fix(net): detect interfaces without following /sys symlinks

/sys/class/net/<if> is always a symlink into /sys/devices; in nested
containers (sysbox runtime) that target is not visible, so the link is
dead. iface_exists() used Path.exists(), which follows the link and
therefore tested the visibility of the target instead of the existence of
the interface. detect_egress_iface() discarded every interface it had
correctly read from the routing table, so automtu aborted with "Could not
detect egress interface" and rc=2 even though `ip -4 route show default`
and `ip link show dev eth0` both worked. The documented escape hatch
--egress-if was equally dead, because core.py validates it through the
same call.

Chosen fix: os.path.lexists() rather than probing netlink for existence.
It asks the right question -- "is there an entry named <if>" -- and costs
no subprocess on a healthy host. Netlink (`ip link show`) is only the
fallback for when /sys/class/net itself is unavailable, so sysfs stays the
preferred path everywhere.

read_iface_mtu() gains the same cascade: sysfs first, MTU parsed from
`ip link show dev <if>` when the sysfs path is unreadable, RuntimeError
naming both sources when neither answers - no silent default. core.py
turns that into an error line plus rc=3 instead of a traceback.

list_ifaces() had the same defect (is_dir() on a dead symlink) and would
have left Docker bridge detection blind in the same environments.

Regression tests build a /sys replacement whose class/net/eth0 points at a
missing target and mock the ip command: they fail if lexists becomes
exists again, if the MTU fallback is removed, or if the symlink filter is
dropped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/automtu/core.py

@@ -2,8 +2,8 @@ from __future__ import annotations
 
 import statistics
 import sys
+from collections.abc import Iterable
 from dataclasses import dataclass
-from typing import Iterable, Optional
 
 from .docker import detect_docker_ifaces
 from .net import (
@@ -28,7 +28,7 @@ class Result:
     wg_mtu: int
 
 
-def _split_targets(items: Optional[list[str]]) -> list[str]:
+def _split_targets(items: list[str] | None) -> list[str]:
     raw: list[str] = []
     for item in items or []:
         raw.extend([x.strip() for x in item.split(",") if x.strip()])
@@ -119,7 +119,11 @@ def run_automtu(args) -> int:
         set_iface_mtu(egress, args.force_egress_mtu, args.dry_run)
         base_mtu = int(args.force_egress_mtu)
     else:
-        base_mtu = int(read_iface_mtu(egress))
+        try:
+            base_mtu = int(read_iface_mtu(egress))
+        except RuntimeError as exc:
+            print(f"[automtu][ERROR] {exc}", file=sys.stderr)
+            return 3
     log(f"[automtu] Egress base MTU: {base_mtu}")
 
     # Targets (explicit + optional WG auto targets)
@@ -140,8 +144,8 @@ def run_automtu(args) -> int:
 
     # PMTU probing
     effective_mtu = base_mtu
-    probe_results: dict[str, Optional[int]] = {}
-    chosen_pmtu: Optional[int] = None
+    probe_results: dict[str, int | None] = {}
+    chosen_pmtu: int | None = None
 
     if targets:
         log(
@@ -186,7 +190,7 @@ def run_automtu(args) -> int:
         f"[automtu] Computed {args.wg_if} MTU: {wg_mtu} (overhead={args.wg_overhead}, min={args.wg_min})"
     )
 
-    wg_mtu_set: Optional[int] = None
+    wg_mtu_set: int | None = None
     wg_mtu_clamped = False
 
     if args.set_wg_mtu is not None:

src/automtu/net.py

@@ -5,7 +5,6 @@ import pathlib
 import re
 import subprocess
 import sys
-from typing import Optional
 
 
 def _run(cmd: list[str]) -> str:
@@ -14,27 +13,55 @@ def _run(cmd: list[str]) -> str:
     ).stdout.strip()
 
 
+SYSFS_NET = pathlib.Path("/sys/class/net")
+
+
+def _ip_link_show(iface: str) -> str:
+    return _run(["ip", "link", "show", "dev", iface])
+
+
 def iface_exists(iface: str) -> bool:
-    return pathlib.Path(f"/sys/class/net/{iface}").exists()
+    """
+    True if the interface exists. Does not follow the /sys/class/net symlink;
+    falls back to netlink when sysfs is unavailable.
+    """
+    if os.path.lexists(SYSFS_NET / iface):
+        return True
+    return bool(_ip_link_show(iface))
 
 
 def list_ifaces() -> list[str]:
     """
-    Return a sorted list of all network interfaces visible under /sys/class/net.
+    Return a sorted list of all network interfaces, netlink as fallback.
+    Plain files in /sys/class/net (e.g. bonding_masters) are not interfaces.
     """
-    base = pathlib.Path("/sys/class/net")
-    if not base.exists():
-        return []
-    names: list[str] = []
-    for p in base.iterdir():
-        if p.is_dir():
-            names.append(p.name)
-    names.sort()
-    return names
+    try:
+        return sorted(
+            p.name for p in SYSFS_NET.iterdir() if p.is_symlink() or p.is_dir()
+        )
+    except OSError:
+        out = _run(["ip", "-o", "link", "show"])
+        return sorted(re.findall(r"^\d+:\s+([^:@\s]+)", out, flags=re.MULTILINE))
 
 
 def read_iface_mtu(iface: str) -> int:
-    return int(pathlib.Path(f"/sys/class/net/{iface}/mtu").read_text().strip())
+    """
+    Read the interface MTU from sysfs, falling back to netlink.
+    Raises RuntimeError if neither source reports an MTU.
+    """
+    try:
+        return int((SYSFS_NET / iface / "mtu").read_text().strip())
+    except (OSError, ValueError):
+        pass
+
+    m = re.search(r"\bmtu\s+(\d+)\b", _ip_link_show(iface))
+    if not m:
+        raise RuntimeError(
+            f"Could not read MTU of {iface}: "
+            f"{SYSFS_NET / iface / 'mtu'} is unreadable and "
+            f"'ip link show dev {iface}' reported no MTU."
+        )
+    return int(m.group(1))
 
 
 def set_iface_mtu(iface: str, mtu: int, dry: bool) -> None:
@@ -53,7 +80,7 @@ def require_root(*, dry: bool, needs_root: bool) -> None:
         raise SystemExit(1)
 
 
-def detect_egress_iface(ignore_vpn: bool = True) -> Optional[str]:
+def detect_egress_iface(ignore_vpn: bool = True) -> str | None:
     devs: list[str] = []
     for cmd in (
         ["ip", "-4", "route", "show", "default"],

tests/unit/test_core.py

@@ -1,12 +1,46 @@
 import io
+import pathlib
+import tempfile
 import unittest
 from contextlib import redirect_stdout
 from types import SimpleNamespace
 from unittest.mock import patch
 
+from automtu import net
 from automtu.core import run_automtu
 
 
+def _args(**over) -> SimpleNamespace:
+    base = {
+        "dry_run": True,
+        "egress_if": None,
+        "prefer_wg_egress": False,
+        "force_egress_mtu": None,
+        "pmtu_target": None,
+        "auto_pmtu_from_wg": False,
+        "pmtu_min_payload": 1200,
+        "pmtu_max_payload": 1472,
+        "pmtu_timeout": 1.0,
+        "pmtu_policy": "min",
+        "apply_egress_mtu": False,
+        "apply_wg_mtu": False,
+        "apply_docker_mtu": False,
+        "apply_all": False,
+        "docker_if": None,
+        "docker_no_user_bridges": False,
+        "wg_if": "wg0",
+        "wg_overhead": 80,
+        "wg_min": 1280,
+        "set_wg_mtu": None,
+        "persist": None,
+        "uninstall": False,
+        "print_mtu": None,
+        "print_json": False,
+    }
+    base.update(over)
+    return SimpleNamespace(**base)
+
+
 class TestCore(unittest.TestCase):
     def test_run_automtu_happy_path_all_mocked(self) -> None:
         args = SimpleNamespace(
@@ -164,5 +198,53 @@ class TestCore(unittest.TestCase):
         mock_set.assert_not_called()
 
 
+class TestCoreInSysboxContainer(unittest.TestCase):
+    """Container with dead /sys/class/net symlinks: routing and ip link work."""
+
+    def setUp(self) -> None:
+        tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(tmp.cleanup)
+        root = pathlib.Path(tmp.name)
+        self.netdir = root / "class" / "net"
+        self.netdir.mkdir(parents=True)
+        for name in ("eth0", "lo"):
+            (self.netdir / name).symlink_to(root / "devices" / "virtual" / "net" / name)
+
+    @staticmethod
+    def _fake_run(cmd: list[str]) -> str:
+        if cmd[:5] == ["ip", "-4", "route", "show", "default"]:
+            return "default via 172.28.0.1 dev eth0"
+        if cmd[:3] == ["ip", "link", "show"] and cmd[-1] == "eth0":
+            return (
+                "2: eth0@if5: <BROADCAST,MULTICAST,UP> mtu 1450 qdisc noqueue state UP"
+            )
+        return ""
+
+    def test_print_mtu_effective_yields_number_and_rc0(self) -> None:
+        with (
+            patch.object(net, "SYSFS_NET", self.netdir),
+            patch.object(net, "_run", side_effect=self._fake_run),
+            patch("automtu.core.probe_pmtu", return_value=1400),
+        ):
+            buf = io.StringIO()
+            with redirect_stdout(buf):
+                rc = run_automtu(_args(pmtu_target=["1.1.1.1"], print_mtu="effective"))
+
+        self.assertEqual(rc, 0)
+        self.assertEqual(int(buf.getvalue().strip()), 1400)
+
+    def test_explicit_egress_if_is_accepted(self) -> None:
+        with (
+            patch.object(net, "SYSFS_NET", self.netdir),
+            patch.object(net, "_run", side_effect=self._fake_run),
+        ):
+            buf = io.StringIO()
+            with redirect_stdout(buf):
+                rc = run_automtu(_args(egress_if="eth0", print_mtu="egress"))
+
+        self.assertEqual(rc, 0)
+        self.assertEqual(int(buf.getvalue().strip()), 1450)
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)

tests/unit/test_net.py

@@ -1,8 +1,120 @@
+import pathlib
+import tempfile
 import unittest
-from pathlib import Path
 from unittest.mock import patch
 
-import automtu.net as net
+from automtu import net
+
+IP_LINK_ETH0 = (
+    "2: eth0@if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1420 qdisc noqueue state UP "
+    "mode DEFAULT group default\n    link/ether 02:42:ac:1c:00:02 brd ff:ff:ff:ff:ff:ff"
+)
+IP_LINK_ALL = (
+    "1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN\n"
+    "2: eth0@if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1420 qdisc noqueue state UP"
+)
+
+
+def _sysbox_sysfs(root: pathlib.Path, ifaces=("eth0", "lo")) -> pathlib.Path:
+    """/sys replacement as seen inside a sysbox container: dead symlinks."""
+    netdir = root / "class" / "net"
+    netdir.mkdir(parents=True)
+    for name in ifaces:
+        (netdir / name).symlink_to(root / "devices" / "virtual" / "net" / name)
+    return netdir
+
+
+class SysboxSysfsBase(unittest.TestCase):
+    def setUp(self) -> None:
+        self._tmp = tempfile.TemporaryDirectory()
+        self.root = pathlib.Path(self._tmp.name)
+        self.addCleanup(self._tmp.cleanup)
+
+
+class TestDeadSymlinkSysfs(SysboxSysfsBase):
+    def test_iface_exists_true_for_dead_symlink_without_netlink(self) -> None:
+        netdir = _sysbox_sysfs(self.root)
+        self.assertFalse((netdir / "eth0").exists())
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", return_value=""),
+        ):
+            self.assertTrue(net.iface_exists("eth0"))
+            self.assertFalse(net.iface_exists("eth9"))
+
+    def test_iface_exists_falls_back_to_netlink_without_sysfs(self) -> None:
+        with (
+            patch.object(net, "SYSFS_NET", self.root / "absent" / "net"),
+            patch.object(net, "_run", return_value=IP_LINK_ETH0),
+        ):
+            self.assertTrue(net.iface_exists("eth0"))
+
+    def test_detect_egress_iface_accepts_dead_symlink_iface(self) -> None:
+        netdir = _sysbox_sysfs(self.root)
+
+        def fake_run(cmd: list[str]) -> str:
+            if cmd[:5] == ["ip", "-4", "route", "show", "default"]:
+                return "default via 172.28.0.1 dev eth0"
+            return ""
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", side_effect=fake_run),
+        ):
+            self.assertEqual(net.detect_egress_iface(), "eth0")
+
+    def test_list_ifaces_keeps_dead_symlinks_and_skips_plain_files(self) -> None:
+        netdir = _sysbox_sysfs(self.root, ("eth0", "lo", "br-abc123"))
+        (netdir / "bonding_masters").write_text("")
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", return_value=""),
+        ):
+            self.assertEqual(net.list_ifaces(), ["br-abc123", "eth0", "lo"])
+
+    def test_list_ifaces_falls_back_to_netlink_without_sysfs(self) -> None:
+        with (
+            patch.object(net, "SYSFS_NET", self.root / "absent" / "net"),
+            patch.object(net, "_run", return_value=IP_LINK_ALL),
+        ):
+            self.assertEqual(net.list_ifaces(), ["eth0", "lo"])
+
+
+class TestReadIfaceMtu(SysboxSysfsBase):
+    def test_prefers_sysfs_over_netlink(self) -> None:
+        netdir = self.root / "class" / "net"
+        (netdir / "eth0").mkdir(parents=True)
+        (netdir / "eth0" / "mtu").write_text("1500\n")
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", return_value=IP_LINK_ETH0) as run,
+        ):
+            self.assertEqual(net.read_iface_mtu("eth0"), 1500)
+            run.assert_not_called()
+
+    def test_netlink_fallback_on_dead_symlink(self) -> None:
+        netdir = _sysbox_sysfs(self.root)
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", return_value=IP_LINK_ETH0),
+        ):
+            self.assertEqual(net.read_iface_mtu("eth0"), 1420)
+
+    def test_raises_when_neither_sysfs_nor_netlink_answers(self) -> None:
+        netdir = _sysbox_sysfs(self.root)
+
+        with (
+            patch.object(net, "SYSFS_NET", netdir),
+            patch.object(net, "_run", return_value=""),
+            self.assertRaises(RuntimeError) as ctx,
+        ):
+            net.read_iface_mtu("eth0")
+
+        self.assertIn("eth0", str(ctx.exception))
 
 
 class TestNet(unittest.TestCase):
@@ -46,18 +158,10 @@ class TestNet(unittest.TestCase):
             self.assertFalse(net.default_route_uses_iface("wg0"))
 
     def test_list_ifaces_returns_sorted_names(self) -> None:
-        fake = [
-            Path("/sys/class/net/eth0"),
-            Path("/sys/class/net/lo"),
-            Path("/sys/class/net/wg0"),
-        ]
-
-        with (
-            patch("automtu.net.pathlib.Path.exists", return_value=True),
-            patch("automtu.net.pathlib.Path.iterdir", return_value=fake),
-            patch.object(Path, "is_dir", return_value=True),
-        ):
-            self.assertEqual(net.list_ifaces(), ["eth0", "lo", "wg0"])
+        with tempfile.TemporaryDirectory() as tmp:
+            netdir = _sysbox_sysfs(pathlib.Path(tmp), ("wg0", "eth0", "lo"))
+            with patch.object(net, "SYSFS_NET", netdir):
+                self.assertEqual(net.list_ifaces(), ["eth0", "lo", "wg0"])
 
 
 if __name__ == "__main__":