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83 lines
3.3 KiB
Plaintext
83 lines
3.3 KiB
Plaintext
2011/12/16 - How ACLs work internally in haproxy - w@1wt.eu
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An ACL is declared by the keyword "acl" followed by a name, followed by a
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matching method, followed by one or multiple pattern values :
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acl internal src 127.0.0.0/8 10.0.0.0/8 192.168.0.0/16
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In the statement above, "internal" is the ACL's name (acl->name), "src" is the
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ACL keyword defining the matching method (acl_expr->kw) and the IP addresses
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are patterns of type acl_pattern to match against the source address.
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The acl_pattern struct may define one single pattern, a range of values or a
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tree of values to match against. The type of the patterns is implied by the
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ACL keyword. For instance, the "src" keyword implies IPv4 patterns.
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The line above constitutes an ACL expression (acl_expr). ACL expressions are
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formed of a keyword, an optional argument for the keyword, and a list of
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patterns (in fact, both a list and a root tree).
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Dynamic values are extracted according to a fetch function defined by the ACL
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keyword. This fetch function fills or updates a struct acl_test with all the
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extracted information so that a match function can compare it against all the
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patterns. The fetch function is called iteratively by the ACL engine until it
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reports no more value. This makes sense for instance when checking IP addresses
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found in HTTP headers, which can appear multiple times. The acl_test is kept
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intact between calls and even holds a context so that the fetch function knows
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where to start from for subsequent calls. The match function may also use the
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context eventhough it was not designed for that purpose.
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An ACL is defined only by its name and can be a series of ACL expressions. The
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ACL is deemed true when any of its expressions is true. They are evaluated in
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the declared order and can involve multiple matching methods.
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So in summary :
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- an ACL is a series of tests to perform on a stream, any of which is enough
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to validate the result.
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- each test is defined by an expression associating a keyword and a series of
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patterns.
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- a keyword implies several things at once :
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- the type of the patterns and how to parse them
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- the method to fetch the required information from the stream
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- the method to match the fetched information against the patterns
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- a fetch function fills an acl_test struct which is passed to the match
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function defined by the keyword
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- the match function tries to match the value in the acl_test against the
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pattern list declared in the expression which involved its acl_keyword.
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ACLs are used by conditional processing rules. A rule generally uses an "if" or
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"unless" keyword followed by an ACL condition (acl_cond). This condition is a
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series of term suites which are ORed together. Each term suite is a series of
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terms which are ANDed together. Terms may be negated before being evaluated in
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a suite. A term simply is a pointer to an ACL.
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We could then represent a rule by the following BNF :
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rule = if-cond
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| unless-cond
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if-cond (struct acl_cond with ->pol = ACL_COND_IF)
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= "if" condition
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unless-cond (struct acl_cond with ->pol = ACL_COND_UNLESS)
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= "unless" condition
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condition
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= term-suite
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| term-suite "||" term-suite
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| term-suite "or" term-suite
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term-suite (struct acl_term_suite)
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= term
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| term term
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term = acl
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| "!" acl
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