Functions
An ALG may offer the following functions:
- allowing client applications to use dynamic ephemeral TCP/ UDP ports to communicate with the known ports used by the server applications, even though a firewall-configuration may allow only a limited number of known ports. In the absence of an ALG, either the ports would get blocked or the network administrator would need to explicitly open up a large number of ports in the firewall — rendering the network vulnerable to attacks on those ports.
- converting the network layer address information found inside an application payload between the addresses acceptable by the hosts on either side of the firewall/NAT. This aspect introduces the term 'gateway' for an ALG.
- recognizing application-specific commands and offering granular security controls over them
- synchronizing between multiple streams/sessions of data between two hosts exchanging data. For example, an FTP application may use separate connections for passing control commands and for exchanging data between the client and a remote server. During large file transfers, the control connection may remain idle. An ALG can prevent the control connection getting timed out by network devices before the lengthy file transfer completes.
Deep packet-inspection of all the packets handled by ALGs over a given network makes this functionality possible. An ALG understands the protocol used by the specific applications that it supports.
For instance, for Session Initiation Protocol (SIP) Back-to-Back User agent (B2BUA), an ALG can allow firewall traversal with SIP. If the firewall has its SIP traffic terminated on an ALG then the responsibility for permitting SIP sessions passes to the ALG instead of the firewall. An ALG can solve another major SIP headache: NAT traversal. Basically a NAT with a builtin ALG can rewrite information within the SIP messages and can hold address bindings until the session terminates.
An ALG is very similar to a proxy server, as it sits between the client and real server, facilitating the exchange. There seems to be an industry convention that an ALG does its job without the application being configured to use it, by intercepting the messages. A proxy, on the other hand, usually needs to be configured in the client application. The client is then explicitly aware of the proxy and connects to it, rather than the real server.
Read more about this topic: Application-level Gateway
Famous quotes containing the word functions:
“Mark the babe
Not long accustomed to this breathing world;
One that hath barely learned to shape a smile,
Though yet irrational of soul, to grasp
With tiny finger—to let fall a tear;
And, as the heavy cloud of sleep dissolves,
To stretch his limbs, bemocking, as might seem,
The outward functions of intelligent man.”
—William Wordsworth (1770–1850)
“Nobody is so constituted as to be able to live everywhere and anywhere; and he who has great duties to perform, which lay claim to all his strength, has, in this respect, a very limited choice. The influence of climate upon the bodily functions ... extends so far, that a blunder in the choice of locality and climate is able not only to alienate a man from his actual duty, but also to withhold it from him altogether, so that he never even comes face to face with it.”
—Friedrich Nietzsche (1844–1900)
“Empirical science is apt to cloud the sight, and, by the very knowledge of functions and processes, to bereave the student of the manly contemplation of the whole.”
—Ralph Waldo Emerson (1803–1882)