Hello Bert,
On 2/24/19 11:18 AM, bert hubert wrote:
> Hi everyone,
>
> I need to get the TTL of UDP datagrams from userspace, so I set the
> IP_RECVTTL socket option. And as promised by ip.7, I then get IP_TTL
> messages from recvfrom. However, unlike what the manpage promises, the TTL
> field gets passed as a 32 bit integer.
>
> The following userspace code works:
>
> uint32_t ttl32;
> for (cmsg = CMSG_FIRSTHDR(msgh); cmsg != NULL; cmsg = CMSG_NXTHDR(msgh,cmsg)) {
> if ((cmsg->cmsg_level == IPPROTO_IP) && (cmsg->cmsg_type == IP_TTL) &&
> CMSG_LEN(sizeof(ttl32)) == cmsg->cmsg_len) {
>
> memcpy(&ttl32, CMSG_DATA(cmsg), sizeof(ttl32));
> *ttl=ttl32;
> return true;
> }
> else
> cerr<<"Saw something else "<<(cmsg->cmsg_type == IP_TTL) <<", "<<(int)cmsg->cmsg_level<<", "<<cmsg->cmsg_len<<", "<<CMSG_LEN(1)<<endl;
> }
>
> The 'else' field was used to figure out I go the length wrong.
Reading the source code also seems to confirm this, from
net/ipv4/ip_sockglue.c:
[[
static void ip_cmsg_recv_ttl(struct msghdr *msg, struct sk_buff *skb)
{
int ttl = ip_hdr(skb)->ttl;
put_cmsg(msg, SOL_IP, IP_TTL, sizeof(int), &ttl);
}
]]
>
> Patch:
>
> diff --git a/man7/ip.7 b/man7/ip.7
> index 02dd47f2a..d664eddb1 100644
> --- a/man7/ip.7
> +++ b/man7/ip.7
> @@ -874,7 +874,7 @@ Expects a boolean integer flag.
> When this flag is set, pass a
> .B IP_TTL
> control message with the time-to-live
> -field of the received packet as a byte.
> +field of the received packet as a 32 bit integer.
> Not supported for
> .B SOCK_STREAM
> sockets.
Thanks,
Patch applied.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
