How to share your terminal session with another user in real time.

Linux has a script command which is mainly used for ‘typescripting’ all output printed on terminal. Commands typed on a terminal and the resulting output can be written to a file for later retrieval.

One little known use of the script command is for sharing your terminal session with another user, this would particularly be useful for telecooperation say between a user and instructor. The instructor can lead the session by executing commands on the shell while the student would observe. Here is one way of doing this –

1. Instructor creates a named pipe using mkfifo

instructor@linubuvma:/$ mkfifo /tmp/shared-screen

instructor@linubuvma:/$ ls -al /tmp/shared-screen 
prw-rw-r-- 1 instructor instructor 0 Mar 31 00:08 /tmp/shared-screen

instructor@linubuvma:/$ script -f /tmp/shared-screen 

2. Student views the session in real time by reading the shared-screen file –

student@linubuvma:/tmp$ cat shared-screen
Script started on Fri 31 Mar 2017 12:09:42 AM EDT

As soon as the student runs the

cat shared-screen

command, the script command also gets started on the instructor’s session.

Whatever is typed on the instructor’s terminal will show up on the student’s screen and the student’s terminal will be restored as soon as the instructor exits or terminates the script session –

instructor@linubuvma:/$ free -m
             total       used       free     shared    buffers     cached
Mem:          3946       3572        374         40        288        996
-/+ buffers/cache:       2288       1658
Swap:         4092        195       3897
instructor@linubuvma:/$ exit
exit

Script done on Fri 31 Mar 2017 12:12:02 AM EDT
student@linubuvma:/tmp$

Note – the student’s screen will show the user id of the instructor at the bash prompt, as it is a replica of the instructors session. Once the instructor terminates the session, the student will get back to their original bash prompt.

References –

http://man7.org/linux/man-pages/man1/script.1.html

User administration: restricting access

1. With the chage command, an account expiration can be set. Once that date is reached, the user cannot log into the system interactively.
Let us run ‘chage’ interactively to set user’s account expiry –

[root@kauai /]# chage sshtest
Changing the aging information for sshtest
Enter the new value, or press ENTER for the default

	Minimum Password Age [0]: 
	Maximum Password Age [99999]: 
	Last Password Change (YYYY-MM-DD) [2015-11-04]: 
	Password Expiration Warning [7]: 
	Password Inactive [-1]: 
	Account Expiration Date (YYYY-MM-DD) [-1]: 2017-03-30

[root@kauai /]# chage -l sshtest
Last password change					: Nov 04, 2015
Password expires					: never
Password inactive					: never
Account expires						: Mar 30, 2017
Minimum number of days between password change		: 0
Maximum number of days between password change		: 99999
Number of days of warning before password expires	: 7

2. In addition to this, the usermod command can “lock” an account with the -L option. Say when a user is no longer with a company, the administrator may lock and expire an account with a single usermod command. The date must be given as the number of days since January 1, 1970. Setting the expiredate to 1 will immediately lock the account –

[student@serverX ~]$ sudo usermod -L -e 1 elvis

[student@serverX ~]$ sudo usermod -L elvis
[student@serverX ~]$ su - elvis
Password: elvis
su: Authentication failure

Locking the account prevents the user from authenticating with a password to the system. It is the recommended method of preventing access to an account by an employee who has left the company. If the employee returns, the account can later be unlocked with

usermod -U USERNAME

. If the account was also expired, be sure to also change the expiration date.

3. The nologin shell. Sometimes a user needs an account with a password to authenticate to a system, but does not need an interactive shell on the system.
For example, a mail server may require an account to store mail and a password for the user to authenticate with a mail client used to retrieve mail.
That user does not need to log directly into the system.

A common solution to this situation is to set the user’s login shell to /sbin/nologin. If the user attempts to log into the system directly,
the nologin “shell” will simply close the connection.

[root@serverX ~]# usermod -s /sbin/nologin student
[root@serverX ~]# su - student
Last login: Tue Feb  4 18:40:30 EST 2014 on pts/0
This account is currently not available.

References –

https://linux.die.net/man/1/chage
https://linux.die.net/man/1/chmod

Linux – Sort IPv4 addresses numerically

A novice user’s first attempt to sort a list of IP addresses would be to use ‘sort -n’, that is a numeric-sort option for sort command. Unfortunately, this will sort only the first quadrant of the IP address preceding the initial dot(‘.’). Definitely the GNU sort command does support sorting IPv4 addresses in numeric order, we just have to specify the right options.

Question to answer –

1. What is our delimiter for IPv4? dot.
2. What type of sorting? numeric.
3. How many fields? four.

Reading the man page for sort provides an option for each – 1) -t. 2) -n 3)-k
The third part might need clarification – since we have dot as a separator, the IP address will have four fields. We need to give sort a key specification (-k), with start and stop positions i.e to story by first quadrant(-k1,1), followed by second(-k2,2), followed by third(-k3,3) and finally by fourth(-k4,4).

The full command looks like this –

sort -t. -n -k1 -k2 -k3 -k4 /tmp/ipv4_file.txt

Let us use ForgeryPy to generate random Ipv4 addresses, we will write a simple python script to generate these random IPs to a file.

First install ForgeryPY –

pip install ForgeryPY

Script to generate IPv4 addresses –

$cat ipv4_generator.py

#!/usr/bin/env python

import forgery_py

uniq_ipv4=set()
for i in range(50):
    uniq_ipv4.add(forgery_py.internet.ip_v4())

with open('/tmp/ipv4_addresses.txt', 'w') as fp:
     for line in uniq_ipv4:
         fp.writelines(line+'\n')

Output –

daniel@linubuvma:/tmp$ cat /tmp/ipv4_addresses.txt
cat: /tmp/ipv4_addresses.txt: No such file or directory
daniel@linubuvma:/tmp$ python ipv4_generator.py
daniel@linubuvma:/tmp$ cat /tmp/ipv4_addresses.txt
222.21.147.97
187.234.9.45
144.101.36.131
31.192.196.59
24.16.131.84
8.52.22.181
17.40.228.224
58.164.169.156
234.78.147.45
254.150.145.225
167.111.243.3
168.168.248.227
68.104.225.196
55.138.152.3
223.30.151.183
235.245.57.76
226.122.222.107
176.199.0.130
13.68.133.125
14.157.155.254
11.155.170.92
249.0.112.141
228.209.60.62
246.130.20.235
113.17.65.20
120.76.166.133
81.191.49.37
17.226.209.151
81.184.136.140
9.172.35.65
129.205.96.54
181.130.8.142
21.78.73.162
5.216.102.88
91.140.115.96
134.140.243.193
177.148.152.60
175.37.63.212
60.175.123.112
176.250.114.170
54.62.22.255
182.78.64.216
238.92.143.140
181.206.65.80
11.139.192.62
38.158.146.36
241.236.161.184
30.223.32.242
233.107.53.70
36.222.68.164
daniel@linubuvma:/tmp$

Let us sort it –

daniel@linubuvma:/tmp$ sort -n -t. -k1,1 -k2,2 -k3,3 -k4,4 /tmp/ipv4_addresses.txt
5.216.102.88
8.52.22.181
9.172.35.65
11.139.192.62
11.155.170.92
13.68.133.125
14.157.155.254
17.40.228.224
17.226.209.151
21.78.73.162
24.16.131.84
30.223.32.242
31.192.196.59
36.222.68.164
38.158.146.36
54.62.22.255
55.138.152.3
58.164.169.156
60.175.123.112
68.104.225.196
81.184.136.140
81.191.49.37
91.140.115.96
113.17.65.20
120.76.166.133
129.205.96.54
134.140.243.193
144.101.36.131
167.111.243.3
168.168.248.227
175.37.63.212
176.199.0.130
176.250.114.170
177.148.152.60
181.130.8.142
181.206.65.80
182.78.64.216
187.234.9.45
222.21.147.97
223.30.151.183
226.122.222.107
228.209.60.62
233.107.53.70
234.78.147.45
235.245.57.76
238.92.143.140
241.236.161.184
246.130.20.235
249.0.112.141
254.150.145.225

Hope this help.

http://man7.org/linux/man-pages/man1/sort.1.html
https://pypi.python.org/pypi/ForgeryPy

How to be your own Certificate Authority(CA) with self signed certificates

This is a hands on tutorial on how you can setup your own Certificate Authority(CA) for internal network use. Once the CA certs are setup, you will generate certificate request(CSR) for your clients and sign them with your CA certs to create SSL certs for your internal network use. If you import your CA certs to your browser, you will be able to visit all internal sites using https without any browser warning, as long as the certs the your internal services are using are signed by your internal CA.

*Demo – Own CA for the home.net internal domain

1. Prepare certificate environment and default parameters to use when creating CSR –

# mkdir /etc/ssl/CA
# mkdir /etc/ssl/newcerts
# sh -c "echo '100000' > /etc/ssl/CA/serial"
# touch /etc/ssl/CA/index.txt

# cat /etc/ssl/openssl.cnf
 dir		= /etc/ssl		# Where everything is kept
 database	= $dir/CA/index.txt	# database index file.
 certificate	= $dir/certs/home_cacert.pem 	# The CA certificate
 serial		= $dir/CA/serial 		# The current serial number
 private_key	= $dir/private/home_cakey.pem  # The private key
 default_days	= 1825			# how long to certify for
 default_bits		= 2048
 countryName_default		= US
 stateOrProvinceName_default	= California
 0.organizationName_default	= Home Ltd

2. Create self signed root certificate and install the root certificate and key –

# openssl req -new -x509 -extensions v3_ca -keyout home_cakey.pem -out home_cacert.pem -days 3650
# mv home_cakey.pem /etc/ssl/private/
# mv home_cacert.pem /etc/ssl/certs/

3. Generate a CSR for the domain you want to issue a certificate –

# openssl genrsa -des3 -out home_server.key 2048
# openssl rsa -in home_server.key -out server.key.insecure
# mv server.key server.key.secure
# mv server.key.insecure server.key

4. Create the CSR now and generate a CA signed certificate –

# openssl req -new -key server.key -out server.csr
# openssl ca -in server.csr -config /etc/ssl/openssl.cnf

Directory structure after signing and issuing certificates –

# ls -l /etc/ssl/CA/
total 24
-rw-r--r-- 1 root root 444 Aug 29 18:20 index.txt
-rw-r--r-- 1 root root  21 Aug 29 18:20 index.txt.attr
-rw-r--r-- 1 root root  21 Aug 29 18:16 index.txt.attr.old
-rw-r--r-- 1 root root 328 Aug 29 18:18 index.txt.old
-rw-r--r-- 1 root root   7 Aug 29 18:20 serial
-rw-r--r-- 1 root root   7 Aug 29 18:19 serial.old

# ls -l /etc/ssl/newcerts/
total 32
-rw-r--r-- 1 root root 4612 Aug 29 16:24 100000.pem
-rw-r--r-- 1 root root 4613 Aug 29 16:51 100001.pem
-rw-r--r-- 1 root root 4574 Aug 29 17:50 100002.pem
-rw-r--r-- 1 root root 4619 Aug 29 18:20 100003.pem

# cat /etc/ssl/CA/index.txt
V	190828202443Z		100000	unknown	/C=US/ST=California/O=Home Ltd/OU=Home/CN=www.home.net/emailAddress=daniel@home.net
V	190828205127Z		100001	unknown	/C=US/ST=California/O=Home Ltd/OU=Home/CN=wiki.home.net/emailAddress=daniel@home.net
V	190828215006Z		100002	unknown	/C=US/ST=California/O=Home Ltd/CN=home.net/emailAddress=daniel@home.net
V	190828222038Z		100003	unknown	/C=US/ST=California/O=Home Ltd/OU=Home/CN=homevm.home.net/emailAddress=daniel@home.net

# cat /etc/ssl/CA/serial
10411A

Now that you have your certificate, in this example /etc/ssl/certs/home_cacert.pem, you can import it to your web client such as a web browser, LDAP client etc.

References –

https://help.ubuntu.com/12.04/serverguide/certificates-and-security.html

Server refused to allocate pty : pseudoterminal in use reached maximum allowed limit.

You are unlikely to encounter this error in most cases, as the default maximum number of pseudoterminal(pty) in a Linux environment is large enough for typical use cases. The error might occur though under either an admin lowering the pty limit or unusual high number of connections to the system, using ssh or GUI terminal. Under those circumstances, you will see the below error during ssh interaction –

$ssh daniel@192.168.10.103
daniel@192.168.10.103's password:
Server refused to allocate pty

GUI terminal error –

There was an error creating the child process for this terminal
getpt failed: No such file or directory

Per the man page –

” The Linux kernel imposes a limit on the number of available UNIX 98
pseudoterminals. In kernels up to and including 2.6.3, this limit is
configured at kernel compilation time (CONFIG_UNIX98_PTYS), and the
permitted number of pseudoterminals can be up to 2048, with a default
setting of 256. Since kernel 2.6.4, the limit is dynamically
adjustable via /proc/sys/kernel/pty/max, and a corresponding file,
/proc/sys/kernel/pty/nr, indicates how many pseudoterminals are
currently in use.
”

To resolve this, get a count of pty currently allocated using either of the below commands –

[root@kauai tmp]# sysctl kernel.pty.nr
kernel.pty.nr = 10

[root@kauai tmp]# cat /proc/sys/kernel/pty/nr 
10

You can list the allocated pts names –

# ps aux |grep -o -P '\s+pts/\d+\s+' |sort -u
 pts/0    
 pts/1    
 pts/2    
 pts/3    
 pts/4    
 pts/5    
 pts/6    
 pts/8    
 pts/9    

If the currently allocated count is closer or less than to the limit, which you can find using

/proc/sys/kernel/pty/max

, go ahead increase the max limit as follows, say to 4096 in this example –

sysctl -w kernel.pty.max=4096

References –

http://man7.org/linux/man-pages/man7/pty.7.html

AIDE (Advanced Intrusion Detection Environment) setup

AIDE is a host-based file and directory integrity checking tool, similar to Tripwire. It creates a snapshot of file details during initialization and stores them in a database. The files that AIDE monitors are user-defined rules, where the admin can specify which directories/files to keep an eye on. The snapshot is basically a message digest of the files/directories information returned by stat command. One AIDE is initialized, it can detect any changes in the future and alert the admin of such changes. AIDE can be configured to run on a scheduled based using cron jobs for instance.

Installation

yum list aide
yum install aide

Initialization

Create AIDE DB – stores snapshot of file or directory stats by scanning the monitored resources.

$ /usr/sbin/aide --init 
$ mv /var/lib/aide/aidb.db.new.gz /var/lib/aide/aide.db.gz

To minimize false positives – Set PRELINKING=no in /etc/sysconfig/prelink and run

 /usr/sbin/prelink -ua 

to restore the binaries to their prelinked state.

Scheduled integrity checks
Add a cron job to check file integrity, say every morning at 8 AM –

echo '0 8 * * * /usr/sbin/aide --check' >> /etc/crontab

Updating DB after making changes or verifying any changes reported during change –

$ aide -c aide.conf --update

References –

AIDE (Advanced Intrusion Detection Environment)