Kernel sends ICMP unreachable for GRE packets even if there is a listening socket

["Steinar H. Gunderson" <sgunderson@bigfoot.com>]

[Resent from Bugzilla]

Hi,

I have a userspace GRE listener, which opens a raw socket (error handling removed):

        int gresock = socket(AF_INET6, SOCK_RAW, IPPROTO_GRE);
        bind(gresock, (sockaddr *)&my_addr, sizeof(my_addr));

and then select()s and recvfrom()s on it, as well as sendto().

This works great. I can send and receive GRE packets. However, Linux _also_
sends ICMPv6 unreachables when the other end sends to me. I assume this is
because I don't have a corresponding kernel GRE tunnel interface. (I don't use
the kernel GRE because I want, among others, stronger reordering functionality
and possibly error correction, which is not in any standard.)

Can this be suppressed when I have such a socket giong? ip6tables -A OUTPUT can
stop them, but it seems very hacky and kludgy.

/* Steinar */
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