interserver_connection_debugging.md

Use dig hostname to check DNS issues

dig redis-host.something.

Look for the presence or absence of the ;; ANSWER SECTION:

Use nping from nmap package to check connection issues

nping -p 6379 redis-host.something.

If Connection refused is received then there are possible problems with the service on the host you’re trying to connect to

Starting Nping 0.6.40 ( http://nmap.org/nping ) at 2016-11-21 12:18 UTC
SENT (0.0022s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (0.0031s) Possible TCP RST received from 10.0.245.90:6379 --> Connection refused
SENT (1.0043s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (1.0053s) Possible TCP RST received from 10.0.245.90:6379 --> Connection refused
SENT (2.0064s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (2.0073s) Possible TCP RST received from 10.0.245.90:6379 --> Connection refused
SENT (3.0084s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (3.0093s) Possible TCP RST received from 10.0.245.90:6379 --> Connection refused
SENT (4.0105s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (4.0115s) Possible TCP RST received from 10.0.245.90:6379 --> Connection refused

Max rtt: N/A | Min rtt: N/A | Avg rtt: N/A
TCP connection attempts: 5 | Successful connections: 0 | Failed: 5 (100.00%)
Nping done: 1 IP address pinged in 4.01 seconds

If completed is received then the connection is AOK

Starting Nping 0.6.40 ( http://nmap.org/nping ) at 2016-11-21 12:23 UTC
SENT (0.0022s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (0.0045s) Handshake with redis-host.something.:6379 (10.0.245.90:6379) completed
SENT (1.0055s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (1.0064s) Handshake with redis-host.something.:6379 (10.0.245.90:6379) completed
SENT (2.0076s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (2.0086s) Handshake with redis-host.something.:6379 (10.0.245.90:6379) completed
SENT (3.0097s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (3.0105s) Handshake with redis-host.something.:6379 (10.0.245.90:6379) completed
SENT (4.0117s) Starting TCP Handshake > redis-host.something.:6379 (10.0.245.90:6379)
RECV (4.0126s) Handshake with redis-host.something.:6379 (10.0.245.90:6379) completed

Max rtt: 0.945ms | Min rtt: 0.074ms | Avg rtt: 0.729ms
TCP connection attempts: 5 | Successful connections: 5 | Failed: 0 (0.00%)
Nping done: 1 IP address pinged in 4.01 seconds

If nothing is received then there are possible issues with security groups

Starting Nping 0.6.40 ( http://nmap.org/nping ) at 2016-11-21 12:24 UTC
SENT (0.0022s) Starting TCP Handshake > redis-host.something.:6370 (10.0.245.90:6370)
SENT (1.0055s) Starting TCP Handshake > redis-host.something.:6370 (10.0.245.90:6370)
SENT (2.0067s) Starting TCP Handshake > redis-host.something.:6370 (10.0.245.90:6370)
SENT (3.0078s) Starting TCP Handshake > redis-host.something.:6370 (10.0.245.90:6370)
SENT (4.0100s) Starting TCP Handshake > redis-host.something.:6370 (10.0.245.90:6370)

Max rtt: N/A | Min rtt: N/A | Avg rtt: N/A
TCP connection attempts: 5 | Successful connections: 0 | Failed: 5 (100.00%)
Nping done: 1 IP address pinged in 5.01 seconds