I have TCP connection between two applications and it gets dropped during certain time of the day for every couple of minutes .I took wireshark traces and see [FIN ACK] from one of the server and it matches with disconnect time . I am not seeing [FIN] at all
everytime disconnect happens I see the following trace. Is there anything am missing
Related
I am using ubuntu 20 and I am connected with wifi to the hotspot of a Galaxy A20e running android 12 which is receiving 4G. I have a connection problem that is not occuring when using my ADSL connection.
Test description : I opened a dozen of shells and established ssh connections to a dozen of servers and I just wait without interacting with the shells.
After some time (usually 5 to 10 minutes), I observe a "client_loop: send disconnect: Broken pipe ssh" message on some of the shells, then after a longer time, I observe the same message on some other shells, until after maybe one hour, all connections are gone.
Using wireshark, I have filtered on packets exchanged with one of the servers and I notice no packets exchanged except the "keep alive" signal that is send from my computer every 5 minutes, (which is conform to my ssh client configuration). At some point, in response to one of those signal, the server returns RST ACK which means that the connection has been interrupted.
I tried to find the root cause of the connection cut by using pings but I did not find anything special during the 5 minutes window when the connection cut happens. Only thing I noticed is that some pings are sometimes not received.
Any idea ?
Thanks
This question belongs to https://serverfault.com/
Try adding this to your .ssh/config:
Host *
ServerAliveInterval 60
ServerAliveCountMax 5
Most probably your 4G router or your 4G operator terminate idle connections.
When I'm a Student, being taught there is 4 process in stop http connection.
However today I test only 3 process. It's Wireshark merge data transition?
You were likely taught that TCP connections require a four way close: FIN/ACK -> ACK, FIN/ACK -> ACK. This is true, but it does not have to take four packets to do it.
In the case that you present, the 192.168.0.106 host begins to close with a FIN/ACK. The other end of the connection, rather than simply ACKing this, takes the opportunity to begin closing as well. So, when it responds with a FIN/ACK, it is both ACKing the FIN that it received and beginning its own close. The final packet is the acknowledgement of the FIN from the 211 host.
What this means is that, in this case, only three packets were used, but we still had a FIN from host A that was acknowledged and a FIN from host B that was acknowledged. That is really the only requirement in the protocol.
We are doing some load testing on our servers and I'm using tshark to capture some data to a pcap file then using the wireshark GUI to see what errors or warnings are showing up by going to Analyze -> expert Info with my pcap loaded in..
I'm seeing various things that I'm not sure or do not completely understand yet..
Under Warnings I have:
779 Warnings for TCP: ACKed segment that wasn't captured (common at capture start)
446 TCP: Previous segment not captured (common at capture start)
An example is :
40292 0.000 xxx xxx TCP 90 [TCP ACKed unseen segment] [TCP Previous segment not captured] 11210 > 37586 [PSH, ACK] Seq=3812 Ack=28611 Win=768 Len=24 TSval=199317872 TSecr=4506547
We also ran the pcap file though a nice command that creates a command line column of data
command
tshark -i 1 -w file.pcap -c 500000
basically just saw a few things in the tcp.analysis.lost_segment column but not many..\
Anyone enlighten what might be going on? tshark not able to keep up with writing data, some other issue? False positive?
That very well may be a false positive. Like the warning message says, it is common for a capture to start in the middle of a tcp session. In those cases it does not have that information. If you are really missing acks then it is time to start looking upstream from your host for where they are disappearing. It is possible that tshark can not keep up with the data and so it is dropping some metrics. At the end of your capture it will tell you if the "kernel dropped packet" and how many. By default tshark disables dns lookup, tcpdump does not. If you use tcpdump you need to pass in the "-n" switch. If you are having a disk IO issue then you can do something like write to memory /dev/shm. BUT be careful because if your captures get very large then you can cause your machine to start swapping.
My bet is that you have some very long running tcp sessions and when you start your capture you are simply missing some parts of the tcp session due to that. Having said that, here are some of the things that I have seen cause duplicate/missing acks.
Switches - (very unlikely but sometimes they get in a sick state)
Routers - more likely than switches, but not much
Firewall - More likely than routers. Things to look for here are resource exhaustion (license, cpu, etc)
Client side filtering software - antivirus, malware detection etc.
Another cause of "TCP ACKed Unseen" is the number of packets that may get dropped in a capture. If I run an unfiltered capture for all traffic on a busy interface, I will sometimes see a large number of 'dropped' packets after stopping tshark.
On the last capture I did when I saw this, I had 2893204 packets captured, but once I hit Ctrl-C, I got a 87581 packets dropped message. Thats a 3% loss, so when wireshark opens the capture, its likely to be missing packets and report "unseen" packets.
As I mentioned, I captured a really busy interface with no capture filter, so tshark had to sort all packets, when I use a capture filter to remove some of the noise, I no longer get the error.
Acked Unseen sampleHi guys! Just some observations from what I just found in my capture:
On many occasions, the packet capture reports “ACKed segment that wasn't captured” on the client side, which alerts of the condition that the client PC has sent a data packet, the server acknowledges receipt of that packet, but the packet capture made on the client does not include the packet sent by the client
Initially, I thought it indicates a failure of the PC to record into the capture a packet it sends because “e.g., machine which is running Wireshark is slow” (https://osqa-ask.wireshark.org/questions/25593/tcp-previous-segment-not-captured-is-that-a-connectivity-issue)
However, then I noticed every time I see this “ACKed segment that wasn't captured” alert I can see a record of an “invalid” packet sent by the client PC
In the capture example above, frame 67795 sends an ACK for 10384
Even though wireshark reports Bogus IP length (0), frame 67795 is
reported to have length 13194
Frame 67800 sends an ACK for 23524
10384+13194 = 23578
23578 – 23524 = 54
54 is in fact length of the
Ethernet / IP / TCP headers (14 for Ethernt, 20 for IP, 20 for TCP)
So in fact, the frame 67796 does represent a large TCP packets (13194
bytes) which operating system tried to put on the wore
NIC driver will fragment it into smaller 1500 bytes pieces in order to transmit over the network
But Wireshark running on my PC fails to understand it is a valid packet and parse it. I believe Wireshark running on 2012 Windows
server reads these captures correctly
So after all, these “Bogus IP
length” and “ACKed segment that wasn't captured” alerts were in fact
false positives in my case
I just came across this and would like to share my observation of TCP ACKed unseen segment. In my case the client was trying to initiate connection on same source port and destination port it used previously and thus the server was confused and replied with old TCP SEQ number saying I havent seen this new packet
Do long BPF filters slow down tcpdump?
I replay a packet trace where all the packets have ttl=k and wait for ICMP messages back. What I've been noticing is that if I use the following filter (on eth0):
(ip and ip[8]=$k and src host $myAddress) or (icmp and dst host $myAddress and icmp[0]=11)
...I always miss 20-30 packets among the sent packets, whereas if I just do:
ip
... and then do the exact above filtering offline on the capture file, I find all the packets I had sent.
Is this a known behaviour?
If tcpdump is not fast enough to pop out captured packets from the queue, the kernel could drop some of them.
Look at the "XXXX packets dropped by kernel" message at the end of the dump to see if effectively some of them is lost.
Ensure to add the -n option to the command line. This will avoid DNS resolving and it will speed up a little (depending on your network)
I am sending HTTP requests from IP_ADDR1 to IP_ADDR2. I observed that HTTP requests are not reaching to application level. When I take wireshark logs I noticed some issue at TCP level. What are these issue? when this occurs ? How to get rid of this? Attaching the Wireshark snapshot here.
'TCP port number reused' means that it saw a successful connection handshake, then the client sent another SYN packet with the same port numbers. If the client hadn't already acknowledged the SYN-ACK, this would have been reported as a retransmission. But since it did acknowlege the SYN-ACK, it shouldn't need to retransmit the SYN. This could mean that something on your network is duplicating packets.
'TCP out-of-order' means that the packets aren't being received in the order that their sequence numbers indicate. It might be a side effect of the duplicate packet that's causing the reused port number error -- that may be resetting the sequence numbers back to the beginning of the connection. Because otherwise it looks like the packet is in order; an HTTP command should be the next thing after the connection handshake.