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PODont fragment flag for IPv6 packets in Linux using C++
 

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  1. PODont fragment flag for IPv6 packets in Linux using C++
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    <p>I can not set the "don't fragment" flag for the IPv6/ICMPv6 packets. I am doing PMTUD and I want to force the router to drop packets bigger then the MTU. Using setsockopt with IPV6_MTU_DISCOVER is not working.</p> <pre><code>int on = IPV6_PMTUDISC_DO; // tried also IPV6_PMTUDISC_PROBE setsockopt(socket, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &amp;on, sizeof(on)); </code></pre> <p>result: <img src="https://i.stack.imgur.com/SBLxw.png" alt="wireshark"></p> <p>I also can not use setosckopt with <code>IPV6_DONTFRAG</code> as described in <em>Unix-Linux Addison-Wesley - Stevens2003 - Unix Network Programming</em> because I have the <code>netinet/in6.h</code> header included and <code>IPV6_DONTFRAG</code> is defined in <code>linux/in6.h</code>. Including <code>linux/in6.h</code> in my source code causes these redefinition errors. </p> <pre><code>In file included from mypmtud.cc:30:0: /usr/include/linux/in6.h:30:8: error: redefinition of ‘struct in6_addr’ In file included from /usr/include/netdb.h:28:0, from mypmtud.cc:23: /usr/include/netinet/in.h:198:8: error: previous definition of ‘struct in6_addr’ In file included from mypmtud.cc:30:0: /usr/include/linux/in6.h:46:8: error: redefinition of ‘struct sockaddr_in6’ In file included from /usr/include/netdb.h:28:0, from mypmtud.cc:23: /usr/include/netinet/in.h:239:8: error: previous definition of ‘struct sockaddr_in6’ In file included from mypmtud.cc:30:0: /usr/include/linux/in6.h:54:8: error: redefinition of ‘struct ipv6_mreq’ In file included from /usr/include/netdb.h:28:0, from mypmtud.cc:23: /usr/include/netinet/in.h:275:8: error: previous definition of ‘struct ipv6_mreq’ make: *** [mypmtud] Error 1 </code></pre> <p>Environment: Ubuntu 12.10 on VirtualBox 4.26 and GNS3 for the virtual network. The virtual Cisco C3660 router has just basic configuration: ip, ipv6 address, no shut and set mtu.</p> <p><strong>EDIT:</strong> I need the IPv6 stack/OS kernel to drop packets that are bigger than the link MTU or to signalize "this packet needs fragmentation". How can I achieve this behavior? </p> <p>I tried <code>setsockopt</code> with <code>IPV6_DONTFRAG</code> (defined it in my code <code>#define IPV6_DONTFRAG 62</code> ), <code>setsockopt</code> with <code>IPV6_MTU_DISCOVER</code>, <code>int on = IPV6_PMTUDISC_DO</code> and <code>setsockopt</code> with <code>IPV6_RECVPATHMTU</code>.</p> <p>But I am not getting the <code>PACKET TOO BIG</code> reply or <code>ancillary data</code> with <code>cmsg_level == IPPROTO_IPV6</code> and <code>cmsg_type == IPV6_PATHMTU.</code></p> <p>Part of my code:</p> <pre><code>/** sending ICMP packet*/ if (((length = sendto(mysocket, packet, lengthBuff, 0, result-&gt;ai_addr, result-&gt;ai_addrlen)) &lt; 0) &amp;&amp; (errno == EMSGSIZE)){ // works for IPv4, doesn't work with IPv6 cout &lt;&lt; "changing maxBuff and lengthBuff size" &lt;&lt; endl; maxBuff = lengthBuff; lengthBuff = (minBuff + maxBuff) / 2; if (packet) { delete(packet); packet = NULL; } } else if (length &lt; 0){ cerr &lt;&lt; "Error: sending data." &lt;&lt; endl; freeaddrinfo(result); close(mysocket); if (packet) { delete(packet); packet = NULL; } exit(1); } else if(((recvmsg(mysocket, &amp;msg, 0)) != -1) &amp;&amp; (errno != EINTR)) { // reading ancillary dada as described in *Unix-Linux Addison-Wesley - Stevens2003 - Unix Network Programming, page 736* cmsgh = CMSG_FIRSTHDR(&amp;msg); if(cmsgh != NULL) { cout &lt;&lt; "getting msg " &lt;&lt; endl; cout &lt;&lt; "msg len " &lt;&lt; msg.msg_controllen &lt;&lt; endl; if(cmsgh-&gt;cmsg_level == IPPROTO_ICMPV6 &amp;&amp; cmsgh-&gt;cmsg_type == IPV6_PATHMTU) { cout &lt;&lt; "CMSGHEADER - GOOD" &lt;&lt; endl; //mtustruct = CMSG_DATA(&amp;msg); maxBuff = lengthBuff; lengthBuff = (minBuff + maxBuff) / 2; if (packet) { delete(packet); packet = NULL; } } else{ cout &lt;&lt; "different ancillary data. " &lt;&lt; endl; cout &lt;&lt; " level " &lt;&lt; cmsgh-&gt;cmsg_level &lt;&lt; " type " &lt;&lt; cmsgh-&gt;cmsg_type &lt;&lt; endl; } } } else { cout &lt;&lt; "no ERROR with sendto and no RESCVMSG" &lt;&lt; endl; } /** receiving ICMP data */ tv.tv_sec = 3; tv.tv_usec = 0; int retval; // select FD_ZERO(&amp;mySet); FD_SET(mysocket, &amp;mySet); retval = select(mysocket + 1, &amp;mySet, NULL, NULL, &amp;tv); if (retval == -1) { cerr &lt;&lt; "select failed" &lt;&lt; endl; //break; exit(1); } else if (retval) { if ((length = recvfrom(mysocket, buffer, MAX, 0, result-&gt;ai_addr, &amp;(result-&gt;ai_addrlen))) == -1) { cerr &lt;&lt; "Error: receiving data." &lt;&lt; endl; } else { icmpRec = (struct icmp6_hdr*) buffer; if((icmpRec-&gt;icmp6_type == ICMP6_PACKET_TOO_BIG)) { cout &lt;&lt; "next hop MTU: " &lt;&lt; ntohl(icmpRec-&gt;icmp6_mtu) &lt;&lt; endl; maxBuff = ntohl(icmpRec-&gt;icmp6_mtu); } else if ((icmpRec-&gt;icmp6_type == ICMP6_ECHO_REPLY) &amp;&amp; (ntohs(icmpRec-&gt;icmp6_id) == pid) &amp;&amp; (ntohs(icmpRec-&gt;icmp6_seq) == (seq - 1))) { cout &lt;&lt; "code " &lt;&lt; ntohs(icmpRec-&gt;icmp6_code) &lt;&lt; endl; cout &lt;&lt; "ICMP ECHO REPLY" &lt;&lt; endl; minBuff = lengthBuff; } } } </code></pre> <p><strong>EDIT2:</strong> I realized, <code>setsockopt</code> with defined <code>IPV6_DONTFRAG</code> is not working for me, but <code>setsockopt</code> with <code>IPV6_MTU_DISCOVER</code> is working for the own interface. The eth1 interface MTU is 1500 (default) and if <code>sendto</code> wants to send packets with bigger size, <code>errno</code> is set to <code>EMSGSIZE</code>. Also after some time I get <code>PACKET TOO BIG</code> message for these not send messages fom the <strong>own kernel/OS</strong>. </p> <p>My real problem is, I am <strong>not</strong> getting (Ubuntu 12.10 running on VirtualBox 4.2.6) <code>PACKET TOO BIG</code> messages from the virtual router (Cisco c3660) running on GNS3.</p>
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