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net/interfaces: better handle multiple interfaces in LikelyHomeRouterIP

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Currently, we get the "likely home router" gateway IP and then iterate
through all IPs for all interfaces trying to match IPs to determine the
source IP. However, on many platforms we know what interface the gateway
is through, and thus we don't need to iterate through all interfaces
checking IPs. Instead, use the IP address of the associated interface.

This better handles the case where we have multiple interfaces on a
system all connected to the same gateway, and where the first interface
that we visit (as iterated by ForeachInterfaceAddress) isn't also the
default internet route.

Updates #8992

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8632f577f1136930f4ec60c76376527a19a47d1f
This commit is contained in:
Andrew Dunham
2023-12-19 16:27:52 -05:00
parent 7a2eb22e94
commit 01286af82b
6 changed files with 83 additions and 25 deletions
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42
net/interfaces/interfaces.go
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@ -15,6 +15,7 @@ import (
"sort"
"strings"
"tailscale.com/envknob"
"tailscale.com/hostinfo"
"tailscale.com/net/netaddr"
"tailscale.com/net/tsaddr"
@ -525,7 +526,21 @@ func HTTPOfListener(ln net.Listener) string {
}
var likelyHomeRouterIP func() (netip.Addr, bool)
// likelyHomeRouterIP, if present, is a platform-specific function that is used
// to determine the likely home router IP of the current system. The signature
// of this function is:
//
// func() (homeRouter, localAddr netip.Addr, ok bool)
//
// It should return a homeRouter IP and ok=true, or no homeRouter IP and
// ok=false. Optionally, an implementation can return the "self" IP address as
// well, which will be used instead of attempting to determine it by reading
// the system's interfaces.
var likelyHomeRouterIP func() (netip.Addr, netip.Addr, bool)
// For debugging the new behaviour where likelyHomeRouterIP can return the
// "self" IP; should remove after we're confidant this won't cause issues.
var disableLikelyHomeRouterIPSelf = envknob.RegisterBool("TS_DEBUG_DISABLE_LIKELY_HOME_ROUTER_IP_SELF")
// LikelyHomeRouterIP returns the likely IP of the residential router,
// which will always be an IPv4 private address, if found.
@ -533,15 +548,30 @@ var likelyHomeRouterIP func() (netip.Addr, bool)
// the LAN using that gateway.
// This is used as the destination for UPnP, NAT-PMP, PCP, etc queries.
func LikelyHomeRouterIP() (gateway, myIP netip.Addr, ok bool) {
if likelyHomeRouterIP != nil {
gateway, ok = likelyHomeRouterIP()
if !ok {
return
}
// If we don't have a way to get the home router IP, then we can't do
// anything; just return.
if likelyHomeRouterIP == nil {
return
}
// Get the gateway next; if that fails, we can't continue.
gateway, myIP, ok = likelyHomeRouterIP()
if !ok {
return
}
// If the platform-specific implementation returned a valid myIP, then
// we can return it as-is without needing to iterate through all
// interface addresses.
if disableLikelyHomeRouterIPSelf() {
myIP = netip.Addr{}
}
if myIP.IsValid() {
return
}
// The platform-specific implementation didn't return a valid myIP;
// iterate through all interfaces and try to find the correct one.
ForeachInterfaceAddress(func(i Interface, pfx netip.Prefix) {
if !i.IsUp() {
// Skip interfaces that aren't up.