fix(network): auto-detect path MTU so large image pulls don't stall with EOF

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>

configs/network/network_docker

@@ -4,7 +4,7 @@
 
 CFG_NETWORK_NAME=vpn                                     # Network Name - Docker network name for container communication
 CFG_NETWORK_SUBNET=10.100.0.0/16                         # Network Subnet - Subnet range for Docker network
-CFG_NETWORK_MTU=1500                                     # Network MTU - Maximum transmission unit for network packets
+CFG_NETWORK_MTU=auto                                     # Network MTU - Maximum transmission unit for network packets. 'auto' probes the real path MTU and clamps to a safe value (fixes large image pulls stalling with EOF on Qubes/NAT/VPN links where the path MTU is below 1500); or set an explicit number like 1500 or 1280. **ADVANCED**
 CFG_REQUIREMENT_DOCKER_NETWORK=true                      # Docker Network - Create and manage the Docker network for container communication
 CFG_REQUIREMENT_DOCKER_NETWORK_PRUNE=true                # Network Cleanup - Automatically prune unused Docker networks
 CFG_REQUIREMENT_DOCKER_SWITCHER=true                     # Docker Switcher - Install the Docker version switching utility

scripts/config/docker/docker_config_setup_data.sh

@@ -93,7 +93,7 @@ dockerConfigSetupFileWithData()
         tagsManagerUpdateUniversalTag "$full_file_path" "DOCKER_NETWORK_TAG" "$CFG_NETWORK_NAME"
         tagsManagerUpdateUniversalTag "$full_file_path" "PUBLIC_IP_TAG" "$public_ip_v4"
         tagsManagerUpdateUniversalTag "$full_file_path" "NETWORK_SUBNET_TAG" "$CFG_NETWORK_SUBNET"
-        tagsManagerUpdateUniversalTag "$full_file_path" "NETWORK_MTU_TAG" "$CFG_NETWORK_MTU"
+        tagsManagerUpdateUniversalTag "$full_file_path" "NETWORK_MTU_TAG" "$(networkEffectiveMtu)"
         ipUpdateComposeTags "$app_name" "$full_file_path"
         portUpdateComposeTags "$app_name" "$full_file_path"
         tagsProcessorTrustedDomains "$full_file_path"

scripts/docker/install/rootless/rootless_docker.sh

@@ -144,11 +144,17 @@ EOF
         local result; result=$(sudo touch $override_conf_file)
         checkSuccess "Create the override.conf in docker.service.d"
 
+        # Resolve CFG_NETWORK_MTU to a concrete, path-safe value (auto = probe).
+        # This is the MTU the daemon PULLS over — a blind 1500 on a lower-path-MTU
+        # link (Qubes/NAT/VPN) black-holes large image-layer downloads (EOF).
+        local resolved_mtu; resolved_mtu="$(networkRedetectMtu)"
+        isNotice "Rootless uplink MTU: $resolved_mtu (CFG_NETWORK_MTU=${CFG_NETWORK_MTU:-auto})"
+
 sudo bash -c "cat <<EOL > '$override_conf_file'
 [Service]
 Environment='DOCKERD_ROOTLESS_ROOTLESSKIT_NET=$rootless_net'
 Environment='DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=$rootless_port_driver'
-Environment='DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=$CFG_NETWORK_MTU'
+Environment='DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=$resolved_mtu'
 EOL"
 
         local result; result=$(sudo chown $CFG_DOCKER_INSTALL_USER:$CFG_DOCKER_INSTALL_USER $override_conf_file)

scripts/network/network_mtu.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+#
+# Network MTU resolution.
+# ---------------------------------------------------------------------------
+# CFG_NETWORK_MTU drives BOTH the rootless daemon's uplink MTU
+# (DOCKERD_ROOTLESS_ROOTLESSKIT_MTU — how image PULLS egress) and every app's
+# container-network MTU (the NETWORK_MTU_TAG baked into each compose). A blind
+# 1500 breaks on links whose real path MTU is lower (Qubes/NAT/VPN with PMTU
+# discovery blocked): small transfers (image manifests, tiny images) succeed but
+# large image-layer downloads stall and reset with "EOF" mid-blob.
+#
+# CFG_NETWORK_MTU accepts:
+#   <number>  — use exactly (explicit override).
+#   auto      — probe the path MTU once, cache a safe value (default).
+#
+# The probe is a don't-fragment ICMP ladder. If ICMP is fully filtered (no probe
+# gets through at all) we can't tell a clean link from a constrained one, so we
+# assume the standard 1500 rather than needlessly shrinking every network — set
+# an explicit number for that case.
+
+networkMtuCacheFile() { echo "${docker_dir%/}/.network_mtu"; }
+
+# Largest standard MTU whose don't-fragment probe reaches the internet.
+# Echoes a number; 1500 when no signal (ping missing / all probes filtered).
+networkDetectMtu() {
+    local host="${1:-1.1.1.1}" m best=""
+    command -v ping >/dev/null 2>&1 || { echo 1500; return 0; }
+    for m in 1500 1492 1400 1300 1280; do
+        # payload = mtu - 28 (IPv4 + ICMP headers)
+        if ping -M do -s $((m - 28)) -c1 -W2 "$host" >/dev/null 2>&1; then
+            best="$m"; break
+        fi
+    done
+    echo "${best:-1500}"
+}
+
+# The concrete MTU to bake into the daemon + composes. Resolves CFG_NETWORK_MTU:
+# a number is used verbatim; "auto"/empty is probed once and cached so we don't
+# re-ping on every app install.
+networkEffectiveMtu() {
+    local want="${CFG_NETWORK_MTU:-auto}"
+    if [[ "$want" =~ ^[0-9]+$ ]]; then echo "$want"; return 0; fi
+
+    local cache cached
+    cache="$(networkMtuCacheFile)"
+    cached="$(cat "$cache" 2>/dev/null | tr -dc '0-9')"
+    if [[ -n "$cached" ]]; then echo "$cached"; return 0; fi
+
+    local detected; detected="$(networkDetectMtu)"
+    if declare -F runInstallWrite >/dev/null 2>&1; then
+        printf '%s\n' "$detected" | runInstallWrite "$cache" 2>/dev/null || true
+    else
+        printf '%s\n' "$detected" > "$cache" 2>/dev/null || true
+    fi
+    echo "$detected"
+}
+
+# Force a fresh probe (clears the cache), echoing the new value. The rootless
+# setup calls this so each (re)install re-detects the current link.
+networkRedetectMtu() {
+    local cache; cache="$(networkMtuCacheFile)"
+    if declare -F runInstallOp >/dev/null 2>&1; then
+        runInstallOp rm -f "$cache" 2>/dev/null || true
+    else
+        rm -f "$cache" 2>/dev/null || true
+    fi
+    networkEffectiveMtu
+}

scripts/source/files/arrays/files_network.sh

@@ -28,6 +28,7 @@ network_scripts=(
     "network/ip/ip_remove_from_db.sh"
     "network/ip/ip_replace_tags.sh"
     "network/monitoring/monitoring.sh"
+    "network/network_mtu.sh"
     "network/ports/allocation/port_allocate.sh"
     "network/ports/allocation/port_store_mapping.sh"
     "network/ports/allocation/port_update_compose_tags.sh"

scripts/source/files/arrays/function_manifest.sh

@@ -681,7 +681,11 @@ declare -gA LP_FN_MAP=(
     [monitoringToggleAppConfig]="network/monitoring/monitoring.sh"
     [moveFile]="function/file/move_file.sh"
     [_netServiceIsRouted]="docker/network/network_conflicts.sh"
+    [networkDetectMtu]="network/network_mtu.sh"
+    [networkEffectiveMtu]="network/network_mtu.sh"
     [networkHealConflicts]="docker/network/network_heal.sh"
+    [networkMtuCacheFile]="network/network_mtu.sh"
+    [networkRedetectMtu]="network/network_mtu.sh"
     [networkScanConflicts]="docker/network/network_conflicts.sh"
     [_nextcloudOcc]="nextcloud/scripts/nextcloud_auth.sh"
     [_nextcloudOccWithPass]="nextcloud/scripts/nextcloud_auth.sh"
@@ -1661,7 +1665,11 @@ declare -gA LP_FN_ROOT=(
     [monitoringToggleAppConfig]="scripts"
     [moveFile]="scripts"
     [_netServiceIsRouted]="scripts"
+    [networkDetectMtu]="scripts"
+    [networkEffectiveMtu]="scripts"
     [networkHealConflicts]="scripts"
+    [networkMtuCacheFile]="scripts"
+    [networkRedetectMtu]="scripts"
     [networkScanConflicts]="scripts"
     [_nextcloudOcc]="containers"
     [_nextcloudOccWithPass]="containers"
@@ -2662,7 +2670,11 @@ monitoringResolveScrapeTags() { source "${install_scripts_dir}network/monitoring
 monitoringToggleAppConfig() { source "${install_scripts_dir}network/monitoring/monitoring.sh"; monitoringToggleAppConfig "$@"; }
 moveFile() { source "${install_scripts_dir}function/file/move_file.sh"; moveFile "$@"; }
 _netServiceIsRouted() { source "${install_scripts_dir}docker/network/network_conflicts.sh"; _netServiceIsRouted "$@"; }
+networkDetectMtu() { source "${install_scripts_dir}network/network_mtu.sh"; networkDetectMtu "$@"; }
+networkEffectiveMtu() { source "${install_scripts_dir}network/network_mtu.sh"; networkEffectiveMtu "$@"; }
 networkHealConflicts() { source "${install_scripts_dir}docker/network/network_heal.sh"; networkHealConflicts "$@"; }
+networkMtuCacheFile() { source "${install_scripts_dir}network/network_mtu.sh"; networkMtuCacheFile "$@"; }
+networkRedetectMtu() { source "${install_scripts_dir}network/network_mtu.sh"; networkRedetectMtu "$@"; }
 networkScanConflicts() { source "${install_scripts_dir}docker/network/network_conflicts.sh"; networkScanConflicts "$@"; }
 _nextcloudOcc() { source "${install_containers_dir}nextcloud/scripts/nextcloud_auth.sh"; _nextcloudOcc "$@"; }
 _nextcloudOccWithPass() { source "${install_containers_dir}nextcloud/scripts/nextcloud_auth.sh"; _nextcloudOccWithPass "$@"; }