fix: dns interceptor affinity

* error was thrown when a single ip address was resolved
* `pick` always started on the second ip address if multiple
were resolved
* affinity was ignored with dual-stack

lib/interceptor/dns.js

@@ -13,7 +13,6 @@ class DNSInstance {
   affinity = null
   lookup = null
   pick = null
-  lastIpFamily = null
 
   constructor (opts) {
     this.#maxTTL = opts.maxTTL
@@ -61,9 +60,7 @@ class DNSInstance {
         const ip = this.pick(
           origin,
           records,
-          // Only set affinity if dual stack is disabled
-          // otherwise let it go through normal flow
-          !newOpts.dualStack && newOpts.affinity
+          newOpts.affinity
         )
 
         cb(
@@ -78,9 +75,7 @@ class DNSInstance {
       const ip = this.pick(
         origin,
         ips,
-        // Only set affinity if dual stack is disabled
-        // otherwise let it go through normal flow
-        !newOpts.dualStack && newOpts.affinity
+        newOpts.affinity
       )
 
       // If no IPs we lookup - deleting old records
@@ -123,36 +118,36 @@ class DNSInstance {
 
   #defaultPick (origin, hostnameRecords, affinity) {
     let ip = null
-    const { records, offset = 0 } = hostnameRecords
-    let newOffset = 0
+    const { records, offset } = hostnameRecords
+
+    let family
+    if (this.dualStack) {
+      if (affinity == null) {
+        // Balance between ip families
+        if (offset == null || offset === maxInt) {
+          hostnameRecords.offset = 0
+          affinity = 4
+        } else {
+          hostnameRecords.offset++
+          affinity = (hostnameRecords.offset & 1) === 1 ? 6 : 4
+        }
+      }
 
-    if (offset === maxInt) {
-      newOffset = 0
+      if (records[affinity] != null && records[affinity].ips.length > 0) {
+        family = records[affinity]
+      } else {
+        family = records[affinity === 4 ? 6 : 4]
+      }
     } else {
-      newOffset = offset + 1
+      family = records[affinity]
     }
 
-    // We balance between the two IP families
-    // If dual-stack disabled, we automatically pick the affinity
-    const newIpFamily = (newOffset & 1) === 1 ? 4 : 6
-    const family =
-      this.dualStack === false
-        ? records[this.affinity] // If dual-stack is disabled, we pick the default affiniy
-        : records[affinity] ?? records[newIpFamily]
-
-    // If no IPs and we have tried both families or dual stack is disabled, we return null
-    if (
-      (family == null || family.ips.length === 0) &&
-      // eslint-disable-next-line eqeqeq
-      (this.dualStack === false || this.lastIpFamily != newIpFamily)
-    ) {
+    // If no IPs we return null
+    if (family == null || family.ips.length === 0) {
       return ip
     }
 
-    family.offset = family.offset ?? 0
-    hostnameRecords.offset = newOffset
-
-    if (family.offset === maxInt) {
+    if (family.offset == null || family.offset === maxInt) {
       family.offset = 0
     } else {
       family.offset++
@@ -172,7 +167,6 @@ class DNSInstance {
       return this.pick(origin, hostnameRecords, affinity)
     }
 
-    this.lastIpFamily = newIpFamily
     return ip
   }
 
@@ -301,12 +295,20 @@ module.exports = interceptorOpts => {
     throw new InvalidArgumentError('Invalid pick. Must be a function')
   }
 
+  const dualStack = interceptorOpts?.dualStack ?? true
+  let affinity
+  if (dualStack) {
+    affinity = interceptorOpts?.affinity ?? null
+  } else {
+    affinity = interceptorOpts?.affinity ?? 4
+  }
+
   const opts = {
     maxTTL: interceptorOpts?.maxTTL ?? 10e3, // Expressed in ms
     lookup: interceptorOpts?.lookup ?? null,
     pick: interceptorOpts?.pick ?? null,
-    dualStack: interceptorOpts?.dualStack ?? true,
-    affinity: interceptorOpts?.affinity ?? 4,
+    dualStack,
+    affinity,
     maxItems: interceptorOpts?.maxItems ?? Infinity
   }