An application layer gateway (ALG) is a network function that understands a particular application protocol and helps its traffic pass through a NAT device or firewall. Unlike basic network address translation, an ALG can inspect or change protocol details inside traffic—such as embedded IP addresses and ports—and coordinate the connection state needed for related data flows.
What does an application layer gateway do?
Network address translation (NAT) changes addressing information at the network layer. Some application protocols also include IP addresses or port numbers in their messages. A basic NAT device may not recognize that information, so the application’s related traffic can fail to connect.
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An ALG is an application-specific translation agent: it recognizes selected protocol messages, adjusts embedded information when needed, and can coordinate NAT or firewall state for additional connections. The term most often refers to this NAT helper function, although “application-level gateway” can also mean a broader application protocol translator.
RFC 2663 defines the NAT-context ALG as a protocol-specific function that allows an application on one address realm to connect transparently to its counterpart in another. Its behavior depends on the protocol it supports and how the network device implements it.
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How does an ALG work with NAT or a firewall?
- It recognizes a supported protocol. The gateway examines application messages for information relevant to the connection.
- It handles embedded addressing or ports. If a message contains an address or port that must change to work across NAT, the ALG may rewrite it.
- It coordinates related traffic. When the application opens a secondary connection or negotiates dynamic ports, the ALG may update NAT state or arrange the firewall handling required for that traffic.
For example, FTP can use a control connection to negotiate a separate data connection. An FTP ALG can adjust the address and port information carried in the control messages and coordinate the corresponding NAT mapping. Without that protocol-aware handling, the separately negotiated connection may not work across the address boundary. RFC 2663 describes this NAT and FTP interaction.
What are common ALG examples?
FTP
FTP is a classic example because control messages can identify the address and port for a separate data flow. An ALG that understands FTP can help that flow work through NAT by handling the embedded connection details.
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SIP and VoIP
Session Initiation Protocol (SIP) and H.323 are used in voice-over-IP systems. A VoIP ALG may parse signaling messages and help configure address and port handling for call traffic. NIST’s 2005 guidance describes this role for SIP and H.323; it is a historical technical example, not a guarantee about every current VoIP deployment. NIST SP 800-58
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How is an ALG different from a proxy?
In RFC 2663’s NAT terminology, an ALG works with the application’s existing communication and does not require a special protocol between the client and gateway or a change to the client. A proxy, by contrast, communicates with a proxy-aware client using a protocol for that purpose and relays data. The terms can be used more broadly in other contexts, so the specific network role matters. RFC 2663
Why can an ALG cause problems?
The same protocol awareness that can restore connectivity also lets an ALG alter application traffic. If the ALG misinterprets a message, does not support the protocol’s encoding, or conflicts with the application’s own NAT-traversal method, its edits can break connections. RFC 3235 notes that building an ALG can be difficult or impossible for some protocols.
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For TCP-affecting ALGs, RFC 5382 recommends that NAT devices disable them by default, with FTP as the exception. This is a recommendation in a standards document, not a universal instruction to switch off every ALG on every current router. The right setting depends on the application, device firmware, and network path. RFC 5382
Should you disable SIP ALG?
There is no reliable yes-or-no answer for every device and service. If calls fail, have one-way audio, or behave inconsistently, check the VoIP provider’s guidance and the router or firewall documentation for the specific firmware. Test the actual network path with the setting changed only when you can safely do so, and restore the prior setting if the change makes service worse. Standards explain general ALG behavior; they do not establish the correct setting for every SIP deployment.
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What does “application-level gateway” mean outside NAT?
The phrase can also describe gateways that mediate or translate application protocols, not just a NAT helper that edits selected fields. RFC 3234 describes a range of middlebox behavior, from mechanical substitution of protocol fields (as in some FTP handling) to translation between application protocols. When reading a network guide, check whether “ALG” means the narrower NAT function or a broader application gateway. RFC 3234
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