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https://github.com/matrix-construct/construct
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114 lines
4.7 KiB
Text
114 lines
4.7 KiB
Text
The hostmask/netmask system.
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Copyright(C) 2001 by Andrew Miller(A1kmm)<a1kmm@mware.virtualave.net>
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Contents
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========
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* Section 1: Motivation
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* Section 2: Underlying mechanism
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- 2.1: General overview.
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- 2.2: IPv4 netmasks.
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- 2.3: IPv6 netmasks.
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- 2.4: Hostmasks.
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* Section 3: Exposed abstraction layer
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- 3.1: Parsing masks.
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- 3.2: Adding configuration items.
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- 3.3: Initialising or rehashing.
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- 3.4: Finding IP/host confs.
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- 3.5: Deleting entries.
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- 3.6: Reporting entries.
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Section 1: Motivation
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=====================
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Looking up config hostnames and IP addresses(such as for I-lines and
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K-lines) needs to be implemented efficiently. It turns out a hash
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based algorithm like that employed here performs well on the average
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case, which is what we should be the most concerned about. A profiling
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comparison with the mtrie code using data from a real network confirmed
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that this algorithm performs much better.
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Section 2: Underlying mechanism
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===============================
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2.1: General overview
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---------------------
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In short, a hash-table with linked lists for buckets is used to locate
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the correct hostname/netmask entries. In order to support CIDR IPs and
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wildcard masks, the entire key cannot be hashed, and there is a need to
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rehash. The means for deciding how much to hash differs between hostmasks
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and IPv4/6 netmasks.
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2.2: IPv4 netmasks
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------------------
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In order to hash IPv4 netmasks for addition to the hash, the mask is first
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processed to a 32 bit address and a number of bits used. All unused bits
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are set to 0. The mask could be in the forms:
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1.2.3.4 => 1.2.3.4 32
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1.2.3.* => 1.2.3.0 24
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1.2 => 1.2.0.0 16
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1.2.3.64/26 => 1.2.3.64 26
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The number of whole bytes is then calculated, and only those bytes are
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hashed. (e.g. 1.2.3.64/26 and 1.2.3.0/24 hash the same).
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When a complete IPv4 address is given so that an IPv4 match can be found,
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the entire IP address is first hashed, and looked up in the table. Then
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the most significant three bytes are hashed, followed by the most
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significant two, the most significant one, and finally the 'identity hash'
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bucket is searched(to match masks like 192/7).
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2.3: IPv6 netmasks
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------------------
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As per IPv4 netmasks, except that instead of rehashing with a one byte
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granularity, a 16 bit(two byte) granularity is used, as 16 rehashes is
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considered too great a fixed offset to be justified for a (possible)
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slight reduction in hash collisions.
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2.4: Hostmasks
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--------------
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On adding a hostmask to the hash, all of the hostmask right of the next
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dot after the last wildcard character in the string is hashed, or in the
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case that there are no wildcards in the hostmask, the entire string is
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hashed.
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On searching for a hostmask match, the entire hostname is hashed, followed
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by the entire hostmask after the first dot, followed by the entire
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hostmask after the second dot, and so on. Finally, the 'identity' hash
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bucket is checked, to catch hostnames like *test*.
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Section 3: Exposed abstraction layer
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====================================
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Section 3.1: Parsing masks
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--------------------------
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Call "parse_netmask()" with the netmask and a pointer to an irc_inaddr
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structure to be filled in, as well as a pointer to an integer where the
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number of bits will be placed.
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Always check the return value. If it returns HM_HOST, it means that the
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mask is probably a hostname mask. If it returns HM_IPV4, it means it was
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an IPv4 address. If it returns HM_IPV6, it means it was an IPv6 address.
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If parse_netmask returns HM_HOST, no change is made to the irc_inaddr
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structure or the number of bits.
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Section 3.2: Adding configuration items
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---------------------------------------
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Call "add_conf_by_address" with the hostname or IP mask, the username,
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and the ConfItem* to associate with this mask.
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Section 3.3: Initialising and rehashing
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----------------------------------------
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To initialise, call init_host_hash(). This only needs to be done once on
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startup.
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On rehash, to wipe out the old unwanted conf, and free them if there are
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no references to them, call clear_out_address_conf().
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Section 3.4: Finding IP/host confs
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----------------------------------
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Call find_address_conf() with the hostname, the username, the address,
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and the address family.
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To find a d-line, call find_dline() with the address and address family.
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Section 3.5: Deletiing entries
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------------------------------
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Call delete_one_address_conf() with the hostname and the ConfItem*.
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Section 3.6: Reporting entries
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------------------------------
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Call report_dlines, report_exemptlines, report_Klines() or report_Ilines()
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with the client pointer to report to. Note these walk the hash, which is
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inefficient, but these are not called often enough to justify the memory
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and maintenance clockcycles to for more efficient data structure.
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