I don't have a good idea how to handle this, but it would be helpful if plugins could get a second chance of a parent proxy fails. The situation now is that you have to call TSHttpTxnParentProxySet and cross your fingers. It would be a lot better if the state machine backed up to a hook when the parent proxy failed. OS-DNS would be a convenient hook, but it will be hard to restart sanely from there since there is a log if state around the DNS result. Maybe a new hook would make sense since that could also be used with TSHttpTxnServerAddrSet.