diff --git a/tests/sys/net/pcp.py b/tests/sys/net/pcp.py
--- a/tests/sys/net/pcp.py
+++ b/tests/sys/net/pcp.py
@@ -61,11 +61,11 @@
 
 	args = parser.parse_args()
 
-	sniffer = Sniffer(args, check_pcp, recvif=args.recvif[0], timeout=20)
+	sniffer = Sniffer(args, check_pcp, args.recvif[0], timeout=20)
 
 	sniffer.join()
 
-	if sniffer.foundCorrectPacket:
+	if sniffer.correctPackets:
 		sys.exit(0)
 
 	sys.exit(1)
diff --git a/tests/sys/net/stp.py b/tests/sys/net/stp.py
--- a/tests/sys/net/stp.py
+++ b/tests/sys/net/stp.py
@@ -100,14 +100,14 @@
 
 	args = parser.parse_args()
 
-	sniffer = Sniffer(args, check_stp)
+	sniffer = Sniffer(args, check_stp, args.recvif[0])
 
 	invalid_stp(args.sendif[0])
 
 	sniffer.join()
 
 	# The 'correct' packet is a corrupt STP packet, so it shouldn't turn up.
-	if sniffer.foundCorrectPacket:
+	if sniffer.correctPackets:
 		sys.exit(1)
 
 if __name__ == '__main__':
diff --git a/tests/sys/netpfil/common/pft_icmp_check.py b/tests/sys/netpfil/common/pft_icmp_check.py
--- a/tests/sys/netpfil/common/pft_icmp_check.py
+++ b/tests/sys/netpfil/common/pft_icmp_check.py
@@ -96,14 +96,14 @@
 	args = parser.parse_args()
 	sniffer = None
 	if not args.recvif is None:
-		sniffer = Sniffer(args, check_icmp_too_big)
+		sniffer = Sniffer(args, check_icmp_too_big, args.recvif[0])
 
 	ping(args.sendif[0], args.to[0], args)
 
 	if sniffer:
 		sniffer.join()
 
-		if sniffer.foundCorrectPacket:
+		if sniffer.correctPackets:
 			sys.exit(0)
 		else:
 			sys.exit(1)
diff --git a/tests/sys/netpfil/common/pft_ping.py b/tests/sys/netpfil/common/pft_ping.py
--- a/tests/sys/netpfil/common/pft_ping.py
+++ b/tests/sys/netpfil/common/pft_ping.py
@@ -317,16 +317,16 @@
 		if args.tcpsyn:
 			checkfn=check_tcpsyn
 
-		sniffer = Sniffer(args, checkfn)
+		sniffer = Sniffer(args, checkfn, args.recvif[0])
 
 	replysniffer = None
 	if not args.replyif is None:
 		checkfn=check_ping_reply
-		replysniffer = Sniffer(args, checkfn, recvif=args.replyif[0])
+		replysniffer = Sniffer(args, checkfn, args.replyif[0])
 
 	dupsniffer = None
 	if args.checkdup is not None:
-		dupsniffer = Sniffer(args, check_dup, recvif=args.checkdup[0])
+		dupsniffer = Sniffer(args, check_dup, args.checkdup[0])
 
 	if args.tcpsyn:
 		tcpsyn(args.sendif[0], args.to[0], args)
@@ -344,7 +344,7 @@
 	if sniffer:
 		sniffer.join()
 
-		if sniffer.foundCorrectPacket:
+		if sniffer.correctPackets:
 			sys.exit(0)
 		else:
 			sys.exit(1)
@@ -352,7 +352,7 @@
 	if replysniffer:
 		replysniffer.join()
 
-		if replysniffer.foundCorrectPacket:
+		if replysniffer.correctPackets:
 			sys.exit(0)
 		else:
 			sys.exit(1)
diff --git a/tests/sys/netpfil/common/sniffer.py b/tests/sys/netpfil/common/sniffer.py
--- a/tests/sys/netpfil/common/sniffer.py
+++ b/tests/sys/netpfil/common/sniffer.py
@@ -31,18 +31,15 @@
 import sys
 
 class Sniffer(threading.Thread):
-	def __init__(self, args, check_function, recvif=None, timeout=3):
+	def __init__(self, args, check_function, recvif, timeout=3):
 		threading.Thread.__init__(self)
 
 		self._sem = threading.Semaphore(0)
 		self._args = args
 		self._timeout = timeout
-		if recvif is not None:
-			self._recvif = recvif
-		else:
-			self._recvif = args.recvif[0]
+		self._recvif = recvif
 		self._check_function = check_function
-		self.foundCorrectPacket = False
+		self.correctPackets = 0
 
 		self.start()
 		if not self._sem.acquire(timeout=30):
@@ -51,7 +48,7 @@
 	def _checkPacket(self, packet):
 		ret = self._check_function(self._args, packet)
 		if ret:
-			self.foundCorrectPacket = True
+			self.correctPackets += 1
 		return ret
 
 	def _startedCb(self):
@@ -59,9 +56,6 @@
 
 	def run(self):
 		self.packets = []
-		try:
-			self.packets = sp.sniff(iface=self._recvif,
-					stop_filter=self._checkPacket, timeout=self._timeout,
-					started_callback=self._startedCb)
-		except Exception as e:
-			print(e, file=sys.stderr)
+		self.packets = sp.sniff(iface=self._recvif,
+			stop_filter=self._checkPacket, timeout=self._timeout,
+			started_callback=self._startedCb)
diff --git a/tests/sys/netpfil/pf/CVE-2019-5598.py b/tests/sys/netpfil/pf/CVE-2019-5598.py
--- a/tests/sys/netpfil/pf/CVE-2019-5598.py
+++ b/tests/sys/netpfil/pf/CVE-2019-5598.py
@@ -72,7 +72,7 @@
 	sp.sendp(udp, iface=args.sendif[0], verbose=False)
 
 	# Start sniffing on recvif
-	sniffer = Sniffer(args, check_icmp_error)
+	sniffer = Sniffer(args, check_icmp_error, args.recvif[0])
 
 	# Send the bad error packet
 	icmp_reachable = sp.Ether() / \
@@ -83,7 +83,7 @@
 	sp.sendp(icmp_reachable, iface=args.sendif[0], verbose=False)
 
 	sniffer.join()
-	if sniffer.foundCorrectPacket:
+	if sniffer.correctPackets:
 		sys.exit(1)
 
 	sys.exit(0)