External UNIX socket

This explains the meaning of external socket is used error message, and the purpose of --external unix[...] 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 --external[...] option should be used. By providing this option for both dump (now) and restore (in plans) 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

Some types of sockets are hard to deal with.

stream/seqpacket

Named unix sockets with stream/seqpacket options can't be dumped/restored, as once we dump one end, the other one will see EOF on the socket and may close it.

The plan for this is to extend the --ext-unix-sk semantics to work like this:

On dump, the --external unix[id] says 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 specified path) (THIS IS IN PLANS!)

socketpair

Unnamed unix sockets created with socketpair() system call can be dumped and restored.

On dump, put inode of this socket in --ext-unix-sk option, for example:

criu dump -D images -o dump.log -v4 --external unix[11890815] -t 16528

On restore, the server should create a new pair and call criu restore asking it to inherit one. For example:

criu restore -d -D images -o restore.log --pidfile restore.pid -v4 -x --inherit-fd fd[3]:socket:[11890815]

Test is available for this feature, see test/socketpairs in criu sources.