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Created by Santiago@santiagorr

dhcrelay -6 does not work if there are interface aliases up in the system

Hi there,

I've been hit by this issue on Debian stretch through sid (the latest including 4.4.1, I've been unable to test on 4.4.2).

dchrelay -6 doesn't start if there are interface aliases up in the system:

    # /usr/sbin/dhcrelay -6  -pf /var/run/dhcrelay6.pid -l vlan.881 -u 2001:db8:cafe::2%vlan.880
    Internet Systems Consortium DHCP Relay Agent 4.4.1
    Copyright 2004-2018 Internet Systems Consortium.
    All rights reserved.
    For info, please visit https://www.isc.org/software/dhcp/
    Bound to *:547
    Listening on Socket/vlan.881:0
    Sending on   Socket/vlan.881:0
    Listening on Socket/vlan.880:0
    Sending on   Socket/vlan.880:0
    Listening on Socket/vlan
    Sending on   Socket/vlan
    setsockopt: IPV6_JOIN_GROUP: Address already in use

    If you think you have received this message due to a bug rather
    than a configuration issue please read the section on submitting
    bugs on either our web page at www.isc.org or in the README file
    before submitting a bug.  These pages explain the proper
    process and the information we find helpful for debugging.

    exiting.

The above error can be reproduced with inside a container with the following /etc/network/interfaces:

    auto lo
    iface lo inet loopback

    auto vlan
    iface vlan inet dhcp

    auto vlan.880
    auto vlan.880:0
    iface vlan.880 inet static
        address 192.168.133.1/24

    iface vlan.880 inet6 static
        address 2001:db8:cafe::1/64

    iface vlan.880:0 inet static
        address 10.0.133.1/16

    auto vlan.881
    auto vlan.881:0
    iface vlan.881 inet static
        address 192.168.131.1/24

    iface vlan.881 inet6 static
        address 2001:db8:cafe:1::1/64

    iface vlan.881:0 inet static
        address 10.1.131.1/16

Just removing (or commenting out) the interface aliases entries makes dchrelay -6 happy (again).

Please note the -l and -u filters do not work. But that's another bug.

This seems similar to #88. The patch attached there doesn't fix the problem.