diff --git a/README.md b/README.md
index f7e5553..d087ffa 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ not PQL) is forwarded verbatim.
 
 | Path | Behaviour |
 |---|---|
-| `GET /pdb/query/v4/nodes` | Fan out to all backends, dedupe by `certname`, keep the record with the newer `report_timestamp`, stamped with the winning backend's name (see provenance). An `extract` query with a `function` column is **combined** instead. |
+| `GET /pdb/query/v4/nodes` | Fan out to all backends, dedupe by `certname`, keep the record with the newer `report_timestamp`, stamped with the winning backend's host (see provenance). An `extract` query with a `function` column is **combined** instead. |
 | `GET /pdb/query/v4/facts` | Fan out to all, and per `certname` keep **all** facts from the backend that owns that node (see merge semantics), plus a synthetic `pdbmux_source` fact naming it. An `extract` query with a `function` column is **combined** instead. |
 | `GET /pdb/query/v4/facts/<name>[/<value>]` | Same fan-out and merge as `/facts`, and an `extract` query with a `function` column is **combined** the same way. The path segment is a `name` constraint, so no synthetic `pdbmux_source` record is added — except on the fact's own path, which is **synthesised** from the `/facts` merge (see provenance). |
 | `GET /pdb/query/v4/fact-names` | Fan out to all and serve the **union** of the flat name arrays, deduped and re-sorted, re-paged across backends, plus the `pdbmux_source` name while injection is on. `order_by` is only valid on `name`. |
@@ -205,8 +205,10 @@ response itself, so nothing has to query each backend to find out:
 
 - **`/facts`** gains one extra fact record per `certname`, alongside the node's
   real facts, in the shape of a real fact record — `certname`, `name`, `value`,
-  `environment` — with `value` set to the **backend name** from `backends` /
-  `PDBMUX_BACKENDS`. `environment` is copied from that node's own facts (all
+  `environment` — with `value` set to the **hostname** of the backend's URL from
+  `backends` / `PDBMUX_BACKENDS` (no scheme or port), e.g.
+  `puppetdb.puppet.svc.cluster.local` for `new=http://puppetdb.puppet.svc.cluster.local:8080`.
+  Two backends may therefore not share a hostname; config validation rejects it. `environment` is copied from that node's own facts (all
   four keys are always present, since clients index them directly).
 - **`/nodes`** gains a `pdbmux_source` **key** on each merged node record. A
   node record carries no facts, so this is a synthetic field, not a fact — the
@@ -268,10 +270,10 @@ response reports, and lets the request's own `query` narrow the result upstream.
 That costs one `/facts` fan-out per request — the widest fan-out `pdbmux`
 makes — on a rare, user-initiated path.
 
-`/facts/pdbmux_source/<value>` pins the backend name, so it answers with the
+`/facts/pdbmux_source/<value>` pins the backend host, so it answers with the
 nodes that backend owns. The `<value>` segment never reaches that fan-out: the
-synthetic record's value is always a backend name, so a value naming none is
-answered `[]` from the configured names alone, with no fan-out at all, and a
+synthetic record's value is always a backend host, so any other value — a
+backend's name included — is answered `[]` from the configured hosts alone, with no fan-out at all, and a
 value naming one filters a record set fetched under a key the value is not part
 of. The record set is a property of the estate rather than of the filter, so
 every value of it — and the unfiltered path — share one entry and one fetch.
diff --git a/cache_test.go b/cache_test.go
index cae13f1..80b6cd3 100644
--- a/cache_test.go
+++ b/cache_test.go
@@ -632,7 +632,7 @@ func TestHandler_LeaderDisconnectDoesNotFailFollowers(t *testing.T) {
 	body := `[` + node("h1", "2026-01-01T00:00:00.000Z") + `]`
 	a := newCountingBackend(t, map[string]string{nodesPath: body})
 	b := newCountingBackend(t, map[string]string{nodesPath: `[]`})
-	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	srv, _ := newCachedServer(t, cacheTestConfig(withHost(a.srv.URL, hostA), withHost(b.srv.URL, hostB)))
 	h := srv.Handler()
 
 	release := make(chan struct{})
@@ -673,7 +673,7 @@ func TestHandler_LeaderDisconnectDoesNotFailFollowers(t *testing.T) {
 	}
 	// The leader built this body, so its provenance names the backend that
 	// answered the leader's fan-out.
-	want := `[` + stamped(t, node("h1", "2026-01-01T00:00:00.000Z"), defaultSourceFact, "a") + `]`
+	want := `[` + stamped(t, node("h1", "2026-01-01T00:00:00.000Z"), defaultSourceFact, hostA) + `]`
 	if got := strings.TrimSpace(follower.Body.String()); !sameJSON(t, got, want) {
 		t.Errorf("follower body = %s, want %s", got, want)
 	}
@@ -1488,16 +1488,16 @@ func stamped(t *testing.T, raw, field, value string) string {
 func TestHandler_CachedNodesKeepSourceStamp(t *testing.T) {
 	a := newCountingBackend(t, map[string]string{nodesPath: `[` + node("h1", "2026-01-02T00:00:00.000Z") + `]`})
 	b := newCountingBackend(t, map[string]string{nodesPath: `[` + node("h1", "2026-01-01T00:00:00.000Z") + `]`})
-	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	srv, _ := newCachedServer(t, cacheTestConfig(withHost(a.srv.URL, hostA), withHost(b.srv.URL, hostB)))
 	h := srv.Handler()
 
 	first := doGet(t, h, nodesPath, "")
-	if got := nodeSources(t, first.Body.Bytes(), defaultSourceFact); got["h1"] != "a" {
+	if got := nodeSources(t, first.Body.Bytes(), defaultSourceFact); got["h1"] != hostA {
 		t.Fatalf("first request stamp = %v, want h1 -> a", got)
 	}
 
 	second := doGet(t, h, nodesPath, "")
-	want := `[` + stamped(t, node("h1", "2026-01-02T00:00:00.000Z"), defaultSourceFact, "a") + `]`
+	want := `[` + stamped(t, node("h1", "2026-01-02T00:00:00.000Z"), defaultSourceFact, hostA) + `]`
 	if got := strings.TrimSpace(second.Body.String()); !sameJSON(t, got, want) {
 		t.Errorf("cached body = %s, want %s", got, want)
 	}
@@ -1512,7 +1512,7 @@ func TestHandler_CachedNodesKeepSourceStamp(t *testing.T) {
 func TestHandler_CachedSourceAgesWithItsData(t *testing.T) {
 	a := newCountingBackend(t, map[string]string{nodesPath: `[` + node("h1", "2026-01-01T00:00:00.000Z") + `]`})
 	b := newCountingBackend(t, map[string]string{nodesPath: `[]`})
-	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	srv, clk := newCachedServer(t, cacheTestConfig(withHost(a.srv.URL, hostA), withHost(b.srv.URL, hostB)))
 	h := srv.Handler()
 
 	doGet(t, h, nodesPath, "")
@@ -1522,7 +1522,7 @@ func TestHandler_CachedSourceAgesWithItsData(t *testing.T) {
 	b.setBody(nodesPath, `[`+node("h1", "2026-01-01T00:00:00.000Z")+`]`)
 
 	cached := doGet(t, h, nodesPath, "")
-	if got := nodeSources(t, cached.Body.Bytes(), defaultSourceFact); got["h1"] != "a" {
+	if got := nodeSources(t, cached.Body.Bytes(), defaultSourceFact); got["h1"] != hostA {
 		t.Errorf("cached stamp = %v, want h1 -> a: the body and its attribution come from the same fetch", got)
 	}
 	if got := a.hitCount(nodesPath); got != 1 {
@@ -1531,7 +1531,7 @@ func TestHandler_CachedSourceAgesWithItsData(t *testing.T) {
 
 	clk.advance(31 * time.Second)
 	rebuilt := doGet(t, h, nodesPath, "")
-	if got := nodeSources(t, rebuilt.Body.Bytes(), defaultSourceFact); got["h1"] != "b" {
+	if got := nodeSources(t, rebuilt.Body.Bytes(), defaultSourceFact); got["h1"] != hostB {
 		t.Errorf("rebuilt stamp = %v, want h1 -> b once the entry expired", got)
 	}
 }
diff --git a/config.go b/config.go
index f7508c2..ca2ef29 100644
--- a/config.go
+++ b/config.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -289,12 +290,22 @@ func (c Config) configHint() string {
 	return ConfigPath()
 }
 
+// backendHost is the upstream's hostname, the value of the source fact.
+func backendHost(raw string) string {
+	u, err := url.Parse(raw)
+	if err != nil {
+		return ""
+	}
+	return strings.ToLower(u.Hostname())
+}
+
 func (c Config) Validate() error {
 	if len(c.Backends) == 0 {
 		return fmt.Errorf("no backends configured: set %sBACKENDS to \"name=url,name=url\" or add a backends list to %s",
 			envPrefix, c.configHint())
 	}
 	seen := map[string]bool{}
+	hosts := map[string]string{}
 	for _, b := range c.Backends {
 		if b.Name == "" || b.URL == "" {
 			return fmt.Errorf("backend requires both name and url: %+v", b)
@@ -303,6 +314,17 @@ func (c Config) Validate() error {
 			return fmt.Errorf("duplicate backend name %q", b.Name)
 		}
 		seen[b.Name] = true
+		if !c.SourceFactEnabled {
+			continue
+		}
+		host := backendHost(b.URL)
+		if host == "" {
+			return fmt.Errorf("backend %q url %q has no host", b.Name, b.URL)
+		}
+		if other, dup := hosts[host]; dup {
+			return fmt.Errorf("backends %q and %q share host %q; %s values would collide", other, b.Name, host, c.SourceFact)
+		}
+		hosts[host] = b.Name
 	}
 	switch c.Merge {
 	case mergeFreshness, mergeStatic:
diff --git a/config_test.go b/config_test.go
index 89c2d9d..aca37a4 100644
--- a/config_test.go
+++ b/config_test.go
@@ -15,7 +15,7 @@ func testConfigValid() Config {
 	cfg := DefaultConfig()
 	cfg.Backends = []Backend{
 		{Name: "a", URL: "http://localhost:18080"},
-		{Name: "b", URL: "http://localhost:18081"},
+		{Name: "b", URL: "http://127.0.0.1:18081"},
 	}
 	return cfg
 }
@@ -57,7 +57,7 @@ func TestLoad_NoBackendsLoadsButFailsValidation(t *testing.T) {
 func TestLoad_BackendsKeepConfiguredOrder(t *testing.T) {
 	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
 	clearEnv(t)
-	t.Setenv(envPrefix+"BACKENDS", "a=http://localhost:18080,b=http://localhost:18081")
+	t.Setenv(envPrefix+"BACKENDS", "a=http://localhost:18080,b=http://127.0.0.1:18081")
 
 	cfg, err := Load("")
 	if err != nil {
@@ -229,8 +229,20 @@ func TestValidate(t *testing.T) {
 	}{
 		{"ok", func(*Config) {}, false},
 		{"no backends", func(c *Config) { c.Backends = nil }, true},
-		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "a", URL: "x"}) }, true},
+		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "a", URL: "http://c"}) }, true},
 		{"missing url", func(c *Config) { c.Backends[0].URL = "" }, true},
+		{"url without host", func(c *Config) { c.Backends[0].URL = "localhost:18080" }, true},
+		{"shared host on another port", func(c *Config) { c.Backends[1].URL = "http://localhost:18081" }, true},
+		{"same host differing by scheme", func(c *Config) { c.Backends[1].URL = "https://localhost" }, true},
+		{"same host differing by case", func(c *Config) { c.Backends[1].URL = "http://LocalHost:18081" }, true},
+		{"shared host while source fact disabled", func(c *Config) {
+			c.SourceFactEnabled = false
+			c.Backends[1].URL = "http://localhost:18081"
+		}, false},
+		{"url without host while source fact disabled", func(c *Config) {
+			c.SourceFactEnabled = false
+			c.Backends[0].URL = "localhost:18080"
+		}, false},
 		{"bad merge", func(c *Config) { c.Merge = "wrong" }, true},
 		{"zero timeout", func(c *Config) { c.Timeout = 0 }, true},
 		{"empty source fact while enabled", func(c *Config) { c.SourceFact = "" }, true},
@@ -512,3 +524,15 @@ func clearEnv(t *testing.T) {
 		t.Setenv(envPrefix+k, "")
 	}
 }
+
+func TestBackendHost(t *testing.T) {
+	for raw, want := range map[string]string{
+		"https://PuppetDB.Example:8081/x": "puppetdb.example",
+		"http://[::1]:8080":               "::1",
+		"localhost:8080":                  "",
+	} {
+		if got := backendHost(raw); got != want {
+			t.Errorf("backendHost(%q) = %q, want %q", raw, got, want)
+		}
+	}
+}
diff --git a/e2e_backend_test.go b/e2e_backend_test.go
index 72eefb3..41aff11 100644
--- a/e2e_backend_test.go
+++ b/e2e_backend_test.go
@@ -154,7 +154,7 @@ func startBackend(ctx context.Context, t fatalf, name, netName string) *backend
 		fail("starting openvoxdb for %s: %v", name, err)
 	}
 
-	return &backend{name: name, url: fmt.Sprintf("http://127.0.0.1:%d", port), pg: pg, pdb: pdb}
+	return &backend{name: name, url: fmt.Sprintf("http://%s.localhost:%d", name, port), pg: pg, pdb: pdb}
 }
 
 // waitForPuppetDBRunning gates on the trapperkeeper status service reporting
diff --git a/e2e_fixture_test.go b/e2e_fixture_test.go
index 20e9fb3..1328152 100644
--- a/e2e_fixture_test.go
+++ b/e2e_fixture_test.go
@@ -42,6 +42,9 @@ func fixtureTime(offset time.Duration) string {
 const (
 	backendAName = "pdb-a"
 	backendBName = "pdb-b"
+	// The source fact's value is the backend host, so each backend gets its own.
+	backendAHost = backendAName + ".localhost"
+	backendBHost = backendBName + ".localhost"
 )
 
 type nodeFixture struct {
diff --git a/e2e_health_test.go b/e2e_health_test.go
index be42a80..dc2db20 100644
--- a/e2e_health_test.go
+++ b/e2e_health_test.go
@@ -77,8 +77,8 @@ func TestBackendDeathAndRecovery(t *testing.T) {
 		if got, _ := factValue(facts, nodeShared, "owner"); got != backendAName {
 			t.Errorf("owner of %s with %s down = %v, want %q", nodeShared, h.b.name, got, backendAName)
 		}
-		if got, _ := factValue(facts, nodeShared, defaultSourceFact); got != backendAName {
-			t.Errorf("%s for %s with %s down = %v, want %q", defaultSourceFact, nodeShared, h.b.name, got, backendAName)
+		if got, _ := factValue(facts, nodeShared, defaultSourceFact); got != backendAHost {
+			t.Errorf("%s for %s with %s down = %v, want %q", defaultSourceFact, nodeShared, h.b.name, got, backendAHost)
 		}
 	})
 
diff --git a/e2e_nodelookup_test.go b/e2e_nodelookup_test.go
index 7420557..07e53d5 100644
--- a/e2e_nodelookup_test.go
+++ b/e2e_nodelookup_test.go
@@ -84,8 +84,8 @@ func TestNodeLookupShowsProvenance(t *testing.T) {
 		if !strings.Contains(line, defaultSourceFact) {
 			continue
 		}
-		if !strings.Contains(line, backendBName) {
-			t.Fatalf("node-lookup reports %q for %s, want the owning backend %q", strings.TrimSpace(line), nodeShared, backendBName)
+		if !strings.Contains(line, backendBHost) {
+			t.Fatalf("node-lookup reports %q for %s, want the owning backend %q", strings.TrimSpace(line), nodeShared, backendBHost)
 		}
 		return
 	}
diff --git a/e2e_query_test.go b/e2e_query_test.go
index 6778f08..4a5ffc7 100644
--- a/e2e_query_test.go
+++ b/e2e_query_test.go
@@ -286,8 +286,8 @@ func TestSharedNodeResolvesToTheFresherBackend(t *testing.T) {
 	if got := node["report_timestamp"]; got != tsSharedOnB {
 		t.Errorf("merged /nodes report_timestamp for %s = %v, want the fresher %q", nodeShared, got, tsSharedOnB)
 	}
-	if got := node[defaultSourceFact]; got != backendBName {
-		t.Errorf("merged /nodes %s for %s = %v, want %q", defaultSourceFact, nodeShared, got, backendBName)
+	if got := node[defaultSourceFact]; got != backendBHost {
+		t.Errorf("merged /nodes %s for %s = %v, want %q", defaultSourceFact, nodeShared, got, backendBHost)
 	}
 
 	rows := get(t, factsPath, query(`["=","certname","`+nodeShared+`"]`)).rows(t)
@@ -769,10 +769,10 @@ func TestSourceFactInjectionAndGating(t *testing.T) {
 	t.Run("facts carry one source record per node", func(t *testing.T) {
 		rows := get(t, factsPath, nil).rows(t)
 		want := map[string]string{
-			nodeAlpha:  backendAName,
-			nodeBeta:   backendBName,
-			nodeGamma:  backendBName,
-			nodeShared: backendBName, // won on freshness, not on configured order
+			nodeAlpha:  backendAHost,
+			nodeBeta:   backendBHost,
+			nodeGamma:  backendBHost,
+			nodeShared: backendBHost, // won on freshness, not on configured order
 		}
 		got := map[string]int{}
 		for _, row := range rows {
@@ -839,10 +839,10 @@ func TestSourceFactInjectionAndGating(t *testing.T) {
 // records /facts carries rather than the empty set the backends hold.
 func TestSourceFactDrilldown(t *testing.T) {
 	want := map[string]string{
-		nodeAlpha:  backendAName,
-		nodeBeta:   backendBName,
-		nodeGamma:  backendBName,
-		nodeShared: backendBName, // won on freshness, not on configured order
+		nodeAlpha:  backendAHost,
+		nodeBeta:   backendBHost,
+		nodeGamma:  backendBHost,
+		nodeShared: backendBHost, // won on freshness, not on configured order
 	}
 	path := factsPath + "/" + defaultSourceFact
 
@@ -882,8 +882,8 @@ func TestSourceFactDrilldown(t *testing.T) {
 			value string
 			want  []string
 		}{
-			{backendAName, []string{nodeAlpha}},
-			{backendBName, []string{nodeBeta, nodeGamma, nodeShared}},
+			{backendAHost, []string{nodeAlpha}},
+			{backendBHost, []string{nodeBeta, nodeGamma, nodeShared}},
 			{"nosuchbackend", nil},
 		} {
 			rows := get(t, path+"/"+tc.value, nil).rows(t)
diff --git a/factroutes_test.go b/factroutes_test.go
index 67635b5..f7e9f75 100644
--- a/factroutes_test.go
+++ b/factroutes_test.go
@@ -259,15 +259,15 @@ func TestHandler_FactsBySourceNamePathSynthesisesOneRecordPerNode(t *testing.T)
 	for _, merge := range []string{mergeStatic, mergeFreshness} {
 		t.Run(merge, func(t *testing.T) {
 			a, b := sourceFactBackends(t)
-			srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, merge))
+			srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, merge))
 
 			// h3 lives in both: static keeps the first configured backend, freshness
 			// the one holding its newer report.
-			wantH3 := "a"
+			wantH3 := hostA
 			if merge == mergeFreshness {
-				wantH3 = "b"
+				wantH3 = hostB
 			}
-			want := map[string]string{"h1": "a", "h2": "b", "h3": wantH3}
+			want := map[string]string{"h1": hostA, "h2": hostB, "h3": wantH3}
 
 			rec := doGet(t, srv.Handler(), sourceFactURL, "")
 			if rec.Code != http.StatusOK {
@@ -315,20 +315,21 @@ func TestHandler_FactsBySourceNamePathCarriesTheOwnersEnvironment(t *testing.T)
 	}
 }
 
-// The pinned value names a backend, so the sub-route is the drilldown filtered
-// by owner; a value naming no backend matches nothing.
+// The pinned value is a backend host, so the sub-route is the drilldown filtered
+// by owner; any other value, a backend's name included, matches nothing.
 func TestHandler_FactsBySourceNameAndValueFiltersByOwner(t *testing.T) {
 	for _, tc := range []struct {
 		value string
 		want  map[string]string
 	}{
-		{"a", map[string]string{"h1": "a"}},
-		{"b", map[string]string{"h2": "b", "h3": "b"}},
+		{hostA, map[string]string{"h1": hostA}},
+		{hostB, map[string]string{"h2": hostB, "h3": hostB}},
+		{"a", map[string]string{}},
 		{"nosuchbackend", map[string]string{}},
 	} {
 		t.Run(tc.value, func(t *testing.T) {
 			a, b := sourceFactBackends(t)
-			srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+			srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 			rec := doGet(t, srv.Handler(), sourceFactURL+"/"+tc.value, "")
 			got, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact)
@@ -377,15 +378,15 @@ func TestHandler_FactsBySourceNameUnknownValueSkipsFanOut(t *testing.T) {
 func TestHandler_FactsBySourceNameValuesFilterOneFetch(t *testing.T) {
 	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "osfamily", "RedHat", "") + `]`})
 	b := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h2", "osfamily", "Debian", "") + `]`})
-	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	srv, _ := newCachedServer(t, cacheTestConfig(withHost(a.srv.URL, hostA), withHost(b.srv.URL, hostB)))
 
 	for _, tc := range []struct {
 		path string
 		want map[string]string
 	}{
-		{sourceFactURL + "/a", map[string]string{"h1": "a"}},
-		{sourceFactURL + "/b", map[string]string{"h2": "b"}},
-		{sourceFactURL, map[string]string{"h1": "a", "h2": "b"}},
+		{sourceFactURL + "/" + hostA, map[string]string{"h1": hostA}},
+		{sourceFactURL + "/" + hostB, map[string]string{"h2": hostB}},
+		{sourceFactURL, map[string]string{"h1": hostA, "h2": hostB}},
 	} {
 		rec := doGet(t, srv.Handler(), tc.path, "")
 		got, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact)
@@ -434,12 +435,12 @@ func TestHandler_FactsBySourceNamePathRespectsQuery(t *testing.T) {
 	a, b := sourceFactBackends(t)
 	a.bodies[factsPath] = `[` + factEnv("h1", "osfamily", "RedHat", "prod") + `]`
 	b.bodies[factsPath] = `[]`
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	const q = `["=","certname","h1"]`
 	rec := doGet(t, srv.Handler(), sourceFactURL, q)
 	got, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact)
-	if n != 1 || got["h1"] != "a" {
+	if n != 1 || got["h1"] != hostA {
 		t.Fatalf("%s?query=%s = %v (%d records), want only h1: %s", sourceFactURL, q, got, n, rec.Body.String())
 	}
 	// The query has to reach the backends for them to narrow anything.
@@ -457,11 +458,11 @@ func TestHandler_FactsBySourceNamePathSuppressesUpstream(t *testing.T) {
 	a.bodies[factsPath] = `[` + factEnv("h1", "osfamily", "RedHat", "prod") + `,` +
 		fact("h1", defaultSourceFact, "stale", "") + `]`
 	b.bodies[factsPath] = `[]`
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	rec := doGet(t, srv.Handler(), sourceFactURL, "")
 	got, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact)
-	if n != 1 || got["h1"] != "a" {
+	if n != 1 || got["h1"] != hostA {
 		t.Fatalf("%s = %v (%d records), want exactly the synthetic one: %s", sourceFactURL, got, n, rec.Body.String())
 	}
 }
@@ -714,20 +715,21 @@ func TestHandler_FactsQuerySelectsSourceFact(t *testing.T) {
 		query string
 		want  map[string]string
 	}{
-		{`["=","name","` + src + `"]`, map[string]string{"h1": "a", "h2": "b", "h3": "b"}},
-		{`["and",["=","name","` + src + `"]]`, map[string]string{"h1": "a", "h2": "b", "h3": "b"}},
-		{`["and",["=","name","` + src + `"],["=","value","b"]]`, map[string]string{"h2": "b", "h3": "b"}},
-		{`["and",["=","name","` + src + `"],["not",["~","value","^b$"]]]`, map[string]string{"h1": "a"}},
-		{`["and",["=","certname","h3"],["=","name","` + src + `"]]`, map[string]string{"h3": "b"}},
-		{`["and",["or",["=","name","osfamily"],["=","name","` + src + `"]]]`, map[string]string{"h1": "a", "h2": "b", "h3": "b"}},
-		{`["in","name",["array",["osfamily","` + src + `"]]]`, map[string]string{"h1": "a", "h2": "b", "h3": "b"}},
-		{`["and",["or",["=","name","` + src + `"]],["=","environment","prod"]]`, map[string]string{"h1": "a"}},
+		{`["=","name","` + src + `"]`, map[string]string{"h1": hostA, "h2": hostB, "h3": hostB}},
+		{`["and",["=","name","` + src + `"]]`, map[string]string{"h1": hostA, "h2": hostB, "h3": hostB}},
+		{`["and",["=","name","` + src + `"],["=","value","` + hostB + `"]]`, map[string]string{"h2": hostB, "h3": hostB}},
+		{`["and",["=","name","` + src + `"],["=","value","b"]]`, map[string]string{}},
+		{`["and",["=","name","` + src + `"],["not",["~","value","^b\\.localhost$"]]]`, map[string]string{"h1": hostA}},
+		{`["and",["=","certname","h3"],["=","name","` + src + `"]]`, map[string]string{"h3": hostB}},
+		{`["and",["or",["=","name","osfamily"],["=","name","` + src + `"]]]`, map[string]string{"h1": hostA, "h2": hostB, "h3": hostB}},
+		{`["in","name",["array",["osfamily","` + src + `"]]]`, map[string]string{"h1": hostA, "h2": hostB, "h3": hostB}},
+		{`["and",["or",["=","name","` + src + `"]],["=","environment","prod"]]`, map[string]string{"h1": hostA}},
 	}
 	for _, tc := range tests {
 		t.Run(tc.query, func(t *testing.T) {
 			a, b := sourceFactBackends(t)
 			a.bodies[factsPath] = `[` + factEnv("h1", "osfamily", "RedHat", "prod") + `,` + fact("h1", src, "stale", "") + `]`
-			srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+			srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 			rec := doGet(t, srv.Handler(), factsPath, tc.query)
 			if rec.Code != http.StatusOK {
@@ -796,12 +798,12 @@ func TestHandler_FactsQuerySourceFactNarrowsUpstream(t *testing.T) {
 			return false
 		}
 	}
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	query := `["and",["=","certname","h3"],["=","environment","dev"],["=","name","` + defaultSourceFact + `"]]`
 	rec := doGet(t, srv.Handler(), factsPath, query)
-	if vals, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact); n != 1 || vals["h3"] != "b" {
-		t.Fatalf("got %v (%d records), want h3=b: %s", vals, n, rec.Body.String())
+	if vals, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact); n != 1 || vals["h3"] != hostB {
+		t.Fatalf("got %v (%d records), want h3=%s: %s", vals, n, hostB, rec.Body.String())
 	}
 	for _, q := range got {
 		if q == "" {
diff --git a/server.go b/server.go
index bc2f1b7..3cff7a0 100644
--- a/server.go
+++ b/server.go
@@ -53,7 +53,9 @@ var errAllBackendsFailed = errors.New("all backends failed")
 const jsonContentType = "application/json;charset=utf-8"
 
 type Server struct {
-	cfg    Config
+	cfg Config
+	// hosts maps backend name to the hostname the source fact reports.
+	hosts  map[string]string
 	client *http.Client
 	log    *log.Logger
 
@@ -83,6 +85,10 @@ func NewServer(cfg Config, logger *log.Logger) *Server {
 		client: &http.Client{Timeout: cfg.Timeout},
 		log:    logger,
 		now:    time.Now,
+		hosts:  map[string]string{},
+	}
+	for _, b := range cfg.Backends {
+		s.hosts[b.Name] = backendHost(b.URL)
 	}
 	if cfg.cacheEnabled() {
 		s.nodeCache = newMemoryCache(cfg.FactsTTL, cfg.CacheBytes)
@@ -476,10 +482,10 @@ func (s *Server) serveFactsByName(w http.ResponseWriter, r *http.Request) {
 // value would be a fresh cache key, a fresh flight and a fresh whole-estate
 // fan-out.
 func (s *Server) serveSourceFact(w http.ResponseWriter, r *http.Request, inject *sourceInjector, value string, valued bool) {
-	// The synthetic record's value is always a backend name, so any other value
+	// The synthetic record's value is always a backend host, so any other value
 	// matches zero records. The response is complete rather than degraded, so it
 	// reports every configured backend.
-	if valued && !s.hasBackend(value) {
+	if valued && !s.hasBackendHost(value) {
 		n := len(s.cfg.Backends)
 		writeCached(w, cachedResponse{Records: -1, Backends: n, Configured: n})
 		return
@@ -505,9 +511,9 @@ func (s *Server) serveSourceFact(w http.ResponseWriter, r *http.Request, inject
 	})
 }
 
-func (s *Server) hasBackend(name string) bool {
-	for _, b := range s.cfg.Backends {
-		if b.Name == name {
+func (s *Server) hasBackendHost(host string) bool {
+	for _, h := range s.hosts {
+		if h == host {
 			return true
 		}
 	}
diff --git a/server_test.go b/server_test.go
index 83d7933..5b06e5c 100644
--- a/server_test.go
+++ b/server_test.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"io"
 	"log"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -163,6 +164,27 @@ func truncate(t *testing.T, body, limit string) string {
 	return string(out)
 }
 
+// Test backends are reached as hostA/hostB, since the source fact's value is the
+// backend host and both httptest servers listen on 127.0.0.1.
+const (
+	hostA = "a.localhost"
+	hostB = "b.localhost"
+)
+
+func withHost(rawURL, host string) string {
+	u, err := url.Parse(rawURL)
+	if err != nil || u.Port() == "" {
+		return rawURL
+	}
+	u.Host = net.JoinHostPort(host, u.Port())
+	return u.String()
+}
+
+// hostedConfig is testConfig with the backends reached as hostA and hostB.
+func hostedConfig(aURL, bURL, merge string) Config {
+	return testConfig(withHost(aURL, hostA), withHost(bURL, hostB), merge)
+}
+
 func testConfig(aURL, bURL, merge string) Config {
 	return Config{
 		Listen:            ":0",
diff --git a/source.go b/source.go
index 9c83da2..d4f1d74 100644
--- a/source.go
+++ b/source.go
@@ -13,6 +13,8 @@ import (
 // callers need no branch.
 type sourceInjector struct {
 	name string
+	// hosts maps backend name to the value emitted.
+	hosts map[string]string
 	// inject is false when the query shape rules synthesis out. Suppression of an
 	// upstream fact of the same name does not depend on it.
 	inject     bool
@@ -25,7 +27,7 @@ func (s *Server) newSourceInjector(query string, factEntity bool) *sourceInjecto
 	if !s.cfg.SourceFactEnabled || s.cfg.SourceFact == "" {
 		return nil
 	}
-	return &sourceInjector{name: s.cfg.SourceFact, inject: injectable(query, factEntity)}
+	return &sourceInjector{name: s.cfg.SourceFact, hosts: s.hosts, inject: injectable(query, factEntity)}
 }
 
 // claims reports whether an upstream record carries the name pdbmux owns, and is
@@ -70,7 +72,7 @@ func (si *sourceInjector) factRecord(certname, backend, environment string) json
 		Environment string `json:"environment"`
 		Name        string `json:"name"`
 		Value       string `json:"value"`
-	}{Certname: certname, Environment: environment, Name: si.name, Value: backend})
+	}{Certname: certname, Environment: environment, Name: si.name, Value: si.hosts[backend]})
 	if err != nil {
 		return nil
 	}
@@ -87,7 +89,7 @@ func (si *sourceInjector) stamp(raw json.RawMessage, backend string) json.RawMes
 	if json.Unmarshal(raw, &obj) != nil || obj == nil {
 		return raw
 	}
-	value, err := json.Marshal(backend)
+	value, err := json.Marshal(si.hosts[backend])
 	if err != nil {
 		return raw
 	}
diff --git a/source_test.go b/source_test.go
index 70895f9..9d22614 100644
--- a/source_test.go
+++ b/source_test.go
@@ -77,7 +77,7 @@ func TestHandler_FactsSourceFollowsMergeOwner(t *testing.T) {
 	b := newFakeBackend(t,
 		`[`+node("h1", "2026-07-01T00:00:00Z")+`,`+node("h2", "2026-07-20T00:00:00Z")+`]`,
 		`[`+factEnv("h1", "role", "web-b", "staging")+`,`+factEnv("h2", "role", "db-b", "staging")+`]`)
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeFreshness))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeFreshness))
 
 	rec := doGet(t, srv.Handler(), factsPath, "")
 	if rec.Code != http.StatusOK {
@@ -87,7 +87,7 @@ func TestHandler_FactsSourceFollowsMergeOwner(t *testing.T) {
 	if n != 2 {
 		t.Fatalf("expected one %s record per certname, got %d: %s", defaultSourceFact, n, rec.Body.String())
 	}
-	if got["h1"] != "a" || got["h2"] != "b" {
+	if got["h1"] != hostA || got["h2"] != hostB {
 		t.Errorf("provenance must name the backend that won the merge, got %v", got)
 	}
 }
@@ -127,14 +127,14 @@ func TestHandler_NodesSourceStamped(t *testing.T) {
 	a := newFakeBackend(t,
 		`[`+node("h1", "2026-07-01T00:00:00Z")+`,`+node("h2", "2026-07-20T00:00:00Z")+`]`, `[]`)
 	b := newFakeBackend(t, `[`+node("h1", "2026-07-20T00:00:00Z")+`]`, `[]`)
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), nodesPath, "")
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
 	}
 	got := nodeSources(t, rec.Body.Bytes(), defaultSourceFact)
-	if got["h1"] != "b" || got["h2"] != "a" {
+	if got["h1"] != hostB || got["h2"] != hostA {
 		t.Errorf("node provenance = %v, want h1=b h2=a", got)
 	}
 }
@@ -186,13 +186,13 @@ func TestHandler_SourceFactNameOverride(t *testing.T) {
 	a := newFakeBackend(t, `[`+node("h1", "2026-07-20T00:00:00Z")+`]`,
 		`[`+fact("h1", "role", "web", "")+`]`)
 	b := newFakeBackend(t, `[]`, `[]`)
-	cfg := testConfig(a.srv.URL, b.srv.URL, mergeStatic)
+	cfg := hostedConfig(a.srv.URL, b.srv.URL, mergeStatic)
 	cfg.SourceFact = "origin_pdb"
 	srv := newTestServer(cfg)
 
 	facts := doGet(t, srv.Handler(), factsPath, "")
 	got, n := sourceValues(t, facts.Body.Bytes(), "origin_pdb")
-	if n != 1 || got["h1"] != "a" {
+	if n != 1 || got["h1"] != hostA {
 		t.Errorf("override name not honoured: %s", facts.Body.String())
 	}
 	if _, n := sourceValues(t, facts.Body.Bytes(), defaultSourceFact); n != 0 {
@@ -200,7 +200,7 @@ func TestHandler_SourceFactNameOverride(t *testing.T) {
 	}
 
 	nodes := doGet(t, srv.Handler(), nodesPath, "")
-	if got := nodeSources(t, nodes.Body.Bytes(), "origin_pdb"); got["h1"] != "a" {
+	if got := nodeSources(t, nodes.Body.Bytes(), "origin_pdb"); got["h1"] != hostA {
 		t.Errorf("override name not honoured on /nodes: %v", got)
 	}
 }
@@ -211,14 +211,14 @@ func TestHandler_UpstreamSourceFactOverridden(t *testing.T) {
 	a := newFakeBackend(t, `[`+node("h1", "2026-07-20T00:00:00Z")+`]`,
 		`[`+fact("h1", "role", "web", "")+`,`+fact("h1", defaultSourceFact, "stale-value", "")+`]`)
 	b := newFakeBackend(t, `[]`, `[]`)
-	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+	srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 	rec := doGet(t, srv.Handler(), factsPath, "")
 	got, n := sourceValues(t, rec.Body.Bytes(), defaultSourceFact)
 	if n != 1 {
 		t.Fatalf("expected exactly 1 %s record, got %d: %s", defaultSourceFact, n, rec.Body.String())
 	}
-	if got["h1"] != "a" {
+	if got["h1"] != hostA {
 		t.Errorf("upstream value survived: %v", got)
 	}
 }
@@ -232,10 +232,10 @@ func TestHandler_UpstreamSourceFactSuppressedOnEveryGateState(t *testing.T) {
 		query string
 		want  string // synthetic value, or "" when the gate blocks injection
 	}{
-		{"injection on", "", "a"},
+		{"injection on", "", hostA},
 		{"gated by extract", `["extract",["certname","name","value"],["=","certname","h1"]]`, ""},
 		{"gated by name filter", `["or",["=","name","role"],["=","name","os"]]`, ""},
-		{"selected by name filter", `["or",["=","name","role"],["=","name","` + defaultSourceFact + `"]]`, "a"},
+		{"selected by name filter", `["or",["=","name","role"],["=","name","` + defaultSourceFact + `"]]`, hostA},
 		{"gated by nested extract", `["and",["=","certname","h1"],["extract",["certname"]]]`, ""},
 	}
 	for _, tc := range tests {
@@ -243,7 +243,7 @@ func TestHandler_UpstreamSourceFactSuppressedOnEveryGateState(t *testing.T) {
 			a := newFakeBackend(t, `[`+node("h1", "2026-07-20T00:00:00Z")+`]`,
 				`[`+fact("h1", "role", "web", "")+`,`+fact("h1", defaultSourceFact, upstream, "")+`]`)
 			b := newFakeBackend(t, `[]`, `[]`)
-			srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+			srv := newTestServer(hostedConfig(a.srv.URL, b.srv.URL, mergeStatic))
 
 			rec := doGet(t, srv.Handler(), factsPath, tc.query)
 			if rec.Code != http.StatusOK {
@@ -560,10 +560,11 @@ func TestMergeFacts_GatedSuppressesUpstream(t *testing.T) {
 func TestMergeFacts_SourceOrderedAfterOwnersFacts(t *testing.T) {
 	a := recs(t, "a", fact("h1", "role", "web-a", ""), fact("h1", "kernel", "Linux", ""))
 	b := recs(t, "b", fact("h1", "role", "web-b", ""))
-	merged := mergeFacts([]backendResult{a, b}, nil, &sourceInjector{name: defaultSourceFact, inject: true})
+	si := &sourceInjector{name: defaultSourceFact, hosts: map[string]string{"a": "a.example", "b": "b.example"}, inject: true}
+	merged := mergeFacts([]backendResult{a, b}, nil, si)
 
 	got := factValues(t, merged)
-	want := []string{"h1:role=web-a", "h1:kernel=Linux", "h1:" + defaultSourceFact + "=a"}
+	want := []string{"h1:role=web-a", "h1:kernel=Linux", "h1:" + defaultSourceFact + "=a.example"}
 	if !slices.Equal(got, want) {
 		t.Errorf("merged = %v, want %v", got, want)
 	}
@@ -638,6 +639,25 @@ func TestSourceInjector_SourceQuery(t *testing.T) {
 	}
 }
 
+// The value is the backend URL's hostname alone; the backend name is not used.
+func TestSourceInjector_ValueIsBackendHostname(t *testing.T) {
+	cfg := testConfig("http://puppetdb.puppet.svc.cluster.local:8080", "https://puppetdbapi.service.consul/pdb", mergeStatic)
+	srv := newTestServer(cfg)
+	si := srv.newSourceInjector("", true)
+	for backend, want := range map[string]string{"a": "puppetdb.puppet.svc.cluster.local", "b": "puppetdbapi.service.consul"} {
+		var rec factFields
+		if err := json.Unmarshal(si.factRecord("h1", backend, ""), &rec); err != nil {
+			t.Fatal(err)
+		}
+		if rec.Value != want {
+			t.Errorf("fact value for %s = %q, want %q", backend, rec.Value, want)
+		}
+		if got := nodeSources(t, []byte(`[`+string(si.stamp([]byte(`{"certname":"h1"}`), backend))+`]`), defaultSourceFact); got["h1"] != want {
+			t.Errorf("node stamp for %s = %v, want %q", backend, got, want)
+		}
+	}
+}
+
 func TestCertnameScope(t *testing.T) {
 	tests := map[string]string{
 		`["=","name","x"]`: ``,