External UNIX socket
This explains the meaning of external socket is used error message, and the purpose of --ext-unix-sk option.
Error meaning
When dumping a process sub-tree, criu checks that the resulting image is consistent and self-contained, meaning if an object A references another object B, and A goes into dump, then B should be dumped as well. For example, if there is a pipe, and criu dumps its one end (object A) because it belongs to a process it dumps, it must also dump the other end of the pipe (object B), meaning it should take a process owning B into the image as well. Same is true for unix sockets: if there is a socket (A) that is connect()ed to another socket (B), and criu dumps socket A (because it is opened by a process it dumps), it must also dump socket B and the task who owns it. Possibly socket B is dumped some time later in the dumping process, but it must be dumped.
This restriction is in place to make sure that upon restore applications will continue to work, rather than, say, receive SIGPIPE signals due to half-closed pipes or sockets.
With that said, "external socket is used" error means criu detected a unix socket connected to another socket which is not being dumped (because it belongs to a process not going into the image).
Using the option
However, sometimes it is possible to dump and successfully restore only one end of a unix socket pair. One particular example is the datagram sockets with on-way connection (client to server) used e.g. by logd. Server opens a datagram socket and waits on it for messages to be written into a log file. Processes using logd also create datagram sockets and connect those to the server. These connections are thus uni-directional. In this case it is possible to dump a program with the client-side socket and on restore the socket needs to be reconnected back to the original server.
This is when --ext-unix-sk option should be used. By providing this option (for both dump and restore commands), user states "I know there may be uni-directional dgram unix connections, and I will make sure the server end will exist on restore".
For criu dump, this option enables dumping datagram unix sockets with additional information about that other ("external") socket it is connected to.
For criu restore, this option asks criu to re-connect such sockets back.
Limitations
Socket screated with socketpair() systemcall are not one-way connected and do not work with --ext-unix-sk
What to do with stream/seqpacket?
Such sockets cannot be just dumped and restored as once we dump one end, the other one seen EOF on socket and may close one. The plan for this is to extend the --ext-unix-sk semantics to work like this
- On dump the
--ext-unix-sk $idsays that socket with $id is OK to be disconnected - On restore the
--ext-unix-sk$id[=$path]says that the socket $id should be reconnected back to the path it say on dump (or to the $path).
This won't work for socketpair()-s also.
What to do with socketpair()-s?
To restore socketpair we need more help from the server -- the server should create new pair and call criu restore asking it to inherit one.