the aa flag means the server is authoritative, i.e., the 科大 DNS server recreated a zone file for xys2.dropin.org locally, so on campus anyone wants to go to the mirror site is directed to the address specified in the local zone file.

;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 1

;; QUESTION SECTION:
;xys2.dropin.org. IN A

;; ANSWER SECTION:
xys2.dropin.org. 14400 IN A 218.22.173.20

;; AUTHORITY SECTION:
xys2.dropin.org. 14400 IN NS ns.ustc.edu.cn.

;; ADDITIONAL SECTION:
ns.ustc.edu.cn. 600 IN A 202.38.64.1

;; Query time: 421 msec
;; SERVER: 202.38.64.1#53(202.38.64.1)

% [whois.apnic.net node-2]
% Whois data copyright terms http://www.apnic.net/db/dbcopyright.html
inetnum: 202.38.64.0 - 202.38.95.255
netname: USTC-CN
descr: The University of Science and Technology of China
descr: Department of Computer Science
descr: Hefei, Anhui 230026
country: CN
admin-c: CER-AP
tech-c: CER-AP
tech-c: CER-AP
remarks: origin AS4538
mnt-by: APNIC-HM
mnt-lower: MAINT-CERNET-AP
status: ALLOCATED PORTABLE
changed: hm-changed@apnic.net 20031014
source: APNIC