Archive for the ‘ How tos ’ Category

Make http request with telnet

How to make an HTTP request with telnet


One of the most frequent interview question for tech professionals, especially system administrators and developers is – “tell us what happens when you type a URL in a browser?”. Skipping the DNS resolution part, we can understand the client to server HTTP communication with telnet. The simplest case is a GET request to a path with a HOST header.As an example, let us make an http request to an AWS service which responds back with our public IP address –

$ telnet checkip.amazonaws.com 80
....
GET / HTTP/1.1
Host: checkip.amazonaws.com
....

Here is the full transaction –


daniel@hidmo:~$ telnet checkip.amazonaws.com 80
Trying 18.214.132.216...
Connected to checkip.us-east-1.prod.check-ip.aws.a2z.com.
Escape character is '^]'.
GET / HTTP/1.1
Host: checkip.amazonaws.com

HTTP/1.1 200 OK
Date: Sat, 14 Sep 2019 12:51:55 GMT
Server: lighttpd/1.4.41
Content-Length: 14
Connection: keep-alive

162.247.79.245
Connection closed by foreign host.

Notice how the server closes the connection after waiting for a few seconds, that is because the keep-alive is enabled on the server side as shown from the server response – “Connection: keep-alive“. With keep-alive we can make additional http calls with out going through the whole 3-way TCP handshake.

Disable keep-alive on client side

If for some reason, we want to close the connection on the client side immediately we can pass “Connection: Close” as part of the http header in the request.


References –

Telnet manpage

https://stackoverflow.com/questions/15772355/how-to-send-an-http-request-using-telnet

https://www.ntu.edu.sg/home/ehchua/programming/webprogramming/HTTP_Basics.html

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Keep-Alive


In Linux, the find command is most commonly used to search files using different criteria such as file name, size and modified time. Did you know that you can search files using inode number as well? Here is how to do it?

With “ls” we can find the inode number –

$ ls -li /etc/hosts
1576843 -rw-r--r-- 1 root root 311 Jan 21  2017 /etc/hosts

Using “-inum” option of find command, we can locate the filename and its path by its inode number.

$ find /etc -type f -inum 1576843 2>/dev/null 
/etc/hosts

$ cat $(find /etc -type f -inum 1576843 2>/dev/null)
127.0.0.1	localhost
127.0.1.1	ubuntu

References

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

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

How to print the file system type of a mount


Linux supports several file systems, including VFAT, ext2, ext3, ext4 and Reiser. The ext* family of file systems are probably the most popular ones.

The quickest way to view the file system on which each FILE resides, or all file systems is the “df” command.

$ df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
udev           devtmpfs  481M  4.0K  481M   1% /dev
tmpfs          tmpfs      99M  1.2M   98M   2% /run
/dev/sda1      ext4       46G   32G   12G  73% /
none           tmpfs     4.0K     0  4.0K   0% /sys/fs/cgroup
none           tmpfs     5.0M     0  5.0M   0% /run/lock
none           tmpfs     494M   12K  494M   1% /run/shm
none           tmpfs     100M   36K  100M   1% /run/user
In the above example, with "df -Th", we can see the file system type
("-T" option) in a human readable ("-h") size format.

Reference

http://linuxcommand.org/lc3_man_pages/df1.html

There are several tools for compressing and decompressing files in Linux, you can get a summary of these tools in this link. Zip is one of the utilities used for packaging, compressing (archive) and decompressing files.

Installation

  • Ubuntu
sudo apt-get update
sudo apt-get install zip unzip
  • RedHat or CentOS
sudo yum install unzip

Compress files

Compress files in a directory named tutorial –

$ zip -r tutorial.zip tutorial/
   adding: tutorial/ (stored 0%)
   adding: tutorial/host.conf (deflated 13%)
   adding: tutorial/hostname (stored 0%)
   adding: tutorial/hosts.deny (deflated 44%)
   adding: tutorial/hosts (deflated 35%)
   adding: tutorial/hosts.allow (deflated 42%)
   adding: tutorial/auth_sa.py (deflated 52%)

View contents of zip files, without uncompressing –

$ zip -sf tutorial
 Archive contains:
   tutorial/
   tutorial/host.conf
   tutorial/hostname
   tutorial/hosts.deny
   tutorial/hosts
   tutorial/hosts.allow
   tutorial/auth_sa.py
 Total 7 entries (2487 bytes)

Unzip or decompress

To decompress a zipped file, use the unzip command –

 $ unzip tutorial.zip
Archive:  tutorial.zip
   creating: tutorial/
  inflating: tutorial/host.conf
 extracting: tutorial/hostname
  inflating: tutorial/hosts.deny
  inflating: tutorial/hosts
  inflating: tutorial/hosts.allow
  inflating: tutorial/auth_sa.py

Search and compress

You can also combine find and zip command to search for certain types of files and compress those files in one command –

 $ find . -type f -name '*.conf' -print | zip confi-files -@
  adding: host.conf (deflated 13%)
  adding: colord.conf (deflated 50%)
  adding: ntp.conf (deflated 56%)

$ zip -sf confi-files
Archive contains:
  host.conf
  colord.conf
  ntp.conf
Total 3 entries (1858 bytes)
References -
https://linux.die.net/man/1/zip

Contents of most text files change during the life of the file , and it is common to find yourself trying to search and replace certain text across multiple files. In Linux, this is a fairly easy task. Let us go through some of the commands you will need to perform this task and then finally construct a single liner to do the job.

  • grep is your best friend when it comes to finding a string in a file. In this case we are looking for the string “REPLACEME” in current directory and across multiple files –
$ grep -r REPLACEME *
host.conf:# The "REPLACEME" line is only used by old versions of the C library.
host.conf:order hosts,REPLACEME,bind
hostname:REPLACEME
hosts.deny:ALL: REPLACEME

If we are interested only in the files which contains this particular text –

$ grep -lr REPLACEME *
host.conf
hostname
hosts.deny
  • sed is a tool of choice for inline editing of files –
$ cat data 
This text will be replaced - REPLACEME
$ sed -i 's/REPLACEME/NEWTEXT/g' data 
$ cat data 
This text will be replaced - NEWTEXT

From here, there are multiple ways to skin the cat – we can loop through the files and do the replacement or we can let the commands do the replacement with a wildcard.

For loop style update -

$ for f in $(grep -lr REPLACEME *); do echo "*** File: ${f} ***" ; sed -i 's/REPLACEME/NEWTEXT/g' $f; done
*** File: host.conf ***
*** File: hostname ***
*** File: hosts.deny ***

$ grep -lr REPLACEME *

$ grep -lr NEWTEXT *
data
host.conf
hostname
hosts.deny

Actually the above for loop is redundant, sed can make changes across multiple files –

 sed -i 's/REPLACEME/NEWTEXT/g' *

How to install Google cloud platform(GCP) sdk – gcloud cli tool


The instructions below were testing in Ubuntu Linux.

gcloud is the command line interface(CLI) tool for interacting with GCP services. Per Google’s product overview page for gcloud – “The Cloud SDK is a set of tools for Cloud Platform. It contains gcloud, gsutil, and bq, which you can use to access Google Compute Engine, Google Cloud Storage, Google BigQuery, and other products and services from the command-line. You can run these tools interactively or in your automated scripts”.

Let us download, install and initialize this tool in an interactive manner, accept all default settings for all prompts –

$ curl https://sdk.cloud.google.com | bash && exec -l $SHELL
$ gcloud init
If above installation steps go well, check its version –
$ gcloud version
Google Cloud SDK 224.0.0
bq 2.0.36
core 2018.11.02
gsutil 4.34
 
A simple way to validate if the CLI is working as expected is to list all the GCP regions –
$ gcloud compute regions list
NAME                     CPUS  DISKS_GB  ADDRESSES  RESERVED_ADDRESSES  STATUS  TURNDOWN_DATE
asia-east1               0/8   0/2048    0/8        0/1                 UP
asia-east2               0/8   0/2048    0/8        0/1                 UP
asia-northeast1          0/8   0/2048    0/8        0/1                 UP
asia-south1              0/8   0/2048    0/8        0/1                 UP
asia-southeast1          0/8   0/2048    0/8        0/1                 UP
australia-southeast1     0/8   0/2048    0/8        0/1                 UP
europe-north1            0/8   0/2048    0/8        0/1                 UP
europe-west1             0/8   0/2048    0/8        0/1                 UP
europe-west2             0/8   0/2048    0/8        0/1                 UP
europe-west3             0/8   0/2048    0/8        0/1                 UP
europe-west4             0/8   0/2048    0/8        0/1                 UP
northamerica-northeast1  0/8   0/2048    0/8        0/1                 UP
southamerica-east1       0/8   0/2048    0/8        0/1                 UP
us-central1              0/8   0/2048    0/8        0/1                 UP
us-east1                 2/8   31/2048   2/8        0/1                 UP
us-east4                 0/8   0/2048    0/8        0/1                 UP
us-west1                 0/8   0/2048    0/8        0/1                 UP
us-west2                 0/8   0/2048    0/8        0/1                 UP

Only the core components of the gcloud sdk are installed during initial installation. For any additional component to interact with GCP, you have to install the additional component. For instance, to install the component for interactive with Google Kubernetes Engine(GKE) you have to install kubectl


gcloud components install kubectl

Many features of GCP are available in Beta only, for that you have to install the beta component –


gcloud components install beta

Stay up to date with  –

gcloud components update 

.

Tab completion and running commands in Beta feature –


$ gcloud beta container  [tab][tab]
binauthz  clusters  get-server-config  images  node-pools  operations  subnets

$ gcloud beta container get-server-config
Fetching server config for us-east1-c
defaultClusterVersion: 1.9.7-gke.7
defaultImageType: COS
validImageTypes:
- COS
- UBUNTU
- COS_CONTAINERD
validMasterVersions:
- 1.11.2-gke.15
- 1.10.9-gke.3
- 1.10.7-gke.9
- 1.10.6-gke.9
- 1.9.7-gke.7
validNodeVersions:
- 1.11.2-gke.15
- 1.11.2-gke.9
- 1.10.9-gke.3
- 1.10.9-gke.0
- 1.10.7-gke.9
- 1.10.7-gke.6
- 1.10.7-gke.2
- 1.10.7-gke.1
- 1.10.6-gke.9
- 1.10.6-gke.6
- 1.10.6-gke.4
- 1.10.6-gke.3
- 1.10.6-gke.2
- 1.10.6-gke.1
- 1.10.5-gke.4
- 1.10.5-gke.3
- 1.10.5-gke.2
- 1.10.5-gke.0
- 1.10.4-gke.3
- 1.10.4-gke.2
- 1.10.4-gke.0
- 1.10.2-gke.4
- 1.10.2-gke.3
- 1.10.2-gke.1
- 1.9.7-gke.7
- 1.9.7-gke.6
- 1.9.7-gke.5
- 1.9.7-gke.4
- 1.9.7-gke.3
- 1.9.7-gke.1
- 1.9.7-gke.0
- 1.9.6-gke.2
- 1.9.6-gke.1
- 1.9.3-gke.0
- 1.8.12-gke.3
- 1.8.12-gke.2
- 1.8.12-gke.1
- 1.8.12-gke.0
- 1.8.10-gke.2
- 1.8.10-gke.0
- 1.8.9-gke.1
- 1.8.8-gke.0
- 1.7.15-gke.0
- 1.7.12-gke.2
- 1.6.13-gke.1

Reference –

Installation – https://cloud.google.com/sdk/docs/downloads-interactive#linux

SDK Components – https://cloud.google.com/sdk/docs/components

Tips and Tricks – https://cloudplatform.googleblog.com/2014/03/tips-and-tricks-command-line-access-to.html