Every visit to a site such as google.com starts with resolving the domain name or FQDN to an IP address. And this resolution is done by a dns service. The domain name to IP address(A record in dns terminology) mapping is cached by both dns servers and client as most domain names do not change IPs that often.

Sometimes though you might know that the A record or IP address of a domain has changed and yet your local cache is holding the old IP. Before clearing the cache, you can view the contents of the dns cache by sending a USR1 signal to systemd-resolved

sudo killall -USR1 systemd-resolved

This will dump the contents of dns cache and name servers to systemd log, which you can view with journalctl command –

sudo journalctl -u systemd-resolved

As the bottom of the log, you should see the CACHE entries –

Oct 30 22:53:04 hidmo systemd-resolved[23811]: CACHE:
Oct 30 22:53:04 hidmo systemd-resolved[23811]:         csi.gstatic.com IN A 209.85.202.120
Oct 30 22:53:04 hidmo systemd-resolved[23811]:         csi.gstatic.com IN A 209.85.202.94
Oct 30 22:53:04 hidmo systemd-resolved[23811]:         connectivity-check.ubuntu.com IN A 35.222.85.5
Oct 30 22:53:04 hidmo systemd-resolved[23811]:         connectivity-check.ubuntu.com IN A 35.224.99.156

....

TLTR; systemd-resolve –flush-caches

systemd-resolve is a CLI tool for resolving domain names, IPv4 and IPv6 addresses, DNS records and services.

It also provides dns resolution statistics, settings and ability to flush cache. Before flushing cache, check the cache size and hit/miss statistics. Additional information such as transactions count is also reported.

$ sudo systemd-resolve --statistics
DNSSEC supported by current servers: no

Transactions                
Current Transactions: 0     
  Total Transactions: 105240
                            
Cache                       
  Current Cache Size: 15     
          Cache Hits: 50425 
        Cache Misses: 66235 
                            
DNSSEC Verdicts             
              Secure: 0     
            Insecure: 0     
               Bogus: 0     
       Indeterminate: 0     

As you can see above, the cache size is 15. In order to clear or flush the dns cache, run below command –

systemd-resolve --flush-caches

Running systemd-resolve --statistics should show a current cache size of 0.

Linux – view HTTP header response using curl, httpie, GET, nmap

Most users are interested in the content they receive when they visit a web site. There is an extra information web clients and servers exchange – HTTP headers. HTTP headers let the client and the server pass additional information with an HTTP request or response.

So how do we view the HTTP response from a remove web server? There are several tools for these

1. Curl : use ‘-I’ flag

   -I, --head
          (HTTP FTP FILE) Fetch the headers only! HTTP-servers feature the command HEAD which this uses to get nothing but  the  header  of  a
          document. When used on an FTP or FILE file, curl displays the file size and last modification time only.

$ curl -I google.com
HTTP/1.1 301 Moved Permanently
Location: http://www.google.com/
Content-Type: text/html; charset=UTF-8
Date: Sun, 02 Aug 2020 13:48:01 GMT
Expires: Tue, 01 Sep 2020 13:48:01 GMT
Cache-Control: public, max-age=2592000
Server: gws
Content-Length: 219
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN

2. httpie : Use ‘-h h’ flag

--print WHAT, -p WHAT
       String specifying what the output should contain:
      'H' request headers
      'B' request body
      'h' response headers
      'b' response body

$ http www.google.com --print h
 HTTP/1.1 200 OK
 Cache-Control: private, max-age=0
 Content-Encoding: gzip
 Content-Length: 5256
 Content-Type: text/html; charset=ISO-8859-1
 Date: Sun, 02 Aug 2020 13:50:50 GMT
 Expires: -1
 P3P: CP="This is not a P3P policy! See g.co/p3phelp for more info."
 Server: gws
 Set-Cookie: 1P_JAR=2020-08-02-13; expires=Tue, 01-Sep-2020 13:50:50 GMT; path=/; domain=.google.com; Secure
 Set-Cookie: NID=TRUNCATED; expires=Mon, 01-Feb-2021 13:50:50 GMT; path=/; domain=.google.com; HttpOnly
 X-Frame-Options: SAMEORIGIN
 X-XSS-Protection: 0

3. GET – lwp-request : ‘Ed’ flag

In many Linux distros, GET is an alias for lwp-request. It gives way more detailed information in the response header, including SSL parameters.

  -E  Print response status chain with full response headers.

  -d  Do not print the content of the response.

$ GET linux.com -Ed
GET http://linux.com
User-Agent: lwp-request/6.31 libwww-perl/6.31

301 Moved Permanently
Cache-Control: public, max-age=86400
Connection: close
Date: Sun, 02 Aug 2020 13:56:35 GMT
Via: 1.1 varnish
Accept-Ranges: bytes
Age: 43368
Location: https://linux.com/
Server: nginx
Vary: Cookie, Cookie
Content-Length: 162
Content-Type: text/html
Client-Date: Sun, 02 Aug 2020 13:56:35 GMT
Client-Peer: REDACTED
Client-Response-Num: 1
Title: 301 Moved Permanently
X-Cache: HIT, HIT
X-Cache-Hits: 1, 1
X-Pantheon-Styx-Hostname: styx-fe3-a-745747b57-x7rhq
X-Served-By: cache-mdw17324-MDW, cache-fty21379-FTY
X-Styx-Req-Id: 01697a62-d463-11ea-a64f-aabcb0e0cfdc
X-Timer: S1596376596.936127,VS0,VE1

GET https://linux.com/
User-Agent: lwp-request/6.31 libwww-perl/6.31

301 Moved Permanently
Cache-Control: public, max-age=86400
Connection: close
Date: Sun, 02 Aug 2020 13:56:36 GMT
Via: 1.1 varnish
Accept-Ranges: bytes
Age: 43368
Location: https://www.linux.com/
Server: nginx
Vary: Cookie, Cookie
Content-Length: 0
Content-Type: text/html; charset=UTF-8
Client-Date: Sun, 02 Aug 2020 13:56:36 GMT
Client-Peer: REDACTED
Client-Response-Num: 1
Client-SSL-Cert-Issuer: /C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
Client-SSL-Cert-Subject: /CN=linux.com
Client-SSL-Cipher: ECDHE-RSA-AES128-GCM-SHA256
Client-SSL-Socket-Class: IO::Socket::SSL
Strict-Transport-Security: max-age=300
X-Cache: HIT, HIT
X-Cache-Hits: 1, 1
X-Pantheon-Styx-Hostname: styx-fe3-b-64d9844f89-tc7zl
X-Served-By: cache-mdw17340-MDW, cache-pdk17820-PDK
X-Styx-Req-Id: 01bf3709-d463-11ea-baef-1ede833e594e
X-Timer: S1596376596.065153,VS0,VE1

GET https://www.linux.com/
User-Agent: lwp-request/6.31 libwww-perl/6.31

200 OK
Cache-Control: public, max-age=1800
Connection: close
Date: Sun, 02 Aug 2020 13:56:36 GMT
Via: 1.1 varnish
Accept-Ranges: bytes
Age: 1659
Server: nginx
Vary: Accept-Encoding, Cookie, Cookie
Content-Length: 126289
Content-Type: text/html; charset=UTF-8
Client-Date: Sun, 02 Aug 2020 13:56:36 GMT
Client-Peer: REDACTED
Client-Response-Num: 1
Client-SSL-Cert-Issuer: /C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
Client-SSL-Cert-Subject: /CN=linux.com
Client-SSL-Cipher: ECDHE-RSA-AES128-GCM-SHA256
Client-SSL-Socket-Class: IO::Socket::SSL
Link: <https://www.linux.com/wp-json/>; rel="https://api.w.org/"
Link: <https://www.linux.com/>; rel=shortlink
Strict-Transport-Security: max-age=300
Title: Linux.com - News For Open Source Professionals
X-Cache: HIT, MISS
X-Cache-Hits: 5, 0
X-Meta-Charset: UTF-8
X-Meta-Description: Linux.com is the go-to resource for open source professionals to learn about the latest in Linux and open source technology, careers, best practices, and industry trends. Get news, information, and tutorials to help advance your next project or career – or just to simply stay informed.
X-Meta-Generator: WordPress 5.4.2
X-Meta-Twitter-Card: summary_large_image
X-Meta-Twitter-Description: Linux.com is the go-to resource for open source professionals to learn about the latest in Linux and open source technology, careers, best practices, and industry trends. Get news, information, and tutorials to help advance your next project or career – or just to simply stay informed.
X-Meta-Twitter-Image: https://www.linux.com/wp-content/uploads/2019/08/ldc_social.jpg
X-Meta-Twitter-Title: Linux.com - News For Open Source Professionals
X-Meta-Viewport: width=device-width, initial-scale=1.0
X-Pantheon-Styx-Hostname: styx-fe3-a-745747b57-mfmk7
X-Served-By: cache-mdw17340-MDW, cache-pdk17866-PDK
X-Styx-Req-Id: 1df2da1b-d4c4-11ea-84e9-925461917092
X-Timer: S1596376596.261327,VS0,VE18

Nmap : –script=http-headers flag

Nmap is a network discovery tool but it can be used for scanning http headers as well. The port number has to be specified, otherwise nmap will scan several common ports.

$ nmap --script=http-headers google.com -p 80

Starting Nmap 7.60 ( https://nmap.org ) at 2020-08-02 10:00 PDT
Nmap scan report for google.com (172.217.15.110)
Host is up (0.026s latency).
rDNS record for 172.217.15.110: iad30s21-in-f14.1e100.net

PORT   STATE SERVICE
80/tcp open  http
| http-headers: 
|   Location: http://www.google.com/
|   Content-Type: text/html; charset=UTF-8
|   Date: Sun, 02 Aug 2020 14:00:10 GMT
|   Expires: Tue, 01 Sep 2020 14:00:10 GMT
|   Cache-Control: public, max-age=2592000
|   Server: gws
|   Content-Length: 219
|   X-XSS-Protection: 0
|   X-Frame-Options: SAMEORIGIN
|   Connection: close
|   
|_  (Request type: GET)

Nmap done: 1 IP address (1 host up) scanned in 0.69 seconds



$ nmap --script=http-headers google.com -p 443

Starting Nmap 7.60 ( https://nmap.org ) at 2020-08-02 10:00 PDT
Nmap scan report for google.com (172.217.15.110)
Host is up (0.027s latency).
rDNS record for 172.217.15.110: iad30s21-in-f14.1e100.net

PORT    STATE SERVICE
443/tcp open  https
| http-headers: 
|   Location: https://www.google.com/
|   Content-Type: text/html; charset=UTF-8
|   Date: Sun, 02 Aug 2020 14:00:13 GMT
|   Expires: Tue, 01 Sep 2020 14:00:13 GMT
|   Cache-Control: public, max-age=2592000
|   Server: gws
|   Content-Length: 220
|   X-XSS-Protection: 0
|   X-Frame-Options: SAMEORIGIN
|   Alt-Svc: h3-29=":443"; ma=2592000,h3-27=":443"; ma=2592000,h3-T050=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"
|   Connection: close
|   
|_  (Request type: GET)

Nmap done: 1 IP address (1 host up) scanned in 0.74 seconds

Deleting files with rm and getting
rm: invalid option –Try ‘rm –help’ for more information.

In Linux, trying to delete a file its name starts with dash(‘-‘) or double dashes(‘–‘) will fails, as dash or ‘-‘ is interpreted as an option by the rm and most linux commands.

Here are two files with filenames starting with ‘-‘ and ‘–‘ and the typical rm command deletion attempt fails with an error –

$ ls -1
-tempfile1
--tempfile2

$ rm -tempfile1
rm: invalid option -- 't'
Try 'rm ./-tempfile1' to remove the file '-tempfile1'.
Try 'rm --help' for more information.

$ rm --tempfile2
rm: unrecognized option '--tempfile2'
Try 'rm ./--tempfile2' to remove the file '--tempfile2'.
Try 'rm --help' for more information.

There are several ways of addressing this – you can precede the file name with ./ OR pass double dash after rm to end all option processing.

$ ls
-tempfile1  --tempfile2

$ rm -- -tempfile1

$ mv -- --tempfile2 tempfile2

$ rm ./-tempfile1

References –

http://www.gnu.org/software/coreutils/faq/coreutils-faq.html#How-do-I-remove-files-that-start-with-a-dash_003f

Creating a text file in Linux

In Unix, everything is a file. In this particular case though we will be demonstrating how to create a text file. Of course, these are some of the many ways of creating a file

touch

Just touch it! – touch command followed by some file name and a file will magically appear.

$ ls linuxfreelancer
ls: cannot access 'linuxfreelancer': No such file or directory

$ touch linuxfreelancer

$ $ ls -l linuxfreelancer 
-rw-rw-r-- 1 daniel daniel 0 Feb  8 16:53 linuxfreelancer

vi

vi or any text editor. In fact, any process which writes to a file.

$ vi linuxfreelancer

Save with “:wq” in vi to save the file.

cat

Write multi-line text with “Here Document” syntax in bash.

$ cat <<EOF>linuxfreelancer
my blog is at https://www.linuxfreelancer.com
EOF

$ cat linuxfreelancer 
my blog is at https://www.linuxfreelancer.com
$

echo

The echo command with some redirection –

$ echo 'My blog is at https://linuxfreelancer.com' > linuxfreelancer

$ cat linuxfreelancer 
My blog is at https://linuxfreelancer.com

Redirection

You can redirect the output of any command to a new file

$ ps > ps.output
$ cat ps.output 
  PID TTY          TIME CMD
 2703 pts/0    00:00:00 bash
 3290 pts/0    00:00:00 ps

tee

$ tee linuxfreelancer
Writing for my blog linuxfreelancer.com
Writing for my blog linuxfreelancer.com
...Crl+X

$ cat linuxfreelancer 
Writing for my blog linuxfreelancer.com

$ 

Why does the tee command repeat what I typed? that is what it does – it reads from standard input and write to standard output and file at the same time.

How to exclude files from being added to docker image

TL;DR – use .dockerignore file, Docker’s equivalent of .gitignore for git

When building docker images, minimizing the size of the image is the goal. During building docker images with Dockerfile, especially with in a git repository, we might unintentionally add all the files into the docker image.

It is not uncommon to have something like “ADD . /app” in your Dockerfile. There are two ways to prevent this –

• Explicitly add only the files you need to Dockfile
• Use .dockerignore file

A typical .dockerignore files in a git repo directory might look like this –

daniel@hidmo:/tmp/myapp$ ls -al
total 56
drwxr-xr-x  4 daniel daniel 4096 Dec 17 17:51 .
drwxrwxrwt 20 root   root   4096 Dec 17 17:51 ..
-rwxr-xr-x  1 daniel daniel  338 Dec 17 17:41 build-image.sh
drwxr-xr-x  2 daniel daniel 4096 Dec 17 17:41 .cache
-rw-r--r--  1 daniel daniel  245 Dec 17 17:41 Dockerfile
-rw-r--r--  1 daniel daniel  102 Dec 17 17:51 .dockerignore
drwxr-xr-x  8 daniel daniel 4096 Dec 17 17:52 .git
-rw-r--r--  1 daniel daniel    6 Dec 17 17:51 .gitignore
-rw-r--r--  1 daniel daniel  133 Dec 17 17:41 README
-rw-r--r--  1 daniel daniel  181 Dec 17 17:41 requirements.txt
-rw-r--r--  1 daniel daniel 7871 Dec 17 17:41 web.py
-rw-r--r--  1 daniel daniel 7871 Dec 17 17:50 web.pyc
daniel@hidmo:/tmp/myapp$ cat .dockerignore 
# Exclude files from being added to docker image
.git
.gitignore
.cache
.pyc
Dockerfile
README
readme

References

https://docs.docker.com/engine/reference/builder/#dockerignore-file