Linux Network Administrators Guide

[Pages:505]Linux Network Administrators Guide

Linux Network Administrators Guide

Table of Contents

1. Purpose and Audience for This Book............................................................................................................1

2. Sources of Information...................................................................................................................................2 2.1. Documentation Available via FTP....................................................................................................3 2.2. Documentation Available via WWW...............................................................................................3 2.3. Documentation Available Commercially.........................................................................................3 2.4. Linux Journal and Linux Magazine..................................................................................................4 2.5. Linux Usenet Newsgroups................................................................................................................4 2.6. Linux Mailing Lists..........................................................................................................................5 2.7. Online Linux Support.......................................................................................................................6 2.8. Linux User Groups............................................................................................................................6 2.9. Obtaining Linux................................................................................................................................7

3. File System Standards....................................................................................................................................9

4. Standard Linux Base....................................................................................................................................10

5. About This Book...........................................................................................................................................11

6. The Official Printed Version........................................................................................................................13

7. Overview........................................................................................................................................................15

8. Conventions Used in This Book...................................................................................................................17

9. Submitting Changes......................................................................................................................................18

10. Acknowledgments.......................................................................................................................................19 10.1. The Hall of Fame..........................................................................................................................19

Chapter 1. Introduction to Networking..........................................................................................................21

1.1. History.........................................................................................................................................................22

1.2. TCP/IP Networks.......................................................................................................................................23 1.2.1. Introduction to TCP/IP Networks................................................................................................23 1.2.2. Ethernets......................................................................................................................................24 1.2.3. Other Types of Hardware.............................................................................................................25 1.2.4. The Internet Protocol...................................................................................................................27 1.2.5. IP Over Serial Lines.....................................................................................................................28 1.2.6. The Transmission Control Protocol.............................................................................................28 1.2.7. The User Datagram Protocol.......................................................................................................29 1.2.8. More on Ports...............................................................................................................................29 1.2.9. The Socket Library......................................................................................................................30

1.3. UUCP Networks.........................................................................................................................................31

1.4. Linux Networking......................................................................................................................................32

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1.4.1. Different Streaks of Development...............................................................................................32 1.4.2. Where to Get the Code.................................................................................................................33

1.5. Maintaining Your System.........................................................................................................................34 1.5.1. System Security...........................................................................................................................34

Chapter 2. Issues of TCP/IP Networking.......................................................................................................36

2.1. Networking Interfaces...............................................................................................................................37

2.2. IP Addresses...............................................................................................................................................38

2.3. Address Resolution....................................................................................................................................40

2.4. IP Routing...................................................................................................................................................41 2.4.1. IP Networks.................................................................................................................................41 2.4.2. Subnetworks.................................................................................................................................41 2.4.3. Gateways......................................................................................................................................42 2.4.4. The Routing Table.......................................................................................................................43 2.4.5. Metric Values...............................................................................................................................45

2.5. The Internet Control Message Protocol...................................................................................................46

2.6. Resolving Host Names...............................................................................................................................47

Chapter 3. Configuringthe NetworkingHardware........................................................................................48

3.1. Kernel Configuration.................................................................................................................................51 3.1.1. Kernel Options in Linux 2.0 and Higher.....................................................................................51 3.1.2. Kernel Networking Options in Linux 2.0.0 and Higher..............................................................53

3.2. A Tour of Linux Network Devices............................................................................................................57

3.3. Ethernet Installation..................................................................................................................................59 3.3.1. Ethernet Autoprobing...................................................................................................................59

3.4. The PLIP Driver........................................................................................................................................62

3.5. The PPP and SLIP Drivers.......................................................................................................................64

3.6. Other Network Types................................................................................................................................65

Chapter 4. Configuring the Serial Hardware.................................................................................................66

4.1. Communications Software for Modem Links.........................................................................................67

4.2. Introduction to Serial Devices...................................................................................................................68

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4.3. Accessing Serial Devices............................................................................................................................69 4.3.1. The Serial Device Special Files...................................................................................................70

4.4. Serial Hardware.........................................................................................................................................72

4.5. Using the Configuration Utilities..............................................................................................................73 4.5.1. The setserial Command................................................................................................................73 4.5.2. The stty Command.......................................................................................................................75

4.6. Serial Devices and the login: Prompt.......................................................................................................78 4.6.1. Configuring the mgetty Daemon.................................................................................................78

Chapter 5. Configuring TCP/IP Networking.................................................................................................81

5.1. Mounting the /proc Filesystem.................................................................................................................82

5.2. Installing the Binaries................................................................................................................................83

5.3. Setting the Hostname.................................................................................................................................84

5.4. Assigning IP Addresses..............................................................................................................................85

5.5. Creating Subnets........................................................................................................................................86

5.6. Writing hosts and networks Files.............................................................................................................87

5.7. Interface Configuration for IP..................................................................................................................89 5.7.1. The Loopback Interface...............................................................................................................89 5.7.2. Ethernet Interfaces.......................................................................................................................91 5.7.3. Routing Through a Gateway........................................................................................................92 5.7.4. Configuring a Gateway................................................................................................................93 5.7.5. The PLIP Interface.......................................................................................................................93 5.7.6. The SLIP and PPP Interfaces.......................................................................................................94 5.7.7. The Dummy Interface..................................................................................................................94 5.7.8. IP Alias........................................................................................................................................95

5.8. All About ifconfig.......................................................................................................................................96

5.9. The netstat Command...............................................................................................................................99 5.9.1. Displaying the Routing Table......................................................................................................99 5.9.2. Displaying Interface Statistics...................................................................................................100 5.9.3. Displaying Connections.............................................................................................................101

5.10. Checking the ARP Tables.....................................................................................................................102

Chapter 6. Name Service and Resolver Configuration...............................................................................104

6.1. The Resolver Library..............................................................................................................................105

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6.1.1. The host.conf File......................................................................................................................105 6.1.1.1. Resolver environment variables.................................................................................106

6.1.2. The nsswitch.conf File...............................................................................................................107 6.1.3. Configuring Name Server Lookups Using resolv.conf..............................................................109 6.1.4. Resolver Robustness..................................................................................................................111

6.2. How DNS Works......................................................................................................................................112 6.2.1. Name Lookups with DNS..........................................................................................................113 6.2.2. Types of Name Servers..............................................................................................................114 6.2.3. The DNS Database.....................................................................................................................115 6.2.4. Reverse Lookups........................................................................................................................116

6.3. Running named........................................................................................................................................118 6.3.1. The named.boot File..................................................................................................................118 6.3.2. The BIND 8 host.conf File.........................................................................................................120 6.3.3. The DNS Database Files............................................................................................................121 6.3.4. Caching-only named Configuration..........................................................................................125 6.3.5. Writing the Master Files............................................................................................................126 6.3.6. Verifying the Name Server Setup..............................................................................................128 6.3.7. Other Useful Tools.....................................................................................................................130

Chapter 7. Serial Line IP................................................................................................................................132

7.1. General Requirements.............................................................................................................................133

7.2. SLIP Operation........................................................................................................................................134

7.3. Dealing with Private IP Networks..........................................................................................................137

7.4. Using dip...................................................................................................................................................138 7.4.1. A Sample Script.........................................................................................................................138 7.4.2. A dip Reference.........................................................................................................................140 7.4.2.1. The modem commands..............................................................................................140 7.4.2.2. The echo command....................................................................................................141 7.4.2.3. The get command.......................................................................................................141 7.4.2.4. The print command....................................................................................................141 7.4.2.5. Variable names..........................................................................................................141 7.4.2.6. The if and goto commands.........................................................................................142 7.4.2.7. send, wait, and sleep..................................................................................................142 7.4.2.8. mode and default........................................................................................................142

7.5. Running in Server Mode.........................................................................................................................144

Chapter 8. The Point-to-Point Protocol......................................................................................................146

8.1. PPP on Linux............................................................................................................................................147

8.2. Running pppd...........................................................................................................................................148

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8.3. Using Options Files..................................................................................................................................149

8.4. Using chat to Automate Dialing..............................................................................................................150

8.5. IP Configuration Options........................................................................................................................152 8.5.1. Choosing IP Addresses..............................................................................................................152 8.5.2. Routing Through a PPP Link.....................................................................................................153

8.6. Link Control Options..............................................................................................................................155

8.7. General Security Considerations............................................................................................................157

8.8. Authentication with PPP.........................................................................................................................158 8.8.1. PAP Versus CHAP....................................................................................................................158 8.8.2. The CHAP Secrets File..............................................................................................................159 8.8.3. The PAP Secrets File.................................................................................................................160

8.9. Debugging Your PPP Setup....................................................................................................................162

8.10. More Advanced PPP Configurations...................................................................................................163 8.10.1. PPP Server...............................................................................................................................163 8.10.2. Demand Dialing.......................................................................................................................164 8.10.3. Persistent Dialing.....................................................................................................................165

Chapter 9. TCP/IP Firewall...........................................................................................................................167

9.1. Methods of Attack....................................................................................................................................168

9.2. What Is a Firewall?..................................................................................................................................170

9.3. What Is IP Filtering?...............................................................................................................................171

9.4. Setting Up Linux for Firewalling............................................................................................................172 9.4.1. Kernel Configured with IP Firewall..........................................................................................172 9.4.2. The ipfwadm Utility...................................................................................................................173 9.4.3. The ipchains Utility...................................................................................................................173 9.4.4. The iptables Utility....................................................................................................................173

9.5. Three Ways We Can Do Filtering..........................................................................................................174

9.6. Original IP Firewall (2.0 Kernels)..........................................................................................................175 9.6.1. Using ipfwadm...........................................................................................................................175 9.6.1.1. A na?ve example........................................................................................................175 9.6.1.2. An important refinement............................................................................................177 9.6.1.3. Listing our rules.........................................................................................................177 9.6.2. A More Complex Example........................................................................................................178 9.6.3. Summary of ipfwadm Arguments..............................................................................................179 9.6.3.1. Categories..................................................................................................................179

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9.6.3.2. Commands.................................................................................................................179 9.6.3.3. Parameters..................................................................................................................180 9.6.3.4. Optional arguments....................................................................................................181 9.6.3.5. ICMP datagram types................................................................................................182

9.7. IP Firewall Chains (2.2 Kernels)............................................................................................................183 9.7.1. Using ipchains............................................................................................................................183 9.7.2. ipchains Command Syntax........................................................................................................183 9.7.2.1. Commands.................................................................................................................184 9.7.2.2. Rule specification parameters....................................................................................185 9.7.2.3. Options.......................................................................................................................186 9.7.3. Our Na?ve Example Revisited...................................................................................................187 9.7.4. Listing Our Rules with ipchains................................................................................................188 9.7.5. Making Good Use of Chains......................................................................................................188 9.7.5.1. User-defined chains..................................................................................................189 9.7.5.2. The ipchains support scripts......................................................................................192

9.8. Netfilter and IP Tables (2.4 Kernels).....................................................................................................194 9.8.1. Backward Compatability with ipfwadmand ipchains................................................................196 9.8.2. Using iptables............................................................................................................................196 9.8.2.1. Commands.................................................................................................................196 9.8.2.2. Rule specification parameters....................................................................................198 9.8.2.3. Options.......................................................................................................................199 9.8.2.4. Extensions..................................................................................................................199 9.8.3. Our Na?ve Example Revisited, Yet Again.................................................................................201

9.9. TOS Bit Manipulation.............................................................................................................................202 9.9.1. Setting the TOS Bits Using ipfwadm or ipchains......................................................................202 9.9.2. Setting the TOS Bits Using iptables..........................................................................................203

9.10. Testing a Firewall Configuration.........................................................................................................205

9.11. A Sample Firewall Configuration.........................................................................................................207

Chapter 10. IP Accounting.............................................................................................................................214

10.1. Configuring the Kernel for IP Accounting..........................................................................................215

10.2. Configuring IP Accounting...................................................................................................................216 10.2.1. Accounting by Address............................................................................................................216 10.2.2. Accounting by Service Port.....................................................................................................218 10.2.3. Accounting of ICMP Datagrams.............................................................................................220 10.2.4. Accounting by Protocol...........................................................................................................221

10.3. Using IP Accounting Results.................................................................................................................222 10.3.1. Listing Accounting Data with ipfwadm...................................................................................222 10.3.2. Listing Accounting Data with ipchains....................................................................................223 10.3.3. Listing Accounting Data with iptables....................................................................................223

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10.4. Resetting the Counters..........................................................................................................................224 10.5. Flushing the Ruleset...............................................................................................................................225 10.6. Passive Collection of Accounting Data.................................................................................................226 Chapter 11. IP Masquerade and Network Address Translation................................................................227 11.1. Side Effects and Fringe Benefits...........................................................................................................229 11.2. Configuring the Kernel for IP Masquerade........................................................................................230 11.3. Configuring IP Masquerade.................................................................................................................232

11.3.1. Setting Timing Parameters for IP Masquerade........................................................................233 11.4. Handling Name Server Lookups..........................................................................................................235 11.5. More About Network Address Translation.........................................................................................236 Chapter 12. ImportantNetwork Features.....................................................................................................237 12.1. The inetd Super Server..........................................................................................................................238 12.2. The tcpd Access Control Facility..........................................................................................................241 12.3. The Services and Protocols Files..........................................................................................................243 12.4. Remote Procedure Call..........................................................................................................................245 12.5. Configuring Remote Loginand Execution...........................................................................................247

12.5.1. Disabling the r; Commands.....................................................................................................247 12.5.2. Installing and Configuring ssh.................................................................................................247

12.5.2.1. The ssh daemon.......................................................................................................248 12.5.2.2. The ssh client...........................................................................................................249 12.5.2.3. Using ssh..................................................................................................................251 Chapter 13. The Network Information System............................................................................................254 13.1. Getting Acquainted with NIS................................................................................................................256 13.2. NIS Versus NIS+....................................................................................................................................259 13.3. The Client Side of NIS...........................................................................................................................260 13.4. Running an NIS Server.........................................................................................................................261 13.5. NIS Server Security...............................................................................................................................262

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