Using the Internet with Blocked Incoming Connections
Understanding Incoming Connection Blocks and Data Reception
A user recently posed a perplexing question: if all incoming connections to a computer are blocked, how is it still possible to receive data or maintain an active network connection?
The core of this issue lies in differentiating between initiating connections and accepting them. A computer can actively establish connections without necessarily allowing incoming connections.
The Role of Outbound Connections
Most internet activity begins with your computer initiating a connection to a server. This is an outbound connection.
Firewalls commonly block unsolicited incoming connections for security reasons, but they generally permit outbound connections.
How Data Reception Works with Blocked Incoming Connections
Data can be received even with blocked incoming connections because the computer is actively requesting it. Think of it like ordering something online.
You initiate the request (outbound connection), and the website sends you the information (data reception). The website didn't need to 'connect' to you first.
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Understanding Incoming Data Despite Blocked Connections
A SuperUser user, Kunal Chopra, has posed a compelling question regarding network communication. He inquires about how a computer can continue to receive data from the internet even when all incoming connections are actively blocked.
The Core Paradox
Kunal highlights a seeming contradiction: if a firewall or Internet Service Provider (ISP) prevents all incoming connections, how can a web server successfully deliver data in response to a user's request? The initial request originates from the user’s computer (an outgoing connection), but the server’s response constitutes incoming data.
Essentially, if all pathways for incoming traffic are closed, how is a reply possible?
The Role of Outgoing Connections
The key lies in understanding that firewalls typically operate by controlling connections, not by simply blocking all data packets. A firewall generally allows outgoing connections to be established.
When you initiate a request to a web server, your computer creates an outgoing connection. The firewall recognizes this as an authorized outbound communication and permits it.
Established Connections and Return Traffic
The web server, upon receiving your request, sends its response back to your computer. This return traffic isn't considered a new, unsolicited incoming connection.
Instead, it's recognized as part of the established connection initiated by your computer. The firewall allows this return traffic because it's directly related to an outgoing connection that was previously authorized.
UDP and Connectionless Protocols
Kunal also raises a valid point about the User Datagram Protocol (UDP), which is connectionless. This means UDP doesn't establish a formal connection before sending data.
However, even with UDP, firewalls can still differentiate between solicited and unsolicited traffic. When your computer sends a UDP packet, the firewall permits the corresponding response packets to return.
Firewall and ISP Handling of UDP
The firewall or ISP examines the destination port of the incoming UDP packets. If these packets are destined for your computer and correspond to a previously sent UDP request, they are allowed through.
This mechanism ensures that responses to your outgoing UDP queries can reach your machine, even in the absence of a traditional connection.
In summary, data reaches Kunal’s computer not by circumventing the firewall, but by leveraging the allowance of return traffic associated with established, outgoing connections, and by correctly handling responses to outgoing UDP packets.
Understanding Blocked Incoming Connections
A SuperUser community member, gowenfawr, provides a clear explanation regarding blocked incoming connections.
The phrase "incoming block" signifies that new connections attempting to initiate from outside are being prevented, while existing, established connections remain unaffected.
Stateful Firewall Operation
Firewalls operate by monitoring the status of network connections, a characteristic that defines them as Stateful Firewalls.
When an outgoing TCP/SYN packet is detected, the firewall permits its transmission.
Subsequently, upon receiving an incoming SYN/ACK packet, the firewall validates its correspondence to the previously observed outbound SYN.
This process continues, allowing packets that are part of an authorized three-way handshake, as defined by the firewall's rules.
Once a connection is terminated – indicated by FIN or RST packets – the firewall removes it from its list of permitted traffic.
UDP Handling
Similar connection tracking is applied to UDP traffic.
Although UDP is inherently connectionless, the firewall simulates a session or connection to manage and filter packets effectively.
This involves the firewall retaining sufficient information to associate UDP packets and determine their validity.
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