Try to use SQLPLUS to connect a oracle database server behind firewall.
$sqlplus
scott/tiger@"(description=(address_list=(address=(protocol=tcp)(port=1521)(hos
t=FireWallHostName))(address=(protocol=tcp)(port=1521)(host=192.168.1.35)))(co
nnect_data=(INSTANCE_NAME=oradb))(source_route=yes))"
Got error message as follow,