Hi! I've noticed that the IP_RECVIF socket option, i.e.:
int y = 1;
setsockopt(fd, IPPROTO_IP, IP_RECVIF, &y, sizeof(y));
does not seem to work if the socket is proxied by a NETransparentProxyProvider type network extension: there's no ancillary data in messages received with recvmsg. As soon as I disable the network extension, recvmsg starts returning ancillary data containing the interface name.
This seems to break some applications which rely on IP_RECVIF in the presence of a transparent proxy, making it, in fact, not transparent. One such example is Apple's own libresolv, which relies on IP_RECVIF and breaks if there's no ancillary data in the recvmsg result.
I don't think that this is the intended behaviour, since IPV6_PKTINFO seems to work fine. I've filed a bug report (FB17586550) about this, however, I would greatly appreciate if someone could point me in the direction of a workaround.