Resend TCP segment modified to the sender

Resend TCP segment modified to the sender

[Kotatsu]

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Hi guys,
    I have a problem. I have a client (192.168.9.2) that send TCP segment to a server (192.168.11.2). Between this PC there is a Linux router that captures all the segment and do forwarding. I want that it takes a client packet (the 10th TCP data packet sended, for example), modify it as i want, and then resend it to the client with ipq_set_verdict.
Is this operation possible? Or netfilter can only send the modified packet at the server? If it's possible, which field must i modify to do this (i've tried to modify some flag but it doesn't work)?

Thanks for your help
Best regards


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Resend TCP segment modified to the sender

[Kotatsu]

Hi guys,
    I have a problem. I have a client (192.168.9.2) that send TCP segment to
a server (192.168.11.2). Between this PC there is a Linux router that
captures all the segment and do forwarding. I want that it takes a client
packet (the 10th TCP data packet sended, for example), modify it as i want,
and then resend it to the client with ipq_set_verdict.
Is this operation possible? Or netfilter can only send the modified packet
at the server? If it's possible, which field must i modify to do this (i've
tried to modify some flag but it doesn't work)?

Thanks for your help
Best regards

Re: Resend TCP segment modified to the sender

[Kotatsu]

>
> It's possible and you can modify any packet you capture. What do u mean by
> "which field"? You just need to write a couple of rules to put packets
into
> QUEUE.

I've maded a C program that take packet from the queue with libipq.h
I've a scipt that queue all the packet sended by my host:

iptables -A FORWARD -j QUEUE

My program catch this packet, see if in this TCP data packet there is a
particular sequence number (for example 10) in the data filed, and if it is
i want that netfilter resend this packet modified to the client. I want to
modify the TCP data packet from data packet to ACK with window advertise =0.
The purpouse of this is to freeze TCP on client (with window probe
thecnique). So i want to know how I must modify my packet to do this.

I've already modified a server data packet to ACK with window size=0, but
now i must reuse a client packet as a server packet.
Best regards
Marco A.

Re: Resend TCP segment modified to the sender

[aksingh]

hi,

   I am trying to do something similar, if anyone wants i can send the code
snippets. I start a telnet session between two hosts and then capture a
packet in between, and try to send back an RST segment, I do checksum
calculation and all my self. (both ip and tcp)
1. NF_IP_LOCAL_OUT queues the packet to user space
2. user space reads it using ipq_read, modifies packet, then sets a verdict
of NF_ACCEPT
3. This packet is now caught at NF_IP_POST_ROUTING, I call netif_rx for
that skb from here and return NF_STOLEN from hook call back function of
NF_IP_POST_ROUTING.
4. what happens after that is not clear to me, surely the RST i sent doesnt
reach the tcp connection it was intended for and hence the packet is
dropped  somewhere by the kernel. when and where is a problem for me, any
idea how do I fgiure that out.

Also Sven, The problem I talked about yesterday, POST_ROUTING not getting
the packet after LOCAL_OUT queued it to the user space. I overcame that
problem, but only after I started setting the mark to a particular value in
the NF_IP_LOCAL_OUT hooks function itself, before queuing it to the user
space. Earlier I was setting ipq_packet_msg->mark = THAT VALUE inside the
user level program.

Hence, these are my doubts :

   ipq_packet_msg-> mark i.e all the fields in ipq_packet msg can we modiy
   them in user space, and inject the "modified" packet back into the
   kernel ? I dont think this is possible with ipq_packet_msg->mark field,
   that is just for reading. Also, if i want to modify the packet before
   setting a verdict of NF_ACCEPT, how do I do it, the ip header starts
   from (unsigned char *)(packet+1), am I right ? Once i modify contents
   here, say interchange the src , dest ip, and then set verdict to
   NF_ACCEPT, the packet that is actually injected has the changed values.

regards
Amit




"Kotatsu" <naughtydog@virgilio.it>@lists.netfilter.org on 06/02/2004
12:09:44 AM

Sent by:    netfilter-admin@lists.netfilter.org


To:    <netfilter@lists.netfilter.org>
cc:

Subject:    Resend TCP segment modified to the sender



Hi guys,
    I have a problem. I have a  client (192.168.9.2) that send TCP segment
to a server (192.168.11.2).  Between this PC there is a Linux router that
captures all the segment  and do forwarding. I want that it takes a client
packet (the 10th TCP data  packet sended, for example), modify it as i
want, and then resend it to the  client with ipq_set_verdict.
Is this operation possible? Or netfilter  can only send the modified packet
at the server? If it's possible, which  field must i modify to do this
(i've tried to modify some flag but it doesn't  work)?

Thanks for your help
Best regards

Re: Resend TCP segment modified to the sender

[Jee J.Z.]

Hi Amit,

A few questions inline...

> hi,
>
>    I am trying to do something similar, if anyone wants i can send the
code
> snippets. I start a telnet session between two hosts and then capture a
> packet in between, and try to send back an RST segment, I do checksum
> calculation and all my self. (both ip and tcp)
> 1. NF_IP_LOCAL_OUT queues the packet to user space
> 2. user space reads it using ipq_read, modifies packet, then sets a
verdict
> of NF_ACCEPT
> 3. This packet is now caught at NF_IP_POST_ROUTING, I call netif_rx for
> that skb from here and return NF_STOLEN from hook call back function of
> NF_IP_POST_ROUTING.

Hm, I don't know much about the kernel routine, but just wonder whether
calling netif_rx means you grab the packet directly from POST ROUTING to the
first step of receiving a packet from the interface? And NF_STOLEN means you
never want the packet going to the interface?

> 4. what happens after that is not clear to me, surely the RST i sent
doesnt
> reach the tcp connection it was intended for and hence the packet is
> dropped  somewhere by the kernel. when and where is a problem for me, any
> idea how do I fgiure that out.

So your RST is supposed to go to the local machine itself?

> Also Sven, The problem I talked about yesterday, POST_ROUTING not getting
> the packet after LOCAL_OUT queued it to the user space. I overcame that
> problem, but only after I started setting the mark to a particular value
in
> the NF_IP_LOCAL_OUT hooks function itself, before queuing it to the user
> space. Earlier I was setting ipq_packet_msg->mark = THAT VALUE inside the
> user level program.

Then it should be my problem of using libnet...

> Hence, these are my doubts :
>
>    ipq_packet_msg-> mark i.e all the fields in ipq_packet msg can we modiy
>    them in user space, and inject the "modified" packet back into the
>    kernel ? I dont think this is possible with ipq_packet_msg->mark field,
>    that is just for reading.

Agree.

>    Also, if i want to modify the packet before
>    setting a verdict of NF_ACCEPT, how do I do it, the ip header starts
>    from (unsigned char *)(packet+1), am I right ?

if
ipq_packet_msg *m = ipq_get_packet(buf)
packet = (unsigned char *)m + sizeof(*m)
then
ip header starts at *packet, not *(packet+1).

>    Once i modify contents
>    here, say interchange the src , dest ip, and then set verdict to
>    NF_ACCEPT, the packet that is actually injected has the changed values.

Yes, just figure out the modified packet length and ipq_set_verdict(handle,
m->packet_id, packet_len, modified_packet).

Jee

> regards
> Amit
>
>
>
>
> "Kotatsu" <naughtydog@virgilio.it>@lists.netfilter.org on 06/02/2004
> 12:09:44 AM
>
> Sent by:    netfilter-admin@lists.netfilter.org
>
>
> To:    <netfilter@lists.netfilter.org>
> cc:
>
> Subject:    Resend TCP segment modified to the sender
>
>
>
> Hi guys,
> I have a problem. I have a  client (192.168.9.2) that send TCP segment
> to a server (192.168.11.2).  Between this PC there is a Linux router that
> captures all the segment  and do forwarding. I want that it takes a client
> packet (the 10th TCP data  packet sended, for example), modify it as i
> want, and then resend it to the  client with ipq_set_verdict.
> Is this operation possible? Or netfilter  can only send the modified
packet
> at the server? If it's possible, which  field must i modify to do this
> (i've tried to modify some flag but it doesn't  work)?
>
> Thanks for your help
> Best regards
>
>
>
>
>
>

Re: Resend TCP segment modified to the sender

[aksingh]

Hi Jee

1. Yes, I return NF_STOLEN cos I dont wat it to go the interface and call
netif_rx cos I want it to appear as it were coming from the interface. this
I do to prevent a loop in my dirver, if I do netif_rx(skb) , the
skb->nfmark field wont be overwritten, otherwise if I allow it to go the
interface by NOT returning NF_STOLEN, a new skb would be allocated for it
and hence ill lose my nfmark field.
2. Yes the RST is supposed to go the local machine itself.
3. So u mean I cannot change m->mark field, such that when the packet gets
reinjected into the kernel the mark field is what I updated it to ?

regards
Amit




"Jee J.Z." <jz105@york.ac.uk> on 06/02/2004 03:58:38 PM

To:    Amit Kumar Singh/HSS@HSS
cc:    <netfilter@lists.netfilter.org>

Subject:    Re: Resend TCP segment modified to the sender


Hi Amit,

A few questions inline...

> hi,
>
>    I am trying to do something similar, if anyone wants i can send the
code
> snippets. I start a telnet session between two hosts and then capture a
> packet in between, and try to send back an RST segment, I do checksum
> calculation and all my self. (both ip and tcp)
> 1. NF_IP_LOCAL_OUT queues the packet to user space
> 2. user space reads it using ipq_read, modifies packet, then sets a
verdict
> of NF_ACCEPT
> 3. This packet is now caught at NF_IP_POST_ROUTING, I call netif_rx for
> that skb from here and return NF_STOLEN from hook call back function of
> NF_IP_POST_ROUTING.

Hm, I don't know much about the kernel routine, but just wonder whether
calling netif_rx means you grab the packet directly from POST ROUTING to
the
first step of receiving a packet from the interface? And NF_STOLEN means
you
never want the packet going to the interface?

> 4. what happens after that is not clear to me, surely the RST i sent
doesnt
> reach the tcp connection it was intended for and hence the packet is
> dropped  somewhere by the kernel. when and where is a problem for me, any
> idea how do I fgiure that out.

So your RST is supposed to go to the local machine itself?

> Also Sven, The problem I talked about yesterday, POST_ROUTING not getting
> the packet after LOCAL_OUT queued it to the user space. I overcame that
> problem, but only after I started setting the mark to a particular value
in
> the NF_IP_LOCAL_OUT hooks function itself, before queuing it to the user
> space. Earlier I was setting ipq_packet_msg->mark = THAT VALUE inside the
> user level program.

Then it should be my problem of using libnet...

> Hence, these are my doubts :
>
>    ipq_packet_msg-> mark i.e all the fields in ipq_packet msg can we
modiy
>    them in user space, and inject the "modified" packet back into the
>    kernel ? I dont think this is possible with ipq_packet_msg->mark
field,
>    that is just for reading.

Agree.

>    Also, if i want to modify the packet before
>    setting a verdict of NF_ACCEPT, how do I do it, the ip header starts
>    from (unsigned char *)(packet+1), am I right ?

if
ipq_packet_msg *m = ipq_get_packet(buf)
packet = (unsigned char *)m + sizeof(*m)
then
ip header starts at *packet, not *(packet+1).

>    Once i modify contents
>    here, say interchange the src , dest ip, and then set verdict to
>    NF_ACCEPT, the packet that is actually injected has the changed
values.

Yes, just figure out the modified packet length and ipq_set_verdict(handle,
m->packet_id, packet_len, modified_packet).

Jee

> regards
> Amit
>
>
>
>
> "Kotatsu" <naughtydog@virgilio.it>@lists.netfilter.org on 06/02/2004
> 12:09:44 AM
>
> Sent by:    netfilter-admin@lists.netfilter.org
>
>
> To:    <netfilter@lists.netfilter.org>
> cc:
>
> Subject:    Resend TCP segment modified to the sender
>
>
>
> Hi guys,
> I have a problem. I have a  client (192.168.9.2) that send TCP segment
> to a server (192.168.11.2).  Between this PC there is a Linux router that
> captures all the segment  and do forwarding. I want that it takes a
client
> packet (the 10th TCP data  packet sended, for example), modify it as i
> want, and then resend it to the  client with ipq_set_verdict.
> Is this operation possible? Or netfilter  can only send the modified
packet
> at the server? If it's possible, which  field must i modify to do this
> (i've tried to modify some flag but it doesn't  work)?
>
> Thanks for your help
> Best regards
>
>
>
>
>
>

Re: Resend TCP segment modified to the sender

[Jee J.Z.]

Hi Amit,

>
> Hi Jee
>
> 1. Yes, I return NF_STOLEN cos I dont wat it to go the interface and call
> netif_rx cos I want it to appear as it were coming from the interface.
this
> I do to prevent a loop in my dirver, if I do netif_rx(skb) , the
> skb->nfmark field wont be overwritten, otherwise if I allow it to go the
> interface by NOT returning NF_STOLEN, a new skb would be allocated for it
> and hence ill lose my nfmark field.
> 2. Yes the RST is supposed to go the local machine itself.
> 3. So u mean I cannot change m->mark field, such that when the packet gets
> reinjected into the kernel the mark field is what I updated it to ?

I didn't try it. I guess you cannot. I think m->any is just for our
information and actually these parameters are assigned and maintained by the
kernel. I even think ipq_set_verdict never passes any of these m->parameters
other than m->packet_id to the kernel. And m->packet_id is just used for
mapping a certain packet block in the kernel, no modifications are allowed.
I tried to modify m->packet_id and the kernel just got confused what that
packet was and error occured --- but this is obvious and not a convincing
evidence. Please anybody correct me if I am wrong.

Jee

> regards
> Amit
>
>
>
>
> "Jee J.Z." <jz105@york.ac.uk> on 06/02/2004 03:58:38 PM
>
> To:    Amit Kumar Singh/HSS@HSS
> cc:    <netfilter@lists.netfilter.org>
>
> Subject:    Re: Resend TCP segment modified to the sender
>
>
> Hi Amit,
>
> A few questions inline...
>
> > hi,
> >
> >    I am trying to do something similar, if anyone wants i can send the
> code
> > snippets. I start a telnet session between two hosts and then capture a
> > packet in between, and try to send back an RST segment, I do checksum
> > calculation and all my self. (both ip and tcp)
> > 1. NF_IP_LOCAL_OUT queues the packet to user space
> > 2. user space reads it using ipq_read, modifies packet, then sets a
> verdict
> > of NF_ACCEPT
> > 3. This packet is now caught at NF_IP_POST_ROUTING, I call netif_rx for
> > that skb from here and return NF_STOLEN from hook call back function of
> > NF_IP_POST_ROUTING.
>
> Hm, I don't know much about the kernel routine, but just wonder whether
> calling netif_rx means you grab the packet directly from POST ROUTING to
> the
> first step of receiving a packet from the interface? And NF_STOLEN means
> you
> never want the packet going to the interface?
>
> > 4. what happens after that is not clear to me, surely the RST i sent
> doesnt
> > reach the tcp connection it was intended for and hence the packet is
> > dropped  somewhere by the kernel. when and where is a problem for me,
any
> > idea how do I fgiure that out.
>
> So your RST is supposed to go to the local machine itself?
>
> > Also Sven, The problem I talked about yesterday, POST_ROUTING not
getting
> > the packet after LOCAL_OUT queued it to the user space. I overcame that
> > problem, but only after I started setting the mark to a particular value
> in
> > the NF_IP_LOCAL_OUT hooks function itself, before queuing it to the user
> > space. Earlier I was setting ipq_packet_msg->mark = THAT VALUE inside
the
> > user level program.
>
> Then it should be my problem of using libnet...
>
> > Hence, these are my doubts :
> >
> >    ipq_packet_msg-> mark i.e all the fields in ipq_packet msg can we
> modiy
> >    them in user space, and inject the "modified" packet back into the
> >    kernel ? I dont think this is possible with ipq_packet_msg->mark
> field,
> >    that is just for reading.
>
> Agree.
>
> >    Also, if i want to modify the packet before
> >    setting a verdict of NF_ACCEPT, how do I do it, the ip header starts
> >    from (unsigned char *)(packet+1), am I right ?
>
> if
> ipq_packet_msg *m = ipq_get_packet(buf)
> packet = (unsigned char *)m + sizeof(*m)
> then
> ip header starts at *packet, not *(packet+1).
>
> >    Once i modify contents
> >    here, say interchange the src , dest ip, and then set verdict to
> >    NF_ACCEPT, the packet that is actually injected has the changed
> values.
>
> Yes, just figure out the modified packet length and
ipq_set_verdict(handle,
> m->packet_id, packet_len, modified_packet).
>
> Jee
>
> > regards
> > Amit
> >
> >
> >
> >
> > "Kotatsu" <naughtydog@virgilio.it>@lists.netfilter.org on 06/02/2004
> > 12:09:44 AM
> >
> > Sent by:    netfilter-admin@lists.netfilter.org
> >
> >
> > To:    <netfilter@lists.netfilter.org>
> > cc:
> >
> > Subject:    Resend TCP segment modified to the sender
> >
> >
> >
> > Hi guys,
> > I have a problem. I have a  client (192.168.9.2) that send TCP segment
> > to a server (192.168.11.2).  Between this PC there is a Linux router
that
> > captures all the segment  and do forwarding. I want that it takes a
> client
> > packet (the 10th TCP data  packet sended, for example), modify it as i
> > want, and then resend it to the  client with ipq_set_verdict.
> > Is this operation possible? Or netfilter  can only send the modified
> packet
> > at the server? If it's possible, which  field must i modify to do this
> > (i've tried to modify some flag but it doesn't  work)?
> >
> > Thanks for your help
> > Best regards
> >
> >
> >
> >
> >
> >
>
>
>
>
>