gnrc_networking example interferes with forwarding rules on the host machine

[OlegHahm]

Description

When building the gnrc_networking example for native, it is interfering with the forwarding rules on the host machine. The reason seems to be that the pseudo-module gnrc_ipv6_router_default will cause the RIOT instance to send out router advertisements (RA). The Linux host receiving the RAs via the bridge interface will add the RIOT instance as a default gateway.

I'm not quite sure whether it is valid behavior of an IPv6 router to announce itself as a router without having an actually routable IPv6 address. In any case this behavior leads to a non-functional IPv6 setup on the host machine.

Steps to reproduce the issue

1. Use latest RIOT master branch.
2. Build and run the gnrc_networking example for native or native64.
3. Wait some time and monitor the Linux kernel IPv6 routing table (e.g., via watch ip -6 r)

Expected results

Only actual useful RAs are sent/processed by the Linux host. (I.e., if it is actually correct behavior of the RIOT router to send the RAs without a routable address we may want to filter them somehow.

Actual results

Additional default routes (depending on the number of native instances) will show up.

Versions

Operating System Environment
----------------------------
         Operating System: "Arch Linux" 
                   Kernel: Linux 6.9.10-arch1-1 x86_64 unknown
             System shell: GNU bash, version 5.2.26(1)-release (x86_64-pc-linux-gnu)
             make's shell: GNU bash, version 5.2.26(1)-release (x86_64-pc-linux-gnu)

Installed compiler toolchains
-----------------------------
               native gcc: gcc (GCC) 14.1.1 20240522
        arm-none-eabi-gcc: arm-none-eabi-gcc (Arch Repository) 14.1.0
                  avr-gcc: avr-gcc (GCC) 14.1.0
           msp430-elf-gcc: msp430-elf-gcc (GCC) 13.1.0
       riscv-none-elf-gcc: missing
  riscv64-unknown-elf-gcc: missing
      riscv32-esp-elf-gcc: missing
     xtensa-esp32-elf-gcc: xtensa-esp32-elf-gcc (crosstool-NG crosstool-ng-1.22.0-80-g6c4433a) 5.2.0
   xtensa-esp32s2-elf-gcc: missing
   xtensa-esp32s3-elf-gcc: missing
   xtensa-esp8266-elf-gcc: missing
                    clang: clang version 18.1.8

Installed compiler libs
-----------------------
     arm-none-eabi-newlib: "4.4.0"
        msp430-elf-newlib: "4.3.0"
    riscv-none-elf-newlib: missing
riscv64-unknown-elf-newlib: missing
   riscv32-esp-elf-newlib: missing
  xtensa-esp32-elf-newlib: "2.2.0"
xtensa-esp32s2-elf-newlib: missing
xtensa-esp32s3-elf-newlib: missing
xtensa-esp8266-elf-newlib: missing
                 avr-libc: "2.2.0" ("20240608")

Installed development tools
---------------------------
                   ccache: ccache version 4.10
                    cmake: cmake version 3.30.1
                 cppcheck: missing
                  doxygen: 1.11.0
                      git: git version 2.45.2
                     make: GNU Make 4.4.1
                  openocd: Open On-Chip Debugger 0.12.0
                   python: Python 3.12.4
                  python2: Python 2.7.18
                  python3: Python 3.12.4
                   flake8: 7.1.0 (mccabe: 0.7.0, pycodestyle: 2.11.1, pyflakes: 3.2.0) CPython 3.12.4 on
               coccinelle: missing