[Dnsmasq-discuss] Support of labels in --interface

[Simon Kelley]

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> You can get the destination address for datagrams from the
> IP_PTKINFO auxiliary data:
> 
> man 7 ip IP_PKTINFO (since Linux 2.2) Pass an IP_PKTINFO ancillary
> message that contains a pktinfo structure that supplies some
> information about the incoming packet. This only works for datagram
> oriented sockets.  The argument is a flag that tells the socket
> whether the IP_PKTINFO message should be passed or not. The message
> itself can only be sent/retrieved as control message with a packet
> using recvmsg(2) or sendmsg(2).
> 
> struct in_pktinfo { unsigned int   ipi_ifindex;  /* Interface index
> */ struct in_addr ipi_spec_dst; /* Local address */ struct in_addr
> ipi_addr;     /* Header Destination address */ };
> 

Indeed. And that gives you, in one hand, the destination address of
the datagram, and in the other hand, a whitelist of interface labels.
The only way to compare the two is to enumerate all the interfaces
using the netlink interface, which gives you all the addresses of each
interface and their associated labels. If the interface set and
configuration can change arbitrarily, you need to do that for every
datagram, which gets expensive.


Cheers,

Simon.
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