Amer Networks E5Web GUI - Manual (Page 399)

be written by a TFTP client. The default value is Allow.
Remove Request Option Specifies if options should be removed from request. The
default is False which means "do not remove".
Allow Unknown Options If this option is not enabled then any option in a request
other than the blocksize, the timeout period and the file
transfer size is blocked. The setting is disabled by default.
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 cOS Core 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 Clavister Security Gateway 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.
Chapter 6: Security Mechanisms
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