APT Hash Mismatches Behind a Caching Proxy

Investigate stale repository data and proxy caching without disabling the hash or signature checks that detected the problem.

APT's “Hash Sum mismatch” means downloaded data differs from its expected checksum. Possible causes include an inconsistent mirror, a stale cache, an incomplete transfer, or content altered along the path.

Asking for a fresh response

With the proxy route already configured, this comparison asks compliant HTTP caches for fresh package-index data:

sudo apt-get \
  -o Acquire::http::No-Cache=true \
  -o Acquire::https::No-Cache=true \
  update

The command updates local APT state. It sends cache-control instructions, not a remote cache purge, and cannot guarantee that every intermediary handles them correctly.

Finding the inconsistent file

Keep the failing repository URL and checksum details. If the mirror is synchronizing, check whether a later retry returns consistent data. An approved alternate route can help compare the same source.

Where diagnostics expose the status and content type, check for a proxy error page returned in place of repository data. A successful TCP connection is insufficient to establish a valid package response.

Keep signature, checksum, and HTTPS verification enabled. Signed metadata identifies the expected repository content; checksums link it to index and package files; TLS protects the connection. After repairing the stale cache, mirror, or transfer path, a verified download establishes that the data is consistent again.

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