Archive for the ‘OS’ Category

Everyday Ubuntu commands

February 4, 2020

Host IP:

hostname -I






ps -ef | grep whatever

Find the largest files and dirs:

sudo du -a / | sort -n -r | head -n 20

to be continued…

cron tricks

April 1, 2019

Edit current user’s cron:

crontab -e

Execute a script once your computer boots up:

@reboot ~/

Check cron log:

grep cron /var/log/syslog



Enabling Jenkins on Ubuntu to ssh to Bitbucket

November 27, 2018

On your workstation use

$ ssh-keygen

to generate a private-public keypair.

In your Bitbucket account Settings -> GENERAL -> Access keys add a key for Jenkins and copy content of a public one in it.

On your Jenkins machine, sign in as jenkins user and do:

$ nano ~/.ssh/id_jenkins_rsa

and copy content of a private key in there. Then:

$ nano .bash_profile

and copy there following:

[ -z "$SSH_AUTH_SOCK" ] && eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_jenkins_rsa

Save and exit. Make sure you have right permissions:

$ chmod 755 .bash_profile

Logout and log back in. Check if it works:

$ ssh -T

Should authenticate you via a deploy key. Then jenkins can use it like:

$ git clone

10 Cool Linux Shell Tricks

August 9, 2018

An extract from this article:

1. Send Output and Errors
command &> file

2. Parallelize Your Loops
for HOST in $(< ListOfHosts); do ssh $HOST ’sudo apt-get update’ & done

3. Catch Memory Leaks By Using Top via Cron
crontab – <<< ‘*/15 * * * * top -n 1 -b’

4. Stdin directly from the command line

5. Set a Random Initial Password, That Must be Changed
umask u=rw,go=
openssl rand -base64 6 | tee -a PasswordFile | passwd –stdin joe
chage -d 0 joe

6. Add Your Public Key to Remote Machines
ssh-copy-id -i .ssh/ hostname

7. Extract an RPM without any additional software
rpm -ivh –root /tmp/deleteme –nodeps –noscripts package.rpm

8. See How a File Has Changed from Factory Defaults
dpkg -S /etc/foo/foo.conf
rpm -qf /etc/foo/foo.conf
diff /etc/foo/foo.conf /tmp/deleteme/etc/foo/foo.conf

9. Undo Your Network Screwups After You’ve Lost the Connection
at now + 5 minutes <<< ‘cp /etc/ssh/sshd_config.old /etc/ssh/sshd_config; service sshd restart’

10. Check if SSH Port is Open
nc -w 3 server 22 ssh <<< ''

Multi hop SSH tunnel

August 6, 2018

There is a private API endpoint listening to port 9999 accessible from EC2 in a private subnet on AWS. Private subnet is accessible from a bastion host which is in a Public subnet and has a Public IP. Both hosts have different keipairs.

The task is to enable developers call API endpoint from their workstations.

Open local port forwarding:

ssh -vtA -i ec2.pem  -L9999:apiendpointIP:9999 ubuntu@ec2IP \
 -o 'ProxyCommand = ssh -vtA -i bastionhost.pem -L9999:localhost:9999 \
 ubuntu@bastionhostIP -W ec2IP:22'

Now developers can access the API endpoint:


The next step is to create a limited user and chroot it to jail.

Troubleshooting network connection

May 24, 2018

How to troubleshoot network connection between AWS Lambda, AWS EC2 through Application Load Balancer and outside world.

First try


returns empty.

Second try

curl --dump-header -

returns something more meaningful but a bit misleading:

HTTP/1.1 302 Found
 Date: Thu, 24 May 2018 01:15:46 GMT
 Content-Type: text/plain; charset=utf-8
 Content-Length: 0
 Connection: keep-alive
 Location: login_page

telnet connects though:

telnet thehost 443
Connected to
Escape character is '^]'.
telnet> status
Connected to
Operating in obsolete linemode
Local character echo
Escape character is '^]'.
Connection closed by foreign host

Third try

curl -svo /dev/null

gives us a hint with TLS info and the root of a problem below:

* Rebuilt URL to:
 * Trying
 * Connected to ( port 443 (#0)
 * found 148 certificates in /etc/ssl/certs/ca-certificates.crt
 * found 592 certificates in /etc/ssl/certs
 * ALPN, offering http/1.1
 * SSL connection using TLS1.2 / ECDHE_RSA_AES_128_GCM_SHA256
 * server certificate verification OK
 * server certificate status verification SKIPPED
 * common name: * (matched)
 * server certificate expiration date OK
 * server certificate activation date OK
 * certificate public key: RSA
 * certificate version: #3
 * subject: CN=*
 * start date: Thu, 22 Mar 2018 00:00:00 GMT
 * expire date: Mon, 22 Apr 2019 12:00:00 GMT
 * issuer: C=US,O=Amazon,OU=Server CA 1B,CN=Amazon
 * compression: NULL
 * ALPN, server accepted to use http/1.1
 > GET / HTTP/1.1
 > Host:
 > User-Agent: curl/7.47.0
 > Accept: */*
 < HTTP/1.1 502 Bad Gateway
 < Server: awselb/2.0
 < Date: Thu, 24 May 2018 01:25:00 GMT
 < Content-Type: text/html
 < Content-Length: 138
 < Connection: keep-alive

Referring to the doco, and here is the answer:

ALB does support HTTP/2, but only for HTTPS listeners. You will not be able to send plaintext HTTP/2 requests.

So, no traffic will go through HTTP/2 AWS ALB unless it’s on 443 port. Damn!




Tricks to try next time:

curl -L
wget -S -O /dev/null
wget -s -O /dev/null --header="Host:"
openssl s_client -connect
curl -v --http2

Another possible reason:

There were no “Content-Length” and “Transfer-Encoding” headers in the response, and the backend used keep-alive and didn’t close connection as ELB was expecting it to do. Placing Apache in between backend and ELB should solve this problem (I haven’t tested it yet).