Linux Network Diagnostics

gistfile1.txt

# PING is all about host reachability

1. Test the reachability

   > ping <destination> 

2. Test packet losses with large packets

   > ping <destination> -s <packet size>  


# Diagnose routes
 
1. Trace the route for a destination

    > traceroute <destination>

    > tracepath <destination>

2. Show all routes

   > route

   Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

   default         192.168.0.8     0.0.0.0         UG    0      0        0 eth1
   link-local      *               255.255.0.0     U     1000   0        0 eth1
   192.168.0.0     *               255.255.255.0   U     2      0        0 eth1
     

   Here 

   U : This flag signifies that the route is up

   G : This flag signifies that the route is to a gateway. If this flag is not present 
       then we can say that the route is to a directly connected destination

   H : This flag signifies that the route is to a host which means that the destination 
      is a complete host address. If this flag      is not present then it can be assumed
      that the route is to a network and destination would be a network address.
 
   D : This flag signifies that this route is created by a redirect.
   
   M : This flag signifies that this route is modified by a redirect. 


# Diagnose firewall issue

1. List all firewall rules

   > iptables --line-numbers -n -L

  

# Diagnose TCP/UDP/SCTP Connectivity

1. TELNET 

   > telnet <destination> <port>



2. NETCAT/ NC

   Server: > netcat -l <source> 8080

   Client: > netcat <destination> 8080



3. Diagnose SCTP Connectivity

   Server: > sctp_test -H <source> -P <source-port> -l

   Client: > sctp_test -H <source> -P <source-port> -h <destination> -p <destination-port> -s