# UNDF: UNDF-2026-000001185
--- a/src/neighbors.cc
+++ b/src/neighbors.cc
@@ -7,6 +7,7 @@

 /* DEBUG: section 15    Neighbor Routines */

+#include <map>
 #include "squid.h"
 #include "acl/FilledChecklist.h"
 #include "anyp/PortCfg.h"
@@ -53,6 +54,19 @@ static int peerHTTPOkay(const CachePeer *, PeerSelector *);

 static CachePeer *whichPeer(const Ip::Address &from);

+/// Maps (ICP-source-address, icp-port) -> CachePeer* for O(log P) lookup.
+/// Built/updated in peerDNSConfigure(); entries removed in DeleteConfigured().
+/// Key: pair<address-with-no-port, icp-port> to avoid port aliasing from
+/// Ip::Address::port() stored inside the address object.
+static std::map<std::pair<Ip::Address, unsigned short>, CachePeer *> WhichPeerMap;
+
+/// Register all addresses for \p p in WhichPeerMap.
+static void
+whichPeerMapAdd(CachePeer *p)
+{
+    for (int j = 0; j < p->n_addresses; ++j)
+        WhichPeerMap[{p->addresses[j], p->icp.port}] = p;
+}
+
+/// Remove all addresses for \p p from WhichPeerMap.
+static void
+whichPeerMapRemove(CachePeer *p)
+{
+    for (int j = 0; j < p->n_addresses; ++j)
+        WhichPeerMap.erase({p->addresses[j], p->icp.port});
+}
+
 static CachePeer *
 whichPeer(const Ip::Address &from)
 {
-    int j;
-
     debugs(15, 3, "whichPeer: from " << from);

-    for (const auto &p: CurrentCachePeers()) {
-        for (j = 0; j < p->n_addresses; ++j) {
-            if (from == p->addresses[j] && from.port() == p->icp.port) {
-                return p.get();
-            }
-        }
-    }
-
-    return nullptr;
+    const auto it = WhichPeerMap.find({from, from.port()});
+    return (it != WhichPeerMap.end()) ? it->second : nullptr;
 }

--- a/src/neighbors.cc (peerDNSConfigure hunk)
+++ b/src/neighbors.cc
@@ -1110,6 +1110,10 @@ peerDNSConfigure(const ipcache_addrs *ia, const Dns::LookupDetails &, void *dat

     CachePeer *p = (CachePeer *)data;

+    // Remove stale map entries for this peer's old addresses before
+    // overwriting p->addresses[] and p->n_addresses below.
+    whichPeerMapRemove(p);
+
     if (p->n_addresses == 0) {
         debugs(15, Important(29), "Configuring " << neighborTypeStr(p) << " " << *p);

@@ -1135,6 +1139,9 @@ peerDNSConfigure(const ipcache_addrs *ia, const Dns::LookupDetails &, void *dat
         }
     }

+    // Rebuild map entries for this peer's new addresses.
+    whichPeerMapAdd(p);
+
     p->in_addr.setEmpty();
     p->in_addr = p->addresses[0];
     p->in_addr.port(p->icp.port);
--- a/src/CachePeers.cc (DeleteConfigured hunk)
+++ b/src/CachePeers.cc
@@ -52,6 +52,7 @@ DeleteConfigured(CachePeer * const peer)
 {
     Assure(Config.peers);
+    whichPeerMapRemove(peer);   // clean up reverse-lookup map entry
     Config.peers->remove(peer);
 }