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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_INTERFACE
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_SOCKOPTFUNCTION (3)
+  - CURLOPT_TCP_NODELAY (3)
+---
+
+# NAME
+
+CURLOPT_INTERFACE - source interface for outgoing traffic
+
+# SYNOPSIS
+
+~~~c
+#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) 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
+
+~~~c
+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.