tyt11fi.htm

tcpip 协议学习电子书籍 第一次上传东西

   /usr/sbin/in.named

fi</FONT></PRE>

<P>The exact directory paths and options might be different in your rc script, but the command should check for the named.boot file and start named if it exists.

<BR>

<BR>

<A ID="E70E46" NAME="E70E46"></A>

<H5 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>Configuring a Client</B></FONT></CENTER></H5>

<BR>

<P>Configuring a UNIX machine to use a primary DNS server for resolution is a quick process. First, the /etc/resolv.conf file is modified to include the primary server's address. For example, a resolv.conf file might look like this:

<BR>

<PRE>

<FONT COLOR="#000080">domain tpci.com

nameserver   143.25.0.1

nameserver   143.25.0.2</FONT></PRE>

<P>The first line establishes the domain name, which is followed by the IP addresses of available name servers. This file points to two name servers on the 143.25 subnet.

<BR>

<BR>

<A ID="E68E103" NAME="E68E103"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>BOOTP Protocol</B></FONT></CENTER></H3>

<BR>

<P>TCP/IP needs to know an Internet address before it can communicate with other machines. This can cause a problem when a machine is initially loaded or has no dedicated disk drive of its own. On Day 2, &quot;TCP/IP and the Internet,&quot; you saw how Reverse Address Resolution Protocol (RARP) can be used to determine an IP address, but an alternative is in common use: the <I>bootstrap protocol</I> or BOOTP. BOOTP uses UDP to enable a diskless machine to determine its IP address without using RARP.

<BR>

<P>Diskless machines usually contain start-up information in their PROMs. Because these must be kept small and consistent between many models of diskless workstations to reduce costs, it is impossible to pack a complete protocol such as TCP/IP into a chip. It is also impossible to embed an IP address, because the chip can be used in many different machines on the same network. This forces a newly booted diskless workstation to determine its own IP address from the other machines on the network. (In practice, the diskless machine also must determine the IP address of the network server it will use, as well as the address of the nearest IP gateway.)

<BR>

<P>BOOTP overcomes a few of RARP's problems. RARP requires direct access to the network hardware, which can cause problems when dealing with servers. Also, RARP supplies only an IP address. When large packets must be sent, this wastes a lot of space that could be used for useful information. BOOTP was developed to use UDP and can be implemented within an application program. BOOTP also requires only a single packet of information to provide all the information a new diskless workstation requires to begin operation. Therefore, BOOTP is more efficient and easier to develop applications for, making it popular.

<BR>

<P>To determine a diskless workstation's IP address, BOOTP uses the broadcast capabilities of IP. (You might recall that IP enables several special network addresses that are broadcast to all machines on the network.) This lets the workstation send a message even when it doesn't know the destination machine's address or even its own.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>IP broadcast addresses such as 255.255.255.255 enable a message to be sent to all machines on a network despite having no source or destination network address. </NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>BOOTP puts all the communications tasks on the diskless workstation. It specifies that all UDP messages sent over the network use checksums and that the Do Not Fragment bit be set. This tends to reduce the number of lost, misinterpreted, or duplicated datagrams.

<BR>

<P>To handle the loss of a message, BOOTP uses a simple set of timers. When a message has been sent, a timer starts. If no reply has been received when the timer runs out, the message is resent. The protocol stipulates that the timer is set to a random value, which increases each time the timer expires until it reaches a maximum value, after which it is randomized again. This prevents massive traffic after several machines fail at once and try to broadcast BOOTP messages at the same time.

<BR>

<P>BOOTP uses the terms <I>client</I> and <I>server</I> to refer to machines. The client is the machine that initiates a query, and the server is the machine that replies to that query. From these definitions, it is easy to see that client and server have no physical relation to any workstation, because the role of each workstation can change with message traffic. Because most systems can handle multiple traffic threads at a time, it is possible for a machine to be both a client and a server simultaneously.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>When considering client/server roles in BOOTP, remember that the machine that sends the first message is the client and the machine that replies is the server. There is no relationship to client/server architecture terms.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<BR>

<A ID="E69E151" NAME="E69E151"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>BOOTP Messages</B></FONT></CENTER></H4>

<BR>

<P>BOOTP messages are kept in fixed formats for simplicity and to enable the BOOTP software to fit in a small space within a PROM. The format of BOOTP messages is shown in Figure 11.9. The OpCode field is used to signal either a request (set to a value of 1) or a reply (set to a value of 2). The HTYPE field indicates the network hardware type. The HLEN field indicates the length of a hardware address. (These last two fields are the same as in ARP.)

<BR>

<P><B><A HREF="11tyt09.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/11tyt09.gif">Figure 11.9. The BOOTP message format.</A></B>

<BR>

<P>The HOPS field keeps track of the number of times the message is forwarded. When the client sends the request message, a value of 0 is put in the HOPS field. If the server decides to forward the message to another machine, it increments the HOPS count. (Forwarding is necessary when bootstrapping a machine across more than one gateway.)

<BR>

<P>The Transaction Number field is an integer assigned by the client to the message and is unchanged from request to reply. This enables matching the replies to the correct request. The Seconds field is the number of seconds the client has been booted, assigned by the client when the message is sent.

<BR>

<P>The Client IP Address field is filled in as much as possible by the client. This might result in a partial network address or no information at all, depending on the client's knowledge of the network it is in. Any information that is unknown is set to 0 (so the field might be 0.0.0.0 if nothing is known about the network address). If the client wants information from a particular server, it can put the address of the server in the Server IP Address field. Similarly, if the client knows the server's name, it puts it in the Server Host Name field. The same applies for the other address fields. If the fields are set to 0, any server can respond. If a specific server or gateway is given, only that machine responds to the message.

<BR>

<P>The Vendor-Specific field is used, as the name suggests, for implementation information that is specific to each vendor. This field is optional. The first 32 bits define the format of the remaining information. These first bits are known as the <I>magic cookie</I> and have a standard value of 99.120.83.99. Following the magic cookie are sets of information in a three-field format: a type, a length, and a value. There are several types identified by the Internet RFC, as shown in Table 11.5. The Length field is not used for types 0 and 255, but it must be present for types 1 and 2. The length can vary depending on the number of entries in the other types of messages.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 11.5. BOOTP vendor-specific types.</B></FONT></CENTER>

<BR>



<CENTER><TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 CELLPADDING=3 >

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Type</I></B>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Code</I></B>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Length</I></B>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Description</I></B>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Padding

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>0

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>--

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Used only for padding messages

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Subnet Mask

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>1

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>4

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Subnet mask for local network

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Time of Day

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>2

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>4

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Time of Day

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Gateways

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>3

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of gateways

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Time Servers

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>4

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of time servers

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IEN116 Server

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>5

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of IEN116 servers

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>DomainName Server

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>6

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of Domain Name Servers

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Log Server

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>7

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of log servers

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Quote Server

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>8

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of quote servers

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>LPR Servers

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>9

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Number of entries

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>IP addresses of lpr servers

<BR>