[Openvpn-devel,v7] Implement block-ipv6

Message ID 20181203164818.15756-1-arne@rfc2549.org
State Accepted
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  • [Openvpn-devel,v7] Implement block-ipv6
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Commit Message

Arne Schwabe Dec. 3, 2018, 4:48 p.m.
This can be used to redirect all IPv6 traffic to the tun interface,
effectively black holing the IPv6 traffic. Without ICMPv6 error
messages this will result in timeouts when the server does not send
error codes.  block-ipv6 allows client side only blocking on all
platforms that OpenVPN supports IPv6. On Android it is only way to do
sensible IPv6 blocking on Android < 5.0 and broken devices (Samsung).

PATCH V6:
- Rebase on master and run uncrustify on the patch

PATCH V5:
- Fix even more style issues by Antonio
- Remove check for dev == tun as this also works for tap

PATCH V4:
- Fix more style issues reported by Antonio
- Clarify parts of the patch in comments and manpage

PATCH V3:
- Fix style iusses reported by Antonio and accidentily commited parts
- merge udp_checksum and ipv6_checkusm into common ip_checksum method
- Use fake ff80::7 address when no other address is configured.
- Make block-ipv6 also work for server  by replying block-ipv6 to all
  ipv6 traffic send to the server

Note for the server the process_ip happens before the ipv6 route
lookup so every ipv6 packet, regardless of its source address is
replyied to with a no route to host packet.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 doc/openvpn.8         |  37 ++++++++++
 src/openvpn/dhcp.c    |  51 +------------
 src/openvpn/forward.c | 165 +++++++++++++++++++++++++++++++++++++++++-
 src/openvpn/forward.h |  12 +--
 src/openvpn/multi.c   |   2 +-
 src/openvpn/options.c |   9 +++
 src/openvpn/options.h |   1 +
 src/openvpn/proto.c   |  52 +++++++++++++
 src/openvpn/proto.h   |  42 ++++++++++-
 9 files changed, 311 insertions(+), 60 deletions(-)

Comments

Gert Doering Dec. 4, 2018, 8:30 p.m. | #1
Acked-by: Gert Doering <gert@greenie.muc.de>

Stared at the code (long and hard, especially the changes to udp_checksum()
- which is a welcome generalization).  Looks all reasonable.  One of the
changes was uncrustify wrapping the call to process_ip_header() - which
looked ugly before, and looks ugly afterwards, but that's not your fault.

What I like quite a lot is that this patch does not auto-add an IPv6 
default route - it looked like a "why, we could do this automatically?"
initially, but is coming handy if you want more selective blocking.

Tested with tun on linux, and tap on FreeBSD.  Works nicely (*).

Stared at the synthesized ICMPv6 packet with tcpdump and wireshark, and
both tools tell me "the packet is good" ;-)

I have test compiled for Windows (mingw).  Builds fine.  Have not 
actually run the resulting binary, but given that the changes are 
fully platform independent, I expect no issues here.


Server mode --block-ipv6 doesn't work correctly for me, though  (but I've 
decided to move this towards a separate patch as it isn't blocking client 
side functionality and not breaking "normal" operation).

I run a "standard tun server" on linux, with --server, and --block-ipv6.

The server assigns a v6 address to the client, the client sends a packet
into the tunnel:

21:15:47.179356 IP6 2001:608:3:814::1000 > 2001:608::2: ICMP6, echo request, seq 1, length 64

.. and I can see it come out of the server's tun interface towards linux:

21:15:47.192493 IP6 2001:608:3:814::1000 > 2001:608::2: ICMP6, echo request, seq 1, length 64

.. the target hosts answers, and the answer is routed into the server's
tun interface:

21:15:47.192993 IP6 2001:608::2 > 2001:608:3:814::1000: ICMP6, echo reply, seq 1, length 64

.. and then the server tells the remote IPv6 host that "no, my client is
not there"

21:15:47.193030 IP6 2001:608:3:814::2 > 2001:608::2: ICMP6, destination unreachable, unreachable route 2001:608:3:814::1000, length 112

.. but no ICMPv6 packet is ever sent towards the client.  Looks like the
multi.c code is somewhat confused about directions...  without block-ipv6
on the server, things work normally...

21:20:28.961067 IP6 2001:608:3:814::1000 > 2001:608::2: ICMP6, echo request, seq 1, length 64
21:20:28.961568 IP6 2001:608::2 > 2001:608:3:814::1000: ICMP6, echo reply, seq 1, length 64
21:20:29.963421 IP6 2001:608:3:814::1000 > 2001:608::2: ICMP6, echo request, seq 2, length 64
21:20:29.963933 IP6 2001:608::2 > 2001:608:3:814::1000: ICMP6, echo reply, seq 2, length 64



Tap mode might need a bit of tweaking as well - it does work, but it
blocks ND packets, instead of "user payload" (TCP SYN)...

21:22:23.923360 IP6 fd00:abcd:194:4::1007 > ff02::1:ff00:1: ICMP6, neighbor solicitation, who has fd00:abcd:194:4::1, length 32
21:22:23.923375 IP6 fd00:abcd:194:4::1 > fd00:abcd:194:4::1007: ICMP6, destination unreachable, unreachable route ff02::1:ff00:1, length 80

.. so you do not get back a "fast fail" on the payload, but it fails 
after ND timeout...

traceroute6 to fd00:abcd:194:4::1 (fd00:abcd:194:4::1) from fd00:abcd:194:4::1007, 64 hops max, 12 byte packets
 1  fd00:abcd:194:4::1007 (fd00:abcd:194:4::1007)  3058.434 ms !A  3038.031 ms !A  3055.928 ms !A

.. so while it works, it's not as nice as it could be - maybe we should
multicast packets (destination ff02::) just pass through in tap mode, so
a ND relationship with the remote can be established?  If I install a
manual ND entry, I get the proper "quick fail from the right source":

21:27:38.686757 IP6 fd00:abcd:194:4::1007.29319 > fd00:abcd:194:4::1.33437: UDP, length 12
21:27:38.686765 IP6 fd00:abcd:194:4::1 > fd00:abcd:194:4::1007: ICMP6, destination unreachable, unreachable route fd00:abcd:194:4::1, length 68

traceroute6 to fd00:abcd:194:4::1 (fd00:abcd:194:4::1) from fd00:abcd:194:4::1007, 64 hops max, 12 byte packets
 1  fd00:abcd:194:4::1 (fd00:abcd:194:4::1)  0.113 ms !N  0.072 ms !N  0.057 ms !N

$ telnet fd00:abcd:194:4::1 22
Trying fd00:abcd:194:4::1...
telnet: connect to address fd00:abcd:194:4::1: No route to host
telnet: Unable to connect to remote host


Anyway.  Room for improvement :-)

Your patch has been applied to the master branch.

commit e11d2d14a9ef5311f791a9a614ab367c6f50ff11 (master)
Author: Arne Schwabe
Date:   Mon Dec 3 17:48:18 2018 +0100

     Implement block-ipv6

     Signed-off-by: Arne Schwabe <arne@rfc2549.org>
     Acked-by: Gert Doering <gert@greenie.muc.de>
     Message-Id: <20181203164818.15756-1-arne@rfc2549.org>
     URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17977.html
     Signed-off-by: Gert Doering <gert@greenie.muc.de>


--
kind regards,

Gert Doering

Patch

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index eae2bd94..11ebdbe5 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -1243,6 +1243,43 @@  Like \-\-redirect\-gateway, but omit actually changing the default
 gateway.  Useful when pushing private subnets.
 .\"*********************************************************
 .TP
+.B \-\-block\-ipv6
+On the client, instead of sending IPv6 packets over the VPN tunnel, all
+IPv6 packets are answered with an ICMPv6 no route host message. On the
+server, all IPv6 packets from clients are answered with an ICMPv6
+no route to host message. This options is intended for cases when IPv6
+should be blocked and other options are not available.
+\.B \-\-block\-ipv6
+will use the remote IPv6 as source address of the ICMPv6 packets if set,
+otherwise will use fe80::7 as source address.
+
+For this option to make sense you actually have to route traffic to the tun
+interface. The following example config block would send all IPv6 traffic to
+OpenVPN and answer all requests with no route to host, effectively blocking
+IPv6.
+
+# client config
+.br
+.B \-\-ifconfig-ipv6
+fd15:53b6:dead::2/64  fd15:53b6:dead::1
+.br
+.B \-\-redirect\-gateway
+ipv6
+.br
+.B \-\-block\-ipv6
+
+# Server config, push a "valid" ipv6 config to the client and block
+# on the server
+.br
+.B \-\-push
+"ifconfig-ipv6 fd15:53b6:dead::2/64  fd15:53b6:dead::1"
+.br
+.B \-\-push
+"redirect\-gateway ipv6"
+.br
+.B \-\-block\-ipv6
+.\"*********************************************************
+.TP
 .B \-\-tun\-mtu n
 Take the TUN device MTU to be
 .B n
diff --git a/src/openvpn/dhcp.c b/src/openvpn/dhcp.c
index fb28b27d..c19370eb 100644
--- a/src/openvpn/dhcp.c
+++ b/src/openvpn/dhcp.c
@@ -147,49 +147,6 @@  do_extract(struct dhcp *dhcp, int optlen)
     return ret;
 }
 
-static uint16_t
-udp_checksum(const uint8_t *buf,
-             const int len_udp,
-             const uint8_t *src_addr,
-             const uint8_t *dest_addr)
-{
-    uint16_t word16;
-    uint32_t sum = 0;
-    int i;
-
-    /* make 16 bit words out of every two adjacent 8 bit words and  */
-    /* calculate the sum of all 16 bit words */
-    for (i = 0; i < len_udp; i += 2)
-    {
-        word16 = ((buf[i] << 8) & 0xFF00) + ((i + 1 < len_udp) ? (buf[i+1] & 0xFF) : 0);
-        sum += word16;
-    }
-
-    /* add the UDP pseudo header which contains the IP source and destination addresses */
-    for (i = 0; i < 4; i += 2)
-    {
-        word16 = ((src_addr[i] << 8) & 0xFF00) + (src_addr[i+1] & 0xFF);
-        sum += word16;
-    }
-    for (i = 0; i < 4; i += 2)
-    {
-        word16 = ((dest_addr[i] << 8) & 0xFF00) + (dest_addr[i+1] & 0xFF);
-        sum += word16;
-    }
-
-    /* the protocol number and the length of the UDP packet */
-    sum += (uint16_t) OPENVPN_IPPROTO_UDP + (uint16_t) len_udp;
-
-    /* keep only the last 16 bits of the 32 bit calculated sum and add the carries */
-    while (sum >> 16)
-    {
-        sum = (sum & 0xFFFF) + (sum >> 16);
-    }
-
-    /* Take the one's complement of sum */
-    return ((uint16_t) ~sum);
-}
-
 in_addr_t
 dhcp_extract_router_msg(struct buffer *ipbuf)
 {
@@ -210,10 +167,10 @@  dhcp_extract_router_msg(struct buffer *ipbuf)
 
             /* recompute the UDP checksum */
             df->udp.check = 0;
-            df->udp.check = htons(udp_checksum((uint8_t *) &df->udp,
-                                               sizeof(struct openvpn_udphdr) + sizeof(struct dhcp) + optlen,
-                                               (uint8_t *)&df->ip.saddr,
-                                               (uint8_t *)&df->ip.daddr));
+            df->udp.check = htons(ip_checksum(AF_INET, (uint8_t *)&df->udp,
+                                              sizeof(struct openvpn_udphdr) + sizeof(struct dhcp) + optlen,
+                                              (uint8_t *)&df->ip.saddr, (uint8_t *)&df->ip.daddr,
+                                              OPENVPN_IPPROTO_UDP));
 
             /* only return the extracted Router address if DHCPACK */
             if (message_type == DHCPACK)
diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
index f8faa810..3b0b58d4 100644
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -1412,7 +1412,9 @@  process_incoming_tun(struct context *c)
          * The --passtos and --mssfix options require
          * us to examine the IP header (IPv4 or IPv6).
          */
-        process_ip_header(c, PIPV4_PASSTOS|PIP_MSSFIX|PIPV4_CLIENT_NAT, &c->c2.buf);
+        unsigned int flags = PIPV4_PASSTOS | PIP_MSSFIX | PIPV4_CLIENT_NAT
+                             | PIPV6_IMCP_NOHOST_CLIENT;
+        process_ip_header(c, flags, &c->c2.buf);
 
 #ifdef PACKET_TRUNCATION_CHECK
         /* if (c->c2.buf.len > 1) --c->c2.buf.len; */
@@ -1423,6 +1425,9 @@  process_incoming_tun(struct context *c)
                                 &c->c2.n_trunc_pre_encrypt);
 #endif
 
+    }
+    if (c->c2.buf.len > 0)
+    {
         encrypt_sign(c, true);
     }
     else
@@ -1433,6 +1438,142 @@  process_incoming_tun(struct context *c)
     gc_free(&gc);
 }
 
+/**
+ * Forges a IPv6 ICMP packet with a no route to host error code from the
+ * IPv6 packetin buf and sends it directly back to the client via the tun device
+ * when used on a client and via the link if used on the server.
+ *
+ * @param buf       - The buf containing the packet for which the icmp6
+ *                    unreachable should be constructed.
+ *
+ * @param client    - determines whether to the send packet back via tun or link
+ */
+void
+ipv6_send_icmp_unreachable(struct context *c, struct buffer *buf, bool client)
+{
+#define MAX_ICMPV6LEN 1280
+    struct openvpn_icmp6hdr icmp6out;
+    CLEAR(icmp6out);
+
+    /*
+     * Get a buffer to the ip packet, is_ipv6 automatically forwards
+     * the buffer to the ip packet
+     */
+    struct buffer inputipbuf = *buf;
+
+    is_ipv6(TUNNEL_TYPE(c->c1.tuntap), &inputipbuf);
+
+    if (BLEN(&inputipbuf) < (int)sizeof(struct openvpn_ipv6hdr))
+    {
+        return;
+    }
+
+    const struct openvpn_ipv6hdr *pip6 = (struct openvpn_ipv6hdr *)BPTR(&inputipbuf);
+
+    /* Copy version, traffic class, flow label from input packet */
+    struct openvpn_ipv6hdr pip6out = *pip6;
+
+    pip6out.version_prio = pip6->version_prio;
+    pip6out.daddr = pip6->saddr;
+
+    /*
+     * Use the IPv6 remote address if we have one, otherwise use a fake one
+     * using the remote address is preferred since it makes debugging and
+     * understanding where the ICMPv6 error originates easier
+     */
+    if (c->options.ifconfig_ipv6_remote)
+    {
+        inet_pton(AF_INET6, c->options.ifconfig_ipv6_remote, &pip6out.saddr);
+    }
+    else
+    {
+        inet_pton(AF_INET6, "fe80::7", &pip6out.saddr);
+    }
+
+    pip6out.nexthdr = OPENVPN_IPPROTO_ICMPV6;
+
+    /*
+     * The ICMPv6 unreachable code worked best in my (arne) tests with Windows,
+     * Linux and Android. Windows did not like the administratively prohibited
+     * return code (no fast fail)
+     */
+    icmp6out.icmp6_type = OPENVPN_ICMP6_DESTINATION_UNREACHABLE;
+    icmp6out.icmp6_code = OPENVPN_ICMP6_DU_NOROUTE;
+
+    int icmpheader_len = sizeof(struct openvpn_ipv6hdr)
+                         + sizeof(struct openvpn_icmp6hdr);
+    int totalheader_len = icmpheader_len;
+
+    if (TUNNEL_TYPE(c->c1.tuntap) == DEV_TYPE_TAP)
+    {
+        totalheader_len += sizeof(struct openvpn_ethhdr);
+    }
+
+    /*
+     * Calculate size for payload, defined in the standard that the resulting
+     * frame should be <= 1280 and have as much as possible of the original
+     * packet
+     */
+    int max_payload_size = min_int(MAX_ICMPV6LEN,
+                                   TUN_MTU_SIZE(&c->c2.frame) - icmpheader_len);
+    int payload_len = min_int(max_payload_size, BLEN(&inputipbuf));
+
+    pip6out.payload_len = htons(sizeof(struct openvpn_icmp6hdr) + payload_len);
+
+    /* Construct the packet as outgoing packet back to the client */
+    struct buffer *outbuf;
+    if (client)
+    {
+        c->c2.to_tun = c->c2.buffers->aux_buf;
+        outbuf = &(c->c2.to_tun);
+    }
+    else
+    {
+        c->c2.to_link = c->c2.buffers->aux_buf;
+        outbuf = &(c->c2.to_link);
+    }
+    ASSERT(buf_init(outbuf, totalheader_len));
+
+    /* Fill the end of the buffer with original packet */
+    ASSERT(buf_safe(outbuf, payload_len));
+    ASSERT(buf_copy_n(outbuf, &inputipbuf, payload_len));
+
+    /* ICMP Header, copy into buffer to allow checksum calculation */
+    ASSERT(buf_write_prepend(outbuf, &icmp6out, sizeof(struct openvpn_icmp6hdr)));
+
+    /* Calculate checksum over the packet and write to header */
+
+    uint16_t new_csum = ip_checksum(AF_INET6, BPTR(outbuf), BLEN(outbuf),
+                                    (const uint8_t *)&pip6out.saddr,
+                                    (uint8_t *)&pip6out.daddr, OPENVPN_IPPROTO_ICMPV6);
+    ((struct openvpn_icmp6hdr *) BPTR(outbuf))->icmp6_cksum = htons(new_csum);
+
+
+    /* IPv6 Header */
+    ASSERT(buf_write_prepend(outbuf, &pip6out, sizeof(struct openvpn_ipv6hdr)));
+
+    /*
+     * Tap mode, we also need to create an Ethernet header.
+     */
+    if (TUNNEL_TYPE(c->c1.tuntap) == DEV_TYPE_TAP)
+    {
+        if (BLEN(buf) < (int)sizeof(struct openvpn_ethhdr))
+        {
+            return;
+        }
+
+        const struct openvpn_ethhdr *orig_ethhdr = (struct openvpn_ethhdr *) BPTR(buf);
+
+        /* Copy frametype and reverse source/destination for the response */
+        struct openvpn_ethhdr ethhdr;
+        memcpy(ethhdr.source, orig_ethhdr->dest, OPENVPN_ETH_ALEN);
+        memcpy(ethhdr.dest, orig_ethhdr->source, OPENVPN_ETH_ALEN);
+        ethhdr.proto = htons(OPENVPN_ETH_P_IPV6);
+        ASSERT(buf_write_prepend(outbuf, &ethhdr, sizeof(struct openvpn_ethhdr)));
+    }
+#undef MAX_ICMPV6LEN
+}
+
 void
 process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
 {
@@ -1454,6 +1595,10 @@  process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
     {
         flags &= ~PIPV4_EXTRACT_DHCP_ROUTER;
     }
+    if (!c->options.block_ipv6)
+    {
+        flags &= ~(PIPV6_IMCP_NOHOST_CLIENT | PIPV6_IMCP_NOHOST_SERVER);
+    }
 
     if (buf->len > 0)
     {
@@ -1489,7 +1634,7 @@  process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
                 /* possibly do NAT on packet */
                 if ((flags & PIPV4_CLIENT_NAT) && c->options.client_nat)
                 {
-                    const int direction = (flags & PIPV4_OUTGOING) ? CN_INCOMING : CN_OUTGOING;
+                    const int direction = (flags & PIP_OUTGOING) ? CN_INCOMING : CN_OUTGOING;
                     client_nat_transform(c->options.client_nat, &ipbuf, direction);
                 }
                 /* possibly extract a DHCP router message */
@@ -1507,8 +1652,18 @@  process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
                 /* possibly alter the TCP MSS */
                 if (flags & PIP_MSSFIX)
                 {
-                    mss_fixup_ipv6(&ipbuf, MTU_TO_MSS(TUN_MTU_SIZE_DYNAMIC(&c->c2.frame)));
+                    mss_fixup_ipv6(&ipbuf,
+                                   MTU_TO_MSS(TUN_MTU_SIZE_DYNAMIC(&c->c2.frame)));
+                }
+                if (!(flags & PIP_OUTGOING) && (flags
+                                                &(PIPV6_IMCP_NOHOST_CLIENT | PIPV6_IMCP_NOHOST_SERVER)))
+                {
+                    ipv6_send_icmp_unreachable(c, buf,
+                                               (bool)(flags & PIPV6_IMCP_NOHOST_CLIENT));
+                    /* Drop the IPv6 packet */
+                    buf->len = 0;
                 }
+
             }
         }
     }
@@ -1689,7 +1844,9 @@  process_outgoing_tun(struct context *c)
      * The --mssfix option requires
      * us to examine the IP header (IPv4 or IPv6).
      */
-    process_ip_header(c, PIP_MSSFIX|PIPV4_EXTRACT_DHCP_ROUTER|PIPV4_CLIENT_NAT|PIPV4_OUTGOING, &c->c2.to_tun);
+    process_ip_header(c,
+                      PIP_MSSFIX | PIPV4_EXTRACT_DHCP_ROUTER | PIPV4_CLIENT_NAT | PIP_OUTGOING,
+                      &c->c2.to_tun);
 
     if (c->c2.to_tun.len <= MAX_RW_SIZE_TUN(&c->c2.frame))
     {
diff --git a/src/openvpn/forward.h b/src/openvpn/forward.h
index 58b75d64..a7913088 100644
--- a/src/openvpn/forward.h
+++ b/src/openvpn/forward.h
@@ -286,11 +286,13 @@  void process_outgoing_tun(struct context *c);
 
 bool send_control_channel_string(struct context *c, const char *str, int msglevel);
 
-#define PIPV4_PASSTOS         (1<<0)
-#define PIP_MSSFIX            (1<<1)         /* v4 and v6 */
-#define PIPV4_OUTGOING        (1<<2)
-#define PIPV4_EXTRACT_DHCP_ROUTER (1<<3)
-#define PIPV4_CLIENT_NAT      (1<<4)
+#define PIPV4_PASSTOS                   (1<<0)
+#define PIP_MSSFIX                      (1<<1)         /* v4 and v6 */
+#define PIP_OUTGOING                    (1<<2)
+#define PIPV4_EXTRACT_DHCP_ROUTER       (1<<3)
+#define PIPV4_CLIENT_NAT                (1<<4)
+#define PIPV6_IMCP_NOHOST_CLIENT        (1<<5)
+#define PIPV6_IMCP_NOHOST_SERVER        (1<<6)
 
 void process_ip_header(struct context *c, unsigned int flags, struct buffer *buf);
 
diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
index 8440f311..d3278938 100644
--- a/src/openvpn/multi.c
+++ b/src/openvpn/multi.c
@@ -2855,7 +2855,7 @@  multi_get_queue(struct mbuf_set *ms)
 
     if (mbuf_extract_item(ms, &item)) /* cleartext IP packet */
     {
-        unsigned int pip_flags = PIPV4_PASSTOS;
+        unsigned int pip_flags = PIPV4_PASSTOS | PIPV6_IMCP_NOHOST_SERVER;
 
         set_prefix(item.instance);
         item.instance->context.c2.buf = item.buffer->buf;
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 9ee1836b..38f668ff 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -225,6 +225,10 @@  static const char usage_message[] =
     "                  Add 'bypass-dns' flag to similarly bypass tunnel for DNS.\n"
     "--redirect-private [flags]: Like --redirect-gateway, but omit actually changing\n"
     "                  the default gateway.  Useful when pushing private subnets.\n"
+    "--block-ipv6     : (Client) Instead sending IPv6 to the server generate\n"
+    "                   ICMPv6 host unreachable messages on the client.\n"
+    "                   (Server) Instead of forwarding IPv6 packets send\n"
+    "                   ICMPv6 host unreachable packets to the client.\n"
     "--client-nat snat|dnat network netmask alias : on client add 1-to-1 NAT rule.\n"
     "--push-peer-info : (client only) push client info to server.\n"
     "--setenv name value : Set a custom environmental variable to pass to script.\n"
@@ -6364,6 +6368,11 @@  add_option(struct options *options,
 #endif
         options->routes->flags |= RG_ENABLE;
     }
+    else if (streq(p[0], "block-ipv6") && !p[1])
+    {
+        VERIFY_PERMISSION(OPT_P_ROUTE);
+        options->block_ipv6 = true;
+    }
     else if (streq(p[0], "remote-random-hostname") && !p[1])
     {
         VERIFY_PERMISSION(OPT_P_GENERAL);
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index 575777f5..93df21b4 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -350,6 +350,7 @@  struct options
     bool route_delay_defined;
     struct route_option_list *routes;
     struct route_ipv6_option_list *routes_ipv6;                 /* IPv6 */
+    bool block_ipv6;
     bool route_nopull;
     bool route_gateway_via_dhcp;
     bool allow_pull_fqdn; /* as a client, allow server to push a FQDN for certain parameters */
diff --git a/src/openvpn/proto.c b/src/openvpn/proto.c
index 87c18e84..3bf22174 100644
--- a/src/openvpn/proto.c
+++ b/src/openvpn/proto.c
@@ -98,6 +98,58 @@  is_ipv6(int tunnel_type, struct buffer *buf)
     return is_ipv_X( tunnel_type, buf, 6 );
 }
 
+
+uint16_t
+ip_checksum(const sa_family_t af, const uint8_t *payload, const int len_payload,
+            const uint8_t *src_addr, const uint8_t *dest_addr, const int proto)
+{
+    uint32_t sum = 0;
+    int addr_len = (af == AF_INET) ? 4 : 16;
+
+    /*
+     * make 16 bit words out of every two adjacent 8 bit words and  */
+    /* calculate the sum of all 16 bit words
+     */
+    for (int i = 0; i < len_payload; i += 2)
+    {
+        sum +=  (uint16_t)(((payload[i] << 8) & 0xFF00)
+                           +((i + 1 < len_payload) ? (payload[i + 1] & 0xFF) : 0));
+
+    }
+
+    /*
+     * add the pseudo header which contains the IP source and destination
+     * addresses
+     */
+    for (int i = 0; i < addr_len; i += 2)
+    {
+        sum += (uint16_t)((src_addr[i] << 8) & 0xFF00) + (src_addr[i + 1] & 0xFF);
+
+    }
+    for (int i = 0; i < addr_len; i += 2)
+    {
+        sum += (uint16_t)((dest_addr[i] << 8) & 0xFF00) + (dest_addr[i + 1] & 0xFF);
+    }
+
+    /* the length of the payload */
+    sum += (uint16_t)len_payload;
+
+    /* The next header or proto field*/
+    sum += (uint16_t)proto;
+
+    /*
+     * keep only the last 16 bits of the 32 bit calculated sum and add
+     * the carries
+     */
+    while (sum >> 16)
+    {
+        sum = (sum & 0xFFFF) + (sum >> 16);
+    }
+
+    /* Take the one's complement of sum */
+    return ((uint16_t) ~sum);
+}
+
 #ifdef PACKET_TRUNCATION_CHECK
 
 void
diff --git a/src/openvpn/proto.h b/src/openvpn/proto.h
index 985aa99b..4ddffc7d 100644
--- a/src/openvpn/proto.h
+++ b/src/openvpn/proto.h
@@ -95,9 +95,10 @@  struct openvpn_iphdr {
 
     uint8_t ttl;
 
-#define OPENVPN_IPPROTO_IGMP 2  /* IGMP protocol */
-#define OPENVPN_IPPROTO_TCP  6  /* TCP protocol */
-#define OPENVPN_IPPROTO_UDP 17  /* UDP protocol */
+#define OPENVPN_IPPROTO_IGMP    2  /* IGMP protocol */
+#define OPENVPN_IPPROTO_TCP     6  /* TCP protocol */
+#define OPENVPN_IPPROTO_UDP    17  /* UDP protocol */
+#define OPENVPN_IPPROTO_ICMPV6 58 /* ICMPV6 protocol */
     uint8_t protocol;
 
     uint16_t check;
@@ -120,6 +121,24 @@  struct openvpn_ipv6hdr {
     struct  in6_addr daddr;
 };
 
+/*
+ * ICMPv6 header
+ */
+struct openvpn_icmp6hdr {
+#define OPENVPN_ICMP6_DESTINATION_UNREACHABLE       1
+#define OPENVPN_ND_ROUTER_SOLICIT                 133
+#define OPENVPN_ND_ROUTER_ADVERT                  134
+#define OPENVPN_ND_NEIGHBOR_SOLICIT               135
+#define OPENVPN_ND_NEIGHBOR_ADVERT                136
+#define OPENVPN_ND_INVERSE_SOLICIT                141
+#define OPENVPN_ND_INVERSE_ADVERT                 142
+    uint8_t icmp6_type;
+#define OPENVPN_ICMP6_DU_NOROUTE                    0
+#define OPENVPN_ICMP6_DU_COMMUNICATION_PROHIBTED    1
+    uint8_t icmp6_code;
+    uint16_t icmp6_cksum;
+    uint8_t icmp6_dataun[4];
+};
 
 /*
  * UDP header
@@ -265,6 +284,23 @@  bool is_ipv4(int tunnel_type, struct buffer *buf);
 
 bool is_ipv6(int tunnel_type, struct buffer *buf);
 
+/**
+ *  Calculates an IP or IPv6 checksum with a pseudo header as required by
+ *  TCP, UDP and ICMPv6
+ *
+ * @param af            - Address family for which the checksum is calculated
+ *                        AF_INET or AF_INET6
+ * @param payload       - the TCP, ICMPv6 or UDP packet
+ * @param len_payload   - length of payload
+ * @param src_addr      - Source address of the packet
+ * @param dest_addr     - Destination address of the packet
+ * @param proto next    - header or IP protocol of the packet
+ * @return The calculated checksum in host order
+ */
+uint16_t
+ip_checksum(const sa_family_t af, const uint8_t *payload, const int len_payload,
+            const uint8_t *src_addr, const uint8_t *dest_addr,  const int proto);
+
 #ifdef PACKET_TRUNCATION_CHECK
 void ipv4_packet_size_verify(const uint8_t *data,
                              const int size,