Differences

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--- doc:appunti:net:ipv6_on_ppp [2024/07/05 11:11] – [Configuring hosts in the LAN] niccolo
+++ doc:appunti:net:ipv6_on_ppp [2024/07/05 15:37] (current) – [Troubleshooting IPv6 problems] niccolo
@@ Line 546: / Line 546: @@
 To give a **persistent addresses** to the client, take note of the DUID received by the server and put it into a **host** section of the server configuration **dhcp6s.conf** (see above). The DUID is composed of a 64 bit random string and the MAC address of the client's network interface; once generated, it is stored into a file named **/var/lib/dhcpv6/dhcp6c_duid**.
-**WARNING**: Enabling only the DHCPv6 address (disabling SLAAC configuration) the IPv6 routing doese not work: we get the //Beyond scope of source address// error message. Sniffing the traffic on the default gateway we can see:
-<code>
-IP6 fe80::225:22ff:fedd:4598 > 2a01:4f8:1c17:7636::1: ICMP6, echo request, id 56698, seq 1, length 64
-IP6 fe80::d003:a6ff:fef2:6fe8 > fe80::225:22ff:fedd:4598: ICMP6, destination unreachable, beyond scope
-a01:4f8:1c17:7636::1, source address fe80::225:22ff:fedd:4598, length 112
-</code>
-So the default gateway on the client is OK (check it with **ip -6 route show**), but the source address is set to the **link scope** address instead of the **global** one.
-FIXME How to solve?
 FIXME: What to do to protect LAN hosts from the internet?
@@ Line 587: / Line 575: @@
 ^ FF02::2            | All-routers multicast address.  |
 ^ FF02::1:FFxx:xxxx  | Duplicate Address Detection multicast group.  |
+===== Troubleshooting IPv6 problems =====
+In general, to troubleshoot any networking issue, you use the **ping** command to check if the remote address is responding:
+<code>
+# ping -6 2a01:4f8:1c17:7636::1
+</code>
+In some cases it is useful also to **ping your own addresses** (you can have more than one). Discover all of them with:
+<code>
+# ip -6 address show dev enp0s7
+</code>
+<code>
+: enp0s7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
+    inet6 2a02:2420:503:1c03::2/128 scope global dadfailed tentative
+       valid_lft forever preferred_lft forever
+    inet6 2a02:2420:503:1c03:225:22ff:fedd:4598/64 scope global dynamic mngtmpaddr
+       valid_lft 86391sec preferred_lft 14391sec
+    inet6 fe80::225:22ff:fedd:4598/64 scope link
+       valid_lft forever preferred_lft forever
+</code>
+In the example above the interface have two **global** scope addresses, one assigned via DHCPv6 and the other obtained via SLAAC. The first one has the **dadfailed** flag, which means that the **duplicate address detection** has detected a conflict in the network. This means that the IP address in question will not be used as the source address and it does not even respond to a self-ping.
+If you have only one global IP address which is **dadfailed**, you cannot reach remote hosts on IPv6. It may be a difficult situation to diagnose, because using the simple **ifconfig** command you see the assigned IPv6 address and the **default route** is OK:
+<code>
+# ip -6 route show
+a02:2420:503:1c03::2 dev enp0s7 proto kernel metric 256 pref medium
+a02:2420:503:1c03::/64 dev enp0s7 proto kernel metric 256 expires 86387sec pref medium
+fe80::/64 dev enp0s7 proto kernel metric 256 pref medium
+default via fe80::d003:a6ff:fef2:6fe8 dev enp0s7 proto ra metric 1024 expires 167sec hoplimit 64 pref medium
+</code>
+**NOTICE**: The default router can be reached via its **link** scope address (as seen above) or via its **global** scope address: both can be used.
+If you sniff the ping request on the router, you can see the problem more clearly:
+<code>
+# tcpdump -i eth0 -n 'icmp6'
+...
+IP6 fe80::225:22ff:fedd:4598 > 2a01:4f8:1c17:7636::1: ICMP6, echo request,
+    id 56698, seq 1, length 64
+IP6 fe80::d003:a6ff:fef2:6fe8 > fe80::225:22ff:fedd:4598: ICMP6, destination unreachable,
+    beyond scope 2a01:4f8:1c17:7636::1, source address fe80::225:22ff:fedd:4598, length 112
+</code>
+The client uses its link scope address as source address, which causes the **beyond scope** error. You can confirm this problem on the client using the ''ip route get'' command:
+<code>
+# ip -6 route get 2a01:4f8:1c17:7636::1
+a01:4f8:1c17:7636::1 from ::
+    via 2a02:2420:503:1c03::1
+    dev enp0s7
+    src fe80::225:22ff:fedd:4598
+    metric 1024 pref medium
+</code>
+You can compare the result when you have at least one working global IP address:
+<code>
+# ip -6 route get 2a01:4f8:1c17:7636::1
+a01:4f8:1c17:7636::1 from ::
+    via fe80::d003:a6ff:fef2:6fe8
+    dev enp0s7 proto ra
+    src 2a02:2420:503:1c03:225:22ff:fedd:4598
+    metric 1024 hoplimit 64 pref medium
+</code>
+In the first case the **src** address is the **link scope** one, not suitable for routing. In the second case it is the one received via SLAAC, which has a **global scope** and indeed it is working. Notice that the address of the router (shown as the **via** address) is not releveant, even the //link scope// one does work.
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