IN_GETIFA(9) NetBSD Kernel Developer's Manual IN_GETIFA(9)
NAME
in_getifa -- Look up the IPv4 source address best matching an IPv4 desti- nation
SYNOPSIS
options IPSELSRC #include <netinet/in_selsrc.h> struct ifaddr * in_getifa(struct ifaddr *ifa, const struct sockaddr *dst0);
DESCRIPTION
in_getifa enforces the IPv4 source-address selection policy. Add the source-address selection policy mechanism to your kernel with options IPSELSRC. options IPSELSRC lets the operator set the policy for choosing the source address of any socket bound to the ``wildcard'' address, INADDR_ANY. Note that the policy is applied after the kernel makes its forwarding decision, thereby choosing the output interface; in other words, this mechanism does not affect whether or not NetBSD is a ``strong ES''. An operator affects the source-address selection using sysctl(8) and ifconfig(8). Operators set policies with sysctl(8). Some policies con- sider the ``preference number'' of an address. An operator may set pref- erence numbers for each address with ifconfig(8). A source-address policy is a priority-ordered list of source-address ranking functions. A ranking function maps its arguments, (source address, source index, source preference, destination address), to inte- gers. The source index is the position of source address in the inter- face address list; the index of the first address is 0. The source preference is the preference number the operator assigned to source address. The destination address is the socket peer / packet destina- tion. Presently, there are four ranking functions to choose from: index ranks by source index; lower indices are ranked more highly. preference ranks by source preference; higher preference numbers are ranked more highly. common-prefix-len ranks each source address by the length of the longest prefix it has in common with destination address; longer common prefixes rank more highly. same-category determines the "categories" of source and destination address. A category is one of private, link-local, or other. If the categories exactly match, same-category assigns a rank of 2. Some sources are ranked 1 by category: a link-local source with a private destina- tion, a private source with a link-local destination, and a private source with an other destination rank 1. All other sources rank 0. Categories are defined as follows. private RFC1918 networks, 192.168/16, 172.16/12, and 10/8 link-local 169.254/16, 224/24 other all other networks---i.e., not private, not link-local To apply a policy, the kernel applies all ranking functions in the policy to every source address, producing a vector of ranks for each source. The kernel sorts the sources in descending, lexicographical order by their rank-vector, and chooses the highest-ranking (first) source. The kernel breaks ties by choosing the source with the least source index. The operator may set a policy on individual interfaces. The operator may also set a global policy that applies to all interfaces whose policy they do not set individually. Here is the sysctl tree for the policy at system startup: net.inet.ip.selectsrc.default = index net.inet.ip.interfaces.ath0.selectsrc = net.inet.ip.interfaces.sip0.selectsrc = net.inet.ip.interfaces.sip1.selectsrc = net.inet.ip.interfaces.lo0.selectsrc = net.inet.ip.interfaces.pflog0.selectsrc = The policy on every interface is the ``empty'' policy, so the default policy applies. The default policy, index, is the ``historical'' policy in NetBSD. The operator may override the default policy on ath0, # sysctl -w net.inet.ip.interfaces.ath0.selectsrc=same-category,common-prefix-len,preference yielding this policy: net.inet.ip.selectsrc.default = index net.inet.ip.interfaces.ath0.selectsrc = same-category,common-prefix-len,preference The operator may set a new default, # sysctl -w net.inet.ip.selectsrc.debug=> same-category,common-prefix-len,preference # sysctl -w net.inet.ip.interfaces.ath0.selectsrc= yielding this policy: net.inet.ip.selectsrc.default = same-category,common-prefix-len,preference net.inet.ip.interfaces.ath0.selectsrc = In a number of applications, the policy above will usually pick suitable source addresses if ath0 is configured in this way: # ifconfig ath0 inet 64.198.255.1/24 # ifconfig ath0 inet 10.0.0.1/24 # ifconfig ath0 inet 169.254.1.1/24 # ifconfig ath0 inet 192.168.49.1/24 preference 5 # ifconfig ath0 inet 192.168.37.1/24 preference 9 A sysctl, net.inet.ip.selectsrc.debug, turns on and off debug messages concerned with source selection. You may set it to 0 (no messages) or 1.
SEE ALSO
ifconfig(8), sysctl(8)
STANDARDS
The family of IPv6 source-address selection policies defined by RFC3484 resembles the family of IPv4 policies that in_getifa enforces.
AUTHORS
David Young <dyoung@NetBSD.org>
BUGS
With options IPSELSRC, a new interface ioctl(2), SIOCSIFADDRPREF, was introduced. It ought to be documented in inet(4). Also, options(4) ought to cross-reference this manual page. This work should be used to set IPv6 source-address selection policies, especially the family of policies defined by RFC3484. NetBSD 10.1 February 22, 2007 NetBSD 10.1
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