File 0002-Change-the-MTU-size-of-tunnel-23.patch of Package icmptunnel

From 5b7bdd5628c02e526cf71f2afdf63037f37782d9 Mon Sep 17 00:00:00 2001
From: YaoYu-Li <forever3580ya@gmail.com>
Date: Thu, 23 Mar 2017 22:25:48 +0800
Subject: [PATCH 2/3] Change the MTU size of tunnel (#23)

* Change the receive buffer in recv_icmp_packet()

    Since the maximum MTU of ethernet is 1500 bytes, the MTU of tunnel
    should be not more than 1500 - iphdr_size - icmphdr_size. So the tunnel
    MTU in both client and server site are up to 1472 bytes.

* fixed tun MTU in script
---
 Makefile  |  2 +-
 client.sh |  2 +-
 icmp.c    | 11 +++++++----
 icmp.h    |  2 +-
 server.sh |  2 +-
 5 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/Makefile b/Makefile
index 8c9b441..5ccc174 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 CC=gcc
-CFLAGS=-I.
+CFLAGS=-I. -O3 -Wall
 DEPS = icmp.h tunnel.h
 
 %.o: %.c $(DEPS)
diff --git a/client.sh b/client.sh
index 4597633..8387fb0 100755
--- a/client.sh
+++ b/client.sh
@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # Assigining an IP address and mask to 'tun0' interface
-ifconfig tun0 mtu 1500 up 10.0.1.2 netmask 255.255.255.0
+ifconfig tun0 mtu 1472 up 10.0.1.2 netmask 255.255.255.0
 
 # Modifying IP routing tables
 route del default
diff --git a/icmp.c b/icmp.c
index 4ce60d1..0cd1aa7 100644
--- a/icmp.c
+++ b/icmp.c
@@ -142,7 +142,7 @@ void send_icmp_packet(int sock_fd, struct icmp_packet *packet_details)
 void receive_icmp_packet(int sock_fd, struct icmp_packet *packet_details)
 {
   struct sockaddr_in src_addr;
-  struct sockaddr_in dest_addr;
+  //struct sockaddr_in dest_addr;
 
   struct iphdr *ip;
   struct icmphdr *icmp;
@@ -151,9 +151,12 @@ void receive_icmp_packet(int sock_fd, struct icmp_packet *packet_details)
   int packet_size;
   char *packet;
 
-  int src_addr_size;
+  socklen_t src_addr_size;
+  int enc_MTU; //encapsulated MTU
 
-  packet = calloc(MTU, sizeof(uint8_t));
+  enc_MTU = MTU + sizeof(struct iphdr) + sizeof(struct icmphdr);
+
+  packet = calloc(enc_MTU, sizeof(uint8_t));
   if (packet == NULL) {
     perror("No memory available\n");
     close_icmp_socket(sock_fd);
@@ -163,7 +166,7 @@ void receive_icmp_packet(int sock_fd, struct icmp_packet *packet_details)
   src_addr_size = sizeof(struct sockaddr_in);
   
   // Receiving packet
-  packet_size = recvfrom(sock_fd, packet, MTU, 0, (struct sockaddr *)&(src_addr), &src_addr_size);
+  packet_size = recvfrom(sock_fd, packet, enc_MTU, 0, (struct sockaddr *)&(src_addr), &src_addr_size);
 
   ip = (struct iphdr *)packet;
   icmp = (struct icmphdr *)(packet + sizeof(struct iphdr));
diff --git a/icmp.h b/icmp.h
index a8302c5..c16ed87 100644
--- a/icmp.h
+++ b/icmp.h
@@ -6,7 +6,7 @@
 #define icmp_guard
 
 // Maximum transmission unit
-#define MTU 1500
+#define MTU 1472
 
 struct icmp_packet
 {
diff --git a/server.sh b/server.sh
index dd1fdad..04dbaf3 100755
--- a/server.sh
+++ b/server.sh
@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # Assigining an IP address and mask to 'tun0' interface
-ifconfig tun0 mtu 1500 up 10.0.1.1 netmask 255.255.255.0 
+ifconfig tun0 mtu 1472 up 10.0.1.1 netmask 255.255.255.0 
 
 # Preventing the kernel to reply to any ICMP pings
 echo 1 | dd of=/proc/sys/net/ipv4/icmp_echo_ignore_all
-- 
2.16.4