mod_asn is an optional extension for MirrorBrain. It is not required.
mod_asn allows a refined mirror selection upon certain network characteristics.
Without mod_asn, MirrorBrain selects an appropriate mirror by geolocation through the GeoIP database. Thus, the client gets his download from a mirror within the same country, or if that isn’t possible from a mirror within the same continent at least. That’s often all that you need.
With mod_asn, MirrorBrain additionally looks at the network prefix that the client’s IP address is contained in, and the autonomous system (AS) that it belongs to, and (if possible) matches that to the networks that the mirrors live in. This can be useful in the following scenarios:
You have a lot of mirrors - so many, that there is a chance that a client might have a mirror on his own network. mod_asn will find it by matching the AS and network prefix. For instance, if there’s a large ISP who run a mirror, and they probably have a significant number of downloaders among their customers, it is generally desirable to send them to their ISPs mirror, instead of somewhere else.
On the contrary, if such “network locality” does not exist, there is no reason to look up the network prefix or autonomous system and match them to the mirrors. For instance, say you have exactly one US mirror, one in India, and one in Germany; then GeoIP is good enough to assign the clients to their closest mirror simply by looking at the client’s country.
You have a certain mirror that has a lot of users on the same network, and you want to make sure that those clients all get sent to that mirror, and not just to any mirror in the same country (what GeoIP alone would do).
You have a certain mirror that doesn’t want to get requests from any network other than its own. Only local traffic is wanted, by policy. MirrorBrain knows which clients are “local”, and you set a flag in the mirror’s config (prefixOnly or asOnly) which tells MirrorBrain that the mirror should not get any other traffic. The mirrors will be happy to know about this possibility.
To install mod_asn, refer to the its documentation.