Now prints a more user-friendly error message when installed as a Snap package and permission errors are encountered. Updated the Snap build process as well.

Makefile.snap

@@ -1,6 +0,0 @@
-all:
-	echo -e "\n\nDid you remember to bump the version number in snapcraft.yaml?\n\n"
-	snapcraft --use-lxd
-
-clean:
-	rm -rf parts/ prime/ snap/ stage/ build/ dist/ src/*.egg-info/ ssh-audit*.snap

PACKAGING.md

@@ -51,27 +51,14 @@ To download from production server and verify:
     $ pip3 install ssh-audit
 ```
 
+
 # Snap
 
 To create the snap package, run a fully-updated Ubuntu Server 20.04 VM.
 
-Install pre-requisites with:
-
+Create the snap package with:
 ```
-    $ sudo apt install make snapcraft
-    $ sudo snap install review-tools lxd
-```
-
-Initialize LXD:
-
-```
-    $ sudo lxd init --auto
-```
-
-Bump the version number in snapcraft.yaml.  Then run:
-
-```
-    $ make -f Makefile.snap
+    $ ./build_snap.sh
 ```
 
 Upload the snap with:

build_snap.sh

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+
+#
+#   The MIT License (MIT)
+#
+#   Copyright (C) 2021 Joe Testa (jtesta@positronsecurity.com)
+#
+#   Permission is hereby granted, free of charge, to any person obtaining a copy
+#   of this software and associated documentation files (the "Software"), to deal
+#   in the Software without restriction, including without limitation the rights
+#   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+#   copies of the Software, and to permit persons to whom the Software is
+#   furnished to do so, subject to the following conditions:
+#
+#   The above copyright notice and this permission notice shall be included in
+#   all copies or substantial portions of the Software.
+#
+#   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+#   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+#   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+#   AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+#   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+#   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+#   THE SOFTWARE.
+#
+
+################################################################################
+# build_snap.sh
+#
+# Builds a Snap package.
+################################################################################
+
+
+# Pre-requisites
+sudo apt install -y make snapcraft
+sudo snap install review-tools lxd 2> /dev/null
+
+# Initialize LXD.
+sudo lxd init --auto
+
+# Reset the filesystem from any previous runs.
+rm -rf parts/ prime/ snap/ stage/ build/ dist/ src/*.egg-info/ ssh-audit*.snap
+git checkout snapcraft.yaml 2> /dev/null
+git checkout src/ssh_audit/globals.py 2> /dev/null
+
+# Get the version from the globals.py file.
+version=$(grep VERSION src/ssh_audit/globals.py | awk 'BEGIN {FS="="} ; {print $2}' | tr -d '[:space:]')
+
+# Strip the quotes around the version (along with the initial 'v' character) and append "-1" to make the default Snap version (i.e.: 'v2.5.0' => '2.5.0-1')
+default_snap_version="${version:2:-1}-1"
+echo -e -n "\nEnter Snap package version [default: ${default_snap_version}]: "
+read -r snap_version
+
+# If no version was specified, use the default version.
+if [[ $snap_version == '' ]]; then
+    snap_version=$default_snap_version
+    echo -e "Using default snap version: ${snap_version}\n"
+fi
+
+# Ensure that the snap version fits the format of X.X.X-X.
+if [[ ! $snap_version =~ ^[0-9]\.[0-9]\.[0-9]\-[0-9]$ ]]; then
+   echo "Error: version string does not match format X.X.X-X!"
+   exit 1
+fi
+
+# Append the version field to the end of the file.  Not pretty, but it works.
+echo -e "\nversion: '${snap_version}'" >> snapcraft.yaml
+
+# Set the SNAP_PACKAGE variable to True so that file permission errors give more user-friendly 
+sed -i 's/SNAP_PACKAGE = False/SNAP_PACKAGE = True/' src/ssh_audit/globals.py
+
+snapcraft --use-lxd && echo -e "\nDone.\n"

snapcraft.yaml

@@ -1,5 +1,5 @@
 name: ssh-audit
-version: '2.5.0-1'
+# 'version' field will be automatically added by build_snap.sh.
 license: 'MIT'
 summary: ssh-audit
 description: |
@@ -12,10 +12,9 @@ confinement: strict
 apps:
   ssh-audit:
     command: bin/ssh-audit
-    plugs: [network,network-bind]
+    plugs: [network,network-bind,home]
 
 parts:
   ssh-audit:
     plugin: python
-    # python-version: python3
     source: .

src/ssh_audit/globals.py

@@ -21,7 +21,20 @@
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
    THE SOFTWARE.
 """
-VERSION = 'v2.6.0-dev'
-SSH_HEADER = 'SSH-{0}-OpenSSH_8.2'  # SSH software to impersonate
-GITHUB_ISSUES_URL = 'https://github.com/jtesta/ssh-audit/issues'  # The URL to the Github issues tracker.
+# The version to display.
+VERSION = 'v2.6.0'
+
+# SSH software to impersonate
+SSH_HEADER = 'SSH-{0}-OpenSSH_8.2'
+
+# The URL to the Github issues tracker.
+GITHUB_ISSUES_URL = 'https://github.com/jtesta/ssh-audit/issues'
+
+# The man page.  Only filled in on Windows systems.
 WINDOWS_MAN_PAGE = ''
+
+# True when installed from a Snap package, otherwise False.
+SNAP_PACKAGE = False
+
+# Error message when installed as a Snap package and a file access fails.
+SNAP_PERMISSIONS_ERROR = 'Error while accessing file.  It appears that ssh-audit was installed as a Snap package.  In that case, there are two options:  1.) only try to read & write files in the $HOME/snap/ssh-audit/common/ directory, or 2.) grant permissions to read & write files in $HOME using the following command: "sudo snap connect ssh-audit:home :home"'

src/ssh_audit/policy.py

@@ -30,6 +30,7 @@ from datetime import date
 from ssh_audit import exitcodes
 from ssh_audit.ssh2_kex import SSH2_Kex  # pylint: disable=unused-import
 from ssh_audit.banner import Banner  # pylint: disable=unused-import
+from ssh_audit.globals import SNAP_PACKAGE, SNAP_PERMISSIONS_ERROR
 
 
 # Validates policy files and performs policy testing
@@ -122,6 +123,13 @@ class Policy:
             except FileNotFoundError:
                 print("Error: policy file not found: %s" % policy_file)
                 sys.exit(exitcodes.UNKNOWN_ERROR)
+            except PermissionError as e:
+                # If installed as a Snap package, print a more useful message with potential work-arounds.
+                if SNAP_PACKAGE:
+                    print(SNAP_PERMISSIONS_ERROR)
+                else:
+                    print("Error: insufficient permissions: %s" % str(e))
+                sys.exit(exitcodes.UNKNOWN_ERROR)
 
         lines = []
         if policy_data is not None:

src/ssh_audit/ssh_audit.py

@@ -36,6 +36,8 @@ import traceback
 from typing import Dict, List, Set, Sequence, Tuple, Iterable  # noqa: F401
 from typing import Callable, Optional, Union, Any  # noqa: F401
 
+from ssh_audit.globals import SNAP_PACKAGE
+from ssh_audit.globals import SNAP_PERMISSIONS_ERROR
 from ssh_audit.globals import VERSION
 from ssh_audit.globals import WINDOWS_MAN_PAGE
 from ssh_audit.algorithm import Algorithm
@@ -560,18 +562,27 @@ def make_policy(aconf: AuditConf, banner: Optional['Banner'], kex: Optional['SSH
     if aconf.policy_file is None:
         raise RuntimeError('Internal error: cannot write policy file since filename is None!')
 
-    # Open with mode 'x' (creates the file, or fails if it already exist).
-    succeeded = True
+    succeeded = False
+    err = ''
     try:
+        # Open with mode 'x' (creates the file, or fails if it already exist).
         with open(aconf.policy_file, 'x', encoding='utf-8') as f:
             f.write(policy_data)
+        succeeded = True
     except FileExistsError:
-        succeeded = False
+        err = "Error: file already exists: %s" % aconf.policy_file
+    except PermissionError as e:
+        # If installed as a Snap package, print a more useful message with potential work-arounds.
+        if SNAP_PACKAGE:
+            print(SNAP_PERMISSIONS_ERROR)
+            sys.exit(exitcodes.UNKNOWN_ERROR)
+        else:
+            err = "Error: insufficient permissions: %s" % str(e)
 
     if succeeded:
         print("Wrote policy to %s.  Customize as necessary, then run a policy scan with -P option." % aconf.policy_file)
     else:
-        print("Error: file already exists: %s" % aconf.policy_file)
+        print(err)
 
 
 def process_commandline(out: OutputBuffer, args: List[str], usage_cb: Callable[..., None]) -> 'AuditConf':  # pylint: disable=too-many-statements
@@ -681,8 +692,16 @@ def process_commandline(out: OutputBuffer, args: List[str], usage_cb: Callable[.
 
     # If a file containing a list of targets was given, read it.
     if aconf.target_file is not None:
+        try:
             with open(aconf.target_file, 'r', encoding='utf-8') as f:
                 aconf.target_list = f.readlines()
+        except PermissionError as e:
+            # If installed as a Snap package, print a more useful message with potential work-arounds.
+            if SNAP_PACKAGE:
+                print(SNAP_PERMISSIONS_ERROR)
+            else:
+                print("Error: insufficient permissions: %s" % str(e))
+            sys.exit(exitcodes.UNKNOWN_ERROR)
 
         # Strip out whitespace from each line in target file, and skip empty lines.
         aconf.target_list = [target.strip() for target in aconf.target_list if target not in ("", "\n")]