# mosh

Mosh keeps a session alive when the network doesn't cooperate: switch from
Wi-Fi to cellular, close the lid, walk out of range — reconnect and the
session is still there. Tessera pairs it with something no other mosh client
on iOS offers: **real scrollback**.

Mosh requires `mosh-server` installed on the host, and UDP ports 60000–61000
open by default. Pick it per host in the
[host editor's transport setting](https://bambouville.com/docs/connections/#transport-ssh-or-mosh).

## Roaming

A mosh session is tied to your identity, not your network path. Tessera stays
attached across Wi-Fi ↔ cellular hops, address changes, and sleep/wake — the
session simply carries on where the packets left off. Over a VPN or a
tailnet this works the same way; mosh only needs UDP to reach the server.

## Real scrollback

Stock mosh has no scrollback — by design it synchronizes only the visible
screen, so anything that scrolls off is never sent. Every other iOS mosh
client inherits that limit and points you at tmux copy-mode.

Tessera synthesizes the history instead. In a tmux-backed mosh session, the
terminal rides UDP while a second SSH side channel speaks to tmux; scrolling
up pulls the real history through that channel and shows it in place —
searchable with [find in terminal](https://bambouville.com/docs/keyboard/#find-in-terminal), selectable,
intact.

![A mosh session in Tessera scrolled back through a long git log with commit messages and diffstats fully rendered, the shell prompt visible below.](https://bambouville.com/docs/assets/img/mosh-scrollback.png)

*A mosh session scrolled deep into a git log — history that stock mosh would have discarded.*

How it behaves while you read:

- Scrolling up reveals the history **frozen in place** — new output doesn't
  yank you back down.
- Returning to the bottom (or typing) dismisses the overlay onto the live,
  up-to-date screen.
- A hard fling keeps going: the first page loads instantly and the rest of
  the history streams in behind it, up to the server's retained depth.

## What it needs

- **A tmux launch mode.** Scrollback synthesis works with **auto-tmux** and
  **named tmux** sessions (auto-tmux is the default). A mosh host launched
  with a custom command has no tmux to ask, so it scrolls like stock mosh.
- **Server history.** Depth is bounded by the server's tmux `history-limit` —
  Tessera can only show what tmux retained.
- **The primary screen.** Full-screen programs like `vim`, `less`, and `htop`
  handle scrolling themselves; Tessera forwards your scroll to them as input,
  as every terminal does.

## Mosh and everything else

The same SSH side channel that fetches history also carries
[tmux windows and panes](https://bambouville.com/docs/tmux/) and
[port forwarding](https://bambouville.com/docs/port-forwarding/#mosh-and-forwarding) for mosh hosts, and
the [files panel](https://bambouville.com/docs/files/) opens its own SSH bridge — so file transfers and
tunnels work identically on mosh and plain SSH.

One caveat: **mosh UDP cannot traverse an SSH jump host**. On a
[jump-chained host](https://bambouville.com/docs/connections/#jump-hosts), Tessera tries mosh first and
falls back to SSH if the server is unreachable over UDP. A target that is
UDP-reachable anyway — over a VPN or tailnet, say — keeps working as mosh.