diff --git a/src/pkg/net/dnsclient.go b/src/pkg/net/dnsclient.go index 3466003fab8..ae9ca843052 100644 --- a/src/pkg/net/dnsclient.go +++ b/src/pkg/net/dnsclient.go @@ -398,10 +398,49 @@ type SRV struct { Weight uint16 } +// byPriorityWeight sorts SRV records by ascending priority and weight. +type byPriorityWeight []*SRV + +func (s byPriorityWeight) Len() int { return len(s) } + +func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] } + +func (s byPriorityWeight) Less(i, j int) bool { + return s[i].Priority < s[j].Priority || + (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight) +} + +// shuffleSRVByWeight shuffles SRV records by weight using the algorithm +// described in RFC 2782. +func shuffleSRVByWeight(addrs []*SRV) { + sum := 0 + for _, addr := range addrs { + sum += int(addr.Weight) + } + for sum > 0 && len(addrs) > 1 { + s := 0 + n := rand.Intn(sum + 1) + for i := range addrs { + s += int(addrs[i].Weight) + if s >= n { + if i > 0 { + t := addrs[i] + copy(addrs[1:i+1], addrs[0:i]) + addrs[0] = t + } + break + } + } + sum -= int(addrs[0].Weight) + addrs = addrs[1:] + } +} + // LookupSRV tries to resolve an SRV query of the given service, // protocol, and domain name, as specified in RFC 2782. In most cases // the proto argument can be the same as the corresponding -// Addr.Network(). +// Addr.Network(). The returned records are sorted by priority +// and randomized by weight within a priority. func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.Error) { target := "_" + service + "._" + proto + "." + name var records []dnsRR @@ -414,6 +453,15 @@ func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os. r := rr.(*dnsRR_SRV) addrs[i] = &SRV{r.Target, r.Port, r.Priority, r.Weight} } + sort.Sort(byPriorityWeight(addrs)) + i := 0 + for j := 1; j < len(addrs); j++ { + if addrs[i].Priority != addrs[j].Priority { + shuffleSRVByWeight(addrs[i:j]) + i = j + } + } + shuffleSRVByWeight(addrs[i:len(addrs)]) return }