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Server Troubleshooting

netstat -taupe | grep httpd

sudo ss -ltp | grep httpd


# Access control. 

man host_acccess

cat /etc/hosts.allow
cat /etc/hosts.deny

For advanced server troubleshooting, the /proc filesystem has settings that affect the network stack:

  • /proc/sys/net/ipv4/ip_forward Allows for network traffic to be forwarded from one interface to another.

  • /proc/sys/net/ipv4/conf/*/accept_redirects Accepting Internet Control Message Protocol (ICMP) redirects from a router to find better routes. This setting has the potential to be exploited by a malicious party to redirect your traffic.

  • /proc/sys/net/ipv4/icmp_echo_ignore_all Changing this setting will affect the host's visibility to ICMP ping packets.

  • /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts This setting will change the host's visibility to broadcast ICMP ping packets.

  • /proc/net/arp Contains the current arp table.

These settings are not persistent across reboots. To make the changes persistent, edit the /etc/sysctl.conf configuration file, or a .conf file in the /etc/sysctl.d directory.

The syntax for /etc/sysctl.conf matches the path for the file in /proc/sys with the . character instead of /.

Common Server-Side Problems​

Common server problems include broken DNS, overzealous firewall rules, incorrect network settings, and the daemon not listening on the right interface/port.

Some access control systems require that Reverse DNS be properly set up.

When enabling new traffic to pass through a firewall, pay attention to the type of protocol (for example, UDP over TCP) used.

Some protocols break when return traffic comes back from a different IP address. Verify that your egress route is correct.