Category Archives: Ubuntu

Configuring TACACS+ Server on Ubuntu 14.04LTS

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It’s all change in the office so far this year, which is quite good as I’m involved in more projects, and who doesn’t enjoy a few projects 😉

The latest thing I was asked to look into was to create a new TACACS+ server as our current server on a HP Proliant BL460c G1 Blade is going to be decommissioned so we need to give it a new home! It was decided that it should be virtualized as there isn’t a need to have a physical server for something that can be slimmed down dramatically. With that being said this post will go over how to configure a TACACS+ server and configure TACACS+ authentication on a Juniper device.

TACACS+ is an improvement on its first version TACACS, as TACACS+ is an entirely new protocol and is not compatible with its predecessors, TACACS and XTACACS. TACACS+ uses TCP. Since TACACS+ uses the authentication, authorisation, and accounting (AAA) architecture, these separate components of the protocol can be segregated and handled on separate servers. TACACS+ allows you to set granular access policies for users and groups, commands, location, subnet, or even device type. The TACACS+ protocol also provides detailed logging of users and what commands have been run on specific devices. In addition, the protocol can run on either Windows or UNIX/Linux.

Although TACACS+ was developed by Cisco Systems, it is actually an open standard as defined by RFC1482 and has been incorporated into a number of different vendors including Alcatel/Lucent, Arbor, Brocade/Foundry, Cisco/Linksys, Extreme, HP/3Com, Huawei, IBM, Juniper/Netscreen, Netgear and any others.

The setup I had for testing was a simple one; I had 2 EXSi Ubuntu 14.04LTS hosts, one as the TACACS+ server with the second being used as Jump-box to access a Juniper SRX220 that will be configured for TACACS authentication.

With all that talk out of the way, let’s get cracking 🙂

You will run sudo/root privileges

Server Configuration

Fortunately, with the newer version of Ubuntu, from apt-get repository you can easily download the tacacs+ package it will also install libtacacs+1

[email protected]:~$ sudo apt-get install tacacs+
[sudo] password for marquk01: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
The following NEW packages will be installed
  libtacacs+1 tacacs+

Having installed the package now we can run the command ps -ef | grep tac_plus and it will show us the location of the configuration file and if the process is running:

[email protected]:~$ ps -ef | grep tac_plus
root      1220     1  0 11:37 ?        00:00:00 /usr/sbin/tac_plus -C /etc/tacacs+/tac_plus.conf
marquk01 22730  2682  0 13:55 pts/0    00:00:00 grep --color=auto tac_plus

As the process is running there’s a few useful binary files that are important to know, these can be seen when you type tac and hit TAB.

[email protected]:~$ tac
tac  tac_plus  tac_pwd

The important files are tac_plus and tac_pwd:

  • tac_plus is the TACACS+ daemon. You can run daemon via the cli
  • tac_pwd is used to generate a Data Encryption Standard (DES) or Message-Digest 5 (MD5) hash from clear text. DES is the defualt, to generate a MD5 hash you need to add -m flag.

We will need to configure the tac_plus.conf file, but firstly we will need to back-up the original file to refer back to if there is any issues

[email protected]:~$ sudo cp /etc/tacacs+/tac_plus.conf /etc/tacacs+/tac_plus.conf.old

I’ll explain from top-down of what my file looks like. The default file has more parameters than I used, as my file doesn’t need too much complexity. My example will also show you how to configure the basis Accounting, Secret Key, Users and Groups. Logically when I look at the layout of the file as I have, it doesn’t make sense… However, all the information is there soooooo it doesn’t matter :p lol


Firstly we’ll need to set the file that the accounting information will be written to. By default this is /var/log/tac_plus.acct, however you can have this file where you like if you don’t want you use the default file and path.

You have to create this file yourself. This can be done by running the command sudo touch /var/log/tac_plus.acct

# Created by Henry-Nicolas Tourneur([email protected])
# See man(5) tac_plus.conf for more details

# Define where to log accounting data, this is the default.

accounting file = /var/log/tac_plus.acct

Secret Key

The Server and Client need to have a matching key so the AAA packets can be encrypted. This key can be anything you wish however, if you’re going to have a key with white-space, key-words, or special characters, you’ll need to use quotation marks

# This is the key that clients have to use to access Tacacs+

key = testing123


You’ll need to define the users that will have access to the device. Each user needs to be associated to a group and have their password defined. The password has to be set as either a MD5 or DES hash. By using tac_pwd use can get your hashed output:

[email protected]:~$ tac_pwd
Password to be encrypted: lab123

There is an additional stanza service = junos-exec that defines an additional group. This is Juniper specific and I’ll explain this later. I created two users kmarquis; will have permission to do anything and second usertest; that will only have Read-Only access. Both have the same password. Usernames ARE case sensitive.

# We also can define local users and specify a file where data is stored.
# That file may be filled using tac_pwd
user = kmarquis {
    name = "Keeran Marquis"
    member = admin
    login = des kBeC6JDjU8icY
		service = junos-exec {
			local-user-name = remote-admin

user = test {
    name = "Test User"
    member = read-only
    login =  des kBeC6JDjU8icY
        service = junos-exec {
            local-user-name = remote-read-only


As you can guess, groups are where you define the level of access and what commands will be used by the group. The commands, for my example, are used to define actions that are largely accepted by most vendors with the expectation of Juniper (from my knowledge but correct me if I’m wrong), although I wont be confirming the configuration works in this post. I have checked with a Cisco device and they worked as expected.

We have a few parameters that are important remember:

  • default service: defines the default permission that the user will have. By default, if this statement isn’t used or left blank, it’s denied. Meaning that each permitted command users of this group will have to be listed. If you want the default permission to allow, then the statement permit is needed
  • service: define services which the group is authorised to execute, these could be commands that the group is authorised to execute. Authorisation must be configured on both the client and the daemon to operate correctly.
  • cmd: This is where you list a command and set an action, it will be either be a permit or deny. Additionally by having the .* this means that any command after the first word is affected. i.e my example below, all show commands will be permitted

In my example I have two groups, admin and read-only, the admin group will have full access permitted and the read-only group, as the name suggests, will have read-only access and will be denied from any configuration, clear or restart commands.

# We can also specify rules valid per group of users.
group = admin {
	default service = permit
	service = exec {
		priv-lvl = 15

group = read-only {
	service = exec {
		priv-lvl = 15
	cmd = show {
		permit .*
	cmd = write {
		permit term
	cmd = dir {
		permit .*
	cmd = admin {
		permit .*
	cmd = terminal {
		permit .*
	cmd = more {
		permit .*
	cmd = exit {
		permit .*
	cmd = logout {
		permit .*

My completed tac_plus file can be seen here.

For more in-depth detail and additional parameters that can be configured in this file, you can find them via the man pages using the command man tac_plus or online Ubuntu tac_plus Manual Documentation

Once you’re happy with everything you can run service tacacs_plus check to make sure the syntax is correct and if you get any errors you will need to restart the daemon using service tacacs_plus restart

TACACS+ Daemon Commands
Additional commands that will be useful to remember:

service tacacs_plus check
service tacacs_plus status
service tacacs_plus stop
service tacacs_plus start
service tacacs_plus restart

With that we have a TACACS+ server configured 🙂

Before getting into the configuration of the SRX, I stated earlier that there’s a Juniper Specific stanza in tac_plus.conf file. When authenticating users against a TACACS+ server on juniper devices and you’ll need to apply Juniper Networks Vendor-Specific TACACS+ Attributes.

These attributes can be either:

  1. Specified in the tac_plus.conf file by using regular expressions to list all the commands that the user has permitted or denied. A user will need to be created on the device with that user being referred under the local-user-name statement. The stanza would look something:
    service = junos-exec {
    	local-user-name = xxx
    	allow-commands =  .*
    	allow-configurations = .*
    	deny-commands = 
    	deny-configuration = 
    	user-permissions = 
  2. Configure a class that has states all the permitted or denied permissions, this class will be linked to a user. Both need to be configured on the device. Once this has been created you’ll need to refer, said user, under the local-user-name

The Junos OS retrieves these attributes through an authorization request of the TACACS+ server after authenticating a user. For my example, I went with the latter. Now we’ll jump onto the SRX220 and get that sorted with TACACS+ AAA configuration.

Juniper Configuration

Firstly, you will have to set the TACACS+ server with its secret key. For standard practice and force of habit, I have set the single connection and forced the source-address of the SRX. By using the single connection statement, this means that instead of multiple TCP sessions connecting to the device from a server, a single session is maintained between them. In addition, for best practice an authentication order should be set so that if there was an issue or loss of connectivity to the TACACS+ server, you’ll be able to fall back to locally defined users.

authentication-order [ tacplus password ];
tacplus-server { {
        secret "$9$SszyMXVb2aGiYgi.fzCAIEcyvWX7-w24"; ## SECRET-DATA

With the TACACS+ server we’re able log different events that take place on the device and get those commands sent to the server. From my experience the accounting events that you would most want logged are logins, configuration changes and interactive commands. This is set under system accounting stanza

accounting {
    events [ login change-log interactive-commands ];
    destination {

Next, under the system login stanza, you need to create a class that has a list of permission available to the user(s) that are going to be associated to it. The user(s) are what are used in the tac_plus.conf file. In my example I created two classes, one with all permission super-user-local and the other user with read-only and basic troubleshooting options (ie ping, traceroute, telnet etc) read-only-user-local. These associated this classes with 2 users remote-admin and remote-read-only

login {
    class read-only-user-local {
        permissions [ network view view-configuration ];
    class super-user-local {            
        permissions all;
    user remote {
        full-name "TACACS User";
        uid 2001;
        class super-user-local;
    user remote-read-only {
        full-name "TACACS read-only user";
        uid 2002;
        class read-only-user-local;
You can learn more about the different permissions flags available here on Juniper TechLibrary


To confirm the configuration is working as expected, I will ssh onto the SRX220 with both the admin user kmarquis and the read-only user test. With both users, I will log in and try to configure the description This is a test on a random port. As you can see below I had no problem with user kmarquis. However, when I logged in with the test user I wasn’t able to enter the configuration mode as the permission wasn’t granted, and for that user the command isn’t even recognized. I ran a show command and you will see that none of the passwords are shown. Again this is due to the permission level granted.

Admin AccessRead Only Access
[email protected]:~$ ssh -l kmarquis
--- JUNOS 12.1X47-D30.4 built 2015-11-13 14:16:02 UTC
[email protected]> configure 
Entering configuration mode
[email protected]# set interfaces ge-0/0/5 description "This is a test" 

[email protected]# commit and-quit 

[email protected]>
[email protected]:~$ ssh -l test
--- JUNOS 12.1X47-D30.4 built 2015-11-13 14:16:02 UTC
[email protected]> configure
unknown command.

[email protected]> show configuration 
## Last commit: 2016-02-01 12:56:23 UTC by kmarquis
version 12.1X47-D30.4;
system {
    host-name v6-testing;
    authentication-order [ tacplus password ];
    root-authentication {
        encrypted-password /* SECRET-DATA */; ## SECRET-DATA

If we check the /var/log/tac_plus.acct file we’ll be able to see all the permitted commands by each user. This is additional confirmation that the users have successfully authenticated against the TACACS+ server and their related permissions authorised to the device.

Feb  1 12:55:38      kmarquis        ttyp0      start   task_id=1       service=shell   process*mgd[38808]      cmd=login
Feb  1 12:55:41      kmarquis        ttyp0      stop    task_id=2       service=shell   process*mgd[38808]      cmd=show configuration 
Feb  1 12:55:44      kmarquis        ttyp0      stop    task_id=3       service=shell   process*mgd[38808]      cmd=edit 
Feb  1 12:56:01      kmarquis        ttyp0      stop    task_id=4       service=shell   process*mgd[38808]      cmd=set: [interfaces ge-0/0/5 de$
Feb  1 12:56:01      kmarquis        ttyp0      stop    task_id=5       service=shell   process*mgd[38808]      cmd=set interfaces ge-0/0/5 desc$
Feb  1 12:56:05      kmarquis        ttyp0      stop    task_id=6       service=shell   process*mgd[38808]      cmd=commit and-quit 
Feb  1 12:56:27      kmarquis        ttyp0      stop    task_id=7       service=shell   process*mgd[38808]      cmd=exit 
Feb  1 12:56:27      kmarquis        ttyp0      stop    task_id=1       service=shell   elapsed_time=49 process*mgd[38808]      cmd=logout
Feb  1 12:56:34      test    ttyp0      start   task_id=1       service=shell   process*mgd[38845]      cmd=login
Feb  1 12:56:44      test    ttyp0      stop    task_id=2       service=shell   process*mgd[38845]      cmd=show configuration 
Feb  1 12:56:53      test    ttyp0      stop    task_id=3       service=shell   process*mgd[38845]      cmd=show system uptime 
Feb  1 12:56:56      test    ttyp0      stop    task_id=4       service=shell   process*mgd[38845]      cmd=exit 
Feb  1 12:56:56      test    ttyp0      stop    task_id=1       service=shell   elapsed_time=22 process*mgd[38845]      cmd=logout

And with that all, we have a fully configured and working AAA TACACS+ server 🙂

Extra Treat 🙂
I have included the set commands below:

set system tacplus-server secret "$9$SszyMXVb2aGiYgi.fzCAIEcyvWX7-w24"
set system tacplus-server single-connection
set system tacplus-server source-address

set system authentication-order tacplus
set system authentication-order password

set system accounting events login
set system accounting events change-log
set system accounting events interactive-commands
set system accounting destination tacplus

set system login class super-user-local permissions all
set system login class read-only-user-local permissions network
set system login class read-only-user-local permissions view
set system login class read-only-user-local permissions view-configuration

set system login user remote-read-only full-name "TACACS read-only user"
set system login user remote-read-only uid 2005
set system login user remote-read-only class read-only-user-local
set system login user remote-admin full-name "TACACS User"
set system login user remote-admin uid 2006
set system login user remote-admin class super-user-local
Extra Extra Treat 😀
P.S. If you want to see what configuration could be used on a Cisco device I have added it below. Although I didn’t test it myself, this is the config we have in production and it works :p

aaa new-model
aaa authentication login default group tacacs+ local enable
aaa authorization exec default group tacacs+ local none 
aaa authorization commands 0 default group tacacs+ local none 
aaa authorization commands 1 default group tacacs+ local none 
aaa authorization commands 15 default group tacacs+ local none 
aaa accounting exec default start-stop group tacacs+
aaa accounting commands 0 default start-stop group tacacs+
aaa accounting commands 1 default start-stop group tacacs+
aaa accounting commands 15 default start-stop group tacacs+
aaa session-id common


Configure TACACS+ Ubuntu 14.04LTS
TACACS+ Accounting
TACACS+ Authenication
TACACS+ Advantages

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Configuring NTP Server on Ubuntu

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As part of a few changes happening I’ve been asked to look into how difficult would it be to configure our own local NTP server. From looking at very useful Ubuntu Man Pages and finding great articles on The Geek Stuff (one of my favourites sites) and Blogging Dragon, it appears that it’s more straightforward than I expected! Soooo this post will note down how to install and configure a NTP Server.

Network Time Protocol (NTP) is a network protocol for clock synchronization between servers, network devices and desktops. NTP is defined in RFC5905 and is described as:

Network Time Protocol version 4 (NTPv4), which is widely used to synchronize system clocks among a set of distributed timeservers and clients…. The NTP subnet model includes a number of widely accessible primary time servers synchronized by wire or radio to national standards. The purpose of the NTP protocol is to convey timekeeping information from these primary servers to secondary time servers and clients via both private networks and the public Internet.

For this test, I’m using ESXi Ubuntu 14.04LTS host as the local NTP server and will be configuring both a Juniper SRX220 and another ESXi Ubuntu 14.04LTS host as a NTP clients. The local NTP server’s IP address is

You will need root and/or sudo privileges

With all the background done, let’s get cracking 😀

As always, when getting anything from Ubuntu’s apt-get repository we’ll need to make sure to run update; to get all the newest version of packages currently installed and run dist-upgrade; to ensure the most important packages are updated as it has a “smart” conflict resolution system.

sudo apt-get update
sudo apt-get dist-upgrade

Having ensured the server’s packages are up to date, we can install the ntp and ntpdate packages by using apt-get install ntp ntpdate

sudo apt-get install ntp ntpdate

Before making any changes we need to make sure that the default time zone to Universal Time Coordinated (UTC). This is because UTC is regarded as the primary time standard by which the world regulates clocks and time. You can check/change the default time zone by running the command dpkg-reconfigure tzdata. You will be prompted to these screens where you can select the time zone: Screen #1 and Screen #2.

Once the time zone has been set, you will get the output below confirming the Time Zone:

[email protected]:~$ sudo dpkg-reconfigure tzdata

Current default time zone: 'Etc/UTC'
Local time is now:      Mon Jan 11 14:56:57 UTC 2016.
Universal Time is now:  Mon Jan 11 14:56:57 UTC 2016.

Next create a backup of the ntp.conf file

sudo cp /etc/ntp.conf /etc/ntp.conf.old

Use a text editor (I prefer nano) to open up the ntp.conf file and find the following lines below:

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See for
# more information.

You can comment them out, delete, keep or replace the lines. The lines are specifying multiple servers to act as a timeserver, which is helpful when one of the timeservers fail. You can use regional pool ntp severs from’s Regional Pools. In my example, as I live in the UK, I’ve used the regional pool of servers closest to the UK

server iburst dynamic
server iburst dynamic
server iburst dynamic
server iburst dynamic

The sytax iburst and dynamic are optional commands but can be useful to have set, depending on your environment.

  • iburst: After every poll a burst of eight packets is sent instead of one. When the server is not responding, packets are sent at 16 seconds intervals. When the server responds, packets are sent every 2 seconds. This means that after reboot or restart ntp synchronizations are established quicker.
  • dynamic: This option tells NTP it can try a configured server again later if it’s unavailable at some point, which can be useful when the server doesn’t always have Internet connectivity.
You can see my full example ntp.conf file here

Having saved and close the updated ntp.conf file, we’ll need to restart the daemon running service ntp restart

[email protected]:~$ sudo service ntp restart
 * Stopping NTP server ntpd                 [ OK ] 
 * Starting NTP server ntpd                 [ OK ]

After restarting the daemon, the server will take around 10-15 minutes for NTP to synchronize with the timeservers and it will automatically set the system clock. By using the command ntpq -p we’ll be able to check the status of the NTP servers that we are synchronized with

[email protected]:~$  ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================    2 u   21   64    3    3.138    0.047   0.506
 resntp-a-vip.lo     3 u   22   64    3    1.723    2.038   0.526
 2a03:b0c0:1:d0:      3 u   19   64    3    2.870    1.632   0.337
 neon.trippett.o    2 u   26   64    3    2.928    1.863   0.473
 golem.canonical   2 u   24   64    3    9.211    3.418   0.387

The table below explains the different parameters from the ntpq -p output:

Parameters Function
Remote Specifics the hostname of the timeserver
Refid This is a 32-bit code identifying the particular reference clock.
St (Stratus) This indicates your physical GPS closeness to the timeserver. Anything under 3 is seen as good
When Number of Seconds passed since the last poll or time check
Poll This is the minimum interval between transmitted messages, in seconds as a power of two. For instance, a value of six indicates a minimum interval of 64 seconds.
Reach How well a clock can maintain a constant frequency.
Delay Provides the capability to launch a message to arrive at the reference clock at a specified time. Relative to a selected reference clock.
Offset The time difference between two clocks, relative to a selected reference clock. Represents the amount to adjust the local clock to bring it into correspondence with the reference clock.
Jitter Short-term variations in Frequency with components greater than 10 Hz. The estimated time error of the system clock measured as an exponential average of RMS time differences.

You will be able start, stop, restart and/or check ntp status by using these commands

service ntp status
service ntp start
service ntp stop
service ntp restart

And with that we have a NTP server configured!

Enabling NTP Client

For testing, I configured a Juniper SRX220 to be the NTP client. It’s quite straightforward to enable ntp on a SRX; you’ll need to set and commit the commands to below and with that you will have NTP enabled. Simple Right! 🙂

set system ntp server prefer
set system ntp server version 4

For verification of NTP on the SRX we can run show ntp associations, show ntp status and show system uptime

show ntp associationsshow system uptimeshow ntp status
[email protected]> show ntp associations no-resolve 
     remote           refid      st t when poll reach   delay   offset  marquk01
*     3 -    8   64    1    1.849    2.498   0.160
[email protected]> show system uptime 
Current time: 2016-01-11 10:15:27 UTC
System booted: 2016-01-08 10:02:36 UTC (3d 00:12 ago)
Protocols started: 2016-01-08 10:05:13 UTC (3d 00:10 ago)
Last configured: 2016-01-11 10:13:34 UTC (00:01:53 ago) by marquk01
10:15AM  up 3 days, 13 mins, 2 users, load averages: 0.05, 0.11, 0.04
[email protected]> show ntp status                     
status=0664 leap_none, sync_ntp, 6 events, event_peer/strat_chg,
version="ntpd 4.2.0-a Fri Nov 13 15:40:48 UTC 2015 (1)",
processor="octeon", system="JUNOS12.1X47-D30.4", leap=00, stratum=4,
precision=-17, rootdelay=13.781, rootdispersion=2.340, peer=31708,
reftime=da3dff68.c47f84ec  Mon, Jan 11 2016 10:16:08.767, poll=6,
clock=da3dff6b.8ee12a6f  Mon, Jan 11 2016 10:16:11.558, state=3,
offset=0.000, frequency=0.000, jitter=0.213, stability=0.000

And on the Ubuntu host, it’s exactly the same as I described above, but in the /etc/ntp.conf file you’ll need to set the server as your local NTP server

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See for
# more information.

server prefer iburst dynamic

We can then run ntpq -p to check the ntp server is the local server!

[email protected]:~$ ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
*  3 u   28   64    1    0.454    0.380   0.315

For more in-depth detailed information on how to use NTP pools see and the Ubuntu Man page for more detail on ntp.conf file.

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IPv6 and Junos – Firewall Filter (ACLs)

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For IPv6 testing I’ve been asked to do one of the more noddy things to test Firewall Filters; these are Stateless Firewall Filters and are what Cisco call Access Control Lists (ACL). Unlike Stateful Firewall Filters, Stateless Filters do not inspect traffic flows, pattern or keep a record of network connections, as such TCP streams and/or UDP communication. Instead, these filters evaluate packet contents statically against a set of packet-matching rules that either permit or deny packets transiting the switch.

Firewall Filter (ACL) is an important feature for a switch to have as it provides some (although limited) protection for devices and host directly connected. I said that this was one of the more noddy things as there’s only one difference between creating a Firewall Filter for IPv4 and IPv6. However, as they say, better to be safe than sorry! With all that said and done, this post will be to show how you’d configure and implement a Stateless Firewall Filter within Junos.

Let’s get cracking 🙂

I had a pretty simple topology, using a Juniper EX Series 4200 switch configured with two Layer-3 vlans, and I’ve set up two Ubuntu 14.04LTS ESXi host; 1 of the host will be configured as a webserver (km-vm1) and the other as a client trying to access the server (km-vm3)

Firewall Filter Topology

Firstly, I created all of the physical and logical connections were expected, by running show ipv6 neighbors and show lldp neighbors

IPv6 SubnetsServers
[email protected]> show ipv6 neighbors 
IPv6 Address                 Linklayer Address  State       Exp Rtr Secure Interface
2001:123:212:1::2            00:0c:29:fc:d5:de  stale       202 no  no      vlan.300       
2001:192:168:2::2            00:0c:29:4f:26:c5  stale       1197 no no      vlan.200
[email protected]> show lldp neighbors 
Local Interface    Parent Interface    Chassis Id          Port info          System Name
ge-0/0/0.0         -                   00:0c:29:4f:26:bb   eth1               km-vm1                          
xe-0/1/0.0         -                   00:0c:29:fc:d5:d4   eth1               km-vm3

The goal of this test is ensure that KM-VM3 can ONLY access KM-VM1 on TCP ports 80 and 443, as these are well-known and IANA defined ports for unsecured (HTTP) and secured (HTTPS) web traffic, and ICMP traffic (ie. ping and traceroute).

Before configuring the Firewall Filter I wanted to see what was accessible for KM-VM3, so I ran a very useful open source utility for network discovery and a security auditing tool called nmap , to produce a port scan of the webserver. From the output we can see that not only are HTTP and HTTPS accessible, but the Port 22 Secure Shell (SSH) is open. As KM-VM1 doesn’t have any firewalling configured on the server level via iptables, KM-VM3 could be used to try and hack KM-VM1 by attacking the SSH port to gain access to the server, which is never good!

Port ScanSSH access
[email protected]:~$ nmap -6 2001:192:168:2::2

Starting Nmap 6.40 ( ) at 2015-11-11 14:17 GMT
Nmap scan report for 2001:192:168:2::2
Host is up (0.0017s latency).
Not shown: 997 closed ports
22/tcp  open  ssh
80/tcp  open  http
443/tcp open  https
[email protected]:~$ ssh 2001:192:168:2::2
The authenticity of host '2001:192:168:2::2 (2001:192:168:2::2)' can't be established.
ECDSA key fingerprint is e3:e3:f7:91:c0:30:a3:02:f9:1f:fd:aa:b7:0d:9c:9d.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '2001:192:168:2::2' (ECDSA) to the list of known hosts.
[email protected]:192:168:2::2's password:

With that security risk in mind, lets add a Firewall Filter that will only allow access to ports 80, 443 and ICMP traffic. I created a prefix-list, webservers, that would list all the prefixes (IP addresses) for the webservers. Although in this example I’ll only have the one prefix, I was always taught to use prefix-lists for ease of configuration. This was configured under edit policy-options prefix-list stanza:

[email protected]# show policy-options       
prefix-list webservers {

Now, for the Filter you’ll need to be under firewall family inet6 filter stanza, (for an IPv4 filter; firewall family inet filter). You can name your filter anything however, it can’t be more than 64 characters and if you have any spaces you’ll need to use ” ” marks. The same goes with naming your rules, which Junos calls terms. The Filter must have at least one term and the term(s) must have from or then statement. The from and then statements provide the actions of the term.

As you can see below, the Firewall Filter is named ALLOW-HTTP/HTTPS and has 3 terms:

{master:0}[edit firewall family inet6 filter ALLOW-HTTP/HTTPS]
[email protected]# show 
term allow-http/https {
    from {
        source-address {
        destination-prefix-list {
        destination-port [ 80 443 ];
    then accept;
term allow-icmp {
    from {
        icmp-type [ echo-reply echo-request neighbor-advertisement packet-too-big destination-unreachable neighbor-solicit ];
    then accept;
term deny-all {
    then {
        count deny-all;

Lets break down each aspect of this Firewall Filter:

  • The first term allow-http/https states from source-address ::/0, for IPv6 is any address to the destination of the webservers, which has been defined in the prefix-list to ports 80 and 443 (HTTP and HTTPS) to then accept those packets
  • The second term allow-icmp states the different type of icmp packets that I want allowed and then those are accepted
  • The final term deny-all states that any other packets should be counted under deny-all, logged and discarded. By using the action discard, this will silently drop all packets without sending an ICMP reply back to the requestor
With Junos, it is important to remember that when creating a Firewall Filter:

  • They works as Top-Down List, so the order of your rules is very significant, because once a rule has been matched, any rules below WILL NOT be checked.
  • Additionally just like with Cisco Firewall Filters come with an Implicit Deny at the end. If any packets don’t match any of the previous terms then they will be dropped automatically. Although this Implicit Deny is there, best practice to add a deny-all term at the end any Firewall Filter or ACL.
  • Finally, you can have only one input and one output filter per interface however have as many terms as you like. You can find all the guidelines that come with Firewall Filters here on Juniper’s TechLibrary page

Having created the filter, it will an input filter, as it is configured to filter traffic coming into the switch. Additionally, it will be place on the outside-facing interface, in this example, that has KM-VM3 in (vlan.300). This is because with any Filter, ACL or Firewall Policy, you want to stop any unnecessary traffic traversing your network at the furthest possible point, which is normally the edge of your network.

So under interface vlan unit 300 family inet6 filter stanza the Firewall Filter is placed as an input filter:

{master:0}[edit interfaces vlan unit 300]
[email protected]# show 
family inet6 {
    filter {
        input ALLOW-HTTP/HTTPS;
    address 2001:123:212:1::1/64;

Having committed the configuration, if we go back onto KM-VM3 and do some testing we’ll be able to see the effect of the Firewall Filter. As we can see below, when the port scan was run again, only ports 80 and 433 are in an OPEN STATE and SSH port 22 isn’t shown at all now, and we’re able to ping. When we try SSH we get nothing, which shows that this filter is working as expected.

Port ScanICMP PingSSH access
[email protected]:~$ nmap -6 2001:192:168:2::2

Starting Nmap 6.40 ( ) at 2015-11-11 15:30 GMT
Nmap scan report for 2001:192:168:2::2
Host is up (0.00072s latency).
Not shown: 998 filtered ports
80/tcp  open  http
443/tcp open  https
[email protected]:~$ ping6 2001:192:168:2::2
PING 2001:192:168:2::2(2001:192:168:2::2) 56 data bytes
9 packets transmitted, 9 received, 0% packet loss, time 7999ms
rtt min/avg/max/mdev = 0.464/0.545/0.662/0.066 ms
[email protected]:~$ ssh 2001:192:168:2::2

For further verification, we can check the counter that was set under the deny-all term to see how many packets have been dropped. By running the command show firewall counter filter ALLOW-HTTP/HTTPS deny-all we’re able to see the counters at that time.

[email protected]> show firewall counter filter ALLOW-HTTP/HTTPS deny-all 

Filter: ALLOW-HTTP/HTTPS                                       
Name                                                Bytes              Packets
deny-all                                              490                    5

As I said, when I started this post, the method of applying a Firewall Filter is exactly same in IPv6 world as it is in IPv4, with the exception of the filter location. Firewall Filters are extremely important in giving protection to hosts and devices connected to the switch if a stateful firewall such as a Juniper SRX or Cisco ASA isn’t suitable and/or available in your network design.

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Useful tcpdump Commands

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Tcpdump is a network debugging tool that runs under the command line. It allows the user to intercept and display TCP/UDP/IP and other packets being transmitted or received over a network to which the computer is attached. Running tcpdump by it’s self will begin recording traffic that is seen on the wire printing the output to the screen.

I found this list by @r_paranoid on their website Rationally Paranoid. Very Very useful set of tcpdump commands that can assist with troubleshooting and/or when a packet capture is needed.

See the list of interfaces on which tcpdump can listen:

tcpdump -D

Listen on interface eth0:

tcpdump -i eth0

Listen on any available interface (cannot be done in promiscuous mode. Requires Linux kernel 2.2 or greater):

tcpdump -i any

Be verbose while capturing packets:

tcpdump -v

Be more verbose while capturing packets:

tcpdump -vv

Be very verbose while capturing packets:

tcpdump -vvv

Be verbose and print the data of each packet in both hex and ASCII, excluding the link level header:

tcpdump -v -X

Be verbose and print the data of each packet in both hex and ASCII, also including the link level header:

tcpdump -v -XX

Be less verbose (than the default) while capturing packets:

tcpdump -q

Limit the capture to 100 packets:

tcpdump -c 100

Record the packet capture to a file called capture.cap:

tcpdump -w capture.cap

Record the packet capture to a file called capture.cap but display on-screen how many packets have been captured in real-time:

tcpdump -v -w capture.cap

Display the packets of a file called capture.cap:

tcpdump -r capture.cap

Display the packets using maximum detail of a file called capture.cap:

tcpdump -vvv -r capture.cap

Display IP addresses and port numbers instead of domain and service names when capturing packets (note: on some systems you need to specify -nn to display port numbers):

tcpdump -n

Capture any packets where the destination host is Display IP addresses and port numbers:

tcpdump -n dst host

Capture any packets where the source host is Display IP addresses and port numbers:

tcpdump -n src host

Capture any packets where the source or destination host is Display IP addresses and port numbers:

tcpdump -n host

Capture any packets where the destination network is Display IP addresses and port numbers:

tcpdump -n dst net

Capture any packets where the source network is Display IP addresses and port numbers:

tcpdump -n src net

Capture any packets where the source or destination network is Display IP addresses and port numbers:

tcpdump -n net

Capture any packets where the destination port is 23. Display IP addresses and port numbers:

tcpdump -n dst port 23

Capture any packets where the destination port is is between 1 and 1023 inclusive. Display IP addresses and port numbers:

tcpdump -n dst portrange 1-1023

Capture only TCP packets where the destination port is is between 1 and 1023 inclusive. Display IP addresses and port numbers:

tcpdump -n tcp dst portrange 1-1023

Capture only UDP packets where the destination port is is between 1 and 1023 inclusive. Display IP addresses and port numbers:

tcpdump -n udp dst portrange 1-1023

Capture any packets with destination IP and destination port 23. Display IP addresses and port numbers:

tcpdump -n "dst host and dst port 23"

Capture any packets with destination IP and destination port 80 or 443. Display IP addresses and port numbers:

tcpdump -n "dst host and (dst port 80 or dst port 443)"

Capture any ICMP packets:

tcpdump -v icmp

Capture any ARP packets:

tcpdump -v arp

Capture either ICMP or ARP packets:

tcpdump -v "icmp or arp"

Capture any packets that are broadcast or multicast:

tcpdump -n "broadcast or multicast"

Capture 500 bytes of data for each packet rather than the default of 68 bytes:

tcpdump -s 500

Capture all bytes of data within the packet:

tcpdump -s 0

Additionally has a great man page on tcpdump commands

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SSH login with 2-Factor Authentication

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During the holiday time, I was discussing with a mate on ways I could make my server more secure and he said why don’t I have 2-Factor Authentication. Of course, I dismissed him as a crazy man saying you can do that on SSH! When I actually looked I saw it could be done and it is a common place to have it done as well. I found a super page that explains how 2-Factor Authentication all works! With this in mind, this post will show how you can enable a SSH server with 2-Factor Authentication.

As always, I’ll be using Ubuntu 14.04 LTS. Because I use Google Authenticator for other things, I was happy to see that you can install Google Authenticator’s time-based one-time password (TOTP) via the apt-get repository. To install 2-factor authentication with Google Authenticator, we’ll need the open-source Google Authenticator PAM module. PAM stands for Pluggable Authentication Modules (PAM) provide dynamic authentication support for applications and services in a Linux. Essentially, it’s a way to easily plug different forms of authentication into a Linux system.

Firstly you will need to have Google Authenticator or Authentication App installed on your phone before doing anything. Personally I use Google’s Authenticator, for iOS App Store, for Android Google Play. Microsoft has their own Authenticator App for Windows Phones.

With the Authenticator installed on your phone, next you will need to install the Google package. You will need to have root and/or sudo access to the server and apt-get libpam-google-authenticator

sudo apt-get install libpam-google-authenticator

With the Module installed, you can set up your users with their OTP token. Run the google-authenticator utility, once ran you will be asked a series of questions that you can answer however best for you environment.

[email protected]:~$ google-authenticator 

Do you want authentication tokens to be time-based (y/n) y|0&cht=qr&chl=otpauth://totp/[email protected]%3Fsecret%3DXYC73MOQV7SMPOSJ
Your new secret key is: XYC73MOQV7SMPOSJ
Your verification code is 194186
Your emergency scratch codes are:

Do you want me to update your "/home/marquk01/.google_authenticator" file (y/n) y

Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) y

By default, tokens are good for 30 seconds and in order to compensate for
possible time-skew between the client and the server, we allow an extra
token before and after the current time. If you experience problems with poor
time synchronization, you can increase the window from its default
size of 1:30min to about 4min. Do you want to do so (y/n) y

If the computer that you are logging into isn't hardened against brute-force
login attempts, you can enable rate-limiting for the authentication module.
By default, this limits attackers to no more than 3 login attempts every 30s.
Do you want to enable rate-limiting (y/n) y
[email protected]:~$ 
Important Notes
You will need to keep safe the Emergency Scratch codes, just in case you lose access or have an issue with your OTP token. Your secret key will be used on the Authenticator app to generate your verification code. You can either manual enter the code or you can use scan QR-code that is generated on the cli to your phone. This is what you should expect to see when you run the google-authenticator utility. Once that’s has been done you will you should get something like this on your app

Next we will need to activate Google Authenticator within the sshd daemon. Firstly you will need to edit /etc/pam.d/sshd file by adding following lines below:

[email protected]:~$ sudo nano /etc/pam.d/sshd 
# To allow Google Authenticator for 2 factor authentication 
auth required

Then you will need to edit the /etc/ssh/sshd_config file. Look for the ChallengeResponseAuthentication and ensure that this is yes

[email protected]:~$ sudo nano /etc/ssh/sshd_config 
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication yes

The full files should look something like this sshd and sshd_config

Now we need to restart the sshd daemon.

[email protected]:~$ sudo service ssh restart

Now that the ssh daemon has been restarted when you try and ssh back onto the server, you will be asked for your password and the OTP verification code

[[email protected] ~]$ ssh
Verification code: 

It also worked with Secure Copy Protocol (SCP), which allows transfer files via Secure Shell (SSH)

[[email protected] ~]$ scp bird.conf.oringial [email protected]:/home/marquk01
Verification code: 
bird.conf.oringial                            100% 6222     6.1KB/s   00:00
ALL Users will need to be configured to have 2-factor authentication before editing the ssh daemon. When I tried this the first time, I assumed it was pre-user enabled the everything to find out my main account was locked out… #GenuisAtWork! In addition, if you have a key-based authentication, they will take supersede 2-Factor Authentication and this will be ignored
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