Add IP_SENDSRCADDR cmsg for UDP sockets. As suggested by sthen@,

IP_SENDSRCADDR == IP_RECVDSTADDR.

OK sthen@ jca@ bluhm@

1 files changed, 30 insertions, 2 deletions

diff --git a/share/man/man4/ip.4 b/share/man/man4/ip.4
index b8daeb47ed8..85f3728064e 100644
--- a/share/man/man4/ip.4
+++ b/share/man/man4/ip.4
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ip.4,v 1.39 2016/06/28 17:32:58 jca Exp $
+.\"	$OpenBSD: ip.4,v 1.40 2016/08/16 22:21:17 vgross Exp $
 .\"	$NetBSD: ip.4,v 1.3 1994/11/30 16:22:19 jtc Exp $
 .\"
 .\" Copyright (c) 1983, 1991, 1993
@@ -30,7 +30,7 @@
 .\"
 .\"     @(#)ip.4	8.2 (Berkeley) 11/30/93
 .\"
-.Dd $Mdocdate: June 28 2016 $
+.Dd $Mdocdate: August 16 2016 $
 .Dt IP 4
 .Os
 .Sh NAME
@@ -289,6 +289,34 @@ cmsg_len = CMSG_LEN(sizeof(u_int))
 cmsg_level = IPPROTO_IP
 cmsg_type = IP_RECVRTABLE
 .Ed
+.Pp
+When sending on a
+.Dv SOCK_DGRAM
+socket with
+.Xr sendmsg 2
+, the source address to be used can be passed as ancillary data with a type code of
+.Dv IP_SENDSRCADDR .
+The
+.Va msg_control
+field in the
+.Vt msghdr
+structure should point to a buffer that contains a
+.Vt cmsghdr
+structure followed by the requested source address.
+The
+.Vt cmsghdr
+fields should have the following values:
+.Bd -literal -offset indent
+cmsg_len = CMSG_LEN(sizeof(struct in_addr))
+cmsg_level = IPPROTO_IP
+cmsg_type = IP_SENDSRCADDR
+.Ed
+.Pp
+The same checks and restrictions as for
+.Xr bind 2
+apply, unless the socket is bound to
+.Dv INADDR_ANY .
+In this case, there is no source address overlap check.
 .Ss "Multicast Options"
 .Tn IP
 multicasting is supported only on