Does Tor take node age into consideration when building a circuit?

Question

I've seen this for a while, but in my experience Tor seems to select well stablished nodes (whose uptime is >60 days) but in the specifications this is not shown. It's just that the current list of nodes doesn't grow/chage? Or is there some other mechanism in play?

Answer 1

From the §2.2 "Path selection and constraints" of the Path Spec (as of commit 6f2919a2):

Additionally, we may be building circuits with one or more requests in mind. Each kind of request puts certain constraints on paths:

• All service-side introduction circuits and all rendezvous paths should be Stable.
• All connection requests for connections that we think will need to stay open a long time require Stable circuits. Currently, Tor decides this by examining the request's target port, and comparing it to a list of "long-lived" ports. (Default: 21, 22, 706, 1863, 5050, 5190, 5222, 5223, 6667, 6697, 8300.)

A "stable" node is defined in §3.1 "Establishing server status" of the dir spec as:

"Stable" -- A router is 'Stable' if it is active, and either its uptime is at least the median uptime for known active routers, or its uptime is at least 30 days. Routers are never called stable if they are running a version of Tor known to drop circuits stupidly. (0.1.1.10-alpha through 0.1.1.16-rc are stupid this way.)

Stable nodes are also required in several other places, so it's likely that you'll often pick nodes with high uptimes when building circuits.