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net/dns: add temporary fallback to quad-9 resolver for split-DNS testing.

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This allows split-DNS configurations to not break clients on OSes that
haven't yet been ported to understand split DNS, by falling back to quad-9
as a global resolver when handed an "impossible to implement"
split-DNS config.

Part of #953. Needs to be removed before shipping 1.8.

Signed-off-by: David Anderson <danderson@tailscale.com>
This commit is contained in:
David Anderson 2021-04-08 23:26:22 -07:00
parent b9e194c14b
commit 2685260ba1
1 changed files with 22 additions and 1 deletions
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23
net/dns/manager.go
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@ -168,7 +168,28 @@ func (m *Manager) compileConfig(cfg Config) (resolver.Config, OSConfig, error) {
} else {
bcfg, err := m.os.GetBaseConfig()
if err != nil {
return resolver.Config{}, OSConfig{}, err
// Temporary hack to make OSes where split-DNS isn't fully
// implemented yet not completely crap out, but instead
// fall back to quad-9 as a hardcoded "backup resolver".
//
// This codepath currently only triggers when opted into
// the split-DNS feature server side, and when at least
// one search domain is something within tailscale.com, so
// we don't accidentally leak unstable user DNS queries to
// quad-9 if we accidentally go down this codepath.
canUseHack := false
for _, dom := range cfg.SearchDomains {
if strings.HasSuffix(dom, ".tailscale.com") {
canUseHack = true
break
}
}
if !canUseHack {
return resolver.Config{}, OSConfig{}, err
}
bcfg = OSConfig{
Nameservers: []netaddr.IP{netaddr.IPv4(9, 9, 9, 9)},
}
}
rcfg.Routes["."] = toIPPorts(bcfg.Nameservers)
ocfg.SearchDomains = append(ocfg.SearchDomains, bcfg.SearchDomains...)