Review nits

firewall.go

@@ -23,9 +23,14 @@ import (
 	"github.com/slackhq/nebula/firewall"
 )
 
-var ErrCannotSNAT = errors.New("cannot snat this packet")
+var ErrCannotSNAT = errors.New("cannot SNAT this packet")
 var ErrSNATIdentityMismatch = errors.New("refusing to SNAT for mismatched host")
 
+const ipv4SourcePosition = 12
+const ipv4DestinationPosition = 16
+const sourcePortOffset = 0
+const destinationPortOffset = 2
+
 type FirewallInterface interface {
 	AddRule(incoming bool, proto uint8, startPort int32, endPort int32, groups []string, host string, cidr, localCidr string, caName string, caSha string) error
 }
@@ -459,7 +464,7 @@ func (f *Firewall) unSnat(data []byte, fp *firewall.Packet) netip.Addr {
 		return netip.Addr{}
 	}
 	oldIP := netip.AddrPortFrom(f.snatAddr, fp.RemotePort)
-	rewritePacket(data, fp, oldIP, c.snat.Src, 16, 2)
+	rewritePacket(data, fp, oldIP, c.snat.Src, ipv4DestinationPosition, destinationPortOffset)
 	return c.snat.SrcVpnIp
 }
 
@@ -496,6 +501,7 @@ func (f *Firewall) findUsableSNATPort(fp *firewall.Packet, c *conn) error {
 		if !ok {
 			//yay, we can use this port
 			//track the snatted flow with the same expiration as the unsnatted version
+			c.snat.SnatPort = fp.RemotePort
 			conntrack.Conns[*fp] = c
 			return nil
 		}
@@ -538,13 +544,12 @@ func (f *Firewall) applySnat(data []byte, fp *firewall.Packet, c *conn, hostinfo
 			c.snat = nil
 			return err
 		}
-		c.snat.SnatPort = fp.RemotePort //may have been updated inside f.findUsableSNATPort
 	} else {
 		return ErrCannotSNAT
 	}
 
 	newIP := netip.AddrPortFrom(f.snatAddr, c.snat.SnatPort)
-	rewritePacket(data, fp, c.snat.Src, newIP, 12, 0)
+	rewritePacket(data, fp, c.snat.Src, newIP, ipv4SourcePosition, sourcePortOffset)
 	return nil
 }
 
@@ -695,18 +700,9 @@ func (f *Firewall) EmitStats() {
 }
 
 func (f *Firewall) peek(fp firewall.Packet) *conn {
-	conntrack := f.Conntrack
-	conntrack.Lock()
-
-	// Purge every time we test
-	ep, has := conntrack.TimerWheel.Purge()
-	if has {
-		f.evict(ep)
-	}
-
-	c := conntrack.Conns[fp]
-
-	conntrack.Unlock()
+	f.Conntrack.Lock()
+	c := f.Conntrack.Conns[fp]
+	f.Conntrack.Unlock()
 	return c
 }