Hi,
When running tcpdump on loopback, I see a currupted packet for each
valid packet.
root@v74:.../~# tcpdump -ni lo0 -X -vvv
tcpdump: listening on lo0, link-type LOOP
21:01:07.645880
0000: e161 0000 ff01 0000 7f00 0001 7f00 0001 .a..............
0010: 0800 00df 3a4f 0000 d257 38fa 0236 0129 ....:O...W8..6.)
0020: b9ec 7ecc 329a e881 4e36 e269 9fe4 1744 ..~.2...N6.i...D
0030: 1819 1a1b 1c1d 1e1f 2021 2223 2425 2627 ........ !"#$%&'
0040: 2829 2a2b 2c2d 2e2f 3031 3233 3435 3637 ()*+,-./01234567
21:01:07.645886 127.0.0.1 > 127.0.0.1: icmp: echo request (id:3a4f seq:0) [icmp
cksum ok] (ttl 255, id 57697, len 84, bad ip cksum 0! -> dc44)
0000: 4500 0054 e161 0000 ff01 0000 7f00 0001 E..T.a..........
0010: 7f00 0001 0800 00df 3a4f 0000 d257 38fa ........:O...W8.
0020: 0236 0129 b9ec 7ecc 329a e881 4e36 e269 .6.)..~.2...N6.i
0030: 9fe4 1744 1819 1a1b 1c1d 1e1f 2021 2223 ...D........ !"#
0040: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
0050: 3435 3637 4567
lo(4) used to dump to bpf only for output. It seems that when
if_bpf_mtap() was introduced, this changed and lo(4) dumps an
additional truncated packet. The default bpf_mtap_ether() is not
suitable for lo(4).
Solution is to install a dummy lo_bpf_mtap() to suppress bpf on
input.
ok?
bluhm
Index: net/if_loop.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/net/if_loop.c,v
retrieving revision 1.96
diff -u -p -r1.96 if_loop.c
--- net/if_loop.c 18 Jul 2023 16:01:20 -0000 1.96
+++ net/if_loop.c 20 Jul 2023 19:06:39 -0000
@@ -146,6 +146,7 @@ void lortrequest(struct ifnet *, int, st
void loinput(struct ifnet *, struct mbuf *);
int looutput(struct ifnet *,
struct mbuf *, struct sockaddr *, struct rtentry *);
+int lo_bpf_mtap(caddr_t, const struct mbuf *, u_int);
int loop_clone_create(struct if_clone *, int);
int loop_clone_destroy(struct ifnet *);
@@ -177,6 +178,7 @@ loop_clone_create(struct if_clone *ifc,
IFCAP_CSUM_TCPv4 | IFCAP_CSUM_UDPv4 |
IFCAP_CSUM_TCPv6 | IFCAP_CSUM_UDPv6 |
IFCAP_LRO | IFCAP_TSOv4 | IFCAP_TSOv6;
+ ifp->if_bpf_mtap = lo_bpf_mtap;
ifp->if_rtrequest = lortrequest;
ifp->if_ioctl = loioctl;
ifp->if_input = loinput;
@@ -228,6 +230,13 @@ loop_clone_destroy(struct ifnet *ifp)
if (rdomain)
rtable_l2set(rdomain, 0, 0);
+ return (0);
+}
+
+int
+lo_bpf_mtap(caddr_t if_bpf, const struct mbuf *m, u_int dir)
+{
+ /* loopback dumps on output, disable input bpf */
return (0);
}
