urllib3 when connecting to https by IP address, client

The Fix

pip install urllib3==1.25

Based on closed urllib3/urllib3 issue #1224 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -0,0 +1,73 @@
@@ -0,0 +1,73 @@
+import mock
+import pytest
+from six import b

repro.py

Frame 31: 571 bytes on wire (4568 bits), 571 bytes captured (4568 bits) on interface 0
Ethernet II, Src: BizlinkK_XX:XX:XX (9c:eb:e8:XX:XX:XX), Dst: JuniperN_XX:XX:XX (08:81:f4:XX:XX:XX)
Internet Protocol Version 4, Src: 192.168.89.18, Dst: 8.8.8.8
Transmission Control Protocol, Src Port: 1156, Dst Port: 443, Seq: 1, Ack: 1, Len: 517
Secure Sockets Layer
    TLSv1.2 Record Layer: Handshake Protocol: Client Hello
        Content Type: Handshake (22)
        Version: TLS 1.0 (0x0301)
        Length: 512
        Handshake Protocol: Client Hello
            Handshake Type: Client Hello (1)
            Length: 508
            Version: TLS 1.2 (0x0303)
            Random
            Session ID Length: 0
            Cipher Suites Length: 148
            Cipher Suites (74 suites)
            Compression Methods Length: 1
            Compression Methods (1 method)
            Extensions Length: 319
            Extension: server_name
                Type: server_name (0x0000)
                Length: 12
                Server Name Indication extension
                    Server Name list length: 10
                    Server Name Type: host_name (0)
                    Server Name length: 7
                    Server Name: 8.8.8.8
            Extension: ec_point_formats
            Extension: elliptic_curves
            Extension: SessionTicket TLS
            Extension: signature_algorithms
            Extension: Heartbeat
            Extension: Padding

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: This fix should not be used if the server requires SNI for IP addresses.\n\n

Why This Fix Works in Production

Trigger: TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Mechanism: The SSL client hello incorrectly includes an IP address in the server_name extension, violating the SNI specification
Why the fix works: Prevents the use of SNI when connecting to an IP address, addressing a known bug in Python's SSL module. (first fixed release: 1.25).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

The SSL client hello incorrectly includes an IP address in the server_name extension, violating the SNI specification
Production symptom (often without a traceback): TLSv1.2 Record Layer: Handshake Protocol: Client Hello

Proof / Evidence

GitHub issue: #1224
Fix PR: https://github.com/urllib3/urllib3/pull/1350
First fixed release: 1.25
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.46

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“We're hitting this in pywinrm/Ansible as well if SNI is enabled”

@nitzmahone · 2017-11-29 · source

“This is a known bug in Python's ssl module. I'll fix it for 3.7 as soon as I can drop support for OpenSSL < 1.0.2.…”

@tiran · 2017-11-29 · source

“@nitzmahone In #1287 you wrote that SNI IP address is causing connection abort with Windows' http.sys. I didn't know that the issue can cause connection…”

@tiran · 2017-11-30 · source

“@tiran done: https://bugs.python.org/issue32185 - I see it's already been default-assigned to you :D”

@nitzmahone · 2017-11-30 · source

Failure Signature (Search String)

TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)

Minimal Reproduction

repro.py

Frame 31: 571 bytes on wire (4568 bits), 571 bytes captured (4568 bits) on interface 0
Ethernet II, Src: BizlinkK_XX:XX:XX (9c:eb:e8:XX:XX:XX), Dst: JuniperN_XX:XX:XX (08:81:f4:XX:XX:XX)
Internet Protocol Version 4, Src: 192.168.89.18, Dst: 8.8.8.8
Transmission Control Protocol, Src Port: 1156, Dst Port: 443, Seq: 1, Ack: 1, Len: 517
Secure Sockets Layer
    TLSv1.2 Record Layer: Handshake Protocol: Client Hello
        Content Type: Handshake (22)
        Version: TLS 1.0 (0x0301)
        Length: 512
        Handshake Protocol: Client Hello
            Handshake Type: Client Hello (1)
            Length: 508
            Version: TLS 1.2 (0x0303)
            Random
            Session ID Length: 0
            Cipher Suites Length: 148
            Cipher Suites (74 suites)
            Compression Methods Length: 1
            Compression Methods (1 method)
            Extensions Length: 319
            Extension: server_name
                Type: server_name (0x0000)
                Length: 12
                Server Name Indication extension
                    Server Name list length: 10
                    Server Name Type: host_name (0)
                    Server Name length: 7
                    Server Name: 8.8.8.8
            Extension: ec_point_formats
            Extension: elliptic_curves
            Extension: SessionTicket TLS
            Extension: signature_algorithms
            Extension: Heartbeat
            Extension: Padding

What Broke

Clients experience SSL handshake failures when connecting to servers using IP addresses.

Why It Broke

The SSL client hello incorrectly includes an IP address in the server_name extension, violating the SNI specification

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install urllib3==1.25

When NOT to use: This fix should not be used if the server requires SNI for IP addresses.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/urllib3/urllib3/pull/1350

First fixed release: 1.25

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the server requires SNI for IP addresses.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
1.25	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:configuration-error urllib3 hub urllib3 best practices All hubs All clusters

Related clusters: Data consistency SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

urllib3 when connecting to https by IP address, client hello message has a server_name SNI block (Fix)