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4.3. IPv6-ready test/debug programs

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.

Usage

# ping6 <hostwithipv6address>
# ping6 <ipv6address>
# ping6 [-I <device>] <link-local-ipv6address>

Example

# 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:

  1. ping6 is not in users path (probably, because ping6 is generally stored in /usr/sbin -> add path (not really recommended)

  2. 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:

# ping6 fe80::212:34ff:fe12:3456 
connect: Invalid argument

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:

# tracepath6 www.6bone.net 
 1?: [LOCALHOST] pmtu 1480 
 1: 3ffe:401::2c0:33ff:fe02:14 150.705ms 
 2: 3ffe:b00:c18::5 267.864ms 
 3: 3ffe:b00:c18::5 asymm 2 266.145ms pmtu 1280 
 3: 3ffe:3900:5::2 asymm 4 346.632ms 
 4: 3ffe:28ff:ffff:4::3 asymm 5 365.965ms 
 5: 3ffe:1cff:0:ee::2 asymm 4 534.704ms 
 6: 3ffe:3800::1:1 asymm 4 578.126ms !N 
Resume: pmtu 1280

4.3.4. IPv6 tcpdump

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:

  • icmp6: filters native ICMPv6 traffic

  • ip6: filters native IPv6 traffic (including ICMPv6)

  • proto ipv6: filters tunneled IPv6-in-IPv4 traffic

  • not port ssh: to suppress displaying SSH packets for running tcpdump in a remote SSH session

Also some command line options are very useful to catch and print more information in a packet, mostly interesting for digging into ICMPv6 packets:

  • "-s 512": increase the snap length during capturing of a packet to 512 bytes

  • "-vv": really verbose output

  • "-n": don't resolve addresses to names, useful if reverse DNS resolving isn't working proper

4.3.4.1. IPv6 ping to 2001:0db8:100:f101::1 native over a local link

# tcpdump -t -n -i eth0 -s 512 -vv ip6 or proto ipv6 
tcpdump: listening on eth0 
2001:0db8:100:f101:2e0:18ff:fe90:9205 > 2001:0db8:100:f101::1: icmp6: echo
� request (len 64, hlim 64) 
2001:0db8:100:f101::1 > 2001:0db8:100:f101:2e0:18ff:fe90:9205: icmp6: echo
� reply (len 64, hlim 64)

4.3.4.2. IPv6 ping to 2001:0db8:100::1 routed through an IPv6-in-IPv4-tunnel

1.2.3.4 and 5.6.7.8 are tunnel endpoints (all addresses are examples)

# tcpdump -t -n -i ppp0 -s 512 -vv ip6 or proto ipv6 
tcpdump: listening on ppp0 
1.2.3.4 > 5.6.7.8: 2002:ffff:f5f8::1 > 2001:0db8:100::1: icmp6: echo request
� (len 64, hlim 64) (DF) (ttl 64, id 0, len 124) 
5.6.7.8 > 1.2.3.4: 2001:0db8:100::1 > 2002:ffff:f5f8::1: icmp6: echo reply (len
� 64, hlim 61) (ttl 23, id 29887, len 124) 
1.2.3.4 > 5.6.7.8: 2002:ffff:f5f8::1 > 2001:0db8:100::1: icmp6: echo request
� (len 64, hlim 64) (DF) (ttl 64, id 0, len 124) 
5.6.7.8 > 1.2.3.4: 2001:0db8:100::1 > 2002:ffff:f5f8::1: icmp6: echo reply (len
� 64, hlim 61) (ttl 23, id 29919, len 124)

 
 
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