Networking Commands
Networking commands
DNS resolv.conf
DNS nameservers and the possible (search) domains are listed in /etc/resolv.conf
$ cat /etc/resolv.conf
If we dont have any nameserver in the /etc/resolv.conf, system will not be able to resolve any host which thinks network is down.
$ dig cluster0.d8v1man.mongodb.net
/AppleInternal/Library/BuildRoots/aaefcfd1-5c95-11ed-8734-2e32217d8374/Library/Caches/com.apple.xbs/Sources/bind9/bind9/lib/isc/unix/socket.c:2132: internal_send: ::1#53: Network is down
; <<>> DiG 9.10.6 <<>> cluster0.d8v1man.mongodb.net
;; global options: +cmd
;; connection timed out; no servers could be reached
DNS cache
DNS lookups cached for perfomance reasons. If you are debugging network, it is good idea to flush the DNS as needed.
Here is the command for MAC to flush the DNS.
$ sudo killall -HUP mDNSResponder;sudo dscacheutil -flushcache
nslookup
$ nslookup cluster0.random.mongodb.net
Server: 12.32.0.1
Address: 12.32.0.1#53
Non-authoritative answer:
*** Can't find cluster0.random.mongodb.net: No answer
To debug add -debug
$ nslookup -debug google.com
Server: 100.64.0.1
Address: 100.64.0.1#53
------------
QUESTIONS:
google.com, type = A, class = IN
ANSWERS:
-> google.com
internet address = 142.250.217.238
ttl = 54
AUTHORITY RECORDS:
ADDITIONAL RECORDS:
------------
Non-authoritative answer:
Name: google.com
Address: 142.250.217.238
dig
dig command helps to identify how a host name resolve into IP address which DNS server help to translate it.
$ dig cluster0.random.mongodb.net
; <<>> DiG 9.10.6 <<>> cluster0.random.mongodb.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14115
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;cluster0.random.mongodb.net. IN A
;; AUTHORITY SECTION:
mongodb.net. 60 IN SOA ns-761.awsdns-31.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 60
;; Query time: 57 msec
;; SERVER: 12.32.0.1#53(12.32.0.1)
;; WHEN: Tue Nov 28 14:07:32 EST 2023
;; MSG SIZE rcvd: 136
Here is an example of we dont get any authritative answers (Just differnt time and differnt network)
$ dig cluster0.random.mongodb.net
; <<>> DiG 9.10.6 <<>> cluster0.random.mongodb.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8323
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;cluster0.random.mongodb.net. IN A
;; Query time: 25 msec
;; SERVER: 12.32.0.1#53(12.32.0.1)
;; WHEN: Wed Nov 29 02:57:23 EST 2023
;; MSG SIZE rcvd: 55
Asking specific DNS server
If you want your host names resolved by checking specific DNS server
dig @50.63.202.40 somehost.company.com
netstat
netstat helps to list all the network connections.
$ netstat -an
Active Internet connections (including servers)
Proto Recv-Q Send-Q Local Address Foreign Address (state)
....
ping
ping command is more frequently used but mostly blocked to keep the bad actors away.
$ ping google.com
traceroute
$ traceroute google.com
References
- https://www.godaddy.com/help/what-is-dns-665
Debugging tools
In linux (Ubuntu), to get some basic commands (ex: dig, nslookup), install them using following commands.
$ sudo apt install net-tools
or in AWS cloudshell…
$ sudo yum install -y bind-utils