Introduction to Linux Networking

 

What is the difference between curl and wget?
curl vs Wget
How to download files in Linux from command line with dynamic url

to download a file when it is pointed by a dynamic url, all you need to to is to use single quotes for the url; -O = specify the output filename

$ wget 'http://some.site.com/download?id=234&status=download' -O output_filename

What to do if ping does not help? 

Options are:

• wget
• curl
• traceroute
• installation: $ sudo apt install traceroute
• lft
• "layer 4 traceroute"
• $ lft -S 192.168.0.20

$ ping example.com
$ traceroute example.com
$ curl -v example.com
$ wget example.com


How to find what DNS servers are used on the local machine?

$ cat /etc/resolv.conf 
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "systemd-resolve --status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 127.0.0.53
options edns0
search whatever.example.com

To find out which DNS servers are used by each network adapter:

$ systemd-resolve --status
Global
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 137 (enxa44cc8e41d0f)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 136 (vethed1f504)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 38 (br-53b4f1b3fbda)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 5 (br-3c8c9487a095)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 4 (docker0)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 3 (wlp2s0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: x.y.z.q
                      x.y.z.v
          DNS Domain: ~.
                      whatever.example.com

Link 2 (enp0s31f6)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
lines 44-81/81 (END)

Another way:

$ nmcli dev show | grep 'IP4.DNS'

IP4.DNS[1]:                             x.y.z.q

IP4.DNS[2]:                             x.y.z.v

DNS on Ubuntu 18.04

How to block some domain?

Add entry e.g.

0.0.0.0 domain.to.be.blocked

to /etc/hosts and then flush DNS cache:

How To Flush Linux / UNIX DNS Cache - nixCraft

How to direct network traffic use Proxy server?

Use environment variables

export http_proxy=10.21.32.70:8080
export https_proxy=10.21.32.70:8080

Domain names can be used instead of IP addresses.

How To Use Proxy Server To Access Internet at Shell Prompt With http_proxy Variable - nixCraft

How to test proxy?

curl -v -x 11.22.33.44:8080 -L 'https://www.example.com/examples/1'

       -x, --proxy [protocol://]host[:port]
              Use the specified proxy.

              The  proxy string can be specified with a protocol:// prefix. No
              protocol specified or http:// will be treated as HTTP proxy. Use
              socks4://, socks4a://, socks5:// or socks5h:// to request a spe‐
              cific SOCKS version to be used.  (The protocol support was added
              in curl 7.21.7)

              HTTPS  proxy  support  via https:// protocol prefix was added in
              7.52.0 for OpenSSL, GnuTLS and NSS.

To let cURL follow redirects (3xx statuses) add -L:

       -L, --location
              (HTTP)  If  the server reports that the requested page has moved
              to a different location (indicated with a Location: header and a
              3XX  response code), this option will make curl redo the request
              on the new place.

To see HTTP status code, use verbose flag:
     -v, --verbose

How to get public IP address of the computer?

$ dig TXT +short o-o.myaddr.l.google.com @ns1.google.com | awk -F'"' '{ print $2}'

3rd party GeoIP service (its web API) can be accessed via curl:

$ curl -m 30 -X GET "https://api.ipify.org?format=json"

{"ip":"197.44.76.233"}

The output of this command (an IP address) can be assigned to a variable

AGENT_IP=$(curl -s -m 30 -X GET "https://api.ipify.org?format=json")

echo "This build agent '%teamcity.agent.name%' has public IP: $AGENT_IP"