This is
especially true when a host is connected to a satellite or DSL line. Consider, for
example, the following three commands:
% dig +noall +answer mypc.example.com
mypc.example.com. 3600 IN A 192.168.45.55
% dig +noall +answer -x 192.168.45.55
55.45.168.192.in-addr.arpa 3600 IN PTR dhcphost12.isp.domain
% dig +noall +answer dhcphost12.isp.domain
dhcphost12.isp.domain 3600 IN A 192.168.45.55
Here, the host mypc.example.com is looked up, yielding its IP address. Next, that IP
address is reverse-looked-up, but instead of yielding mypc.example.com as expected,
it yields dhcphost12.isp.domain. This is a simplified example of a PC in someone??™s
home connected to a telephone company??™s DSL service. Note that when this new
hostname is looked up, that lookup reveals the original IP address.
Although such false or misleading lookups may seem dishonest, there is actually no
restriction in the RFCs against them.
9.4.3 Look Up MX Records with dig(1)
Recall that an MX record is a Mail eXchanger record. MX records list the hosts that
should receive email for a host or a domain. A handy way to look up MX records
with dig(1) is to use its +short command-line argument and pipe the result through
sort(1):
% dig +short mx example.gov | sort -n
5 amx.example.gov.
5 bmx.example.gov.
5 cmx.example.gov.
100 backup1.example.gov.
100 backup2.
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