Archive for the ‘ Linux ’ Category


The net-tools set of packages had been deprecated years back, although the commands are still being in use. Tools such as netstat and ifconfig are part of the net-tools. The alternatives can be installed from iproute2 package.

Which Ubuntu package provides a file/command

daniel@hidmo:/tmp$ sudo dpkg -S $(which ss)
iproute2: /bin/ss

daniel@hidmo:/tmp$ sudo dpkg -S $(which netstat)
net-tools: /bin/netstat

daniel@hidmo:/tmp$ sudo dpkg -S $(which ifconfig)
net-tools: /sbin/ifconfig

daniel@hidmo:/tmp$ sudo dpkg -S $(which ip)
iproute2: /sbin/ip

Not all features of netstat can be replace with ss, but ss combined with ip can do the job.

There is lots of similarity between netstat and ss flags or options. Let us see how we can use ss to substitute for one of the most common uses of netstat – viewing TCP connections and their state, including the process name and ID associated with the socket.

Below list is for IPv4 only (-4 ) flag –

daniel@hidmo:/tmp$ sudo netstat -plant4
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      675/systemd-resolve
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      230810/cupsd
tcp        0      0 127.0.0.1:6379          0.0.0.0:*               LISTEN      853/redis-server 12
tcp        0      0 192.168.10.44:51328       74.6.143.25:443       ESTABLISHED 39005/chrome --type 
tcp        0      0 192.168.10.44:56610       74.6.143.25:5228      ESTABLISHED 39005/chrome --type 
tcp        0      0 192.168.10.44:57920       64.233.177.138:443      ESTABLISHED 39005/chrome --type


The equivalent ss comand is below –

daniel@hidmo:/tmp$ sudo ss -pant4
State          Recv-Q         Send-Q                 Local Address:Port                    Peer Address:Port         Process
LISTEN         0              4096                   127.0.0.53%lo:53                           0.0.0.0:*             users:(("systemd-resolve",pid=675,fd=13))
LISTEN         0              5                          127.0.0.1:631                          0.0.0.0:*             users:(("cupsd",pid=230810,fd=7))
LISTEN         0              511                        127.0.0.1:6379                         0.0.0.0:*             users:(("redis-server",pid=853,fd=6))
ESTAB          0              0                        192.168.10.44:51328                  74.6.143.25:443         users:(("chrome",pid=39005,fd=35))
ESTAB          0              0                        192.168.10.44:56610                  74.6.143.25:5228        users:(("chrome",pid=39005,fd=37))
ESTAB          0              0                        192.168.10.44:57920                 64.233.177.138:443         users:(("chrome",pid=39005,fd=32))

ss has very helpful filtering features, for instance we can filter by source or destination IP address or port and tcp states. In below example, we are looking for TCP connections in TIMEWAIT state to a an http or https port and destined to specific IP CIDR block –

daniel@hidmo:/tmp$ sudo ss -o state time-wait '( dport = :http or dport = :https )' dst 162.247.78.0/24
Netid             Recv-Q             Send-Q                           Local Address:Port                            Peer Address:Port              Process                               
tcp               0                  0                                  192.168.10.44:59318                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59312                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59322                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59328                           162.247.78.1:https              timer:(timewait,59sec,0)             
tcp               0                  0                                  192.168.10.44:59304                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59326                           162.247.78.1:https              timer:(timewait,59sec,0)             
tcp               0                  0                                  192.168.10.44:59320                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59306                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59334                           162.247.78.1:https              timer:(timewait,59sec,0)             
tcp               0                  0                                  192.168.10.44:59314                           162.247.78.1:https              timer:(timewait,58sec,0)             
tcp               0                  0                                  192.168.10.44:59308                           162.247.78.1:https              timer:(timewait,58sec,0)    


References –

https://www.redhat.com/sysadmin/ss-command

https://man7.org/linux/man-pages/man8/ss.8.html

https://linux.die.net/man/8/netstat


How to generate a sequence of numbers in Bash scripting

  • Use Bash 4’s brace expansion
$ echo {1..10}
1 2 3 4 5 6 7 8 9 10

$ echo {1..10..2}
1 3 5 7 9
  • Use “seq” command
$ seq 1 10
1
2
3
4
5
6
7
8
9
10

$ seq 1 2 10
1
3
5
7
9

$ echo $(seq 1 10)
1 2 3 4 5 6 7 8 9 10


Show the changelog of package with apt-get


In Ubuntu, the command “dpkg” is a tool to install, build, remove and manage Debian packages. And aptitude(“apt”) is commonly used a frontend for dpkg. For instance, to install a package most users would use “apt-get install package”.

The “changelog” sub-command for apt-get can be used to download and display the changelog for the given package.

$ apt-get changelog dnsutils

bind9 (1:9.16.1-0ubuntu2.7) focal; urgency=medium

  * Fix a race between deactivating socket handle and processing
    async callbacks, which can lead to sockets not being closed
    properly, exhausting TCP connection limits. (LP: #1909950) 
    - d/p/lp-1909950-fix-race-between-deactivating-handle-async-callback.patch

 -- Matthew Ruffell <matthew.ruffell@canonical.com>  Thu, 18 Feb 2021 16:28:44 +1300

bind9 (1:9.16.1-0ubuntu2.6) focal-security; urgency=medium

  * SECURITY UPDATE: off-by-one bug in ISC SPNEGO implementation
    - debian/patches/CVE-2020-8625.patch: properly calculate length in
      lib/dns/spnego.c.
    - CVE-2020-8625
  * This update does _not_ contain the changes from 1:9.16.1-0ubuntu2.5 in
    focal-proposed.

 -- Marc Deslauriers <marc.deslauriers@ubuntu.com>  Tue, 16 Feb 2021 15:08:33 -0500

....

Note: this will download the changelog for the package to the most recent version of the package, not just the installed version. You can view the installed version and candidate version with below command –

$ apt-cache policy dnsutils
dnsutils:
  Installed: 1:9.16.1-0ubuntu2.3
  Candidate: 1:9.16.1-0ubuntu2.7
  Version table:
     1:9.16.1-0ubuntu2.7 500
        500 http://archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu focal-updates/universe i386 Packages
     1:9.16.1-0ubuntu2.6 500
        500 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu focal-security/universe i386 Packages
 *** 1:9.16.1-0ubuntu2.3 100
        100 /var/lib/dpkg/status
     1:9.16.1-0ubuntu2 500
        500 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu focal/universe i386 Packages

References


  1. https://linux.die.net/man/8/apt-get
  2. https://linux.die.net/man/1/dpkg

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.

Get HTTP headers

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