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Understanding
LINUX
NETWORK
INTERNALS
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Understanding
LINUX
NETWORK
INTERNALS
Christian Benvenuti
Beijing • Cambridge • Farnham • Köln • Paris • Sebastopol • Taipei • Tokyo
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Understanding Linux Network Internals
by Christian Benvenuti
Copyright © 2006 O’Reilly Media, Inc. All rights reserved.
Printed in the United States of America.
Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472.
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Editor: Andy Oram
Production Editor: Philip Dangler
Cover Designer: Karen Montgomery
Interior Designer: David Futato
Printing History:
December 2005: First Edition.
Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of
O’Reilly Media, Inc. The Linux series designations, Understanding Linux Network Internals, images of
the American West, and related trade dress are trademarks of O’Reilly Media, Inc.
Many of the designations used by manufacturers and sellers to distinguish their products are claimed as
trademarks. Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a
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While every precaution has been taken in the preparation of this book, the publisher and author assume
no responsibility for errors or omissions, or for damages resulting from the use of the information
contained herein.
[M]
ISBN: 978-0-596-00255-8 [5/08]
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v
Table of Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
Part I. General Background
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Basic Terminology 3
Common Coding Patterns 4
User-Space Tools 18
Browsing the Source Code 19
When a Feature Is Offered as a Patch 20
2. Critical Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
The Socket Buffer: sk_buff Structure 22
net_device Structure 43
Files Mentioned in This Chapter 57
3. User-Space-to-Kernel Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Overview 58
procfs Versus sysctl 60
ioctl 67
Netlink 70
Serializing Configuration Changes 71
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vi | Table of Contents
Part II. System Initialization
4. Notification Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Reasons for Notification Chains 75
Overview 77
Defining a Chain 78
Registering with a Chain 78
Notifying Events on a Chain 79
Notification Chains for the Networking Subsystems 81
Tuning via /proc Filesystem 82
Functions and Variables Featured in This Chapter 83
Files and Directories Featured in This Chapter 83
5. Network Device Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
System Initialization Overview 84
Device Registration and Initialization 86
Basic Goals of NIC Initialization 86
Interaction Between Devices and Kernel 87
Initialization Options 93
Module Options 93
Initializing the Device Handling Layer: net_dev_init 94
User-Space Helpers 96
Virtual Devices 100
Tuning via /proc Filesystem 103
Functions and Variables Featured in This Chapter 104
Files and Directories Featured in This Chapter 105
6. The PCI Layer and Network Interface Cards . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Data Structures Featured in This Chapter 106
Registering a PCI NIC Device Driver 108
Power Management and Wake-on-LAN 109
Example of PCI NIC Driver Registration 110
The Big Picture 112
Tuning via /proc Filesystem 114
Functions and Variables Featured in This Chapter 114
Files and Directories Featured in This Chapter 115
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Table of Contents | vii
7. Kernel Infrastructure for Component Initialization . . . . . . . . . . . . . . . . . . . . 116
Boot-Time Kernel Options 116
Module Initialization Code 122
Optimized Macro-Based Tagging 125
Boot-Time Initialization Routines 128
Memory Optimizations 130
Tuning via /proc Filesystem 134
Functions and Variables Featured in This Chapter 134
Files and Directories Featured in This Chapter 135
8. Device Registration and Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
When a Device Is Registered 137
When a Device Is Unregistered 138
Allocating net_device Structures 138
Skeleton of NIC Registration and Unregistration 140
Device Initialization 141
Organization of net_device Structures 145
Device State 147
Registering and Unregistering Devices 149
Device Registration 154
Device Unregistration 156
Enabling and Disabling a Network Device 159
Updating the Device Queuing Discipline State 161
Configuring Device-Related Information from User Space 166
Virtual Devices 169
Locking 171
Tuning via /proc Filesystem 171
Functions and Variables Featured in This Chapter 172
Files and Directories Featured in This Chapter 173
Part III. Transmission and Reception
9. Interrupts and Network Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Decisions and Traffic Direction 178
Notifying Drivers When Frames Are Received 178
Interrupt Handlers 183
softnet_data Structure 206
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10. Frame Reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
Interactions with Other Features 211
Enabling and Disabling a Device 211
Queues 212
Notifying the Kernel of Frame Reception: NAPI and netif_rx 212
Old Interface Between Device Drivers and Kernel: First Part of netif_rx 219
Congestion Management 225
Processing the NET_RX_SOFTIRQ: net_rx_action 228
11. Frame Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
Enabling and Disabling Transmissions 241
12. General and Reference Material About Interrupts . . . . . . . . . . . . . . . . . . . . . 261
Statistics 261
Tuning via /proc and sysfs Filesystems 262
Functions and Variables Featured in This Part of the Book 263
Files and Directories Featured in This Part of the Book 265
13. Protocol Handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
Overview of Network Stack 266
Executing the Right Protocol Handler 274
Protocol Handler Organization 278
Protocol Handler Registration 279
Ethernet Versus IEEE 802.3 Frames 281
Tuning via /proc Filesystem 293
Functions and Variables Featured in This Chapter 293
Files and Directories Featured in This Chapter 294
Part IV. Bridging
14. Bridging: Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Repeaters, Bridges, and Routers 297
Bridges Versus Switches 299
Hosts 300
Merging LANs with Bridges 300
Bridging Different LAN Technologies 302
Address Learning 302
Multiple Bridges 305
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Table of Contents | ix
15. Bridging: The Spanning Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
Basic Terminology 311
Example of Hierarchical Switched L2 Topology 311
Basic Elements of the Spanning Tree Protocol 314
Bridge and Port IDs 321
Bridge Protocol Data Units (BPDUs) 323
Defining the Active Topology 328
Timers 335
Topology Changes 340
BPDU Encapsulation 344
Transmitting Configuration BPDUs 346
Processing Ingress Frames 347
Convergence Time 349
Overview of Newer Spanning Tree Protocols 350
16. Bridging: Linux Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
Bridge Device Abstraction 355
Important Data Structures 359
Initialization of Bridging Code 360
Creating Bridge Devices and Bridge Ports 361
Creating a New Bridge Device 362
Bridge Device Setup Routine 362
Deleting a Bridge 364
Adding Ports to a Bridge 364
Enabling and Disabling a Bridge Device 367
Enabling and Disabling a Bridge Port 368
Changing State on a Bridge Port 370
The Big Picture 371
Forwarding Database 373
Handling Ingress Traffic 375
Transmitting on a Bridge Device 380
Spanning Tree Protocol (STP) 380
netdevice Notification Chain 389
17. Bridging: Miscellaneous Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
User-Space Configuration Tools 391
Tuning via /proc Filesystem 396
Tuning via /sys Filesystem 396
Statistics 398
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Data Structures Featured in This Part of the Book 398
Functions and Variables Featured in This Part of the Book 403
Files and Directories Featured in This Part of the Book 405
Part V. Internet Protocol Version 4 (IPv4)
18. Internet Protocol Version 4 (IPv4): Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 409
IP Protocol: The Big Picture 409
IP Header 411
IP Options 414
Packet Fragmentation/Defragmentation 420
Checksums 432
19. Internet Protocol Version 4 (IPv4): Linux Foundations and Features . . . . . 439
Main IPv4 Data Structures 439
General Packet Handling 443
IP Options 453
20. Internet Protocol Version 4 (IPv4): Forwarding and Local Delivery . . . . . . 466
Forwarding 466
Local Delivery 472
21. Internet Protocol Version 4 (IPv4): Transmission . . . . . . . . . . . . . . . . . . . . . . 473
Key Functions That Perform Transmission 474
Interface to the Neighboring Subsystem 510
22. Internet Protocol Version 4 (IPv4): Handling Fragmentation . . . . . . . . . . . 511
IP Fragmentation 512
IP Defragmentation 521
23. Internet Protocol Version 4 (IPv4): Miscellaneous Topics . . . . . . . . . . . . . . . 536
Long-Living IP Peer Information 536
Selecting the IP Header’s ID Field 540
IP Statistics 541
IP Configuration 545
IP-over-IP 550
IPv4: What’s Wrong with It? 551
Tuning via /proc Filesystem 553
Data Structures Featured in This Part of the Book 555
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Table of Contents | xi
Functions and Variables Featured in This Part of the Book 565
Files and Directories Featured in This Part of the Book 568
24. Layer Four Protocol and Raw IP Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569
Available L4 Protocols 569
L4 Protocol Registration 571
L3 to L4 Delivery: ip_local_deliver_finish 574
IPv4 Versus IPv6 582
Tuning via /proc Filesystem 583
Functions and Variables Featured in This Chapter 583
Files and Directories Featured in This Chapter 583
25. Internet Control Message Protocol (ICMPv4) . . . . . . . . . . . . . . . . . . . . . . . . . 585
ICMP Header 586
ICMP Payload 587
ICMP Types 588
Applications of the ICMP Protocol 595
The Big Picture 598
Protocol Initialization 599
Data Structures Featured in This Chapter 600
Transmitting ICMP Messages 602
Receiving ICMP Messages 611
ICMP Statistics 617
Passing Error Notifications to the Transport Layer 619
Tuning via /proc Filesystem 620
Functions and Variables Featured in This Chapter 622
Files and Directories Featured in This Chapter 622
Part VI. Neighboring Subsystem
26. Neighboring Subsystem: Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625
What Is a Neighbor? 625
Reasons That Neighboring Protocols Are Needed 628
Linux Implementation 634
Proxying the Neighboring Protocol 637
When Solicitation Requests Are Transmitted and Processed 640
Neighbor States and Network Unreachability Detection (NUD) 642
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27. Neighboring Subsystem: Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651
Main Data Structures 651
Common Interface Between L3 Protocols and Neighboring Protocols 655
General Tasks of the Neighboring Infrastructure 666
Reference Counts on neighbour Structures 670
Creating a neighbour Entry 671
Neighbor Deletion 673
Acting As a Proxy 679
L2 Header Caching 683
Protocol Initialization and Cleanup 687
Interaction with Other Subsystems 688
Interaction Between Neighboring Protocols and L3 Transmission
Functions 692
Queuing 696
28. Neighboring Subsystem: Address Resolution Protocol (ARP) . . . . . . . . . . . 699
ARP Packet Format 700
Example of an ARP Transaction 702
Gratuitous ARP 702
Responding from Multiple Interfaces 707
Tunable ARP Options 708
ARP Protocol Initialization 714
Initialization of a neighbour Structure 716
Transmitting and Receiving ARP Packets 722
Processing Ingress ARP Packets 726
Proxy ARP 735
Examples 740
External Events 742
ARPD 744
Reverse Address Resolution Protocol (RARP) 746
Improvements in ND (IPv6) over ARP (IPv4) 748
29. Neighboring Subsystem: Miscellaneous Topics . . . . . . . . . . . . . . . . . . . . . . . 749
System Administration of Neighbors 749
Tuning via /proc Filesystem 752
Data Structures Featured in This Part of the Book 757
Files and Directories Featured in This Part of the Book 774
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Part VII. Routing
30. Routing: Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777
Routers, Routes, and Routing Tables 778
Essential Elements of Routing 781
Routing Table 793
Lookups 798
Packet Reception Versus Packet Transmission 800
31. Routing: Advanced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
Concepts Behind Policy Routing 802
Concepts Behind Multipath Routing 808
Interactions with Other Kernel Subsystems 815
Routing Protocol Daemons 819
Verbose Monitoring 821
ICMP_REDIRECT Messages 822
Reverse Path Filtering 828
32. Routing: Linux Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 830
Kernel Options 830
Main Data Structures 834
Route and Address Scopes 837
Primary and Secondary IP Addresses 841
Generic Helper Routines and Macros 842
Global Locks 843
Routing Subsystem Initialization 844
External Events 845
Interactions with Other Subsystems 858
33. Routing: The Routing Cache. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861
Routing Cache Initialization 861
Hash Table Organization 862
Major Cache Operations 864
Multipath Caching 873
Interface Between the DST and Calling Protocols 879
Flushing the Routing Cache 885
Garbage Collection 886
Egress ICMP REDIRECT Rate Limiting 896
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