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[GH-ISSUE #2951] --holepunch=false (relay mode) fails when UDP 21820 is blocked — server still requires recent hole punch for registration #11041

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opened 2026-05-06 15:43:48 -05:00 by GiteaMirror · 3 comments
GiteaMirror commented 2026-05-06 15:43:48 -05:00
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Originally created by @z10n-dev on GitHub (May 1, 2026).
Original GitHub issue: https://github.com/fosrl/pangolin/issues/2951

Describe the Bug

When UDP port 21820 is blocked by a firewall (e.g. school or corporate network), the --holepunch=false flag does not work as expected. Even with holepunch disabled on the client, the server still requires a successful hole punch within the last 5 seconds before it will respond to olm/wg/register. This makes it impossible to use relay mode on networks that block UDP 21820.

Actual behavior
Server-side validation in handleOlmRegisterMessage checks if (now - (client.lastHolePunch || 0) > 5 && sitesCount > 0) and skips the register regardless of whether the client requested relay mode.

Additional context

  • The connection works fine on a hotspot (UDP 21820 unblocked)
  • Port 21820 UDP is hardcoded in Gerbil and not yet configurable via config.yml (clients_start_port does not affect Gerbil's actual listening port)
  • This is a regression from pre-0.8.0 behavior, where relay fallback worked silently when hole punch failed
  • tcpdump on the server confirms hole punch packets from the school network never reach port 21820, but do reach 51820

Environment

  • Pangolin server: 1.18.1
  • Olm client: 0.8.0
  • OS: Arch Linux (kernel 7.0.2-arch1-1)
  • Network: School WiFi — UDP 21820 blocked, UDP 51820 allowed (confirmed via tcpdump)

To Reproduce

  1. Block outbound UDP port 21820 (or be on a network that does, e.g. school/corporate WiFi)
  2. Run: pangolin up --holepunch=false
  3. The websocket connects successfully, but the server keeps responding with:
    Client last hole punch is too old and we have sites to send; skipping this register
  4. The client times out and disconnects — relay mode never establishes

Expected Behavior

When --holepunch=false is passed, the server should accept the registration without requiring a recent hole punch and fall back to relay mode, just as it did in versions prior to 0.8.0.

Originally created by @z10n-dev on GitHub (May 1, 2026). Original GitHub issue: https://github.com/fosrl/pangolin/issues/2951 ### Describe the Bug When UDP port 21820 is blocked by a firewall (e.g. school or corporate network), the `--holepunch=false` flag does not work as expected. Even with holepunch disabled on the client, the server still requires a successful hole punch within the last 5 seconds before it will respond to `olm/wg/register`. This makes it impossible to use relay mode on networks that block UDP 21820. **Actual behavior** Server-side validation in `handleOlmRegisterMessage` checks `if (now - (client.lastHolePunch || 0) > 5 && sitesCount > 0)` and skips the register regardless of whether the client requested relay mode. **Additional context** - The connection works fine on a hotspot (UDP 21820 unblocked) - Port 21820 UDP is hardcoded in Gerbil and not yet configurable via config.yml (`clients_start_port` does not affect Gerbil's actual listening port) - This is a regression from pre-0.8.0 behavior, where relay fallback worked silently when hole punch failed - tcpdump on the server confirms hole punch packets from the school network never reach port 21820, but do reach 51820 ### Environment - Pangolin server: 1.18.1 - Olm client: 0.8.0 - OS: Arch Linux (kernel 7.0.2-arch1-1) - Network: School WiFi — UDP 21820 blocked, UDP 51820 allowed (confirmed via tcpdump) ### To Reproduce 1. Block outbound UDP port 21820 (or be on a network that does, e.g. school/corporate WiFi) 2. Run: `pangolin up --holepunch=false` 3. The websocket connects successfully, but the server keeps responding with: `Client last hole punch is too old and we have sites to send; skipping this register` 4. The client times out and disconnects — relay mode never establishes ### Expected Behavior When `--holepunch=false` is passed, the server should accept the registration without requiring a recent hole punch and fall back to relay mode, just as it did in versions prior to 0.8.0.
GiteaMirror commented 2026-05-06 15:43:48 -05:00
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@AstralDestiny commented on GitHub (May 1, 2026):

Relay is done via 21820 Out to the pangolin stack then it connects down to site over you going directly.

<!-- gh-comment-id:4358731769 --> @AstralDestiny commented on GitHub (May 1, 2026): Relay is done via 21820 Out to the pangolin stack then it connects down to site over you going directly.
GiteaMirror commented 2026-05-06 15:43:49 -05:00
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@z10n-dev commented on GitHub (May 1, 2026):

So there is no other way, than using my Hotspot, when the port 21820 is blocked in school wifi?

<!-- gh-comment-id:4358766476 --> @z10n-dev commented on GitHub (May 1, 2026): So there is no other way, than using my Hotspot, when the port 21820 is blocked in school wifi?
GiteaMirror commented 2026-05-06 15:43:49 -05:00
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@oschwartz10612 commented on GitHub (May 1, 2026):

Not right now sorry. Relay over UDP is the only option though I know some work is being done on websocket relaying which we might be able to take.

<!-- gh-comment-id:4360921345 --> @oschwartz10612 commented on GitHub (May 1, 2026): Not right now sorry. Relay over UDP is the only option though I know some work is being done on websocket relaying which we might be able to take.
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Reference: github-starred/pangolin#11041
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