不要使用IPv6为Linux中的C++数据包设置碎片标志

Question

我不能为IPv6 6/ICMPv6 6数据包设置“不要碎片”标志。我正在做PMTUD，我想迫使路由器丢包比MTU更大。在IPV6_MTU_DISCOVER中使用setsockopt不起作用。

int on = IPV6_PMTUDISC_DO; // tried also IPV6_PMTUDISC_PROBE
setsockopt(socket, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &on, sizeof(on));

结果：

​

我也不能像中描述的那样，在IPV6_DONTFRAG中使用setosckopt，因为我包含了netinet/in6.h头，而IPV6_DONTFRAG是在linux/in6.h中定义的。在我的源代码中包括linux/in6.h会导致这些重新定义错误。

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

环境: VirtualBox 4.26上的Ubuntu12.10和虚拟网络的GNS3。虚拟思科C3660路由器只有基本的配置: ip，ipv6地址，没有关闭和设置mtu。

编辑：--我需要IPv6堆栈/OS内核来丢弃大于链接MTU的数据包，或者表示“此数据包需要碎片”。我怎样才能达到这种行为呢？

我尝试了setsockopt与IPV6_DONTFRAG (在代码#define IPV6_DONTFRAG 62中定义了它)、setsockopt与IPV6_MTU_DISCOVER、int on = IPV6_PMTUDISC_DO和setsockopt与IPV6_RECVPATHMTU。

但是我没有收到PACKET TOO BIG的回复，也没有收到cmsg_level == IPPROTO_IPV6和cmsg_type == IPV6_PATHMTU.的ancillary data

我的代码之一是：

/** sending ICMP packet*/
 if (((length = sendto(mysocket, packet, lengthBuff, 0, result->ai_addr, result->ai_addrlen)) < 0) && (errno == EMSGSIZE)){

     // works for IPv4, doesn't work with IPv6
     cout << "changing maxBuff and lengthBuff size" << endl;

        maxBuff = lengthBuff;
        lengthBuff = (minBuff + maxBuff) / 2;

        if (packet) {
            delete(packet);
            packet = NULL;
        }


    } else if (length < 0){

     cerr << "Error: sending data." << endl;

            freeaddrinfo(result);
            close(mysocket);

            if (packet) {
                delete(packet);
                packet = NULL;
            }

            exit(1);
    } else if(((recvmsg(mysocket, &msg, 0)) != -1) && (errno != EINTR)) {

        // reading ancillary dada as described in  *Unix-Linux Addison-Wesley - Stevens2003 - Unix Network Programming, page 736*
        cmsgh = CMSG_FIRSTHDR(&msg);    

        if(cmsgh != NULL) {
            cout << "getting msg " << endl;
            cout << "msg len " << msg.msg_controllen << endl;


            if(cmsgh->cmsg_level == IPPROTO_ICMPV6 && cmsgh->cmsg_type == IPV6_PATHMTU)
            {
                cout << "CMSGHEADER - GOOD" << endl;
                //mtustruct = CMSG_DATA(&msg); 

            maxBuff = lengthBuff;
            lengthBuff = (minBuff + maxBuff) / 2;

            if (packet) {
                delete(packet);
                packet = NULL;
            }

            }
            else{

                cout << "different ancillary data. " << endl;
                cout << " level " << cmsgh->cmsg_level << " type " << cmsgh->cmsg_type << endl;
            }
        }

    } else {

        cout << "no ERROR with sendto and no RESCVMSG" << endl;
    } 

    /** receiving ICMP data */
    tv.tv_sec = 3;
    tv.tv_usec = 0;

    int retval; // select

        FD_ZERO(&mySet);
        FD_SET(mysocket, &mySet);

        retval = select(mysocket + 1, &mySet, NULL, NULL, &tv);

        if (retval == -1) {
            cerr << "select failed" << endl;
            //break;
            exit(1);
        } else if (retval) {
            if ((length = recvfrom(mysocket, buffer, MAX, 0, result->ai_addr, &(result->ai_addrlen))) == -1) {
                cerr << "Error: receiving data." << endl;
            } else {
                icmpRec = (struct icmp6_hdr*) buffer;

                if((icmpRec->icmp6_type == ICMP6_PACKET_TOO_BIG)) {


                    cout << "next hop MTU: " << ntohl(icmpRec->icmp6_mtu) << endl;
                    maxBuff = ntohl(icmpRec->icmp6_mtu);                       

                } else if ((icmpRec->icmp6_type == ICMP6_ECHO_REPLY) && (ntohs(icmpRec->icmp6_id) == pid) && (ntohs(icmpRec->icmp6_seq) == (seq - 1))) {
                    cout << "code " << ntohs(icmpRec->icmp6_code) << endl;
                    cout << "ICMP ECHO REPLY" << endl;
                    minBuff = lengthBuff;

                }
            }
        }

EDIT2: --我意识到，带有定义IPV6_DONTFRAG的setsockopt并不适合我，但是setsockopt与IPV6_MTU_DISCOVER一起使用的是自己的接口。eth1接口MTU是1500 (默认的)，如果sendto想要发送更大的数据包，则errno设置为EMSGSIZE。另外，过了一段时间，我得到了PACKET TOO BIG消息，因为这些消息不是针对自己的内核/OS发送的。

我真正的问题是，我是而不是not(运行在VirtualBox 4.2.6上的Ubuntu12.10)来自运行在GNS3上的虚拟路由器(Ciscoc3660)的PACKET TOO BIG消息。

Answer 1

我想强迫路由器丢弃比MTU大的数据包

在IPv6中，大于MTU的数据包将始终被丢弃。与IPv4不同的是，IPv6路由器不对数据包进行分段。相反，预计源将执行PMTU，并：

• 传输层协议是否产生足够大小的数据报？
• 片段包本地并附加片段扩展

Linux确实完全支持IPV6_DONTFRAG (我认为它是在2.6.35中添加的)，尽管它只影响本地行为。