DNS and Hosts Resolution

====================================================================================

DNS and Hosts Testing

------------------------------------------------------------------------------------------------------------------------------------------------

dig domainname

dig from a specific DNS server

dig domainname @DNS_ServerIP

====================================================================================

DNS and Host Resolution

------------------------------------------------------------------------------------------------------------------------------------------------

/etc/resolv.conf

The /etc/resolv.conf file is used to configure DNS server that your server will use for DNS lookups. 

Important Note; The below documentation is related to the /etc/resolve.conf file. This is not the primary file that Linux machines will use for the resolution configuration. Instead, this file is symlinked to /run/systemd/resolve/stub-resolve.conf which is referenced by systemd-resolvd. The primary configuration file used by systemd-resolvd is /run/systemd/resolve/resolve.conf, but stub-resolv.conf is also referenced. TLDR; /etc/resolve.conf is still used, but it's not the primary place referenced by systemd-resolvd.

root@test:~# ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Aug 10  2023 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf

The systemd-resolved service listens on port 53 locally: this port needs to be open in order for DNS resolution to function.

root@test:~# lsof -i:53
COMMAND      PID            USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
systemd-r 127318 systemd-resolve   13u  IPv4 1028696      0t0  UDP localhost:domain
systemd-r 127318 systemd-resolve   14u  IPv4 1028697      0t0  TCP localhost:domain (LISTEN)

/etc/Resolve.conf config:

Below is a typical default configuration you might see on a Linux system:

# This file configures your system's DNS resolution.

nameserver 127.0.0.53  # Local DNS server (systemd-resolved)
options edns0 trust-ad  # Enables EDNS for performance 
search b4sed.xyz        # Search domain to append to incomplete names

nameserver - specifies where the system looks for DNS resolution

search - This is the default search domain. For example, if a lookup is made to google, this option would append .b4sed.xyz to the end: google.b4sed.xyz 

options:

    edns0 -  enables a potentially performance-enhancing feature.

    trust-ad - instructs your resolver to accept and potentially use the information in the Additional Records section without                                   further verification.

View current DNS configuration:

resolvectl status

------------------------------------------------------------------------------------------------------------------------------------------------

/etc/hosts

The /etc/hosts file can be thought of as essentially a local DNS configuration. This means that DNS entries can be mapped here, overwriting any DNS entries provided by an external service.

Entries into the /etc/hosts file can be formatted as follows:

IP domainname

------------------------------------------------------------------------------------------------------------------------------------------------