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charset="us-ascii" Content-Transfer-Encoding: quoted-printable We notice some reorgs in Bitcoin testnet, while reorgs in testnet are commo= n and may be part of different tests and experiments, it seems the forks ar= e not created by a single user and multiple blocks were mined by different = users in each chain. My first impression was that the problem was related = to network issues but some Bitcoin explorers were following one chain while= others follow the other one. Nonetheless, well established explorers like= blocktrail.com or blockr.io were following different chains at different h= eights which led to me to believe that it was not a network issue. After so= me time, a reorg occurs and it all comes to normal state as a single chain. We started investigating more and we identified that the fork occurs with n= odes 0.12; in some situations, nodes 0.12 has longer/different chains. The = blocks in both chains are valid so something must be occurring in the commu= nication between nodes but not related with the network itself. Long story short, when nodes 0.13+ receive blocks from 0.13+ nodes all is o= k, and those blocks propagate to older nodes with no issues. But when a blo= ck tries to be propagated from bitcoind 0.12.+ to newer ones those blocks a= re NOT being propagated to the peers with newer versions while these newer = blocks are being propagated to peers with older versions with no issues. My conclusion is that we have a backward compatibility issue between 0.13.X= + and older versions. The issue is simple to replicate, first, get latest version of bitcoind, co= mplete the IBD after is at current height, then force it to use exclusively= one or more peers of versions 0.12.X and older, and you will notice that t= he latest version node will never receive a new block. Probably some alternative bitcoin implementations act as bridges between th= ese two versions and facilitate the chain reorgs. I have not yet found any way where/how it can be used in a malicious way or= be exploited by a miner but in theory Bitcoin 0.13.X+ should remain compat= ible with older ones, but a 0.13+ node may become isolated by 0.12 peers, a= nd there is not notice for the node owner. --_000_SC1P152MB1648D0F9DF4279C49D755233F53F0SC1P152MB1648LAMP_ Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

We notice some reorgs in = Bitcoin testnet, while reorgs in testnet are common and may be part of diff= erent tests and experiments, it seems the forks are not created by a single= user and multiple blocks were mined by different users in each chain.  My first impression was that the p= roblem was related to network issues but some Bitcoin explorers were follow= ing one chain while others follow the other one.  Nonetheless, well es= tablished explorers like blocktrail.com or blockr.io were following different chains at different heights which led t= o me to believe that it was not a network issue. After some time, a reorg o= ccurs and it all comes to normal state as a single chain.

We started investigating = more and we identified that the fork occurs with nodes 0.12; in some situat= ions, nodes 0.12 has longer/different chains. The blocks in both chains are= valid so something must be occurring in the communication between nodes but not related with the network itself= .

Long story short, when no= des 0.13+ receive blocks from 0.13+ nodes all is ok, and those bloc= ks propagate to older nodes with no issues. But when a block tries to be pr= opagated from bitcoind 0.12.+ to newer ones those blocks are NOT being propagated to the peers with newer versions whi= le these newer blocks are being propagated to peers with older versions wit= h no issues.

My conclusion is that we = have a backward compatibility issue between 0.13.X+ and older versions.=

The issue is simple to re= plicate, first, get latest version of bitcoind, complete the IBD after is a= t current height, then force it to use exclusively one or more peers of ver= sions 0.12.X and older, and you will notice that the latest version node will never receive a new block.

Probably some alternative= bitcoin implementations act as bridges between these two versions and faci= litate the chain reorgs.

I have not yet found any = way where/how it can be used in a malicious way or be exploited by a miner = but in theory Bitcoin 0.13.X+ should remain compatible with older ones,= but a 0.13+ node may become isolated by 0.12 peers, and there is not notice for the node owner.

 

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