1/7/2024 0 Comments Web server ephemeral ports![]() ![]() Allowing "inbound port 22" will allow connections to your server, while allowing "outbound port 22" will allow connections from your server, and in both cases the stateful firewall will implicitly allow responses to go in the opposite direction. Other software vendors are supposed to respect these registered values and register their own server application port numbers from the pool of unused values. Vendors can register their application’s ports with ICANN. if you're looking at "Inbound" and "Outbound" tabs in AWS security groups, you can think in terms of whole connections, not individual packets. Clients should choose ephemeral port numbers from this range, but many systems do not. (For example, with a Linux server using iptables or ufw as the firewall, you could see its own state table using conntrack -L.) They also do the same for UDP (keeping track of ephemeral ports that were used by "recent" packets). ![]() Scenario 2: VPC with Public and Private Subnets (NAT) Scenario 3: VPC with Public and Private Subnets and AWS Managed VPN Access. Usually you don't need to explicitly allow outbound packets to ephemeral ports because many firewalls are stateful – they keep track of src/dst ports used by active TCP connections and will automatically allow packets that look like they belong to an "established" connection. Scenario 1: VPC with a Single Public Subnet. "Outbound TCP Port 22" means packets sent from your server's ephemeral ports to another host's port 22, so it never matches the replies that your server sends to inbound connections.) The Operating System (OS) of the application selects this port randomly to become the port that it will use to send / receive data for that server and application. (This applies equally to both "Inbound" and "Outbound" rules – e.g. In almost all cases, if you see a field labelled only as "Port", it really means "Destination port" – the source port is very rarely (if ever) checked for new connections, so "Port 22" really means "any→22" and not "22→22". Your firewall rules allow SSH because they only check one port, not both. Response traffic isnt on port 80, its on one of the ephemeral ports. You can see this for yourself using ss -tn/ netstat -tn to list active sockets (either on the client or the server), or using a packet capture tool ( wireshark, termshark, tcpdump -n) to see the actual TCP packets – with their ports and other parameters. You do not need to open 80 (or 443) on the outbound for your web server to work. Blocking outbound for the ephemeral ports is blocking that response. The client chose that when it sent the first packet to the web server. You need to open ephemeral ports 1024-65535 (assuming a Linux server is being used) Your server will receive requests on 80 (or 443) but send the response over one of those ephemeral. For packets from the web server, the destination port is whatever port the client is sending its packets from. For packets to the web server, the destination port is 80. An ephemeral port is a communications endpoint ( port) of a transport layer protocol of the Internet protocol suite that is used for only a short period of time for the duration of a communication session. They are used to distinguish between different client connections to a server and are released once the connection is closed. That's the default behavior of TCP sockets unless a program explicitly binds to some local port. Which port is the 'destination' port depends on which direction the packets are going. Any TCP client, including SSH clients, will use ephemeral ports on the 'client' side. ![]()
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