---

c: Copyright (C) Daniel Stenberg, daniel@haxx.se, et al. SPDX-License-Identifier: curl Title: CURLOPT_INTERFACE Section: 3 Source: libcurl Protocol:

• All See-also:
• CURLOPT_SOCKOPTFUNCTION (3)
• CURLOPT_TCP_NODELAY (3)
• CURLOPT_LOCALPORT (3)

---

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 you prefer one of these, you can use the following special prefixes:

• if!\<name\> - Interface name
• host!\<name\> - IP address or hostname
• ifhost!\<interface\>!\<host\> - Interface name and IP address or hostname

If if! or ifhost! 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) 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

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.

The ifhost! syntax was added in 8.9.0.

RETURN VALUE

Returns CURLE_OK on success or CURLE_OUT_OF_MEMORY if there was insufficient heap space.