<p>The <a href="https://bitcoin.stackexchange.com/questions/29554/explanation-of-what-an-op-return-transaction-looks-like"><code>OP_RETURN</code></a> feature can be used to accompany a transaction with arbitrary data.  A standard transaction can contain an <code>OP_RETURN</code> output with up to 80 bytes of data (40 bytes prior to Bitcoin Core 0.11).</p>

<p>You could have a client that includes an <code>OP_RETURN</code> output with every transaction, containing whatever meta data is desired (client software version, info about client settings, friendly greetings, Bible verses, etc).  </p>

<p>However, since this would make each transaction larger, the users of this client software would on average pay higher transaction fees (depending on how often the extra data pushes your transaction past a 1 KB boundary, since the fee is per KB).  Also, since every full node has to store the extra data forever, some people might consider this feature to be "unfriendly" to the block chain, even though it is legal in the protocol.</p>

<p>Given these tradeoffs, one would probably not implement such a feature without a pretty compelling reason to do so, beyond simple curiosity.</p>

CC BY-SA 3.0
2015-08-07T17:38:52.053Z
2015-08-07T17:38:52.053Z
2017-04-13T12:47:33.610Z
Nate Eldredge
<p>Would it be possible for a client (eg the core client) to leave its mark in the form of META data on the blockchain or even in the mempool so that we know that this client was used to create or send that transaction? </p>

<p>This is very similar to what web browsers do (for example this website tells you all about your browser by reading some META info: <a href="https://www.whatismybrowser.com/" rel="nofollow">https://www.whatismybrowser.com/</a>)? </p>

<blockchain><client>
Can a client leave its mark on the blockchain?
Doug Peters
# Can a client leave its mark on the blockchain?

(Asked by: username_0 on 2015-08-07)

Would it be possible for a client (eg the core client) to leave its mark in the form of META data on the blockchain or even in the mempool so that we know that this client was used to create or send that transaction?

This is very similar to what web browsers do (for example this website tells you all about your browser by reading some META info: <https://www.whatismybrowser.com/>)?

**Tags:** blockchain, client


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## 1 answers

### Answer by username_1 on 2015-08-07. Score: 2 | Accepted answer

The [`OP_RETURN`](https://bitcoin.stackexchange.com/questions/29554/explanation-of-what-an-op-return-transaction-looks-like) feature can be used to accompany a transaction with arbitrary data. A standard transaction can contain an `OP_RETURN` output with up to 80 bytes of data (40 bytes prior to Bitcoin Core 0.11).

You could have a client that includes an `OP_RETURN` output with every transaction, containing whatever meta data is desired (client software version, info about client settings, friendly greetings, Bible verses, etc).

However, since this would make each transaction larger, the users of this client software would on average pay higher transaction fees (depending on how often the extra data pushes your transaction past a 1 KB boundary, since the fee is per KB). Also, since every full node has to store the extra data forever, some people might consider this feature to be "unfriendly" to the block chain, even though it is legal in the protocol.

Given these tradeoffs, one would probably not implement such a feature without a pretty compelling reason to do so, beyond simple curiosity.


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