"*" means any hosts which are not defined in DNS will be sent to the configured dns entry ( host)
in example any hosts not defined in the domain dns like newhost.mydomain.com will be resolved to IP address of dmz.mydomain.com
( CNAME is this example is alias)
"@" symbol in any DNS entry means 'Primary' which means just the domain name need to be specified. For hosts where you dont want a dns host name to have to specified in front of the your domain for a particular purpose. This is useful for email hosts ( mail exchangers or MX records in DNS)
so no other name is needed in front of the domain name
example dns entries
* CNAME dmz.myisp.com
@ MX mxchange.mydomain.com 5
@ MX backupmx.mydomain.com 10
poormx MX mydomain.com 20
mxchange A 52.25.134.234
backupmx CNAME backup.external-ispdomain.com
vpn-hosts A 52.25.134.235
In the example, two mail servers are enabled for mydoamin.com. Should mxchange.mydomain.com fail then backupmx.mydomain would receive email untill mxchange ( the primary server comes back online. notice the DNS priority Preference no's . The lower the number the higher the priority or the order in which the servers should be contacted.
backupmx.mydomain.com in example above is infact an alias for an ISP mail server definined in CNAME ( canonical name) entry lower down backup.external-ispdomain.com. So when the companies internal mail server fails their isp mail server takes over, until the private mail server is back online.
Mail backup servers or relays configuration are not part of this note.
@ is typically used in MX Mail Exchange record where you dont want the hostname to proceed the domain like poormx (above) otherwise your email address would look like someone@poormx.mydomain.com instead of someone@mydomain.com
to check dns entries which you have made have taken effect. use 'dig' on linux/unix hosts
or on windows 'nslookup'
then type
'set type =mx'
'mydomain.com'
will show current like mx records and their route values
mydomain.com MX Preference = 5, mail exchanger =mxchange.mydomain.com
mydomain.com MX Preference = 10 mail exchanger = backupmx.mydomain.com
mydomain.com MX Preerence = 20 mail exchanger = poormx.mydomain.com
REMEMBER DNS replication outside the UK to other Internet facing DNS servers might take upto 5 hours to replicate !
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