Re: IPv6 UDP not working

[Juan Orti <jorti@pm.me>]

------- Original Message -------
El domingo, 28 de mayo de 2023 a las 12:12, Juan Orti <jorti@pm.me> escribió:

> I don't understand why it's not working, as the service listens on the :: address. Maybe it's using a socket option that it's causing this? I need to investigate this further.
> 
> # netstat -putan
> Active Internet connections (servers and established)
> Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
> tcp 0 0 :::3003 :::* LISTEN 2/AdGuardHome
> tcp 0 0 :::54 :::* LISTEN 2/AdGuardHome
> udp 0 0 :::54 :::* 2/AdGuardHome
> 
> # cat /proc/net/udp6
> sl local_address remote_address st tx_queue rx_queue tr tm->when retrnsmt uid timeout inode ref pointer drops
> 
> 4718: 00000000000000000000000000000000:0036 00000000000000000000000000000000:0000 07 00000000:00000000 00:00000000 00000000 0 0 38510801 2 0000000073476783 0


After stracing the AdGuardHome process, I can see that the UDP packet is indeed reaching the service but it's the reply that fails to be sent:

11    10:25:04.445902 recvmsg(25<UDPv6:[38993134]>,  <unfinished ...>
11    10:25:04.446238 <... recvmsg resumed>{msg_name={sa_family=AF_INET6, sin6_port=htons(33308), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "fddc:f797:78ef:10::b46", &sin6_addr), sin6_scope_id=0}, msg_namelen=112 => 28, msg_iov=[{iov_base="\246\245\1 \0\1\0\0\0\0\0\1\3www\6google\3com\0\0\1\0\1\0\0)\4\320\0\0\0\0\0\f\0\n\0\10\207a\315\224\245\253\v\37", iov_len=65535}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32}], msg_controllen=40, msg_flags=0}, 0) = 55 <0.000059>
11    10:25:04.446371 futex(0xc000064548, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
11    10:25:04.446415 <... futex resumed>) = 1 <0.000032>
11    10:25:04.446461 recvmsg(25<UDPv6:[38993134]>,  <unfinished ...>
11    10:25:04.446658 <... recvmsg resumed>{msg_namelen=112}, 0) = -1 EAGAIN (Resource temporarily unavailable) <0.000097>
11    10:25:04.447130 sendmsg(25<UDPv6:[38993134]>, {msg_name={sa_family=AF_INET6, sin6_port=htons(33308), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "fddc:f797:78ef:10::b46", &sin6_addr), sin6_scope_id=0}, msg_namelen=28, msg_iov=[{iov_base="\246\245\201\200\0\1\0\1\0\0\0\0\3www\6google\3com\0\0\1\0\1\300\f\0\1\0\1\0\0\0\n\0\4\330\357&x", iov_len=48}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32}], msg_controllen=40, msg_flags=0}, 0) = -1 EINVAL (Invalid argument) <0.000020>


It's not clear to me what's wrong with the sendmsg syscall. Any ideas?

Thanks.