Initial support for figuring out inbound interface for inetd connections

This patch adds initial support (only SOCK_STREAM verified!) for
figuring out the inbound interface for inetd service connections.

The intention is to use this later on for a very simple tcpwrappers
replacement, e.g: `deny telnet eth0 eth3`

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

inetd.c

@@ -21,8 +21,8 @@
  * THE SOFTWARE.
  */
 
-//#include <unistd.h>
-//#include <sys/types.h>
+#include <ifaddrs.h>
+#include <net/if.h>
 #include <netinet/in.h>
 #include <sys/socket.h>
 
@@ -76,11 +76,18 @@ static void spawn_socket(uev_ctx_t *ctx, svc_t *svc)
 		return;
 	}
 
-	if (svc->port && svc->sock_type != SOCK_DGRAM) {
-		if (-1 == listen(sd, 20)) {
-			FLOG_PERROR("Failed listening to inetd service %s", svc->service);
-			close(sd);
-			return;
+	if (svc->port) {
+		if (svc->sock_type == SOCK_STREAM) {
+			if (-1 == listen(sd, 20)) {
+				FLOG_PERROR("Failed listening to inetd service %s", svc->service);
+				close(sd);
+				return;
+			}
+		} else {           /* SOCK_DGRAM */
+			int opt = 1;
+
+			/* Set extra sockopt to get ifindex from inbound packets */
+			setsockopt(sd, SOL_IP, IP_PKTINFO, &opt, sizeof(opt));
 		}
 	}
 
@@ -89,6 +96,64 @@ static void spawn_socket(uev_ctx_t *ctx, svc_t *svc)
 	uev_io_init(ctx, &svc->watcher, socket_cb, svc, sd, UEV_READ);
 }
 
+/* Peek into SOCK_DGRAM socket to figure out where an inbound packet comes from. */
+int inetd_dgram_peek(int sd, char *ifname)
+{
+	struct msghdr msgh;
+	struct cmsghdr *cmsg;
+
+	if (recvmsg(sd, &msgh, MSG_PEEK) < 0)
+		return -1;
+
+	for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg; cmsg = CMSG_NXTHDR(&msgh,cmsg)) {
+		struct in_pktinfo *ipi = (struct in_pktinfo *)CMSG_DATA(cmsg);
+
+		if (cmsg->cmsg_level != SOL_IP || cmsg->cmsg_type != IP_PKTINFO)
+			continue;
+
+		if_indextoname(ipi->ipi_ifindex, ifname);
+
+		return 0;
+	}
+
+	return -1;
+}
+
+/* Peek into SOCK_STREAM on accepted client socket to figure out inbound interface */
+int inetd_stream_peek(int sd, char *ifname)
+{
+	struct ifaddrs *ifaddr, *ifa;
+	struct sockaddr_in sin;
+	socklen_t len = sizeof(sin);
+
+	if (-1 == getsockname(sd, (struct sockaddr *)&sin, &len))
+		return -1;
+
+	if (-1 == getifaddrs(&ifaddr))
+		return -1;
+
+	for (ifa = ifaddr; ifa; ifa = ifa->ifa_next) {
+		size_t len = sizeof(struct in_addr);
+		struct sockaddr_in *iin;
+
+		if (!ifa->ifa_addr)
+			continue;
+
+		if (ifa->ifa_addr->sa_family != AF_INET)
+			continue;
+
+		iin = (struct sockaddr_in *)ifa->ifa_addr;
+		if (!memcmp(&sin.sin_addr, &iin->sin_addr, len)) {
+			strncpy(ifname, ifa->ifa_name, IF_NAMESIZE);
+			break;
+		}
+	}
+
+	freeifaddrs(ifaddr);
+
+	return 0;
+}
+
 /* Inetd monitor, called by svc_monitor() */
 int inetd_respawn(pid_t pid)
 {

inetd.h

@@ -26,6 +26,9 @@
 
 #include "libuev/uev.h"
 
+int  inetd_dgram_peek  (int sd, char *ifname);
+int  inetd_stream_peek (int sd, char *ifname);
+
 int  inetd_respawn (pid_t pid);
 void inetd_runlevel(uev_ctx_t *ctx, int runlevel);
 

svc.c

@@ -26,6 +26,7 @@
 #include <sys/wait.h>
 #include <utmp.h>
 #include <netdb.h>
+#include <net/if.h>
 
 #include "finit.h"
 #include "helpers.h"
@@ -517,6 +518,7 @@ void svc_monitor(pid_t lost)
 int svc_start(svc_t *svc)
 {
 	int respawn, sd = 0;
+	char ifname[IF_NAMESIZE] = "UNKNOWN";
 	pid_t pid;
 	sigset_t nmask, omask;
 
@@ -551,9 +553,17 @@ int svc_start(svc_t *svc)
 			}
 
 			_d("New client socket %d accepted for inetd service %d/tcp", sd, svc->port);
+
+			/* Find ifname by means of getsockname() and getifaddrs() */
+			inetd_stream_peek(sd, ifname);
+		} else {           /* SOCK_DGRAM */
+			/* Find ifname by means of IP_PKTINFO sockopt --> ifindex + if_indextoname() */
+			inetd_dgram_peek(sd, ifname);
 		}
 
-		FLOG_INFO("Starting inetd service %s ...", svc->service);
+		/* XXX: Add poor man's tcpwrappers here, we now know inbound ifname ... */
+
+		FLOG_INFO("Starting inetd service %s for requst from iface %s ...", svc->service, ifname);
 	}
 	else if (SVC_CMD_SERVICE != svc->type)
 		print_desc("", svc->desc);