TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)

9.4 To Forward or Not to Forward

We've mentioned a few times that hosts are not supposed to forward IP datagrams unless they have been specifically configured as a router. How is this configuration done?

Most Berkeley-derived implementations have a kernel variable named ipforwarding , or some similar name . (See Appendix E.) Some systems (BSD/386 and SVR4, for example) only forward datagrams if this variable is nonzero. SunOS 4.1.x allows three values for the variable: -1 means never forward and never change the value of the variable, 0 means don't forward by default but set this variable to 1 when two or more interfaces are up, and 1 means always forward. Solaris 2.x changes the three values to be 0 (never forward), 1 (always forward), and 2 (only forward when two or more interfaces are up).

Older 4.2BSD hosts forwarded datagrams by default, which caused lots of problems for systems configured improperly. That's why this kernel option must always default to "never forward" unless the system administrator specifically enables forwarding.