Testing NetBIOS over TCP/IP services during authorized engagements. Covers the NetBIOS Name Service (137/udp, 137/tcp), Datagram Service (138/udp), and Session Service (139/tcp). Focuses on name and MAC enumeration with nmblookup, nbtscan and nmap nbstat, identifying workgroups/domains, and pivoting to SMB over NetBIOS (139) for share and user enumeration.
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Testing NetBIOS over TCP/IP services during authorized engagements. Covers the NetBIOS Name Service (137/udp, 137/tcp), Datagram Service (138/udp), and Session Service (139/tcp). Focuses on name and MAC enumeration with nmblookup, nbtscan and nmap nbstat, identifying workgroups/domains, and pivoting to SMB over NetBIOS (139) for share and user enumeration.
During authorized internal network assessments when UDP 137, UDP 138, or TCP 139 is reachable
When mapping Windows hosts on a LAN by NetBIOS name, workgroup/domain, and MAC address
When TCP 445 is filtered but 139 is open (SMB over NetBIOS still works)
As an early reconnaissance step before SMB/Active Directory enumeration
When investigating NBT-NS/LLMNR poisoning opportunities for credential capture
Quick Enumeration
# Names the server uses + MAC address (Name Service, 137)
nmblookup -A <IP>
nbtscan <IP>/30
nbtscan -r 192.168.0.1/24 # scan a /24 quicklysudo nmap -sU -sV -T4 --script nbstat.nse -p137 -Pn -n <IP>
# Session service banner (139) — confirms SMB-over-NetBIOS
nmap -p139 -sV <IP>
# Expected: 139/tcp open netbios-ssn Microsoft Windows netbios-ssn# Name service banner (137/udp)# Expected: 137/udp open netbios-ns Samba nmbd netbios-ns (workgroup: WORKGROUP)
Critical: Checks Most Often Missed
Workgroup vs domain disclosure — the NetBIOS name table reveals the workgroup or NT domain (<1C>/<00> group entries), giving you the AD/workgroup name for free before touching SMB.
MAC address leak — nbtscan/nbstat return the server MAC, useful for vendor fingerprinting and pivoting.
Unique vs group name suffixes — suffix bytes identify roles: <00> workstation, <20> File Server (SMB enabled), <1B> Domain Master Browser (often the PDC), <1C> Domain Controllers group. A <20> means the host serves SMB — pivot to share enumeration.
SMB still reachable on 139 — if 445 is filtered, 139 (NBT over IP) frequently still exposes the full SMB attack surface. Point your SMB tooling at port 139.
NBT-NS poisoning surface — broadcast name resolution on 137/udp can be poisoned with Responder to capture NetNTLM hashes on the segment.
How to CONFIRM: NetBIOS is confirmed when nmblookup -A <IP> returns a name table (lines with <XX> suffixes) or nbtscan lists the name and MAC. A suffix confirms the host runs an SMB file server reachable over 139.
<20>
Workflow
Step 1: Enumerate (names, workgroup/domain, MAC)
# Resolve the name table for a single host
nmblookup -A <IP>
# Read the suffixes:# <00> UNIQUE -> workstation / hostname# <00> GROUP -> workgroup / domain name# <20> UNIQUE -> SMB file server service running# <1B> UNIQUE -> Domain Master Browser (likely PDC)# <1C> GROUP -> Domain Controllers# <1D> UNIQUE -> Master Browser# Sweep a subnet for live NetBIOS hosts
nbtscan -r 192.168.0.0/24
sudo nmap -sU --script nbstat.nse -p137 -Pn -n 192.168.0.0/24
Step 2: Identify roles and reachable SMB
# Confirm session service (SMB over NetBIOS) on 139
nmap -p139,445 -sV <IP>
# If 445 is filtered but 139 is open, drive SMB tooling at 139
smbclient -p 139 --no-pass -L //<IP>
smbmap -P 139 -H <IP>
rpcdump.py -port 139 [[domain/]username[:password]@]<IP> # map RPC endpoints over 139
Step 3: Pivot to SMB enumeration
# Once a <20> (file server) host is found, enumerate shares/users over NetBIOS
enum4linux-ng -A <IP>
smbclient --no-pass -L //<IP>
crackmapexec smb <IP> --port 139 -u '' -p '' --shares
# See the pentesting-smb skill for full share/user/credential workflow
# Poison broadcast NBT-NS/LLMNR requests to capture NetNTLM hashessudo responder -I <iface> -wv
# Crack captured hashes offline or relay (see SMB relay in pentesting-smb)
hashcat -m 5600 captured_netntlmv2.txt wordlist.txt
Key Concepts
Concept
Description
Name Service (137)
Registers and resolves NetBIOS names; queried to obtain names and MAC (137/udp and 137/tcp)
Datagram Service (138)
Connectionless communication and broadcast messaging (138/udp)
Session Service (139)
Connection-oriented service carrying SMB over NetBIOS (139/tcp)
NetBIOS name
Up to 16 characters; the 16th byte is a suffix identifying the service/role
Name suffixes
<00> workstation/workgroup, <20> file server, <1B> domain master browser, <1C> DC group
NBT over IP vs SMB over IP
Port 139 is NBT over IP; port 445 is SMB directly over TCP/IP
NBT-NS poisoning
Spoofing broadcast name responses to capture NetNTLM authentication
Tools & Systems
Tool
Purpose
nmblookup
Resolve a host's NetBIOS name table and roles (-A <IP>)
nbtscan
Fast subnet sweep returning NetBIOS names and MAC addresses
nmap (nbstat.nse)
Scriptable name-table enumeration over UDP 137
enum4linux-ng
Aggregated NetBIOS/SMB enumeration once a file server is found
smbclient / smbmap
SMB access over port 139 when 445 is filtered
Responder
NBT-NS/LLMNR poisoning to capture NetNTLM hashes
impacket rpcdump.py
Map MSRPC endpoints over NetBIOS (port 139)
Common Scenarios
Scenario 1: Domain Discovery on a Flat LAN
nbtscan -r 10.0.0.0/24 returns each host's NetBIOS name and a <1C> group entry exposing the AD domain name, immediately scoping the Active Directory environment.
Scenario 2: 445 Filtered, 139 Open
A hardened host blocks 445 but leaves 139 open. Pointing smbclient -p 139 --no-pass -L //<IP> at the session service still enumerates shares, bypassing the partial filtering.
Scenario 3: File Server Identification
nmblookup -A <IP> shows a <20> suffix, confirming the host runs an SMB file server. The tester pivots to share enumeration and finds a readable backup share.
Scenario 4: Credential Capture via NBT-NS
Responder answers broadcast NetBIOS name queries for mistyped hostnames, capturing NetNTLMv2 hashes that are cracked offline to recover a domain user password.
Output Format
## NetBIOS Finding
**Service**: NetBIOS over TCP/IP
**Severity**: <Medium|Low|Informational>
**Host**: <IP> (137/udp, 138/udp, 139/tcp)
### Summary
<What was disclosed: hostname, workgroup/domain, MAC, file-server role, poisoning exposure>
### Name Table
| Name | Suffix | Type | Meaning |
|------|--------|------|---------|
| WIN-DC01 | <00> | UNIQUE | Hostname |
| CORP | <1C> | GROUP | Domain Controllers (domain = CORP) |
| WIN-DC01 | <20> | UNIQUE | SMB file server active |
MAC address: <xx:xx:xx:xx:xx:xx>
### Evidence
- Command: nmblookup -A <IP>
- Output: <name table excerpt>
### Recommendation
1. Disable NetBIOS over TCP/IP where not required (use DNS/SMB direct on 445)
2. Disable LLMNR and NBT-NS broadcast resolution to prevent poisoning
3. Restrict 137-139 to trusted management segments
4. Enable SMB signing to mitigate relay of captured NetNTLM hashes
