Monitoring Packet Transfers With the snoop Command
You can use the snoop command to monitor the state of data transfers. snoop captures network packets and displays their contents in the format that you specify. Packets can be displayed as soon as they are received, or saved to a file. When snoop writes to an intermediate file, packet loss under busy trace conditions is unlikely. snoop itself is then used to interpret the file.
To capture packets to and from the default interface in promiscuous mode, you must assume the Network Management role or become superuser. In summary form, snoop displays only the data that pertains to the highest-level protocol. For example, an NFS packet only displays NFS information. The underlying RPC, UDP, IP, and Ethernet frame information is suppressed but can be displayed if either of the verbose options is chosen.
Use snoop frequently and consistently to become familiar with normal system behavior. For assistance in analyzing packets, look for a recent white paper and RFC, and seek the advice of an expert in a particular area, such as NFS or NIS. For details on using snoop and its options, refer to the snoop(1M) man page.
How to Check Packets From All Interfaces
The basic snoop command returns output that resembles the following, for a dual-stack host.
% snoop Using device /dev/hme (promiscuous mode) farhost.remote.com -> myhost RLOGIN C port=993 myhost -> farhost.remote.com RLOGIN R port=993 Using device /dev/hme router5.local.com -> router5.local.com ARP R 10.0.0.13, router5.local.com is 0:10:7b:31:37:80 router5.local.com -> BROADCAST TFTP Read "network-confg" (octet) myhost -> DNSserver.local.com DNS C 192.168.10.10.in-addr.arpa. Internet PTR ? DNSserver.local.com myhost DNS R 192.168.10.10.in-addr.arpa. Internet PTR niserve2. . . farhost.remote.com-> myhost RLOGIN C port=993 myhost -> farhost.remote.com RLOGIN R port=993 fe80::a00:20ff:febb: . fe80::a00:20ff:febb:e09 -> ff02::9 RIPng R (5 destinations)
The packets that are captured in this output show a remote login section, including lookups to the NIS and DNS servers for address resolution. Also included are periodic ARP packets from the local router and advertisements of the IPv6 link-local address to in.ripngd.
How to Capture snoop Output Into a File
The following output shows a variety of captures such as you might receive as output from the snoop -i command.
# snoop -i /tmp/cap 1 0.00000 fe80::a00:20ff:fee9:2d27 -> fe80::a00:20ff:fecd:4375 ICMPv6 Neighbor advertisement 2 0.16198 farhost.com -> myhost RLOGIN C port=985 3 0.00008 myhost -> farhost.com RLOGIN R port=985 10 0.91493 10.0.0.40 -> (broadcast) ARP C Who is 10.0.0.40, 10.0.0.40 ? 34 0.43690 nearserver.here.com -> 22.214.171.124 IP D=126.96.36.199 S=10.0.0.40 LEN=28, ID=47453, TO =0x0, TTL=1 35 0.00034 10.0.0.40 -> 188.8.131.52 IP D=184.108.40.206 S=10.0.0.40 LEN=28, ID=57376, TOS=0x0, TTL=47
How to Check Packets Between an IPv4 Server and a Client
How to Monitor IPv6 Network Traffic
You can use the snoop command to display only IPv6 packets.
The following example shows typical output such as you might receive from running the snoop ip6 command on a node.
# snoop ip6 fe80::a00:20ff:fecd:4374 -> ff02::1:ffe9:2d27 ICMPv6 Neighbor solicitation fe80::a00:20ff:fee9:2d27 -> fe80::a00:20ff:fecd:4375 ICMPv6 Neighbor solicitation fe80::a00:20ff:fee9:2d27 -> fe80::a00:20ff:fecd:4375 ICMPv6 Neighbor solicitation fe80::a00:20ff:febb:e09 -> ff02::9 RIPng R (11 destinations) fe80::a00:20ff:fee9:2d27 -> ff02::1:ffcd:4375 ICMPv6 Neighbor solicitation