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Line 1: − This page describes how we handle established TCP connections+ − − == TCP repair mode in kernel == − − The sockoption called TCP_REPAIR was recently added to the kernel and helps with doing C/R for TCP sockets. − − When set this option turn the socket into a special state in which any action performed on it doesn't − result in any defined by protocol actions, but instead directly puts the socket into a state, which − should be at the end of the successfully finished operation. − − E.g. calling connect() on a repaired socket just switches one to the ESTABLISHED state with the peer set as requested. − The bind() call forcibly binds the socket to a given address (ignoring any potential conflicts). Close()-ing the − socket under repair happens without any transient FIN_WAIT/TIME_WAIT/etc states. Socket is silently killed. − − === Sequences === − − In order to restore the connection properly only binding and connecting it is not enough. One also needs to restore the − TCP sequence numbers. To do so the TCP_REPAIR_QUEUE and TCP_QUEUE_SEQ options were introduced. − − The former one selects which queue (input or output) will be repaired and the latter gets/sets the sequence. Note, that − setting the sequence is only possible on CLOSE-d socket. − − === Packets in queue === − − When set the queue to repair as described above, one can call recv or send syscalls on a repaired socket. Both calls − result on peeking or poking data from/to the respective queue. This sounds funny, but yes, for repaired socket one − can receve the outgoing and send the incoming queues. Using the MSG_PEEK flag for recv is required. − − === Options === − − There are 4 options that are negotiated by the socket at the connecting stage. These are − − * mss_clamp -- the maximum size of the segment peer is ready to accept − * snd _scale -- the scale factor for a window − * sack -- whether selective acks are permitted or not − * tstamp -- whether timestamps on packets are supported − − All four can be read with getsockopt calls to a socket and in order to restore them the TCP_REPAIR_OPTIONS sockoption − is introduced. − − == Checkpoint and restore TCP connection == − − With the above sockoptions dumping and restoring TCP connection becomes possible. The crtools just reads the socket − state and restores it back letting the protocol resurrect the data sequence. − − One thing to note here -- while the socket is closed between dump and restore the connection should be "locked", i.e. − no packets from peer should enter the stack, otherwise the RST will be sent by a kernel. In order to do so a simple − netfilter rule is configured that drops all the packets from peer to a socket we're dealing with. This rule sits − − crtools restore one. − − That said, the command line option --tcp-established should be used when calling crtools to explicitly state, that the − caller is aware of this "transitional" state of the netfilter.
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in the host netfilter tables after the crtools dump command finishes and it should be there when you issue the