Narrow the egress claims to non-public metadata addresses

The ALLOWED_EGRESS_CIDRS startup warning no longer calls the listed
blocks private/reserved, and says a public cloud metadata address such
as 168.63.129.16 is reachable once it, or a block covering it, is
listed. The README, the AllowedEgressCIDRs field comment and the
checkIP doc comment now say the allowlist cannot open metadata
endpoints at non-public addresses, not every metadata endpoint.

Model: opus-5-5

README.md

@@ -200,15 +200,16 @@ Two things this setting cannot do:
   the list is always an allowlist; an empty list (the default) means
   every private and reserved range stays refused. Note that
   `0.0.0.0/0` gets you most of the way there anyway, per above.
-- **It cannot open link-local, or a cloud metadata endpoint that
-  discloses credentials or user data.** An address is on the list below
-  when both of these hold: the provider fixes it, so it cannot collide
-  with anything you run; and reaching it hands out credentials, user
-  data or bootstrap material. Those stay blocked no matter what you
-  list, including when you list them outright or list a supernet such
-  as `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`. Treat this as
-  best effort rather than a guarantee — it is a hand-maintained list
-  and the caveat below the table applies:
+- **It cannot open link-local, or a cloud metadata endpoint at a
+  non-public address that discloses credentials or user data.** An
+  address is on the list below when it is not a public address and both
+  of these hold: the provider fixes it, so it cannot collide with
+  anything you run; and reaching it hands out credentials, user data or
+  bootstrap material. Those stay blocked no matter what you list,
+  including when you list them outright or list a supernet such as
+  `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`. Treat this as best
+  effort rather than a guarantee — it is a hand-maintained list and the
+  caveat below the table applies:
 
   | Blocked unconditionally | What it is |
   | ----------------------- | ---------- |

internal/config/config.go

@@ -192,9 +192,10 @@ type Config struct {
 	// alwaysBlockedNetworks stays blocked no matter what is listed
 	// here. That set is link-local plus the cloud metadata
 	// endpoints outside it that disclose credentials or user data
-	// at a provider-fixed address; it is not exhaustive of every
-	// cloud's metadata address. See alwaysBlockedNetworks for the
-	// authoritative list and the criterion it is built from.
+	// at a provider-fixed, non-public address; it is not
+	// exhaustive of every cloud's metadata address. See
+	// alwaysBlockedNetworks for the authoritative list and the
+	// criterion it is built from.
 	AllowedEgressCIDRs []netip.Prefix
 
 	params *ConfigParams
@@ -746,12 +747,14 @@ func (c *Config) warnEgressAllowlist(log *slog.Logger) {
 
 	log.Warn(
 		"ALLOWED_EGRESS_CIDRS lets delivery targets reach these "+
-			"otherwise-blocked private/reserved networks. Anyone "+
-			"who can create a delivery target can now make this "+
-			"process issue requests into them, and read back the "+
-			"response. Link-local and the known cloud instance "+
-			"metadata endpoints outside it stay blocked "+
-			"regardless of what is listed here.",
+			"otherwise-blocked networks. Anyone who can create a "+
+			"delivery target can now make this process issue "+
+			"requests into them, and read back the response. Only "+
+			"the addresses the README lists as blocked "+
+			"unconditionally stay blocked regardless of what is "+
+			"listed here; a public cloud metadata address such as "+
+			"168.63.129.16 is reachable once it, or a block "+
+			"covering it, is listed.",
 		"allowedEgressCIDRs",
 		strings.Join(PrefixStrings(c.AllowedEgressCIDRs), ","),
 	)

internal/config/config_test.go

@@ -834,12 +834,13 @@ func TestEgressAllowlistWarning(t *testing.T) {
 			// to be able to read back which networks are open.
 			assert.Contains(t, logged, "10.0.0.0/8")
 			assert.Contains(t, logged, "127.0.0.0/8")
-			// What stays shut. Asserted on the clause naming the
-			// wider set rather than on "Link-local" alone, so the
-			// string cannot narrow back to link-local only while
-			// the always-blocked set covers ULA, CGNAT and two
-			// public metadata addresses as well.
-			assert.Contains(t, logged, "metadata endpoints outside it")
+			// What stays shut is the whole unconditional set, not
+			// link-local alone; a public metadata address is not in
+			// it, so a listed block covering it opens it.
+			assert.Contains(t, logged, "blocked unconditionally")
+			assert.Contains(t, logged, "168.63.129.16 is reachable")
+			// The listed blocks need not be private or reserved.
+			assert.NotContains(t, logged, "private/reserved")
 		})
 	}
 }

internal/delivery/ssrf.go

@@ -323,7 +323,7 @@ func (g *Guard) allows(ip net.IP) bool {
 //
 //  1. alwaysBlockedNetworks is refused before the allowlist is
 //     consulted, so no configured CIDR reaches link-local or a
-//     cloud instance metadata endpoint.
+//     cloud metadata endpoint at a non-public address.
 //  2. The allowlist is consulted next, so a listed private
 //     network becomes reachable.
 //  3. Everything else keeps the default blocklist's answer.