If you pick a particular DNS server, can you suck the entire contents out of it, without particular privileged or non-standardised access?
Some, not all, DNS servers support "zone transfer" (AXFR and an incremental version IXFR) to pull all the entries for a domain (sanitised example extracts below for my home system). This is normally used to allow slave DNS servers to catch up with the master when a change is made. Normally only permitted from certain IP addresses, such as the slaves. See https://en.wikipedia.org/wiki/DNS_zone_transfer
I can envisage an organisation's security people having fits if all the publicly addressable machines names and IP addresses were available as a neat list to all and sundry.
I can also envisage them thinking themselves under attack if you were to start doing a lot of lookups against their name servers, with reactions ranging from rate limiting you to hunting you down.
Also:
While some ISP's have good DNS servers for their customers, some have very poor ones with their wiser customers using Google, OpenDNS or other name servers. If an organization is not prepared to run a few good customer name servers, are they likely to pay out to enhance what they have?
There is not a simple 1:1 mapping between a name and address. Consider CNAMEs (a sort of alias), pools (where one name is supported by several addresses), the games played by content delivery networks, ...
An interesting project though. I seem to recall scorn being paid on the ideas of indexing and archiving the entire web, so don't be too put off.
Example zone transfer
Made on my home slave name server - a Beaglebone Black running FreeBSD and BIND.
[aw1@beaglebone ~]$ dig @localhost mydomain axfr
; <<>> DiG 9.10.3-P4 <<>> @localhost mydomain axfr
; (1 server found)
;; global options: +cmd
mydomain. 60 IN SOA swelter.mydomain. myemail. 2016123000 60 60 3600000 86400
mydomain. 60 IN NS titus.mydomain.
...
access4.mydomain. 60 IN A 192.168.1.12
access4.mydomain. 60 IN MX 1 access4.mydomain.
... (ad nauseaum - A and often MX entries for many hosts and all DHCP pool addresses))
mydomain. 60 IN SOA swelter.mydomain. myemail. 2016123000 60 60 3600000 86400
;; Query time: 12 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sat Jan 28 06:04:27 GMT 2017
;; XFR size: 325 records (messages 1, bytes 6834)