From fdc836786e82a355da43a67a64b447a9dced2df7 Mon Sep 17 00:00:00 2001
From: vgross <vgross@openbsd.org>
Date: Tue, 16 Aug 2016 22:21:17 +0000
Subject: [PATCH] Add IP_SENDSRCADDR cmsg for UDP sockets. As suggested by
 sthen@, IP_SENDSRCADDR == IP_RECVDSTADDR.

OK sthen@ jca@ bluhm@
---
 share/man/man4/ip.4      | 32 ++++++++++++++++++++++++++++++--
 sys/netinet/in.h         |  4 +++-
 sys/netinet/udp_usrreq.c | 31 ++++++++++++++++++++++++++++---
 3 files changed, 61 insertions(+), 6 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
diff --git a/sys/netinet/in.h b/sys/netinet/in.h
index 31b2a26b453..a390574da3a 100644
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: in.h,v 1.117 2016/06/28 17:18:24 chris Exp $	*/
+/*	$OpenBSD: in.h,v 1.118 2016/08/16 22:21:17 vgross Exp $	*/
 /*	$NetBSD: in.h,v 1.20 1996/02/13 23:41:47 christos Exp $	*/
 
 /*
@@ -307,6 +307,8 @@ struct ip_opts {
 #define IP_RECVRTABLE		35   /* bool; receive rdomain w/dgram */
 #define IP_IPSECFLOWINFO	36   /* bool; IPsec flow info for dgram */
 #define IP_IPDEFTTL		37   /* int; IP TTL system default */
+#define IP_SENDSRCADDR		IP_RECVDSTADDR  /* struct in_addr; */
+						/* source address to use */
 
 #define IP_RTABLE		0x1021	/* int; routing table, see SO_RTABLE */
 #define IP_DIVERTFL		0x1022	/* int; divert direction flag opt */
diff --git a/sys/netinet/udp_usrreq.c b/sys/netinet/udp_usrreq.c
index 6e2589ea607..01b6c5b64ef 100644
--- a/sys/netinet/udp_usrreq.c
+++ b/sys/netinet/udp_usrreq.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: udp_usrreq.c,v 1.217 2016/08/04 20:46:24 vgross Exp $	*/
+/*	$OpenBSD: udp_usrreq.c,v 1.218 2016/08/16 22:21:17 vgross Exp $	*/
 /*	$NetBSD: udp_usrreq.c,v 1.28 1996/03/16 23:54:03 christos Exp $	*/
 
 /*
@@ -909,6 +909,7 @@ udp_output(struct inpcb *inp, struct mbuf *m, struct mbuf *addr,
 	struct sockaddr_in *sin = NULL;
 	struct udpiphdr *ui;
 	u_int32_t ipsecflowinfo = 0;
+	struct sockaddr_in src_sin;
 	int len = m->m_pkthdr.len;
 	struct in_addr *laddr;
 	int error = 0;
@@ -927,6 +928,8 @@ udp_output(struct inpcb *inp, struct mbuf *m, struct mbuf *addr,
 		goto release;
 	}
 
+	memset(&src_sin, 0, sizeof(src_sin));
+
 	if (control) {
 		u_int clen;
 		struct cmsghdr *cm;
@@ -960,9 +963,20 @@ udp_output(struct inpcb *inp, struct mbuf *m, struct mbuf *addr,
 			    cm->cmsg_level == IPPROTO_IP &&
 			    cm->cmsg_type == IP_IPSECFLOWINFO) {
 				ipsecflowinfo = *(u_int32_t *)CMSG_DATA(cm);
-				break;
-			}
+			} else
 #endif
+			if (cm->cmsg_len == CMSG_LEN(sizeof(struct in_addr)) &&
+			    cm->cmsg_level == IPPROTO_IP &&
+			    cm->cmsg_type == IP_SENDSRCADDR) {
+				memcpy(&src_sin.sin_addr, CMSG_DATA(cm),
+				    sizeof(struct in_addr));
+				src_sin.sin_family = AF_INET;
+				src_sin.sin_len = sizeof(src_sin);
+				/* no check on reuse when sin->sin_port == 0 */
+				if ((error = in_pcbaddrisavail(inp, &src_sin,
+				    0, curproc)))
+					goto release;
+			}
 			clen -= CMSG_ALIGN(cm->cmsg_len);
 			cmsgs += CMSG_ALIGN(cm->cmsg_len);
 		} while (clen);
@@ -1000,6 +1014,17 @@ udp_output(struct inpcb *inp, struct mbuf *m, struct mbuf *addr,
 			if (error)
 				goto release;
 		}
+
+		if (src_sin.sin_len > 0 &&
+		    src_sin.sin_addr.s_addr != INADDR_ANY &&
+		    src_sin.sin_addr.s_addr != inp->inp_laddr.s_addr) {
+			src_sin.sin_port = inp->inp_lport;
+			if (inp->inp_laddr.s_addr != INADDR_ANY &&
+			    (error =
+			    in_pcbaddrisavail(inp, &src_sin, 0, curproc)))
+				goto release;
+			laddr = &src_sin.sin_addr;
+		}
 	} else {
 		if (inp->inp_faddr.s_addr == INADDR_ANY) {
 			error = ENOTCONN;
-- 
2.20.1