How to find the scope_id for an IPv6 socket

Question

I use a IPv6 UDP socket connection in my code. The execution fails, because the device does not exist.

strerror(errno)

responded with

No such device

I use a link local address.

Server

fe80::213:afff:fe94:d75

Client

fe80::f2d5:bfff:fe10:f1b1

I got the scope_id by ifconfig:

bt0: flags=4177<UP,POINTOPOINT,RUNNING,MULTICAST>  mtu 1280
        inet6 fe80::f2d5:bfff:fe10:f1b1  prefixlen 64  scopeid 0x20<link>
        unspec F0-D5-BF-FF-FE-10-F1-B1-00-00-00-00-00-00-00-00  txqueuelen 1000  (UNSPEC)
        RX packets 3  bytes 84 (84.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 11  bytes 370 (370.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Since another interface has the same scope_id, I guess the scope_id means here the IPv6 scope. 0x20 = link_local.

enp0s31f6: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.4  netmask 255.255.255.0  broadcast 192.168.0.255
        inet6 fe80::fb58:43b9:10fa:101  prefixlen 64  scopeid 0x20<link>
        inet6 2a02:8109:b6bf:ca24:ae7f:b967:932e:b2f4  prefixlen 64  scopeid 0x0<global>
        inet6 2a02:8109:b6bf:ca24:51f:6c37:66c4:5282  prefixlen 64  scopeid 0x0<global>
        ether c8:5b:76:98:55:bf  txqueuelen 1000  (Ethernet)
        RX packets 16413820  bytes 20299047086 (20.2 GB)
        RX errors 18  dropped 0  overruns 0  frame 16
        TX packets 6110718  bytes 714537636 (714.5 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 16  memory 0xf1200000-f1220000

So, I used this scope_id in my code:

#define CLIENT_ADDRESS { 0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf2, 0xd5, 0xbf, 0xff, 0xfe, 0x10, 0xf1, 0xb1 };

struct sockaddr_in6 client_addr;
unsigned int client_addr_len = sizeof(client_addr);
memset(&client_addr, 0, client_addr_len);
client_addr.sin6_family = AF_INET6;
client_addr.sin6_port = htons(PORT);
client_addr.sin6_scope_id = 0x20;
uint8_t address[16] = CLIENT_ADDRESS;
memcpy(&client_addr.sin6_addr, address, 16);

This is not the correct interface.

So my question: Where can I find the correct scope_id for my interface?

Is there some linux tool to get it?

SKi SKi · Accepted Answer · 2019-05-09T09:01:46

Maybe if_nametoindex() is what you are looking for:

client_addr.sin6_scope_id = if_nametoindex("tun0");

More information of if_nametoindex in man page.

How to find the scope_id for an IPv6 socket

2 Answers