Refactor - now wireguard based. (#7)

peer/hosts.go

@@ -0,0 +1,128 @@
+package peer
+
+import (
+	"fmt"
+	"log"
+	"net/netip"
+	"os"
+	"sort"
+	"strings"
+	"syscall"
+
+	"git.crumpington.com/lib/go/flock"
+)
+
+const (
+	hostsFile  = "/etc/hosts"
+	hostsBegin = "# BEGIN vppn"
+	hostsEnd   = "# END vppn"
+)
+
+// hostMarkers returns the begin/end marker lines that delimit the managed
+// section for localDomain. The domain is wrapped in parentheses so one domain's
+// marker can never be a prefix of another's (e.g. "net" vs "net2") when
+// multiple vppn instances share /etc/hosts.
+func hostMarkers(localDomain string) (begin, end string) {
+	return hostsBegin + "(" + localDomain + ")", hostsEnd + "(" + localDomain + ")"
+}
+
+// updateHosts rewrites the managed vppn section in /etc/hosts using the
+// current peersByIP map. Peers without a Name are skipped.
+func (a *App) updateHosts() {
+	if a.localDomain == "" {
+		return
+	}
+	if err := updateHosts(hostsFile, a.localDomain, a.peersByIP); err != nil {
+		log.Printf("Failed to update hosts file: %v", err)
+	}
+}
+
+func updateHosts(hostsPath, localDomain string, peers map[netip.Addr]*Peer) error {
+	lockFile, err := flock.Lock(hostsPath + ".vppn.lock")
+	if err != nil {
+		return err
+	}
+	defer lockFile.Close()
+
+	begin, end := hostMarkers(localDomain)
+
+	info, err := os.Stat(hostsPath)
+	if err != nil {
+		return err
+	}
+
+	raw, err := os.ReadFile(hostsPath)
+	if err != nil {
+		return err
+	}
+
+	data := string(raw)
+
+	before := strings.TrimSpace(data)
+	after := ""
+
+	if idxBegin := strings.Index(data, begin); idxBegin != -1 {
+		idxEnd := strings.Index(data[idxBegin:], end)
+		if idxEnd != -1 {
+			after = strings.TrimSpace(data[idxBegin+idxEnd+len(end):])
+		}
+		before = strings.TrimSpace(data[:idxBegin])
+	}
+
+	b := strings.Builder{}
+	b.WriteString(before)
+	b.WriteRune('\n')
+	b.WriteString(after)
+	b.WriteRune('\n')
+	b.WriteRune('\n')
+
+	b.WriteString(begin)
+	b.WriteRune('\n')
+
+	// Collect entries so we can sort by IP for stable output. Pad the IP
+	// column to the width of the widest possible address ("255.255.255.255")
+	// for readability.
+	type entry struct {
+		ip   netip.Addr
+		host string
+	}
+	var entries []entry
+	for ip, p := range peers {
+		if p.Name == "" {
+			continue
+		}
+		entries = append(entries, entry{ip: ip, host: p.Name + "." + localDomain})
+	}
+	sort.Slice(entries, func(i, j int) bool {
+		return entries[i].ip.Less(entries[j].ip)
+	})
+
+	for _, e := range entries {
+		b.WriteString(fmt.Sprintf("%-15s %s\n", e.ip.String(), e.host))
+	}
+
+	b.WriteString(end)
+	b.WriteRune('\n')
+
+	// Write to a temp file in the same directory, then rename over the
+	// original so readers never observe a partial file. Preserve the
+	// original's mode and ownership, since rename replaces the inode.
+	tmpPath := hostsPath + ".vppn.tmp"
+	if err := os.WriteFile(tmpPath, []byte(b.String()), info.Mode().Perm()); err != nil {
+		return err
+	}
+
+	if st, ok := info.Sys().(*syscall.Stat_t); ok {
+		if err := os.Chown(tmpPath, int(st.Uid), int(st.Gid)); err != nil {
+			os.Remove(tmpPath)
+			return err
+		}
+	}
+
+	if err := os.Rename(tmpPath, hostsPath); err != nil {
+		os.Remove(tmpPath)
+		return err
+	}
+
+	return nil
+}