Summary: sr-ally(1) sr-ally(1)
sr-ally - IP Alias resolution pairwise test
Ally is the tool that executes the IP identifier-based pairwise alias test to discover whether two IP addresses
belong to interfaces on same machine. The source-address based test is also supported, and if undns is
installed, name-based alias resolution is used when routers are unresponsive or unreachable. We describe
each technique in turn, then present example output.
IP-identifier based alias resolution seeks evidence that the two IP addresses share a single IP-id counter.
The IP identifier field is intended to make each packet unique so that it can be reassembled after fragmenta-
tion. It is commonly implemented as a counter, or not used at all, though some have made a case that it
should be randomized. If packets generated by two different IP addresses have in-order IP identifiers, it is
likely, but not certain, that those IP addresses are aliases. This is the technique developed in the Rocketfuel
Source-address based alias resolution relies on routers that use the outgoing interface address as the source
of ICMP port unreachable messages. The source address to use is unspecified and up to the implementor,
so sometimes this is not used. It is, however, the most accurate way to show that addresses are aliases.
Name-based alias resolution exploits the structure of DNS names in certain ISPs to determine whether