Hello @Bob Cochran, @James Moe
Sorry for the very late reply. I was not able to work on this until yesterday.
@Bob Cochran, I gave right argument (my network interface name was eno1) for tcpdump and have got very long output. Below are the first few lines.
And I got reply from my ISP that they wont block any incoming requests. And they gave the reason, if port 80 was blocked I would not be able to use internet at all. I thought blocking port and blocking requests are 2 different things. So I've again asked them for clarification on blocking incoming requests.
Output of tcpdump
sudo tcpdump -i eno1 -vvvSXn port 80
tcpdump: listening on eno1, link-type EN10MB (Ethernet), capture size 262144 bytes
08:34:36.311747 IP (tos 0x0, ttl 64, id 2906, offset 0, flags [DF], proto TCP (6), length 60)
192.168.0.106.53348 > 35.222.85.5.80: Flags [S], cksum 0x3a24 (incorrect -> 0x2c43), seq 3571338894, win 64240, options [mss 1460,sackOK,TS val 3757642427 ecr 0,nop,wscale 7], length 0
0x0000: 4500 003c 0b5a 4000 4006 f56c c0a8 006a E..<.Z@.@..l...j
0x0010: 23de 5505 d064 0050 d4de 4e8e 0000 0000 #.U..d.P..N.....
0x0020: a002 faf0 3a24 0000 0204 05b4 0402 080a ....:$..........
0x0030: dff9 12bb 0000 0000 0103 0307 ............
08:34:36.542112 IP (tos 0x0, ttl 57, id 0, offset 0, flags [DF], proto TCP (6), length 60)
35.222.85.5.80 > 192.168.0.106.53348: Flags [S.], cksum 0x5b44 (correct), seq 2985042357, ack 3571338895, win 28160, options [mss 1420,sackOK,TS val 3577197868 ecr 3757642427,nop,wscale 7], length 0
0x0000: 4500 003c 0000 4000 3906 07c7 23de 5505 E..<..@.9...#.U.
0x0010: c0a8 006a 0050 d064 b1ec 21b5 d4de 4e8f ...j.P.d..!...N.
0x0020: a012 6e00 5b44 0000 0204 058c 0402 080a ..n.[D..........
0x0030: d537 b52c dff9 12bb 0103 0307 .7.,........
08:34:36.542189 IP (tos 0x0, ttl 64, id 2907, offset 0, flags [DF], proto TCP (6), length 52)
192.168.0.106.53348 > 35.222.85.5.80: Flags [.], cksum 0x3a1c (incorrect -> 0xf50b), seq 3571338895, ack 2985042358, win 502, options [nop,nop,TS val 3757642658 ecr 3577197868], length 0
0x0000: 4500 0034 0b5b 4000 4006 f573 c0a8 006a E..4.[@.@..s...j
0x0010: 23de 5505 d064 0050 d4de 4e8f b1ec 21b6 #.U..d.P..N...!.
0x0020: 8010 01f6 3a1c 0000 0101 080a dff9 13a2 ....:...........
0x0030: d537 b52c
I checked the status of Ubuntu firewall. Its not yet active. Hence all requests from any host and any port should be allowed.
In my Apache I've even given the server name of my free registered domain.
And Listen is only given with port 80. Hence any IP address should be allowed.
Listen 80
Finally,
1. There is no problem with OS and firewall.
2. There is no problem with web server configuration.
3. There is no problem with my ISP.
Still not able to reach my server from internet.
Can anyone think of any other reasons please.
Thank you.
