Why Did Some Tender Files Show 127.0.0.1? Uncovering a Hidden Bug in Electronic Bidding Tools

While browsing online, the author noticed a government notice about a tender project where the IP addresses recorded in the bid files of five companies were all "127.0.0.1". The notice, intended as a simple misconduct warning, unintentionally revealed a puzzling technical anomaly.

Netizens quickly mocked the oversight, pointing out that 127.0.0.1 is the standard loopback address and questioning how such a basic mistake could pass review.

投标文件 IP 地址一致，均为 127.0.0.1

Further investigation showed that the tender required the use of an official electronic bidding tool, which records the computer's IP, MAC address, and serial number when generating the bid document. Normally this ensures each bid is tied to a unique machine.

The author examined the official announcement, which emphasized the need for the latest version of the electronic bidding tool, implying the tool itself captures the IP address.

If the tool were malfunctioning, all bidders would see the same address, but only five did, while the other twelve showed normal external IPs, suggesting the tool worked correctly for most users.

A reverse‑engineered .NET DLL from the tool (found by a user on Zhihu) revealed the logic for obtaining the IP: it first tries to read the network adapter's IP, and if none is found, it falls back to DNS reverse lookup of the hostname.

Further code inspection showed that a valid IP is required to generate a bid file. One user discovered that disabling the "Internet Protocol Version 4 (TCP/IPv4)" network adapter forces the tool to record 127.0.0.1, effectively reproducing the anomaly.

https://www.zhihu.com/question/655664715/answer/3496103428

This demonstrates that, under specific conditions, the electronic bidding tool can indeed capture the loopback address, which the author classifies as a bug.

The article then speculates whether the five companies deliberately exploited this bug to conceal their true IPs or simply fell victim to it, and notes that the official notice only highlighted the abnormal IPs, leaving room for interpretation.

In conclusion, the incident illustrates how a seemingly minor software flaw can become a source of controversy in procurement processes.