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wgengine/router: reduce routing rule priority for openWRT + mwan3

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Fixes #3659

Signed-off-by: Tom DNetto <tom@tailscale.com>
Co-authored-by: Ian Foster <ian@vorsk.com>
This commit is contained in:
Tom DNetto 2022-09-09 14:23:48 -07:00 committed by Tom
parent c14361e70e
commit ed2b8b3e1d
1 changed files with 80 additions and 16 deletions
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96
wgengine/router/router_linux.go
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@ -105,6 +105,9 @@ type linuxRouter struct {
v6Available bool
v6NATAvailable bool
// ipPolicyPrefBase is the base priority at which ip rules are installed.
ipPolicyPrefBase int
ipt4 netfilterRunner
ipt6 netfilterRunner
cmd commandRunner
@ -163,6 +166,7 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, linkMon *monit
cmd: cmd,
ipRuleFixLimiter: rate.NewLimiter(rate.Every(5*time.Second), 10),
ipPolicyPrefBase: 5200,
}
if r.useIPCommand() {
r.ipRuleAvailable = (cmd.run("ip", "rule") == nil)
@ -176,6 +180,29 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, linkMon *monit
}
}
// A common installation of OpenWRT involves use of the 'mwan3' package.
// This package installs ip-tables rules like:
// -A mwan3_fallback_policy -m mark --mark 0x0/0x3f00 -j MARK --set-xmark 0x100/0x3f00
//
// which coupled with an ip rule:
// 2001: from all fwmark 0x100/0x3f00 lookup 1
//
// has the effect of gobbling tailscale packets, because tailscale by default installs
// its policy routing rules at priority 52xx.
//
// As such, if we are running on openWRT, detect a mwan3 config, AND detect a rule
// with a preference 2001 (corresponding to the first interface wman3 manages), we
// shift the priority of our policies to 13xx. This effectively puts us betwen mwan3's
// permit-by-src-ip rules and mwan3 lookup of its own routing table which would drop
// the packet.
isMWAN3, err := checkOpenWRTUsingMWAN3()
if err != nil {
r.logf("error checking mwan3 installation: %v", err)
} else if isMWAN3 {
r.ipPolicyPrefBase = 1300
r.logf("mwan3 on openWRT detected, switching policy base priority to 1300")
}
return r, nil
}
@ -219,7 +246,7 @@ func (r *linuxRouter) useIPCommand() bool {
// about the priority number. We could just do this in response to any netlink
// change. Filtering by known priority ranges cuts back on some logspam.
func (r *linuxRouter) onIPRuleDeleted(table uint8, priority uint32) {
if priority < 5200 || priority >= 5300 {
if int(priority) < r.ipPolicyPrefBase || int(priority) >= (r.ipPolicyPrefBase+100) {
// Not our rule.
return
}
@ -870,6 +897,8 @@ func mustRouteTable(num int) routeTable {
)
// ipRules are the policy routing rules that Tailscale uses.
// The priority is the value represented here added to r.ipPolicyPrefBase,
// which is usually 5200.
//
// NOTE(apenwarr): We leave spaces between each pref number.
// This is so the sysadmin can override by inserting rules in
@ -886,14 +915,14 @@ func mustRouteTable(num int) routeTable {
// Packets from us, tagged with our fwmark, first try the kernel's
// main routing table.
{
Priority: 5210,
Priority: 10,
Mark: tailscaleBypassMarkNum,
Table: mainRouteTable.num,
},
// ...and then we try the 'default' table, for correctness,
// even though it's been empty on every Linux system I've ever seen.
{
Priority: 5230,
Priority: 30,
Mark: tailscaleBypassMarkNum,
Table: defaultRouteTable.num,
},
@ -901,7 +930,7 @@ func mustRouteTable(num int) routeTable {
// then packets from us should be aborted rather than falling through
// to the tailscale routes, because that would create routing loops.
{
Priority: 5250,
Priority: 50,
Mark: tailscaleBypassMarkNum,
Type: unix.RTN_UNREACHABLE,
},
@ -911,7 +940,7 @@ func mustRouteTable(num int) routeTable {
// it takes precedence over all the others, ie. VPN routes always
// beat non-VPN routes.
{
Priority: 5270,
Priority: 70,
Table: tailscaleRouteTable.num,
},
// If that didn't match, then non-fwmark packets fall through to the
@ -928,6 +957,7 @@ func (r *linuxRouter) justAddIPRules() error {
}
var errAcc error
for _, family := range r.addrFamilies() {
for _, ru := range ipRules {
// Note: r is a value type here; safe to mutate it.
ru.Family = family.netlinkInt()
@ -936,6 +966,7 @@ func (r *linuxRouter) justAddIPRules() error {
ru.SuppressIfgroup = -1
ru.SuppressPrefixlen = -1
ru.Flow = -1
ru.Priority += r.ipPolicyPrefBase
err := netlink.RuleAdd(&ru)
if errors.Is(err, errEEXIST) {
@ -954,19 +985,19 @@ func (r *linuxRouter) addIPRulesWithIPCommand() error {
rg := newRunGroup(nil, r.cmd)
for _, family := range r.addrFamilies() {
for _, r := range ipRules {
for _, rule := range ipRules {
args := []string{
"ip", family.dashArg(),
"rule", "add",
"pref", strconv.Itoa(r.Priority),
"pref", strconv.Itoa(rule.Priority + r.ipPolicyPrefBase),
}
if r.Mark != 0 {
args = append(args, "fwmark", fmt.Sprintf("0x%x", r.Mark))
if rule.Mark != 0 {
args = append(args, "fwmark", fmt.Sprintf("0x%x", rule.Mark))
}
if r.Table != 0 {
args = append(args, "table", mustRouteTable(r.Table).ipCmdArg())
if rule.Table != 0 {
args = append(args, "table", mustRouteTable(rule.Table).ipCmdArg())
}
if r.Type == unix.RTN_UNREACHABLE {
if rule.Type == unix.RTN_UNREACHABLE {
args = append(args, "type", "unreachable")
}
rg.Run(args...)
@ -1040,14 +1071,14 @@ func (r *linuxRouter) delIPRulesWithIPCommand() error {
// That leaves us some flexibility to change these values in later
// versions without having ongoing hacks for every possible
// combination.
for _, r := range ipRules {
for _, rule := range ipRules {
args := []string{
"ip", family.dashArg(),
"rule", "del",
"pref", strconv.Itoa(r.Priority),
"pref", strconv.Itoa(rule.Priority + r.ipPolicyPrefBase),
}
if r.Table != 0 {
args = append(args, "table", mustRouteTable(r.Table).ipCmdArg())
if rule.Table != 0 {
args = append(args, "table", mustRouteTable(rule.Table).ipCmdArg())
} else {
args = append(args, "type", "unreachable")
}
@ -1548,6 +1579,39 @@ func checkIPRuleSupportsV6(logf logger.Logf) error {
return netlink.RuleAdd(rule)
}
// Checks if the running openWRT system is using mwan3, based on the heuristic
// of the config file being present as well as a policy rule with a specific
// priority (2000 + 1 - first interface mwan3 manages) and non-zero mark.
func checkOpenWRTUsingMWAN3() (bool, error) {
if distro.Get() != distro.OpenWrt {
return false, nil
}
if _, err := os.Stat("/etc/config/mwan3"); err != nil {
if os.IsNotExist(err) {
return false, nil
}
return false, err
}
rules, err := netlink.RuleList(netlink.FAMILY_V4)
if err != nil {
return false, err
}
for _, r := range rules {
// We want to match on a rule like this:
// 2001: from all fwmark 0x100/0x3f00 lookup 1
//
// We dont match on the mask because it can vary, or the
// table because I'm not sure if it can vary.
if r.Priority == 2001 && r.Mark != 0 {
return true, nil
}
}
return false, nil
}
func nlAddrOfPrefix(p netip.Prefix) *netlink.Addr {
return &netlink.Addr{
IPNet: netipx.PrefixIPNet(p),