dns query默认走udp，大小有限制，每个datagram 512byte，对于超过限制的dns response会用tcp mode重试，但是经测试surge dns server好像不支持。
创建了两个测试SRV record，_s._tcp.edge.polarbox.win response 在limit内是成功的，_large._tcp.edge.polarbox.win response 超过limit用tcp mode重试一直失败。 尝试了设置dns-server = system或者域名加入always-real-ip都没用。
case 1a: ali dns works as expected in udp mode
% nslookup -type=srv _s._tcp.edge.polarbox.win 223.5.5.5
Server: 223.5.5.5
Address: 223.5.5.5#53
Non-authoritative answer:
_s._tcp.edge.polarbox.win service = 0 0 443 edge2.polarbox.win.
_s._tcp.edge.polarbox.win service = 0 0 443 edge1.polarbox.win.
Authoritative answers can be found from:
case 1b: surge dns server works as expected in udp mode
% nslookup -type=srv _s._tcp.edge.polarbox.win
Server: 198.18.0.2
Address: 198.18.0.2#53
Non-authoritative answer:
_s._tcp.edge.polarbox.win service = 0 0 443 edge1.polarbox.win.
_s._tcp.edge.polarbox.win service = 0 0 443 edge2.polarbox.win.
Authoritative answers can be found from:
case 2a: ali dns works as expected in tcp mode
% nslookup -type=srv _large._tcp.edge.polarbox.win 223.5.5.5
;; Truncated, retrying in TCP mode.
Server: 223.5.5.5
Address: 223.5.5.5#53
Non-authoritative answer:
_large._tcp.edge.polarbox.win service = 0 0 8443 large006.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large009.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large003.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large007.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large011.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large008.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large002.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large012.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large001.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large005.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large004.edge.polarbox.win.
_large._tcp.edge.polarbox.win service = 0 0 8443 large010.edge.polarbox.win.
Authoritative answers can be found from:
case 2b: surge dns server fails in tcp mode
% nslookup -type=srv _large._tcp.edge.polarbox.win
;; Truncated, retrying in TCP mode.
;; Connection to 198.18.0.2#53(198.18.0.2) for _large._tcp.edge.polarbox.win failed: timed out.
;; Connection to 198.18.0.2#53(198.18.0.2) for _large._tcp.edge.polarbox.win failed: timed out.
;; connection timed out; no servers could be reached
;; Connection to 198.18.0.2#53(198.18.0.2) for _large._tcp.edge.polarbox.win failed: timed out.
