From 2ca483cd4b3da0a7b2c775db69f812f6a1a6fa1c Mon Sep 17 00:00:00 2001
From: bert hubert <bert.hubert@powerdns.com>
Date: Sun, 24 Feb 2019 11:18:11 +0100
Subject: [PATCH] ip.7: IP_RECVTTL error fixed

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.

Note from mtk:

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);
}
]]

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
---
 man7/ip.7 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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.