Re: Is it possible to get device information via CMSG?
Michal Kubecek : > > 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). Thanks, that confirms something I found on Stack Overflow after I queried your list. > 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. NTP is weird that way. My group, NTPsec, inherited the reference Mills codebase (what we now call "NTP Classic") which really does have a filter-by-interface-name feature *in addition to* local address filtering. We want to simplify the way it works without discarding that feature, because we've made promises about backward compatibility that we mean to keep. We don't throw away features unless either they're security holes or we are *dead certain* they are obsolete. In case it's of interest to you, NTPsec is a drop-in replacement for NTP Classic that solves its chronic security problems by stripping out unused features and legacy code. We've actually shrunk the codebase size by a factor of 4x. We have better monitoring and admin tools, too. -- http://www.catb.org/~esr/;>Eric S. Raymond My work is funded by the Internet Civil Engineering Institute: https://icei.org Please visit their site and donate: the civilization you save might be your own.
Re: Is it possible to get device information via CMSG?
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
Is it possible to get device information via CMSG?
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? 2. Is the set of possible cmsg_level and cmsg_type values documented anywhere? If not, how would one go about assemnbling such information? (I would be willing to write a man page about this.) -- http://www.catb.org/~esr/;>Eric S. Raymond You [should] not examine legislation in the light of the benefits it will convey if properly administered, but in the light of the wrongs it would do and the harm it would cause if improperly administered -- Lyndon Johnson, former President of the U.S.
