D-Link DFL-260E User Manual for DFL-260E - Page 280
The SMTP ALG, Remove Request Option, Maximum Blocksize, Block Directory Traversal
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6.2.5. The SMTP ALG Chapter 6. Security Mechanisms TFTP Request Options As long as the Remove Request Option described above is set to false (options are not removed) then the following request option settings can be applied: Maximum Blocksize The maximum blocksize allowed can be specified. The allowed range is 0 to 65,464 bytes. The default value is 65,464 bytes. Maximum File Size The maximum size of a file transfer can be restricted. By default this is the absolute maximum allowed which 999,999 Kbytes. Block Directory Traversal This option can disallow directory traversal through the use of filenames containing consecutive periods (".."). Allowing Request Timeouts The NetDefendOS TFTP ALG blocks the repetition of an TFTP request coming from the same source IP address and port within a fixed period of time. The reason for this is that some TFTP clients might issue requests from the same source port without allowing an appropriate timeout period. 6.2.5. The SMTP ALG Simple Mail Transfer Protocol (SMTP) is a text based protocol used for transferring email between mail servers over the Internet. Typically the local SMTP server will be located on a DMZ so that mail sent by remote SMTP servers will traverse the NetDefend Firewall to reach the local server (this setup is illustrated later in Section 6.2.5.1, "Anti-Spam Filtering"). Local users will then use email client software to retrieve their email from the local SMTP server. SMTP is also used when clients are sending email and the SMTP ALG can be used to monitor SMTP traffic originating from both clients and servers. SMTP ALG Options Key features of the SMTP ALG are: Email rate limiting A maximum allowable rate of email messages can be specified. This rate is calculated on a per source IP address basis, in other words it is not the total rate that is of interest but the rate from a certain email source. This is a very useful feature to have since it is possible to put in a block against either an infected client or an infected server sending large amounts of malware generated emails. Email size limiting A maximum allowable size of email messages can be specified. This feature counts the total amount of bytes sent for a single email which is the header size plus body size plus the size of any email attachments after they are encoded. It should be kept in mind that an email with, for example, an attachment of 100 Kbytes, will be larger than 100 Kbytes. The transferred size might be 120 Kbytes or more since the encoding which takes place automatically for attachments may substantially increase the transferred attachment size. 280