Teardown tcp connection что означает

Не могу понять log файл

И последнее, пожалуйста не пишите что-то типа "Ну здесь ведь ясно написано . ". Мне не ясно. Помогите разобраться в этом.

Не могу создать LOG-файл
Добрый вечер. Нужна помощь, а именно — в домашнем задании необходимо создать LOG-файл, в который.

Файл: Не могу понять, почему данные не записываются в файл, он остается пустым.
Здравствуйте , помогите пожалуйста , с записью данных в файл. #include "stdafx.h".

bat файл, не могу понять
Пакетный файл предназначенный для углубленный проверки жесткого диска с созданием файла отчета.

Не могу понять как прочитать файл
Вродь как двоичный файл, но прочитать не могу его.

Cisco ASA 0 SYN Timeout

I was having issues connecting to my load balancers and we seeing in the Cisco logs that the TCP connection was being built but then getting torn down with the following error:

The syslog message indicates the connection closed because the of SYN timeout. This tells the administrator that no application X server responses were received by the ASA. Syslog message termination reasons can vary.

The SYN timeout gets logged because of a forced connection termination after 30 seconds that occurs after the three-way handshake completion. This issue usually occurs if the server fails to respond to a connection request, and, in most cases, is not related to the configuration on PIX/ASA.

OVS Conntrack Tutorial¶

OVS can be used with the Connection tracking system where OpenFlow flow can be used to match on the state of a TCP, UDP, ICMP, etc., connections. (Connection tracking system supports tracking of both statefull and stateless protocols)

This tutorial demonstrates how OVS can use the connection tracking system to match on the TCP segments from connection setup to connection tear down. It will use OVS with the Linux kernel module as the datapath for this tutorial. (The datapath that utilizes the openvswitch kernel module to do the packet processing in the Linux kernel) It was tested with the “master” branch of Open vSwitch.

Definitions¶

conntrack: is a connection tracking module for stateful packet inspection.

pipeline: is the packet processing pipeline which is the path taken by the packet when traversing through the tables where the packet matches the match fields of a flow in the table and performs the actions present in the matched flow.

network namespace: is a way to create virtual routing domains within a single instance of linux kernel. Each network namespace has it’s own instance of network tables (arp, routing) and certain interfaces attached to it.

flow: used in this tutorial refers to the OpenFlow flow which can be programmed using an OpenFlow controller or OVS command line tools like ovs-ofctl which is used here. A flow will have match fields and actions.

Conntrack Related Fields¶

Match Fields¶

OVS supports following match fields related to conntrack:

1. ct_state: The state of a connection matching the packet. Possible values:

• new
• est
• rel
• rpl
• inv
• trk
• snat
• dnat

Each of these flags is preceded by either a “+” for a flag that must be set, or a “-” for a flag that must be unset. Multiple flags can also be specified e.g. ct_state=+trk+new. We will see the usage of some of these flags below. For a detailed description, please see the OVS fields documentation at: http://openvswitch.org/support/dist-docs/ovs-fields.7.txt

2. ct_zone: A zone is an independent connection tracking context which can be set by a ct action. A 16-bit ct_zone set by the most recent ct action (by an OpenFlow flow on a conntrack entry) can be used as a match field in another flow entry.

3. ct_mark: The 32-bit metadata committed, by an action within the exec parameter to the ct action, to the connection to which the current packet belongs.

4. ct_label: The 128-bit label committed by an action within the exec parameter to the ct action, to the connection to which the current packet belongs.

5. ct_nw_src / ct_ipv6_src: Matches IPv4/IPv6 conntrack original direction tuple source address.

6. ct_nw_dst / ct_ipv6_dst: Matches IPv4/IPv6 conntrack original direction tuple destination address.

7. ct_nw_proto: Matches conntrack original direction tuple IP protocol type.

8. ct_tp_src: Matches on the conntrack original direction tuple transport source port.

9. ct_tp_dst: Matches on the conntrack original direction tuple transport destination port.

Actions¶

OVS supports “ct” action related to conntrack.

The ct action sends the packet through the connection tracker.

The following arguments are supported:

1. commit: Commit the connection to the connection tracking module which will be stored beyond the lifetime of packet in the pipeline.

2. force: The force flag may be used in addition to commit flag to effectively terminate the existing connection and start a new one in the current direction.

3. table=number: Fork pipeline processing in two. The original instance of the packet will continue processing the current actions list as an untracked packet. An additional instance of the packet will be sent to the connection tracker, which will be re-injected into the OpenFlow pipeline to resume processing in table number, with the ct_state and other ct match fields set.

4. zone=value OR zone=src[start..end]: A 16-bit context id that can be used to isolate connections into separate domains, allowing over‐lapping network addresses in different zones. If a zone is not provided, then the default is to use zone zero.

5. exec([action][,action…]): Perform restricted set of actions within the context of connection tracking. Only actions which modify the ct_mark or ct_label fields are accepted within the exec action.

6. alg=<ftp/tftp>: Specify alg (application layer gateway) to track specific connection types.

7. nat: Specifies the address and port translation for the connection being tracked.

Sample Topology¶

This tutorial uses the following topology to carry out the tests.

The steps for creation of the setup are mentioned below.

Create “left” network namespace:

Create “right” network namespace:

Create first pair of veth interfaces:

Add veth_l1 to “left” network namespace:

Create second pair of veth interfaces:

Add veth_r1 to “right” network namespace:

Create a bridge br0:

Add veth_l0 and veth_r0 to br0:

Packets generated with src/dst IP set to 192.168.0.X / 10.0.0.X in the “left” and the inverse in the “right” namespaces will appear to OVS as hosts in two networks (192.168.0.X and 10.0.0.X) communicating with each other. This is basically a simulation of two networks / subnets with hosts communicating with each other with OVS in middle.

Tool used to generate TCP segments¶

You can use scapy to generate the TCP segments. We used scapy on Ubuntu 16.04 for the steps carried out in this testing. (Installation of scapy is not discussed and is out of scope of this document.)

You can keep two scapy sessions active on each of the namespaces:

Note: In case you encounter this error:

run the command:

Matching TCP packets¶

TCP Connection setup¶

Two simple flows can be added in OVS which will forward packets from “left” to “right” and from “right” to “left”:

Instead of adding these two flows, we will add flows to match on the states of the TCP segments.

We will send the TCP connection setup segments namely: syn, syn-ack and ack between hosts 192.168.0.2 in the “left” namespace and 10.0.0.2 in the “right” namespace.

First, let’s add a flow to start “tracking” a packet received at OVS.

How do we start tracking a packet?

To start tracking a packet, it first needs to match a flow, which has action as “ct”. This action sends the packet through the connection tracker. To identify that a packet is an “untracked” packet, the ct_state in the flow match field must be set to “-trk”, which means it is not a tracked packet. Once the packet is sent to the connection tracker, then only we will know about its conntrack state. (i.e. whether this packet represents start of a new connection or the packet belongs to an existing connection or it is a malformed packet and so on.)

Let’s add that flow:

A TCP syn packet sent from “left” namespace will match flow #1 because the packet is coming to OVS from veth_l0 port and it is not being tracked. This is because the packet just entered OVS. When a packet enters a namespace for the first time, a new connection tracker context is entered, hence, the packet will be initially “untracked” in that namespace. When a packet (re)enters the same datapath that it already belongs to there is no need to discard the namespace and other information associated with the conntrack flow. In this case the packet will remain in the tracked state. If the namespace has changed then it is discarded and a new connection tracker is created since connection tracking information is logically separate for different namespaces. The flow will send the packet to the connection tracker due to the action “ct”. Also “table=0” in the “ct” action forks the pipeline processing in two. The original instance of packet will continue processing the current action list as untracked packet. (Since there are no actions after this, the original packet gets dropped.) The forked instance of the packet will be sent to the connection tracker, which will be re-injected into the OpenFlow pipeline to resume processing in table number, with the ct_state and other ct match fields set. In this case, the packet with the ct_state and other ct match fields comes back to table 0.

Next, we add a flow to match on the packet coming back from conntrack:

Now that the packet is coming back from conntrack, the ct_state would have the “trk” set. Also, if this is the first packet of the TCP connection, the ct_state “new” would be set. (Which is the condition here as there does not exist any TCP connection between hosts 192.168.0.2 and 10.0.0.2) The ct argument “commit” will commit the connection to the connection tracking module. The significance of this action is that the information about the connection will now be stored beyond the lifetime of the packet in the pipeline.

Let’s send the TCP syn segment using scapy (at the “left” scapy session) (flags=0x02 is syn):

This packet will match flow #1 and flow #2.

The conntrack module will now have an entry for this connection:

Note: At this stage, if the TCP syn packet is re-transmitted, it will again match flow #1 (since a new packet is untracked) and it will match flow #2. The reason it will match flow #2 is that although conntrack has information about the connection, but it is not in “ESTABLISHED” state, therefore it matches the “new” state again.

Next for the TCP syn-ack from the opposite/server direction, we need following flows at OVS:

flow #3 matches untracked packets coming back from server (10.0.0.2) and sends this to conntrack. (Alternatively, we could have also combined flow #1 and flow #3 into one flow by not having the “in_port” match)

The syn-ack packet which has now gone through the conntrack has the ct_state of “est”.

Note: Conntrack puts the ct_state of the connection to “est” state when it sees bidirectional traffic, but till it does not get the third ack from client, it puts a short cleanup timer on the conntrack entry.

Sending TCP syn-ack segment using scapy (at the “right” scapy session) (flags=0x12 is ack and syn):

This packet will match flow #3 and flow #4.

The conntrack state is “ESTABLISHED” on receiving just syn and syn-ack packets, but at this point if it does not receive the third ack (from client), the connection gets cleared up from conntrack quickly.

Next, for a TCP ack from client direction, we can add following flows to match on the packet:

Send the third TCP ack segment using scapy (at the “left” scapy session) (flags=0x10 is ack):

This packet will match on flow #1 and flow #5.

The conntrack state stays in “ESTABLISHED” state, but now since it has received the ack from client, it will stay in this state for a longer time even without receiving any data on this connection.

TCP Data¶

When a data segment, carrying one byte of TCP payload, is sent from 192.168.0.2 to 10.0.0.2, the packet carrying the segment would hit flow #1 and then flow #5.

Send a TCP segment with one byte data using scapy (at the “left” scapy session) (flags=0x10 is ack):

Send the TCP ack for the above segment using scapy (at the “right” scapy session) (flags=0x10 is ack):

The acknowledgement for the data would hit flow #3 and flow #4.

TCP Connection Teardown¶

There are different ways to tear down TCP connection. We will tear down the connection by sending “fin” from client, “fin-ack” from server followed by the last “ack” by client.

All the packets from client to server would hit flow #1 and flow #5. All the packets from server to client would hit flow #3 and flow #4. Interesting point to note is that even when the TCP connection is going down, all the packets (which are actually tearing down the connection) still hits “+est” state. A packet, for which the conntrack entry is or was in “ESTABLISHED” state, would continue to match “+est” ct_state in OVS.

Note: In fact, when the conntrack connection state is in “TIME_WAIT” state (after all the TCP fins and their acks are exchanged), a re-transmitted data packet (from 192.168.0.2 -> 10.0.0.2), still hits flows #1 and #5.

Sending TCP fin segment using scapy (at the “left” scapy session) (flags=0x11 is ack and fin):

This packet hits flow #1 and flow #5.

Sending TCP fin-ack segment using scapy (at the “right” scapy session) (flags=0x11 is ack and fin):

This packet hits flow #3 and flow #4.

Sending TCP ack segment using scapy (at the “left” scapy session) (flags=0x10 is ack):

This packet hits flow #1 and flow #5.

Summary¶

Following table summarizes the TCP segments exchanged against the flow match fields

TCP Segment	ct_state(flow#)
Connection Setup
192.168.0.2 → 10.0.0.2 [SYN] Seq=0	-trk(#1) then +trk+new(#2)
10.0.0.2 → 192.168.0.2 [SYN, ACK] Seq=0 Ack=1	-trk(#3) then +trk+est(#4)
192.168.0.2 → 10.0.0.2 [ACK] Seq=1 Ack=1	-trk(#1) then +trk+est(#5)
Data Transfer
192.168.0.2 → 10.0.0.2 [ACK] Seq=1 Ack=1	-trk(#1) then +trk+est(#5)
10.0.0.2 → 192.168.0.2 [ACK] Seq=1 Ack=2	-trk(#3) then +trk+est(#4)
Connection Teardown
192.168.0.2 → 10.0.0.2 [FIN, ACK] Seq=2 Ack=1	-trk(#1) then +trk+est(#5)
10.0.0.2 → 192.168.0.2 [FIN, ACK] Seq=1 Ack=3	-trk(#3) then +trk+est(#4)
192.168.0.2 → 10.0.0.2 [ACK] Seq=3 Ack=2	-trk(#1) then +trk+est(#5)

Note: Relative sequence number and acknowledgement numbers are shown as captured from tshark.

Objective behind TCP connection termination

The socket connection facilitates continuous data exchange and establishes a session between nodes, while TCP connection ensures reliable transmission of data segments between nodes. In my perspective, socket connection relies on TCP connection, but they are distinct from each other.

Purpose of TCP connection teardown

The Tcp protocol is connection-oriented, which means it establishes a virtual connection. However, it raises the question of why we should terminate the connection once the data exchange is complete.

Solution:

TCP, being a byte stream, requires the peers to be aware of the end of the stream. This is accomplished through the use of TCP segments, which inform the peer that all data in the stream has been received and no further data will be transmitted.

21 — TCP Connection Establishment and Teardown

TCP-Connection — keep up or reconnect?

I would like to inquire about TCP-IP communication in general.

Currently, my aim is to establish a brief connection between an ATMega and a Raspberry Pi. I plan on periodically transmitting data, such as 100 bytes, every 5 minutes using the TCP/IP Protocol.

Is it logical to maintain the connection, or should I establish a new connection for every dataset?

Thanks for your help.

Solution:

My inclination is to maintain the TCP connection instead of creating a new one each time. """.

There are several reasons for this. Firstly, using the same connection eliminates the need to send SYN-based messages (such as TCP handshake ) and FIN-based messages for teardown. This is particularly advantageous in your case, where transmitting 100 bytes every 5 minutes could incur more overhead from SYN/FIN messages. Secondly, if the connection is already open, you can save time by avoiding the need for reconnection. Thirdly, every time you initiate a new connection, TCP may go to slow-start and start its send window with 1 MSS. While this may not be an issue with 100 bytes, if you need to send more, reusing an existing existing connection would allow TCP to use the current window.

In addition, it should be noted that an open connection only utilizes resources in terms of the ports held on both devices, without consuming any bandwidth or other resources. Essentially, any TCP connection that remains unclosed after being opened is still considered open, unless there are unintentional disconnections, etc. Moreover, whether you choose to keep the connection open or reopen it does not affect the detection of dropped connections. In either case, you will receive a similar error message if the connection has been interrupted during that time.

Cisco ASA: which side is "inside" with same security-level?

I'm experiencing a problem where an FWSM is seemingly terminating RDP sessions after an unpredictable duration, and the log entry appears as follows:

I comprehend that in this specific firewall, when both interfaces have a security-level of 0, the term " TCP Reset -I" signifies the reception of a TCP Reset on the 'inside' interface. However, I am unsure which interface is actually considered the inside.

Solution:

I comprehend that the term "TCP Reset-I" refers to the reception of a TCP Reset on the 'inside' interface. However, in this specific firewall, both interfaces possess a security level of 0. Therefore, it is unclear which interface is considered the inside.

For the past 30 minutes, I've been reviewing the PIX and FWSM documentation, but I haven't come across a clear source explaining the behavior of the PIX/FWSM in this particular scenario.

Regrettably, I don't believe there is a foolproof method to determine the source of the reset without a packet capture .

FYI, the Cisco ASA flow-drop rules suggest that TCP Reset-I is applicable to same-security traffic. Given the close relationship between the FWSM and ASA, it is highly likely that the syslogs do not provide sufficient information in this particular scenario.

Any difference between socket connection and tcp connection?

Do these 2 concepts denote the same thing or do they possess distinction?

From my perspective, there is a distinction between them, where a socket connection relies on a TCP connection. A socket comprises an IP address and port, allowing it to connect exclusively to another socket. In contrast, an IP address and port on the same machine can establish connections with numerous other IP addresses and ports using TCP. Is my understanding correct?

Solution 1:

TCP/IP is a communication protocol stack, where a socket serves as an endpoint ( bidirectional) communication . While a socket can use protocols other than TCP, TCP-based sockets are commonly used. The term socket is also used to describe the operating system's API, such as the Winsock API on Windows, which facilitates connections over the TCP/IP stack.

The operating system manages the ports, ensuring that each application has a unique mapping to a socket. """.

For additional information, refer to the following links: http://en.wikipedia.org/wiki/Internet_socket and http://en.wikipedia.org/wiki/Winsock.

Solution 2:

The Socket protocol, located at layer 5 in the OSI Model, is independent of the underlying layers such as TCP, UDP or MPTCP, which are protocols at the Transport layer (layer 4). Socket connections facilitate the continuous exchange of data between nodes, establishing a session between them. In contrast, TCP connections ensure the reliable transmission of data segments between nodes.

Solution 3:

Sockets, which serve as an application programming interface (API) for communication, typically involve library functions that your application will link with, system calls, and the implementation within the operating system kernel. The two most widely used APIs for sockets are Berkely socket and Winsock (Windows).

There are other types of socket classifications based on the access they provide.

• To establish TCP connections , TCP sockets are used.
• UDP sockets are used for facilitating UDP communication.
• packet socket s: for immediate entry to layer 2
• Unix domain sockets allow for inter-process communication (IPC) by using files within the same node.
• Utilize a raw socket to directly access the IP layer.
• Routing sockets
• SCTP sockets
• Other types of sockets

TCP, UDP, and Unix domain sockets are available in Linux, Unix, and Windows operating systems. However, I am unsure if the other socket types mentioned earlier are also supported in Windows.

A TCP connection is a concept in TCP where it links two endpoints, typically two processes (or one process to itself). It is defined by the combination of IP addresses and ports (IPAddress1, Port1, IPAddress2, Port2). The establishment of a TCP connection occurs following the TCP 3-way handshake.

A TCP socket is established by the elements of a TCP connection between two processes, namely IPAddress1, Port1, IPAddress2, and Port2. Apart from the regular socket, there is also a listening socket, which enables a process to receive incoming connections from other processes via the network.

Please refer to the webpage on Berkeley sockets available at the following URL: "https://en.wikipedia.org/wiki/Berkeley_sockets".

Please refer to the Winsock page on Wikipedia at https://en.wikipedia.org/wiki/Winsock.

Solution 4:

A socket connection refers to the connection between two peers, where the protocol used can be either TCP or UDP. Therefore, the term "connection" is a generic term that does not specify the type of connection.

The mention of a TCP connection denotes the connection of two nodes using the TCP protocol.