chore(firewall): split apart iptables specific code in internal/firewall/iptables

internal/firewall/iptables/tcp.go

@@ -0,0 +1,98 @@
+package iptables
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/netip"
+	"os"
+)
+
+type tcpFlags struct {
+	mask       []tcpFlag
+	comparison []tcpFlag
+}
+
+type tcpFlag uint8
+
+const (
+	tcpFlagFIN tcpFlag = 1 << iota
+	tcpFlagSYN
+	tcpFlagRST
+	tcpFlagPSH
+	tcpFlagACK
+	tcpFlagURG
+	tcpFlagECE
+	tcpFlagCWR
+)
+
+func (f tcpFlag) String() string {
+	switch f {
+	case tcpFlagFIN:
+		return "FIN"
+	case tcpFlagSYN:
+		return "SYN"
+	case tcpFlagRST:
+		return "RST"
+	case tcpFlagPSH:
+		return "PSH"
+	case tcpFlagACK:
+		return "ACK"
+	case tcpFlagURG:
+		return "URG"
+	case tcpFlagECE:
+		return "ECE"
+	case tcpFlagCWR:
+		return "CWR"
+	default:
+		panic(fmt.Sprintf("%s: %d", errTCPFlagUnknown, f))
+	}
+}
+
+var errTCPFlagUnknown = errors.New("unknown TCP flag")
+
+func parseTCPFlag(s string) (tcpFlag, error) {
+	allFlags := []tcpFlag{
+		tcpFlagFIN, tcpFlagSYN, tcpFlagRST, tcpFlagPSH,
+		tcpFlagACK, tcpFlagURG, tcpFlagECE, tcpFlagCWR,
+	}
+	for _, flag := range allFlags {
+		if s == fmt.Sprintf("%#02x", uint8(flag)) || s == flag.String() {
+			return flag, nil
+		}
+	}
+	return 0, fmt.Errorf("%w: %s", errTCPFlagUnknown, s)
+}
+
+var ErrMarkMatchModuleMissing = errors.New("kernel is missing the mark module libxt_mark.so")
+
+// TempDropOutputTCPRST temporarily drops outgoing TCP RST packets to the specified address and port,
+// for any TCP packets not marked with the excludeMark given.
+// This is necessary for TCP path MTU discovery to work, as the kernel will try to terminate the connection
+// by sending a TCP RST packet, although we want to handle the connection manually.
+func (c *Config) TempDropOutputTCPRST(ctx context.Context,
+	src, dst netip.AddrPort, excludeMark int) (
+	revert func(ctx context.Context) error, err error,
+) {
+	_, err = os.Stat("/usr/lib/xtables/libxt_mark.so")
+	if err != nil && errors.Is(err, os.ErrNotExist) {
+		return nil, fmt.Errorf("%w", ErrMarkMatchModuleMissing)
+	}
+
+	const template = "%s OUTPUT -p tcp -s %s --sport %d -d %s --dport %d " +
+		"--tcp-flags RST RST -m mark ! --mark %d -j DROP" //nolint:dupword
+	instruction := fmt.Sprintf(template, "--append", src.Addr(), src.Port(), dst.Addr(), dst.Port(), excludeMark)
+	revertInstruction := fmt.Sprintf(template, "--delete", src.Addr(), src.Port(), dst.Addr(), dst.Port(), excludeMark)
+	run := c.runIptablesInstruction
+	if dst.Addr().Is6() {
+		run = c.runIP6tablesInstruction
+	}
+	revert = func(ctx context.Context) error {
+		return run(ctx, revertInstruction)
+	}
+	err = run(ctx, instruction)
+	if err != nil {
+		return nil, fmt.Errorf("running instruction: %w", err)
+	}
+	return revert, nil
+}