I’m on at least 2 blocklists at this point for the crime of not having reverse DNS set up. I don’t know how rDNS works. No amount of reading Wikipedia is helping me understand what I have to do.
- I have a domain at a registrar which gives me bog standard DNS.
- I have Apache running on my network.
- I have PiHole running on my network.
My understanding is that rDNS is not set up at my registrar, but somewhere in my network. What do I do?
Thank you for your time.
It’s not a DNS issue, as the afflicted users can get the correct IPs from nslookup.
You are correct; looking at the resolutions via Google’s toolbox, it seems to be resolving fine.
Traceroutes time out at consistent hops but it’s different per afflicted network. The only recurring name has been costumer.tdc.net
I did a couple of traceroute tests from a couple locations (two data centres, my local WiFi, and via cellular data), and they all seem to end at
cpe.ae20-0.khk7nqp8.dk.customer.tdc.net. In teleco terms,cpeusually means “Client/Customer Premise Equipment”, so it wouldn’t surprise me if that is the address assigned to the network equipment closest to your server’s local network (think neighborhood hub, or PON on premise, something super close to the demarcation point). If everyone else is able to get that far, then I’m more inclined to think the drop is happening on your equipment, not your ISP’s equipment; but having said that, if this is a residential network (regardless if ISP is provisioning you gigabit connectivity via fibre or whatever), there will always be a likelihood for ISP to be doing more filtering.If you don’t mind, what is the gateway you’re using? Is there an ISP gateway and then your own, or straight from ISP equipment into your network? Are there any (single/double) NAT going on? Are there’s any security/filtering options on (either) equipment(s)?
I have an ISP issued fiber modem that goes straight into my pfsense box. For IPv4 I have NAT and Firewall, for v6 it’s just the firewall. It should be noted that I’ve been hosting with this setup for years and it wasn’t a problem until I got this ISP at this location. I’ve had this ISP elsewhere without problems and another ISP here where it also wasn’t a problem.
The traceroutes getting dropped was a misconfiguration in my firewall on my part, it should work now!
My friend across town can now ping me successfully but going to my website still gives him ERR_CONNECTION_TIMED_OUT in browser.
EDIT ----------
I got another troublesome user to run a quick test with the new firewall config. Also can’t reach my website. Pings DO time out for him! Traceroute expectedly also fails, last hop was cpe.ae20-0.khk7nqp8.dk.costumer.tdc.net [87.61.121.169]
From my side, I now see 3
???s between the CPE and your IP address, which is also responding, so that’s great.Can your friend do something like
curl -vvv https://drkt.euor whatever to see if the time out happens before/after SSL handshake etc.? Also, do they have any firewalls / security appliances configured to filter content? I’d be curious to seedigornslookupresult,pingortracerouteresult, andcurl -vvvresult, just to understand where it is breaking down.Also, do you have a login to your ISP’s equipment? Are you able to set it to bridge mode to bypass it altogether? Just throwing ideas out there, to see if there is anything else on the go. That
cpedevice is also pretty curious for sure.Edit: Also, if they can get a response from ping, then it is probably not routing, but something else on the connection to the service / port itself. That’s what I’m hoping we can figure out from the various outputs.
Office friend still can’t ping me. The campus friend can ping me but can’t reach my website. I never trusted campus friend to be on a clean network so it’s a bit fruitless to test his connection.
My ISP’s equipment is entirely a modem right now, bridging straight to my pfsense router. It’s not doing any weird filtering.
Here are the results of those from office friend
Server: unifi.localdomain Address: 192.168.0.1 Non-authoritative answer: Name: drkt.eu Addresses: 2a05:f6c7:8039::1337 89.150.135.135 Tracing route to drkt.eu [89.150.135.135] over a maximum of 30 hops: 1 <1 ms <1 ms <1 ms unifi.localdomain [192.168.0.1] 2 1 ms <1 ms <1 ms 5.186.33.65.static.fibianet.dk [5.186.33.65] 3 4 ms 4 ms 3 ms 89.150.66.104 4 4 ms 4 ms 4 ms ae2.core01-tkbg.bb.fibianet.dk [89.150.64.25] 5 * * * Request timed out. 6 3 ms 3 ms 3 ms 89.150.69.66 7 2 ms 4 ms 2 ms 217.74.211.104 8 3 ms 4 ms 4 ms 194.182.97.132 9 4 ms 4 ms 3 ms ae22-0.khk7nqp7.dk.ip.tdc.net [195.215.109.46] 10 4 ms 4 ms 4 ms ae1-0.khk7nqp8.dk.ip.tdc.net [83.88.12.15] 11 3 ms 4 ms 4 ms cpe.ae20-0.khk7nqp8.dk.customer.tdc.net [87.61.121.169] 12 * * * Request timed out. 13 * * * Request timed out. 14 * * * Request timed out. 15 * * * Request timed out. 16 * * * Request timed out. 17 * * * Request timed out. 18 * * * Request timed out. 19 * * * Request timed out. 20 * * * Request timed out. 21 * * * Request timed out. 22 * * * Request timed out. 23 * * * Request timed out. 24 * * * Request timed out. 25 * * * Request timed out. 26 * * * Request timed out. 27 * * * Request timed out. 28 * * * Request timed out. 29 * * * Request timed out. 30 * * * Request timed out. Trace complete. * Rebuilt URL to: https://drkt.eu/ * Trying 89.150.135.135... * TCP_NODELAY set * connect to 89.150.135.135 port 443 failed: Timed out * Failed to connect to drkt.eu port 443: Timed out * Closing connection 0 curl: (7) Failed to connect to drkt.eu port 443: Timed out— UPDATE
Office friends pinged me 4 times, all timed out on his end. His IP showed up in my state table and I captured these 2 packets
State table:
Interface Protocol Source -> Destination State Packets Bytes WAN icmp 5.186.33.87:1 -> 89.150.135.135:1 0:0 4 / 4 240 B / 240 BPacket capture:
(the only 2, despite the 4 pings, and they have bad checksums?)All 8 expected packets show up but all do have bad checksums. I’m seeing a LOT of bad checksums. A known working connection from fourth friend did not show bad checksums in this same test.5.186.33.87 > 89.150.135.135: ICMP echo request, id 1, seq 43476, length 40 12:38:19.843890 d0:50:99:81:48:17 > 4c:6d:58:4a:97:d4, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 21715, offset 0, flags [none], proto ICMP (1), length 60, bad cksum 0 (->1dc0)!) 89.150.135.135 > 5.186.33.87: ICMP echo reply, id 1, seq 43476, length 40 12:38:20.219177 d0:50:99:81:48:17 > 4c:6d:58:4a:97:d4, ethertype IPv4 (0x0800), length 43: (tos 0x0, ttl 64, id 32958, offset 0, flags [none], proto ICMP (1), length 29, bad cksum 0 (->3f6d)!)He went to my domain while I was collecting packets from ANY protocol filtering his IP It’s a bit long, so here’s the cap file https://u.drkt.eu/iiUsH7.cap
I’ve learned that the bad checksums are to to be ignored because it’s the NIC that’s responsible for it, so tcpdump sees the wrong checksums and it doesn’t matter. I have also learned that the Apache container actually does see the incoming connections. Here’s an example of my working connection and his connection.
root ~ -> tcpdump -n port 443 ## My machine to https://drkt.eu 15:44:50.985650 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [S], seq 2232908482, win 64800, options [mss 1440,nop,wscale 8,nop,nop,sackOK], length 0 15:44:50.985656 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [S.], seq 1276898910, ack 2232908483, win 64800, options [mss 1440,nop,nop,sackOK,nop,wscale 7], length 0 15:44:50.985780 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 1, win 8235, length 0 15:44:50.987032 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [P.], seq 1:518, ack 1, win 8235, length 517 15:44:50.987040 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [.], ack 518, win 503, length 0 15:44:50.987333 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 1:4097, ack 518, win 503, length 4096 15:44:50.987566 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 4097, win 8235, length 0 15:44:50.988400 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 4097:5381, ack 518, win 503, length 1284 15:44:50.988531 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 5381, win 8229, length 0 15:44:50.992091 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [P.], seq 518:582, ack 5381, win 8229, length 64 15:44:50.992095 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [P.], seq 582:1087, ack 5381, win 8229, length 505 15:44:50.992173 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [.], ack 1087, win 501, length 0 15:44:50.992274 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 5381:5668, ack 1087, win 501, length 287 15:44:50.992342 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 5668:5955, ack 1087, win 501, length 287 15:44:50.992488 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 5955, win 8235, length 0 15:44:50.993404 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 5955:10902, ack 1087, win 501, length 4947 15:44:50.993647 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 10902, win 8235, length 0 15:44:55.998521 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [P.], seq 10902:10926, ack 1087, win 501, length 24 15:44:55.998547 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [F.], seq 10926, ack 1087, win 501, length 0 15:44:55.998727 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [P.], seq 1087:1111, ack 10926, win 8234, length 24 15:44:55.998733 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [.], ack 10927, win 8234, length 0 15:44:55.998734 IP6 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868 > 2a05:f6c7:8039::1337.443: Flags [F.], seq 1111, ack 10927, win 8234, length 0 15:44:55.998736 IP6 2a05:f6c7:8039::1337.443 > 2a05:f6c7:8039:0:c859:8de4:257f:b4ff.53868: Flags [.], ack 1112, win 501, length 0 ## Office dude to https://drkt.eu 15:45:06.759198 IP 5.186.33.87.59842 > 192.168.78.164.443: Flags [S], seq 3042763137, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:06.759210 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:07.009444 IP 5.186.33.87.59843 > 192.168.78.164.443: Flags [S], seq 1050721154, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:07.009455 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:07.761986 IP 5.186.33.87.59842 > 192.168.78.164.443: Flags [S], seq 3042763137, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:07.761994 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:08.011014 IP 5.186.33.87.59843 > 192.168.78.164.443: Flags [S], seq 1050721154, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:08.011023 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:08.774730 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:09.030829 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:09.772318 IP 5.186.33.87.59842 > 192.168.78.164.443: Flags [S], seq 3042763137, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:09.772327 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:10.020444 IP 5.186.33.87.59843 > 192.168.78.164.443: Flags [S], seq 1050721154, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:10.020452 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:11.782726 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:12.038818 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:13.778722 IP 5.186.33.87.59842 > 192.168.78.164.443: Flags [S], seq 3042763137, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:13.778730 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:14.026072 IP 5.186.33.87.59843 > 192.168.78.164.443: Flags [S], seq 1050721154, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 15:45:14.026082 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:18.026717 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:18.282740 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:26.214732 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:26.474741 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:42.342747 IP 192.168.78.164.443 > 5.186.33.87.59842: Flags [S.], seq 4039717320, ack 3042763138, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 15:45:42.598745 IP 192.168.78.164.443 > 5.186.33.87.59843: Flags [S.], seq 1810250599, ack 1050721155, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0We’re making progress!
Looks like connection are being made, so it isn’t routing after all!
Looking at the first recording, based on the different packet length, I’d guess it is doing SSL handshake properly; whereas the second one seems to be all 0 length and so something is not working out. At least on a cursory glance, your settings seems to be pretty permissive, so unless your friend’s using a super old system, it shouldn’t be an issue. Do you know what OS your friend is using, and if it has up-to-date root certificates? Are they on a system with
opensslcli available? Judging by the unifi network, probably? Tryopenssl s_client -connect drkt.eu:443 -prexit(andctrl + cto quit after it stops) and see if you can see any oddities with the SSL handshake process.He’s running Windows 10, unfortunately- but wouldn’t SSL errors show up in Apache logs? His IP appears 0 times in all apache error and access logs dating back 8 months (the beginning of recorded logs).
Here’s another example of a working request to https://drkt.eu, and his non-working request respectively.
21:06:10.038213 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [S], seq 383581601, win 65535, options [mss 1460,nop,nop,sackOK,nop,wscale 11], length 0 21:06:10.038225 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [S.], seq 1228608203, ack 383581602, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:06:10.062959 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [.], ack 1228608204, win 32, length 0 21:06:10.068861 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [P.], seq 383581602:383582119, ack 1228608204, win 32, length 517 21:06:10.068878 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [.], ack 383582119, win 501, length 0 21:06:10.069190 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228608204:1228611124, ack 383582119, win 501, length 2920 21:06:10.069272 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228611124:1228612300, ack 383582119, win 501, length 1176 21:06:10.069905 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228612300:1228613496, ack 383582119, win 501, length 1196 21:06:10.093915 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [.], ack 1228611124, win 35, length 0 21:06:10.094156 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [.], ack 1228612300, win 37, length 0 21:06:10.095161 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [.], ack 1228613496, win 38, length 0 21:06:10.096357 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [P.], seq 383582119:383582245, ack 1228613496, win 38, length 126 21:06:10.096588 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228613496:1228613547, ack 383582245, win 501, length 51 21:06:10.121482 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [P.], seq 383582245:383582345, ack 1228613547, win 38, length 100 21:06:10.121749 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228613547:1228614978, ack 383582345, win 501, length 1431 21:06:10.146792 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [P.], seq 383582345:383582376, ack 1228614978, win 40, length 31 21:06:10.146907 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [P.], seq 1228614978:1228615009, ack 383582376, win 501, length 31 21:06:10.146931 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [F.], seq 1228615009, ack 383582376, win 501, length 0 21:06:10.147568 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [F.], seq 383582376, ack 1228614978, win 40, length 0 21:06:10.147573 IP 192.168.78.164.443 > 185.21.217.51.34034: Flags [.], ack 383582377, win 501, length 0 21:06:10.171696 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [R], seq 383582376, win 0, length 0 21:06:10.171722 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [R], seq 383582376, win 0, length 0 21:06:10.172482 IP 185.21.217.51.34034 > 192.168.78.164.443: Flags [R], seq 383582377, win 0, length 0 21:49:09.999249 IP 5.186.33.87.51592 > 192.168.78.164.443: Flags [S], seq 2577736519, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:09.999412 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:10.254112 IP 5.186.33.87.51594 > 192.168.78.164.443: Flags [S], seq 2174498243, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:10.254425 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:11.000511 IP 5.186.33.87.51592 > 192.168.78.164.443: Flags [S], seq 2577736519, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:11.000624 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:11.262009 IP 5.186.33.87.51594 > 192.168.78.164.443: Flags [S], seq 2174498243, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:11.262135 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:12.010686 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:12.266608 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:13.014793 IP 5.186.33.87.51592 > 192.168.78.164.443: Flags [S], seq 2577736519, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:13.014920 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:13.265504 IP 5.186.33.87.51594 > 192.168.78.164.443: Flags [S], seq 2174498243, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:13.265645 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:15.046677 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:15.274640 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:17.022305 IP 5.186.33.87.51592 > 192.168.78.164.443: Flags [S], seq 2577736519, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:17.022424 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:17.277539 IP 5.186.33.87.51594 > 192.168.78.164.443: Flags [S], seq 2174498243, win 64240, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 21:49:17.277663 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:21.226692 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:21.482700 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:29.414663 IP 192.168.78.164.443 > 5.186.33.87.51592: Flags [S.], seq 3425825559, ack 2577736520, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 21:49:29.670628 IP 192.168.78.164.443 > 5.186.33.87.51594: Flags [S.], seq 1029590757, ack 2174498244, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0See this page here that explains the
Flags: https://opensource.com/article/18/10/introduction-tcpdumpTypically, in a TCP connection, you’d SYN, SYN+ACK, ACK, then transfer actual data over. In the successful sequence, you see this happening as expected.
In the unsuccessful sequence, it seems to be stuck in SYN, SYN+ACK, but there is no ACK that follows (
Flags [.]).Where is the second one captured? On the user’s system, or on your system? Something in between is determining the packet isn’t intended for the destination and dropping it. It may be a firewall, it may be something else.
Thinking about this some more, could it be asymmetric routing?
If so, does this help?
