # recovers from delay, and never panics.
           - suite: stun-faults
             type: stun-faults
-          # ── Real-deb install across target distros ─────────────────────
-          # Boots a privileged systemd container per distro, runs
-          # `apt install ./fips_*.deb` with the locally-built package,
-          # then asserts end-to-end `.fips` resolution + the
-          # gateway/daemon default-pairing. The most thorough single
-          # test surface — exercises packaging, maintainer scripts,
-          # systemd unit ordering, real TUN, and the DNS responder
-          # filter on a per-distro resolver backend.
-          - suite: deb-install-debian12
-            type: deb-install
-            scenario: debian12
-          - suite: deb-install-debian13
-            type: deb-install
-            scenario: debian13
-          - suite: deb-install-ubuntu22
-            type: deb-install
-            scenario: ubuntu22
-          - suite: deb-install-ubuntu24
-            type: deb-install
-            scenario: ubuntu24
-          - suite: deb-install-ubuntu26
-            type: deb-install
-            scenario: ubuntu26
           # ── DNS resolver multi-backend coverage ────────────────────────
           # Exercises every fips-dns-setup backend (resolved, dnsmasq,
           # NM+dnsmasq, dns-delegate, no-resolver) across five distros,
@@ -734,36 +712,6 @@ jobs:
           docker compose -f testing/static/docker-compose.yml \
             --profile gateway down --volumes --remove-orphans
 
-      # ── Real-deb install integration ────────────────────────────────────
-      # The deb-install harness builds its own .deb from source in a
-      # cargo-deb builder image; the pre-built Linux binary from the
-      # build job is intentionally not used here so the test exercises
-      # the full packaging pipeline. ~5-7 min cold-cache on a fresh
-      # runner (.deb build dominates), ~1-2 min warm-cache.
-      - name: Run deb-install scenario
-        if: matrix.type == 'deb-install'
-        timeout-minutes: 25
-        run: bash testing/deb-install/test.sh ${{ matrix.scenario }}
-
-      - name: Collect logs on failure (deb-install)
-        if: matrix.type == 'deb-install' && failure()
-        run: |
-          docker ps -a --filter "name=fips-deb-test-${{ matrix.scenario }}" --format '{{.Names}}' | while read -r c; do
-            echo "--- ${c} fips.service ---"
-            docker exec "$c" journalctl -u fips.service --no-pager 2>&1 | tail -100 || true
-            echo "--- ${c} fips-dns.service ---"
-            docker exec "$c" journalctl -u fips-dns.service --no-pager 2>&1 | tail -100 || true
-            echo "--- ${c} fips-gateway.service ---"
-            docker exec "$c" journalctl -u fips-gateway.service --no-pager 2>&1 | tail -100 || true
-          done
-
-      - name: Stop containers (deb-install)
-        if: matrix.type == 'deb-install' && always()
-        run: |
-          docker ps -a --filter "name=fips-deb-test-${{ matrix.scenario }}" --format '{{.Names}}' | while read -r c; do
-            docker rm -f "$c" >/dev/null 2>&1 || true
-          done
-
       # ── Native datagram API ─────────────────────────────────────────────
       # Reads FIPS_TEST_IMAGE rather than defaulting to a name, so it runs
       # against the image this workflow built. The two-node check creates and
@@ -817,3 +765,113 @@ jobs:
           docker ps -a --filter "name=fips-dns-test-" --format '{{.Names}}' | while read -r c; do
             docker rm -f "$c" >/dev/null 2>&1 || true
           done
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Job 4 – The .deb the install suite installs
+#
+# Built once, here, by the same script the release workflow and a local run
+# call, so the package the suite installs is built the way the shipped one is.
+# That was not true before: each install leg built its own package on a fresh
+# runner with no cache, so one run performed five complete Rust release builds
+# and four were waste — and none of them was built the way the release is, so
+# the suite could not exhibit a defect that only the release environment
+# produced.
+#
+# The script builds in the pinned container from packaging/build-floor.env and
+# runs testing/check-glibc-floor.sh on the result, so this job is also where a
+# floor violation stops the run.
+# ─────────────────────────────────────────────────────────────────────────────
+  deb-package:
+    name: Build .deb
+    runs-on: ubuntu-latest
+    needs: [build, test]
+    if: ${{ !inputs.skip_integration }}
+
+    steps:
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6
+
+      - name: Build the .deb in the pinned build container
+        timeout-minutes: 30
+        run: bash packaging/debian/build-deb-container.sh --output-dir deploy
+
+      - name: Upload the .deb
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
+        with:
+          name: fips-deb
+          path: deploy/fips_*.deb
+          if-no-files-found: error
+          retention-days: 1
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Job 5 – Real-deb install across target distros
+#
+# Boots a privileged systemd container per distro, runs `apt install
+# ./fips_*.deb` with the package job 4 built, then asserts end-to-end `.fips`
+# resolution + the gateway/daemon default-pairing. The most thorough single
+# test surface — exercises packaging, maintainer scripts, systemd unit
+# ordering, real TUN, and the DNS responder filter on a per-distro resolver
+# backend.
+#
+# A job of its own rather than legs of the integration matrix: the install legs
+# are the only ones that need the package, and as integration legs every other
+# integration suite would wait on the package build.
+#
+# The legs keep `type: deb-install` and `scenario:` because
+# testing/check-ci-parity.sh reads those to match this matrix against the local
+# suite's distro list; the steps below use `scenario:` only.
+# ─────────────────────────────────────────────────────────────────────────────
+  deb-install:
+    name: Deb install (${{ matrix.scenario }})
+    runs-on: ubuntu-latest
+    needs: [deb-package]
+    if: ${{ !inputs.skip_integration }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - type: deb-install
+            scenario: debian12
+          - type: deb-install
+            scenario: debian13
+          - type: deb-install
+            scenario: ubuntu22
+          - type: deb-install
+            scenario: ubuntu24
+          - type: deb-install
+            scenario: ubuntu26
+
+    steps:
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6
+
+      - name: Download the .deb
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
+        with:
+          name: fips-deb
+          path: _deb
+
+      - name: Run deb-install scenario
+        timeout-minutes: 25
+        run: |
+          deb=$(find _deb -maxdepth 1 -type f -name 'fips_*.deb' | sort | head -1)
+          [ -n "$deb" ] || { echo "no .deb in the downloaded artifact" >&2; exit 1; }
+          bash testing/deb-install/test.sh --deb "$deb" ${{ matrix.scenario }}
+
+      - name: Collect logs on failure
+        if: failure()
+        run: |
+          docker ps -a --filter "name=fips-deb-test-${{ matrix.scenario }}" --format '{{.Names}}' | while read -r c; do
+            echo "--- ${c} fips.service ---"
+            docker exec "$c" journalctl -u fips.service --no-pager 2>&1 | tail -100 || true
+            echo "--- ${c} fips-dns.service ---"
+            docker exec "$c" journalctl -u fips-dns.service --no-pager 2>&1 | tail -100 || true
+            echo "--- ${c} fips-gateway.service ---"
+            docker exec "$c" journalctl -u fips-gateway.service --no-pager 2>&1 | tail -100 || true
+          done
+
+      - name: Stop containers
+        if: always()
+        run: |
+          docker ps -a --filter "name=fips-deb-test-${{ matrix.scenario }}" --format '{{.Names}}' | while read -r c; do
+            docker rm -f "$c" >/dev/null 2>&1 || true
+          done
diff --git a/.github/workflows/package-linux.yml b/.github/workflows/package-linux.yml
index b0d53271..d4526c4d 100644
--- a/.github/workflows/package-linux.yml
+++ b/.github/workflows/package-linux.yml
@@ -49,6 +49,11 @@ jobs:
     runs-on: ${{ matrix.os }}
     needs: determine-versioning
 
+    # Both legs build in the same pinned container. Nothing passes --platform,
+    # so the arm runner resolves the arm64 variant of the base image and builds
+    # natively; the floor check runs on that package too, so an aarch64 build
+    # above the floor fails the leg rather than shipping. What the runner
+    # supplies is Docker and the checkout -- neither leg compiles on the host.
     strategy:
       fail-fast: false
       matrix:
@@ -68,32 +73,76 @@ jobs:
       - name: Set SOURCE_DATE_EPOCH from git
         run: echo "SOURCE_DATE_EPOCH=$(git log -1 --format=%ct)" >> "$GITHUB_ENV"
 
-      - name: Install system dependencies
-        run: sudo apt-get update && sudo apt-get install -y --no-install-recommends libdbus-1-dev llvm
+      # The host no longer compiles anything: the container carries the
+      # toolchain and the build dependencies. llvm is here only for llvm-strip,
+      # which build-tarball.sh uses on the binaries recovered from the package.
+      - name: Install host packaging tools
+        run: sudo apt-get update && sudo apt-get install -y --no-install-recommends llvm
 
-      - name: Install Rust toolchain
-        uses: actions-rust-lang/setup-rust-toolchain@166cdcfd11aee3cb47222f9ddb555ce30ddb9659 # v1
-        with:
-          cache: false
-          rustflags: ''
+      # Build in the pinned container rather than on the runner. The runner's
+      # glibc is what put a GLIBC_2.39 requirement into every Linux artifact
+      # from v0.3.0 onward, so the package installed cleanly and then could not
+      # load on Debian 12 or Ubuntu 22.04. packaging/build-floor.env declares
+      # the base image and the floor; the script builds there and runs
+      # testing/check-glibc-floor.sh on the package it produced, so a build that
+      # would ship an unloadable binary fails here instead of at the user.
+      #
+      # This is the same script ci.yml and a local run call, so the package that
+      # passes the five-distro suite is built the way this one is.
+      - name: Build Debian package in the pinned container
+        id: deb
+        shell: bash
+        run: |
+          set -euo pipefail
+          : ${GITHUB_OUTPUT:=/tmp/github_output}
 
-      - name: Cache Cargo registry + build
-        if: ${{ env.ACT != 'true' }}
-        uses: actions/cache@caa296126883cff596d87d8935842f9db880ef25 # v5
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: linux-release-${{ runner.os }}-${{ matrix.artifact_arch }}-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            linux-release-${{ runner.os }}-${{ matrix.artifact_arch }}-
+          packaging/debian/build-deb-container.sh \
+            --version "${{ needs.determine-versioning.outputs.linux_package_version }}" \
+            --output-dir deploy \
+            | tee /tmp/build-deb-container.log
 
-      - name: Install cargo-deb
-        run: cargo install cargo-deb --version 3.6.3 --locked
+          # The script prints the package path as its last line of stdout.
+          # Only stdout is captured; its diagnostics go to stderr and straight
+          # to the job log, so nothing can land after the path.
+          DEB_FILE=$(tail -n 1 /tmp/build-deb-container.log)
+          if [[ ! -f "$DEB_FILE" ]]; then
+            echo "build-deb-container.sh did not name a package: '$DEB_FILE'" >&2
+            exit 1
+          fi
+          case "$DEB_FILE" in
+            *_${{ matrix.deb_arch }}.deb) ;;
+            *)
+              echo "Package $DEB_FILE is not ${{ matrix.deb_arch }}" >&2
+              exit 1
+              ;;
+          esac
 
-      - name: Build release binaries
-        run: cargo build --release
+          # Record it relative to the checkout: upload-artifact derives the
+          # archive layout from the common ancestor of its paths, and an
+          # absolute path here would nest the package under directories the
+          # release job's dist/*.deb glob does not look in.
+          echo "deb=${DEB_FILE#"$PWD"/}" >> "$GITHUB_OUTPUT"
+
+      # The container writes its target directory to a Docker volume, so the
+      # runner's target/release is empty. Recover the four binaries from the
+      # package instead: they are the container-built ones, so the tarball ships
+      # what the package ships rather than a second, runner-built set that the
+      # floor check never saw and that no package manager would refuse.
+      - name: Stage container-built binaries for the tarball
+        shell: bash
+        run: |
+          set -euo pipefail
+          UNPACK=$(mktemp -d)
+          dpkg-deb -x "${{ steps.deb.outputs.deb }}" "$UNPACK"
+          mkdir -p target/release
+          for bin in fips fipsctl fipstop fips-gateway; do
+            if [[ ! -f "$UNPACK/usr/bin/$bin" ]]; then
+              echo "Package is missing usr/bin/$bin" >&2
+              exit 1
+            fi
+            install -m 0755 "$UNPACK/usr/bin/$bin" "target/release/$bin"
+          done
+          rm -rf "$UNPACK"
 
       - name: Build systemd tarball
         env:
@@ -104,11 +153,23 @@ jobs:
             --arch "${{ matrix.artifact_arch }}" \
             --no-build
 
-      - name: Build Debian package
+      # The tarball has no package manager to refuse it, so nothing at install
+      # time would notice a bad floor. Check the binaries out of the finished
+      # tarball, after the strip, rather than trusting that they are the same
+      # objects the package check already passed.
+      - name: Check the tarball against the declared glibc floor
+        shell: bash
         run: |
-          packaging/debian/build-deb.sh \
-            --version "${{ needs.determine-versioning.outputs.linux_package_version }}" \
-            --no-build
+          set -euo pipefail
+          TARBALL="deploy/fips-${{ needs.determine-versioning.outputs.linux_package_version }}-linux-${{ matrix.artifact_arch }}.tar.gz"
+          UNPACK=$(mktemp -d)
+          tar -xzf "$TARBALL" -C "$UNPACK"
+          testing/check-glibc-floor.sh \
+            "$UNPACK"/*/fips \
+            "$UNPACK"/*/fipsctl \
+            "$UNPACK"/*/fipstop \
+            "$UNPACK"/*/fips-gateway
+          rm -rf "$UNPACK"
 
       - name: Resolve Linux asset paths
         id: linux-assets
@@ -122,14 +183,8 @@ jobs:
             exit 1
           fi
 
-          DEB_FILE=$(find deploy -maxdepth 1 -type f -name "fips_*_${{ matrix.deb_arch }}.deb" | sort | head -n 1)
-          if [[ -z "$DEB_FILE" ]]; then
-            echo "Missing Debian package for ${{ matrix.deb_arch }}" >&2
-            exit 1
-          fi
-
           echo "tarball=$TARBALL" >> "$GITHUB_OUTPUT"
-          echo "deb=$DEB_FILE" >> "$GITHUB_OUTPUT"
+          echo "deb=${{ steps.deb.outputs.deb }}" >> "$GITHUB_OUTPUT"
 
       - name: SHA-256 hashes
         run: |
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c026b52f..dc67cdb7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `fipstop` as "Own Loopback". `req_duplicate` returns to meaning only what it
   says.
 
+#### Packaging
+
+- The Linux `.deb` and the systemd tarball now install and run on Debian 12 and
+  Ubuntu 22.04. Every Linux artifact from v0.3.0 through v0.5.0 was built on the
+  newest available runner, whose C library made the standard library's `pidfd`
+  references a hard `GLIBC_2.39` version requirement instead of the weak,
+  runtime-checked ones it is meant to compile to. The loader refuses an image on
+  that entry alone, so `fips`, `fipstop` and `fips-gateway` could not start;
+  `fipsctl` was unaffected, which is why an install that was checked by running
+  it looked healthy while the daemon was dead. No source code caused this and
+  none was changed. The Linux artifacts are now built in a container pinned to
+  the oldest supported distribution, declared with the floor in
+  `packaging/build-floor.env`, and every producer runs
+  `testing/check-glibc-floor.sh` on what it made, so a package or a tarball that
+  would not load fails the build rather than reaching a user. The declared
+  dependency is derived from the binaries instead of hand-written, so it states
+  the floor it was built against.
+
 ## [0.5.0] - 2026-08-30
 
 ### Added
diff --git a/Cargo.toml b/Cargo.toml
index 7dc3f8f9..99025faf 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -76,7 +76,15 @@ copyright = "2026 Johnathan Corgan"
 license-file = ["LICENSE", "0"]
 section = "net"
 priority = "optional"
-depends = "libc6, systemd, libdbus-1-3"
+# "$auto" runs dpkg-shlibdeps over each packaged binary and derives the real
+# dependencies, including the libc6 version floor. Written by hand this field
+# said only "libc6", which no glibc fails to satisfy, so a package whose
+# binaries needed 2.39 installed happily on a system with 2.35 and the daemon
+# then could not start. Deriving it also picks up a libdbus floor the hand
+# written list lacked, and re-derives per architecture, which matters because
+# arm64 links libdbus in all four binaries where amd64 links it in one.
+# systemd stays by hand: nothing links it, so nothing can derive it.
+depends = "$auto, systemd"
 recommends = "bluez"
 extended-description = """\
 FIPS is a distributed, decentralized network routing protocol for mesh \
diff --git a/packaging/Makefile b/packaging/Makefile
index f83e3ea6..69bb106f 100644
--- a/packaging/Makefile
+++ b/packaging/Makefile
@@ -4,7 +4,8 @@
 # All outputs are placed in deploy/ at the project root.
 #
 # Usage:
-#   make deb        Build a Debian/Ubuntu .deb package
+#   make deb        Build a Debian/Ubuntu .deb package in the pinned container
+#   make deb-host   Build a .deb with the host toolchain (see below)
 #   make tarball    Build a systemd install tarball
 #   make ipk        Build an OpenWrt .ipk package (opkg, OpenWrt 24.x and earlier)
 #   make apk        Build an OpenWrt .apk package (apk-tools, mandatory on OpenWrt 25+)
@@ -19,11 +20,23 @@ SHELL := /bin/bash
 PACKAGING_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
 PROJECT_ROOT  := $(abspath $(PACKAGING_DIR)/..)
 
-.PHONY: all deb tarball ipk apk aur pkg freebsd zip clean
+.PHONY: all deb deb-host tarball ipk apk aur pkg freebsd zip clean
 
 all: deb tarball
 
+# `deb` builds in the pinned container so the package carries the declared glibc
+# floor and can install on every supported distribution. It also checks that
+# floor before it hands the package back.
 deb:
+	@bash $(PACKAGING_DIR)/debian/build-deb-container.sh
+
+# `deb-host` builds with whatever toolchain and C library the host has. It is
+# for iterating locally and NOT for anything anyone else installs: on a modern
+# host it produces a package that installs cleanly and then cannot start on
+# Debian 12 or Ubuntu 22.04, which is the defect that made the container build
+# necessary. Nothing checks its floor, deliberately, so the check stays
+# attached to the artifact that ships.
+deb-host:
 	@bash $(PACKAGING_DIR)/debian/build-deb.sh
 
 tarball:
diff --git a/packaging/README.md b/packaging/README.md
index 44588744..3859942e 100644
--- a/packaging/README.md
+++ b/packaging/README.md
@@ -7,7 +7,7 @@ and `make apk` write to `dist/` instead.
 ## Quick Start
 
 ```sh
-make deb        # Debian/Ubuntu .deb
+make deb        # Debian/Ubuntu .deb (built in the pinned container)
 make tarball    # systemd install tarball
 make ipk        # OpenWrt .ipk (opkg, OpenWrt 24.x and earlier)
 make apk        # OpenWrt .apk (apk-tools, mandatory on OpenWrt 25+)
@@ -18,8 +18,29 @@ make zip        # Windows .zip package
 make all        # deb + tarball (default)
 ```
 
+## The two Debian build paths
+
+`make deb` builds in a container pinned to the oldest supported
+distribution, named with the glibc floor in
+[build-floor.env](build-floor.env), and checks the package it produced
+against that floor before handing it back. Its only host prerequisite is
+docker: the toolchain and the build dependencies live in the image. This
+is the path the release workflow, the integration suite and the internal
+builder all take, so a package that passes locally is built the way the
+shipped one is.
+
+`make deb-host` is the old path. It builds on the host, at whatever glibc
+the host has, and it is checked against nothing. Use it for local
+iteration only. A package built on a current distribution records a
+version dependency that the loader refuses on Debian 12 and Ubuntu 22.04,
+which is what shipped in every Linux artifact from v0.3.0 through v0.5.0,
+so it must not produce anything anyone else installs.
+
 ## Build Prerequisites
 
+The prerequisites below apply to the host-build targets. `make deb` needs
+docker and nothing else.
+
 These targets build FIPS from source, so the host needs a build
 environment in addition to a Rust toolchain (the version pinned in
 `rust-toolchain.toml` is auto-installed by rustup).
diff --git a/packaging/build-floor.env b/packaging/build-floor.env
new file mode 100644
index 00000000..08be956c
--- /dev/null
+++ b/packaging/build-floor.env
@@ -0,0 +1,35 @@
+# The glibc floor for the Linux release artifacts, and the image that produces it.
+#
+# Sourced by packaging/debian/build-deb-container.sh and by
+# testing/check-glibc-floor.sh. It exists so the floor is a decision written
+# down in one place rather than a side effect of whichever build host ran last.
+#
+# The rule it encodes: FIPS installs on every version of a supported operating
+# system that its distributor still supports for free. As of 2026-09-05 that is
+#
+#   Ubuntu 22.04   glibc 2.35   free support ends April 2027
+#   Debian 12      glibc 2.36   LTS ends 2028-06-30
+#   Ubuntu 24.04   glibc 2.39
+#   Debian 13      glibc 2.41   LTS ends 2030-06-30
+#   Ubuntu 26.04   glibc 2.43
+#
+# so the lowest is Ubuntu 22.04 and the floor is its 2.35. Debian 11 left the
+# set on 2026-08-31 and is deliberately not counted.
+#
+# Deliberately NOT a GitHub runner label. Runner availability follows GitHub's
+# rule of supporting the newest two images; the floor follows distributors'
+# support windows. Those are different clocks, and ubuntu-26.04 being in preview
+# means the ubuntu-22.04 runner will very likely retire before Canonical's date.
+# A container base outlives the runner label and builds the same way on a
+# developer's machine, which is what lets local and GitHub CI do identical work.
+#
+# Changing FIPS_GLIBC_FLOOR drops support for every distribution below it. Check
+# the table above first, and expect check-glibc-floor.sh to hold you to it.
+
+# Base image for the build. Pinned to the oldest supported distribution.
+FIPS_BUILD_IMAGE="ubuntu:22.04"
+
+# Highest glibc symbol version any shipped binary may require. Building on
+# FIPS_BUILD_IMAGE currently yields 2.34, one step below this, so there is a
+# little headroom: the check is an upper bound, not an equality.
+FIPS_GLIBC_FLOOR="2.35"
diff --git a/packaging/debian/Dockerfile.build b/packaging/debian/Dockerfile.build
new file mode 100644
index 00000000..52182848
--- /dev/null
+++ b/packaging/debian/Dockerfile.build
@@ -0,0 +1,49 @@
+# Build image for the Linux release artifacts.
+#
+# Pinned to the oldest distribution FIPS supports, because the glibc a binary is
+# linked against decides the glibc it will run on. See packaging/build-floor.env
+# for the floor and the reasoning; BASE is passed from there, not written here,
+# so there is one place to change it.
+#
+# This image carries the toolchain and the build dependencies only. It never
+# carries the source: the source is mounted at run time, so editing a file does
+# not invalidate the image and a warm rebuild costs seconds rather than minutes.
+ARG BASE=ubuntu:22.04
+FROM ${BASE}
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# dpkg-dev is not in a bare ubuntu:22.04 and is what provides dpkg-shlibdeps,
+# which cargo-deb's "$auto" dependency resolution shells out to. Without it the
+# declared dependencies would silently lose their versions again.
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        build-essential \
+        pkg-config \
+        libdbus-1-dev \
+        libclang-dev \
+        clang \
+        binutils \
+        dpkg-dev \
+        curl \
+        ca-certificates \
+    && apt-get clean && rm -rf /var/lib/apt/lists/*
+
+# The toolchain version is passed in, read from rust-toolchain.toml by the
+# calling script, and the image tag carries it -- so the image cannot drift from
+# the compiler the rest of CI uses, and bumping the pin rebuilds the image. The
+# builder this replaces installed `stable` and never copied rust-toolchain.toml,
+# so it compiled with a different compiler from the release and nothing said so.
+ARG RUST_TOOLCHAIN
+ENV RUSTUP_HOME=/usr/local/rustup \
+    CARGO_HOME=/usr/local/cargo \
+    PATH=/usr/local/cargo/bin:$PATH
+RUN test -n "${RUST_TOOLCHAIN}" \
+    && curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
+      | sh -s -- -y --profile minimal --default-toolchain "${RUST_TOOLCHAIN}" \
+    && chmod -R a+w "$RUSTUP_HOME" "$CARGO_HOME"
+
+ARG CARGO_DEB_VERSION=3.6.3
+RUN cargo install cargo-deb --version "${CARGO_DEB_VERSION}" --locked \
+    && chmod -R a+w "$CARGO_HOME"
+
+WORKDIR /src
diff --git a/packaging/debian/build-deb-container.sh b/packaging/debian/build-deb-container.sh
new file mode 100755
index 00000000..1b147a54
--- /dev/null
+++ b/packaging/debian/build-deb-container.sh
@@ -0,0 +1,135 @@
+#!/bin/bash
+# Build the Debian package in the pinned build container, then check its floor.
+#
+# This is the one place the Linux artifacts are produced. The release workflow,
+# the CI integration job and a local run all call it, so all three build the
+# same way and a package that passes locally is the package that ships. That was
+# not true before: the test suite built its own package inside a Debian 12 image
+# while the release built on the newest GitHub runner, so the suite could not
+# exhibit a defect that only the release environment produced -- and for five
+# releases it did not.
+#
+# Usage: build-deb-container.sh [--output-dir DIR] [--version V] [--features LIST]
+#                               [--rebuild-image]
+#
+# Requires docker. The image is cached between runs and rebuilt only when the
+# Dockerfile or the floor changes; the source is mounted rather than copied, so
+# editing code does not invalidate it.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+
+# shellcheck source=../build-floor.env
+. "$REPO_ROOT/packaging/build-floor.env"
+
+DEST_DIR="$REPO_ROOT/deploy"
+VERSION=""
+FEATURES=""
+REBUILD_IMAGE=0
+
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        --output-dir)    DEST_DIR="${2:?missing value for --output-dir}"; shift 2 ;;
+        --version)       VERSION="${2:?missing value for --version}"; shift 2 ;;
+        --features)      FEATURES="${2:?missing value for --features}"; shift 2 ;;
+        --rebuild-image) REBUILD_IMAGE=1; shift ;;
+        -h|--help)       sed -n '2,17p' "$0"; exit 0 ;;
+        *)               echo "Unknown option: $1" >&2; exit 2 ;;
+    esac
+done
+
+command -v docker >/dev/null 2>&1 || {
+    echo "build-deb-container: docker is required and was not found." >&2
+    exit 2
+}
+
+# Read the toolchain from the pin rather than choosing one here, and put it in
+# the tag so a bump rebuilds the image instead of silently reusing a stale one.
+RUST_TOOLCHAIN=$(awk -F'"' '/^channel *=/{print $2; exit}' "$REPO_ROOT/rust-toolchain.toml")
+[ -n "$RUST_TOOLCHAIN" ] || {
+    echo "build-deb-container: could not read channel from rust-toolchain.toml" >&2
+    exit 2
+}
+
+IMAGE_TAG="fips-deb-builder:${FIPS_BUILD_IMAGE//[:\/]/-}-rust${RUST_TOOLCHAIN}"
+
+if [ "$REBUILD_IMAGE" -eq 1 ] || ! docker image inspect "$IMAGE_TAG" >/dev/null 2>&1; then
+    echo "=== Building $IMAGE_TAG from $FIPS_BUILD_IMAGE with Rust $RUST_TOOLCHAIN ===" >&2
+    docker build \
+        --build-arg "BASE=$FIPS_BUILD_IMAGE" \
+        --build-arg "RUST_TOOLCHAIN=$RUST_TOOLCHAIN" \
+        -t "$IMAGE_TAG" \
+        -f "$SCRIPT_DIR/Dockerfile.build" \
+        "$SCRIPT_DIR"
+else
+    echo "=== Using cached $IMAGE_TAG ===" >&2
+fi
+
+# Derive the version and the timestamp on the host, where git works, and pass
+# both in. The container then never runs git, which matters for two reasons: a
+# worktree's .git is a file pointing outside the mount and would not resolve,
+# and a bind-mounted repository trips git's dubious-ownership check.
+if [ -z "$VERSION" ]; then
+    CRATE_VERSION=$(awk -F'"' '/^version = /{print $2; exit}' "$REPO_ROOT/Cargo.toml")
+    if [[ "$CRATE_VERSION" == *-dev ]]; then
+        GIT_DATE=$(git -C "$REPO_ROOT" log -1 --format=%cs | tr -d '-')
+        GIT_SHA=$(git -C "$REPO_ROOT" rev-parse --short HEAD)
+        DIRTY=""
+        [ -n "$(git -C "$REPO_ROOT" status --porcelain 2>/dev/null)" ] && DIRTY=".dirty"
+        VERSION="${CRATE_VERSION%-dev}~dev+git${GIT_DATE}.${GIT_SHA}${DIRTY}-1"
+    else
+        VERSION="$CRATE_VERSION"
+    fi
+fi
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git -C "$REPO_ROOT" log -1 --format=%ct)}"
+
+mkdir -p "$DEST_DIR"
+DEST_ABS="$(cd "$DEST_DIR" && pwd)"
+
+echo "=== Building fips $VERSION in $IMAGE_TAG ===" >&2
+
+# A feature build hands the whole job to build-deb.sh rather than pre-building:
+# --features has to reach cargo, and build-deb.sh is also what marks the version
+# so a feature package is distinguishable from the default build of the same
+# commit. It refuses --features with --no-build for exactly that reason, so the
+# two cases cannot share one command.
+if [ -n "$FEATURES" ]; then
+    # build-deb.sh does the whole job here: --features has to reach cargo, and
+    # it is also what marks the version so a feature package is distinguishable
+    # from the default build of the same commit. It refuses --features with
+    # --no-build for that reason, so the two cases cannot share one command.
+    # The version still comes from the host, because the image has no git.
+    BUILD_CMD="packaging/debian/build-deb.sh --features '$FEATURES' --version '$VERSION' --output-dir /out"
+else
+    BUILD_CMD="cargo build --release --locked
+        packaging/debian/build-deb.sh --no-build --version '$VERSION' --output-dir /out"
+fi
+
+# The source is mounted read-only so a build cannot leave artifacts in the tree.
+# CARGO_TARGET_DIR and the registry live in named volumes, which is what makes a
+# second run fast; they are per-base-image so a floor change does not reuse
+# objects linked against the wrong C library.
+VOL_SUFFIX="${FIPS_BUILD_IMAGE//[:\/]/-}"
+docker run --rm \
+    -v "$REPO_ROOT":/src:ro \
+    -v "$DEST_ABS":/out \
+    -v "fips-deb-target-${VOL_SUFFIX}":/target \
+    -v "fips-deb-registry-${VOL_SUFFIX}":/usr/local/cargo/registry \
+    -e CARGO_TARGET_DIR=/target \
+    -e SOURCE_DATE_EPOCH="$SOURCE_DATE_EPOCH" \
+    -w /src \
+    "$IMAGE_TAG" \
+    bash -euo pipefail -c "$BUILD_CMD" >&2
+
+DEB=$(find "$DEST_ABS" -maxdepth 1 -name "fips_*_*.deb" -newermt '-10 minutes' -print | sort | tail -1)
+[ -n "$DEB" ] || { echo "build-deb-container: no .deb was produced." >&2; exit 1; }
+
+# Check the artifact here rather than in one workflow, so every producer is
+# gated: the release, the CI job, a local run and packaging/Makefile all reach
+# the check through this script.
+"$REPO_ROOT/testing/check-glibc-floor.sh" "$DEB" >&2
+
+echo "=== Built $DEB ===" >&2
+printf '%s\n' "$DEB"
diff --git a/packaging/debian/build-deb.sh b/packaging/debian/build-deb.sh
index 188255ee..f1088f92 100755
--- a/packaging/debian/build-deb.sh
+++ b/packaging/debian/build-deb.sh
@@ -23,6 +23,9 @@ Options:
   --features <list>   Cargo features to build with (comma-separated). Marks the
                       auto-derived Version so the package is distinguishable
                       from a default build of the same commit.
+  --output-dir <dir>  Where to put the finished .deb. Defaults to deploy/ under
+                      the project root. Exists so the container build can write
+                      to a mount and leave the source tree read-only.
   -h, --help          Show this help
 EOF
 }
@@ -31,6 +34,7 @@ TARGET_TRIPLE=""
 VERSION_OVERRIDE=""
 NO_BUILD=0
 FEATURES=""
+DEST_DIR=""
 
 while [[ $# -gt 0 ]]; do
     case "$1" in
@@ -50,6 +54,10 @@ while [[ $# -gt 0 ]]; do
             FEATURES="${2:?missing value for --features}"
             shift 2
             ;;
+        --output-dir)
+            DEST_DIR="${2:?missing value for --output-dir}"
+            shift 2
+            ;;
         -h|--help)
             usage
             exit 0
@@ -124,9 +132,20 @@ if [[ -z "${VERSION_OVERRIDE}" ]]; then
         echo "Auto-derived dev Version: ${VERSION_OVERRIDE}"
     fi
 elif [[ -n "${FEATURES}" ]]; then
-    echo "Warning: --version was given with --features, so the Version carries no" >&2
-    echo "feature marker and this package is indistinguishable from a default" >&2
-    echo "build of the same commit. Mark it yourself if that matters." >&2
+    # An explicit version needs the same marker for the same reason, and it is
+    # the only way a caller that cannot derive the version here can get one.
+    # The container build is that caller: it derives the version on the host
+    # because the image has no git, and the source is mounted read-only from a
+    # worktree whose .git is a file pointing outside the mount.
+    if [[ "${VERSION_OVERRIDE}" == *"+$(printf '%s' "${FEATURES}" | tr -c 'a-zA-Z0-9.' '.')"* ]]; then
+        : # already marked by the caller
+    elif [[ "${VERSION_OVERRIDE}" == *-* ]]; then
+        # Split off the Debian revision so the marker lands on the upstream part.
+        VERSION_OVERRIDE="${VERSION_OVERRIDE%-*}+$(printf '%s' "${FEATURES}" | tr -c 'a-zA-Z0-9.' '.')-${VERSION_OVERRIDE##*-}"
+    else
+        VERSION_OVERRIDE="${VERSION_OVERRIDE}+$(printf '%s' "${FEATURES}" | tr -c 'a-zA-Z0-9.' '.')"
+    fi
+    echo "Feature-marked Version: ${VERSION_OVERRIDE}"
 fi
 
 # Build the .deb package
@@ -149,8 +168,11 @@ if [[ -n "${FEATURES}" ]]; then
 fi
 cargo "${cargo_args[@]}"
 
-# Move output to deploy/
-mkdir -p deploy
+# Move output to the requested directory, or deploy/ by default. Note the
+# distinction from OUTPUT_DIR above, which is cargo-deb's temporary staging
+# directory and is removed by the EXIT trap.
+: "${DEST_DIR:=deploy}"
+mkdir -p "${DEST_DIR}"
 DEB_FILE=$(find "${OUTPUT_DIR}" -maxdepth 1 -name '*.deb' -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)
 
 if [ -z "${DEB_FILE}" ]; then
@@ -158,10 +180,10 @@ if [ -z "${DEB_FILE}" ]; then
     exit 1
 fi
 
-cp "${DEB_FILE}" deploy/
+cp "${DEB_FILE}" "${DEST_DIR}/"
 BASENAME=$(basename "${DEB_FILE}")
-echo "Package built: deploy/${BASENAME}"
+echo "Package built: ${DEST_DIR}/${BASENAME}"
 echo ""
-echo "Install with: sudo dpkg -i deploy/${BASENAME}"
+echo "Install with: sudo dpkg -i ${DEST_DIR}/${BASENAME}"
 echo "Remove with:  sudo dpkg -r fips"
 echo "Purge with:   sudo dpkg -P fips  (removes config and identity keys)"
diff --git a/testing/README.md b/testing/README.md
index f2b20f65..9658b5c6 100644
--- a/testing/README.md
+++ b/testing/README.md
@@ -125,11 +125,12 @@ machines. Not a Docker harness.
 
 [`ci-local.sh`](ci-local.sh) runs the full local CI pipeline — build,
 clippy, unit tests, and the integration suites (including the chaos
-scenarios) — mirroring the GitHub `ci.yml` integration matrix. Run
+scenarios) — mirroring the GitHub `ci.yml` integration matrices. Run
 `./ci-local.sh --help` for the full option list and `--list` for the
 available suites. Every run starts with a parity check that verifies the
 local suite set covers the same work as the GitHub matrix, per scenario for
-chaos and per distro for deb-install; a divergence fails the run. GitHub
+chaos and per distro for deb-install, across every job that carries a
+matrix; a divergence fails the run. GitHub
 runs the same check as its own `ci-parity` job. `--check-parity` runs it
 alone (see [check-ci-parity.sh](check-ci-parity.sh)).
 
diff --git a/testing/check-ci-parity.sh b/testing/check-ci-parity.sh
index a3690636..9c2fd897 100755
--- a/testing/check-ci-parity.sh
+++ b/testing/check-ci-parity.sh
@@ -1,10 +1,10 @@
 #!/bin/bash
 # ── CI parity invariant guard ───────────────────────────────────────────────
-# The GitHub integration matrix (.github/workflows/ci.yml) and the local
-# default suite set (ci-local.sh) MUST run the same integration suites,
-# EXCEPT for the deliberate local-only entries listed below. Adding a suite
-# to one runner without the other means "local green" and "GitHub green" stop
-# being equivalent claims.
+# The GitHub integration matrices (.github/workflows/ci.yml, swept across every
+# job) and the local default suite set (ci-local.sh) MUST run the same
+# integration suites, EXCEPT for the deliberate local-only entries listed below.
+# Adding a suite to one runner without the other means "local green" and
+# "GitHub green" stop being equivalent claims.
 #
 # Deliberate local-only (NOT on the GitHub gate), with reason:
 #   tor-socks5     — requires live Tor network; opt-in via --with-tor,
@@ -19,8 +19,9 @@
 #                    names differ cosmetically between runners and are ignored
 #                    — `scenario:` is the identity.
 #   deb-install    — per distro. GitHub splits into per-distro legs carrying
-#                    `scenario:`; local runs the same distro set in one suite,
-#                    enumerated by ALL_SCENARIOS in deb-install/test.sh.
+#                    `scenario:`, in a job of their own; local runs the same
+#                    distro set in one suite, enumerated by ALL_SCENARIOS in
+#                    deb-install/test.sh.
 #   everything else — per suite name.
 #
 #   dns-resolver is the one leg still compared at leg granularity rather than
@@ -134,7 +135,21 @@ for name, entries in arrays.items():
 with open(ci_yml_path, encoding="utf-8") as fh:
     doc = yaml.safe_load(fh)
 
-include = doc["jobs"]["integration"]["strategy"]["matrix"]["include"]
+# Sweep every job's matrix rather than one named job: the install legs live in
+# a job of their own so the rest of the integration matrix does not wait on the
+# package build, and a guard keyed to a job name reports them as local-only the
+# moment they move. Identity comes from the leg's own fields, so where a leg
+# lives does not matter; a leg deleted from every job still shows up as
+# local-only, because the local side is the reference.
+include = []
+for job in (doc.get("jobs") or {}).values():
+    legs = (((job.get("strategy") or {}).get("matrix") or {}).get("include") or [])
+    if isinstance(legs, list):
+        include += [leg for leg in legs if isinstance(leg, dict)]
+if not include:
+    print("check-ci-parity: no matrix include: found in any job of "
+          f"{ci_yml_path}; cannot verify CI parity", file=sys.stderr)
+    sys.exit(2)
 github_chaos, github_deb, github = {}, set(), set()
 malformed = []
 for leg in include:
diff --git a/testing/check-glibc-floor.sh b/testing/check-glibc-floor.sh
new file mode 100755
index 00000000..1ca81deb
--- /dev/null
+++ b/testing/check-glibc-floor.sh
@@ -0,0 +1,147 @@
+#!/bin/bash
+# Fail when a shipped binary needs a newer glibc than the declared floor.
+#
+# The defect this exists to catch installs cleanly and then cannot run. Rust's
+# standard library references pidfd_spawnp and pidfd_getpid as weak undefined
+# symbols guarded by a runtime check, so a binary is meant to fall back where
+# the C library lacks them. Linking against a glibc that HAS them records a
+# version dependency instead, and the loader refuses the whole image on that
+# entry alone regardless of the symbols being weak. Every Linux artifact from
+# v0.3.0 to v0.5.0 shipped that way and could not start on Debian 12 or Ubuntu
+# 22.04, while apt reported success and fipsctl -- which never spawns a process
+# and so never referenced those symbols -- ran perfectly.
+#
+# Usage: check-glibc-floor.sh <artifact|binary>...
+#   A .deb is unpacked and every executable under usr/bin is checked.
+#   Anything else is treated as a single ELF binary.
+#
+# Reads the floor from packaging/build-floor.env unless FIPS_GLIBC_FLOOR is set.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+if [ -z "${FIPS_GLIBC_FLOOR:-}" ]; then
+    # shellcheck source=../packaging/build-floor.env
+    . "$REPO_ROOT/packaging/build-floor.env"
+fi
+FLOOR="${FIPS_GLIBC_FLOOR:?no floor declared}"
+
+for tool in readelf dpkg dpkg-deb; do
+    command -v "$tool" >/dev/null 2>&1 || {
+        echo "check-glibc-floor: $tool is not installed; cannot check anything." >&2
+        echo "                   Refusing to report a pass I did not establish." >&2
+        exit 2
+    }
+done
+
+# The maximum glibc version a binary requires.
+#
+# Reads the `Version needs` section, which is the table the dynamic loader
+# enforces and the one that carries the fatal entry. Do NOT compute this from
+# `objdump -T | grep GLIBC_ | sort -V | tail -1`: that sorts whole lines, the
+# version is not the leading field, and a data symbol at GLIBC_2.2.5 therefore
+# sorts last. Measured against the shipped v0.5.0 fips binary, that pipeline
+# reports 2.2.5 for a binary whose real floor is 2.39 -- it would have passed
+# every affected release.
+#
+# Prints nothing and returns 1 when it finds no GLIBC requirement at all, so an
+# unreadable or non-dynamic input cannot be scored as a pass.
+max_glibc_need() {
+    local bin="$1" found
+    found=$(readelf -VW "$bin" 2>/dev/null \
+        | awk '/Version needs section/,0' \
+        | grep -oE 'GLIBC_[0-9.]+' \
+        | sed 's/GLIBC_//' \
+        | sort -V \
+        | tail -1) || true
+    [ -n "$found" ] || return 1
+    printf '%s\n' "$found"
+    # Explicit: the caller tests this status to tell "no requirement" from a
+    # value, so it must not be whatever printf happened to return.
+    return 0
+}
+
+FAILED=0
+CHECKED=0
+
+is_elf() { readelf -hW "$1" >/dev/null 2>&1; }
+
+check_binary() {
+    local bin="$1" label="$2" need
+    if ! need=$(max_glibc_need "$bin"); then
+        # A static binary is a legitimate no-requirement case, so distinguish
+        # it from a file that could not be read rather than passing both.
+        if readelf -hW "$bin" >/dev/null 2>&1; then
+            echo "  ok    $label (no glibc version requirement)"
+            CHECKED=$((CHECKED + 1))
+            return
+        fi
+        echo "  ERROR $label is not a readable ELF object" >&2
+        FAILED=$((FAILED + 1))
+        return
+    fi
+    CHECKED=$((CHECKED + 1))
+    if dpkg --compare-versions "$need" gt "$FLOOR"; then
+        echo "  FAIL  $label needs glibc $need, above the declared floor $FLOOR" >&2
+        FAILED=$((FAILED + 1))
+    else
+        echo "  ok    $label needs glibc $need"
+    fi
+}
+
+check_deb() {
+    local deb="$1" tmp
+    tmp=$(mktemp -d)
+    # shellcheck disable=SC2064
+    trap "rm -rf '$tmp'" RETURN
+    dpkg-deb -x "$deb" "$tmp"
+    # A package legitimately ships executable shell scripts alongside its
+    # binaries -- fips-dns-setup and its teardown are two -- so filter to ELF
+    # objects rather than treating a script as an unreadable binary. A .deb
+    # with no ELF object at all is still an error: it means the glob or the
+    # layout moved and this check examined nothing.
+    local found=0 f
+    while IFS= read -r -d '' f; do
+        is_elf "$f" || continue
+        found=1
+        check_binary "$f" "$(basename "$deb"):$(basename "$f")"
+    done < <(find "$tmp" -type f -perm -u+x -print0)
+    if [ "$found" -eq 0 ]; then
+        echo "  ERROR $(basename "$deb") contains no ELF executables" >&2
+        FAILED=$((FAILED + 1))
+    fi
+}
+
+[ $# -gt 0 ] || {
+    echo "usage: check-glibc-floor.sh <artifact|binary>..." >&2
+    exit 2
+}
+
+echo "=== glibc floor check (declared floor: $FLOOR) ==="
+for arg in "$@"; do
+    [ -e "$arg" ] || { echo "  ERROR $arg does not exist" >&2; FAILED=$((FAILED + 1)); continue; }
+    case "$arg" in
+        *.deb) check_deb "$arg" ;;
+        *)     check_binary "$arg" "$(basename "$arg")" ;;
+    esac
+done
+
+# Nothing examined is a failure, not a pass. An argument list that matched no
+# binary means the caller's glob went stale, and reporting that as green is how
+# a guard quietly stops guarding.
+if [ "$CHECKED" -eq 0 ] && [ "$FAILED" -eq 0 ]; then
+    echo "check-glibc-floor: examined no binaries; refusing to report a pass." >&2
+    exit 2
+fi
+
+if [ "$FAILED" -ne 0 ]; then
+    echo "check-glibc-floor: $FAILED problem(s) across $CHECKED binaries." >&2
+    echo "  A binary above the floor installs cleanly and then fails to start." >&2
+    echo "  Build through packaging/debian/build-deb-container.sh, which pins" >&2
+    echo "  the build image to the oldest supported distribution." >&2
+    exit 1
+fi
+
+echo "=== glibc floor check passed ($CHECKED binaries, all at or below $FLOOR) ==="
diff --git a/testing/ci-local.sh b/testing/ci-local.sh
index d6923301..9b097be5 100755
--- a/testing/ci-local.sh
+++ b/testing/ci-local.sh
@@ -1006,11 +1006,49 @@ run_dns_resolver() {
 # that wedges -- a docker daemon that stops answering, a container that never
 # boots. 40 minutes is well above the observed cold-cache cost of a full
 # five-distro run and is not a performance budget.
+#
+# It bounds the container build and the five installs separately rather than
+# both together: the budget was sized for the installs, and a cold build that
+# ate into it would shrink theirs. Neither phase is left unbounded, which is
+# the property this exists for.
 DEB_INSTALL_TIMEOUT=${DEB_INSTALL_TIMEOUT:-2400}
 run_deb_install() {
-    info "[deb-install] Running multi-distro test (slow — builds .deb + per-distro install)"
-    local rc=0
-    timeout "$DEB_INSTALL_TIMEOUT" bash testing/deb-install/test.sh 2>&1 || rc=$?
+    # Build once through the shared container script, then install that one
+    # artifact into every distro. The harness would build its own package if
+    # handed none, and that fallback goes through the same script -- but the
+    # GitHub job builds explicitly and passes `--deb`, so doing it explicitly
+    # here too makes the two systems read as the same work rather than leaving
+    # a reader to discover that a fallback happens to match.
+    info "[deb-install] Building the package in the pinned container"
+    local build_log deb rc=0
+    build_log=$(mktemp "/tmp/ci-deb-install-build.XXXXXX")
+    # stdout carries the package path on its last line, so it is captured;
+    # stderr stays on the console so build progress is still visible live.
+    timeout "$DEB_INSTALL_TIMEOUT" \
+        bash packaging/debian/build-deb-container.sh | tee "$build_log" || rc=$?
+    if [[ $rc -ne 0 ]]; then
+        if [[ $rc -eq 124 ]]; then
+            echo "  ERROR: the container build exceeded ${DEB_INSTALL_TIMEOUT}s and was killed;" >&2
+            echo "         no package was produced, so this is not an assertion failure." >&2
+        else
+            echo "  ERROR: the container build failed (exit $rc); no package to install." >&2
+        fi
+        rm -f "$build_log"
+        record "deb-install" "$rc"
+        return
+    fi
+    deb=$(tail -n 1 "$build_log")
+    rm -f "$build_log"
+    if [[ -z "$deb" || ! -f "$deb" ]]; then
+        echo "  ERROR: the container build reported success but its last line of stdout" >&2
+        echo "         was not a package path: '${deb}'" >&2
+        record "deb-install" 1
+        return
+    fi
+
+    info "[deb-install] Running multi-distro install test against $deb"
+    rc=0
+    timeout "$DEB_INSTALL_TIMEOUT" bash testing/deb-install/test.sh --deb "$deb" 2>&1 || rc=$?
     if [[ $rc -eq 124 ]]; then
         # Say so explicitly. A bare red here reads as a failed assertion, and
         # the difference matters: a timeout means no assertion was reached.
diff --git a/testing/deb-install/test.sh b/testing/deb-install/test.sh
index 81f4de65..3eaab9ca 100755
--- a/testing/deb-install/test.sh
+++ b/testing/deb-install/test.sh
@@ -51,6 +51,11 @@ PASS=0
 FAIL=0
 SKIP=0
 
+# Set by --deb. DEB_PREPARED keeps the copy-into-cache to a single operation
+# however many scenarios run in one process.
+SUPPLIED_DEB=""
+DEB_PREPARED=0
+
 # ─────────────────────────────────────────────────────────────────────
 # Helpers
 # ─────────────────────────────────────────────────────────────────────
@@ -169,6 +174,25 @@ container_systemd_version() {
 build_deb() {
     mkdir -p "$DEB_CACHE_DIR"
 
+    # A supplied package wins outright and bypasses the staleness check below.
+    # That check compares mtimes, so a cached package could otherwise beat the
+    # artifact the caller explicitly handed over, which is exactly the silent
+    # substitution this suite exists to stop making.
+    if [ -n "$SUPPLIED_DEB" ]; then
+        if [ "$DEB_PREPARED" -eq 1 ]; then
+            return 0
+        fi
+        if [ ! -f "$SUPPLIED_DEB" ]; then
+            echo "  ERROR: --deb $SUPPLIED_DEB does not exist" >&2
+            return 1
+        fi
+        rm -f "$DEB_CACHE_DIR"/*.deb
+        cp "$SUPPLIED_DEB" "$DEB_CACHE_DIR/"
+        DEB_PREPARED=1
+        log "Installing the supplied package $(basename "$SUPPLIED_DEB")"
+        return 0
+    fi
+
     local cached_deb
     cached_deb=$(ls "$DEB_CACHE_DIR"/fips_*_amd64.deb 2>/dev/null | head -1)
 
@@ -188,45 +212,25 @@ build_deb() {
         log "No cached .deb, building"
     fi
 
-    local builder_tag="fips-deb-test:builder"
-    log "Building Debian 12 cargo-deb builder image (slow on first run)"
-    docker build -t "$builder_tag" -f - "$REPO_ROOT" <<'DOCKERFILE' >/dev/null
-FROM debian:12
-ENV DEBIAN_FRONTEND=noninteractive
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    build-essential pkg-config libdbus-1-dev curl ca-certificates \
-    libclang-dev clang && \
-    apt-get clean && rm -rf /var/lib/apt/lists/*
-RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
-    sh -s -- -y --default-toolchain stable --profile minimal
-ENV PATH="/root/.cargo/bin:${PATH}"
-RUN cargo install cargo-deb --version 3.6.3 --locked
-WORKDIR /src
-COPY Cargo.toml Cargo.lock build.rs LICENSE README.md ./
-COPY src ./src
-COPY packaging ./packaging
-COPY docs ./docs
-RUN cargo build --release && cargo deb --no-build
-DOCKERFILE
-
-    if [ ! "$(docker images -q "$builder_tag" 2>/dev/null)" ]; then
-        echo "  ERROR: builder image build failed"
+    # Build through the shared container script rather than a builder defined
+    # here. This suite used to compile its own package inside a Debian 12 image
+    # while the release compiled on the newest GitHub runner, so the package it
+    # tested had a lower glibc floor than the package users received and could
+    # not exhibit a defect that only the release environment produced. It stayed
+    # green through five releases that could not start on two of the five
+    # distributions in its own matrix.
+    log "Building the .deb in the pinned build container (slow on first run)"
+    if ! bash "$REPO_ROOT/packaging/debian/build-deb-container.sh" \
+            --output-dir "$DEB_CACHE_DIR" >&2; then
+        echo "  ERROR: container build failed" >&2
         return 1
     fi
 
-    log "Extracting .deb from builder image"
-    rm -f "$DEB_CACHE_DIR"/*.deb
-    local cid
-    cid=$(docker create "$builder_tag")
-    docker cp "$cid:/src/target/debian/." "$DEB_CACHE_DIR/" >/dev/null 2>&1
-    docker rm "$cid" >/dev/null
-    # Cargo-deb leaves intermediate artifacts; keep just the .deb.
-    find "$DEB_CACHE_DIR" -mindepth 1 -not -name 'fips_*_amd64.deb' -delete 2>/dev/null || true
     cached_deb=$(ls "$DEB_CACHE_DIR"/fips_*_amd64.deb 2>/dev/null | head -1)
     if [ -n "$cached_deb" ]; then
         log "Cached at $cached_deb ($(stat -c %s "$cached_deb") bytes)"
     else
-        echo "  ERROR: no .deb produced by cargo-deb"
+        echo "  ERROR: no .deb produced by the container build" >&2
         return 1
     fi
 }
@@ -565,6 +569,35 @@ test_ubuntu26() { _run_deb_install_scenario ubuntu26 ubuntu:26.04;  }
 
 ALL_SCENARIOS="debian12 debian13 ubuntu22 ubuntu24 ubuntu26"
 
+# `--deb PATH` installs a package the caller already built, which is how one
+# build serves all five distributions and how GitHub CI and a local run come to
+# do the same work: both build once through the container script and hand the
+# result here. Keep ALL_SCENARIOS above on its own line at column zero;
+# check-ci-parity.sh reads it to compare this matrix against the GitHub one.
+_args=()
+while [ $# -gt 0 ]; do
+    case "$1" in
+        --deb)
+            SUPPLIED_DEB="${2:?--deb requires a path}"
+            shift 2
+            ;;
+        -h|--help)
+            echo "usage: test.sh [--deb PATH] [scenario ...]"
+            echo "scenarios: $ALL_SCENARIOS"
+            exit 0
+            ;;
+        -*)
+            echo "Unknown option: $1" >&2
+            exit 1
+            ;;
+        *)
+            _args+=("$1")
+            shift
+            ;;
+    esac
+done
+set -- ${_args[@]+"${_args[@]}"}
+
 if [ $# -eq 0 ]; then
     scenarios="$ALL_SCENARIOS"
 else