mirror of https://github.com/CyberMind-FR/secubox-deb.git synced 2026-07-29 18:36:55 +00:00

CyberMind-FR 787c6b03e8 fix(nextcloud): give LXC systemd-resolved a DNS upstream; document LXC DNS gotcha

Static resolv.conf in the container is hijacked by systemd-resolved's empty
127.0.0.53 stub, killing DNS (no appstore/updates/external mounts) while NAT
egress still works. Drop a resolved.conf.d upstream at install. Add
docs/FAQ-LXC-DNS.md describing symptom/cause/fix. Live NC already patched.

2026-05-29 07:10:39 +02:00

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FAQ — LXC container has no DNS / no internet

Symptom

An app in an LXC (Nextcloud, PhotoPrism, PeerTube…) behaves as if offline:

Nextcloud: "Ce serveur ne peut se connecter à Internet", app store / optional addons don't show, update checks and external-storage mounts fail.
getent hosts apps.nextcloud.com → nothing; curl https://… → 000.
But reaching the internet by IP works (curl https://1.1.1.1 → 301).

That last point is the tell: routing/NAT is fine, only name resolution is broken.

Cause

The container's /etc/resolv.conf points at the systemd-resolved stub 127.0.0.53, but systemd-resolved has no upstream configured (the containers are statically addressed, so there's no DHCP to hand one over). A static resolv.conf written at install time gets hijacked by systemd-resolved on boot — it replaces the file with a symlink to its stub.

Working containers (e.g. grafana) instead carry a direct nameserver 1.1.1.1 in resolv.conf and resolve fine.

Fix (live, minimal)

Give systemd-resolved an explicit upstream inside the container:

lxc-attach -n <container> -- bash -c '
  mkdir -p /etc/systemd/resolved.conf.d
  printf "[Resolve]\nDNS=1.1.1.1 9.9.9.9\nFallbackDNS=8.8.8.8\n" \
    > /etc/systemd/resolved.conf.d/secubox-dns.conf
  systemctl restart systemd-resolved'
# verify
lxc-attach -n <container> -- getent hosts apps.nextcloud.com

NAT egress for the LXC bridge is already in the host ruleset (ip saddr 10.100.0.0/24 oifname != "br-lxc" masquerade), so no firewall change is needed — this is purely a resolver fix.

Fix (persistent, in the installer)

New container builds must drop the same resolved.conf.d/secubox-dns.conf into the rootfs alongside the static resolv.conf. Reference implementation: packages/secubox-nextcloud/sbin/nextcloudctl (the # DNS block in the base install). Apply the same pattern to every install-lxc.sh that provisions a systemd container.

Rule of thumb

When provisioning a statically-addressed LXC, always configure systemd-resolved's upstream, don't rely on a static resolv.conf — it will be overwritten.

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