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Linux already has a model to give a hint for source address selection via route table "hints". When adding routes (either manually via `ip route' or via things like bird2 BGP daemon), Linux supports setting a source IP for when that route is used. Interestingly, JunOS (which I believe is based on FreeBSD) also supports a way to specify a default IPv4 source address, preferring the primary address on lo0 that is not 127.0.0.1. It is a common practice for BGP systems to attach their announced IPs to the loopback interface. https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/default-address-selection-edit-system.html For the Linux and bird (BGP) documentation: Linux ----- http://linux-ip.net/html/tools-ip-route.html#ex-tools-ip-route-add-src "The src option provides a hint to the kernel for source address selection. When you are working with multiple routing tables and different classes of traffic, you can ease your administrative burden, by hosting several different IPs on your linux machine and setting the source address differently, depending on the type of traffic. In the example below, let's assume that our masquerading host also runs a DNS resolver for the internal network and we have selected all of the outbound DNS packets to be routed according to table 7 [53]. Now, any packet which originates on this box (or is masqueraded through this table) will have its source IP set to 205.254.211.198. Example D.19. Using src in a routing command with route add [root@masq-gw]# ip route add default via 205.254.211.254 src 205.254.211.198 table 7 " man ip-route "src ADDRESS the source address to prefer when sending to the destinations covered by the route prefix." Bird (BGP Daemon) ---- "The Kernel protocol defines several attributes. These attributes are translated to appropriate system (and OS-specific) route attributes. We support these attributes: ... ip krt_prefsrc (Linux) The preferred source address. Used in source address selection for outgoing packets. Has to be one of the IP addresses of the router."
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