ssh and snmp allow list

build_debian.sh

@@ -224,6 +224,9 @@ sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y do
 
 sudo mv $FILESYSTEM_ROOT/grub-pc-bin*.deb $FILESYSTEM_ROOT/$PLATFORM_DIR/x86_64-grub
 
+sudo dpkg --root=$FILESYSTEM_ROOT -i target/debs/libwrap0_*.deb || \
+    sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y install -f
+
 ## Disable kexec supported reboot which was installed by default
 sudo sed -i 's/LOAD_KEXEC=true/LOAD_KEXEC=false/' $FILESYSTEM_ROOT/etc/default/kexec
 

dockers/docker-snmp-sv2/Dockerfile.j2

@@ -43,6 +43,8 @@ RUN apt-get update && apt-get install -y libperl5.20 libpci3 libwrap0 \
 COPY ["start.sh", "/usr/bin/"]
 COPY ["supervisord.conf", "/etc/supervisor/conf.d/"]
 COPY ["*.j2", "/usr/share/sonic/templates/"]
+COPY ["snmpd-config-updater", "/usr/bin/snmpd-config-updater"]
+RUN chmod +x /usr/bin/snmpd-config-updater
 
 ## Although exposing ports is not needed for host net mode, keep it for possible bridge mode
 EXPOSE 161/udp 162/udp

dockers/docker-snmp-sv2/snmpd-config-updater

@@ -0,0 +1,132 @@
+#!/usr/bin/env python
+
+# Daemon that listens to updates about the source IP prefixes from which snmp access
+# is allowed. In case of change, it will update the snmp configuration file accordingly. 
+# Also, after a change, it will notify snmpd to re-read its config file (service reload).
+
+import os
+import re
+import sys
+import time
+import redis
+
+service="snmpd"
+config_file_path="/etc/snmp"
+redis_key="SNMP_ALLOW_LIST"   # the redis list we listen to
+subscription='__keyspace@0__:%s' % redis_key
+temporization_duration = 3 # how long we wait for changes to settle (ride out a bursts of changes in redis_key)
+fake_infinite = 9999 # How often we wake up when nothing is going on --get_message()'s timeout has no 'infinite' value
+# after these operations we may need to revisit existing ssh connections because they removed or modified existing entries
+delete_operations = ["lrem", "lpop", "rpop", "blpop", "brpop", "brpoplpush", "rpoplpush", "ltrim", "del", "lset"]
+
+r = redis.StrictRedis(host='localhost')
+p = r.pubsub()
+
+# If redis is not up yet, this can fail, so wait for redis to be available
+while True:
+   try:
+      p.subscribe(subscription)
+      break
+   except redis.exceptions.ConnectionError:
+     time.sleep(3)
+
+# We could loose contact with redis at a later stage, in which case we will exit with
+# return code -2 and supervisor will restart us, at which point we are back in the
+# while loop above waiting for redis to be ready.
+try:
+
+  # By default redis does enable events, so enable them
+  r.config_set("notify-keyspace-events", "KAE")
+
+
+  # To update the configuration file 
+  #
+  # Example config file for reference:
+  #   root@sonic:/# cat /etc/snmp/snmpd.conf
+  #   <...some snmp config, like udp port to use etc...>
+  #   rocommunity public 172.20.61.0/24
+  #   rocommunity public 172.20.60.0/24
+  #   rocommunity public 127.00.00.0/8
+  #   <...some more snmp config...>
+  #   root@sonic:/#
+  #
+  # snmpd.conf supports include file, like so:
+  #  includeFile /etc/snmp/community.conf
+  #  includeDir /etc/snmp/config.d
+  # which could make file massaging simpler, but even then we still deal with lines 
+  # that have shared "masters", since some other entity controls the community strings 
+  # part of that line.
+  # If other database attributes need to be written to the snmp config file, then
+  # it should be done by this daemon as well (sure, we could inotify on the file
+  # and correct it back, but that's glitchy).
+
+  def write_configuration_file(v):
+    filename="%s/%s.conf" % (config_file_path, service)
+    filename_tmp = filename + ".tmp"
+    f=open(filename, "r")
+    snmpd_config = f.read()
+    f.close()
+    f=open(filename_tmp, "w")
+    this_community = "not_a_community"
+    for l in snmpd_config.split('\n'):
+      m = re.match("^(..)community (\S+)", l)
+      if not m:
+          f.write(l)
+          f.write("\n")
+      else:
+        if not l.startswith(this_community): # already handled community (each community is duplicated per allow entry)
+           this_community="%scommunity %s" % (m.group(1), m.group(2))   
+           if len(v):
+             for value in v:
+               f.write("%s %s\n" % (this_community, value))
+           else:
+             f.write("%s\n" % this_community)
+    f.close()
+    os.rename(filename_tmp, filename)
+    os.system("kill -HUP $(pgrep snmpd) > /dev/null 2> /dev/null || :")
+
+  # write initial configuration
+  write_configuration_file(r.lrange(redis_key, 0, -1))
+
+  # listen for changes and rewrite configuration file if needed, after some temporization
+  #
+  # How those subscribed to messages look like, for reference:
+  #   {'pattern': None, 'type': 'subscribe', 'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 1L}
+  #   {'pattern': None, 'type': 'message',   'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 'rpush'}
+  #   {'pattern': None, 'type': 'message',   'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 'lpush'}
+  #   {'pattern': None, 'type': 'message',   'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 'lrem'}
+  #   {'pattern': None, 'type': 'message',   'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 'lset'}
+  #   {'pattern': None, 'type': 'message',   'channel': '__keyspace@0__:SNMP_PERMIT_LIST', 'data': 'del'}
+
+  select_timeout = fake_infinite
+  config_changed = False
+  while True:
+     try:
+       m = p.get_message(timeout=select_timeout)
+     except Exception:
+       sys.exit(-2)
+     # temporization: no change after 'timeout' seconds -> commit any accumulated changes
+     if not m and config_changed:
+        write_configuration_file(r.lrange(redis_key, 0, -1))
+        config_changed = False
+        select_timeout = fake_infinite
+     if m and m['type'] == "message":
+        if m['channel'] != subscription:
+           print "WTF: unexpected case"
+           continue
+        config_changed = True
+        select_timeout = temporization_duration
+        # some debugs for now
+        print "-------------------- config change: ",
+        if m["data"] in delete_operations:
+           print "DELETE"
+        else:
+           print ""
+        v = r.lrange(redis_key, 0, -1)
+        for value in v:
+           print value
+
+except redis.exceptions.ConnectionError as e:
+  sys.exit(-2)
+
+

dockers/docker-snmp-sv2/supervisord.conf

@@ -11,6 +11,15 @@ autorestart=false
 stdout_logfile=syslog
 stderr_logfile=syslog
 
+[program:snmpd-config-updater]
+command=/usr/bin/snmpd-config-updater
+priority=1
+autostart=true
+autorestart=unexpected
+startsecs=0
+stdout_logfile=syslog
+stderr_logfile=syslog
+
 [program:rsyslogd]
 command=/usr/sbin/rsyslogd -n
 priority=2

files/build_templates/sonic_debian_extension.j2

@@ -224,6 +224,18 @@ if [ "$image_type" = "aboot" ]; then
     sudo sed -i 's/udevadm settle/udevadm settle -E \/sys\/class\/net\/eth0/' $FILESYSTEM_ROOT/etc/init.d/networking
 fi
 
+# Service to update the sshd config file based on database changes
+sudo cp $IMAGE_CONFIGS/ssh/sshd-config-updater.service $FILESYSTEM_ROOT/etc/systemd/system
+sudo mkdir -p $FILESYSTEM_ROOT/etc/systemd/system/multi-user.target.wants
+cd $FILESYSTEM_ROOT/etc/systemd/system/multi-user.target.wants/
+sudo ln -s ../sshd-config-updater.service sshd-config-updater.service
+cd -
+sudo cp $IMAGE_CONFIGS/ssh/sshd-config-updater $FILESYSTEM_ROOT/usr/bin/
+sudo chmod +x $FILESYSTEM_ROOT/usr/bin/sshd-config-updater
+sudo cp $IMAGE_CONFIGS/ssh/sshd-clear-denied-sessions $FILESYSTEM_ROOT/usr/bin
+sudo chmod +x $FILESYSTEM_ROOT/usr/bin/sshd-clear-denied-sessions
+sudo cp src/libwrap/tcp-wrappers-7.6.q/tcpdmatch $FILESYSTEM_ROOT/usr/bin
+
 ## copy platform rc.local
 sudo cp $IMAGE_CONFIGS/platform/rc.local $FILESYSTEM_ROOT/etc/
 

files/image_config/ssh/sshd-clear-denied-sessions

@@ -0,0 +1,82 @@
+#!/usr/bin/env python
+
+"""
+This utility will find the ip addresses of all hosts that have connected to
+this device via ssh, then validate they are still in the list of allowed prefixes,
+and if not kill the ssh session with a SIGHUP.
+"""
+
+import os
+import re
+import subprocess
+
+# Run utmpdump, capture and return its output
+def run_utmpdump(_utmpFilename):
+   devnull = file("/dev/null", "w" )
+   p = subprocess.Popen(args=["utmpdump", _utmpFilename], stdout=subprocess.PIPE, stderr=devnull)
+   (stdout, stderr) = p.communicate()
+   rc = p.returncode
+   assert rc is not None # because p.communicate() should wait.
+   out = (stdout or '') + (stderr or '')
+   if rc:
+      e = SystemCommandError("%r: error code %d" % (" ".join(argv), rc))
+      e.error = rc
+      e.output = out
+      raise e
+   return stdout
+
+# Run utmpdump and parse its output into a list of dicts and return that
+def get_utmp_data(utmpFileName=None):
+   """Reads the specified utmp file.
+   Returns a list of dictionaries, one for each utmp entry.
+   All dictionary keys and values are strings
+   Values are right padded with spaces and may contain all
+   spaces if that utmp field is empty.
+   Dictionary keys:
+      "type":     See UTMP_TYPE_* above
+      "pid":      Process ID as a string
+      "tty":      TTY (line) name - device name of tty w/o "/dev/"
+      "tty4":     4 char abbreivated TTY (line) name
+      "user":     User ID
+      "host":     Hostname for remote login,
+                  kernel release for Run Level and Boot Time
+      "ipAddr":   IP Address
+      "time":     Time and date entry was made
+   See linux docs on utmp and utmpdemp for more info.
+   Example output from utmpdump:
+         pid     tty4   user       tty            host          ipAddr            time
+     [7] [22953] [/238] [myname  ] [pts/238     ] [example.com] [253.122.98.159 ] [Mon Dec 18 21:08:09 2017 PST]
+   """
+   if not utmpFileName:
+      utmpFileName = os.environ.get( "DEFAULT_UTMP_FILE", "/var/run/utmp" )
+   if not os.path.exists(utmpFileName):
+      return []
+   output = run_utmpdump(utmpFileName)
+   lines  = re.split("\n", output)
+   regExp = re.compile(
+      r"\[(?P<type>"   r"[^\]]*?)\s*\] \[(?P<pid>"  r"[^\]]*?)\s*\] " \
+      r"\[(?P<tty4>"   r"[^\]]*?)\s*\] \[(?P<user>" r"[^\]]*?)\s*\] " \
+      r"\[(?P<tty>"    r"[^\]]*?)\s*\] \[(?P<host>" r"[^\]]*?)\s*\] " \
+      r"\[(?P<ipAddr>" r"[^\]]*?)\s*\] \[(?P<time>" r"[^\]]*?)\s*\]"  )
+   entries = []
+   for line in lines:
+      m = regExp.match(line)
+      if not m:
+         # Skip header and any other lines we don't recognize
+         continue
+      entry = m.groupdict()
+      entries.append(entry)
+   return entries
+
+# Find the source ip addresses of all ssh sessions, verify they are still allowed, and if not kill the ssh session
+if __name__ == '__main__':
+   for e in get_utmp_data():
+     if e["host"] and e["ipAddr"] != "0.0.0.0": # entry is for a live connection
+        # check allowness
+        r = os.system('tcpdmatch sshd %s | grep "access.*granted" > /dev/null' % e["ipAddr"])
+        # print some debugs
+        print "From:", e["ipAddr"], "ssh pid:", e["pid"], "allowed" if r == 0 else "denied"
+        # if needed go for the kill
+        if r != 0:
+           os.system("kill -1 %s" % e["pid"])
+