Refuse host entries that are not a host name or an IP address

An entry of allowlist_hosts or referer_blocklist that is neither a host
name (letters, digits, hyphens and dots, with at most one leading dot)
nor an IP address now aborts startup naming the setting and the entry,
so a `*.` wildcard or a port no longer loads and silently matches
nothing. README.md, configs/config.example.yml and the TODO.md entry now
say the Referer check comes before the signature, the cache and the
upstream fetch, since maintenance mode answers first; the example config
says the list does not cover the login and generator pages.

Model: opus-5-5

README.md

@@ -177,11 +177,12 @@ path under `/v1/` answers 200, in maintenance mode too.
   the image's `ETag`; 400 for a URL or parameter that is not valid; 401 for a
   missing or wrong signature, a missing `exp` or an `exp` in the past; 403 when
   the request's `Referer` names a host in `referer_blocklist`, checked before
-  anything else; 403 when the upstream host, or a host it redirects to, is
-  `localhost`, ends in `.localhost` or `.local`, or has an address in a blocked
-  network (see `blocked_networks`); 502 when the upstream answered with an error
-  status, and for 5 minutes after that for the same source URL; 503 when pixa is
-  busy or in maintenance mode; 500 for any other failure.
+  the signature, the cache and the upstream fetch; 403 when the upstream host,
+  or a host it redirects to, is `localhost`, ends in `.localhost` or `.local`,
+  or has an address in a blocked network (see `blocked_networks`); 502 when the
+  upstream answered with an error status, and for 5 minutes after that for the
+  same source URL; 503 when pixa is busy or in maintenance mode; 500 for any
+  other failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
   (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
   `If-None-Match` matches the image's `ETag`; 400 for a token that does not
@@ -394,6 +395,11 @@ and the URL is
 - **Suffix match**: `.example.com` — matches `cdn.example.com`,
   `images.example.com`, and `example.com`
 
+An IP address is matched exactly; write an IPv6 address without brackets. An
+entry that is neither a host name (letters, digits, hyphens and dots, with at
+most one leading dot) nor an IP address, such as one with a port or a `*.`
+wildcard, aborts startup.
+
 ### Configuration
 
 Every setting can be given as an environment variable, in a YAML config
@@ -461,11 +467,12 @@ Key settings in more detail:
 - `allowlist_hosts` — list of allowed upstream hosts
 - `referer_blocklist` — list of hosts whose pages may not show pixa's images, to
   stop other sites hotlinking them. Entries are written and matched as for
-  `allowlist_hosts` (see Allowlist patterns); one that is not a bare host aborts
-  startup. A request to `/v1/image/` or `/v1/e/` whose `Referer` header names a
-  listed host is refused with 403 before anything else is done for it, so it
-  fetches nothing from the upstream host, and it is refused even when the image
-  is cached. A request with no `Referer`, or one that does not parse as a URL
+  `allowlist_hosts` (see Allowlist patterns), and an entry that is neither a
+  host name nor an IP address aborts startup. A request to `/v1/image/` or
+  `/v1/e/` whose `Referer` header names a listed host is refused with 403 before
+  its signature or token is checked and before the cache or the upstream host is
+  used, so it fetches nothing, and it is refused even when the image is cached.
+  A request with no `Referer`, or one that does not parse as a URL
   with a host, is served, as many clients send none. So this is easily got
   around: a site whose pages send no `Referer` (for example with
   `Referrer-Policy: no-referrer`) is not stopped. It does not apply to the login

TODO.md

@@ -31,6 +31,18 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-04 referer blocklist (closes #90): `referer_blocklist`
+  (`PIXA_REFERER_BLOCKLIST`) lists hosts, written and matched as for
+  `allowlist_hosts` with the same matcher; an entry of either list that is
+  neither a host name (letters, digits, hyphens and dots, with at most one
+  leading dot) nor an IP address, such as one with a port or a `*.` wildcard,
+  aborts startup naming the setting and the entry. Both image routes refuse a
+  request whose `Referer` names a listed host with 403 and a JSON error before
+  the signature, the cache and the upstream fetch, so it fetches nothing and is
+  refused whether or not the image is cached. A request with no `Referer`, or
+  one that does not parse as a URL with a host, is served, so the list is easily
+  got around; `README.md` and `configs/config.example.yml` say so. It does not
+  apply to the login and generator pages.
 - 2026-10-04 fewer files in the repository root (closes #97):
   `config.example.yml` moved unchanged to `configs/config.example.yml`, and
   `README.md`, the comments in `internal/config/config.go` and the startup error
@@ -47,16 +59,6 @@ P2: security: per-IP rate limiting on the image routes
   for it, and the default `db_url` turns on WAL mode with
   `_pragma=journal_mode(WAL)`. The old default's `_journal_mode=WAL` is not a
   parameter the driver reads, so the database was never in WAL mode.
-- 2026-10-04 referer blocklist (closes #90): `referer_blocklist`
-  (`PIXA_REFERER_BLOCKLIST`) lists hosts, written and matched as for
-  `allowlist_hosts` with the same matcher; an entry that is not a bare host
-  aborts startup naming the setting and the entry. Both image routes refuse a
-  request whose `Referer` names a listed host with 403 and a JSON error before
-  anything else is done for it, so it fetches nothing and is refused whether or
-  not the image is cached. A request with no `Referer`, or one that does not
-  parse as a URL with a host, is served, so the list is easily got around;
-  `README.md` and `config.example.yml` say so. It does not apply to the login
-  and generator pages.
 - 2026-10-04 `TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup`
   only passes through a periodic pass (closes #189): it slept for three
   eviction intervals before writing its file, and a startup pass still running

configs/config.example.yml

@@ -46,6 +46,8 @@ signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
 # Hosts that don't require signatures (default: none)
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
+# An entry that is neither a host name nor an IP address (IPv6 without
+# brackets), such as one with a port or a "*." wildcard, aborts startup.
 allowlist_hosts:
   - s3.sneak.cloud
   - static.sneak.cloud
@@ -58,7 +60,7 @@ allowlist_hosts:
 # names one of them is answered 403 before anything is fetched, even when
 # the image is cached. A request with no Referer, or one that does not
 # parse, is served, so a site whose pages send no Referer is not stopped.
-# An entry that is not a host aborts startup. (default: none)
+# The login and generator pages are not covered. (default: none)
 # referer_blocklist:
 #   - leech.example
 #   - .hotlinker.example

internal/config/config.go

@@ -11,6 +11,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"sort"
 	"strconv"
@@ -98,12 +99,11 @@ var (
 		"value is null; omit the key entirely to use the default")
 	errValuesNull = errors.New(
 		"value is null; omit a key entirely to use its default")
-	errNotBareHostname = errors.New(
-		"must be a bare hostname without scheme, path, or whitespace")
-	errNoHostnameLabels = errors.New("contains no hostname labels")
-	errNotADuration     = errors.New("not a duration such as 30s or 2m")
-	errMustBePositive   = errors.New("must be positive")
-	errNotAnOrigin      = errors.New(
+	errNotAHost = errors.New("must be a host name such as " +
+		"cdn.example.com or .example.com, or an IP address")
+	errNotADuration   = errors.New("not a duration such as 30s or 2m")
+	errMustBePositive = errors.New("must be positive")
+	errNotAnOrigin    = errors.New(
 		`not "*" or an origin such as https://example.com`)
 )
 
@@ -742,25 +742,23 @@ func (c *Config) validateConcurrencyLimits() error {
 	return nil
 }
 
+// hostNamePattern matches a host name: letters, digits, hyphens and dots,
+// optionally after one leading dot.
+var hostNamePattern = regexp.MustCompile(`^\.?[A-Za-z0-9-][A-Za-z0-9.-]*$`)
+
 // validateHostPattern checks that an entry of the named key, allowlist_hosts
-// or referer_blocklist, is a bare hostname, optionally with a leading dot for
-// suffix matching. URLs, paths, and whitespace indicate a misconfigured entry.
-// An entry with no hostname labels (such as ".") is rejected: the allowlist
-// matcher treats a leading dot as a suffix pattern, so a bare "." would match
-// any host written in FQDN trailing-dot form, and in allowlist_hosts
-// effectively disable URL signing.
+// or referer_blocklist, is an IP address or a host name, the host name
+// optionally with one leading dot for suffix matching. Anything else, such as
+// a URL, a port or a "*." wildcard, can never match a host, so it is refused.
+// So is "." alone: the allowlist matcher would match it against any host
+// written with a trailing dot, which in allowlist_hosts disables URL signing.
 func validateHostPattern(key, host string) error {
-	if strings.Contains(host, "://") || strings.ContainsAny(host, "/ \t") {
-		return fmt.Errorf("%s: entry %q %w",
-			settingName(key), host, errNotBareHostname)
+	_, err := netip.ParseAddr(host)
+	if err == nil || hostNamePattern.MatchString(host) {
+		return nil
 	}
 
-	if strings.Trim(host, ".") == "" {
-		return fmt.Errorf("%s: entry %q %w",
-			settingName(key), host, errNoHostnameLabels)
-	}
-
-	return nil
+	return fmt.Errorf("%s: entry %q %w", settingName(key), host, errNotAHost)
 }
 
 // loadConfigFile loads configuration from the PIXA_CONFIG_PATH env var