fix(mcp): pin outbound connections to IPv4 to avoid unreachable-IPv6 hangs

apps/sim/lib/core/security/input-validation.server.ts

@@ -105,7 +105,10 @@ export async function validateUrlWithDNS(
   }
 
   try {
-    const { address } = await dns.lookup(cleanHostname, { verbatim: true })
+    // Prefer IPv4: pinning strips Happy Eyeballs' fallback, and a pinned IPv6 address hangs
+    // on IPv4-only egress (e.g. AWS NAT gateways). See validateMcpServerSsrf.
+    const resolved = await dns.lookup(cleanHostname, { all: true, verbatim: true })
+    const { address } = resolved.find((entry) => entry.family === 4) ?? resolved[0]
 
     const resolvedIsLoopback =
       ipaddr.isValid(address) &&

apps/sim/lib/mcp/domain-check.test.ts

@@ -364,12 +364,31 @@ describe('validateMcpServerSsrf', () => {
   })
 
   it('returns resolved IP for URLs that resolve to public IPs', async () => {
-    mockDnsLookup.mockResolvedValue({ address: '93.184.216.34' })
+    mockDnsLookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }])
     await expect(validateMcpServerSsrf('https://example.com/mcp')).resolves.toBe('93.184.216.34')
   })
 
+  it('prefers IPv4 over IPv6 for a dual-stack host (verbatim returns IPv6 first)', async () => {
+    // Cloudflare-fronted hosts resolve IPv6-first; pinning that IPv6 hangs on an IPv4-only
+    // egress. The guard must pin the reachable IPv4 instead.
+    mockDnsLookup.mockResolvedValue([
+      { address: '2606:4700:3037::ac43:cc5f', family: 6 },
+      { address: '104.21.22.105', family: 4 },
+    ])
+    await expect(validateMcpServerSsrf('https://app.withgauge.com/mcp')).resolves.toBe(
+      '104.21.22.105'
+    )
+  })
+
+  it('pins the sole IPv6 address for an IPv6-only host', async () => {
+    mockDnsLookup.mockResolvedValue([{ address: '2606:4700:3037::ac43:cc5f', family: 6 }])
+    await expect(validateMcpServerSsrf('https://ipv6-only.example/mcp')).resolves.toBe(
+      '2606:4700:3037::ac43:cc5f'
+    )
+  })
+
   it('returns resolved IP for HTTP URLs on non-localhost hosts', async () => {
-    mockDnsLookup.mockResolvedValue({ address: '93.184.216.34' })
+    mockDnsLookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }])
     await expect(validateMcpServerSsrf('http://example.com:3000/mcp')).resolves.toBe(
       '93.184.216.34'
     )
@@ -405,12 +424,12 @@ describe('validateMcpServerSsrf', () => {
   })
 
   it('throws McpSsrfError for URLs resolving to private IPs', async () => {
-    mockDnsLookup.mockResolvedValue({ address: '10.0.0.5' })
+    mockDnsLookup.mockResolvedValue([{ address: '10.0.0.5', family: 4 }])
     await expect(validateMcpServerSsrf('https://internal.corp/mcp')).rejects.toThrow(McpSsrfError)
   })
 
   it('throws McpSsrfError for URLs resolving to link-local IPs', async () => {
-    mockDnsLookup.mockResolvedValue({ address: '169.254.169.254' })
+    mockDnsLookup.mockResolvedValue([{ address: '169.254.169.254', family: 4 }])
     await expect(validateMcpServerSsrf('https://metadata.internal/latest')).rejects.toThrow(
       McpSsrfError
     )
@@ -424,7 +443,7 @@ describe('validateMcpServerSsrf', () => {
   })
 
   it('returns resolved IP for URLs resolving to loopback on self-hosted (localhost alias)', async () => {
-    mockDnsLookup.mockResolvedValue({ address: '127.0.0.1' })
+    mockDnsLookup.mockResolvedValue([{ address: '127.0.0.1', family: 4 }])
     await expect(validateMcpServerSsrf('http://my-local-alias:3000/mcp')).resolves.toBe('127.0.0.1')
   })
 
@@ -450,14 +469,14 @@ describe('validateMcpServerSsrf', () => {
     })
 
     it('rejects URLs resolving to loopback on hosted', async () => {
-      mockDnsLookup.mockResolvedValue({ address: '127.0.0.1' })
+      mockDnsLookup.mockResolvedValue([{ address: '127.0.0.1', family: 4 }])
       await expect(validateMcpServerSsrf('http://my-local-alias:3000/mcp')).rejects.toThrow(
         McpSsrfError
       )
     })
 
     it('returns resolved IP for public IP resolutions on hosted', async () => {
-      mockDnsLookup.mockResolvedValue({ address: '93.184.216.34' })
+      mockDnsLookup.mockResolvedValue([{ address: '93.184.216.34', family: 4 }])
       await expect(validateMcpServerSsrf('https://example.com/mcp')).resolves.toBe('93.184.216.34')
     })
 
@@ -483,7 +502,7 @@ describe('validateMcpServerSsrf', () => {
     })
 
     it('still blocks DNS resolutions to private IPs on hosted (regression)', async () => {
-      mockDnsLookup.mockResolvedValue({ address: '10.0.0.5' })
+      mockDnsLookup.mockResolvedValue([{ address: '10.0.0.5', family: 4 }])
       await expect(validateMcpServerSsrf('https://internal.corp/mcp')).rejects.toThrow(McpSsrfError)
     })
 
@@ -508,7 +527,7 @@ describe('validateMcpServerSsrf', () => {
     })
 
     it('returns resolved loopback IP for DNS aliases (caller pins)', async () => {
-      mockDnsLookup.mockResolvedValue({ address: '127.0.0.1' })
+      mockDnsLookup.mockResolvedValue([{ address: '127.0.0.1', family: 4 }])
       await expect(validateMcpServerSsrf('http://my-local-alias/mcp')).resolves.toBe('127.0.0.1')
     })
   })

apps/sim/lib/mcp/domain-check.ts

@@ -185,8 +185,10 @@ export async function validateMcpServerSsrf(url: string | undefined): Promise<st
 
   let address: string
   try {
-    const lookup = await dns.lookup(cleanHostname, { verbatim: true })
-    address = lookup.address
+    // Prefer IPv4: pinning strips Happy Eyeballs' fallback, and a pinned IPv6 address
+    // (which `verbatim` returns first for dual-stack hosts) hangs on IPv4-only egress.
+    const resolved = await dns.lookup(cleanHostname, { all: true, verbatim: true })
+    address = (resolved.find((entry) => entry.family === 4) ?? resolved[0]).address
   } catch (error) {
     logger.warn('DNS lookup failed for MCP server URL', {
       hostname,