[Security] DNS queries allowed to any IP on port 53 (data exfiltration risk)

[Mossaka]

Priority

High

Description

The firewall currently allows DNS queries (port 53) to ANY IP address, not just the configured DNS servers. This creates a data exfiltration vector.

Current behavior

• host-iptables.ts:181-192 allows UDP/TCP port 53 to any destination
• An attacker could send data to evil-dns-server.com:53 even if that domain isn't in the allowlist
• The --dns-servers flag is documented but not enforced in the host-level filtering rules

Expected behavior

• DNS queries should only be allowed to explicitly allowlisted DNS server IPs
• Default: 8.8.8.8 and 8.8.4.4
• User-configurable via --dns-servers flag

Code locations

• src/host-iptables.ts:181-192 - Need to restrict by destination IP
• src/host-iptables.ts:setupHostIptables() - Add dnsServers parameter
• src/cli.ts - Pass DNS server IPs to setupHostIptables()

Suggested fix

// Instead of allowing port 53 to any IP:
for (const dnsServer of dnsServers) {
  await execa('iptables', [
    '-t', 'filter', '-A', chainName,
    '-p', 'udp', '-d', dnsServer, '--dport', '53',
    '-j', 'ACCEPT',
  ]);
  // Same for TCP
}

Related documentation

• CLAUDE.md:226-253 documents DNS allowlisting but implementation is incomplete