/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 guide explains how to determine and set correct MTU values with and without WireGuard.
It includes examples for automatic detection, PMTU probing, egress application, and WireGuard overrides.
Reference: conversation about wg-mtu-auto practical usage and testing at https://chatgpt.com/share/68efc179-1a10-800f-9656-1e8731b40546
- Added main.py: automatic WireGuard MTU calculation and PMTU probing
- Added test.py: unittests covering base, PMTU, and fallback scenarios
- Added Makefile: includes test target and install guidance
- Added README.md: usage, pkgmgr installation, and MIT license
Reference: https://chatgpt.com/share/68efc179-1a10-800f-9656-1e8731b40546
