resolv_conf: make /etc/resolv.conf selectable

By refactoring the previous --host-resolv flag into a more generic
--resolv flag which accepts auto, guest, and host as its parameter, one
can select the /etc/resolv.conf to be used. By default, the auto
behavior is chosen and prefers the guest's resolv.conf over an overlay
from the host.

Related to #57.

CHANGELOG.md

@@ -17,7 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ### Added
-- New --host-resolv flag to enforce using the host's /etc/resolv.conf.
+- New -r/--resolv flag to select which /etc/resolv.conf to use.
 
 ### Fixed
 - Apply `ENV` variables also to `HOST` commands.

README.md

@@ -38,13 +38,20 @@ For detailed information [read our paper](https://jonashoechst.de/assets/papers/
 pimod pimod.sh -h
 Usage: pimod.sh [Options] Pifile
 
+Options:
 Options:
   -c --cache DEST   Define cache location.
   -d --debug        Debug on failure; run an interactive shell before tear down.
   -h --help         Print this help message.
-     --host-resolv  Always uses the host's /etc/resolv.conf file.
-                    Be aware, that when run within Docker this might be Docker's
-                    resolv.conf file.
+  -r --resolv TYPE  Specify which /etc/resolv.conf file to use for networking.
+                    By default, TYPE "auto" is used, which prefers an already
+                    existing resolv.conf, only to be replaced by the host's if
+                    missing.
+                    TYPE "guest" never mounts the host's file within the guest,
+                    even when such a file is absent within the image.
+                    TYPE "host" always uses the host's file within the guest.
+                    Be aware that when run within Docker, the host's file might
+                    be Docker's resolv.conf file.
   -t --trace        Trace each executed command for debugging.
 ```
 

modules/resolv_conf.sh

@@ -1,24 +1,40 @@
 if [ -z "${PIMOD_HOST_RESOLV+x}" ]; then
-  PIMOD_HOST_RESOLV=0
+  PIMOD_HOST_RESOLV_TYPE="auto"
 fi
 
 # resolv_conf_setup checks the /etc/resolv.conf file within an image and remaps
 # it, if necessary.
 resolv_conf_setup() {
   local resolv_conf="${CHROOT_MOUNT}/etc/resolv.conf"
 
-  if [[ -f "${resolv_conf}" ]] || (RUN test -e "/etc/resolv.conf"); then
-    [[ "${PIMOD_HOST_RESOLV}" -eq "0" ]] && return
-  fi
+  case "${PIMOD_HOST_RESOLV_TYPE}" in
+    auto)
+      # Do not mount the host's file when a /etc/resolv.conf already exists.
+      ((test -f "${resolv_conf}") || (RUN test -e "/etc/resolv.conf")) && return
+      ;;
+
+    guest)
+      # Never mount the host's file.
+      return
+      ;;
+
+    host)
+      # Always mount the host's file as an overlay.
+      ;;
+
+    *)
+      echo -e "\033[0;31m### Error: unknown resolv type ${PIMOD_HOST_RESOLV_TYPE} \033[0m"
+      return 1
+  esac
 
   if [[ -L "${resolv_conf}" ]]; then
     RESOLV_CONF_BACKUP=$(mktemp -u)
     mv "${resolv_conf}" "${RESOLV_CONF_BACKUP}"
   fi
 
   if ! touch "${resolv_conf}"; then
-    echo -e "\033[0;33m### Warning: Mounting ${resolv_conf} failed.\033[0m"
-    return
+    echo -e "\033[0;31m### Error: Mounting ${resolv_conf} failed.\033[0m"
+    return 1
   fi
   mount -o ro,bind /etc/resolv.conf "${resolv_conf}"
 
@@ -27,7 +43,7 @@ resolv_conf_setup() {
 
 # resolv_conf_teardown resets the actions done by resolv_conf_setup.
 resolv_conf_teardown() {
-  [[ -z ${RESOLVE_MOUNT+x} ]] && return 0
+  [[ -z ${RESOLVE_MOUNT+x} ]] && return
 
   local resolv_conf="${CHROOT_MOUNT}/etc/resolv.conf"
 

pimod.sh

@@ -26,14 +26,19 @@ Options:
   -c --cache DEST   Define cache location.
   -d --debug        Debug on failure; run an interactive shell before tear down.
   -h --help         Print this help message.
-     --host-resolv  Always uses the host's /etc/resolv.conf file.
-                    Be aware, that when run within Docker this might be Docker's
-                    resolv.conf file.
+  -r --resolv TYPE  Specify which /etc/resolv.conf file to use for networking.
+                    By default, TYPE "auto" is used, which prefers an already
+                    existing resolv.conf, only to be replaced by the host's if
+                    missing.
+                    TYPE "guest" never mounts the host's file within the guest,
+                    even when such a file is absent within the image.
+                    TYPE "host" always uses the host's file within the guest.
+                    Be aware that when run within Docker, the host's file might
+                    be Docker's resolv.conf file.
   -t --trace        Trace each executed command for debugging.
 EOF
 }
 
-
 main() {
   local pifile
 
@@ -56,9 +61,19 @@ main() {
         exit 0
         ;;
 
-      --host-resolv)
-        # PIMOD_HOST_RESOLV is defined in modules/resolv_conf.sh
-        PIMOD_HOST_RESOLV=1
+      -r|--resolv)
+        [[ "$#" -le "2" ]] && (echo "Usage: $0 --resolv KIND"; exit 1)
+        case "$2" in
+          auto|guest|host)
+            # PIMOD_HOST_RESOLV_TYPE is defined in modules/resolv_conf.sh
+            PIMOD_HOST_RESOLV_TYPE="$2"
+            ;;
+
+          *)
+            echo "Usage: $0 --resolv KIND"
+            exit 1
+        esac
+        shift
         ;;
 
       -t|--trace)