Why Wireshark does not recognize this HTTP response?

I have a trivial CGI script that outputs simple text content. It's written in Perl and using CGI module and it specifies only the most basic headers:

print $q->header(
    -type               => 'text/plain',
    -Content_length     => $length,
);
print $stuff;

There's no apparent issue with functionality—neither User Agent nor server do complain a bit, but I'm confused about the fact that Wireshark does not recognize the HTTP response as HTTP—it's marked as TCP.

Here is request and response, as shown in "Follow TCP Stream" dialog box:

GET /cgi-bin/memfile/memfile.pl?mbytes=1 HTTP/1.1
Host: 10.0.0.1
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20100101 Firefox/11.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: cs,en-us;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive

HTTP/1.1 200 OK
Date: Thu, 05 Apr 2012 18:52:23 GMT
Server: Apache/2.2.15 (Win32) mod_ssl/2.2.15 OpenSSL/0.9.8m
Content-length: 1048616
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/plain; charset=ISO-8859-1

XXXXXXXX...

And here is the packet overview)

No.     Time        Source      srcp   Destination  dstp    Protocol    Info                                tcp.stream  abstime
5       0.112749    10.0.0.1    80     10.0.0.2     48072   TCP         [TCP segment of a reassembled PDU]  0           20:52:23.228063

Frame 5: 1514 bytes on wire (12112 bits), 1514 bytes captured (12112 bits)
Ethernet II, Src: 00:12:34:56:78:9a, Dst: 00:12:34:56:78:9b
Internet Protocol Version 4, Src: 10.0.0.1 (10.0.0.1), Dst: 10.0.0.2 (10.0.0.2)
Transmission Control Protocol, Src Port: http (80), Dst Port: 48072 (48072), Seq: 1, Ack: 330, Len: 1460

Now when I open the stream in Wireshark:

• first three packets are usual TCP handshake
• fourth packet the GET request shown as HTTP with this in "Info" field GET /cgi-bin/memfile/memfile.pl, as I expect
• fifth packet contains the response, but is not marked as an HTTP response. I would expect Info field to be something like "HTTP 200 OK", but there's only a generic "[TCP segment of a reassembled PDU]". The real inconvenience is that this packet is not caught by "http.response" filter

Can somebody explain why Wireshark does not recognize HTTP nature of the fifth packet? Is there something wrong with the response?

I've had the same issue myself and found the answer at Wireshark Q&A:

You can make Wireshark show the HTTP response straight away by disabling the "Allow Subdissector to reassemble TCP streams" in the TCP protocol preferences" (you need to restart Wireshark after disabling the option).

To explain the why of Zorel's answer ....

When the WireShark preference is to reassemble multi-packet messages, it doesn't know that the messages is HTTP until it has seen it all and the reassembly is complete.

So the packets are first reported as TCP (which they are) and after reassembly reported again as HTTP.

In the reassembled line you should see something like:

Frame 10: ...
...
[3 Reassembled TCP segments (2496 bytes): #8(316), #9(1324), #10(856)]
Hypertext Transfer Protocol
...

Which indicates which record numbers, and how many bytes each provided to make the reassembled entry.

In your case, the response is 1048616 bytes, so it will be MANY packets before it knows it is HTTP. Your indicated entry does say:

[TCP segment of a reassembled PDU]

Which is another indication that WireShark is postponing the final display for that message.