util: Allow sock_l4() to open dual stack sockets

Currently, when instructed to open an IPv6 socket, sock_l4() explicitly
sets the IPV6_V6ONLY socket option so that the socket will only respond to
IPv6 connections.  Linux (and probably other platforms) allow "dual stack"
sockets: IPv6 sockets which can also accept IPv4 connections.

Extend sock_l4() to be able to make such sockets, by passing AF_UNSPEC as
the address family and no bind address (binding to a specific address would
defeat the purpose).  We add a Makefile define 'DUAL_STACK_SOCKETS' to
indicate availability of this feature on the target platform.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

2 files changed, 20 insertions, 2 deletions

diff --git a/Makefile b/Makefile
index ce159e5..5a0f96a 100644
--- a/Makefile
+++ b/Makefile
@@ -11,6 +11,10 @@
 
 VERSION ?= $(shell git describe --tags HEAD 2>/dev/null || echo "unknown\ version")
 
+# Does the target platform allow IPv4 connections to be handled via
+# the IPv6 socket API? (Linux does)
+DUAL_STACK_SOCKETS := 1
+
 RLIMIT_STACK_VAL := $(shell /bin/sh -c 'ulimit -s')
 ifeq ($(RLIMIT_STACK_VAL),unlimited)
 RLIMIT_STACK_VAL := 1024
@@ -36,6 +40,7 @@ FLAGS += -DPASST_AUDIT_ARCH=AUDIT_ARCH_$(AUDIT_ARCH)
 FLAGS += -DRLIMIT_STACK_VAL=$(RLIMIT_STACK_VAL)
 FLAGS += -DARCH=\"$(TARGET_ARCH)\"
 FLAGS += -DVERSION=\"$(VERSION)\"
+FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
 
 PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c icmp.c igmp.c \
 	isolation.c lineread.c log.c mld.c ndp.c netlink.c packet.c passt.c \
diff --git a/util.c b/util.c
index 2125a67..59c6641 100644
--- a/util.c
+++ b/util.c
@@ -22,6 +22,8 @@
 #include <string.h>
 #include <time.h>
 #include <errno.h>
+#include <stdbool.h>
+#include <assert.h>
 
 #include "util.h"
 #include "passt.h"
@@ -112,6 +114,7 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 		0, IN6ADDR_ANY_INIT, 0,
 	};
 	const struct sockaddr *sa;
+	bool dual_stack = false;
 	struct epoll_event ev;
 	int fd, sl, y = 1;
 
@@ -119,6 +122,13 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 	    proto != IPPROTO_ICMP && proto != IPPROTO_ICMPV6)
 		return -1;	/* Not implemented. */
 
+	if (af == AF_UNSPEC) {
+		if (!DUAL_STACK_SOCKETS || bind_addr)
+			return -1;
+		dual_stack = true;
+		af = AF_INET6;
+	}
+
 	if (proto == IPPROTO_TCP)
 		fd = socket(af, SOCK_STREAM | SOCK_NONBLOCK, proto);
 	else
@@ -158,8 +168,11 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 		sa = (const struct sockaddr *)&addr6;
 		sl = sizeof(addr6);
 
-		if (setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &y, sizeof(y)))
-			debug("Failed to set IPV6_V6ONLY on socket %i", fd);
+		if (!dual_stack)
+			if (setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY,
+				       &y, sizeof(y)))
+				debug("Failed to set IPV6_V6ONLY on socket %i",
+				      fd);
 	}
 
 	if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &y, sizeof(y)))