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

1. Print information about the interfaces that are attached to the system.

   # ipadm show-if

   The snoop command normally uses the first non-loopback device, typically the primary network interface.

2. Begin packet capture by typing snoop without arguments, as shown in Example 5-15.
3. Use Control-C to halt the process.

Example 5-15 Output From the snoop Command

The basic snoop command returns output that resembles the following, for a dual-stack host.

% snoop
Using device /dev/net (promiscuous mode)
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.
.
.
.
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

1. Capture a snoop session into a file.

   # snoop -o filename

   For example:

   # snoop -o /tmp/cap
   Using device /dev/eri (promiscuous mode)
   30 snoop: 30 packets captured

   In the example, 30 packets have been captured in a file named /tmp/cap. The file can be in any directory with enough disk space. The number of packets that are captured is displayed on the command line, enabling you to press Control-C to abort at any time.

   snoop creates a noticeable networking load on the host machine, which can distort the results. To see the actual results, run snoop from a third system.

2. Inspect the snoop output captures file.

   # snoop -i filename

Example 5-16 Contents of a snoop Output Captures 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
...
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  -> 224.0.1.1  IP  D=224.0.1.1 S=10.0.0.40 LEN=28, 
      ID=47453, TO =0x0, TTL=1
35  0.00034  10.0.0.40 -> 224.0.1.1    IP  D=224.0.1.1 S=10.0.0.40 LEN=28, ID=57376, 
     TOS=0x0, TTL=47  

How to Check Packets Between an IPv4 Server and a Client

1. Establish a snoop system off a hub that is connected to either the client or the server.

   The third system (the snoop system) checks all the intervening traffic, so the snoop trace reflects what is actually happening on the wire.

2. Type snoop with options and save the output to a file.
3. Inspect and interpret the output.

   Refer to RFC 1761, Snoop Version 2 Packet Capture File Format for details of the snoop capture file.

How to Monitor IPv6 Network Traffic

You can use the snoop command to display only IPv6 packets.

• Capture IPv6 packets.

  # snoop ip6

  For more information on the snoop command, see the snoop(1M) man page.

Example 5-17 Displaying Only IPv6 Network Traffic

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

Monitoring Packets by Using IP Layer Devices

IP layer devices are introduced in Oracle Solaris to enhance IP observability. These devices provide access to all packets with addresses that are associated with the system's network interface. The addresses include local addresses as well as addresses that are hosted on non-loopback interfaces or logical interfaces. The observable traffic can be both IPv4 and IPv6 addresses. Thus, you can monitor all traffic that is destined to the system. The traffic can be loopback IP traffic, packets from remote machines, packets that are being sent from the system, or all forwarded traffic.

With IP layer devices, an administrator for a global zone can monitor traffic between zones as well as within a zone. An administrator of a non-global zone can also observe traffic that is sent and received by that zone.

To monitor traffic on the IP layer, a new option, -I, is added to the snoop command. This option specifies for the command to use the new IP layer devices instead of the underlying link-layer device to display traffic data.

How to Check Packets on the IP Layer

1. If necessary, print the information about the interfaces that are attached to the system.

   # ipadm show-if

2. Capture IP traffic on a specific interface.

   # snoop -I interface [-V | -v]

Examples of Checking Packets

All the examples are based on the following system configuration:

# ipadm show-addr
ADDROBJ      TYPE       STATE   ADDR
lo0/v4       static     ok      127.0.0.1/8
net0/v4      static     ok      192.68.25.5/24
lo0/?        static     ok      127.0.0.1/8
net0/?       static     ok      172.0.0.3/24
net0/?       static     ok      172.0.0.1/24
lo0/?        static     ok      127.0.0.1/8
 

Suppose that two zones, sandbox and toybox, are using the following IP addresses:

• sandbox – 172.0.0.3

• toybox – 172.0.0.1

You can issue the snoop -I command on the different interfaces on the system. The packet information that is displayed depends on whether you are an administrator for the global zone or for the non-global zone.

Example 5-18 Traffic on the Loopback Interface

# snoop -I lo0
Using device ipnet/lo0 (promiscuous mode)
   localhost -> localhost    ICMP Echo request (ID: 5550 Sequence number: 0)
   localhost -> localhost    ICMP Echo reply (ID: 5550 Sequence number: 0)

To generate a verbose output, use the -v option.

# snoop -v -I lo0
Using device ipnet/lo0 (promiscuous mode)
IPNET:  ----- IPNET Header -----
IPNET:  
IPNET:  Packet 1 arrived at 10:40:33.68506
IPNET:  Packet size = 108 bytes
IPNET:  dli_version = 1
IPNET:  dli_type = 4
IPNET:  dli_srczone = 0
IPNET:  dli_dstzone = 0
IPNET:  
IP:   ----- IP Header -----
IP:   
IP:   Version = 4
IP:   Header length = 20 bytes
...

Support for observing packets on the IP layer introduces a new ipnet header that precedes the packets that are being observed. Both the source and destination IDs are indicated. The '0' ID indicates that the traffic is being generated from the global zone.

Example 5-19 Packet Flow in the net0 Device in Local Zones

# snoop -I net0
Using device ipnet/net0 (promiscuous mode)
toybox -> sandbox TCP D=22 S=62117 Syn Seq=195630514 Len=0 Win=49152 Options=<mss
sandbox -> toybox TCP D=62117 S=22 Syn Ack=195630515 Seq=195794440 Len=0 Win=49152
toybox -> sandbox TCP D=22 S=62117 Ack=195794441 Seq=195630515 Len=0 Win=49152
sandbox -> toybox TCP D=62117 S=22 Push Ack=195630515 Seq=195794441 Len=20 Win=491

The output shows traffic that occurs in the different zones within the system. You can see all packets that are associated with the net0 IP addresses, including packets that are locally delivered to other zones. If you generate a verbose output, you can see the zones that are involved in the flow of packets.

# snoop -I net0 -v port 22
IPNET:  ----- IPNET Header ----- 
IPNET: 
IPNET:  Packet 5 arrived at 15:16:50.85262 
IPNET:  Packet size = 64 bytes 
IPNET:  dli_version = 1 
IPNET:  dli_type = 0 
IPNET:  dli_srczone = 0 
IPNET:  dli_dstzone = 1 
IPNET: 
IP:   ----- IP Header ----- 
IP: 
IP:   Version = 4 
IP:   Header length = 20 bytes 
IP:   Type of service = 0x00 
IP:         xxx. .... = 0 (precedence) 
IP:         ...0 .... = normal delay 
IP:         .... 0... = normal throughput 
IP:         .... .0.. = normal reliability 
IP:         .... ..0. = not ECN capable transport 
IP:         .... ...0 = no ECN congestion experienced 
IP:   Total length = 40 bytes 
IP:   Identification = 22629 
IP:   Flags = 0x4 
IP:         .1.. .... = do not fragment 
IP:         ..0. .... = last fragment 
IP:   Fragment offset = 0 bytes 
IP:   Time to live = 64 seconds/hops 
IP:   Protocol = 6 (TCP) 
IP:   Header checksum = 0000 
IP:   Source address = 172.0.0.1, 172.0.0.1 
IP:   Destination address = 172.0.0.3, 172.0.0.3 
IP:   No options 
IP: 
TCP:  ----- TCP Header ----- 
TCP: 
TCP:  Source port = 46919 
TCP:  Destination port = 22 
TCP:  Sequence number = 3295338550 
TCP:  Acknowledgement number = 3295417957 
TCP:  Data offset = 20 bytes 
TCP:  Flags = 0x10 
TCP:        0... .... = No ECN congestion window reduced 
TCP:        .0.. .... = No ECN echo 
TCP:        ..0. .... = No urgent pointer 
TCP:        ...1 .... = Acknowledgement 
TCP         .... 0... = No push 
TCP         .... .0.. = No reset 
TCP:        .... ..0. = No Syn 
TCP:        .... ...0 = No Fin 
TCP:  Window = 49152 
TCP:  Checksum = 0x0014 
TCP:  Urgent pointer = 0 
TCP:  No options 
TCP:

The ipnet header indicates that the packet is coming from the global zone (ID 0) to Sandbox (ID 1).

Example 5-20 Observing Traffic by Identifying the Zone

# snoop -I hme0 sandboxsnoop -I net0 sandbox
Using device ipnet/hme0 (promiscuous mode)
toybox -> sandbox TCP D=22 S=61658 Syn Seq=374055417 Len=0 Win=49152 Options=<mss
sandbox -> toybox TCP D=61658 S=22 Syn Ack=374055418 Seq=374124525 Len=0 Win=49152
toybox -> sandbox TCP D=22 S=61658 Ack=374124526 Seq=374055418 Len=0 Win=49152
#

The ability to observe packets by identifying zone is useful in systems that have multiple zones. Currently, you can only identify zone by using the zone ID. Using snoop with zone names is not supported.