Application Layer Gateway
The Application Layer Gateway ( ALG or Application-Level Gateway for short ) is a security component in a computer network. When transmitting data in a network, some protocols such as TFTP or RTP use a randomly selected port , which means that on a Server must have a large number of ports open. This fact makes attacks on the system easier. In order to close this gap, the ALG enables dynamic port sharing, which means that only those ports are opened that are also required for current sessions .
The functionality of an application layer gateway represents a stateful inspection firewall , which should not be confused with an application level firewall !
DNS ALG
Another task of the ALG is to ensure in an internal network that works with NAT that an accessible IP address is transmitted during name lookups . This function is called DNS ALG . For example, if an internal network with a DNS server is connected to the Internet via a router and there is an external request for a host from the internal network, the DNS server of the internal network would also specify an internal IP address. The host cannot be reached via this address for the client requesting the address , as it may be in a private IP address range that is not routed on the Internet. If the DNS server now sends a data packet with the internal IP address to the requesting client, the DNS ALG on the router changes the data segment so that an address that can be reached from the Internet from the address range that is also used by NAT is used for the requested host is entered. Furthermore, the DNS ALG initializes that the corresponding host is also routed for a certain time via the external address assigned to it, as well as the associated timer.
literature
- Ulrich Trick, Frank Weber: SIP and telecommunications networks. 5th edition, Walter De Gruyter GmbH, Berlin 2015, ISBN 978-3-486-77853-3 .