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

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/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>
This commit is contained in:
Kevin Veen-Birkenbach
2026-09-08 18:35:25 +02:00
parent a0db5f797f
commit 499e61346d
4 changed files with 251 additions and 34 deletions
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82
tests/unit/test_core.py
View File

@@ -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)

132
tests/unit/test_net.py
View File

@@ -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__":