I have an old server on CentOS 5, with cPanel installed, hosted on Amazon AWS EC2:

uname -a
Linux 2.6.32-573.22.1.el6.x86_64 #1 SMP Wed Mar 23 03:35:39 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

It wasn't touched for a very long time. Today, suddenly, it's not able to make outgoing connections on port 443:

curl -v https://google.com 
* About to connect() to google.com port 443 (#0)
*   Trying 142.250.217.78... Connection timed out
*   Trying 2607:f8b0:400a:80b::200e... Failed to connect to 2607:f8b0:400a:80b::200e: Network is unreachable
* Success
* couldn't connect to host
* Closing connection #0
curl: (7) Failed to connect to 2607:f8b0:400a:80b::200e: Network is unreachable

while the connections to port 80 go through:

curl -v http://google.com                                                                                                                                                         ⏎
* About to connect() to google.com port 80 (#0)
*   Trying 142.251.211.238... connected
* Connected to google.com (142.251.211.238) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.44 zlib/1.2.3 libidn/1.18 libssh2/1.4.2
> Host: google.com
> Accept: */*
> 

301 Moved
301 Moved
The document has moved
here.

* Connection #0 to host google.com left intact
* Closing connection #0

I've cleaned iptables to ensure that's they're not a problem:

iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

The only relevant note in /var/log/messages I can find, during the moment when problems started to occur, logs when datadog agent starts to fail at 11:25:

ov 25 11:04:50 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944863: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:05:00 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944864: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:05:05 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145520
Nov 25 11:05:10 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944865: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:06:00 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145540
Nov 25 11:07:00 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145560
Nov 25 11:07:29 xxx dd.collector: INFO (collector.py:571): Finished run #491350. Collection time: 4.23s. Emit time: 0.02s
Nov 25 11:08:05 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145580
Nov 25 11:09:03 xxx pure-ftpd: (?@127.0.0.1) [INFO] New connection from 127.0.0.1
Nov 25 11:09:03 xxx pure-ftpd: (?@127.0.0.1) [INFO] __cpanel__service__auth__ftpd__xxx is now logged in
Nov 25 11:09:03 xxx pure-ftpd: (__cpanel__service__auth__ftpd__xxx@127.0.0.1) [INFO] Logout.
Nov 25 11:09:10 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145600
Nov 25 11:10:05 xxx dd.forwarder: INFO (transaction.py:177): Flushing 2 transactions during flush #3145620
Nov 25 11:10:43 xxx dd.collector: INFO (collector.py:571): Finished run #491360. Collection time: 4.12s. Emit time: 0.02s
Nov 25 11:11:02 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145640
Nov 25 11:12:05 xxx dd.forwarder: INFO (transaction.py:177): Flushing 3 transactions during flush #3145660
Nov 25 11:13:20 xxx dd.forwarder: INFO (transaction.py:177): Flushing 3 transactions during flush #3145680
Nov 25 11:13:55 xxx dd.collector: INFO (collector.py:571): Finished run #491370. Collection time: 4.23s. Emit time: 0.02s
Nov 25 11:14:07 xxx pure-ftpd: (?@127.0.0.1) [INFO] New connection from 127.0.0.1
Nov 25 11:14:07 xxx pure-ftpd: (?@127.0.0.1) [INFO] __cpanel__service__auth__ftpd__xxx is now logged in
Nov 25 11:14:07 xxx pure-ftpd: (__cpanel__service__auth__ftpd__xxx@127.0.0.1) [INFO] Logout.
Nov 25 11:14:45 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145700
Nov 25 11:16:00 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944930: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:16:09 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145720
Nov 25 11:16:10 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944931: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:16:20 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944932: flushed 26 metrics, 0 events, and 0 service check runs
Nov 25 11:16:30 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944933: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:16:40 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944934: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:16:50 xxx dd.dogstatsd: INFO (dogstatsd.py:276): Flush #944935: flushed 25 metrics, 0 events, and 0 service check runs
Nov 25 11:17:07 xxx dd.collector: INFO (collector.py:571): Finished run #491380. Collection time: 4.12s. Emit time: 0.02s
Nov 25 11:17:26 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145740
Nov 25 11:18:30 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145760
Nov 25 11:18:32 xxx dhclient: DHCPREQUEST on eth0 to 10.0.0.1 port 67 (xid=0x24b5d86f)
Nov 25 11:18:32 xxx dhclient: DHCPACK from 10.0.0.1 (xid=0x24b5d86f)
Nov 25 11:18:34 xxx dhclient: bound to 10.0.0.247 -- renewal in 1642 seconds.
Nov 25 11:19:30 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145780
Nov 25 11:20:08 xxx pure-ftpd: (?@127.0.0.1) [INFO] New connection from 127.0.0.1
Nov 25 11:20:08 xxx pure-ftpd: (?@127.0.0.1) [INFO] __cpanel__service__auth__ftpd__xxx is now logged in
Nov 25 11:20:08 xxx pure-ftpd: (__cpanel__service__auth__ftpd__xxx@127.0.0.1) [INFO] Logout.
Nov 25 11:20:19 xxx dd.collector: INFO (collector.py:571): Finished run #491390. Collection time: 4.22s. Emit time: 0.02s
Nov 25 11:20:30 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145800
Nov 25 11:21:36 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145820
Nov 25 11:22:26 xxx auditd: Audit daemon rotating log files
Nov 25 11:22:35 xxx dd.forwarder: INFO (transaction.py:177): Flushing 2 transactions during flush #3145840
Nov 25 11:23:31 xxx dd.forwarder: INFO (transaction.py:177): Flushing 1 transaction during flush #3145860
Nov 25 11:23:31 xxx dd.collector: INFO (collector.py:571): Finished run #491400. Collection time: 4.13s. Emit time: 0.02s
Nov 25 11:24:40 xxx dd.forwarder: INFO (transaction.py:190): No transaction to flush during flush #3145880
Nov 25 11:25:07 xxx pure-ftpd: (?@127.0.0.1) [INFO] New connection from 127.0.0.1
Nov 25 11:25:07 xxx pure-ftpd: (?@127.0.0.1) [INFO] __cpanel__service__auth__ftpd__xxx is now logged in
Nov 25 11:25:07 xxx pure-ftpd: (__cpanel__service__auth__ftpd__xxx@127.0.0.1) [INFO] Logout.
Nov 25 11:25:47 xxx dd.forwarder: ERROR (ddagent.py:282): Response: HTTPResponse(_body=None,buffer=None,code=599,effective_url='https://5-32-8-app.agent.datadoghq.com/intake/?api_key=*************************55166 ',error=HTTPError('HTTP 599: Timeout',),headers={},reason='Unknown',request=,request_time=20.000658988952637,time_info={})
Nov 25 11:25:47 xxx dd.forwarder: WARNING (transaction.py:268): Transaction 2419202 in error (1 error), it will be replayed after 2022-11-25 11:26:07
Nov 25 11:26:10 xxx dd.forwarder: ERROR (ddagent.py:282): Response: HTTPResponse(_body=None,buffer=None,code=599,effective_url='https://5-32-8-app.agent.datadoghq.com/api/v1/check_run/?api_key=*************************55166 ',error=HTTPError('HTTP 599: Timeout',),headers={},reason='Unknown',request=,request_time=20.00121808052063,time_info={})
Nov 25 11:26:10 xxx dd.forwarder: WARNING (transaction.py:268): Transaction 2419209 in error (1 error), it will be replayed after 2022-11-25 11:26:30
Nov 25 11:26:10 xxx dd.forwarder: WARNING (transaction.py:212): Flush 3145896 is taking more than 10s, stopping it
Nov 25 11:26:31 xxx dd.forwarder: ERROR (ddagent.py:282): Response: HTTPResponse(_body=None,buffer=None,code=599,effective_url='https://5-32-8-app.agent.datadoghq.com/api/v1/series/?api_key=*************************55166 ',error=HTTPError('HTTP 599: Timeout',),headers={},reason='Unknown',request=,request_time=20.00036597251892,time_info={})
Nov 25 11:26:31 xxx dd.forwarder: WARNING (transaction.py:268): Transaction 2419215 in error (1 error), it will be replayed after 2022-11-25 11:26:51
Nov 25 11:26:31 xxx dd.forwarder: WARNING (transaction.py:212): Flush 3145897 is taking more than 10s, stopping it
Nov 25 11:26:43 xxx dd.collector: INFO (collector.py:571): Finished run #491410. Collection time: 4.24s. Emit time: 0.02s
Nov 25 11:26:51 xxx dd.forwarder: ERROR (ddagent.py:282): Response: HTTPResponse(_body=None,buffer=None,code=599,effective_url='https://5-32-8-app.agent.datadoghq.com/api/v1/series/?api_key=*************************55166 ',error=HTTPError('HTTP 599: Timeout',),headers={},reason='Unknown',request=,request_time=20.00032901763916,time_info={})

No AWS configuration regarding security and networking has been changed. It seems that all HTTPS connections time out, hanging in SYN_SENT state.. Any ideas are very much appreciated. Regards