CFG_NETWORK_MTU=1500 was baked blindly into the rootless daemon uplink
(DOCKERD_ROOTLESS_ROOTLESSKIT_MTU) and every app's container network. On links
whose real path MTU is below 1500 (Qubes/NAT/VPN with PMTU discovery blocked),
image manifests + tiny images pull fine but large image LAYERS stall and reset
mid-blob with "httpReadSeeker: ... EOF" — apps silently fail to install. Probed
here: path MTU ~1328, Docker at 1500 → black hole.
- New scripts/network/network_mtu.sh: networkDetectMtu (don't-fragment ICMP
ladder → largest standard MTU that gets through, 1500 when ICMP gives no
signal) + networkEffectiveMtu (CFG_NETWORK_MTU: a number is verbatim, "auto"
probes once and caches $docker_dir/.network_mtu) + networkRedetectMtu.
- CFG_NETWORK_MTU default 1500 -> auto. Rootless setup now writes the resolved
MTU into the override (and re-detects per install); the per-app NETWORK_MTU_TAG
uses the resolved value too. Explicit numbers still win.
Verified on this box: auto -> 1300; alpine (multi-MB) + the full ~100MB navidrome
image now pull to completion where they previously EOF'd. Applied live (override
1300, config=auto, cache=1300, rootless docker restarted).
Signed-off-by: librelad <librelad@digitalangels.vip>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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