tcpdump

To debug the packet flows.

Command	Desc
tcpdump host <IP>	Traffic that’s going from and to the <IP>
tcpdump src <IP>	Only traffics from the IP
tcpdump dst <IP>	Only traffics to the IP
tcpdump net <IP>/CIDR	Traffic going to or coming from a subnet or network
tcpdump port <PORT_NO>	Packets flowing from and to via PORT_NO
tcpdump -nnvvS src <IP> and dst port <PORT_NO>	Finds all traffic from <IP> going to any host on port <PORT_NO>
tcpdump -n -vvv -i any port 53	Any traffic that’s going to DNS Server
tcpdump -nnSX port 443	Picks all the traffic on port 443
tcpdump -i eth0	Everything on the eth0 interface

time

To debug the response time. Create a txt file as following and save it as time.txt

    time_namelookup:  %{time_namelookup}\n
       time_connect:  %{time_connect}\n
    time_appconnect:  %{time_appconnect}\n
   time_pretransfer:  %{time_pretransfer}\n
      time_redirect:  %{time_redirect}\n
 time_starttransfer:  %{time_starttransfer}\n
                    ----------\n
         time_total:  %{time_total}\n

Now add the following in the normal curl like below.

curl -w "@time.txt" -o /dev/null -s "https://google.com"

The above command should give the time splits as follows.

    time_namelookup:  0.012 
       time_connect:  0.014
    time_appconnect:  0.727
   time_pretransfer:  0.727
      time_redirect:  0.000
 time_starttransfer:  0.785
                    ----------
         time_total:  0.785

nslookup

nslookup is used to resolve the IP address to a FQDN (Fully Qualified Domain Name).

nslookup <IP>

dig

domain information groper used to query DNS

dig google.com

; <<>> DiG 9.10.6 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38902
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;google.com.			IN	A

;; ANSWER SECTION:
google.com.		227	IN	A	172.217.164.110

;; Query time: 15 msec
;; SERVER: 2001:558:feed::1#53(2001:558:feed::1)
;; WHEN: Sun Nov 24 12:36:49 PST 2019
;; MSG SIZE  rcvd: 55

traceroute

traceroute or tracert used to display the hops and measure latency in the network

ping

ping is used to check the reachability