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TCP/IP Network Administration

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Troubleshooting TCP/IP
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11.7 Analyzing Protocol Problems

Problems caused by bad TCP/IP configurations are much more common than problems caused by bad TCP/IP protocol implementations. Most of the problems you encounter will succumb to analysis using the simple tools we have already discussed. But on occasion, you may need to analyze the protocol interaction between two systems. In the worst case, you may need to analyze the packets in the data stream bit by bit. Protocol analyzers help you do this.

snoop is the tool we'll use. It is provided with the Solaris operating system. [11] Although we use snoop in all of our examples, the concepts introduced in this section should be applicable to the analyzer that you use, because most protocol analyzers function in basically the same way. Protocol analyzers allow you to select, or filter, the packets you want to examine, and to examine those packets byte by byte. We'll discuss both of these functions.

[11] If you don't use Solaris, try tcpdump. It is available via anonymous FTP on the Internet and is similar to snoop.

Protocol analyzers watch all the packets on the network. Therefore, you only need one system that runs analyzer software on the affected part of the network. One Solaris system with snoop can monitor the network traffic and tell you what the other hosts are (or aren't) doing. This, of course, assumes a shared media network. If you use an Ethernet switch, only the traffic on an individual segment can be seen. Some switches provide a monitor port. For others you may need to take your monitor to the location of the problem.

11.7.1 Packet Filters

snoop reads all the packets on an Ethernet. It does this by placing the Ethernet interface into promiscuous mode. Normally, an Ethernet interface only passes packets up to the higher layer protocols that are destined for the local host. In promiscuous mode, all packets are accepted and passed to the higher layer. This allows snoop to view all packets and to select packets for analysis, based on a filter you define. Filters can be defined to capture packets from, or to, specific hosts, protocols, and ports, or combinations of all these. As an example, let's look at a very simple snoop filter. The following snoop command displays all packets sent between the hosts almond and peanut:

# snoop host almond and host peanut
Using device /dev/le (promiscuous mode)
peanut.nuts.com -> almond.nuts.com ICMP Echo request
almond.nuts.com -> peanut.nuts.com ICMP Echo reply
peanut.nuts.com -> almond.nuts.com RLOGIN C port=1023
almond.nuts.com -> peanut.nuts.com RLOGIN R port=1023
^C

The filter "host almond and host peanut" selects only those packets that are from peanut to almond, or from almond to peanut. The filter is constructed from a set of primitives, and associated hostnames, protocol names, and port numbers. The primitives can be modified and combined with the operators and, or, and not. The filter may be omitted; this causes snoop to display all packets from the network.

Table 11.2 shows the primitives used to build snoop filters. There are a few additional primitives and some variations that perform the same functions, but these are the essential primitive. See the snoop manpage for additional details.

Table 11.2: Expression Primitives
PrimitiveMatches Packets
dst host | net | port destinationTo destination host, net, or port
src host | net | port sourceFrom source host, net, or port
host destinationTo or from destination host
net destinationTo or from destination network
port destinationTo or from destination port
ether addressTo or from Ethernet address
protocolOf protocol type (icmp, udp, or tcp)

Using these primitives with the operators and and or, complex filters can be constructed. However, filters are usually simple. Capturing the traffic between two hosts is probably the most common filter. You may further limit the data captured to a specific protocol, but often you're not sure which protocol will reveal the problem. Just because the user sees the problem in ftp or telnet does not mean that is where the problem actually occurs. Analysis must often start by capturing all packets, and can only be narrowed after test evidence points to some specific problem.

11.7.1.1 Modifying analyzer output

The example in the previous section shows that snoop displays a single line of summary information for each packet received. All lines show the source and destination addresses, and the protocol being used (ICMP and RLOGIN in the example). The lines that summarize the ICMP packets identify the packet types (Echo request and Echo reply in the example). The lines that summarize the application protocol packets display the source port and the first 20 characters of the packet data.

This summary information is sufficient to gain insight into how packets flow between two hosts and into potential problems. However, troubleshooting protocol problems requires more detailed information about each packet. snoop has options that give you control over what information is displayed. To display the data contained in a packet, use the -x option. It causes the entire contents of the packet to be dumped in hex and ASCII. In most cases, you don't need to see the entire packet; usually, the headers are sufficient to troubleshoot a protocol problem. The -v option displays the headers in a well-formatted and very detailed manner. Because of the large number of lines displayed for each packet, only use -v when you need it.

The following example shows an ICMP Echo Request packet displayed with the -v option. The same type of packet was summarized in the first line of the previous example.

# snoop -v host almond and host macadamia
Using device /dev/le (promiscuous mode)
ETHER:  ----- Ether Header -----
ETHER:  
ETHER:  Packet 3 arrived at 16:56:57.90
ETHER:  Packet size = 98 bytes
ETHER:  Destination = 8:0:20:22:fd:51, Sun
ETHER:  Source      = 0:0:c0:9a:d0:db, Western Digital
ETHER:  Ethertype = 0800 (IP)
ETHER:  
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:   Total length = 84 bytes
IP:   Identification = 3049
IP:   Flags = 0x0
IP:         .0.. .... = may fragment
IP:         ..0. .... = last fragment
IP:   Fragment offset = 0 bytes
IP:   Time to live = 64 seconds/hops
IP:   Protocol = 1 (ICMP)
IP:   Header checksum = fde0
IP:   Source address = 172.16.55.106, macadamia.nuts.com
IP:   Destination address = 172.16.12.1, almond.nuts.com
IP:   No options
IP:   
ICMP:  ----- ICMP Header -----
ICMP:  
ICMP:  Type = 8 (Echo request)
ICMP:  Code = 0
ICMP:  Checksum = ac54
ICMP:

The detailed formatting done by snoop maps the bytes received from the network to the header structure. Look at the description of the various header fields in Chapter 1, and Appendix F, Selected TCP/IP Headers, for more information.


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