Linux IPv6 HOWTO (en)
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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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