After you have prepared your system for IPv6, you now want to use IPv6 for network communications. First you should learn how to examine IPv6 packets with a sniffer program. This is strongly recommended because for debugging/troubleshooting issues this can aide in providing a diagnosis very quickly.
4.3.1. IPv6 ping
This program is normally included in package iputils. It is designed for simple transport tests sending ICMPv6 echo-request packets and wait for ICMPv6 echo-reply packets.
# ping6 -c 1 ::1
PING ::1(::1) from ::1 : 56 data bytes
64 bytes from ::1: icmp_seq=0 hops=64 time=292 usec
--- ::1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/mdev = 0.292/0.292/0.292/0.000 ms
Hint: ping6 needs raw access to socket and therefore root permissions. So if non-root users cannot use ping6 then there are two possible problems:
ping6 is not in users path (probably, because ping6 is generally stored in /usr/sbin -> add path (not really recommended)
ping6 doesn't execute properly, generally because of missing root permissions -> chmod u+s /usr/sbin/ping6
4.3.1.1. Specifying interface for IPv6 ping
Using link-local addresses for an IPv6 ping, the kernel does not know through which (physically or virtual) device it must send the packet - each device has a link-local address. A try will result in following error message:
In this case you have to specify the interface additionally like shown here:
# ping6 -I eth0 -c 1 fe80::2e0:18ff:fe90:9205
PING fe80::212:23ff:fe12:3456(fe80::212:23ff:fe12:3456) from
� fe80::212:34ff:fe12:3478 eth0: 56 data bytes
64 bytes from fe80::212:23ff:fe12:3456: icmp_seq=0 hops=64 time=445 usec
--- fe80::2e0:18ff:fe90:9205 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss round-trip
� min/avg/max/mdev = 0.445/0.445/0.445/0.000 ms
4.3.1.2. Ping6 to multicast addresses
An interesting mechanism to detect IPv6-active hosts on a link is to ping6 to the link-local all-node multicast address:
# ping6 -I eth0 ff02::1
PING ff02::1(ff02::1) from fe80:::2ab:cdff:feef:0123 eth0: 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.104 ms
64 bytes from fe80::212:34ff:fe12:3450: icmp_seq=1 ttl=64 time=0.549 ms (DUP!)
Unlike in IPv4, where replies to a ping on the broadcast address can be disabled, in IPv6 currently this behavior cannot be disable except by local IPv6 firewalling.
4.3.2. IPv6 traceroute6
This program is normally included in package iputils. It's a program similar to IPv4 traceroute. Below you will see an example:
# traceroute6 www.6bone.net
traceroute to 6bone.net (3ffe:b00:c18:1::10) from 2001:0db8:0000:f101::2, 30
� hops max, 16 byte packets
1 localipv6gateway (2001:0db8:0000:f101::1) 1.354 ms 1.566 ms 0.407 ms
2 swi6T1-T0.ipv6.switch.ch (3ffe:2000:0:400::1) 90.431 ms 91.956 ms 92.377 ms
3 3ffe:2000:0:1::132 (3ffe:2000:0:1::132) 118.945 ms 107.982 ms 114.557 ms
4 3ffe:c00:8023:2b::2 (3ffe:c00:8023:2b::2) 968.468 ms 993.392 ms 973.441 ms
5 3ffe:2e00:e:c::3 (3ffe:2e00:e:c::3) 507.784 ms 505.549 ms 508.928 ms
6 www.6bone.net (3ffe:b00:c18:1::10) 1265.85 ms * 1304.74 ms
Note: unlike some modern versions of IPv4 traceroute, which can use ICMPv4 echo-request packets as well as UDP packets (default), current IPv6-traceroute is only able to send UDP packets. As you perhaps already know, ICMP echo-request packets are more accepted by firewalls or ACLs on routers inbetween than UDP packets.
4.3.3. IPv6 tracepath6
This program is normally included in package iputils. It's a program like traceroute6 and traces the path to a given destination discovering the MTU along this path. Below you will see an example:
On Linux, tcpdump is the major tool for packet capturing. Below you find some examples. IPv6 support is normally built-in in current releases of version 3.6.
tcpdump uses expressions for filtering packets to minimize the noise:
