DragonFlyBSD

I spent some time playing with LDNS example code and comparing the net.c with resolv/res_send.c in libc, and found that LDNS sends the query with sendto, while the libc resolver uses connect+send, unless RES_INSECURE1 is specified.

So with the code at the bottom of this comment, this succeeds
./r aaaa leaf.dragonflybsd.org.

while this fails
./r -1 aaaa leaf.dragonflybsd.org.

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <err.h>
#include <errno.h>
#include <resolv.h>
#include <stdio.h>
#include <unistd.h>

int main(int ac, char **av)
{
        u_char answer[1024];
        int ch;
        const char *me = av[0];

        if (res_init() != 0)
                err(-1, "res_init");
        while ((ch = getopt(ac, av, "12t")) != -1) {
                switch (ch) {
                case '1':
                        _res.options |= RES_INSECURE1;
                        break;
                case '2':
                        _res.options |= RES_INSECURE2;
                        break;
                case 't':
                        _res.options |= RES_USEVC;
                        break;
                default:
                        errx(-1, "unknown switch %c", ch);
                }
        }
        ac -= optind, av += optind;
        if (ac < 2)
                errx(-1, "usage: %s resource type domain...", me);

        int ok = 0;
        int query_type = res_nametotype(*av, &ok);
        if (!ok)
                errx(-1, "unknown query type: %s", *av);

        fp_resstat(&_res, stdout);
        while (++av, --ac > 0) {
                int l = res_query(*av, C_IN, query_type, answer, sizeof answer);
                if (l == -1) {
                        warnx("res_query: %s", *av);
                        continue;
                }
                res_pquery(&_res, answer, l, stdout);
        }
}

On DragonFlyBSD, the UDP packet sent from sendto has no flowlabel (0x00000) even if net.inet6.ip6.auto_flowlabel is left set (the default).

On DragonFlyBSD, tcpdump shows the empty flowlabel (the 3 octets following the first 0x60)

08:58:15.240037 IP6 (hlim 64, next-header UDP (17) payload length: 16) ::1.2082 > ::1.3456: [udp sum ok] udp/vt 8 69 / 21 [|vat]
        0x0000:  6000 0000 0010 1140 0000 0000 0000 0000  `......@........
        0x0010:  0000 0000 0000 0001 0000 0000 0000 0000  ................
        0x0020:  0000 0000 0000 0001 0822 0d80 0010 c00c  ........."......
        0x0030:  5445 5354 4041 4243                      TEST@ABC

while on another system (namely, WSL)

23:51:23.201149 IP6 (flowlabel 0x33ba2, hlim 64, next-header UDP (17) payload length: 16) ::1.40700 > ::1.3456: [bad udp cksum 0x0023 -> 0x2932!] udp/vt 8 69 / 21 [|vat]
        0x0000:  6003 3ba2 0010 1140 0000 0000 0000 0000  `.;....@........
        0x0010:  0000 0000 0000 0001 0000 0000 0000 0000  ................
        0x0020:  0000 0000 0000 0001 9efc 0d80 0010 0023  ...............#
        0x0030:  5445 5354 4041 4243                      TEST@ABC

sendto.c:

#include <sys/socket.h>
#include <sys/types.h>
#include <err.h>
#include <errno.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main(int ac, char **av)
{
        struct addrinfo hint, *ai = NULL;
        int s = -1;

        if (ac < 1 + 3)
                errx(1, "usage: %s host port msg", av[0]);
        const char *msg = av[3];
        size_t msglen = strlen(msg);
        memset(&hint, 0, sizeof hint);
        hint.ai_family = AF_UNSPEC;
        hint.ai_socktype = SOCK_DGRAM;
        hint.ai_flags = AI_PASSIVE;
        hint.ai_protocol = 0;
        hint.ai_canonname = NULL;
        hint.ai_addr = NULL;
        hint.ai_next = NULL;
        if (getaddrinfo(av[1], av[2], &hint, &ai) != 0)
                err(errno, "getaddrinfo");
        if ((s = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol)) == -1)
                err(errno, "socket");
        ssize_t sent = sendto(s, msg, msglen, 0, ai->ai_addr, ai->ai_addrlen);
        if (sent == -1)
                err(errno, "sendto");
        close(s);
        freeaddrinfo(ai);
        return 0;
}

make sendto CFLAGS='-W -Wall' && ./sendto ::1 3456 TEST@ABC

sudo tcpdump -Xnvs 1500 -i lo0 udp 3456

Hmm, forget about the flowlabel, the C code in the first comment works with an empty flowlabel by setting net.inet6.ip6.auto_flowlabel=0, so it's simply a matter of connect+send vs sendto (as I mentioned earlier). Not sure how it used to be OK, as apparently there hasn't been a dport-specific patch to convert sendto to use connect+send in dns/ldns or dns/dnsmasq.

sendto(which ends up calling udp_send or udp6_send) behaves differently in the point of view of the hash tables involved.

When it auto-binds the local port
- udp_send() calls in_pcbbind(nam = NULL), which calls in_pcbsetlport(), then udp_inswildcardhash(), which populates both localgroup hash and wildcard hash.
- udp6_send() only calls in6_pcbsetlport (in udp6_output)

so sendto() populates the localgroup hash, the port hash, and the wildcard hash for IPv4, whereas only the port hash for IPv6.

For receiving replies(which probably is not a multicast or a broadcast) on the socket used by sendto()
- udp_input() uses in_pcblookup_pkthash(wildcard = TRUE), which looks up on hashbase, calls inp_localgroup_lookup(), then looks up on wildcardhashbase
- udp6_input() uses in6_pcblookup_hash(wildcard = 1), which looks up on hashbase, then wildcardhashbase

so the possible fixes are:
- call in_pcbinswildcardhash() in udp6_output() for local port auto-bind, or
- create in_pcbinslocalgrphash() and call it in udp_output for auto-bind; call inp_localgroup_lookup() in in6_pcblookup_hash()

The change for the first fix seems to work for me.

diff --git a/sys/netinet6/udp6_output.c b/sys/netinet6/udp6_output.c
index 355735e7f9..d2627a7c96 100644
--- a/sys/netinet6/udp6_output.c
+++ b/sys/netinet6/udp6_output.c
@@ -186,9 +186,11 @@ udp6_output(struct in6pcb *in6p, struct mbuf *m, struct sockaddr *addr6,
                 error = EADDRNOTAVAIL;
             goto release;
         }
-        if (in6p->in6p_lport == 0 &&
-            (error = in6_pcbsetlport(laddr, in6p, td)) != 0)
-            goto release;
+        if (in6p->in6p_lport == 0) {
+            if ((error = in6_pcbsetlport(laddr, in6p, td)) != 0)
+                goto release;
+            in_pcbinswildcardhash(in6p);
+        }
     } else {
         if (IN6_IS_ADDR_UNSPECIFIED(&in6p->in6p_faddr)) {
             error = ENOTCONN;

Ok, reproduced the bug over here (I had to configure an IPV6 dns server in my /etc/resolv.conf to test it), and the patch fixes it. I think you can go ahead and commit it.

-Matt