Why did snmpd show "Failure in sendto" when requested with a broadcast address?

Issue

• When we requested SNMP with a broadcast source IP address '10.x.x.255', nothing is returned and following error message was shown in /var/log/messages at the snmpd server:

snmpd[xxxx]: send response: Failure in sendto

In the strace of snmpd service, sendmsg returned ENETUNREACH before the message was output.

887241 11:20:29.860297 sendmsg(11, {msg_name={sa_family=AF_INET, sin_port=htons(36750), sin_addr=inet_addr("10.x.x.yyy")}, msg_namelen=16, msg_iov=[{iov_base="...", iov_len=53}], msg_iovlen=1, msg_control=[{cmsg_len=28, cmsg_level=SOL_IP, cmsg_type=IP_PKTINFO, cmsg_data={ipi_ifindex=0, ipi_spec_dst=inet_addr("10.x.x.255"), ipi_addr=inet_addr("0.0.0.0")}}], msg_controllen=32, msg_flags=0}, MSG_DONTWAIT|MSG_NOSIGNAL) = -1 ENETUNREACH (Network is unreachable) <0.000011>
887241 11:20:29.860389 getpid()         = 887241 <0.000005>
887241 11:20:29.860410 sendto(8, "<27>Mar 15 11:20:29 snmpd[887241]: send response: Failure in sendto\n", 68, MSG_NOSIGNAL, NULL, 0) = 68 <0.002007>
887241 11:20:29.862764 getpid()         = 887241 <0.000006>
887241 11:20:29.862798 sendto(8, "<27>Mar 15 11:20:29 snmpd[887241]:     -- IP-MIB::ipAdEntAddr.10.x.x.yyy\n", 74, MSG_NOSIGNAL, NULL, 0) = 74 <0.001213>

According to the source code of net-snmp, sendmsg is supposed to return EINVAL when it requested with a broadcast address. We think that sendmsg returning ENETUNREACH instead of EINVAL was the reason why the error message was shown.

net-snmp-5.8-14.el8.x86_64/snmplib/transports/snmpUDPBaseDomain.c

         rc = sendmsg(fd, &m, MSG_NOSIGNAL|MSG_DONTWAIT);
        if (rc >= 0 || errno != EINVAL)
            return rc;

        /*
         * The error might be caused by broadcast srcip (i.e. we're responding
         * to a broadcast request) - sendmsg does not like it. Try to resend it
         * using the interface on which it was received
         */

        ...

        rc = sendmsg(fd, &m, MSG_NOSIGNAL|MSG_DONTWAIT);

Could you please tell us why sendmsg returned ENETUNREACH instead of EINVAL and the error message was shown?