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mod_asn looks up the AS and network prefix of IP address.

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mod_asn is an Apache module doing lookups of the autonomous system (AS) and the

network prefix that an IP address is contained in.

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It is written with scalability in mind. To do high-speed lookups, it uses the

PostgreSQL ip4r datatype that is indexable with a Patricia Trie algorithm to

store network prefixes.

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It comes with script to create such a database and update it with snapshots from

router's "view of the world".

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The module sets the looked up data as env table variables, for use by other

Apache module to do things with it, or for logging -- and it can add the data

as response headers to the client.

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Example HTTP response headers:

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HTTP/1.1 200 OK

Date: Thu, 12 Feb 2009 23:24:33 GMT

Server: Apache/2.2.11 (Linux/SUSE)

X-Prefix: 83.133.0.0/16

X-AS: 13237

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Performance

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The database with all ~250.000 prefixes is about 20-30MB in size in the form of

a PostgreSQL database. Without any tuning, it is able to to >3000 lookups per

second on a MacBook Pro (tested with random IPs, a single connection, and

client written in Python running on the same machine).

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The Apache module is extremely lightweight. 

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Design notes

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Performed with a Patricia Trie algorithm, the lookup is very efficient. The

Patricia Trie is a radix tree that works it way from bit to bit, starting at

the most significant bit. At each bit, there are two alternative "paths". Or

put another way, the space of prefixes is roughly divided in two halfs at each

point. The ip4r datatype achieves this by implementing an index that works this

way. Without the index, a full table scan would be required, plus bitmask

prefix match for each of the ~250.000 candidate rows.

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"Conventional" storage in databases is possible with a workaround, e.g. with

two long integers denoting each prefix in a MySQL database. But this would

require an SQL "between" query. An additional column would be needed to store

the prefix length, in order to find the closest match (the most narrow prefix).

The built-in inet/cidr data type in PostgreSQL doens't help either because it

can't be indexed. With conventional methods, only about 30 lookups per second

can be achieved with a database.

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Having the data in a real database makes it accessible for other means as well;

it is easily possible to query it the list of prefixes that an AS announces,

for instance. In addition, the storage in the database offers the possibility

to change and update the data (or even completely replace it) in a simple way,

by doing this in transaction, without blocking running queries.

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For usage outside of Apache, a small libpq-based standalone daemon could be

written that queries the database. Alternatively, a small handler could be

written for mod_asn that does nothing than read an IP address from a request

body (or URL) and return the result.

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One argument for the ip4r data type in PostgreSQL is that it is IPv6-ready.

Some IPv6 autonomous systems already exist (about 800 as of the beginning of

2009).

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Usage with MirrorBrain

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mod_asn can support mod_mirrorbrain (see http://mirrorbrain.org).

mod_mirrorbrain can use the data (set in the subprocess environment) for its

mirror selection algorithm.

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In addition, the database can be queried with the MirrorBrain tool set:

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 # mb iplookup mirror.susestudio.com

130.57.19.0/24 (AS3680)

 # mb iplookup mirror.susestudio.com --all-prefixes

130.57.19.0/24 (AS3680)

130.57.0.0/16, 130.57.0.0/20, 130.57.19.0/24, 130.57.32.0/21, 137.65.0.0/16,

147.2.0.0/17, 151.155.0.0/16, 164.99.0.0/16, 192.31.114.0/24, 192.94.118.0/24,

192.108.102.0/24, 192.149.26.0/24, 195.109.215.0/24, 212.153.69.0/24

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