---
name: "CURLOPT_INTERFACE"
section: "3"
platform: "macos"
package: "curl"
version: "166.0.0.0.1"
release: "macOS 26.5"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_INTERFACE"
---

# CURLOPT\_INTERFACE(3)

source interface for outgoing traffic

## NAME

CURLOPT\_INTERFACE - source interface for outgoing traffic

## SYNOPSIS

```
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_INTERFACE, char *interface);
```

## DESCRIPTION

Pass a char pointer as parameter. This sets the *interface* name to use as outgoing network interface. The name can be an interface name, an IP address, or a hostname.

If the parameter starts with "if!" then it is treated only as an interface name. If the parameter starts with &"host!" it is treated as either an IP address or a hostname.

If "if!" is specified but the parameter does not match an existing interface, *CURLE\_INTERFACE\_FAILED* is returned from the libcurl function used to perform the transfer.

libcurl does not support using network interface names for this option on Windows.

We strongly advise against specifying the interface with a hostname, as it causes libcurl to do a blocking name resolve call to retrieve the IP address. That name resolve operation does **not** use DNS-over-HTTPS even if *[CURLOPT\_DOH\_URL(3)](/CURLOPT_DOH_URL/3.md)* is set.

The application does not have to keep the string around after setting this option.

## DEFAULT

NULL, use whatever the TCP stack finds suitable

## PROTOCOLS

All

## EXAMPLE

```
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/foo.bin");
    curl_easy_setopt(curl, CURLOPT_INTERFACE, "eth0");
    res = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}
```

## AVAILABILITY

The "if!" and "host!" syntax was added in 7.24.0.

## RETURN VALUE

Returns CURLE\_OK on success or CURLE\_OUT\_OF\_MEMORY if there was insufficient heap space.

## SEE ALSO

[**CURLOPT\_SOCKOPTFUNCTION**(3)](/CURLOPT_SOCKOPTFUNCTION/3.md), [**CURLOPT\_TCP\_NODELAY**(3)](/CURLOPT_TCP_NODELAY/3.md)