diff --git a/pds-check/package.json b/pds-check/package.json
index 28803e1..db2b464 100644
--- a/pds-check/package.json
+++ b/pds-check/package.json
@@ -9,8 +9,10 @@
 	},
 	"devDependencies": {
 		"@atcute/lexicons": "^2.1.1",
+		"@types/node": "^26.6.5",
 		"better-result": "^3.0.1",
 		"hono": "4.13.13",
+		"ipaddr.js": "^2.5.0",
 		"valibot": "^1.5.0",
 		"wrangler": "4.151.3"
 	},
diff --git a/pds-check/src/hosts.ts b/pds-check/src/hosts.ts
new file mode 100644
index 0000000..7005a88
--- /dev/null
+++ b/pds-check/src/hosts.ts
@@ -0,0 +1,56 @@
+import { resolve4, resolve6 } from 'node:dns/promises';
+import { Result } from 'better-result';
+import ipaddr from 'ipaddr.js';
+
+function isIPv6(addr: ipaddr.IPv4 | ipaddr.IPv6): addr is ipaddr.IPv6 {
+	return addr.kind() === 'ipv6';
+}
+
+function isPublicIp(ip: string) {
+	if (!ipaddr.isValid(ip)) return false;
+	let addr = ipaddr.parse(ip);
+	if (isIPv6(addr) && addr.isIPv4MappedAddress()) {
+		addr = addr.toIPv4Address(); // ::ffff:127.0.0.1 -> 127.0.0.1
+	}
+	return addr.range() === 'unicast'; // everything special-purpose is rejected
+}
+
+const BLOCKED_SUFFIXES = [
+	'.local',
+	'.localhost',
+	'.internal',
+	'.lan',
+	'.home.arpa',
+	'.test',
+	'.invalid',
+];
+
+async function resolvesPublic(host: string) {
+	// oxlint-disable-next-line unicorn/no-await-expression-member
+	const addrs = (await Promise.allSettled([resolve4(host), resolve6(host)]))
+		.filter((r) => r.status === 'fulfilled')
+		.flatMap((r) => r.value);
+
+	return addrs.length > 0 && addrs.every((a) => isPublicIp(a));
+}
+
+/**
+ * Check whether a given hostname is a public domain, or ip address.
+ * This can be used to protect against `fetch` calls to these addresses,
+ * however since we can't use the result of the dns lookup we perform
+ * DNS rebinding attacks are still possible - although the risk is acceptable.
+ *
+ * @param hostname - hostname to check
+ */
+export async function isPublicHost(hostname: string) {
+	// oxlint-disable-next-line e18e/prefer-static-regex
+	let host = hostname.toLowerCase().replace(/\.$/, '');
+	if (host.startsWith('[') && host.endsWith(']')) host = host.slice(1, -1);
+
+	if (ipaddr.isValid(host)) return Result.ok(isPublicIp(host));
+	if (!host.includes('.')) return Result.ok(false);
+	if (host === 'localhost') return Result.ok(false);
+	if (BLOCKED_SUFFIXES.some((s) => host.endsWith(s))) return Result.ok(false);
+
+	return await Result.tryPromise(async () => await resolvesPublic(host));
+}
diff --git a/pds-check/src/index.ts b/pds-check/src/index.ts
index 7a2e7f7..1871d02 100644
--- a/pds-check/src/index.ts
+++ b/pds-check/src/index.ts
@@ -1,6 +1,7 @@
+import { describeServer } from './xrpc';
+import { isPublicHost } from './hosts';
 import { logger } from 'hono/logger';
 import { Hono } from 'hono';
-import { describeServer } from './xrpc';
 
 const app = new Hono<{ Bindings: Env }>();
 
@@ -23,6 +24,15 @@ app.get('/v0/check', async (c) => {
 		});
 	}
 
+	const publicHost = await isPublicHost(url.hostname);
+
+	if (publicHost.isErr() || !publicHost.value) {
+		return c.json({
+			valid: false,
+			reason: 'url is invalid',
+		});
+	}
+
 	const result = await describeServer(url);
 	if (!result.valid) return c.json(result);
 
diff --git a/pds-check/tsconfig.json b/pds-check/tsconfig.json
index 1ffd8b0..0d20943 100644
--- a/pds-check/tsconfig.json
+++ b/pds-check/tsconfig.json
@@ -9,7 +9,7 @@
 		"checkJs": true,
 		"resolveJsonModule": true,
 		"skipLibCheck": true,
-		"types": ["./worker-configuration.d.ts"],
+		"types": ["./worker-configuration.d.ts", "node"],
 		"lib": ["esnext"]
 	},
 	"include": ["./src/**.ts", "./tests/**.ts", "./worker-configuration.d.ts"],
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c266d13..a54aa99 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -89,12 +89,18 @@ importers:
       '@atcute/lexicons':
         specifier: ^2.1.1
         version: 2.1.1
+      '@types/node':
+        specifier: ^26.6.5
+        version: 26.6.5
       better-result:
         specifier: ^3.0.1
         version: 3.0.1
       hono:
         specifier: 4.13.13
         version: 4.13.13
+      ipaddr.js:
+        specifier: ^2.5.0
+        version: 2.5.0
       valibot:
         specifier: ^1.5.0
         version: 1.5.0(typescript@6.0.3)
@@ -1342,6 +1348,10 @@ packages:
     resolution: {integrity: sha512-5Hh7Y1wQbvY5ooGgPbDaL5iYLAPzMTUrjMulskHLH6wnv/A+1q5rgEaiuqEjB+oxGXIVZs1FF+R/KPN3ZSQYYg==}
     engines: {node: '>=12'}
 
+  ipaddr.js@2.5.0:
+    resolution: {integrity: sha512-aq+t5NAc+cS6rZQQVWC2x98CPqGtKKTMDd4Gaodv0wShnItdKg/51djkGJ1hqH+Oy0ivDftCbSLCQob8zso01w==}
+    engines: {node: '>= 10'}
+
   is-reference@3.0.3:
     resolution: {integrity: sha512-ixkJoqQvAP88E6wLydLGGqCJsrFUnqoH6HnaczB8XmDH1oaWU+xxdptvikTgaEhtZ53Ky6YXiBuUI2WXLMCwjw==}
 
@@ -2733,6 +2743,8 @@ snapshots:
 
   internmap@2.0.3: {}
 
+  ipaddr.js@2.5.0: {}
+
   is-reference@3.0.3:
     dependencies:
       '@types/estree': 1.0.9