当前 master branch 的 DNS 缓存有问题（缓存 TypeA 结果包含了 IPv6 地址）

[Mukou-Aoi]

master branch ( 编译时最新 commit 67cd3ac )

$ nslookup www.qq.com. 127.0.0.2
Server:  UnKnown
Address:  127.0.0.2

Non-authoritative answer:
Name:    www.qq.com
Addresses:  IPv6::1
          IPv6::2
          4.0.0.1

$ nslookup www.qq.com. 127.0.0.2
Server:  UnKnown
Address:  127.0.0.2

Non-authoritative answer:
Name:    www.qq.com
Addresses:  IPv6::1
          IPv6::2
          4.0.0.1
          IPv6::1
          IPv6::2

log

  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 querying DNS for: www.qq.com.
  [Info] app/dns: UDP:223.5.5.5:53 got answer: www.qq.com. TypeA -> [4.0.0.1] 22.0012ms
  [Debug] app/dns: UDP:223.5.5.5:53 updating IP records for domain:www.qq.com.
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 querying DNS for: www.qq.com.
  [Info] app/dns: UDP:223.5.5.5:53 got answer: www.qq.com. TypeAAAA -> [[IPv6::1] [IPv6::2]] 21.0012ms
  [Debug] app/dns: UDP:223.5.5.5:53 updating IP records for domain:www.qq.com.
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [4.0.0.1 IPv6::1 IPv6::2]
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [IPv6::1 IPv6::2]
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
- [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [4.0.0.1 IPv6::1 IPv6::2]
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [IPv6::1 IPv6::2]

---

Release v4.41.1 （正常）

$ nslookup www.qq.com.
Server:  UnKnown
Address:  127.1.1.1

Non-authoritative answer:
Name:    www.qq.com
Addresses:  IPv6::1
          IPv6::2
          4.0.0.1

$ nslookup www.qq.com.
Server:  UnKnown
Address:  127.1.1.1

Non-authoritative answer:
Name:    www.qq.com
Addresses:  IPv6::1
          IPv6::2
          4.0.0.1

log（Release 4.41.1）

  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 querying DNS for: www.qq.com.
  [Info] app/dns: UDP:223.5.5.5:53 got answer: www.qq.com. TypeA -> [4.0.0.1] 26.0015ms
  [Debug] app/dns: UDP:223.5.5.5:53 updating IP records for domain:www.qq.com.
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 querying DNS for: www.qq.com.
  [Info] app/dns: UDP:223.5.5.5:53 got answer: www.qq.com. TypeAAAA -> [[IPv6::2] [IPv6::1]] 25.0014ms
  [Debug] app/dns: UDP:223.5.5.5:53 updating IP records for domain:www.qq.com.
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
+ [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [4.0.0.1]
  [Debug] app/dns: domain www.qq.com will use DNS in order: [UDP:223.5.5.5:53]
  [Debug] app/dns: UDP:223.5.5.5:53 cache HIT www.qq.com -> [IPv6::2 IPv6::1]

[digglife]

这应该是 PR #1183 引入的一个 Bug，涉及到所有协议的 nameserver。

fallthrough语句会导致无论 option.IPv6Enable 是真是假， IPv6 地址都会加入到 ips 里。

v2ray-core/app/dns/nameserver_tcp.go

Lines 286 to 293 in 70e3415

	switch {
	case option.IPv4Enable:
	ips, err4 = record.A.getIPs()
	fallthrough
	case option.IPv6Enable:
	ip6, err6 = record.AAAA.getIPs()
	ips = append(ips, ip6...)
	}

所以如果查询 AAAA 记录只会获取到 IPv6，但是查询 A 记录会同时获取到 v4 和 v6。我也能复现这个问题。

cc @Loyalsoldier