Automatically detect DNS interface when empty in pihole.toml #2456

DL6ER · 2025-05-15T17:39:58Z

What does this implement/fix?

Follow-up on similar PRs across Pi-hole's repositories, see #2448 (review)

You can use

pihole-FTL --default-gateway

to get which interface would be used on your particular machine

Related issue or feature (if applicable): #2448 (review)

Pull request in docs with documentation (if applicable): N/A

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I have commented my proposed changes within the code.
I am willing to help maintain this change if there are issues with it later.
It is compatible with the EUPL 1.2 license
I have squashed any insignificant commits. (git rebase)

Checklist:

The code change is tested and works locally.
I based my code and PRs against the repositories developmental branch.
I signed off all commits. Pi-hole enforces the DCO for all contributions
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I have read the above and my PR is ready for review.

Signed-off-by: DL6ER <[email protected]>

yubiuser

Leave empty for auto-detection.

I'm not sure if I like that. That makes it unpredictable. I think we should have this auto detecting code only for the migration and then write it into pihole.toml. If the user wants to change it, they happily can do this afterwards.

DL6ER · 2025-05-15T20:18:33Z

I thought about this as well. In the end, it is not unpredictable. You have the Ethernet cable only in one of the sockets so the automatic detection should always return the same. If you happen to use another one (for whatever reason), it may not be that bad that the automatism automatically changes the interface for you.

Writing it statically into the file would also make the tests harder because this would make the toml be different based on the machine the CI tests are running one. This wouldn't be a show-stopper and could be coded around, but still, on second and third thought, I still think the "empty is auto" is the best approach here. And I think we do this elsewhere, too.

rdwebdesign · 2025-05-20T20:01:20Z

Will the API still return an empty string (just like the pihole.toml file) or it will return the interface FTL is actually using?

In this case, the Web Interface will show eth0.

DL6ER · 2025-05-21T11:27:04Z

Yes. And this is a very good point. So we need to add an additional AJAX call to get the interface in this case from the API (instead of blindly substituting with "eth0")

rdwebdesign · 2025-05-21T16:25:35Z

So we need to add an additional AJAX call to get the interface in this case from the API

We don't need another AJAX call.
The admin/settings/dns page already calls the correct API endpoint, but the API sends an empty string:

The API needs to be changed to send the interface FTL is actually using, but I don't know how to change that without messing the other settings pages and pihole.toml.

Maybe we need another API "virtual value" that doesn't need to be written from (or read to) the config file, like dns.realInterface, but with a better name.

DL6ER · 2025-05-21T19:31:12Z

We don't need another AJAX call.

Yes, we do need one. This field you are mentioning has to stay empty when it is empty in pihole.toml. Otherwise, when the user hits "save & apply", the empty value would be replaced by the substituted one.

We can use network/gateway here and just take gateway.0.interface. This is guaranteed to be the same FTL will by pick by default for an empty sting.

rdwebdesign · 2025-05-21T20:45:55Z

We can use network/gateway here and just take gateway.0.interface. This is guaranteed to be the same FTL will by pick by default for an empty sting.

OK.
Then we need a new API call to get gateway.0.interface and adjust the code in scripts/js/settings-dns.js.

I will try to open a new PR later.

rdwebdesign · 2025-05-22T00:55:22Z

Done.

I think "empty is auto" approach is fine.

Core repo will use the selected interface, on new installs;
FTL will use the interface set in pihole.toml, or automatically detect the interface if empty;
Web will replicate FTL (use the interface set in pihole.toml, or use the interface automatically detected by FTL).

rdwebdesign

From my point of view this PR is approved, unless yubiuser has something else to add.

DL6ER · 2025-05-22T13:14:57Z

@yubiuser any objections?

yubiuser

No objections to the proposed idea.
Just for the implementation:

We can use network/gateway here and just take gateway.0.interface.

If we have code already at an other place, why not use this directly, but instead create a new function here?

src/args.c

src/config/dnsmasq_config.c

pralor-bot · 2025-05-30T20:05:20Z

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/interfqces-not-shown-can-not-select-wg0/80185/2

github-actions · 2025-06-30T11:16:49Z

This pull request has conflicts, please resolve those before we can evaluate the pull request.

Signed-off-by: DL6ER <[email protected]>

github-actions · 2025-07-07T17:30:15Z

Conflicts have been resolved.

yubiuser · 2025-07-22T14:06:09Z

@DL6ER

Did you see my question above?

If we have code already at an other place, why not use this directly, but instead create a new function here?

DL6ER · 2025-07-22T18:20:07Z

Well, that's basically what we do: get_gateway_name() calls the existing nlroutes(routes, false); function and what follows is basically extracting said gateway.0.oif. A bit more because we iterate over each of the array elements and pick the interface name (oif) where gateway.X.dst == 'default'. I see now that my explanation above was too oversimplified.

yubiuser · 2025-07-22T19:05:26Z

I was just asking because this also what get_gateway() does

FTL/src/api/network.c

Lines 47 to 70 in 632a90d

    
           cJSON *gateway = JSON_NEW_ARRAY(); 
        
           // Search through routes for the default gateway 
        
           // They are the ones with "dst" == "default" 
        
           cJSON *route = NULL; 
        
           cJSON_ArrayForEach(route, routes) 
        
           { 
        
           	cJSON *dst = cJSON_GetObjectItem(route, "dst"); 
        
           	if(dst != NULL && 
        
           	   cJSON_IsString(dst) && 
        
           	   strcmp(cJSON_GetStringValue(dst), "default") == 0) 
        
           	{ 
        
           		cJSON *gwobj = JSON_NEW_OBJECT(); 
        
           		// Extract and add family 
        
           		const char *family = cJSON_GetStringValue(cJSON_GetObjectItem(route, "family")); 
        
           		JSON_REF_STR_IN_OBJECT(gwobj, "family", family); 
        
           		// Extract and add interface name 
        
           		const char *iface_name = cJSON_GetStringValue(cJSON_GetObjectItem(route, "oif")); 
        
           		JSON_COPY_STR_TO_OBJECT(gwobj, "interface", iface_name); 
        
           		// Extract and add gateway address 
        
           		const char *gw_addr = cJSON_GetStringValue(cJSON_GetObjectItem(route, "gateway")); 
        
           		JSON_COPY_STR_TO_OBJECT(gwobj, "address", gw_addr);

But it might be more code to extract part of the information from get_gateway() instead of just doing what we really need in get_gateway_name()

DL6ER · 2025-07-22T19:26:06Z

Yes

Automatically detect DNS interface when empty in pihole.toml

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Signed-off-by: DL6ER <[email protected]>

DL6ER requested a review from a team as a code owner May 15, 2025 17:39

yubiuser reviewed May 15, 2025

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rdwebdesign mentioned this pull request May 22, 2025

Use the same interface name used by FTL in DNS Settings page pi-hole/web#3481

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yubiuser reviewed May 23, 2025

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github-actions bot added the Merge conflicts label Jun 30, 2025

DL6ER added 2 commits July 7, 2025 19:29

Merge branch 'development' into tweak/auto_interface

71fe15f

Signed-off-by: DL6ER <[email protected]>

Remove redundant double-net

51ab321

Signed-off-by: DL6ER <[email protected]>

DL6ER dismissed rdwebdesign’s stale review via 51ab321 July 7, 2025 17:30

github-actions bot removed the Merge conflicts label Jul 7, 2025

DL6ER requested a review from a team July 7, 2025 17:42

rdwebdesign approved these changes Jul 19, 2025

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yubiuser approved these changes Jul 22, 2025

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DL6ER merged commit 151a343 into development Jul 22, 2025
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DL6ER deleted the tweak/auto_interface branch July 22, 2025 19:26

DL6ER mentioned this pull request Aug 15, 2025

Fix logic in automatic interface determination (when dns.interface = "") #2607

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PromoFaux mentioned this pull request Oct 25, 2025

FTL v6.3 #2616

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Automatically detect DNS interface when empty in pihole.toml #2456

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DL6ER commented May 15, 2025

What does this implement/fix?

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