cnp3-ebook/exercises/ipv6 — English @ Weblate

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81 ../../exercises/ipv6.rst:743

	English The ``links`` command lists the links that have been instantiated and shows that mapping between the named interfaces on each node. 131/10000 · 131
82 ../../exercises/ipv6.rst:760

	English It is possible to execute any of the standard Linux commands to configure the network stack on any of the hosts by prefixing the command with the corresponding host. Remember to always specify ``inet6`` as the address family to retrieve the IPv6 information. 258/10000 · 258
83 ../../exercises/ipv6.rst:770

	English Host ``a`` has two interfaces: the standard loopback interface and a network interface named ``a-eth0`` that is attached to router ``r1``. We can also verify how the IPv6 addresses have been configured: 202/10000 · 202
84 ../../exercises/ipv6.rst:784

	English On its ``a-eth0`` interface, host ``a`` uses IPv6 address ``2001:db8:1341:1::a/64``. The link local address (``fe80::c44e:26ff:fed9:de6d/64``) will be described in another chapter. Finally, we can check the forwarding table of host ``a``. 238/10000 · 238
85 ../../exercises/ipv6.rst:794

	English There are three routes in this table. The first two correspond to the two prefixes that are used over the ``a-eth0`` interface. These routes are automatically created when an IPv6 address is configured on an interface. The last route is the default route (``::/0``) which points towards ``2001:db8:1341:1::1``, i.e. router ``r1``. 330/10000 · 330
86 ../../exercises/ipv6.rst:796

	English Another useful command is ``xterm`` 'node' that allows to launch a terminal on the specified node. This gives you a interactive shell on any node. You can use it to capture packets with tcpdump_. As an example, let us use :manpage:`traceroute6(8)` to trace the path followed by packets from host ``a`` towards the IPv6 address of host ``b`` i.e. ``2001:db8:1341:3::b``. The output of this command shows that the path passes through routers ``r1``, ``r2`` and ``r3``. 466/10000 · 466
87 ../../exercises/ipv6.rst:808

	English Another interesting mininet_ command is ``pingall`` it allows to check that any host can reach any other host inside the network. It executes a ping from any host to any other host inside the network topology. 209/10000 · 209
88 ../../exercises/ipv6.rst:819

	English When debugging a network, it can be interesting to capture packets using tcpdump_ on specific links to check that they follow the expect. If you use tcpdump_ without any filter, you will capture the packets generated by xterm. To capture packets, you need to specify precise filters that will match the packets of interest. For traceroute6, you need to match the IPv6 packets that contain UDP segments and some ICMPv6 packets. The script below provides a simple filter that you can reuse. It takes one argument: the name of the interface on which tcpdump_ needs to run. 569/10000 · 569
89 ../../exercises/ipv6.rst:827

	English Starting from the :download:`/exercises/ipmininet_scripts/static-1.py` IPMininet_ script, we can explore classical problems when networks are configured with static routes. A first problem is when a router has an incomplete forwarding table. We configure the static routes as shown below. The entire script is available from :download:`/exercises/ipmininet_scripts/static-1-hole.py`. 383/10000 · 383
90 ../../exercises/ipv6.rst:841

	English We first check with ``pingall`` whether the network works correctly. 68/10000 · 68
91 ../../exercises/ipv6.rst:851

	English The problem can be detected by using :manpage:`traceroute6(8)`. 63/10000 · 63
92 ../../exercises/ipv6.rst:861

	English In the output of :manpage:`traceroute6(8)`, a ``!N`` indicates that host ``a`` received from ``2001:db8:1341:12::2``, i.e. router ``r2``, a Network unreachable ICMPv6 message. The forwarding table of ``r2`` confirms the root cause of this problem. 247/10000 · 247
93 ../../exercises/ipv6.rst:873

	English A second problem is when there is a forwarding loop inside the network, i.e. packets sent to a specific destination loop through several routers. With the static routes shown below, router ``r2`` forwards the packets towards ``2001:db8:1341:3::b`` via router ``r1``. The entire script is available from :download:`/exercises/ipmininet_scripts/static-1-loop.py`. 361/10000 · 361
94 ../../exercises/ipv6.rst:888

	English The ``pingall`` command reveals that there is a problem in this network. 72/10000 · 72
95 ../../exercises/ipv6.rst:898

	English We can analyze this configuration problem in more details by using ``traceroute6``. The loop appears clearly. 109/10000 · 109
96 ../../exercises/ipv6.rst:919

	English On host ``b``, the problem is different. The packets that it sends towards host ``a`` do not seem to go beyond router ``r3``. 125/10000 · 125
97 ../../exercises/ipv6.rst:933

	English To debug this problem, let us look at the forwarding table of ``r3``. This router forwards the packets sent to host ``a`` to router ``r1`` that is directly connected to host ``a``. 180/10000 · 180
98 ../../exercises/ipv6.rst:946

	English Unfortunately, when router ``r1`` sends its ICMP HopLimit exceeded message, the destination of this IP packet is ``2001:db8:1341:3::b``. This packet is forward to router ``r2`` that returns the packet back to router ``r1``. The packet loops between the two routers until their HopLimit reaches zero. 299/10000 · 299
99 ../../exercises/ipv6.rst:965

	English IPv6 packets 12/10000 · 12
100 ../../exercises/ipv6.rst:967

	English To correctly understand the operation of IPv6, it is sometimes important to remember the packet format and how the different fields are used. 141/10000 · 141

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