Everytime I see AWS rules that just say "port" I always have to think wait, do they mean source or destination port here? From the documentation: The established keyword I mentioned earlier helps this a lot, but still it's just too easy to make an error.ĪWS NACLs always hurt my head a bit just because I'm used to ACLs always having source and destination. That's probably not what you intended to permit. The person originating connections on the server just has to source packets from port 22 and boom they get through the ACL and have unrestricted access to all high ports on your client. guess what? It also allows serverIP to connect to all ports 1024 and above on your client. So, from the client to server you'd need to permit source yourIP:1024-65535 destination serverIP:22, and then in the other direction you'd need to permit source serverIP:22 destination yourIP:1024-65535. Stateless is fine for controlling what IPs can talk to what other IPs, but once you get into ports you really need to be doing stateful ACLs to make it secure and not absurdly complex and prone to accidents.Įxample of the security troubles of stateless ACLs: In general, stateless ACLs are a terrible way to do security that includes ports. In other words, it denies initial connection requests, but permits everything else TCP. That last line permits all TCP packets that are not SYN-only flagged. Server to client: permit tcp any any established For example, on an IOS router you can do:Ĭlient to server: permit tcp yourIP eq any serverIP eq 22 However, in most situations (other than AWS NACLs) we have a better way of handling this. In a stateless firewall that is filtering in both directions, you have to permit the return packets. Technically, there are a few protocols that allocate temporary ports for incoming connections, but those are rare. Thus, they're unique, and can both exist at the same time.Įphemeral ports are those allocated temporarily, generally for outbound connection source ports. So, connection 1 is yourIP:40263 to serverIP:22 and connection 2 is yourIP:10642 to serverIP:22. So, the way it actually works is that an outgoing port is allocated that is unique. What if you wanted to make 2 connections at the same time? Connection 1 would be from yourIP:22 to serverIP:22 and connection 2 would be. But what should the source be? You might think 22, but now think about that a little more. The destination port of the original packet is 22. If you're posting a technical query, please include the following details, so that we can help you more efficiently:ĭoes this sidebar need an addition or correction? Tell us hereĮach connection has both a source and destination port. public IP addresses or hostnames, account numbers, email addresses) before posting! ✻ Smokey says: MITIGATION TEAM ACTIVE Note: ensure to redact or obfuscate all confidential or identifying information (eg. News, articles and tools covering Amazon Web Services (AWS), including S3, EC2, SQS, RDS, DynamoDB, IAM, CloudFormation, AWS-CDK, Route 53, CloudFront, Lambda, VPC, Cloudwatch, Glacier and more.
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