irmd: Pass MTU from IPCP to process for FRCT

FRCT needs to know the MTU for fragmentation. The MTU is now passed
from the layer serving the flow to the process as part of flow
allocation.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

1 files changed, 55 insertions, 2 deletions

diff --git a/src/ipcpd/udp/udp.c b/src/ipcpd/udp/udp.c
index 168ea718..8fba5a77 100644
--- a/src/ipcpd/udp/udp.c
+++ b/src/ipcpd/udp/udp.c
@@ -47,6 +47,10 @@
 #include <stdlib.h>
 #include <sys/wait.h>
 #include <fcntl.h>
+#include <unistd.h>
+#if defined(__linux__)
+#include <netinet/ip.h>
+#endif
 
 #define FLOW_REQ                 1
 #define FLOW_REPLY               2
@@ -130,6 +134,53 @@ static const char * __inet_ntop(const struct __ADDR * addr,
         return inet_ntop(__AF, addr, buf, __ADDRSTRLEN);
 }
 
+#if defined(BUILD_IPCP_UDP4)
+#define UDP_MTU_FALLBACK IPCP_UDP4_MTU
+#define UDP_IP_OVERHEAD  28U /* IPv4 + UDP */
+#else
+#define UDP_MTU_FALLBACK IPCP_UDP6_MTU
+#define UDP_IP_OVERHEAD  48U /* IPv6 + UDP */
+#endif
+
+static uint32_t udp_query_mtu(const struct __SOCKADDR * saddr)
+{
+#if defined(__linux__) && (defined(IP_MTU) || defined(IPV6_MTU))
+        int       sock;
+        int       mtu = 0;
+        socklen_t len = sizeof(mtu);
+
+        sock = socket(__AF, SOCK_DGRAM, IPPROTO_UDP);
+        if (sock < 0)
+                return UDP_MTU_FALLBACK;
+
+        if (connect(sock, (const struct sockaddr *) saddr,
+                    sizeof(*saddr)) < 0)
+                goto fallback;
+
+#if defined(BUILD_IPCP_UDP4) && defined(IP_MTU)
+        if (getsockopt(sock, IPPROTO_IP, IP_MTU, &mtu, &len) < 0)
+                goto fallback;
+#elif defined(BUILD_IPCP_UDP6) && defined(IPV6_MTU)
+        if (getsockopt(sock, IPPROTO_IPV6, IPV6_MTU, &mtu, &len) < 0)
+                goto fallback;
+#else
+        goto fallback;
+#endif
+        close(sock);
+
+        if (mtu <= (int) UDP_IP_OVERHEAD)
+                return UDP_MTU_FALLBACK;
+
+        return (uint32_t) mtu - UDP_IP_OVERHEAD;
+
+ fallback:
+        close(sock);
+#else
+        (void) saddr;
+#endif
+        return UDP_MTU_FALLBACK;
+}
+
 static int udp_data_init(void)
 {
         int                i;
@@ -285,7 +336,8 @@ static int udp_ipcp_port_req(struct __SOCKADDR * c_saddr,
 {
         int fd;
 
-        fd = ipcp_wait_flow_req_arr(dst, qs, IPCP_UDP_MPL, data);
+        fd = ipcp_wait_flow_req_arr(dst, qs, IPCP_UDP_MPL,
+                                    udp_query_mtu(c_saddr), data);
         if (fd < 0) {
                 log_err("Could not get new flow from IRMd.");
                 return -1;
@@ -332,7 +384,8 @@ static int udp_ipcp_port_alloc_reply(const struct __SOCKADDR * saddr,
 
         pthread_rwlock_unlock(&udp_data.flows_lock);
 
-        if (ipcp_flow_alloc_reply(s_eid, response, mpl, data) < 0) {
+        if (ipcp_flow_alloc_reply(s_eid, response, mpl,
+                                  udp_query_mtu(saddr), data) < 0) {
                 log_err("Failed to reply to flow allocation.");
                 return -1;
         }