Sender Policy Framework (SPF), is an e-mail validation system designed to prevent e-mail spam by addressing a common vulnerability, source address spoofing. SPF allows administrators to specify which hosts are allowed to send e-mail from a given domain by creating a specific DNS SPF record in the public DNS..
example.org. IN SPF "v=spf1 a mx -all"
"v=" defines the version of SPF used. The following words provide mechanisms to use to determine if a domain is eligible to send mail. The "a" and "mx" specify the systems permitted to send messages for the given domain. The "-all" at the end specifies that, if the previous mechanisms did not match, the message should be rejected.
[edit] Mechanisms
Eight mechanisms are defined:
ALL Matches always; used for a default result like -all for all IPs not matched by prior mechanisms.
A If the domain name has an address record (A or AAAA) that can be resolved to the sender's address, it will match.
IP4 If the sender is in a given IPv4 address range, match.
IP6 If the sender is in a given IPv6 address range, match.
MX If the domain name has an MX record resolving to the sender's address, it will match (i.e. the mail comes from one of the domain's mail servers).
PTR If the domain name (PTR record) for the client's address is in the given domain and that domain name resolves to the client's address (forward-confirmed reverse DNS), match.
EXISTS If the given domain name resolves to any address, match (no matter the address it resolves to). This is rarely used. Along with the SPF macro language it offers more complex matches like DNSBL-queries.
INCLUDE If the included (a misnomer) policy passes the test this mechanism matches. This is typically used to include policies of more than one ISP.
2010/03/18
Ext2 Vs ext3
Ext3 filesystem is nothing but next version of ext2 filesystem with journaling support. Ext3 has been structurally implemented same as ext2 so they have same data structures. he most important difference between Ext2 and Ext3 is that Ext3 supports journaling which allows fast recovery from disk problems. You also get reliability and a better performance with ext3. Ext3 is designed to take care of both metadata and data.
Ext3 is a tiny bit slower than ext2 is, but it holds tremendous advantages. There is really only one difference between ext2 and ext3, and that is that ext3 uses a journal to prevent filesystem corruption in the case of an unclean shutdown (ie. before the filesystem is synced to disk). That makes ext3 a bit slower than ext2 since all metadata changes are written to the journal, and then flushed to disk, but on the other hand you don't risk having the entire filesystem destroyed at power failure or if an unwitted person turns the computer off uncleanly.
Ext3 is a tiny bit slower than ext2 is, but it holds tremendous advantages. There is really only one difference between ext2 and ext3, and that is that ext3 uses a journal to prevent filesystem corruption in the case of an unclean shutdown (ie. before the filesystem is synced to disk). That makes ext3 a bit slower than ext2 since all metadata changes are written to the journal, and then flushed to disk, but on the other hand you don't risk having the entire filesystem destroyed at power failure or if an unwitted person turns the computer off uncleanly.
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