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cmd/cli: watch changes to /etc/resolv.conf

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On some routers, change to network may trigger re-rendering
/etc/resolv.conf file, causing requests from router itself stop using
ctrld.

Fixing this by watching changes to /etc/resolv.conf, then revert them.
This commit is contained in:
Cuong Manh Le
2023-11-22 19:07:53 +07:00
committed by Cuong Manh Le
parent 2bebe93e47
commit a2cb895cdc
1 changed files with 67 additions and 1 deletions
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68
cmd/cli/os_linux.go
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@@ -9,10 +9,12 @@ import (
"net"
"net/netip"
"os/exec"
"path/filepath"
"strings"
"syscall"
"time"
"github.com/fsnotify/fsnotify"
"github.com/insomniacslk/dhcp/dhcpv4/nclient4"
"github.com/insomniacslk/dhcp/dhcpv6"
"github.com/insomniacslk/dhcp/dhcpv6/client6"
@@ -23,7 +25,10 @@ import (
"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
)
const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
const (
resolvConfPath = "/etc/resolv.conf"
resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
)
// allocate loopback ip
// sudo ip a add 127.0.0.2/24 dev lo
@@ -64,6 +69,11 @@ func setDNS(iface *net.Interface, nameservers []string) error {
Nameservers: ns,
SearchDomains: []dnsname.FQDN{},
}
defer func() {
if r.Mode() == "direct" {
go watchResolveConf(osConfig)
}
}()
trySystemdResolve := false
for i := 0; i < maxSetDNSAttempts; i++ {
@@ -299,3 +309,59 @@ func sliceIndex[S ~[]E, E comparable](s S, v E) int {
}
return -1
}
// watchResolveConf watches any changes to /etc/resolv.conf file,
// and reverting to the original config set by ctrld.
func watchResolveConf(oc dns.OSConfig) {
mainLog.Load().Debug().Msg("start watching /etc/resolv.conf file")
watcher, err := fsnotify.NewWatcher()
if err != nil {
mainLog.Load().Warn().Err(err).Msg("could not create watcher for /etc/resolv.conf")
return
}
// We watch /etc instead of /etc/resolv.conf directly,
// see: https://github.com/fsnotify/fsnotify#watching-a-file-doesnt-work-well
watchDir := filepath.Dir(resolvConfPath)
if err := watcher.Add(watchDir); err != nil {
mainLog.Load().Warn().Err(err).Msg("could not add /etc/resolv.conf to watcher list")
return
}
r, err := dns.NewOSConfigurator(func(format string, args ...any) {}, "lo") // interface name does not matter.
if err != nil {
mainLog.Load().Error().Err(err).Msg("failed to create DNS OS configurator")
return
}
for {
select {
case event, ok := <-watcher.Events:
if !ok {
return
}
if event.Name != resolvConfPath { // skip if not /etc/resolv.conf changes.
continue
}
if event.Has(fsnotify.Write) || event.Has(fsnotify.Create) {
mainLog.Load().Debug().Msg("/etc/resolv.conf changes detected, reverting to ctrld setting")
if err := watcher.Remove(watchDir); err != nil {
mainLog.Load().Error().Err(err).Msg("failed to pause watcher")
continue
}
if err := r.SetDNS(oc); err != nil {
mainLog.Load().Error().Err(err).Msg("failed to revert /etc/resolv.conf changes")
}
if err := watcher.Add(watchDir); err != nil {
mainLog.Load().Error().Err(err).Msg("failed to continue running watcher")
return
}
}
case err, ok := <-watcher.Errors:
if !ok {
return
}
mainLog.Load().Err(err).Msg("could not get event for /etc/resolv.conf")
}
}
}