---
title: "Formatting MAC addresses in dhcpd"
slug: "aa-01039"
description: "The binary-to-ascii function strips leading zeroes when printing numeric values."
tags: ["isc dhcp"]
updated: 2018-10-25T18:39:15Z
published: 2018-10-25T18:39:15Z
canonical: "kb.isc.org/aa-01039"
stale: true
---

> ## Documentation Index
> Fetch the complete documentation index at: https://kb.isc.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Formatting MAC addresses in dhcpd (or 'why does binary-to-ascii strip leading zeroes?')

This question arises from time to time when an administrator uses binary-to-ascii in `dhcpd.conf`, hoping to obtain the hardware address of a device that has been assigned a lease. The problem is that where a component of the address has a leading zero, this zero will be omitted in the output.

Here's a typical example of usage that doesn't work quite as expected:

```
set foo = binary-to-ascii(16, 8, ":", substring(hardware, 1, 6));
```

Instead of the expected hardware address format - for example f8:<span style="background-color: #FFFF00">0c</span>:f3:\*\*:\*\*:\*\*, foo will instead contain f8:<span style="background-color: #FFFF00">c</span>:f3:\*\*:\*\*:\*\*.

This is not a bug. The problem is that the binary-to-ascii function doesn't "know" anything about the intended use of the converted binary digits and it's unusual to include leading zeroes when printing numeric values.

However, with a bit of additional manipulation it's still possible to get the desired result:

```
set foo = concat (
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,1,1))),2), ":",
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,2,1))),2), ":",
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,3,1))),2), ":",
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,4,1))),2), ":",
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,5,1))),2), ":",
suffix (concat ("0", binary-to-ascii (16, 8, "", substring(hardware,6,1))),2)
);
```

*(It operates by converting each "component" separately, adding a leading zero to it (in case one is needed); taking the last two hex characters, and then concatenating them all together again.)*