DirServer is being replaced by
DirAuthority - see the ticket. And - according to an official manual - it says:
FallbackDir address:port orport=port id=fingerprint [weight=num] [ipv6=address:orport]
When we’re unable to connect to any directory cache for directory info (usually because we don’t know about any yet) we try a directory authority. Clients also simultaneously try a FallbackDir, to avoid hangs on client startup if a directory authority is down. Clients retry FallbackDirs more often than directory authorities, to reduce the load on the directory authorities. By default, the directory authorities are also FallbackDirs. Specifying a FallbackDir replaces Tor’s default hard-coded FallbackDirs (if any). (See the DirAuthority entry for an explanation of each flag.)
Use Tor’s default hard-coded FallbackDirs (if any). (When a FallbackDir
line is present, it replaces the hard-coded FallbackDirs, regardless of
the value of UseDefaultFallbackDirs.) (Default: 1)
seem to be your solution, and
DirAuthority [nickname] [flags] address:port fingerprint
Use a nonstandard authoritative directory server at the provided address and port, with the specified key fingerprint. This option can be repeated many times, for multiple authoritative directory servers. Flags are separated by spaces, and determine what kind of an authority this directory is. By default, an authority is not authoritative for any directory style or version unless an appropriate flag is given. Tor will use this authority as a bridge authoritative directory if the "bridge" flag is set. If a flag "orport=port" is given, Tor will use the given port when opening encrypted tunnels to the dirserver. If a flag "weight=num" is given, then the directory server is chosen randomly with probability proportional to that weight (default 1.0). If a flag "v3ident=fp" is given, the dirserver is a v3 directory authority whose v3 long-term signing key has the fingerprint fp. Lastly, if an "ipv6=address:orport" flag is present, then the directory authority is listening for IPv6 connections on the indicated IPv6 address and OR Port.
Tor will contact the authority at address:port (the DirPort) to download directory documents. If an IPv6 address is supplied, Tor will also download directory documents at the IPv6 address on the DirPort.
If no DirAuthority line is given, Tor will use the default directory authorities. NOTE: this option is intended for setting up a private Tor network with its own directory authorities. If you use it, you will be distinguishable from other users, because you won’t believe the same authorities they do.
so - yes - there's no use in adding DirAuthority lines, unless you're making your own testnet. But if you need to use some explicit directories - you can add their descriptions as
FallbackDir lines and trying to disable default ones via
UseDefaultFallbackDirs 0 and blocking an access to them - you can try to force Tor to use your provided list of directory mirrors.
But - I'm curious - what do you need this setup for? Can you add some more details about your task?