I have a second CentOS 7 box that's having a lot of trouble connecting. Basically, we were running WHMCS, but a domain lookup kept failing. So the issue is IPv6 connectivity.
[root@da01 ~]# yum install telnet -y -q -e 0
[root@da01 ~]# telnet whois.crsnic.net 43
Trying 2001:503:e8ef:1000::74...
It's trying to connect through IPv6. We've been moving our colo servers and got an IPv6 range (10 addresses) but this hasn't been setup in the system yet – thus the connection times out.
[root@da01 ~]# telnet whois.crsnic.net 43
Trying 2001:503:e8ef:1000::74...
telnet: connect to address 2001:503:e8ef:1000::74: Connection timed out
The resolution is to temporarily disable IPv6 connectivity throughout the whole system. So simply go ahead and pick the network interface you wanna disable IPv6 on:
[root@da01 ~]# vi /etc/sysconfig/network-scripts/ifcfg-em2
Then change all IPv6-related config values to "no" as follows:
IPV6INIT="no"
IPV6_AUTOCONF="no"
IPV6_DEFROUTE="no"
IPV6_FAILURE_FATAL="no"
IPV6_PEERDNS="no"
IPV6_PEERROUTES="no"
IPV6_PRIVACY="no"
Finally, restart the networking-related service.
[root@da01 ~]# service network restart
It will now pass trying to connect through IPv6, then on timeout retry with IPv4:
[root@da01 ~]# telnet whois.crsnic.net 43
Trying 199.7.74.74...
Connected to whois.crsnic.net.
Escape character is '^]'.
Connection closed by foreign host.
So, I changed these values in the system configuration.
[root@da01 network-scripts]# sysctl -w net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.all.disable_ipv6 = 1
[root@da01 network-scripts]# sysctl -w net.ipv6.conf.default.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6 = 1
[root@da01 network-scripts]# sysctl -p /etc/sysctl.conf
[root@da01 network-scripts]# sysctl --system
HOWEVER, I am now experiencing network issues.
Can't ping the server
iMac-van-William-4:~ williamdavidedwards$ ping
PING : 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1
Traceroute from the server is weird
[root@da01 domains]# traceroute google.nl
traceroute to google.nl (74.125.136.94), 30 hops max, 60 byte packets
1 178.21.23.3 (178.21.23.3) 11.069 ms * *
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 * * *
14 * * *
15 * * *
16 * * *
17 * * *
18 * * *
19 * * *
20 * * *
21 * * *
22 * * *
23 * * *
24 * * *
25 * * *
26 * * *
27 * * *
28 * * *
29 * * *
30 * * *
Can't ping anything, except the server IP itself, the gateway, 8.8.8.8 and 8.8.4.4
[root@da01 domains]# ping nu.nl
PING nu.nl (62.69.166.254) 56(84) bytes of data.
^C
--- nu.nl ping statistics ---
2 packets transmitted, 0 received, 100% packet loss, time 999ms
route -n
[root@da01 ~]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 5.255.90.1 0.0.0.0 UG 100 0 0 em2
5.255.90.0 0.0.0.0 255.255.255.0 U 100 0 0 em2
/etc/resolv.conf
[root@da01 ~]# cat /etc/resolv.conf
# Generated by NetworkManager
search -removed- -removed-
nameserver 8.8.8.8
nameserver 8.8.4.4
ifconfig
[root@da01 ~]# ifconfig
em1: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether 00:21:9b:a7:c3:fd txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
em2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet serveripv4 netmask 255.255.255.0 broadcast 5.255.90.255
inet6 serveripv6 prefixlen 32 scopeid 0x0<global>
inet6 ipv6 prefixlen 64 scopeid 0x20<link>
inet6 ipv6 prefixlen 64 scopeid 0x0<global>
ether ipv6 txqueuelen 1000 (Ethernet)
RX packets 42827922 bytes 4801530409 (4.4 GiB)
RX errors 0 dropped 30874 overruns 0 frame 0
TX packets 2972081 bytes 1060556603 (1011.4 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
em3: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether 00:21:9b:a7:c4:01 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
em4: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether 00:21:9b:a7:c4:03 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
loop txqueuelen 0 (Local Loopback)
RX packets 13560475 bytes 5100699012 (4.7 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 13560475 bytes 5100699012 (4.7 GiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
When I ping an inexistent address (dsjahdasjdsa.com) it automatically pings my own server by pinging dsjahdasjdsa.com.example.com where example.com is my DOMAIN for the network interface.
Unfortunately, this is still happening. I must admit I'm stuck… HELP
Best Answer
The issue was caused by the DDoS protection software from the datacenter. It contained a bug which caused big latency for DNS lookups. I reinstalled the OS, disabled the DDoS protection (they're fixing the bug) and everything is working now.