Comment by cryptonector

Registration of such voters can be audited and checked carefully to limit their numbers (since they must be inherently limited), and ballots can be delivered to them, or voting equipment can be brought to them (e.g., elderly care homes). I'm not sure much can be done to make voting by the permanently confined secure. For military the military itself can help a great deal. For civilians overseas, either come back to vote, vote at a consulate, or don't vote -- I don't think it's necessary to bend over backwards for civilians who choose to be overseas unless a country has a huge proportion of its population residing abroad.