[Openvpn-devel,v4] Implement block-ipv6

Message ID 20180703144636.6704-1-arne@rfc2549.org
State Rejected
Delegated to: Antonio Quartulli
Headers show
Series
  • [Openvpn-devel,v4] Implement block-ipv6
Related show

Commit Message

Arne Schwabe July 3, 2018, 2:46 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 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.
---
 doc/openvpn.8         |  36 +++++++++++
 src/openvpn/dhcp.c    |  51 ++--------------
 src/openvpn/forward.c | 138 +++++++++++++++++++++++++++++++++++++++++-
 src/openvpn/forward.h |   4 +-
 src/openvpn/multi.c   |   2 +-
 src/openvpn/options.c |  11 ++++
 src/openvpn/options.h |   1 +
 src/openvpn/proto.c   |  45 ++++++++++++++
 src/openvpn/proto.h   |  52 ++++++++++++++++
 src/openvpn/socket.h  |  19 ------
 10 files changed, 288 insertions(+), 71 deletions(-)

Comments

Antonio Quartulli Oct. 28, 2018, 9 a.m. | #1
Hi,

On 03/07/18 22:46, Arne Schwabe wrote:
> 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 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.
> ---
>  doc/openvpn.8         |  36 +++++++++++
>  src/openvpn/dhcp.c    |  51 ++--------------
>  src/openvpn/forward.c | 138 +++++++++++++++++++++++++++++++++++++++++-
>  src/openvpn/forward.h |   4 +-
>  src/openvpn/multi.c   |   2 +-
>  src/openvpn/options.c |  11 ++++
>  src/openvpn/options.h |   1 +
>  src/openvpn/proto.c   |  45 ++++++++++++++
>  src/openvpn/proto.h   |  52 ++++++++++++++++
>  src/openvpn/socket.h  |  19 ------
>  10 files changed, 288 insertions(+), 71 deletions(-)
> 
> diff --git a/doc/openvpn.8 b/doc/openvpn.8
> index 8a987b37..bad66754 100644
> --- a/doc/openvpn.8
> +++ b/doc/openvpn.8
> @@ -1240,6 +1240,42 @@ 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..24c45c76 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,

please remove the space after the cast.

> +                                  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 00112275..d3e6eede 100644
> --- a/src/openvpn/forward.c
> +++ b/src/openvpn/forward.c
> @@ -1194,7 +1194,8 @@ 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; */
> @@ -1205,6 +1206,9 @@ process_incoming_tun(struct context *c)
>                                  &c->c2.n_trunc_pre_encrypt);
>  #endif
>  
> +    }
> +    if (c->c2.buf.len > 0)
> +    {
>          encrypt_sign(c, true);
>      }
>      else
> @@ -1215,6 +1219,123 @@ 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 contrusted.
> + *
> + *  @param client determines whether to the send packet back via tun or link

This @param is misaligned. On top of that, we normally use a tab to
separate the description and we align all the descriptions. Check
ssl_backend.h as a reference.

> + */
> +void
> +ipv6_send_icmp_unreachable(struct context *c, struct buffer *buf, bool client)
> +{
> +#define MAX_ICMPV6LEN  1280

We have

/usr/include/linux/ipv6.h:11:#define IPV6_MIN_MTU	1280

Don't you think it may make sense to define ICMPV6LEN as IPV6_MIN_MTU to
give it a better meaning?

> +    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 prefered since it makes debugging and understanding
> +     * where the ICMPv6 error originates easier */

Keep the multiline comment style consistent. Put the final '*/' on a new
line please.

> +    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);

We agreed on using curly brackets also for branches having one line only.

> +
> +    pip6out.nexthdr = OPENVPN_IPPROTO_ICMPV6;
> +
> +    /*
> +     * The ICMPv6 unreachable code worked best in my tests with Windows, Linux and Android (arne)
> +     * 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 payload_len = min_int(
> +        min_int(MAX_ICMPV6LEN, TUN_MTU_SIZE(&c->c2.frame) - icmpheader_len),
> +        BLEN(&inputipbuf));

This is kind of funky to read - how about adding another temp variable
instead of writing all those operations within the min_int() call ?
Temp variable names can help understanding what you are doing and they
are all optimized out by the compiler anyway (in most of the cases).

> +
> +    pip6out.payload_len = htons(sizeof(struct openvpn_icmp6hdr) + payload_len);
> +
> +    /* Construct the packet as outgoing packet back to the client */
> +    struct buffer * outbuf;

No space after *

> +    if (client) {

bracket goes on the new line

> +        c->c2.to_tun = c->c2.buffers->aux_buf;
> +        outbuf = &(c->c2.to_tun);
> +    } else {

bracket goes on the new line

> +        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 */
> +    ((struct openvpn_icmp6hdr*) BPTR(outbuf))->icmp6_cksum =
> +            htons(ip_checksum(AF_INET6, BPTR(outbuf), BLEN(outbuf), (const uint8_t*)&pip6out.saddr,
> +                               (uint8_t*)&pip6out.daddr, OPENVPN_IPPROTO_ICMPV6));

not sure about this, but maybe you can use the same temp variable trick
I suggested above?

> +
> +    /* 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;

brackets

> +
> +        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)
>  {
> @@ -1236,6 +1357,10 @@ 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)
>      {
> @@ -1271,7 +1396,7 @@ 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 */
> @@ -1291,6 +1416,13 @@ process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
>                  {
>                      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)))

the | should be surrounded by spaces.

> +                {
> +                    ipv6_send_icmp_unreachable(c, buf, (bool)(flags & PIPV6_IMCP_NOHOST_CLIENT));
> +                    /* Drop the IPv6 packet */
> +                    buf->len=0;
> +                }
> +
>              }
>          }
>      }
> @@ -1471,7 +1603,7 @@ 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);

same here. I know a lot of these things were there already, but I think
while touching the same line it is good to fix little style hitchups
like this.

>  
>      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 924cc5e2..6d0414f6 100644
> --- a/src/openvpn/forward.h
> +++ b/src/openvpn/forward.h
> @@ -251,9 +251,11 @@ bool send_control_channel_string(struct context *c, const char *str, int msgleve
>  
>  #define PIPV4_PASSTOS         (1<<0)
>  #define PIP_MSSFIX            (1<<1)         /* v4 and v6 */
> -#define PIPV4_OUTGOING        (1<<2)
> +#define PIP_OUTGOING          (1<<2)
>  #define PIPV4_EXTRACT_DHCP_ROUTER (1<<3)

I know this one above was already misaligned, but now that you are
adding new constants, you could re-align them all with another tab.

>  #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 3da8c110..fcb54f19 100644
> --- a/src/openvpn/multi.c
> +++ b/src/openvpn/multi.c
> @@ -2838,7 +2838,7 @@ 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;

spaces around |

>  
>          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 11b9dc15..d2944ccb 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -224,6 +224,8 @@ 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 only) Instead sending IPv6 to the server generate\n"
> +    "                   ICMPv6 host unreachable messages.\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"
> @@ -2085,6 +2087,10 @@ options_postprocess_verify_ce(const struct options *options, const struct connec
>          msg(M_USAGE, "--lladdr can only be used in --dev tap mode");
>      }
>  
> +    if (options->block_ipv6 && dev != DEV_TYPE_TUN)
> +    {
> +        msg(M_USAGE, "--block-ipv6 can be only used in --dev tun mode");
> +    }
>      /*
>       * Sanity check on MTU parameters
>       */
> @@ -6336,6 +6342,11 @@ 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 3a6c33f8..8cfc95fb 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -336,6 +336,7 @@ 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..f1bddeaa 100644
> --- a/src/openvpn/proto.c
> +++ b/src/openvpn/proto.c
> @@ -98,6 +98,51 @@ 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,

I think this is longer than 80 chars.

> +            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));

no space after the cast. And maybe you could split it on two lines.

> +
> +    }
> +
> +    /* 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);

no space after the cast

> +
> +    }
> +    for (int i = 0; i < addr_len; i += 2)
> +    {
> +        sum += (uint16_t) ((dest_addr[i] << 8) & 0xFF00) + (dest_addr[i + 1] & 0xFF);

no space after the cast

> +    }
> +
> +    /* the length of the payload */
> +    sum += (uint16_t) len_payload;

no space after the cast

> +
> +    /* The next header or proto field*/
> +    sum += (uint16_t) proto;

no space after the cast

> +
> +    /* 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);

no space after the cast

> +}
> +
>  #ifdef PACKET_TRUNCATION_CHECK
>  
>  void
> diff --git a/src/openvpn/proto.h b/src/openvpn/proto.h
> index 985aa99b..7a634490 100644
> --- a/src/openvpn/proto.h
> +++ b/src/openvpn/proto.h
> @@ -98,6 +98,7 @@ struct openvpn_iphdr {
>  #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 */

I think the constants above were all aligned - keep them that way please.

>      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

not sure why you are adding a space between the # and the word 'define',
we never do that.

> +	uint8_t		icmp6_type;
> +# define OPENVPN_ICMP6_DU_NOROUTE 0
> +# define OPENVPN_ICMP6_DU_COMMUNICATION_PROHIBTED 1

same as above

> +	uint8_t		icmp6_code;
> +	uint16_t	icmp6_cksum;
> +	uint8_t		icmp6_dataun[4];
> +};
>  
>  /*
>   * UDP header
> @@ -265,6 +284,39 @@ bool is_ipv4(int tunnel_type, struct buffer *buf);
>  
>  bool is_ipv6(int tunnel_type, struct buffer *buf);
>  
> +static inline int
> +af_addr_size(sa_family_t af)
> +{
> +    switch (af)
> +    {
> +        case AF_INET: return sizeof(struct sockaddr_in);
> +
> +        case AF_INET6: return sizeof(struct sockaddr_in6);

since you are moving this function I'd suggest to make it look slightly
better: move the return on their own line.

> +
> +        default:
> +#if 0
> +            /* could be called from socket_do_accept() with empty addr */
> +            msg(M_ERR, "Bad address family: %d\n", af);
> +            ASSERT(0);
> +#endif

... and maybe we can get rid of this #if 0 section ?

> +            return 0;
> +    }
> +}
> +
> +/**
> + *  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 calculated checksum in host order

This doc has to be completely re-aligned. Please keep ssl_backend.h as
reference.

> + */
> +uint16_t
> +ip_checksum(const sa_family_t af, const uint8_t *payload, const int len_payload, const uint8_t *src_addr,

too long line :)

> +            const uint8_t *dest_addr,  const int proto);
> +
>  #ifdef PACKET_TRUNCATION_CHECK
>  void ipv4_packet_size_verify(const uint8_t *data,
>                               const int size,
> diff --git a/src/openvpn/socket.h b/src/openvpn/socket.h
> index 479d1150..294dddef 100644
> --- a/src/openvpn/socket.h
> +++ b/src/openvpn/socket.h
> @@ -848,25 +848,6 @@ addr_inet4or6(struct sockaddr *addr)
>  
>  int addr_guess_family(sa_family_t af,const char *name);
>  
> -static inline int
> -af_addr_size(sa_family_t af)
> -{
> -    switch (af)
> -    {
> -        case AF_INET: return sizeof(struct sockaddr_in);
> -
> -        case AF_INET6: return sizeof(struct sockaddr_in6);
> -
> -        default:
> -#if 0
> -            /* could be called from socket_do_accept() with empty addr */
> -            msg(M_ERR, "Bad address family: %d\n", af);
> -            ASSERT(0);
> -#endif
> -            return 0;
> -    }
> -}
> -
>  static inline bool
>  link_socket_actual_match(const struct link_socket_actual *a1, const struct link_socket_actual *a2)
>  {
> 


I think we need to focus a bit more on style, otherwise I'll always have
something to complain about :-P


Cheers,

Patch

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index 8a987b37..bad66754 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -1240,6 +1240,42 @@  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..24c45c76 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 00112275..d3e6eede 100644
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -1194,7 +1194,8 @@  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; */
@@ -1205,6 +1206,9 @@  process_incoming_tun(struct context *c)
                                 &c->c2.n_trunc_pre_encrypt);
 #endif
 
+    }
+    if (c->c2.buf.len > 0)
+    {
         encrypt_sign(c, true);
     }
     else
@@ -1215,6 +1219,123 @@  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 contrusted.
+ *
+ *  @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 prefered 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 tests with Windows, Linux and Android (arne)
+     * 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 payload_len = min_int(
+        min_int(MAX_ICMPV6LEN, TUN_MTU_SIZE(&c->c2.frame) - icmpheader_len),
+        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 */
+    ((struct openvpn_icmp6hdr*) BPTR(outbuf))->icmp6_cksum =
+            htons(ip_checksum(AF_INET6, BPTR(outbuf), BLEN(outbuf), (const uint8_t*)&pip6out.saddr,
+                               (uint8_t*)&pip6out.daddr, OPENVPN_IPPROTO_ICMPV6));
+
+    /* 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)
 {
@@ -1236,6 +1357,10 @@  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)
     {
@@ -1271,7 +1396,7 @@  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 */
@@ -1291,6 +1416,13 @@  process_ip_header(struct context *c, unsigned int flags, struct buffer *buf)
                 {
                     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;
+                }
+
             }
         }
     }
@@ -1471,7 +1603,7 @@  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 924cc5e2..6d0414f6 100644
--- a/src/openvpn/forward.h
+++ b/src/openvpn/forward.h
@@ -251,9 +251,11 @@  bool send_control_channel_string(struct context *c, const char *str, int msgleve
 
 #define PIPV4_PASSTOS         (1<<0)
 #define PIP_MSSFIX            (1<<1)         /* v4 and v6 */
-#define PIPV4_OUTGOING        (1<<2)
+#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 3da8c110..fcb54f19 100644
--- a/src/openvpn/multi.c
+++ b/src/openvpn/multi.c
@@ -2838,7 +2838,7 @@  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 11b9dc15..d2944ccb 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -224,6 +224,8 @@  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 only) Instead sending IPv6 to the server generate\n"
+    "                   ICMPv6 host unreachable messages.\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"
@@ -2085,6 +2087,10 @@  options_postprocess_verify_ce(const struct options *options, const struct connec
         msg(M_USAGE, "--lladdr can only be used in --dev tap mode");
     }
 
+    if (options->block_ipv6 && dev != DEV_TYPE_TUN)
+    {
+        msg(M_USAGE, "--block-ipv6 can be only used in --dev tun mode");
+    }
     /*
      * Sanity check on MTU parameters
      */
@@ -6336,6 +6342,11 @@  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 3a6c33f8..8cfc95fb 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -336,6 +336,7 @@  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..f1bddeaa 100644
--- a/src/openvpn/proto.c
+++ b/src/openvpn/proto.c
@@ -98,6 +98,51 @@  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..7a634490 100644
--- a/src/openvpn/proto.h
+++ b/src/openvpn/proto.h
@@ -98,6 +98,7 @@  struct openvpn_iphdr {
 #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 +284,39 @@  bool is_ipv4(int tunnel_type, struct buffer *buf);
 
 bool is_ipv6(int tunnel_type, struct buffer *buf);
 
+static inline int
+af_addr_size(sa_family_t af)
+{
+    switch (af)
+    {
+        case AF_INET: return sizeof(struct sockaddr_in);
+
+        case AF_INET6: return sizeof(struct sockaddr_in6);
+
+        default:
+#if 0
+            /* could be called from socket_do_accept() with empty addr */
+            msg(M_ERR, "Bad address family: %d\n", af);
+            ASSERT(0);
+#endif
+            return 0;
+    }
+}
+
+/**
+ *  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 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,
diff --git a/src/openvpn/socket.h b/src/openvpn/socket.h
index 479d1150..294dddef 100644
--- a/src/openvpn/socket.h
+++ b/src/openvpn/socket.h
@@ -848,25 +848,6 @@  addr_inet4or6(struct sockaddr *addr)
 
 int addr_guess_family(sa_family_t af,const char *name);
 
-static inline int
-af_addr_size(sa_family_t af)
-{
-    switch (af)
-    {
-        case AF_INET: return sizeof(struct sockaddr_in);
-
-        case AF_INET6: return sizeof(struct sockaddr_in6);
-
-        default:
-#if 0
-            /* could be called from socket_do_accept() with empty addr */
-            msg(M_ERR, "Bad address family: %d\n", af);
-            ASSERT(0);
-#endif
-            return 0;
-    }
-}
-
 static inline bool
 link_socket_actual_match(const struct link_socket_actual *a1, const struct link_socket_actual *a2)
 {