How to bind cURL to a network interface

Directing cURL traffic through a specific network interface keeps HTTP requests pinned to a predictable source address on multi-homed systems. This approach simplifies testing of firewalls and load balancers, enforces VPN-only egress for sensitive APIs, and avoids surprises when multiple outbound links exist.

Underneath the command-line flags, cURL exposes the libcurl option CURLOPT_INTERFACE, which binds the outgoing socket to an interface name, local IPv4 address, or local IPv6 address before connecting. When an interface such as eth0 or wlp2s0 is used, the kernel chooses an appropriate address from that device; when a literal address is provided, the kernel binds exactly that source IP and uses standard routing to reach the target.

Binding succeeds only when the selected interface is up, configured with the requested address, and part of a route capable of reaching the remote host. Attempts to bind to an absent or mismatched address typically fail immediately with errors such as curl: (45) bind failed with errno 99: Cannot assign requested address, and elevated privileges may be needed when experimenting with low-level or policy-restricted interfaces on Linux.

Steps to bind cURL to a network interface:

Display available network interfaces and their assigned IP addresses on the host where cURL runs.
```
$ ip -brief addr
lo               UNKNOWN        127.0.0.1/8 ::1/128
##### snipped #####
eth0@if344       UP             172.17.0.3/16
```
Interface names such as eth0, enp3s0, or wlp2s0 work as values for --interface when binding cURL traffic to a device.
Send an HTTP request bound to a specific interface name to confirm that outbound connectivity works over that path.
```
$ curl --silent --interface lo http://api.example.net/
<!doctype html><html><title>Mock API</title><h1>OK</h1></html>
```
Binding to an interface name allows the kernel to select whichever address on that device has a valid route to the remote server.
Send an HTTP request bound to a specific local IP address to force traffic from a single source address.
```
$ curl --silent --interface 127.0.0.1 http://api.example.net/
<!doctype html><html><title>Mock API</title><h1>OK</h1></html>
```
Binding to an IP address that is not configured locally results in an immediate failure such as curl: (45) bind failed with errno 99: Cannot assign requested address and prevents the connection from being established.
Verify the source address used for the request while binding traffic to the chosen interface.
```
$ curl --interface lo --silent --output /dev/null --write-out '%{local_ip}\n' http://api.example.net/
127.0.0.1
```
Success signal: local_ip matches the address assigned to the selected interface in the interface listing.
Compare the reported source IP when repeating the request without an interface binding or with a different interface to confirm isolation between paths.
```
$ curl --silent --output /dev/null --write-out '%{local_ip}\n' http://api.example.net/
127.0.0.1
```
On single-interface hosts the source address remains unchanged, while multi-homed systems report different values for bound versus unbound requests.

Author: Mohd Shakir Zakaria
Mohd Shakir Zakaria is a cloud architect with deep roots in software development and open-source advocacy. Certified in AWS, Red Hat, VMware, ITIL, and Linux, he specializes in designing and managing robust cloud and on-premises infrastructures.