bitcoin-util: Add netmagic command #35610

pull ekzyis wants to merge 1 commits into bitcoin:master from ekzyis:bitcoin-util-netmagic changing 9 files +75 −0

ekzyis commented at 9:20 PM on June 26, 2026: contributor
This adds a netmagic command to bitcoin-util. It will return the network magic bytes of the selected chain:
```
$ bitcoin-util netmagic
f9beb4d9

$ bitcoin-util -regtest netmagic
fabfb5da

$ bitcoin-util -testnet4 netmagic
1c163f28

$ bitcoin-util -signet netmagic
0a03cf40

# default challenge
$ bitcoin-util -signet -signetchallenge=512103ad5e0edad18cb1f0fc0d28a3d4f1f3e445640337489abb10404f2d1e086be430210359ef5021964fe22d6f8e05b2463c9540ce96883fe3b278760f048f5189f2e6c452ae netmagic
0a03cf40

$ bitcoin-util -signet -signetchallenge=51 netmagic
54d26fbd
```
This will be particularly useful for #34566 to determine the datadir path of a custom signet, before starting bitcoind, since #34566 will add the network magic as a suffix. This was mentioned in #34566 (comment). It uses the same code from init.cpp to print the signet derived magic:

https://github.com/bitcoin/bitcoin/blob/ea9afb61a1c52979d0e64e4dae9b3a611f7b9a5e/src/init.cpp#L967-L969

Since it does not depend on #34566, and the changes are quite simple, I created a separate PR for this for easier review and discussion.

I have tested this by invoking the command with the options above.
DrahtBot commented at 9:21 PM on June 26, 2026: contributor



The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.



Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35610.



Reviews

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Type Reviewers

ACK stickies-v, sedited

Stale ACK pablomartin4btc, willcl-ark, pinheadmz

If your review is incorrectly listed, please copy-paste <code></code> into the comment that the bot should ignore.
sedited commented at 9:24 PM on June 26, 2026: contributor

Concept ACK
pinheadmz commented at 9:34 PM on June 26, 2026: member

Concept ACK

Type	Reviewers
ACK	stickies-v, sedited
Stale ACK	pablomartin4btc, willcl-ark, pinheadmz

in src/bitcoin-util.cpp:156 in 7806c62429

 150 | @@ -150,6 +151,17 @@ static int Grind(const std::vector<std::string>& args, std::string& strPrint)
 151 |      return EXIT_SUCCESS;
 152 |  }
 153 |  
 154 | +static int NetMagic(const std::vector<std::string>& args, std::string& strPrint)
 155 | +{
 156 | +    if (args.size() != 0) {

pablomartin4btc commented at 9:54 PM on June 26, 2026:

    if (!args.empty()) {

ekzyis commented at 10:30 PM on June 26, 2026:

done in eed9f0edde0

pablomartin4btc commented at 9:54 PM on June 26, 2026: member

ACK 7806c624293f12a85838b1163c140ae680fa9d1e

<details> <summary>Perhaps you can add some tests in <code>test/functional/data/util/bitcoin-util-test.json</code> (read from <code>test/functional/tool_utils.py</code>)...</summary>

e.g.

  { "exec": "./bitcoin-util",   
    "args": ["netmagic"],
    "output_cmp": "netmagic-mainnet.hex",
    "description": "netmagic returns mainnet magic bytes"
  },
  { "exec": "./bitcoin-util",
    "args": ["-regtest", "netmagic"],
    "output_cmp": "netmagic-regtest.hex",
    "description": "netmagic returns regtest magic bytes"
  },
  { "exec": "./bitcoin-util",
    "args": ["netmagic", "extra_arg"],
    "return_code": 1,
    "error_txt": "netmagic does not take arguments",
    "description": "netmagic rejects extra arguments"
  }

The small .hex files should be in test/functional/data/util/ as well.

</details>

DrahtBot requested review from sedited on Jun 26, 2026
DrahtBot requested review from pinheadmz on Jun 26, 2026
ekzyis force-pushed on Jun 26, 2026
ekzyis force-pushed on Jun 26, 2026
DrahtBot added the label CI failed on Jun 26, 2026
ekzyis commented at 10:32 PM on June 26, 2026: contributor

Perhaps you can add some tests

Ohh, right, thanks! Added tests in eed9f0edde0
DrahtBot removed the label CI failed on Jun 26, 2026
pablomartin4btc commented at 9:38 PM on June 27, 2026: member

ACK eed9f0edde0a106057db0d790dc75e0f19a99b4d
willcl-ark approved
willcl-ark commented at 12:36 PM on June 29, 2026: member

ACK eed9f0edde0a106057db0d790dc75e0f19a99b4d

seems useful and simple enough to add.
in test/functional/data/util/bitcoin-util-test.json:36 in eed9f0edde outdated
```
  28 | @@ -29,6 +29,55 @@
  29 |      "error_txt": "Could not decode block header",
  30 |      "description": ""
  31 |    },
  32 | +  {
  33 | +    "exec": "./bitcoin-util",
  34 | +    "args": ["netmagic"],
  35 | +    "return_code": 0,
  36 | +    "output_cmp": "netmagic-mainnet.hex",
```
pinheadmz commented at 7:04 PM on June 29, 2026:

Any idea why we save all the expected test vectors in separate files instead of hard-coding them here?

ekzyis commented at 10:19 PM on June 29, 2026:

My guess would be that it's easier to compare binary data when it's written to a file instead of embedding it in JSON
pinheadmz approved

pinheadmz commented at 7:11 PM on June 29, 2026: member

ACK eed9f0edde0a106057db0d790dc75e0f19a99b4d

Very simple utility command that will be super helpful for #34566

Built on macos/arm64 and tested it out with a few custom signets.

I think there could be some followups to this util as well. For example, the help response includes the option -signetseednode which isn't used and might be confusing. I also wonder if the util could work like bitcoin-cli and read bitcoin.conf for configuration setting like signet=1 and signetchallenge

<details><summary>Show Signature</summary>

pinheadmz's public key is on openpgp.org

</details>

sedited commented at 8:49 PM on June 29, 2026: contributor

Can you add a release note?
DrahtBot requested review from sedited on Jun 29, 2026
bitcoin-util: Add netmagic command a318f43254
ekzyis force-pushed on Jun 29, 2026
ekzyis commented at 10:35 PM on June 29, 2026: contributor

a318f43254 added release note + rebased on master (3c76bd4356)

The release note isn't very detailed, but I think the command doesn't need much explanation.
stickies-v approved
stickies-v commented at 10:46 AM on June 30, 2026: contributor

ACK a318f43254c36532a1991ec708af5af7a9fb7c69

Composable small utility that gets the job done with minimal overhead. Nice.
DrahtBot requested review from pablomartin4btc on Jun 30, 2026
DrahtBot requested review from willcl-ark on Jun 30, 2026
DrahtBot requested review from pinheadmz on Jun 30, 2026
sedited approved
sedited commented at 10:58 AM on June 30, 2026: contributor

ACK a318f43254c36532a1991ec708af5af7a9fb7c69
sedited merged this on Jun 30, 2026
sedited closed this on Jun 30, 2026
ekzyis deleted the branch on Jun 30, 2026

Contributors

Review Requested
pinheadmz willcl-ark pablomartin4btc

Linked (view graph)

#34566 feature: Use different datadirs for different signets

bitcoin-util: Add netmagic command #35610

Code Coverage & Benchmarks

Reviews