On Sat, May 26, 2018 at 05:39:12AM -0400, Eric S. Raymond wrote: > I'm trying to untangle some nasty code in the Mills implementation of > NTP. I could simplify it a lot if there were a way to query a packet > to find out the name of the network interface it arrived on. (At the > moment the code has to iterate over all interfaces checking for > traffic on each one just so it doesn't lose that information.) > > This seems like the kind of thing the CMSG macros are intended to > support, but I can't find anywhere a specification of what cmsg_level > and cmsg_type values are valid and what their semantics are. > > So I have two questions: > > 1. Is there a cmsg_level/cmsg_type combination that will return the > name of the device the packet arrived through?
Not name directly, AFAIK, but you can set SOL_IP / IP_PKTINFO (or SOL_IPV6 / IPV6_RECVPKTINFO) socket option and get IP_PKTINFO (IPV6_PKTINFO) message with recvmsg(). This will tell you incoming interface index so that you can look the name up. See ip(7) or ipv6(7) for format of the message (struct ip_pktinfo, struct in6_pktinfo). However, I suspect that userspace application is not really interested in incoming interface name but rather in destination address of the incoming packet which is also provided in IP_PKTINFO / IPV6_PKTINFO message. Michal Kubecek
