chore: allow DNS queries from Uncloud containers to the embedded DNS server

internal/machine/firewall/iptables_linux.go

@@ -0,0 +1,74 @@
+package firewall
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/docker/docker/libnetwork/iptables"
+	"github.com/psviderski/uncloud/internal/machine/network"
+)
+
+const (
+	DockerUserChain   = "DOCKER-USER"
+	UncloudInputChain = "UNCLOUD-INPUT"
+)
+
+// ConfigureIptablesChains sets up custom iptables chains and initial firewall rules for Uncloud networking.
+func ConfigureIptablesChains() error {
+	// Ensure iptables UNCLOUD-INPUT chain with a RETURN rule exists. All existing rules are flushed.
+	ipt := iptables.GetIptable(iptables.IPv4)
+	if _, err := ipt.NewChain(UncloudInputChain, iptables.Filter); err != nil {
+		return fmt.Errorf("create iptables chain '%s': %w", UncloudInputChain, err)
+	}
+	if err := ipt.RawCombinedOutput("-t", string(iptables.Filter), "-F", UncloudInputChain); err != nil {
+		return fmt.Errorf("flush iptables chain '%s': %w", UncloudInputChain, err)
+	}
+	if err := ipt.AddReturnRule(UncloudInputChain); err != nil {
+		return fmt.Errorf("add the RETURN rule for iptables chain '%s': %w", UncloudInputChain, err)
+	}
+
+	// Ensure the main iptables INPUT chain has a jump rule to the UNCLOUD-INPUT chain before any DROP/REJECT rules.
+	jumpRule := []string{"-m", "comment", "--comment", "Uncloud-managed", "-j", UncloudInputChain}
+	if !ipt.Exists(iptables.Filter, "INPUT", jumpRule...) {
+		// Look for the first DROP/REJECT rule in the INPUT chain.
+		out, err := ipt.Raw("-t", string(iptables.Filter), "-L", "INPUT", "--line-numbers")
+		if err != nil {
+			return fmt.Errorf("get iptables rules for chain '%s': %w", UncloudInputChain, err)
+		}
+
+		firstRejectRuleNum := 0
+		for _, line := range strings.Split(string(out), "\n") {
+			fields := strings.Fields(line)
+			if len(fields) < 2 {
+				continue
+			}
+			if fields[1] == "DROP" || fields[1] == "REJECT" {
+				if ruleNum, err := strconv.Atoi(fields[0]); err == nil {
+					firstRejectRuleNum = ruleNum
+					break
+				}
+			}
+		}
+
+		var addJumpRule []string
+		if firstRejectRuleNum > 0 {
+			addJumpRule = append([]string{"-t", string(iptables.Filter), "-I", "INPUT", strconv.Itoa(firstRejectRuleNum)},
+				jumpRule...)
+		} else {
+			addJumpRule = append([]string{"-t", string(iptables.Filter), "-A", "INPUT"}, jumpRule...)
+		}
+		if err = ipt.RawCombinedOutput(addJumpRule...); err != nil {
+			return fmt.Errorf("add iptables rule '%s': %w", strings.Join(addJumpRule, " "), err)
+		}
+	}
+
+	// Allow WireGuard traffic to the machine.
+	acceptWireGuardRule := []string{"-p", "udp", "--dport", strconv.Itoa(network.WireGuardPort), "-j", "ACCEPT"}
+	err := ipt.ProgramRule(iptables.Filter, UncloudInputChain, iptables.Insert, acceptWireGuardRule)
+	if err != nil {
+		return fmt.Errorf("insert iptables rule '%s': %w", strings.Join(acceptWireGuardRule, " "), err)
+	}
+
+	return nil
+}