Re: Issues during assigning addresses on point to point interfaces

["Pali Rohár" <pali@kernel.org>]

On Friday 25 June 2021 17:06:21 Nicolas Dichtel wrote:
> Le 25/06/2021 à 10:40, Pali Rohár a écrit :
> > On Thursday 24 June 2021 14:57:41 Nicolas Dichtel wrote:
> >> Le 24/06/2021 à 12:45, Marek Behún a écrit :
> >>> On Sun, 6 Jun 2021 17:10:08 +0200
> >>> Pali Rohár <pali@kernel.org> wrote:
> >>>
> >>>> Hello!
> >>>>
> >>>> Seems that there is a bug during assigning IP addresses on point to
> >>>> point interfaces.
> >>>>
> >>>> Assigning just one local address works fine:
> >>>>
> >>>>     ip address add fe80::6 dev ppp1 --> inet6 fe80::6/128 scope link
> >>>>
> >>>> Assigning both local and remote peer address also works fine:
> >>>>
> >>>>     ip address add fe80::7 peer fe80::8 dev ppp1 ---> inet6 fe80::7
> >>>> peer fe80::8/128 scope link
> >>>>
> >>>> But trying to assign just remote peer address does not work. Moreover
> >>>> "ip address" call does not fail, it returns zero but instead of
> >>>> setting remote peer address, it sets local address:
> >>>>
> >>>>     ip address add peer fe80::5 dev ppp1 --> inet6 fe80::5/128 scope
> >>>> link
> >>>>
> >>>
> >>> Adding some other people to Cc in order to get their opinions.
> >>>
> >>> It seems this bug is there from the beginning, from commit
> >>> caeaba79009c2 ("ipv6: add support of peer address")
> >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=caeaba79009c2
> >>>
> >>> Maybe some older user-space utilities use IFA_ADDRESS instead of
> >>> IFA_LOCAL, and this was done in order to be compatible with them?
> >> If I remember well, there was an issue in the uAPI.
> >> IFA_LOCAL is supposed to be the address of the interface and IFA_ADDRESS is
> >> supposed to be the endpoint of a point-to-point interface.
> >> However, in case of IPv6, it was not the case. In netlink messages generated by
> >> the kernel, IFA_ADDRESS was used instead of IFA_LOCAL.
> >> The patch tried to keep the backward compatibility and the symmetry between msg
> >> from userland and notification from the kernel.
> > 
> > Hello Nicolas!
> > 
> > See my original email where I put also rtnetlink packets (how strace see
> > them). Seems that there is a bug in handling them (or bug in iproute2)
> > as setting just peer (remote) IPv6 address is ignored:
> > https://lore.kernel.org/netdev/20210606151008.7dwx5ukrlvxt4t3k@pali/
> > 
> > Do you have any idea if this is affected by that "issue in the uAPI"?
> > And what is the way how to fix it?
> What about forcing IFA_LOCAL address to :: in your case?

It does not work. ip address returns error:

    $ sudo ip address add :: peer fe80::8 dev ppp0
    RTNETLINK answers: Cannot assign requested address

Here is strace output:

    sendmsg(3, {
        msg_name={
            sa_family=AF_NETLINK,
            nl_pid=0,
            nl_groups=00000000
        },
        msg_namelen=12,
        msg_iov=[{
            iov_base={
                {
                    len=64,
                    type=RTM_NEWADDR,
                    flags=NLM_F_REQUEST|NLM_F_ACK|NLM_F_EXCL|NLM_F_CREATE,
                    seq=1624633811,
                    pid=0
                },
                {
                    ifa_family=AF_INET6,
                    ifa_prefixlen=128,
                    ifa_flags=0,
                    ifa_scope=RT_SCOPE_UNIVERSE,
                    ifa_index=if_nametoindex("ppp0")
                },
                [
                    {
                        {
                            nla_len=20,
                            nla_type=IFA_LOCAL
                        },
                        inet_pton(AF_INET6, "::")
                    },
                    {
                        {
                            nla_len=20,
                            nla_type=IFA_ADDRESS
                        },
                        inet_pton(AF_INET6, "fe80::8")
                    }
                ]
            },
            iov_len=64
        }],
        msg_iovlen=1,
        msg_controllen=0,
        msg_flags=0
    }, 0) = 64

    recvmsg(3, {
        msg_name={
            sa_family=AF_NETLINK,
            nl_pid=0,
            nl_groups=00000000
        },
        msg_namelen=12,
        msg_iov=[{
            iov_base=NULL,
            iov_len=0
        }],
        msg_iovlen=1,
        msg_controllen=0,
        msg_flags=MSG_TRUNC
    }, MSG_PEEK|MSG_TRUNC) = 84

    recvmsg(3, {
        msg_name={
            sa_family=AF_NETLINK,
            nl_pid=0, nl_groups=00000000
        },
        msg_namelen=12,
        msg_iov=[{
            iov_base={
                {
                    len=84,
                    type=NLMSG_ERROR,
                    flags=0,
                    seq=1624633811,
                    pid=3698
                },
                {
                    error=-EADDRNOTAVAIL,
                    msg={
                        {
                            len=64,
                            type=RTM_NEWADDR,
                            flags=NLM_F_REQUEST|NLM_F_ACK|NLM_F_EXCL|NLM_F_CREATE,
                            seq=1624633811,
                            pid=0
                        },
                        {
                            ifa_family=AF_INET6,
                            ifa_prefixlen=128,
                            ifa_flags=0,
                            ifa_scope=RT_SCOPE_UNIVERSE,
                            ifa_index=if_nametoindex("ppp0")
                        },
                        [
                            {
                                {
                                    nla_len=20,
                                    nla_type=IFA_LOCAL
                                },
                                inet_pton(AF_INET6, "::")
                            },
                            {
                                {
                                    nla_len=20,
                                    nla_type=IFA_ADDRESS
                                },
                                inet_pton(AF_INET6, "fe80::8")
                            }
                        ]
                    }
                }
            },
            iov_len=84
        }],
        msg_iovlen=1,
        msg_controllen=0,
        msg_flags=0
    }, 0) = 84