debugging-snippets

Network
Processes
Java

Useful debugging snippets for everyday operations. ⚙️

1. Network

Show all open TCP connections

Show open TCP connections with source and destination addresses and ports. ℹ️️ Replace tcp with tcp6 at the end to show IPv6 connections.

awk 'function hextodec(str,ret,n,i,k,c){
    ret = 0
    n = length(str)
    for (i = 1; i <= n; i++) {
        c = tolower(substr(str, i, 1))
        k = index("123456789abcdef", c)
        ret = ret * 16 + k
    }
    return ret
}
function getIP(str,ret){
    ret=hextodec(substr(str,index(str,":")-2,2)); 
    for (i=5; i>0; i-=2) {
        ret = ret"."hextodec(substr(str,i,2))
    }
    ret = ret":"hextodec(substr(str,index(str,":")+1,4))
    return ret
} 
NR > 1 {{if(NR==2)print "Local - Remote";local=getIP($2);remote=getIP($3)}{print local" - "remote}}' /proc/net/tcp

Show all HTTP requests on a port

Show the HTTP path of a request on a port. ℹ️ Replace the port number accordingly.

sudo tcpdump -s 0 -v -n -l port 8080 | egrep -i "POST /|GET /|HEAD /|PUT /|DELETE /|CONNECT /|OPTIONS /|TRACE /|PATH /"

2. Processes

Find PID by port

Find a PID which has opened a port. ℹ️ Replace the port number with the port you want to find.

sudo ss -lptn 'sport = :8080'

Find port by PID

Find the ports belonging to a PID. ℹ️ Replace the PID with the PID you want to find.

sudo netstat -ltnup | grep 'LISTEN' | grep '123/'

3. Java

Inspecting class JVM bytecode

Find the minimum required JRE required to run the bytecode. E.g. 51 means Java 1.7. ℹ️ Replace the jar and class references accordingly.

javap -v -classpath myjar.jar com.example.Main | grep major

whirlwin/debugging-snippets