curl: bind failed with errno 22: Invalid argument when using binding interface for IPv6 link-scope address

[adeadman]

I did this

When using a link-scope ipv6 address (based on hardware address and only addressable from the link neighbours, not globally), I have to specify the interface to bind. After looking at #3993 (which I believe was looking at a related issue), I tried this:

curl --interface eth0 -vg -6 'http://[fe80:928d:xxff:fexx:xxxx]:8080/healthz'

(this is the link-local ipv6 address on the network interface of another machine on the same LAN, which works over IPv4).

I expected the following

It should have successfully bound to the eth0 interface and made the connection, but instead I got the following:

*    Trying [fe80::928d:xxff:fexx:xxxx]:8080...
* Local Interface eth0 is ip fe80::ec4:xxff:fexx:xxx%2 using address family 10
* Name 'fe80::ec4:xxff:fexx:xxx' family 10 resolved to 'fe80::ec4:xxff:fexx:xxx' family 10
* bind failed with errno 22: Invalid argument
* Failed to connect to fe80::928d:xxff:fexx:xxxx port 8080 after 0ms: Failed binding local connection end
* closing connection #0
curl: (45) bind failed with errno 22: Invalid argument

I suspect it is trying to bind to the local interface with an ipv4 socket even though the address and protocol for the remote end is ipv6, but I don't really know (hence why the reference to #3993 ).

I know the connection is working as ping6 -I eth0 fe80::928d:xxff:fexx:xxxx binds and sends/receives the packets.

possible workaround

I can work around this --interface specification by using the interface scope in the URI - this works as expected:

curl -vg -6 'http://[fe80::928d:xxff:fexx:xxxx%eth0]:8080/healthz'
*    Trying [fe80::928d:xxff:fexx:xxxx]:8080...
* Connected to fe80::928d:xxff:fexx:xxxx (fe80::928d:xxff:fexx:xxxx) port 8080
> GET /healthz HTTP/1.1
> Host: [fe80::928d:xxff:fexx:xxxx]:8080
> User-Agent: curl/8.9.1
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Length: 17
< Content-Type: application/json
< Date: Wed, 04 Sep 2024 11:04:38 GMT
<
{"healthy":true}
* Connection #0 to host fe80::928d:xxff:fexx:xxxx left intact

curl/libcurl version

curl 8.9.1 (x86_64-pc-linux-gnu) libcurl/8.9.1 OpenSSL/1.0.2k-fips zlip/1.2.7 OpenLDAP/2.4.44
Release-Date 2024-07-31
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS HSTS HTTPS-proxy IPv6 Largefile libz NTLM SSL threadsafe UnixSockets

operating system

Linux hostname 3.10.0-1160.81.1.el7.x86_64 #1 SMP
CentOS Linux release 7.9.2009 (Core)

This is the same with both the system-provided curl and the latest curl that I built from source.