diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 43242d6..fc63c55 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -76,6 +76,14 @@ jobs:
         run: .tools/almide/almide check src/main.almd
       - name: Build
         run: .tools/almide/almide build src/main.almd -o target/porta
+      # TLS is rustls only (reqwest without its default features), so the Linux
+      # binary needs no libssl.so.3 from whatever image it lands in (#35). A
+      # dependency that turns native-tls back on shows up here.
+      - name: No OpenSSL linked
+        if: runner.os == 'Linux'
+        run: |
+          ldd target/porta
+          ! ldd target/porta | grep -E 'libssl|libcrypto'
       - name: Test
         run: .tools/almide/almide test --ci
       # The memory ceiling is a cgroup v2 scope asked of the systemd user
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 24aa1f8..18a479f 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -39,9 +39,19 @@ jobs:
           python-version: "3.12"
       - uses: dtolnay/rust-toolchain@stable
       - name: Install pinned Almide
+        if: runner.os == 'macOS'
         run: bash scripts/install-almide.sh
       - name: Build
+        if: runner.os == 'macOS'
         run: .tools/almide/almide build src/main.almd -o target/porta
+      # Linux builds in Debian bullseye, not on the runner: a binary linked on
+      # Ubuntu 24.04 asks for glibc 2.39 and does not start on Debian 12,
+      # Ubuntu 22.04 or the python:*-slim images, which is most containers an
+      # agent runs in (#35). The script refuses a result past glibc 2.31 or one
+      # linking OpenSSL.
+      - name: Build (Debian bullseye, glibc 2.31)
+        if: runner.os == 'Linux'
+        run: bash scripts/build-linux.sh target/porta
       # A binary nobody checked is worse than no binary: the same suite that
       # gates a commit gates a release, on the machine that produced the file.
       - name: Verify the binary that will ship
diff --git a/README.ja.md b/README.ja.md
index b7b31da..4b3b37d 100644
--- a/README.ja.md
+++ b/README.ja.md
@@ -46,7 +46,7 @@ curl -fsSL https://raw.githubusercontent.com/almide/porta/main/scripts/install.s
 
 | 環境 | 方法 |
 |---|---|
-| macOS (Apple silicon)、Linux (x86-64, arm64) | 上のスクリプト。SHA-256 を検証し、`cosign` があれば Sigstore 署名も検証 |
+| macOS (Apple silicon)、Linux (x86-64, arm64。glibc 2.31 以降：Debian 11、Ubuntu 20.04 以降) | 上のスクリプト。SHA-256 を検証し、`cosign` があれば Sigstore 署名も検証 |
 | Debian、Ubuntu | [リリース](https://github.com/almide/porta/releases)の `sudo apt install ./porta_<version>_amd64.deb`。Ubuntu 23.10 以降では namespace に必要な AppArmor プロファイルも読み込む |
 | それ以外の方法で入れた Ubuntu | 一度だけ `sudo porta setup` で同じプロファイルを入れる |
 | GitHub Actions | `- uses: almide/porta@v0.6.16` の後、どのステップでも `porta run …` |
diff --git a/README.md b/README.md
index 43c9a7e..ed5f321 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@ curl -fsSL https://raw.githubusercontent.com/almide/porta/main/scripts/install.s
 
 | Where | How |
 |---|---|
-| macOS (Apple silicon), Linux (x86-64, arm64) | the script above: checks the SHA-256, and the Sigstore signature where `cosign` is installed |
+| macOS (Apple silicon), Linux (x86-64, arm64; glibc 2.31+: Debian 11, Ubuntu 20.04 and later) | the script above: checks the SHA-256, and the Sigstore signature where `cosign` is installed |
 | Debian, Ubuntu | `sudo apt install ./porta_<version>_amd64.deb` from a [release](https://github.com/almide/porta/releases); on Ubuntu 23.10+ it also loads the AppArmor profile porta's namespaces need |
 | Ubuntu, installed any other way | `sudo porta setup` once, for the same profile |
 | GitHub Actions | `- uses: almide/porta@v0.6.16`, then `porta run …` in any step |
diff --git a/README.zh-CN.md b/README.zh-CN.md
index d9ac93c..25a30fa 100644
--- a/README.zh-CN.md
+++ b/README.zh-CN.md
@@ -45,7 +45,7 @@ curl -fsSL https://raw.githubusercontent.com/almide/porta/main/scripts/install.s
 
 | 平台 | 方法 |
 |---|---|
-| macOS (Apple silicon)、Linux (x86-64, arm64) | 上面的脚本：校验 SHA-256，安装了 `cosign` 时还会校验 Sigstore 签名 |
+| macOS (Apple silicon)、Linux (x86-64, arm64；glibc 2.31+：Debian 11、Ubuntu 20.04 及以上) | 上面的脚本：校验 SHA-256，安装了 `cosign` 时还会校验 Sigstore 签名 |
 | Debian、Ubuntu | 从 [release](https://github.com/almide/porta/releases) 下载后 `sudo apt install ./porta_<version>_amd64.deb`；在 Ubuntu 23.10+ 上还会加载命名空间所需的 AppArmor 配置 |
 | 以其他方式安装的 Ubuntu | 运行一次 `sudo porta setup`，加载同样的配置 |
 | GitHub Actions | `- uses: almide/porta@v0.6.16`，之后任意步骤中都可 `porta run …` |
diff --git a/almide.toml b/almide.toml
index e5adf37..592520f 100644
--- a/almide.toml
+++ b/almide.toml
@@ -8,7 +8,7 @@ allow = ["FS.read", "FS.write", "IO", "Env", "Time", "Net"]
 [native-deps]
 wasmtime = "47.0.3"
 wasmtime-wasi = { version = "47.0.3", features = ["p3"] }
-reqwest = { version = "0.12.28", features = ["blocking", "rustls-tls"] }
+reqwest = { version = "0.12.28", default-features = false, features = ["blocking", "rustls-tls"] }
 serde_json = "1"
 sha2 = "0.10"
 jsonschema = { version = "=0.56.0", default-features = false }
diff --git a/scripts/build-linux.sh b/scripts/build-linux.sh
new file mode 100755
index 0000000..6223f65
--- /dev/null
+++ b/scripts/build-linux.sh
@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# Builds the Linux porta that ships: inside Debian bullseye, so the binary asks for
+# nothing newer than glibc 2.31 and runs on Debian 11 and later, Ubuntu 20.04 and later,
+# and the python:*-slim images (#35). A binary built on the runner itself (Ubuntu 24.04)
+# needed glibc 2.39 and did not start on bookworm.
+#
+#   scripts/build-linux.sh [OUT]      (default: target/porta)
+#
+# The pinned Almide is built from its tag in the same container, because its own Linux
+# release needs glibc 2.39 too. The result is checked before it is handed back: no glibc
+# symbol past 2.31, and no libssl or libcrypto (porta speaks TLS through rustls only).
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+out="${1:-target/porta}"
+tag="${ALMIDE_RELEASE_TAG:-$(sed -n 's/^tag="\${ALMIDE_RELEASE_TAG:-\(.*\)}"$/\1/p' scripts/install-almide.sh)}"
+[ -n "$tag" ] || { echo "no pinned Almide tag in scripts/install-almide.sh" >&2; exit 2; }
+image="${PORTA_LINUX_BUILD_IMAGE:-rust:1-bullseye}"
+mkdir -p "$(dirname "$out")" .tools/linux-build
+
+# As the caller's user, so what lands in target/ can be stripped and packaged after.
+docker run --rm --user "$(id -u):$(id -g)" -e HOME=/tmp -v "$PWD:/src" -w /src \
+  -e ALMIDE_TAG="$tag" -e OUT="$out" -e CARGO_HOME=/src/.tools/linux-build/cargo \
+  ${CARGO_BUILD_JOBS:+-e CARGO_BUILD_JOBS="$CARGO_BUILD_JOBS"} "$image" bash -euo pipefail -c '
+    almide=/src/.tools/linux-build/almide-$ALMIDE_TAG
+    if [ ! -x "$almide" ]; then
+      git init -q /tmp/almide && cd /tmp/almide
+      git remote add origin https://github.com/almide/almide
+      git fetch -q --depth 1 origin "refs/tags/$ALMIDE_TAG:refs/tags/$ALMIDE_TAG"
+      git checkout -q "$ALMIDE_TAG"
+      cargo build --release --locked --bin almide > /tmp/almide-build.log 2>&1 || { tail -30 /tmp/almide-build.log; exit 1; }
+      install -m 0755 target/release/almide "$almide"
+      cd /src
+    fi
+    "$almide" --version
+    "$almide" build src/main.almd -o "$OUT"
+    newest=$(objdump -T "$OUT" | grep -o "GLIBC_[0-9.]*" | sort -uV | tail -1)
+    echo "needs $newest"
+    [ "$(printf "%s\nGLIBC_2.31\n" "$newest" | sort -V | tail -1)" = GLIBC_2.31 ] || { echo "needs $newest, past bullseye" >&2; exit 1; }
+    if ldd "$OUT" | grep -E "libssl|libcrypto"; then echo "links OpenSSL; porta is rustls only" >&2; exit 1; fi
+  '
+echo "== built $out"