[client] Add stateful userspace firewall and remove egress filters (#3093)

- Add stateful firewall functionality for UDP/TCP/ICMP in userspace firewalll
- Removes all egress drop rules/filters, still needs refactoring so we don't add output rules to any chains/filters.
- on Linux, if the OUTPUT policy is DROP  then we don't do anything about it (no extra allow rules). This is up to the user, if they don't want anything leaving their machine they'll have to manage these rules explicitly.

client/firewall/nftables/acl_linux.go

@@ -5,7 +5,6 @@ import (
 	"encoding/binary"
 	"fmt"
 	"net"
-	"net/netip"
 	"strconv"
 	"strings"
 	"time"
@@ -28,7 +27,6 @@ const (
 
 	// filter chains contains the rules that jump to the rules chains
 	chainNameInputFilter   = "netbird-acl-input-filter"
-	chainNameOutputFilter  = "netbird-acl-output-filter"
 	chainNameForwardFilter = "netbird-acl-forward-filter"
 	chainNamePrerouting    = "netbird-rt-prerouting"
 
@@ -441,18 +439,6 @@ func (m *AclManager) createDefaultChains() (err error) {
 		return err
 	}
 
-	// netbird-acl-output-filter
-	// type filter hook output priority filter; policy accept;
-	chain = m.createFilterChainWithHook(chainNameOutputFilter, nftables.ChainHookOutput)
-	m.addFwdAllow(chain, expr.MetaKeyOIFNAME)
-	m.addJumpRule(chain, m.chainOutputRules.Name, expr.MetaKeyOIFNAME) // to netbird-acl-output-rules
-	m.addDropExpressions(chain, expr.MetaKeyOIFNAME)
-	err = m.rConn.Flush()
-	if err != nil {
-		log.Debugf("failed to create chain (%s): %s", chainNameOutputFilter, err)
-		return err
-	}
-
 	// netbird-acl-forward-filter
 	chainFwFilter := m.createFilterChainWithHook(chainNameForwardFilter, nftables.ChainHookForward)
 	m.addJumpRulesToRtForward(chainFwFilter) // to netbird-rt-fwd
@@ -619,45 +605,6 @@ func (m *AclManager) addDropExpressions(chain *nftables.Chain, ifaceKey expr.Met
 	return nil
 }
 
-func (m *AclManager) addFwdAllow(chain *nftables.Chain, iifname expr.MetaKey) {
-	ip, _ := netip.AddrFromSlice(m.wgIface.Address().Network.IP.To4())
-	dstOp := expr.CmpOpNeq
-	expressions := []expr.Any{
-		&expr.Meta{Key: iifname, Register: 1},
-		&expr.Cmp{
-			Op:       expr.CmpOpEq,
-			Register: 1,
-			Data:     ifname(m.wgIface.Name()),
-		},
-		&expr.Payload{
-			DestRegister: 2,
-			Base:         expr.PayloadBaseNetworkHeader,
-			Offset:       16,
-			Len:          4,
-		},
-		&expr.Bitwise{
-			SourceRegister: 2,
-			DestRegister:   2,
-			Len:            4,
-			Xor:            []byte{0x0, 0x0, 0x0, 0x0},
-			Mask:           m.wgIface.Address().Network.Mask,
-		},
-		&expr.Cmp{
-			Op:       dstOp,
-			Register: 2,
-			Data:     ip.Unmap().AsSlice(),
-		},
-		&expr.Verdict{
-			Kind: expr.VerdictAccept,
-		},
-	}
-	_ = m.rConn.AddRule(&nftables.Rule{
-		Table: chain.Table,
-		Chain: chain,
-		Exprs: expressions,
-	})
-}
-
 func (m *AclManager) addJumpRule(chain *nftables.Chain, to string, ifaceKey expr.MetaKey) {
 	expressions := []expr.Any{
 		&expr.Meta{Key: ifaceKey, Register: 1},